end-to-end performance analysis of the paris in-orbit demonstrator ocean altimeter

IGARSS-2011, 25-29 July, Vancouver (Canada)

END-TO-END PERFORMANCE ANALYSISEND-TO-END PERFORMANCE ANALYSISOF THE OF THE

PARIS IN-ORBIT DEMONSTRATOR PARIS IN-ORBIT DEMONSTRATOR OCEAN ALTIMETEROCEAN ALTIMETER

Salvatore D’Addio,Salvatore D’Addio, Manuel Martin-NeiraManuel Martin-Neira, Chris Buck, Chris Buck

Acknowledgment: Nicolas Floury, Roberto Pietro Acknowledgment: Nicolas Floury, Roberto Pietro CerdeiraCerdeira

TEC-ETP, Electrical Engineering DepartmentTEC-ETP, Electrical Engineering DepartmentEuropean Space AgencyEuropean Space Agency


The PARIS ConceptThe PARIS Concept

PARIS = PARIS =

Passive Reflectometry and Interferometry SystemPassive Reflectometry and Interferometry System


PARIS IOD Key FeaturesPARIS IOD Key Features• Direct Cross-correlation (DxR)Direct Cross-correlation (DxR)

– High gain beams for direct signals (D)High gain beams for direct signals (D)– High gain beams for reflected signals (R)High gain beams for reflected signals (R)– Implicit use of full GNSS bandwidth (3x40 Implicit use of full GNSS bandwidth (3x40

MHz)MHz)• Dual or Trial Frequency for precise estimation of Dual or Trial Frequency for precise estimation of

ionospheric delay ionospheric delay • Precise on-board delay calibrationPrecise on-board delay calibration• Precise on-board amplitude calibrationPrecise on-board amplitude calibration

L1L5 L2

40 MHz 40 MHz 40 MHz


PARIS IoD L1 Power WaveformPARIS IoD L1 Power Waveform• GPS L1 Composite, Orbit Height: 500Km, Down-Looking GPS L1 Composite, Orbit Height: 500Km, Down-Looking

Antenna Gain: Antenna Gain: 19dBi, 19dBi, Nadir Looking GeometryNadir Looking Geometry


PARIS IoD Mission Summary TablePARIS IoD Mission Summary TableInstrument PARIS Altimeter

Main Scientific Product Mesoscale Ocean Altimetry

Secondary Scientific Product

Polar Ice Thickness

Scientific by-products

Ionospheric total electron contentWind speed and wind direction over oceanSignificant Wave HeightMean Square Slope of ocean surfaceSea Ice ExtentRiver and lake water level

Particular Application Tsunami detection

In-orbit Demonstrator

Operational Mission

Orbit Polar Sun

SynchronousPolar Sun

Synchronous

Orbital Height 800 km 1500 km

Swath 900 km 1500 km

Revisit Time 3 days 2 days

Spatial Resolution 100 km < 100 km

Antenna Diameter / Gain 0.9 m / 19 dB 2.4 m / 30 dB

Number of Beams 4 16

FrequenciesGPS L1+L5

Galileo E1+E5

All 3 bands ofGPS, GLONASS,

GALILEO, BEIDOU

Total Altimetry Accuracy (1)

< 17-20 cm < 5-7.5 cm

Platform PROBA-like PROTEUS-like

Launcher Configuration Piggy back Main passenger


Factors affecting altimetry Factors affecting altimetry performanceperformance

Factors affecting Factors affecting altimetry performancealtimetry performance

CommentComment

PrecisionPrecision Allowed incidence angle of specular point

• Higher incidence angle reduces the height precision, but increases swath• Scanning reduces antenna gain

Instrument Characteristics:1. Antenna Gain, Receiver NF 2. XC Coherent integration

time

1. Improve SNR2. Optimized for maximum

precision

Waveform Retracking Performance

Optimum retracker to be defined

AccuracyAccuracy Uncertainties on Propagation:1. Ionosphere2. Troposphere3. EM Bias4. Skewness Bias

1. Use of 2 or 3 Frequencies2. Estimated or measured3. Frequency Dependent4. Fraction of EM Bias

Uncertainties on Geometry/Orbit

Residual instrument errors after calibration

On-board Delay/Amplitude Calibration needed


Max Incidence Angle (1/2)Max Incidence Angle (1/2)

P

G

O

s

Ionosphere

i i

The incidence angle plays an important role in the definition of The incidence angle plays an important role in the definition of critical parameters of the PARIS mission, such as:critical parameters of the PARIS mission, such as:

(a) Precision(a) Precision

(b) Sampling and Coverage(b) Sampling and Coverage


Max Incidence Angle (2/2)Max Incidence Angle (2/2)

a) Altimetric performance degrades with incidence anglea) Altimetric performance degrades with incidence angle

b) Ionospheric delay and antenna loss increase with incidence b) Ionospheric delay and antenna loss increase with incidence angleangle

The incidence angle should be restricted (<35The incidence angle should be restricted (<35).).Coverage and sampling obtained by raising orbital height. Coverage and sampling obtained by raising orbital height.

(a(a)) (b)(b)

dh

P

G

O

s

Ionosphere

i i

i

ddh

cos2

i

d

Antenna gain




CommentComment


• Higher incidence angle reduces the height precision, but increases swath• Scanning reduces antenna gain


time


precision









Interferometric Processing SNRInterferometric Processing SNR

• Up/Down-looking Antenna Gain and Up/Down-looking Antenna Gain and Up/Down RX NF shall be such to ensure sufficient Up/Down RX NF shall be such to ensure sufficient SNRSNR

• In PARIS IoD with interferometric processing, the In PARIS IoD with interferometric processing, the final power waveform SNR is given as:final power waveform SNR is given as:

int

11

cr

R

D

SNRSNR

SNR

SNR


Interferometric Processing SchematicInterferometric Processing Schematic

DELAYDOPP SHIFT

XC

DOCONBPF

fo

DOCONBPF

fo

cT

dt

UPCHAIN

DOWNCHAIN

|.|2W2(τ)

W1(τ)

W3(τ)

WN(τ)

iN

W fs

Ts

For a given target along track spatial resolution, e.g. For a given target along track spatial resolution, e.g. 100Km100Km, , the coherent integration time and the incoherent averaging the coherent integration time and the incoherent averaging are are inversely proportionalinversely proportional: :

Tc Ninc

100 Km c incT T N

Cross-Correlation Integration Time Cross-Correlation Integration Time (Tc)(Tc)


t SP

P

t

Power Altimetry Waveform

1 chip delay

22

, ,

,

sin cS R

c

fTZ P ACF

fT

Pulse limited footprint

θs

φs

P=(θ,φ) SP

Iso-Doppler

RX PARISsatellite

TX GNSSsatellite

direct path “d”

Specularreflected path

Generic reflected path “r”

Iso-Delay



22

, ,

,

sin cS R

c

fTZ P ACF

fT

PA

RIS

Flig

ht D

ir

RX

Pulse Limited

FootprintDoppler FootrpintFor Low Tc

3dB Antenna Footprint

Low TcLow TcDoppler

Limited

Footprint

PA

RIS

Flig

ht D

ir

RX

Doppler FootrpintFor High Tc


High High TcTc



2

, ,

,

sin cSP R c

c

fTSNR P ACF T

fT

SNRSP

TC

PA

RIS

Flig

ht D

ir

RX

PA

RIS

Flig

ht D

ir

RX

PA

RIS

Flig

ht D

ir

RX


PA

RIS

Flig

ht D

ir

RX




2 2,

, ,

1 1 11

2cos

Z S

hinc

inc SP Z S

c P

SNR SNRNP

cSNR T100 Kminc

c

TN

T

2 2

12 2

1 11

h c

c c

const Tconst T const T

• Analytical model for altimetry precision:Analytical model for altimetry precision:

• Tc shall be chosen such to have a sufficient SNR (e.g. 6dB) but cannot be increased further otherwise the altimetric precision is degraded because speckle is not averaged enoughTc shall be chosen such to have a sufficient SNR (e.g. 6dB) but cannot be increased further otherwise the altimetric precision is degraded because speckle is not averaged enough• However, Tc cannot be too low otherwise consecutive XC samples may be correlated and speckle would anyway not be averagedHowever, Tc cannot be too low otherwise consecutive XC samples may be correlated and speckle would anyway not be averaged




CommentComment


• Higher incidence angle reduces the height precision, but increases swath• Scanning reduce antenna gain


time


precision









Waveform RetrackingWaveform Retracking

• The retracking has the objective of estimating the sea state The retracking has the objective of estimating the sea state parameters and the sea mean surface height by fitting a parameters and the sea mean surface height by fitting a theoretical model to measured waveformstheoretical model to measured waveforms

• Maximum Likelihood Estimation (MLE) or on weighted least Maximum Likelihood Estimation (MLE) or on weighted least squares estimation has been extensively used in Conventional squares estimation has been extensively used in Conventional Radar Altimetry.Radar Altimetry.

• The MLE method estimates the parameters by determining which The MLE method estimates the parameters by determining which values maximize the probability of obtaining the recorded values maximize the probability of obtaining the recorded waveform shape in the presence of noise of a given statistical waveform shape in the presence of noise of a given statistical distribution. distribution. 0( ) , , spW f SWH

SWH

delay




CommentComment


• Higher incidence angle reduces the height precision, but increases swath• Scanning reduce antenna gain


time


precision









The impact of IonosphereThe impact of Ionosphere

• At L-band, the ionosphere is a major contributor to the At L-band, the ionosphere is a major contributor to the propagation propagation delay delay Iih cos2

TECf

I2

3.40

21

1 cos2f

Iih

25

5 cos2f

Iih

iih

cos226.2

cos226.1 15

15

59.215

h

withwith

• Multi-frequency observations allow to remove the Multi-frequency observations allow to remove the ionospheric effectionospheric effect

• However this is at the expense of a severe error amplification factorHowever this is at the expense of a severe error amplification factor


Possible Ionospheric Correction Possible Ionospheric Correction Method Method

• Multi-frequency observations are taken:Multi-frequency observations are taken:

• A regression is performed over N samplesA regression is performed over N samples of ionospheric delay of ionospheric delay

•The smoothed ionospheric delay is used in the range equations, The smoothed ionospheric delay is used in the range equations, resulting in an improved error amplification factor resulting in an improved error amplification factor

x

h

f

f

y

y2

5

21

2

1

1

1

• The ionospheric delays are retrieved with the following The ionospheric delays are retrieved with the following uncertainties:uncertainties:

yxf 78.1

12

1

yxf 2.3

12

5

yxNf

78.112

1

yxNf

2.312

5

yh NN

4

2.3

4

78.1

2

1 22

2

xy h

f

22cos N

xy h

i f

512

1

1.26 2.262cos 2cos

x

f i i

2 21 515 1 5

1 1

2 2cos 2 2cosN

Ih f f

i i

151.27 for 3 (linear regression)h y N

1251.08 for 5 (4th order regression)h y N

• Starting from range equation:Starting from range equation:


Ionospheric Correction: Example Ionospheric Correction: Example 1/21/2

• What matters in mesoscale ocean altimetry is the difference What matters in mesoscale ocean altimetry is the difference between between the ionospheric delay at two specular points the ionospheric delay at two specular points Worst case is between nadir (i=0 Worst case is between nadir (i=0) and edge of swath (i=30) and edge of swath (i=30))

P

G2

O

s2

Ionosphere

i i

G1

s1


Ionospheric Correction: Example Ionospheric Correction: Example 2/22/2

0.000

2.000

4.000

6.000

8.000

10.000

12.000

-8000.0 -6000.0 -4000.0 -2000.0 0.0 2000.0 4000.0 6000.0

-10

-8

-6

-4

-2

0

2

4

6

-8000.0 -6000.0 -4000.0 -2000.0 0.0 2000.0 4000.0 6000.0

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

-8000.0 -6000.0 -4000.0 -2000.0 0.0 2000.0 4000.0 6000.0

Vertical Delay (m)Vertical Delay (m)

Mesoscale Delay (m)Mesoscale Delay (m)

Residual Delay (m)Residual Delay (m)

200 km averaging of mesoscale delay applied200 km averaging of mesoscale delay applied 0.15 m0.15 m

+0.15 m+0.15 m

+6 m+6 m

10 m10 m

+12 m+12 m

0 m0 m

Derived from RA-2 real dataDerived from RA-2 real data


PARIS IoD Preliminary Error BudgetPARIS IoD Preliminary Error Budget

ParameterIOD Height Total Accuracy on 100Km, G=20dBi, H=800Km

Instrument Noise and Speckle 12.5 cm

Ionosphere Averaging Noise 9.5 cm (2 frequencies,

N=3)

Ionosphere Residual 5 cm

Troposphere (Wet and Dry) 5 cm

EM Bias 2 cm

Skewness Bias 1 cm

Orbit / Geometry 5 cm

Instrument error residuals 2 cm

Total RMS Height Accuracy 18 cm at Edge of Swath


CONCLUSIONSCONCLUSIONS

• The PARIS altimeter can provide synoptic views of the ocean being thus The PARIS altimeter can provide synoptic views of the ocean being thus well suited for mesoscale ocean altimetrywell suited for mesoscale ocean altimetry

• The interferometric processing between direct and reflected signals The interferometric processing between direct and reflected signals maximises the altimetric performance of the systemmaximises the altimetric performance of the system

• A demonstration mission should be able to achieve 18 cm total accuracyA demonstration mission should be able to achieve 18 cm total accuracy

• An operational mission could be targeted for 5 cmAn operational mission could be targeted for 5 cm

• A particular application of PARIS IoD is the direct observation of A particular application of PARIS IoD is the direct observation of tsunamis:tsunamis:

• enhance our knowledge on tsunamis and complement early warning enhance our knowledge on tsunamis and complement early warning systemssystems

end-to-end performance analysis of the paris in-orbit demonstrator ocean altimeter

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