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SAMOSA
The SAMOSA project The SAMOSA project main achievements and their contribution to Coastal Altimetry Cristina Martin-Puig; STARLAB [email protected] In collaboration with: Philippa Berry – DMU David Cotton - SATOC Christine Gommenginger – NOC Lars Stenseng - DTU
Project funded by ESA under the STSE programme:
The SAMOSA project 2
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Motivation and Mission
MOTIVATION • CryoSat-2 à SIRAL − First SAR altimeter on board a Satellite
MISSION • To quantify the improvements
of SAR altimetry compared to conventional altimetry for observations over ocean, coastal zones and inland waters
IMAGES COURTESY OF ESA
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Project description and objectives overview
Full Name: Development of SAR Mode Altimetry studies and Applications over Ocean, coastal zones and inland waters Objectives: 1 To establish a state of the art review for SAR altimetry
capabilities to observe water surfaces 2 To reduce SARM data into LRM data and perform a scienti!c study of the
potential improved capability of SAR data compared to conventional altimetry over water surfaces
3 To develop a theoretical model for the SAR altimetry mode echoes over water surfaces
4 To de!ne a new re-tracking method for the SAR altimetry mode
5 To perform a scienti!c study of the potential capabilities of SAR altimetry data to characterise coastal zones, estuaries, rivers and lakes
6 To evaluate the SAMOSA re-tracker with ASIRAS data
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TEAM
STARLAB
SATOC
NOC
DMU
DTU-SPACE
ESA ESRIN
JHU - APL
Expert Advice and Support:
R.K.Raney
ESA - ESTEC R.Cullen
MSSL CRYMPS
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O1: State of the art review
• The SAMOSA team de!ned a SAR Altimetry state of the art review available at:
http://www.satoc.eu/projects/samosa/
Along-‐track
Time delay
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O2: RDSAR
• The SIRAL modes are mutually exclusive
• For the quantitative comparison of SARM and LRM data the only way around is through the reduction of SAR data such that it emulates conventional altimetry data
• RDSAR allows for the retrieve of LRM data and SARM data from a single operating mode
In addition to the previous, it also allow for the simpli!cation of hardware/mission planning and risks estimation of offset/gaps between modes
By:
In collaboration:
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SIRAL processing modes
Etc…
Low Resolution Mode - LRM
PRFLRM = 1970 Hz
Decorrelated echoes time
SAR Mode - SARM
PRFSAR = 17.8 KHz
Correlated echoes
Burst period 11.7 ms
Etc …
time
Full Bit
Rate
CryoSat-2 operating modes are mutually exclusive, thus for quantitative comparison we will need to reduce SARM FBR data to emulate LRM L1b data (step 1). We will refer hereafter to reduced SARM data as pseudo-LRM. Once we achieve the pseudo-LRM sequence (power) we need to transform it to L1b (~20Hz incoh. Integrated data; step 2), and compare performance with L1b SARM data (step 3).
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Step 1: correlated vs decorrelated pulses
• The return echoes in SARM are correlated. This is an effect from the high PRF of this mode. In LRM correlation between pulses is not present, nor desired
• We need to !nd a solution to combine, or intelligently select, SARM echoes such that the resulting sequence of this satis!es decorrelation between pulses
Pseudo-LRM (at this stage we have a complex sequence with I,Q components)
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Step 2: Transform pseudo-LRM to LRM L1b
• Apply tracking corrections − Rx correction − Gain compensation
• Convert to IFFT • Power transformation • Incoherent integration
LRM Block diagram
SARM Block diagram SARM
FBR data
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Step 3: Quantitative comparison
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O3: SAR altimeter theoretical model
• An semi-analytical waveform model has been derived for: • Elliptical antenna pattern
• Mispointing error in elevation and azimuth
• Errors in range cell migration correction
• Surface Scattering pattern
• Non-linear ocean wave statistics
• Spherical Earth surface effects
The multi-looked waveform shall be achieved by adding all available looks (single-look contributions) per Doppler cells.
Figure 6: Single-look model examples for different SWH
By:
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Derivation of the waveform model
• Given the new shape of SARM echoes there is a need to de!ne a new theoretical model to retrack waveforms
• The SARM single-look waveform has been de!ned (per Burst) as the convolution:
• Where τ refers to delay time, or range window, with respect MSL; fa corresponds to a Doppler bin; PFS is the average $at surface response; SPTR the radar system point target response; and Pz the surface elevation pdf. The major difference with respect conventional altimetry is that SPTR is de!ned as:
• Where T is the long-track boxcar time, τu
the useful pulse length and s the chirp slope.
Along-track time
!
W (" i, fa ) = PFS (" i, fa ) **SPTR (" i, fa ) *c2#
$ % &
' ( Pz
c2#
$ % &
' (
!
SPTR (" i, fa ) = sinc2 Tfa( ) sinc2 "us"( )
!!" !!# !" # " !# !"#
#$!
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#$(
#$)
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#$+
!
,-./012345160780941:;<1=>?
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HHI1E-F0GA6>1GA61D3GG060.41;EJ1G6A>1#14A1*$"1>
1
1
;EJK#
;EJK!
;EJK%
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;EJK'
;EJK"
;EJK"$"
;EJK($"
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;EJK*$"
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O4: SAR Altimeter re-tracker
• A SAR altimeter re-tracker has been developed based on the analytical expression derived in the previous objective
• Good agreement was found between theoretical & numerical SAR waveforms
• A DPM for the Sentinel-3 SRAL retracker has been delivered.
By:
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LRM & SAR L1B Scenario C1
Along-track time LRM: slow decaying trailing edge & increased tilt of leading edge for high SWH SAR: peaky waveforms, fast decaying trailing edge, peak broadening at high SWH
LRM
SAR
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Multi-looking
!
Multi-looked SAR L1B Waveform at 18Hz
!"
#!
"
#$
surfaceantenna beamwidth
Doppler-beam selection & incoherent integration over multiple bursts
… … … …
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How good are O4 and O5 for the coastal altimetry community?
See related movie StarSARAltCO.mp4
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O5: Coastal & Inland Waters
• What improvements does SARM altimetry offer for coastal and inland waters? Two types of scenario have been modelled: − Lakes − Coastal Zones
By:
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Coastal zone results
DEM picture courtesy of D.Brockley MSSL
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O6: ASIRAS and SAMOSA
• Evaluate the SAMOSA re-tracker against real SAR altimetry data
• Gain information on the differences between space- and airborne data
• Processing scheme for airborne data to allow comparison with space-borne data
By:
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SIRAL vs. ASIRAS
SIRAL ASIRAS
Along-track beam width 1.0766° 10°
Cross-track beam width 1.2016° 2.5°
Transmitted power 25 W 5 W
Center frequency 13.575 GHz 13.5 GHz
Transmitted bandwidth 350 MHz 1,000 MHz (100-1,000MHz)
Transmitted pulse length 49 ms 4 µs (4-80 µs)
Pulse repetition frequency 18.182 kHz 2.5 kHz (2-15 kHz)
Approximate footprint size 15 km x 250 m 120 m x 10 m
Measurement range gate 0.46875 m 0.08783 m (0.109787 m)
Average platform velocity 7,389 m/s 65 m/s
Average range to surface 717 km 2.7 km (0.2-10 km)
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Along-track geometry (Doppler selection)
0.0
0.2
0.4
0.6
0.8
1.0N
orm
aliz
ed
am
plit
ud
e
60 70 80 90 100 110 120 130 140
Range bin
6070
8090 100 110 120 130 140 150
Range bin
0
25
50
75
100
125
150Beam bin
0.2
0.4
0.6
0.8
Nor
mal
ized
pow
er
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SAMOSA main progress
• Investigations to reduce SAR mode to LRM • De!nition of SAR Altimetry waveform model • Development of SAR retracker • Investigations with simulated data • Analysis of performance of SAR retracker with ASIRAS
data
