RA-2 ABSOLUTE RANGE CALIBRATION

M. Roca(1), R. Francis(1), J. Font(2), A. Rius(3), E. Cardellach(3), T. Schuler(4), G. Hein(4), F. Lefèvre(5),

J. Durandeu(5), P-Y. LeTraon(5), C. Bouzinac(5), D. Gomis(6), S. Ruiz(6), M. Marcos(6), S. Monserrat(6),

R. Scharroo(7), E. Doornbos(7), A.Richter(8), G.Liebsch(8), R.Dietrich(8), A. Martellucci(1)

(1)ESA/ESTEC, Keplerlaan 1, 2200AG Noordwijk, (Email: mroca@jw.estec.esa.nl)(2)Institut de Ciències del Mar, ICM-CSIC, Barcelona

(3)Institut d'Estudis Espacials de Catalunya, IEEC-CSIC, Barcelona(4)Institute of Geodesy and Navigation, IfEN, Munchen

(5)CLS Direction Océanographie Spatiale, Toulouse(6)Institut Mediterrani d'Estudis Avançats, IMEDEA, Palma de Mallorca

(7)DEOS, TUDelft, Delft(8)TU Dresden, Inst. f. Planetare Geodäsie, Technische Universitaet, Dresden

ABSTRACT

The EnviSat altimeter, RA-2, is intended to continue an uninterrupted series of measurements of sea-level and ice-sheetelevation started by ERS-1 in 1991. To fully exploit these measurements, an absolute reference in the time series, and adistinction between instrumental artifacts and significant geophysical signals, is necessary. Therefore, the range bias andinstrument drift shall be determined with a accuracy of 1 cm and 1 mm/year respectively.

Such accuracies can only be achieved by using:

• a large number of measurements to reduce random errors; • a diversity of measurement techniques and independent data analysis to reduce susceptibility to systematic errors.

Due to the limited temporal sampling of the 35-day repeat orbit of EnviSat, the resulting overall concept is a regional cal-ibration which makes use of the north-western Mediterranean basin as a reference surface, with a number of particular ofhigh-confidence “super-sites”.

1. INTRODUCTION

The EnviSat-1 satellite embarks an innovative radar altimeter, the RA-2 which represents a new generation of radar al-timeters compared to previous instruments such as the ERS altimeters and TOPEX/Poseidon. This is due to its integrationof many important features.

In order to fully exploit the measurements from the RA-2 it is necessary to be able to relate this measurements to a generalreference system and to have knowledge about their stability. In other words it is necessary to calibrate the measurementsand determine their drift with time. Calibration can be performed in an absolute sense, by reference to independent meas-urements or it can be performed relative to similar measurements from other satellites when one is interested primarily incontinuity of a data-set rather than its absolute value. The calibration objectives for RA-2 are given in Table 1.

Parameter Bias Relative Bias Drift Dynamic Range

range 10 mm 1 mm 1 mm/yr not applicable

sigma-0 (dual freq) ±0.2 dB - - 5 - 20 dB

Table 1: Calibration objectives for RA-2. The Relative Bias is the bias with respect to other altimeters. The slope in windspeed and waveheight refers to the slope of a regression line when comparing RA-2 data to other sources.

__________________________________________________________________________________________________________Proc. of Envisat Validation Workshop, Frascati, Italy, 9 – 13 December 2002 (ESA SP-531, August 2003)

As in many previous instruments the RA-2 geophysical parameters will be cross-calibrated and the range measurementswill be absolutely calibrated. Uniquely among satellite altimeters the backscatter coefficient measurements will also beabsolutely calibrated.

Before the data is ready to be used for any calibration purpose an important activity has to be performed. This is the RA-2 In-flight Instrument Calibration and Level 1b Verification, the objective of which is to ensure the correct and optimisedfunctionality of the instrument in-flight and its operations, and the quality of the data to be used for calibration and vali-dation purposes.

2. OBJECTIVES OF THE ABSOLUTE RANGE CALIBRATION

The RA-2 altimeter is intended to continue an uninterrupted series of measurements of sea-level and ice-sheet elevationstarted by ERS-1 in 1991. To fully exploit these measurements it is necessary to determine the range bias and drift of theinstrument, both to provide an absolute reference in the time series and to distinguish between instrumental artifacts andsignificant geophysical signals. To satisfy these needs the required accuracy in the bias determination is extremely chal-lenging: 1 cm in bias error and 1 mm/year in bias drift.

Such accuracies can only be achieved by an experiment design which includes the following characteristics: a largenumber of measurements to reduce random errors; diversity of measurement techniques to reduce systematic errors andindependent data analysis, again to reduce the susceptibility to systematic errors. The design also has to take account ofpractical limitations: limited temporal sampling due to the 35-day repeat orbit of EnviSat; logistic constraints which limitthe geographical scope and limited resources.

The resulting overall concept is a regional calibration which effectively makes use of the north-western Mediterraneanbasin as a reference surface. However this concept has been modified by a number of particular and serendipitous circum-stances such that an additional layer of high-confidence “super-sites” have evolved within the overall framework.

The document that describes in detail the tasks to be performed regarding this activity and its plan is “RA-2 In-Orbit Ab-solute Calibration Plan: Range” (see [1]).

The team is composed by ESA members, P-I’s and Co-I’s from each of the AO proposal participating in the RA-2 absoluterange calibration and some scientists and engineers requested to join the team due to specific skills or resources, despitenot responding to the AO.

3. CONCEPT

The basics of the overall approach were frozen after the Muntanyà Workshop in March 1998 [2]. Later, modificationshave been incorporated on details of specific subjects, but the concept and the approach have been kept since. This conceptof the range absolute calibration and approach are summarised below.

The requirement on absolute range calibration is 1 cm; this is very stringent, being significantly more demanding than theabsolute bias calibration of ERS-1. Additionally the goal for determination of system drift is 1 mm/yr. These requirementsindicate:

• the need for a large number of measurements to reduce random errors;• multiple measurement sites to reduce the susceptibility to systematic errors;

windspeed ~0.05 m/s - - 3 - 20 m/s

significant waveheight 25 cm few cm - 1 - 10 m

Parameter Bias Relative Bias Drift Dynamic Range

Table 1: Calibration objectives for RA-2. The Relative Bias is the bias with respect to other altimeters. The slope in windspeed and waveheight refers to the slope of a regression line when comparing RA-2 data to other sources.

e sites

• drift is insensitive to systematic errors and may be determined at a single high-quality site.

These indications combined with the mission constraints, particularly the 35-day orbit which provides a dense coveragewith infrequent revisits, have led to the concept of range calibration over a region; the selected region is the north-westernMediterranean. This region benefits from a particularly dense coverage by laser ranging systems and reference GPS sites.The weather conditions are generally good and the ocean currents and tides are relatively weak leading to small altimetricsignals. The region, and the EnviSat-1 ground-tracks (corresponding to ascending and descending passes), as well as thenetwork of tide gauges (existing and foreseen) and a number of representative sites for GPS-equipped buoys, are shownin Fig.1.

The calibration will be based on the determination of sea-level. There are three key measurements (or vectors) as shownin Fig.2. These are:

• The altimeter range, corrected for propagation effects and sea-state bias.• The orbital height above the ellipsoid.• The sea-surface height above the ellipsoid.

Details of these primary measurements are given in the following sections.

42 43 44 45 46 47 48 49 50 51Mean Sea Surface [m]

52Tide GaugeCasablanca Platform

GPS BuoyWave Buoy 100 km

2000

2000

2000

2000

100

100

100

100

100

50

50

50

50

1000

1000

1000

1000

1000

1000

1000

50

50

4949

4948

48

48

48

48

47

47

47

47

47

46

46

46

46

46

46

45

45

45 45

47

44

44

43

44

Barcelona

Palma

MarseillesToulon

NiceMonaco

GenovaLa Spezia

44

43

42

41

40

39

11109876543210

Fig. 1: Absolute range calibration region showing the EnviSat ground-tracks and the tide-gauge network, including somproposed in the framework of the calibration.

4. RANGE

Altimeter measurements over the ocean surface have been retracked, as all RA-2 data, and corrected for environmentaleffects: propagation and sea-state bias. The retracker used is based on Hayne’s retracker with some corrections and mod-ifications [3]. Propagation corrections have been derived from in-situ measurements in the calibration region, supportedby modelling. Dedicated instrumentation include dual-frequency GPS receivers (some mounted in buoys) and upward-looking microwave radiometers.

4.1. Neutral atmosphere

The excess path length, introduced by air refractivity in the troposphere, results from the combined effects of water vapourand of other atmospheric components (mainly oxygen). The latter gives the major contribution to the total delay, never-theless the delay due to water vapour is characterized by larger temporal and spatial fluctuations and it is loosely correlatedto ground parameters. Therefore the assessment of delay introduced by vapour is critical for getting an accurate estimateof the total atmospheric delay. To this end the following techniques have been used during the Envisat RA-2 cal/val cam-paign:

• Spatial Meteorological Fields produced by Numerical Weather Prediction (NWP) systems.• Radiosonde data.• Microwave radiometers.• GPS receivers.

The propagation parameters relevant for this analysis have been assessed using the Liebe's model for air refractivityMPM93 [5]. In the following the total zenith delay, TZD, introduced by the atmosphere is calculated as the sum of thetotal zenith delay due to dry air, TDD, and with total zenith delay due to water vapour, TVD.

Fig. 2: Key measurement in the RA-2 Absolute Range calibration.

Ellipsoid

OrbitalHeight

Satellite Orbit

SLR

Sea SurfaceHeight

TideGauge

Measurement

RangeBenchmarks

rent

4.1.1.Spatial fields of Tropospheric delay derived from ECMWF products

The European Centre for Meteorological Weather Forecast (ECMWF) performs routinely an analysis process, based onthe assimilation of various meteorological measurements (e.g. ground, radiosonde and satellite data), and is used to ini-tialise the forecast process. The ECMWF analysis products can be used to calculate spatial fields of air refractivity.

The ECMWF analysis data used for RA2 Cal/Val campaign covered the period from 23rd of March to the 28th of October2002 and are applicable at 06:00, 12:00, 18:00 and 24:00 UTC time of the day. During this period the ENVISAT passedover the Cal/Val region about 185 times (considering both ascending and descending orbits). The lat-long resolution ofthe original data is 1 x 1 (deg) (corresponding to about 100 x 100 [km]).

The main limitation of this approach is represented by the temporal and spatial resolution. Nevertheless it has been usedin the following as reference for checking the actual accuracy of the other techniques (radiosonde, GPS and radiometers).The ECMWF products have been processed in order to: derive ground meteorological parameters; to increase the verticalresolution; to calculate tropospheric delay by means of the Liebe's model. The values of tropospheric delay at surfacepoints different from the original ECMWF spatial grid have been derived by spatial bi-linear interpolation. An exampleof the spatial fields of the total zenith delay during October 2002 over the calibration region is given in the Fig. 3.

The analysis of TDD spatial fields over sea revealed that the spatial fluctuations of the total zenith delay (TZD) are mainlydue to vapour effects. Accordingly, for the evaluation of total zenith delay in sites where the radiometers were installed,the value of the dry air delay, TDD, derived from the ECMWF spatial fields has been added to the radiometric estimationof the total zenith delay to due vapour.

4.1.2.Total zenith delay derived from Radiosonde Data in Palma de Mallorca

The data collected by the Radiosonde launched from Palma de Mallorca (Lat = 39.55 N [deg], Long: 2.62E [deg], altitude3 m) have been made available by the British Atmospheric Data Centre (BADC, United Kingdom). The radiosonde inPalma was usually launched every 12 hours but occasionally launches every 6 hours have been performed. The BADCarchive of measurements covers the period from 1990 to 2002.

The radiosonde data collected in Palma from April to October 2002 have been used as input to tropospheric model. Duringthe processing the profile vertical resolution has been increased using the same criteria adopted for ECMWF data analysisand when needed the values at surface were extrapolated from upper levels. The original data have been thoroughlychecked to remove errors and to select the profiles containing reliable data at surface and in the first levels near surface.As a result about the 80% of the overall radiosonde launches during the period (424) have been selected.

0 2 4 6 8 10 12

Longitude [degree]

38

40

42

44

Lati

tud

e [

deg

ree

]

1 9

2 0

2 0

2 1

2 1

2 2

2 2

2 3

2 3

2 4

2 4

Zenith Total Delay (ZTD)ECMWF Analysis Product - 28/10/2002 12:00 UTC

Fig. 3: Example of the spatial distribution of zenith atmospheric delay derived from ECMWF products. Location of concurradiosonde and radiometric measurement is also shown (++++ = radiometer, <> = radiosonde)

d

The time series of tropospheric delay during the Cal/Val Campaign (both total and dry) are given in Fig. 5. A clear sea-sonal pattern of the total zenith delay, due to the increase of total water vapour content during the summer, is clearly vis-ible. The mean value of the total tropospheric zenith delay in Palma was about 245 cm and the standard deviation valuewas about 4 [cm]. For dry delay the mean values and the std resulted to be 231 and 1 [cm] respectively.

Accuracy of radiosonde data has been derived by comparison with ECMWF data. Radiosonde and ECMWF data havebeen correlated and averaged, by determining ECMWF samples contained within a temporal window of about four hoursfrom the radiosonde measurement. The corresponding accuracy values are given in Table 2, on a monthly basis.

The average accuracy of radiosonde data with respect to ECMWF data is about 1% of the average value of the total zenithdelay and it may be ascribed to the combination of the radiosonde instrumental accuracy and to the effect of spatial aver-aging of the ECMWF data over a region that spans about 100 x 110 [km].

The bias value, that is constantly lower than 7 mm, confirms indirectly the similarity of the employed radiosonde pre-processing procedures with respect to the ones used by ECMWF to assimilate measurements.

Bias [mm] Stand. Dev. [mm]

April -1.2 27.6

May 2.8 21.7

June -6.6 18.3

July 3.2 19.6

August -1.9 20.5

September -6.4 25.6

October -1.1 21.2

Mean Value -2.7 22.6

Table 2: Relative accuracy of radiosonde and of ECMWF based assessment of the total zenith delay in Palma de Mallorca.

220222224226228230232234236238240242244246248250252254256258260

60 80 100 120 140 160 180 200 220 240 260 280 300 320 340

Zenit

h Dela

y [cm

]

Day of the Year

Palma de Mallorca, Radiosonde Data, April - October 2002

TotalDry

Fig. 5: Time series of the total zenith delay (TZD, upper curve) and total dry delay (TDD, lower curve) in Palma de Mallorca, erived from radiosonde data.

4.1.3.Total zenith delay derived from Radiometric Measurements

For the Envisat RA2 Cal/Val campaign, three ESA microwave radiometers have been installed at the beginning of 2002in the region (see fig. 1) at Eivissa (Lat = 38.98 N [deg], Long = 1.32 E [deg], altitude 3 m), Menorca (Lat = 40.02 N [deg],Long = 3.8 E [deg], altitude 3 m) and on the Casablanca Oil Platform (Lat = 39.55 N [deg], Long = 1.36 E [deg], altitude3 m). The radiometers' setting up is shown in Fig. 6.

During the Cal/Val campaign the mean value of the total tropospheric zenith delay in Menorca and The three radiometersshare a similar design. The main characteristics of the instruments are given in Table 2.

The radiometers installed in Menorca and Eivissa operated within nominal conditions. On the contrary the radiometer in-stalled on the Casablanca platform, due to failure of the data transmission system, measured only few days during the cal/val campaign period. Therefore it was excluded from the present analysis.

For the assessment of total delay due to water vapour, TVD, a linear retrieval algorithm based on atmospheric attenuationat two frequencies has been adopted. The accuracy of the algorithm for the 23.8 and 31.7 GHz frequency pair, estimated13 years of radiosonde data collected in Palma de Mallorca, is about 0.2 cm. The atmospheric attenuation is derived fromthe measured sky noise brightness using the monthly mean of the atmospheric mean radiating temperature, derived fromradiosonde data for each frequency. During the processing of radiometric data also the total atmospheric total liquid watercontent, Lt [mm], was also estimated using a linear retrieval algorithm and measurements with a value Lt higher than 1[mm] were excluded from delay analysis.

The results of the radiometric assessment of delay have been compared with ECMWF based estimation. The results aresummarised in Table 4.

Frequencies 21.3, 23.8 31.7 GHz

Integration Time 4 sec

Range of Measurement 0 - 313 K

Accuracy 1 K

Antenna Beam-width (3 dB) 1.9 [deg]

Manufacturer Rescom A/S, Denmark

Antenna Movement 0-90 [deg] Elevation

Table 3: Characteristics of the ESA microwave radiometers

Fig. 6: Pictures of the radiometers installed on the a) Casablanca Platform, b) Eivissa and c) Menorca

(a) (b) (c)

Regular calibration of the microwave radiometers, by means of the 'tipping curve' technique, have been performed duringthe Cal/Val period. Nevertheless the instruments exhibited drift of the circuit parameters, that has been revealed by thebias of the radiometric accuracy. The comparison of sky noise measurements with theoretical values calculated by usingthe MPM93 model and the radiosonde data, confirmed that, apart the slowly varying bias, the instrument were correctlyoperating. Therefore the error bias has been removed, on a monthly basis, from radiometric measurements.

Concerning the error standard deviation, it remained fairly constant during the whole period and it was similar to the errorstandard deviation of Radiosonde data in Palma de Mallorca (see table 1). Therefore, like in Palma, the differences be-tween radiometric measurements and ECMWF can be ascribed both to instrumental accuracy and the effect of ECMWFspatial averaging.

With regard to the calculation of the total zenith delay in Eivissa and Menorca, the total delay due to vapour, as derivedfrom radiometric measurements, has been corrected using the bias values given in table 4 and combined with the totaldelay due to dry air, derived by ECMWF spatial fields.

Eivissa was about 246.2 cm and the standard deviation was about 3.8 and 4 [cm] in Menorca and Eivissa respectively.

The time series of total zenith delay in Eivissa and Menorca during the measurement period are shown in Fig. 7.

4.2. Ionosphere

The ionospheric effect on the dual-frequency RA-2 would normally be compensated by using the two frequencies (at 13.5GHz in Ku-band and 3.2 GHz in S-band). The range measured at S-band must be calibrated independently so that it canbe used for this ionospheric correction. For the ionospheric refractive index the total electron content in the vertical col-umn needs to be determined by independent means. This is done by assimilation of high-density dual-frequency GPS data(plus other available data types such as DORIS) in the calibration region into ionospheric models.

4.3. Sea-state bias

The correction for sea-state bias at both Ku and S-band needs to established. Sea-state bias includes effects related to theradar interaction with the sea-surface as well as contributions due to the processing of the echo. These effects will be com-pensated for, as far as possible, in the processor. Further theoretical and empirical studies are in progress to amelioratethis major contribution to the overall error budget. An important consideration is to use only data for which the sea-stateeffect is negligible. Studies showed that around 60% of the data within the calibration region have a SWH lower than a 1m, which means that the sea-state bias can be considered negligible. This approach considerably simplifies the situation.

Menorca Eivissa

Month Bias [mm] Stand. Dev. [mm] Bias [mm] Stand. Dev. [mm]

April +4 27.8 51 21.4

May +3 17.6 53 25.1

June -2 21.6 45 22.0

July +40 18.6 67 20.7

August +52 20.8 68 22.4

September +56 20.1 55 19.9

October +93 10.7 70 17.8

Table 4: Relative accuracy of radiosonde and ECMWF based assessment of the total zenith delay in Menorca and Eivissa.

Ze

nith T

otal D

elay [

cm]

enorc

5. ORBITAL HEIGHT

The local trajectory has been obtained by performing a local fit to range measurements obtained by simultaneously track-ing laser rangers. The fit will be constrained by the restituted global orbit. The results are dependent on the particular com-binations of European lasers which may be used. Good results (variance of order 1cm over the calibration region) can beobtained with a set of 4 lasers, in particular the best combination is with the laser station at Graz, Grasse, Herstmonceuxand San Fernando. Tests show that the formal errors are consistent with actual ERS tracking results [4].

The number of passes for which coverage by at least three stations is present depends on the season. ERS-2 practical ex-perience shows that one out of three ascending (night-time) passes has tracking from 3 or more stations. With the availa-bility of Zimmerwald (and San Fernando) this is expected to be closer to 50% during the EnviSat Commissioning Phase.

Night-time passes normally are better tracked than day-time passes due to the inherent performance of the lasers. Howeverday-time passes are also taken into account for the calibration exercise with the advantage of being able to do crossovererror analysis between ascending and descending tracks.

6. SEA SURFACE HEIGHT

The sea-surface position, on average, lies on the surface of the geoid. It has an instantaneous deviation from the geoid dueto the influence of tides, currents and atmospheric effects.

Existing altimeter data combined with models of the time-varying effects mentioned above will be used to generate a meanprofile and its residual variability. An important problem is to link this mean profile to the tide gauge measurements as therelative geoid between the two is unknown. This problem may be solved by installing a GPS buoy on the profile in an areaof low variability, within a minimum range for high accuracy kinematic GPS positioning.

Individual RA-2 tracks will be compensated by local tide gauge measurements applying the models of tides, currents andatmospheric effects.

The evaluation of the actual bias value is depicted in Fig.8, which shows:

230

232

234

236

238

240

242

244

246

248

250

252

254

256

258

260

60 80 100 120 140 160 180 200 220 240 260 280 300 320 340

Day of the Year

April - October 2002

EivissaMenorca

Fig. 7: Time series of total zenith delay, derived from radiometric measurements and ECMWF spatial fields, in Eivissa and Ma during the Cal/Val period.

• determination of predicted surface on case by case basis (shown with variability derived from repeat track analy-sis in the sketch);

• determination of the confidence in this surface (combination of variability, distance from references, local condi-tions etc.);

• determination, over elementary averaging lengths (20 km in the sketch), of elementary bias values (bi);

This large-scale approach has been complemented by in-situ measurements from a number of high-quality, open-sea sitessuch as GPS buoys (mobile and fixed) and an oil-platform (Casablanca platform) in the Ebre mouth. These sites will beequipped with instruments for the in-situ measurements of local environmental parameters as well as precise positioningof the instantaneous sea-surface. Each of those sites is described in details in the following sections.

Within this very synthetic overview there are some important details:

• exploitation of other altimeter satellites: it has been recognised that by placing the highest quality measurement systems at cross-overs between EnviSat and Jason (for example) and performing relative calibration globally between these two altimeters, then the effective number of measurements of EnviSat is increased. This important characteristic unites absolute and relative calibration activities;

• determination of drift: a dedicated site on the ground-track fully equipped with instrumentation. Drift in the geo-physical parameters may also be measured by the use of regional/global tide gauge networks.

6.1. Light GPS Buoys off the Catalan coast

The objective of the light buoys experiments was to provide instantaneous geocentric sea level measurements at 10 dif-ferent ENVISAT sub-track points in the Northwestern Mediterranean area. A series of 45 experiments were performedalong the Catalan coast from April to October 2002. The following sections describe the hardware used, the campaignsperformed and the results obtained.

6

5

4

3

2

1

0

-1

Sea

Surf

ace

Heig

ht

100806040200Along-Track Distance

1.0

0.8

0.6

0.4

0.2

0.0

Confidence

Predicted Sea Surface “Altimeter” Sea Surface Normalised Confidence

Mean Predicted Surface

Mean Altimeter Measurement

bi

Fig. 8: Evaluation of the elementary bias value along a single track.

6.1.1.Hardware preparation

The observation system was composed by a bi-buoy system (seeFig.9) and a Reference Station (see Fig.10). The Refer-ence Station consists of:

One GPS receiver at double frequency, high sampling rate (ZFX, Ashtech);

One GPS Choke Ring antenna (Ashtech) mounted on a tripod; and

One low losses coaxial cable.

The instrumentation used at the bi-buoy system consist of:

Two double frequency GPS Choke Ring antennas (Ashtech);

Fig. 9: The bi-buoy system. Left: The buoys on board. Right: The buoys over the sea.

Fig. 10: Reference Station. Left: The antenna placed in the coast. Right: The receiver running.

Two coaxial cables of low losses, 50 meters each;

Two GPS receivers at double frequency, high sampling rate (ZXtreme, Ashtech),

Batteries, flash cards, ropes, floats,...

The bi-buoy system was constructed by ICM (Institut de Ciències del Mar, CSIC). The distance between the buoys was1.61 meters (see Fig.11), with an error of 20mm. The bone of the structure was an iron bar. The system was able to followthe sea movements because it tended to locate the main axis parallel to the wave front.

A measurement of the flotation line of the system was realized. The day before to the beginning of the campaigns (8th ofApril) the buoys were left in a tank of sea water and were noted the sinking marks of every buoy. This measurement pro-vides the distance between the bottom of the antenna and the sea level, 3.75cm. The GPS solutions provides the height ofthe bottom of the antenna with respect to the reference ellipsoid and not the level of the sea. By subtracting this quantityto the solutions the sea level at each point could be obtained.

During the campaigns several changes of the equipment occurred: the receiver of the ground station was changed one timeby another one with same characteristics, from the campaign number 33 (27th August) to the last (8th October).

Due to the increase of losses in the cables of the buoys, these were replaced by others with the same characteristics. Aprotection of a buoy was broken due to the collision of a buoy with the bottom of the boat, but was replaced by anotherone. All these changes did not affect the measurements.

6.1.2.Description of the campaigns

To fill the WVR Envisat requirements at minimum operational effort, the measurement points were chosen over the tracks(see Fig.12 and Table 5) at 15 km from the shore, approximately. The departure harbours for the different points werefrom the North to South: Palamós, Mataró, Port Ginesta, Cambrils, Vinaròs and El Grau de Castelló. The buoys were leftto drift freely around the nominal point within a radius of 2 km. The receivers collected data during a period of 3 hours,centred in the EnviSat overpass time. The reference station was set to collect data during a period of 4 hours overlappingthe sea operation. The systems were checked at ICM after each campaign.

78 cm

28 cm

8.5 cm

161 cm

83 cm

Fig. 11: Bi-buoy system design.

6.1.3.Data Process

The semi automatic software used for processing the data was AUTOGRAC. This software was developed at IEEC andpresents three main parts: Data preparation, Data Processing and Analysis of the results.

Data preparation The binary files were converted to RINEXASCII files. With these new files a Quality Check was per-formed. The data type conversion and the quality control were performed using UNAVCO TEQC.

Data Processing Once the rinex files were generated and the quality check passed. We used a Precise Point Positioning(PPP) mode by using JPL GIPSY/OASIS software and their products, to process the data. We follow the next steps:

• Calculation of the position and the tropospheric delay in the area of the Reference Station. The Reference Station was processed with the Earth tides models of Gipsy.

• Application of this tropospheric delay to the buoys, obtaining the buoys’ phase centre antenna position.

The buoys’ solution was processed without using the Ocean Loading and Earth tides models.

Point ID 344a 237d 115a 387a 008d

Lat 4140.8’ 4128.2’ 4124.3’ 4106.0’ 4105.3’

Lon 311.9’ 245.9’ 234.4’ 157.5’ 155.0’

Point ID 158a 280d 430a 201a 051d

Lat 4056.0’ 4054.0’ 4024.8’ 3953.5’ 3945.3’

Lon 117.7’ 108.1’ 045.0’ 012.2’ 002.3’

Table 5: Latitude and longitude coordinates of the EnviSat tracks.

Fig. 12: The Northwestern Mediterranean area and the Envisat tracks.

Analysis of the results AUTOGRAC used information from the campaign notes to exclude data obtained during the ex-periment. At the end of the process AUTOGRAC generated automatically a pdf report including the quality check and theresults. These reports contain the following quality indicators:

• Distance between buoys. • Difference in the vertical component of the buoys’ position (sea level). • Post-fit residuals. • Cost functional values for the inversion procedure and • Formal errors of the positioning.

The reports were posted on the GRACII web page.Table 6 shows the main parameters of the solutions averaged for thewhole campaign.

6.1.4.Results

The campaigns started 9th of April 2002 and finished on 8th of October. Of the 53 possible ENVISAT visits, 45 experi-ments were performed and 44 were processed satisfactorily. The number of visits per track were: 6 (10% of tracks), 5(40% of tracks), 4 (30% of tracks) and 3 (20% of tracks). 8 of the 44 solutions were calculated with only 1 buoy, becauseinstrumental problems. We had obtained the height with respect to the Reference Ellipsoid WGS84 (Earth Radius:6.378,137 km. Earth flattening: 1/298.257223563). It is important to note that the solutions presented here have to be cor-rected by the distance between the antenna reference point and the sea (3.75 cm). First of all we obtained the solution ofthe ground station. The relevant parameter obtained was the tropospheric delay. We applied this information to the processthe buoys data. Only twice measurements in the ground station failed, on the 9th April and on 21th August. In these casesa close reference station (permanent station) of the ICC (Institut Cartogràfic de Catalunya) network was used. Fig.13shows an example of the Total Zenith Tropospheric Delay obtained of a ground station.

Fig.14 shows the trajectory described by the buoys. In this figure, several events of the experiment are marked differences depending on the buoys directions. Fig.15 shows the height with respect to the Reference Ellipsoid WGS84 and the different events are marked. These events are related to the actions done during the experiment.

• Motor ON: Action related to the movement of the boat to the ENVISAT point.

• Motor OFF: Action related to stop the boat motor close to the ENVISAT point.

Another important parameter is showed at the bottom part of Fig.15, the distance between the buoys, it is important tonotice that the buoys direction. When the buoys were adrift, the distance between the buoys was constant and when theywere not adrift (Motor ON), this distance changed due to the tension done on the buoys.

Fig.16 shows the height of the antenna above the Reference Ellipsoid WGS84 for all the experiments. The solutions pre-sented a dispersion and some values are out of the range of the solution (compared with data of the ERS in the same point).With the purpose of reducing the dispersion of our solutions we have applied a crude inverse barometer correction (IB)and tides correction. In the next paragraphs are explained.

Formal uncertainty of 1 Hz positioning <3cm

Post-fit residuals of phase observables <1cm

Distance between buoys 2 cm agreement

RMS of the distance time series <3 cm

Difference between the 15 minute sea level solution obtained from each buoy <4 cm

Difference in the ’apparent’ sea roughness (vertical position RMS) <1cm

Table 6: Main parameters of the solutions

pri

Preliminary corrections

Inverse barometer correction The Inverse barometer correction takes into account the response of the sea surface tochanges in atmospheric pressure. It exists a simple form for this correction and it can be easily computed from the valueof the sea level pressure.

The inverted barometer correction is then:

IB(mm)=9.948 (P(mbar)

1013.3) (1)

The values of the pressure were obtained from the web site: http://www.gencat.es/servmet. By applying to the campaignsperformed we have found corrections ranging from 3 mm to 14 cm.

Tides correction The fourth generation CSR ocean tide model (CSR 4.0) obtained from analysis of TOPEX / POSEIDONaltimeter data, was applied. This model is based upon 239 cycles (6.4 years) of T / P altimetry. The smoothed ortoweightcorrections have a resolution of 0.5 x 0.5 degrees.

By applying to the campaigns performed we have found corrections ranging from 0 cm to 13 cm. In Fig.17 the solutionswith the corrections are showed.

6.1.5.Verification and conclusions

The results obtained present a similar height and dispersion to the values of the ERS. Without the corrections, some of thevalues are out of the ERS range. Applying the corrections these values bring near to the range.

These corrections have been done to verify that can correct some strange values with a mean sea level different to theothers. The various results for every point (track) provide redundancy in the solutions and provide a large quantity of datato calibrate the Altimeter of the Envisat. The quality controls used in the solutions (Quality check, heights of the buoys,

Fig. 13: Time evolution of the delay suffered by the microwave signal due to the troposphere in the ground station on 16th Al in Mataró harbour. It is called the Total Zenith Tropospheric Delay

l

is a

distance between the buoys) guarantee the solutions with small errors. The campaign notes written during all the experi-ments provided important information to the data process and they were very useful to detect some irregularities.

6.2. Moored GPS Buoys NW of the Menorca coast (Torben, et al.)

The method applied here to derive the altimeter bias makes use of differential GPS carrier phase measurements in orderto precisely determine the position of GPS-equipped buoys. The diameter of the radar altimeter footprint usually lies be-tween 3 and 7 km depending on the geographical latitude and the roughness of the sea. Several buoys are placed on posi-tions being covered by the ENVISAT ground tracks near the island of Menorca (Balearic Islands, Spain, Europe). If thesatellite orbit is precisely determined, a reference value for the altimeter measurements can be computed with help of thebuoy (by GPS) and the satellite position (by orbit determination).

Advantages. An advantage of this method is that a direct comparison is possible if the buoy is positioned within the al-timeter footprint. The fact that an altimeter measures the sea surface height over the area of the whole footprint whereasthe buoy only allows us to derive an instantaneous sea height at a distinct point within this footprint can be compensatedby methods of smoothing in time domain. Tide gauges are often used for this calibration work, too. However, the GPSbuoy approach offers an important advantage in comparison to such gauges: The buoys can be fixed to a cross-over posi-tion in the open sea whereas tides gauges are by nature bound to the coastal sites in most cases. The geophysical correc-tions to be applied when computing the radar altimeter bias make use of models which fit best in the open sea and ratherpoorly nearby the coastal areas (pile up effects) often leading to undesired systematic errors. This means that high-seaGPS buoys are of rather significant importance for the calibration work.

Objectives. The Global Positioning System has become a mature technology meanwhile that is no longer solely used forpositioning only. As the ENVISAT satellite also carries a microwave radiometer onboard and GPS allows to determinethis quantity, too, the mission goals can be extended accordingly to a radiometer check and calibration. In summary, the

Fig. 14: Trajectory described by the antennas of the buoys during the experiment. The time labels indicate the switch to a puling or to a free floating phase. SWFF for time of beginning of free floating phase and SWP when starts a power pulling phase. The nominal

location of the ENVISAT observation (predicted point) is marked with its ID: EGP#, while the approximate time of over pass lso listed in the legend

lour e is the as of

Fig. 15: Top: Time evolution of the height of the antennas reference point with respect to the Reference Ellipsoid. Each cocorresponds to the positioning of a single antenna. The red points represent BUOY2 and green ones BUOY3. the solid lin

smoothed solution through a spline interpolation. Bottom: Time evolution of the distance between the phase centre of the antennthe buoys.

nlysed ’ s

contributions of the Institute of Geodesy and Navigation within ESA's calibration and validation activities cover the fol-lowing items:

1 Derivation of precise sea surface heights at the satellite's cross-over points using GPS-equipped buoys and a nearby reference station that is to be installed at a proper location.

2 Investigation of long-baseline kinematic data processing and water vapour estimation. If this method proved to be effective, the expensive installation and maintenance of the nearby reference station would become superfluous and many more locations, not necessarily very near the coast, would become candidates for further calibration missions.

3 Integrated water vapour estimation by GPS using data of the nearby CIUX reference station installed by the Univer-sity FAF Munich.

4 Extraction of relevant meteorological quantities like total pressure, dry temperature, mean temperature and inte-grated water vapour from numerical weather fields to support the data analysis.

This section focuses on item 1, 3 and 4, but does not feature the work regarding the long-baseline kinematic GPS dataprocessing approach.

Fig. 16: Height above the Reference Ellipsoid WGS84 without corrections for all the experiments. The labels are the identity ame of the track placed in their correct latitude. Red circles represent the minimum and maximum values of the ERS solutions ana(6

cycles). The black triangles show the mean solution of the buoys for every day, the black lines are the sigma of every buoysolution.

inimuvery

6.2.1.Calibration Site

The calibration concept adopted by the European Space Agency ESA leads to a concentration of all different calibrationactivities to a particular region featuring major advantages for the bias determination. The calibration region should pro-vide ideal boundary conditions such as a dense coverage by laser tracking systems (orbit determination), a dense networkof tide gauges, small and well-known signals due to tides and currents, a well-known response to meteorological condi-tions, low average wave heights, a smooth and precisely known marine geoid, a dense network of meteorological sensors(pressure, water vapour), high-resolution and high-quality atmospheric models, just to mention the most important crite-ria. The European region was favoured not only for logistic reasons. The Western Mediterranean Sea appeared to fit therequirements best, as well as being the largest sea, and was finally chosen for the calibration campaign.

With the commissioning phase being shorter than 9 months during which all calibration activities are to be carried out,the number of satellite cross-over events over a particular station is rather limited. For this region, only cross-over sitescombining both ascending and descending ENVISAT tracks are of interest in order to gain an optimal redundancy.Fig. 18(a) illustrates the situation over the Balearic Islands. It should be noted that the distance between a GPS referencestation and the buoys is to be minimised in order to reduce atmospheric - in particular ionospheric - errors (and also, butof minor concern, GPS orbit uncertainties) and thereby to allow ambiguity fixing with a certain degree of integrity beingthe guarantor for a high-precision position determination. Additionally, the buoys need to be appropriately moored, i.e.the sea depth is also limited to approximately 100 metres. The cross-over point in the northern direction off the coast ofMallorca, for instance, is already in a region of large sea depth of more than 200 metres.

Finally, the cross-over point in the north-western part of Menorca was selected where the GPS buoy systems are locatednot to far away from the coast in order to allow precise differential positioning over a reasonably short baseline and, at thesame time, could be deployed in the open sea.

Fig. 17: Height above the Reference Ellipsoid WGS84 with corrections for all the experiments. Red circles represent the mm and maximum values of the ERS solutions analysed (6 cycles). The black triangles show the mean solution of the buoys for eday,

the black lines are the sigma of every buoys’ solution

of the ion

Fig. 18(b) shows that the buoys are arranged symmetrically around the nominal cross-over point. Ideally, the distance tothe reference site "CIUX" is in the range of 8 to 9 kilometres. The sea depth, however, is between 120 and 150 metresrequiring a special light-weight mooring system using stable fibre ropes.

6.2.2.The Reference Station

The principle of differential GPS implies to set up a reference station nearby the GPS buoys since the deduction of posi-tions of highest precision requires to take care of the error budget: atmospheric errors (troposphere and ionosphere) as wellas orbit uncertainties of the GPS satellites depend on the length of the baseline. Particularly the ionospheric delays quicklydegrade the measurements such that a reliable ambiguity fixing is no longer feasible. Reliable position estimates with cen-timetre accuracy can only be guaranteed up to distances in the range of 10 to approximately 20 kilometres between refer-ence station and buoy. Data processing using longer baselines is under investigation at the University and seems to befeasible as well, but has not yet become as mature and reliable as necessary.

Fig. 19 presents the scenery in which the reference station is located: Preference was given to a small but modern purifi-cation plant situated in the north of the town of Ciutadella. Apart from the conveniently small distance to the GPS plat-forms, the power supply was the least problematic issue here in contrast to all other available sites.

The following tasks of the CIUX reference station can be identified:

• Continuous GPS data recording at an interval of 30 seconds (IGS standard) for static self-positioning and inte-grated water vapour estimation.

• High-frequency (1 Hz) data logging during all cross-over events scheduled for the commissioning phase.• Collection of all relevant meteorological surface quantities to aid in the data analysis.• Use of a radio transceiver system for data and position reception from the buoys as well as to provide uplink capa-

bilities in order to update the observation schedule and to install firm- and software updates if necessary.

All mission-critical devices are redundant, i.e. two GPS reference station receivers were in use [6] and the same yields forthe logging and communication servers as well as for the telemetry units that can be either operated via a directional (Yagi)antenna or an omni-directional 2-metres long antenna [7]. Un interruptible power supplies (UPS) operated as surge filtersand allowed to bridge power outages of up to 15 minutes. All incoming data were primarily stored on a logging host com-puter, additionally stored on external discs and finally transmitted to the communication server from where the data weresent to the analysis centre at the Institute of Geodesy and Navigation once per day using GSM 900 communication chan-nels.

Fig. 18: (a) Extracted fro Fig. 1, EnviSat ground tracks over the region of the Balearic Islands, in the Western MediterraneanSea [1]. The large island in the middle is Mallorca, the eastward one where the reference station was set up is Menorca. (b) Map calibration area in the north-western part of Menorca island near the town of Ciutadella. The location of the reference stat is

indicated by the abbreviation "CIUX".

tat

sen

6.2.3.Buoy Sensor System

The University FAF Munich developed a high-precision high-sea GPS buoy system that is capable of autonomous oper-ation (see Fig. 20). The weight of one buoy (without payload) is around 2,400 kg with a volume of approximately 9.4 m3.The buoy’s attitude is stabilized by a heavy V-shaped steel chain. It is a system of heavy, moored buoys that can be placedat a favourable calibration site - a unique approach that has never been tried before as. In contrast to the light buoys whichare towed to the desired locations each time of an event, many more challenges are to be met in order to allow for accuratesea surface height determination.

Some of the major points that were to be considered are:

Autonomous Operation: The sensor platform, i.e. the buoy, must be able to withstand the conditions of rough sea con-ditions. It must be of sufficient size and weight, and must allow to carry not only all sensors and other electronic parts, butalso instruments meeting the security requirements such as radar reflectors as well as sea lights and visual signs. For this

Fig. 19: The purification plant (Ciutadella Nord, latitude: 40° 01’04” N, longitude: 3°47’45” E) where a dual-GPS reference sion (CIUX) is located. The distance to the GPS buoys is around 8 to 9 kilometres

Fig. 20: (a) Buoy “ODAS FTB2” (type “Große Fass-tonne”) as visited during the maintenance in June 2002. (b) A look into thesor box onboard the buoy. The GPS receiver can be seen on the very left side.

,Z)

reason, small and light-weight buoys are not usable in this context. The large sea depth of 120 to 140 metres places anadditional challenge with respect to the system design - conventional steel chains cannot be used as they would tear thebuoys down under the sea level due to their heavy weight. Finally, the system should be designed to operate autonomous-ly, i.e. energy supply using solar cells and rechargeable batteries is needed. Careful computation of the system’s energyconsumption is required to come up with an optimal solution for the energy system.

Communication and Remote Control: Once the system is deployed, it will be relatively difficult to access the buoy inorder to perform modifications related to hard- or software. For this reason, communication capabilities are a vital aspectof the measurement systems allowing for downlink of the collected data to the nearby reference station in order to performquality checks and data processing as well as for uplink capabilities in order to update the buoy’s software components ifnecessary and, should interactive intervention be required, for remote control of the buoy’s computer.

Sensor Fusion and Synchronization: Albeit a GPS receiver with antenna constitutes the heart of the buoy system, sev-eral additional sensors are needed. Attitude control is necessary since the GPS antenna is to be mounted at a certain heightin order to overcome multipath-corruption and to yield an unmasked satellite visibility. Further-more, the heavy buoy willnot respond immediately to sea waves passing through its position. In order to sense these natural sea height variations,water level measurements are needed. All these measurements need to be referenced to the same time frame and are to befused properly.

6.2.4.Static Data Processing

Static data processing comprises all data analysis tasks related to the CIUX reference station, namely GPS and meteoro-logical data quality checks, reference station coordinate processing and tropospheric delay estimation.

Data Quality

Fig. 21 illustrates that the GPS reference station receivers apparently showed unwanted variations of the signal quality.The indications of the carrier-to-noise ratio on the second GPS frequency were partly extremely poor. The reason for thisstrange behaviour still remains unclear. It is assumed that temperature effects on the GPS antenna or the receivers led tothese performance variations. However, fears that the poor C/N0 indications drastically decreased the performance of thepositioning and tropospheric delay estimation accuracy did not become true. All processing tasks could be performed withsufficient accuracy by forcing the analysis software to accept also measurements of low C/N0.

Reference Station Positioning

The final coordinate solution for the CIUX reference station antennas using data from March to November 2002 is givenin Fig. 22.

Daily coordinate solutions were processed by the TropAC analysis software which was developed at the University FAFMunich [8]. Weekly combinations were calculated from these solution files. The final coordinates represent the mean po-sitions of the GPS antennas during the calibration campaign.

Fig. 22: Final coordinate solutions for the two CIUX reference station GPS antennas. The geocentric coordinate triples (X,Yare given in units of metres with their standard deviations (red colour). The ellipsoidal height is around 80 metres. The geoid height in this

region is slightly less than 50 metres

son

ma

ris

Tropospheric Propagation Delay Estimation

Zenith total delays were estimated in the TropAC Kalman filter during the reference station processing as additional pa-rameters in order to aid in the validation process of the Envisat radiometer as well as ESA’s extra radiometer deployednext to the CIUX reference station.

The tropospheric delay estimates deduced via the two GPS receivers were compared with each other (see Figure 8, firstdata line) showing a typical RMS of about 5 to 6 millimetres. Additionally, the results were compared to troposphericdelays derived from NOAA NCEP GDAS 1° x 1° weather fields showing a RMS of about 1.5 centimetres which is thetypical accuracy of tropospheric delays for mid-latitude regions extracted from this weather model. All in all, it can bestated that the precision level of all results is within acceptable limits.

Fig. 21: Graphical illustration of RINEX Carrier-to-Noise flags (1: poor signal-to-noise ratio, 9: excellent ratio, 4-5: threshold from poor to good signal-to-noise) describing the reference station’s signal quality. The plot shows which observations had a reaably

good C/N0 indication depending on the elevation of the satellite (the lower the satellite is seen the smaller the C/N0 is by nature). The higher the elevation is until the signal is tracked with acceptable C/N0 the more problematic the situation becomes which isinly

true for the second GPS frequency in this example.

Fig. 23: Internal and external comparison of tropospheric zenith total delays. The first data line depicts the internal compaon between both GPS receivers, the last two lines describe the comparison with a 1° x 1° numerical weather model

the bia

6.2.5.Kinematic Data Processing

The instantaneous sea surface height is derived in several subsequent processing steps. The first step comprises a GPSkinematic data analysis in order to obtain the GPS antenna position during the ENVISAT cross-over. The primary process-ing suite was the commercial software package GeoGenius which was originally developed at the University FAF Munichunder the name TOPAS and later further improved and ported to the MS Windows operating system. Fig. 24 shows com-parisons of the GPS buoy antenna positions of GeoGenius with the software package TropAC. The TropAC-results werederived using the L1-frequency with LAMBDA-ambiguity fixing. Most processing solutions are in acceptable agreement.The L1-solution for DoY 147 exhibits some analysis problems and was replaced by the TropAC LW-solution (wide-lane).

For the determination of the sea surface height several more datasets are necessary. Additionally to the GPS data from thereference station and the buoys, the buoy sensor datasets (inclinometer, pressure gauge), a clock file for the time synchro-nisation and the precise ephemeris for the GPS satellites are needed.

An overview of the processing chain shows Figure 10. First of all the position of the reference station in Ciutadella (CIUX)was determined with help of the recorded GPS data using the TropAC software (see preceding paragraph). After theprocessing of data of two weeks the first reference coordinates were available with a standard deviation of several milli-metres. With these fixed coordinates the positions of the buoys could be calculated with the integrated surveying softwareGeoGenius2000. The kind of determination was kinematic double differences. The results of this processing were ellip-soidal heights (based on the International Terrestrial Reference Frame ITRF). Fig. 26 shows the trajectory of the day ofyear 227 (15 August 2002).

The measurement range of the processed heights amounts about 40 centimetres in this example. The small movement ofthe buoy depends on the good weather conditions (no rain and small wind force) on that day. Movement ranges of morethan 2 metres were reached on days with worse weather conditions.

An overview of the graphical presentation possibilities of the GeoGenius software is given in Fig. 27 for 8 May 2002.With the function ‘Trajectory’ it is possible to look at the ellipsoidal heights, the PDOP values, the root mean square ofthe heights and the number of received satellites. With these items of information one can review whether the results areuseable or if some processing settings must be changed. On the left side of the picture one can see the approximate positionof the buoy (symbol: square) and the movement of the buoy (green cluster of points) during the observation period of onehour.

Till now the results are the heights of the GPS-antenna on the buoys. But the position of the antenna is not identical withthe sea surface. Therefore some reductions are still to be applied.

For the reduction the measured inclination angles and the immersion depths of the buoys must be considered. The pressuregauge data were transformed into heights as a first step. Afterwards both sensor datasets must be merged. In an internalprocessing step the recorded data must be transformed from PC-time to GPS-time. The transformation parameters were

Fig. 24: Comparison of kinematic processing results of TropAC with GeoGenius in order to check for problems regardingpositioning results. (DoY: day of year; Bias: systematic error, mean difference; Sigma: standard deviation, bias-free, RMS: s is

included).

calculated with a software developed at the University. After the mentioned arrangements the reduction from the GPS-antenna to the instantaneous sea surface can be determined (see Fig. 28).

The main result of the calculation is the instantaneous sea surface height at any point of time of the observation period.But the time window of interest is centred on the cross-over time. Therefore, only a smaller number of the results is takeninto consideration for the final sea surface height taking into account the wave propagation. To filter the motion of thewaves a mean value was determined. To get a reasonable time value for forming of a mean value the motion of the waveswas analysed, i.e. power spectral densities of the declination angles which are correlated with the movement of the waveswere estimated. With help of the main frequency of the wave movement, the time a wave needs to move through the foot-print of the radar altimeter (effective diameter of about 3 km was assumed) could be determined yielding a approximatemean value of about 320 seconds around the cross-over time. Thus, all heights used for the calibration are arithmetic meanvalues over a time window of 320 seconds set symmetrically around the time of the satellite's cross-over.

Fig. 25: Data processing chain (overview

Fig. 26: ITRF heights after the processing with GeoGenius (FTB2, 15 August 2002

The quality of the results depends on the one hand on the accuracy of the particular measurements (GPS, inclination angle,pressure) and on the other hand on the fluctuation of the buoys caused by the weather conditions (wave heights). Fig. 29shows the mean sea height for the buoy ‘FTB2’ for all cross-over days as well as the corresponding standard deviation(error bar). A dependency of the quality (standard deviation) on the wave heights is represented in shades of green andred. The last results in autumn 2002 had to be determined with a mean value for the dip-in-depth due to a failure of thepressure sensor and are marked by blue dots instead of black ones.

During the measurement and the analysis of the data some different problems appeared. In order to reach continuous andcycle slip free GPS-solutions the elevation angle had to be decreased on several days within the GeoGenius data process-ing. This led to higher multipath errors and a poorer average signal-to-noise ratio. Several overflows of the pressure gaugecaused some outliers, too, that could be easily detected. A certain time delay of the recorded inclination data appeared thatis caused by the internal low-pass filtering of the inclinometer showing a dynamic-dependent error behaviour. The windyweather causing rougher sea conditions during spring and autumn than expected led to higher wave heights correspondingto high declination angles which partly caused higher standard deviations of the results. Hence the heights on days withhigher wave heights than approximately 0.75 metres are considered as less reliable as well as less accurate. Usable resultsare marked in green colours in Fig. 29.

A comparison between the two buoys FTB2 and FTB4 is shown in Figure 15. After removing the systematic offsets dueto the difference in location, the agreement between both buoys is in the level of a few centimetres.

Fig. 27: Graphical presentation with GeoGenius (FTB2, 05 August 2002

Fig. 28: Height reduction from GPS-antenna to instantaneous sea surface.

Two outliers are visible: The one on DoY 087 can be explained with high wave heights (about 1.8 metres) on this day.

6.2.6.Concluding Remarks

A lot of experience was gained during the ENVISAT calibration campaign that is of high value for future missions of thisand similar kind. All components of the reference station always met the expectations and, thus, no changes for future

Fig. 29: Mean heights with standard deviation (FTB2

Fig. 30: Height differences between FTB2 and FTB4; systematic offset due to differences in location removed

missions are necessary, but the GPS receivers should be replaced by devices showing a more reliable tracking perform-ance.

Regarding the buoy systems, the use of pressure sensors for dip-in-depth measurements proved to be partly problematic.A replacement by a different technology, e.g. magneto-restrictive or capacitive devices, infrared or sonar sensors is rec-ommended. Finally, the attitude correction should be improved. The best approach seems to be a dual GPS-antenna systemwith the dip-in-depth sensor mounted on one rod.

6.3. The Casablanca Tower

The Casablanca tower is an oil drilling rig belonging to REPSOL YPF situated in the NW Mediterranean Sea, close to thecontinental shelf break 40 km off the Ebro river delta, where the water depth is 165 m. Due to its specific geographicalposition (out of the coastal zone, near the shelf/slope front and current, area restricted to navigation, often under the influ-ence of the Ebro fresh water plume) it was used from the early eighties to deploy oceanographic instruments. Recently ithas hosted the WISE (Wind and Salinity Experiment) 2000 and 2001 campaigns in support of the ESA's SMOS (SoilMoisture and Ocean Salinity) mission.

It is located at 40° 43.02’ N, 1° 21.50’ E, within the ENVISAT RA-2 field-of-view on track 158a and 22 km away fromtrack 280d. The tower was chosen to install a tide gauge, a GPS station for long-term tracking and a microwave radiometerto contribute to the RA-2 range calibration during a period of two years. The instruments were deployed in early February2002. A meteorological station is operational in the platform on top of a 69 m high telecommunications tower. Due to missfunctioning of its relative humidity sensor a new one was installed in October 2002.

The GPS choke-ring antenna was installed at the highest level available (helicopter deck) and provides two-frequency dataevery second for a precise absolute vertical referentiation.

6.3.1.Tide Gauge Measurements

The Casablanca sea level recorder is an Aanderaa WLR7 pressure sensor, the same model used in the Eivissa island (seesection 6.4). It was fixed by divers at about 7 m depth in a vertical tube in the northern part of the platform. An AanderaaP280 atmospheric pressure sensor was installed for correction in a deck at 30 m above sea level. Both instruments wereset at a 10 min. sampling interval. They had been bought for this specific operation and inter-compared with the Eivissaunits under controlled conditions in Mallorca before deployment.

To compute the absolute sea level the atmospheric pressure data recorded at 30 m were reduced to 0 m using air temper-ature and humidity information. Then the pressure of the water column was obtained by subtracting this surface air pres-

Fig. 31: Location of the Casablanca oil rig in the NW Mediterranean and the nearby ENVISAT tracks

sure from the WLR7 pressure records. The transformation from pressure to water height was done using the hydrostaticequation, with a value of 9.0822 m/s2 for the local gravity acceleration according to the Normal Gravity Formula and theGeodetic Reference System GRS80. The water density is an important parameter in this transformation and its preciseknowledge is not easy from the routine measurements at Casablanca. The WLR7 records water temperature and conduc-tivity (the latter only after September 2002) at 7 m, but the mean density of the column can change due to the upper layervertical stratification, mainly in temperature. To complement this information we have used the surface temperature pro-vided by a wave buoy moored 35 km away, and occasionally vertical temperature and salinity profiles recorded from theplatform itself or from boats in nearby locations during the light GPS buoys measurements (see Section 6.1.).

The above described procedure gives the sea level with respect to the WLR7 pressure sensor location. The vertical dis-tance from this point to a reference in the lowest deck of the platform was mechanically measured by divers during theinstrument deployment. New measurements will be done in further operations to check the uncertainty of this distance,supposed to be of the order of 1 cm. Using optical means and GPS records the Institut Cartogràfic de Catalunya in July2002 provided an absolute vertical coordinate for this reference point with respect to the WGS84 ellipsoid with an uncer-tainty of 0.45 cm. Then the absolute sea level referred to WGS84 was finally computed at 10 min. intervals.

The error budget, as explained in section 6.4.2, has to consider the different data sources. In our case the systematic errorof the pressure sensors produces an uncertainty of 0.55 cm in the sea level, the transformation of air pressure to surface0.4 cm, the effect of water density determination is 0.1 to 0.4 cm depending on the season, while the vertical referentiationhas still to be checked. In total the absolute sea level measured at Casablanca will have an estimated error of the order of2 cm.

Fig. 32 shows the computed sea level during the ENVISAT calibration period.

Fig. 32: Absolute sea level measured at Casablanca tower from mid April until mid November 2002

6.4. Coastal Tide Gauges in Eivissa

Tide gauges were deployed following two complementary strategies. On one hand, three tide gauges were deployed atopen sea (two in Menorca, close to the GPS buoys and one in Eivissa). The advantage of open-ocean deployment is thatthey can be located just at satellite cross-overs, so that no spatial extrapolation is required for the calibration of the altim-eter. Their major drawback is that they cannot be vertically referenced, and therefore they cannot provide a measure ofabsolute sea level, but only of sea level variations. A secondary drawback is the difficulty to measure in a continuous waythe density of the whole water column, which is in principle requested to convert the water pressure measured by the in-struments into sea surface height. Density profiles were periodically obtained (using a CTD probe) in the vicinity of theinstruments, but some kind of time interpolation is required in between CTD measurements.

On the other hand, a coastal tide gauge was deployed in Eivissa, namely in Sant Antoni harbour. The main advantages ofcoastal instruments is that they can be vertically referenced, and also that the density of the water column can be easilyderived from complementary sensors of the tide gauge. Therefore, coastal tide gauges provide an accurate measure of ab-solute sea level height. Their major drawback is that they are usually not located over the altimeter ground track, so thatsome kind of spatial extrapolation is required. A secondary drawback is that if they are located in semi-enclosed waterbodies such as bays or inlets, then the seiche or eigenmode oscillations must be filtered out (or at least damped) for themeasurements to be representative of open sea level.

6.4.1.Open-ocean Tide Gauge Measurements

Open ocean pressure tide gauge measurements have been carried out at two sites: at the location of the moored buoysnorth-west of Menorca, and near Es Vedrà, a small island south-west of Eivissa.

The principal aims of these measurements were the determination of

• tidal constituents• atmospheric effects on sea-level variations (effects of the inverse barometer)• relative sea-level variations (within the duration of an EnviSat transition)

at the open sea in the close vicinity of EnviSat crossover points.

The pressure gauges were operated on the sea floor in a depth of 130 m at Menorca, and 50 m at Es Vedrà.The determi-nation of sea-level heights with pressure gauges is based on the hydrostatic relationship

(2)

Whereas the gravity g is known with sufficient accuracy, the air pressure (AP) and the density of the water column abovethe pressure gauge (ρ) have to be determined in order to derive sea-level heights (H) from the measured pressure data (P).For the reduction of the atmospheric pressure, the continuous air pressure recordings at the Ciutadella GPS reference sta-tion (8 km from Menorca tide gauge) and at San Antonio (Eivissa, 15 km from Es Vedrà tide gauge) were taken. In addi-tion to the oceanographic database of the Mediterranean Sea (MEDAR/MEDATLAS), data gathered from four CTD castsat each location during the tide gauge operation as well as temperature and salinity measurements from the tide gauges atthe sea bottom have been used to determine the vertical water density profile and its temporal behaviour.

The analysis of the tide gauge data results in sea-level time series of about 240 days duration and a sampling rate of 10minutes (Fig. 33). The preliminary results for both sites are very similar to each other and will be demonstrated exempla-rily for the Menorca tide gauge.

An analysis of the time series using the POL/PSMSL Tidal Analysis Software Kit (TASK 2000) revealed the amplitudesand phases of the major tidal constituents. The largest tide (M2) shows an amplitude of 4.8 cm (see Table 7). It is inter-esting to note that the tide model FES99 reveals an amphidrome of the M2 tide just west of Eivissa. Fig. 34 illustrates themeasured sea-level variations at Menorca with the dominating tidal signal.

H P AP–ρ g⋅

-----------------=

Menorca Es Vedrà

Tide Amplitude (mm) Phase (°) Amplitude (mm) Phase (°)

M2 48 217.6 15 179.3

K1 39 166.7 34 157.3

MM 39 160.0 45 160.7

O1 21 103.1 22 103.7

S2 19 238.8 4 209.9

P1 12 149.6 9 148.8

MF 11 64.7 12 28.0

N2 10 206.5 4 177.6

Table 7: Amplitudes and phases of major tidal constituents derived from tide gauge measurements

Fig. 33: Sea-level record of Menorca tide gauge. Heights are relative to the position of the bottom gauge

Fig. 34: Sea level and tidal signal at Menorca tide gauge location during July 2002

The residuals of the non-tidal sea-level changes indicate a high correlation to the air pressure time series. This is due tothe effect of the “inverse barometer” Fig. 35). Most of the atmospheric pressure variations occur in a frequency band from0.02 to 0.33 cycles/day, corresponding to cycle duration of 3 to 50 days. In these frequencies, they are well reflected bythe measured sea-level oscillations (see Fig. 36).

The comparison of the measured sea-level time series from Menorca with the tide gauge in San Antonio (vertically stablemounted on the harbour dock, distance from Menorca: 240 km) shows a slight trend, which could be explained by a sink-ing of the Menorca tide gauge of 10 cm maximum. However, the comparison of the Es Vedrà tide gauge with San Antonio(distance: 15 km) reveals no significant linear trend. This allows the assumption, that the Es Vedrà tide gauge has not suf-fered a larger rate of sinking into the sea ground.

Fig. 35: The “inverse barometer” effect: Air-pressure and sea-level variations (tides removed) at Menorca.

Fig. 36: Fourier analysis of sea-level variation and air pressure. The air pressure is converted to mm of equivalent water column. The upper graph indicates the ratio air pressure/sea level for the particular frequencies

ssion on

For the assessment of the error budget it is necessary to distinguish between the reliability of the sea-level variations overthe entire time span, where the influences of water density variations, the accuracy of the air pressure data as well as apossible sinking of the tide gauge have to be considered, and the accuracy of short-term changes (e.g. one hour during thepass of EnviSat). The respective contributions of the different sources of uncertainty (pessimistic estimates) are given indetail in Table 8.

The tide gauge measurements at Menorca can be compared to the results of the moored buoys data. Whereas the buoysare sensitive to high frequent sea-level changes such as waves due to their high sampling rate, the tide gauges monitoronly variations over longer periods (Fig. 37).

In this manner, differences of the mean sea level of GPS buoy and tide gauge during the duration of the GPS measurementscan be computed for each EnviSat pass. Apart from the influence of a possible sinking of the tide gauge, this differenceshould remain within the limits of the measurement accuracies during the entire tide gauge operation (Fig. 38).

It can be summarised that continuous time series of sea-level variations have been recorded successfully at two sites atopen sea. In Menorca the achieved accuracies amount to 5.5 cm over the entire measurement duration, and 4 cm within ashort period (1 h). The shift of the tide gauge’s zero point due to sinking contributes less than 10 cm over the entire oper-ation time. The Es Vedrà tide gauge features accuracies of 3.5 cm resp. 2 cm, and a insignificant sinking signal.

The results have been used to determine in-situ tidal and atmospheric contributions to sea-level variations and to verifyrespective models as well as for independent short-time comparisons with measurements of other sensors (GPS-buoys,coastal tide gauges).

Menorca Es Vedrà

Determination of: Long-term variation

Short-term variation

Long-term variation

Short-term variation

Accuracy of Pressure Sensor: ±40 mm ±40 mm ±20 mm ±20 mm

Reduction of Air Pressure: ±2 mm 0 ±2 mm 0

Density Variation of Water Column: ±35 mm 0 ±25 mm 0

Shift of Zero Point (sinking): <100 mm 0 <20 mm 0

Table 8: Error Budget of Menorca (130 m depth) and Es Vedrà (50 m depth) tide gauges

Fig. 37: Comparison of Menorca tide gauge measurements with sea level heights derived from GPS buoy FTB2 for one seSeptember, 19 2002

eights

6.4.2.Costal Tide Gauge Measurements

A bottom-pressure tide gauge was deployed at Sant Antoni harbour, on the NW coast of Eivissa Island, about 8 nm awayfrom an EnviSat cross-over and 4 nm away from a JASON ground track. The instrument was deployed at about 2 m depth,exactly at 47.1713 m above the reference ellipsoid, according to the vertical referencing carried out from a combinationof 5 days of GPS data and optoelectronic positioning.

The principal aims of this tide gauge deployment were to:

• provide an accurate measure of absolute sea level relatively close to satellite ground tracks• allow a cross-validation with the open-ocean tide gauge deployed at Es Vedrà (13 nm to the SW)

A key issue regarding the first objective is the derivation of the error budget. As for open-ocean tide gauges, the determi-nation of sea-level height from water pressure bases on the hydrostatic relationship (Eq.2). The differences with respectto the latter are that the density derived from the temperature and conductivity sensors of the instrument can be very rea-sonably assumed to be the same for the whole (2 m) water column, and that the atmospheric pressure sensor was collocatedwith the tide gauge. Under this framework, the main error source is the accuracy of the instrument sensors, namely: theaccuracy of a single measurement, which affects random errors, and the calibration accuracy (0.02% of actual pressure =2.4 mm), which affects systematic errors. An example of the several types of data involved in the process are shown inFig. 39.

Regarding random errors, water pressure was recorded every 2 min with an accuracy of 0.01% of the measurement range(60 dbar) = 0.6 mbar, and atmospheric pressure was recorded every 10 minutes with an accuracy of 0.2 mbar. Hence, therandom errors of a single measurement could be up to 6.5 mm. However, errors reduce significantly when averaging evena small number of measurements: total random errors for a time series of sea level averaged every 10 minutes would beof the order of 3.5 mm, decreasing to 1.3 mm for hourly averages.

The effect of systematic errors is probably more relevant to the calibration experiment, as they can lead to an absolutebias. These errors come mainly from the calibration accuracy of the tide gauge (0.02% of actual pressure = 0.24 mbar),the accuracy of the atmospheric pressure sensor (0.2 mbar) and from the vertical referencing. The first two sources alto-gether provide a maximum bias of less than 5 mm. Instead, the vertical referencing, had an associated error of the orderof 8 mm. This makes a total of 1.3 cm, which is higher that the targeted bias. Despite the inter-comparison of satellite datawith a whole set of sites would still allow to match the targeted bias, actions have been undertaken to decrease systematicerrors. Namely, a continuous GPS station will be installed close to the tide gauge by the beginning of 2003 in the frame-work of a European project [1]. This will allow a significant reduction of the systematic error, as well as a longer termmonitoring of eventual crustal movements (this sites is planned to be operative at least until the end of 2005, partly fundedby an ESA contract and partly by the European Project [9]).

Fig. 38: The temporal behaviour of the zero point of the Menorca tide gauge as computed from comparisons to sea-level hmeasured by buoy FTB2. The variation remains within the measurement accuracy.

At the time of the Validation Meeting, five time series of absolute sea level height covering from the 25th of March to the16th of November 2002 were delivered for calibration purposes. The second phase of the data processing is presently be-ing undertaken. This will consist of two main processes, namely the filtering of the harbour seiche and the cross-validationwith the open-ocean tide gauge located close to Es Vedrà.

Regarding the first process, the periodicity of the seiche had been determined to be of the order of 12 minutes, accordingto preliminary measurements. Hence, the sampling step of the instrument was set to 2 minutes, in order to resolve theseiche. An example of these short-period oscillations is shown in Fig. 40, superimposed on the tidal oscillations. For thatevent, they reached of the order of 10 cm and the external forcing lasted for a few days.

Regarding the inter-comparison with the open-ocean tide gauge deployed at Es Vedrà, an example of the agreement be-tween the two time series is shown in Fig. 41. Tidal and atmospheric effects have been removed from both series prior tothe comparison, so that the obtained differences are due to spatial sea-level variations (as well as to measurement errors

Fig. 39: Example of the different types of data involved in the computation of sea level from bottom pressure

Fig. 40: Example of a forced seiche in Sant Antoni harbour (3-8 June 2002)

of both pressure gauges). Combining the two time series it is expected to provide an improved sea level measure at thesatellite cross over.

6.5. Coastal Tide Gauges in the NW Mediterranean Coast

So as to complete the high confidence “super-site” tide gauges which were specifically moored for the calibration study,a set of coastal tide gauges was also selected. The measurements of these tide gauges were referenced to the ENVISATgeoid so as to compare the data with the remote sensing ones. Table 9 indicates the selected tide gauges with their owncharacteristics. In particular, the last column provides the end date of availability at the date of the validation workshop.The differences between dates are essentially the consequence of technical problems due the time gap which is necessaryto retrieve the data, to process them and to validate them.

Several tide gauges which were previously selected for the comparison are not listed there. Capria sea level station wasnot available for the study because of technical problems. Rough weather conditions damaged the instrument. Toulon,Nice and Monaco sea level stations are not absolute referenced at the present time. Their data are not useful for the RA-2absolute calibration. Sète tide gauge, initially selected, has been removed from the listing because of data acquisition prob-lems. Barcelona tide gauge was initially added in the listing but administrative problems prevent from accessing the data.However, essentially due to Spanish cooperation, Palma tide gauge measurements were introduced although they werenot foreseen at the beginning of the study.

To compare the in situ data with RA-2, absolute references of each tide gauge was necessary. The references were col-lected from each responsible organism and converted to correspond to the ENVISAT geoid (WGS84).

Name Longitude Latitude Provider Available until

Palma 2.624600 39.552600 ICM 25 November 2002

Sant Antoni 1.301749 38.978470 IMEDIA/ESA 12 July 2002

Casablanca 1.358330 40.717000 ICM/ESA 15 November 2002

Marseille 5.353667 43.278833 SHOM/LEGOS 24 November 2002

Ajaccio 8.764000 41.920633 SHOM/LEGOS 24 November 2002

Senetosa 8.814967 41.54985 CNES/LEGOS 16 September 2002

Table 9: Coastal tide gauges utilised in the calibration study

Fig. 41: Comparison between the tide gauges deployed at Es Vedrà and Sant Antoni harbour (about 13 nm apart)

6.6. Data Processing for Sea Surface Height Determination

As explained in the chapter before, the RA-2 measurements were compared to a set of tide gauge measurements Figure 1presents the location of the utilised tide gauges.

RA-2 data can be compared directly to in situ data if an ENVISAT track passes just over the sea level station. But a prob-lem remains if no ENVISAT track passes over. For instance Casablanca is about 3 km of the nearest theoretical ascendingtrack and about 25 km of the nearest theoretical descending track. Fig.43 focused on the Casablanca location. The crossesare the projections of the theoretical ENVISAT tracks on the ocean surface.

Thus, the problem is to compare ENVISAT data with in situ station which is not under ENVISAT tracks. The solutionwhich was taken, was to propagate in-situ measurements towards the satellite tracks through geophysical models.

A specific software was developed and improved to automatically compute the Sea Surface Height (SSH) estimations atin-situ gauges location and at the nearest point on ENVISAT track. SSH is approximated by the sum of the Mean SeaSurface (MSS), the tidal elevation and the sea level elevation due to atmospheric effects. Those three geophysical correc-tions allow to link in situ measurements to ENVISAT measurements. New geophysical models were used:

MSS: the CLS.01 MSS model (10)

Atmospheric effects: the Mediterranean MOG2D LEGOS atmospheric model (11)

Tides: the FES99 LEGOS/CLS tide model (12).

Fig. 42: Calibration region with tide gauges utilised in the comparisons and ENVISAT tracks

(3)

u + (4)

7. DETERMINATION OF THE ABSOLUTE RANGE BIAS AND RESULTS

7.1. Determination of the Bias

With the three geophysical models introduced before, the EnviSat bias can be computed according to the simple formula:

Bias = ∆EnviSat - ∆TG (+/- errors on models and measurements)

Fig.44 summarizes the processing to deduce the RA-2 bias at a location compared with the nearest tide gauge by propa-gating the in situ measurements toward the remote sensing measurements with the geophysical models.

7.2. Errors and Corrections

It is necessary to determine the confidence in the computed surface with the models. Indeed errors introduced by the mod-els need to be evaluated so as to provide a confidence on the computed bias. This is the reason why, specific model errorswere computed for the study for the CLS.01 MSS model and or the FES99 tide model. For the MOG2 atmospheric model,no error was computed.

CLS01 MSS error is a formal error computed according to an objective analysis algorithm (Fig.45).

FES99 error on the Mediterranean area is computed by taking into account bathymetry and tide gauge error (Fig.46).

It is noticeable that errors are stronger along shorelines.

Several corrections have to be applied to retrieve SSH from ENVISAT. Indeed, the SSH Hsat can be computed has:

Hsat = alt_cog_ellip–(ku_band_ocean_range + mod_dry_tropo_corr + mod_wet_tropo_corr + ra2_ion_corr_ksea_bias_ku + solid_earth_tide_ht + geocen_pole_tide_ht)

If available, these fields can be extracted from the ESA PDS files (Table 10). But in order to perform better comparison,specific corrections were set for the calibration study.

Fig. 43: Zoom on the area Casablanca tide gauge where ENVISAT tracks are represented

Mean Sea Surface

Tidal effects

Atmospheric effects

Tide gauge Nearest point on ENVISATtrack

Distance between the 2 measurementsEllipsoid

Sea surface

Computedby models

Computedby models

MeasuredSSHby

tide gauge

MeasuredSSHby

ENVISAT

∆∆∆∆TG ∆∆∆∆ENVISAT

Fig. 44: Determination of the RA-2 bias compared with in situ tide gauge measurement

Fig. 45: CLS.01 MSS error model

Fig. 46: FES99 error model

Specific orbits (16 April to 13 September 2002) were computed on the calibration area. The different orbits were used inthe order of importance: short arcs if available, long arcs (DORIS orbit) if short arcs are not available and orbit extractedform ESA PDS files if no other orbits are supplied. The dry and wet tropospheric corrections were optimised by takinginto account better models so as to provide a Tropospheric Zenith Delay (TZD). TZD is the sum of dry and wet tropo-spheric correction and is derived from ECMWF fields. The TZD provides a global coverage during the calibration phase.At the date of the validation workshop (December 2002), no specific correction were computed for ionospheric field andthe sea state bias

7.3. Computation of the Bias

A global processing system was developed to access the SSH deduced from the in situ measurements and the SSH com-puted from RA-2 measurements. Thus, specific modules were developed:

• To compute the nearest point of a tide gauge location on EnviSat track• To select EnviSat measurements• To interpolate tide gauges measurements at EnviSat time passes• To compute the SSH at EnviSat and tide gauge locations• To interpolate orbits provided by DEOS• To interpolate TZD provided by ESTEC

According to these different modules, a unique bias is calculated for each tide gauge measurement compared to EnviSatdata. For each tide gauge and each correction, an error was introduced to provide a confidence on each comparison. Bycollecting the whole set of altimetric measurements available and computed at ESRIN, biases with error bars were calcu-lated (Fig. 47).

At a first glance, it can be seen that three tide gauges are not around 0 meter (which corresponds to no bias!): Marseille,Palma and Ajaccio. However, these comparisons for each of the three tide gauges are grouped around a value (~-1m forMarseille, ~1.2m for Ajaccio and ~2.8m for Palma). This appears to be a systematic bias which is certainly the conse-quence of inadequate geophysical corrections. Indeed these three tide gauges are coastal and it is known that the geophysi-cal model are less accurate in such area mainly due to MSS computation which is an extrapolation of satellitemeasurements to the coast and which is linked to the geoid on the coasts. Unfortunately, no EnviSat measurements to com-pare with Sant Antoni tide gauge.

Field number Field name Description in ENVISAT datasets Correction

1 dsr_time MDSR Time stamp. Time fields based on UTC are computed for each record and referred to the centre

of the averaged waveform

4 lat Geodetic Latitude (positive N, negative S)

5 lon Longitude (positive E, 0 at Greenwich, negative W)

9 alt_cog_ellip Altitude of CoG above reference ellipsoid +

17 ku_band_ocean_range Ku-band ocean range -

39 mod_dry_tropo_corr Model dry tropospheric correction -

41 mod_wet_tropo_corr Model wet tropospheric correction -

43 ra2_ion_corr_ku RA-2 ionospheric correction on Ku-band -

49 sea_bias_ku Sea state bias on Ku-band -

103 solid_earth_tide_ht Solid earth tide height -

104 geocen_pole_tide_ht Geocentric pole tide height -

Table 10: Corrections to apply to retrieve SSH from RA-2 measurements

This is the reason why, the study was focused on the open ocean tide gauges (ICM light GPS buoys, IoGN moored GPSbuoys, Casablanca tide gauges) and represented Fig. 48.

-2

-1

0

1

2

3

4

19160 19180 19200 19220 19240 19260 19280 19300

Date in julian day (19160=17 jun and 19300=4 nov)

Bia

s in

mete

rs

Casablanca

FTB2

FTB4

ICM

Marseille

Palma

Ajaccio

Fig. 47: Complete comparisons of RA-2 measurements with tide gauge measurements

-1

-0.5

0

0.5

1

1.5

19145 19180 19215 19250 19285 19320

Date in julian day (19160=17 jun and 19300=4 nov)

Bia

s in

mete

rs Casablanca

FTB2

FTB4

ICM

Fig. 48: Deep ocean comparisons of RA-2 measurements with tide gauge measurements

For each bias computation a formal error was computed. The formal error is the sum of:

• A systematic error (which is the same for each tide gauge location)• A random error: the whole set of measurements (temporal and spatial) is assumed not to be correlated (so if the

number of observations is infinite the random error tends to be zero).

The total mean error of the bias depends on the number n' of different tide gauges and the number n of different observa-tions (n >>n'):

(5)

By taking into account only the open ocean tide gauges:

• The computed bias is ~41 cm• The computed error on the bias is ~6cm• For information the standard deviation of the bias is ~30cm

7.4. Remarks and Conclusions

Several fields were not taken into account for the computation of the errors: the atmospheric corrections, the sea state bias,the solid earth tides and the polar tides. A specific ionospheric correction for the study has to be set. The sea state bias hasto be further validated and an error has to be set to provide a confidence on the correction. Coastal tide gauges need to befurther exploited by computing a specific coastal MSS to better link the measured sea level elevation to the nearest EnviSatlocation and by adding other near real time tide gauges measurements so as to increase the number of independent obser-vations.

The bias will be further tuned by adding new in situ measurements (longer time series and added tide gauges) and by usingmore EnviSat RA-2data.

The assessment of the of the RA-2 bias estimation shall be improved. A better knowledge of errors will allow to reducesusceptibility to systematic errors and, thus, to better estimate the altimeter bias.

All these improvement described above shall be performed by spring 2003, when a new RA-2 absolute range bias will bepublished.

8. REFERENCES

1. Francis R. and Roca M., “RA-2 In-Orbit Absolute Calibration Plan: Range” - PO-PL-ESA-GS-0714, Issue 2a, March 2000.

2. Francis R. and Roca M., “RA-2 Absolute Calibration Workshop minutes”, el Muntanyà, Barcelona, March 1998.

3. CLS Level 2 DPM.

4. Scharroo R., “The Contribution of Radial Orbit Errors to the EnviSat RA-2 Range Calibration Accuracy” Report for ESA, 1997.

5. Liebe H. J., Hufford G. A. and Cotton M. G., ”Propagation modelling of moist air and suspended water/ice particles at frequencies below 1000 GHz”, AGARD 52nd Specialists’Meeting of The EM Wave Propagation Panel, Palma De Mallorca, Spain, 17-21 May 1993.

6. Haines B. J. and Ménard Y., “Jason-1 Calibration/Validation”; The Earth Observer, Volume12, Number 5, pp. 14-17, September/October 2001

7. Ménard Y., Jeansou E., and Vincent P., “Calibration of the TOPEX/POSEIDON altimeters at Lampedusa”; Journal of Geophysical Research, Vol. 99, No. C12, pp. 24,487-24,504, December 15, 1994

ε fεs

n′---------

εr

n-------+=

8. Schuler T., “On Ground-Based GPS Tropospheric Delay Estimation”; Doctor’s Thesis, Studiengang Geodä-sie und Geo-information, Universität der Bundeswehr München (University FAF Munich), Germany, Vol-ume 73, Neubiberg, 2001, you may browse on-line: http://137.193.32.1/Forschung/TropAC/docs / phd / index. html or http://137.193.32.1/Forschung/TropAC/ docs/phd/ full.html

9. Project “ESEAS-RI: European Sea Level Service – Research Infrastructure” (U.E., EvR1-2001-00042).

10. Hernandez F., and Schaeffer P., “The CLS01 Mean Sea Surface: A validation with the GSFC00.1 surface”, pp. 14, CLS, Toulouse, 2001.

11. Lefèvre F., Lyard F., Le Provost C. and Schrama E.J.O., “FES99: a tide finite element solution assimilating tide gauge and altimetric information”, J. Atm. Oceano. Tech., 19 (9), 1345-1356, 2002.

12. Lyard F. and Carrere L., “Mediterranean MOG2D Legos atmospheric model”, Personal communication, 2002.