6th plenary meeting (#17)ra-2/mwr cross-cal & val team, esrin 25-27/03/031 sixth plenary envisat...

6th Plenary Meeting (#17) RA-2/MWR Cross-Cal & Val Team, ESRIN 25-27/03/03 1

Sixth Plenary ENVISAT RADAR ALTIMETRY

CROSS-CALIBRATION and VALIDATION TEAM Meeting (#17)

DISCUSSION, RECOMMENDATIONS and

CONCLUSIONS

Jérôme Benveniste

ENVISAT RA-2/MWR Cross-Calibration andValidation Team Coordinator

European Space Agency,

Earth Observation Applications Department

ESRIN, Via Galileo Galilei, I-00044, Frascati (RM), Italy,

(e-mail: [email protected])


Table of content - Product validation summary status - Product and algorithm evolution Update of parameters in processors (application of calibration values) Update of algorithms Update of auxiliary files - DEM+Bathymetry (Diane Defrenne) - Tides, MSS, GEOID, ... - Product enhancement - Product Handbook - Future recommended studies - New products? (e.g. SSH, SLA, X-overs, Coastal zone,...) - New Algorithms? - Recommendations for reprocessing commissioning phase data - Recommendations for the dual ERS-2/ENVISAT Mission - EGS-AGU-EUG Session (JB) - Final Report - Conclusion


Product validation summary status

• SWH: Improved histogram, particularly for low waveheights (<1 m). Zero-clipping much reduced with updated SWH algorithm. Still some values set to zero (2%). Differs by 20 cm or 3% (higher) than ERS-2. (see separate table for comparison with buoys and model)

• Sigma0/wind speed: Improved histogram, Ku-band sigma0 aligned on ERS-2 to satisfy MWC wind model. Implementation of the MWC Look-up table should be revisited (same “features” for all other altimeters). Estimated absolute calibration bias around 0.95 dB (RA-2 higher). Passive Calibration confirms for Ku Band (0.91 dB) and will produce S band bias (after further validation). Wind limited to 20 m/s: set to default value or extend the wind model. Accumulation of values near 1 m/s (15 dB) to be investigated.



• Range: The range bias change has been pinned down: RA2_CHD_AX update error of eps_ground produces a fixed range offset of 73.68 cm. To correct: add 73.68 cm to range. Correct RA2_CHD_AX produced.Low noise (1.8 cm at 1Hz). Improved noise spectrum: higher frequency signal (than in ERS) is present (~50 vs. 70) km . (Range cross-calibration: see table.)

• Time-tag bias: Not significantly different from 0. Relative time tag bias to (corrected) ERS-2 is 0 (on FRAPPE and since IPF v4.53, 27/11/02, i.e, in CCVT data and FDMAR).

• Mispointing: mispointing squared, 2, estimated in IECF is less than 0.01 deg2. In products, 2 = 0.026 deg2 -> may depend on trailing edge window used (too near end of leading edge), to be investigated.



• Geophysical corrections: – Dual frequency iono validated against JPL GIM. Impacted by range bias

of 73.68 cm (corrected) , but depends on the SSB which is not yet final on the limited data set available.

– Doris iono not validated, needs further evolution.

– Need to investigate differences in ECMWF wet tropo in Envisat and E2

– Dry tropo on non-ocean data needs investigation (gives different results compared the NCEP and measured elevation up to 20cm).

– Solid earth tide diff b/w En and E2, <-1 cm>, but need to check with Ph. Moore about how the comparison was made.

– All other geophysical correction validated.



• Orbit error– Agreement on 2-3 cm (radial) from CNES, NCL, DUT, ESOC on the GDR

– To investigate improvement to be gained by using reduced dynamic orbit determination

– Orbit propagated into FDMAR is unacceptably poor: 20 to 30 meter in the radial component with jumps of several meters between orbit state vectors



• RA-2 tracking Vs. geophysical applications

Impact of resolution mode switching on the geophysical applications over land: The river and lake level application is less performant than with ERS-2 ice mode. It is of same low quality as for Topex. Over rivers the complete waveform is not fully in the window and as it takes 4 events in need of switch before switching (in order to avoid unnecessary mode oscillations) and the river level acquisition is missed when finally entering the 80 MHz mode. It may be worth studying the setting to 3 or 2 events and verify that it has no detrimental effect on the ice mission (which holds a higher priority in the mission objectives than land applications).



• MCD flags: – MCD flags are not properly set in GDR/SGDRs. To be corrected.

• Doppler correction– Doppler correction are sometimes set to zero when valid orbit data are

present. To be corrected.


Cross-calibration error formalism

• Major improvement since ERS-1 Cross-calibration on ERS-1. Simultaneous solutions taking into account correlation between different data sets (RA-2, RA, model, buoys). Formal error produced. See P. Challenor and P. Janssen presentations for details.

• See cross-calibration tables to be generated right after the meeting


Update of parameters in processors

• Application of calibration values– Ku band Sigma0 was adjusted on ERS-2:

» need to introduce absolute calibration when enough data and stable bias produced (by July 03)

» need to develop the use of the absolute calibration: wind model, wave period, mean square slope, sea state bias, MWR retrieval algo.

--> Futher studies, theoretical and empirical, needed (see B.Greco’s note in the further studies section)

--> At this point the relative calibration between ENVISAT and ERS-2 is not interesting in itself, what counts is the absolute calibration: to obtain the absolute sigma0 from the data add 3.5 dB to return to the unmodified ENVISAT RA-2 Ku-band sigma0 and remove 0.95 dB (preliminary result).

• Ice2 mispointing evaluation tuning


Update of algorithms

• In Level 1B: PTR correction on altimeter range and AGC.• Ice2 leading edge detection improvement (already defined by

B.Legresy, to be implemented)• SWH zero-clipping needs further improvement and output on

negative values• Doris iono needs further development• Rain flag update: Update Aux file entries (f, rms)

– 1 byte for likelihood, 1 byte for flags: RAIN from Dual-Freq , RAIN likelihood, MWR data used in the algo or not, (GQ and JT will specify new algorithm)

• Orbit Flag: clarify bit 28-31 and add in the products an orbit quality flag for degraded orbits due to manoeuvres and DORIS data drop-outs


Update of auxiliary files (1)

• SSB: update pre-launch (ERS-2) model to current (preliminary ) Model: NP/direct (S. Labroue). Then update with absolute sigma0 (or new wind model) and when estimate is stable with 8-10 cycles of data.

• Tides: use GOT00.2 and FES00 (or better FES02) (FES99 has known drawbacks that are improved in this new version; Ch.Le Provost to make available validation documentation and model before any recommendation on FES02 can be issued). – There is only one load tide in the product. This is because the differences

between model are small (sub-millimeter for M2). Investigate if GOT load tide can be added in the GOT ocean tide instead of the “generic” load tide.


Update of auxiliary files (2)

• MSS: Recommendation to use CLS01 (already in use by most CCVT range teams). Among other improved features, CLS01 is built with ocean average mean pressure in the IB and this makes it compatible with the IB correction in the products (if not there would be a 3.2 cm offset, as in the current one, GSFC00.1)

• DEM+Bathymetry (see presentation by Diane Defrenne)– An new model has been generated by ESRIN based on ACE (Berry

2001) and Sandwell and Smith (1998). Same resolution (1/12 deg). Another higher resolution model (1/30 deg) will be generated.

– Update AUX_DEM_AX: need acceptance of ASAR, MERIS and AATSR experts. JB/DD to present this new model to them and convince them to update.

– Update model in CMA, may be with higher resolution than 1/12 deg CMA DEM is DTM2000.1 (Pavlis) and is based on Globe. It thus contain the same gross errors as shown in Berry 2000 (Envisat symposium) and in D Defrenne’s presentation?


New geophysical models

• Need a iono model in the GDR/SGDR. Best candidate is JPL GIM ionospheric correction model (scaled to 800 km by IRI95). Should also be also introduced FDGDR.

• Question from J. Dorandeu: Do we need to add in the product the MSS cross-track value (MSS-CT was generated for T-P/Jason but doesn’t exist yet for the ERS/ENVISAT ground track)?

• Atmospheric tides– not discussed during meeting, but this subject should be addressed in the

future.


Product enhancement

• Add OSV in Off-line products (to compute the long of ANX)– Check compatibility with ENVISAT-wide specification:

» The Orbit State Vector (OSV) corresponds to the Ascending Node crossing if the OSV is generated from the DORIS Navigator Level 0 product (DOR_NAV_OP), or from the FOS Predicted Orbit (AUX_FPO_AX). The OSV corresponds to sensing start time of the product if the OSV is generated from the DORIS preliminary product (DOR_POR_2P), DORIS precision product (DOR_VOR_2P), or from the FOS Restituted Orbit (AUX_FRO_AX).

• Add a sea-ice flag– MWR Tbs + peakiness + delta_meteo (Dorandeu/Faugere proposal)

– Publish the sea-ice algo in the IECF to the CCVT

• Add peakiness in FDMAR (and sea- ice flag) (any other fields?)


Product enhancement

• Add a rain likelihood flag (2-byte) and/or a “don’t know” flag– What if no MWR? Set to “don’t know”

(see “update of algorithms” slide).

• Add pass number in IGDR/IMAR, GDR, SGDR– Where? - In place of orbit number? - In place of sequential number in

product name (last four characters before “.N1”)

– It seems that adding the pass number in product name would solve the double product system bug. One stone, two birds!


Miscellaneous

• Investigate difference in range bias b/w asc and desc passes

• Is there a need to flag the S band hardware anomaly– Is it really inhibited to happen again?

– Is there any tell-tale flag or parameter to look at?

– Need Dual freq iono flag?

--> the Rain flag may be the place to find the solution

• Clarify the on-land dry-tropo computation• Pay attention to the IB: different models in ERS-2 and Envisat

(improved with daily mean pressure over ocean)– check for any steps in the mean pressure


Product Handbook

• Clarify unclear statements• Add missing information

– Quality flag use

– Typical data screening criteria for different applications

– Use of U and V component of ECMWF wind vector (?)

• Add differences between NRT and off-line processing– Based on the maintained NRT/OFL “delta-doc”

• Add Product evolution information • Add CCVT recommendations to apply or not calibration results (e.g. SWH)

---> Please send me your “red marked” pages of your copy of the Product Handbook


DIODE full-rate data for NRT Products

• Before FDMAR is released to the Users, the NRT orbit needs to be updated to use the full rate DORIS Navigator data. If this is not done, CCVT Users will discontinue using FDMAR due to the unacceptably large orbit error. This update, already requested at the RA-2/MWR Products and Algorithms Verification Workshop in June 1999, shall be implemented without further delays. Today its development and implementation status is not clearly known (on-going implementation…)

PS: After payload switch-off, turn DORIS back on before of same time as RA-2 and MWR (not several orbits later). Request already sent to Misson Manager.


Individual echoes

• Individual echoes collection strategy– Targets (plan 2 weeks in advance)

– 1/20th sec. (short but more homogeneous in space)

– 1 sec every 180 sec (180*7=1260 km)

(already discussed at CCVT #5, see PF presentation… anything to add?)

• Publish individual echoes processing algorithm.


Future recommended studies

• New Algorithms?– MWR proposed new algorithms (neural network, side lobe)– Need an Inverted Barometer Model (Barotropic model).– A fifth retracker? Land? To be developed with more expertise from RA2

waveforms– Coastal zone altimetry

» MWR “de-pollution” near coast» Echoes selection» Individual echoes application

• New products? e.g. SSH, SLA, X-Overs, Position of XO– Requirements for such products? Yes these products were useful for the

cal/val phase and should be extended to the users during the mission.– Freezing level (to be developed)– Rain rate (to be developed)– Wave period (to be developed)– SSM/I ice maps as aux data (should be added in the product)– Coastal zone Product (to be developed)


Future recommended studies - Sigma0 Absolute Calibration

By Bruno Greco• Strong arguments for attempting an absolute calibration of Radar Altimeter backscatter have been discussed and expressed during the

preparation of the ENVISAT cal/val phase (e.g at the ERS Symposium in Florence) to access wave period, wind speed and stress and the sea state bias. An accuracy of 0.1 dB is required to derive wave period with an accuracy of 14% (jn the range 5-10 s) (Cotton and Challenor). It was also established that without an absolute calibration radar altimeter data cannot be directly analysed in terms of surface slope statistics and the derivation of air-sea interaction (B. Chapron). The main limitation is clearly the lack of compatibility between the theory and the data collected by radar altimeters so far. It was further brought to the RA2 Science Advisory Group and a recommendation was issued . This work was also presented at the CEOS Wind and Wave Validation Workshop on 3-5 June 1997 and again a recommendation was issued. The exploitation of an absolute sigma0 required a dedicated research development both theoretical and empirical with the acquisition of in situ measurement. This was proposed to ESA/CNES/ASI but it was unfortunately not funded. Nevertheless three methods were set-up: a novel passive calibration method was developed (Greco and Pierdicca), a ku-band transponder was built by ESA and an approach to monitor any sigma0 drifts during the mission lifetime using natural target was developed and tested on ERS-2 (Berry).

• The present results of the passive calibration show a good agreement with the Transponder results for the Ku-band, and hence can be considered now in the phase of convergence to a final result, with future updates in the order of fractions of a dB. That raises two points. First the reputed altimeters handicap of overestimating the mean square slope (mss) appears due, at least partly, to the poor calibration of the past altimeters. The ENVISAT calibrated Ku-band data are 2.5 dB (preliminary result) higher than GEOSAT. This should deliver much more realistic mss values using existing theoretical model. Secondly, the sigma0 in S-band appears to be lower than in Ku-band, contrary to the expectations. This might open new breakthroughs in the understanding of the backscattering processes.

• Both results are preliminary and need more data to be confirmed. One way is to conduct a similar calibration experiment with Jason (including active calibration) and to compare the results. Another more comprehensive way is to carry out multi-frequency calibrated sea backscattering measurements with a dedicated campaign, and use the collected data to extensively verify and/or tune the models.

• It is time to explore and exploit the new features offered by the third generation instruments, if necessary revisiting some assumptions based on the data of the first generation ones. It is recommended to perform now the research development and in-situ data acquisition mentioned above.


Reprocessing recommendations

• Recommendations for reprocessing commissioning phase data– Continue processing with real-time data?

– Re-process earlier part of the mission?

» RA-2 was in a commissioned state by Cal review (Sept 02)

– Re-process cycles 10-11-12-13-14?

-- > Update IPF and go on with cycle 15, 16, …


Dual mission recommendations

• Recommendations for the dual ERS-2/ENVISAT Mission

– Enhance the s/t sampling– 7 day separation (use the flying formation of ERS-1 and ERS-2)

– For Ice mission, increase time revisit…

– Input from D. Stammer and J.schroter (pers. comm. to JB) to be added here


Future Altimetry Workshop recommendation

• Recommendation for ESA to hold an Altimetry Workshop beginning 2004


EGS-AGU-EUG

• A small oral session to introduce CCVT Approach and results and to point to all the posters for detailed results.

• A HUGE Poster session to publish detailed results

---> Please upload your EGS Posters into the CCVT web site! (so others can see them after EGS)

(Do we need a mini CCVT meeting in Nice? - will happen de facto around session G13)


Final RA-2/MWR CCVT report

• Objective and approach exported from the Plan• Similar table of content to the agenda of this meeting

----> Please upload your final report by 9 May 03

the Final report will be published after a peer-review cycle


Conclusion

• Product validation status

• Recommendations for future Product and Algorithm evolution

• Recommendations for future studies

• Recommendation for the tandem ERS-2/ENVISAT Mission

• Publications– EGS-AGU-EUG (April 03)

– Final report (May 03)

– IGARSS (July 03)


Please upload your presentation, poster, final report


Thank you for your participation!

See you in Nice!


http://envisat.esa.int

6th plenary meeting (#17)ra-2/mwr cross-cal & val team, esrin 25-27/03/031 sixth plenary envisat...

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