SIRAL Beam-formation Verification using Transponders Mònica Roca and Mercedes Reche PiLDo Labs, Barcelona

1. SIRAL Beam-formation Verification using Transponders Mònica Roca and Mercedes Reche PiLDo Labs, Barcelona

2. ToC • Introduction • Relevant aspects of the SIRAL on-ground processing and data • The ESA transponder • SIRAL calibration using transponder data • Conclusions and way forward

3. 2. Relevant aspects of the SIRAL ground processing and data 2. SIRAL ground processing and data

4. 2. SIRAL ground processing and data • SIRAL has 3 operating modes: • Low-resolution mode (LRM): Conventional, pulse-limited altimeter, used over ice sheet interiors and over ocean. • Synthetic Aperture mode (SAR): used over sea ice. One single antenna. • SAR-Interferometric mode (SARin):Used over themargins of the ice sheets. Interferometry between echoes received on each antenna. • In LRM, the range compression and incoherent averaging are performed on-board, and the resulting averaged echo forms the level 0 data. • The FBR data in the SAR and SARin modes consists of the individual, complex (I and Q) echoes. In SARin mode, there are two such echoes, one for each antenna of the interferometer. • In SAR and SARin modes, the radar echoes must first be synthetic aperture processed (performed on-ground) before incoherent multi-looking (in SARin mode also phase multi-looking will also be applied). This forms the Level 1b data. 2. SIRAL ground processing and data

5. 2. SIRAL ground processing and data Instrument-corrected and geo-located bursts FBR Steering: shifting the orientation of the beams Beamssteered to specific surface • Range compression • 64 Doppler beams with an equal angular separation • FFT over elapsed time • FFT across the burst Computed for each Doppler beam, to align them with their respective surface samples. Slant range correction Multi-look of each stack (+ phase multilooking for SARin) To reduce speckle. Echoes from beams of successive bursts, directed at the same location, are summed. SAR L1b SARin L1b 2. SIRAL ground processing and data

6. 3. The ESA Transponder 3. The ESA transponder

7. 3.1. Introduction • A transponder is seen by a radar as a point target (well known). • 1 transponder will be available for the CryoSat project (refurbished old ESA transponder developed for the ERS altimeter calibration). • Located in the ESA Svalbard station. • Initially 2 TRP’s were located 250 meters a part. This strategic location would have allowed the retrieval of many passes with transponder signal taken with different along-track and across-track separation. • Finally only 1 transponder. We can still perform the analysis and interpretation of the data over the transponder, acting as a point target, for retrieval of the beam formation. • We have identified several specific studies or calibrations that will make use of the transponder data (after reconstruction of doppler beams). 3. The ESA transponder

8. 3.2. Transponder location and passes • Transponder Location: SVALBARD (selected due to its high latitude) • CryoSat Orbit repeat cycle is 369 days. Not a sun-synchronous orbit. • SARin mode over the transponder • The passes can be used so long as the power decay (with respect to the peak) is not grater than about -7dB, which implies a separation from the nadir track of about about 5 kilometres. • Days for CryoSat passes over Svalbard In 6 months: 13 days In 16 months: 37 days Irregular interval,averaging 12 days 3. The ESA transponder

9. 3.2. Transponder location and passes 3. The ESA transponder

10. 3.3. TRP characteristics Transponder developed by RAL (UK) in 1987 curtsey of ESA 3. The ESA transponder

11. 4. SIRAL calibration using Transponder data

12. 4. SIRAL calibration using TRP data Primarily used for calibration of the interferometer baseline. • Other possible calibrations over the transponder: • Range bias: the measured range is compared with the expected one • Input to orbit studies: comparison of the range measured by the radar altimeter with that calculated from an ephemeris • Sigma-0 bias: altimeter measured received power when flying over the transponder is compared with the theoretical power. The transponder radar cross-section has to be accurately characterised • Datation: the expected time of minimum range is compared with the actual time of minimum range seen by the altimeter over the TRP • Mispointing: traditionally by measuring the slope of the echo trailing edge. New methods using SARin mode echoes over a TRP • SIRAL Doppler beam formation consolidation through the TRP. 4. SIRAL calibration using transponder data

13. 4.1. Range bias 4.1. Range Bias: • Over TRP: • using Level 1b data (after multi-looking) • using stack (after beam steering, 2D FFT, but before slant-range correction and multi-looking) • Over a natural target 4. SIRAL calibration using transponder data

14. 4.1. Range bias Over TRP using Level 1b data • Principle: the range is evaluated by retracking the one single multi-looked echo (from the Level 1b data) at the transponder position and compared with the theoretical range at that same point. • We need to pay particular attention to the atmospheric corrections (ionosphere and troposphere). • We do not have in-situ measurements so we should rely on modelling. • Ionospheric correction smaller at high latitudes. • We need the internal TRP delay characterised up to a value that depends on the range calibration requirement. This is measured by an TRP internal calibration (about 12 ns). • We can also characterise it through the RA-2 (use RA-2 well calibrated over the TRP to determine TRP internal delay). 4. SIRAL calibration using transponder data

15. 4.1. Range bias • EnviSat orbits • CryoSat orbits • Need to evaluate TRP distance to EnviSat ground track 4. SIRAL calibration using transponder data

16. 4.1. Range bias Over TRP using a stack of beams Principle: the range is evaluated by modelling the theoretical function that describes the range distance between the altimeter and the TRP, accounting for the satellite trajectory and velocity [r(t) hyperbola]. This range is compared with the measured range. 4. SIRAL calibration using transponder data

17. 4.1. Range bias • We can use SARin mode, although we only need one antenna received echo. • In order to retrieve the range measured by the altimeter, the individual I&Q echoes shall be processed by performing the 2D-FFT (Doppler beams and range compression), and retracking. • We will compare the above with the computed theoretical range and retrieve the range bias. • Same considerations about the characterisation of the internal delay of the transponder apply. • Same considerations about the atmospheric corrections apply. 4. SIRAL calibration using transponder data

18. 085 360 ICESat EnviSat TOPEX 4.1. Range bias Over natural targets: the salar de Uyuni, Bolivia (PI- H.A. Fricker) • measurement of the surface height retrieved by the altimeter is compared with independent measurement (GPS survey) • we will use SAR In • we will reproduce the processing performed for RA-2: retrack the waveforms with a gaussian retracker • we can cross-calibrate RA-2 and SIRAL SAR In • we can also use the salar to calibrate the interferometric baseline 4. SIRAL calibration using transponder data

19. 4.2. Sigma-0 bias 4.2. Sigma-0 bias • The TRP can be used to calibrate the radar measurements of the surface backscatter • 2 ways of approaching the problem • using Level 1b data (after multi-looking) • using stack (after beam steering, 2D FFT, but before slant-range correction and multi-looking) • The altimeter measures the power of a target, the transponder, which has a well known radar cross section (RCS). • Therefore the RCS shall be extremely well characterised, depending on sigma-0 calibration requirements (aprox. ±0.1 dB). • By applying the radar equation it is possible to compute the theoretical power the altimeter is supposed to measure. 4. SIRAL calibration using transponder data

20. CryoSat or TRP 4.2. Sigma-0 bias • Over a TRP, the altimeter is looking at a ground fixed point target and therefore the altimeter is not measuring at nadir but at a certain angle given by the geometry. At the same time, the TRP is not seen by the altimeter from zenith. • Since the radar cross section of the transponder depends on the transponder antenna gain, the angle in which the transponder is seen by the altimeter also becomes important. 4. SIRAL calibration using transponder data

21. 4.2. Sigma-0 bias • Power received by the altimeter over the TRP 4. SIRAL calibration using transponder data

22. 4.2. Sigma-0 bias Processing (conceptually similar to range bias): • We can use SARin mode, although we only need one antenna received echo. • If using the stack beams: in order to retrieve the power measured by the altimeter, the individual I&Q echoes shall be processed by performing: the 2D-FFT (Doppler beams and range compression), and a retracking (gaussian fitting and integration). • If using the Level 1b: the multi-looked echo shall be retracked. • We will compare the above with the computed theoretical power and retrieve the power bias. • We need the TRP RCS characterised up to a value that depends on the sigma-0 calibration requirement (about ± 0.1 dB). • Otherwise we will characterise it through the RA-2 (use RA-2 well calibrated over the TRP to determine TRP internal delay). 4. SIRAL calibration using transponder data

23. 4.3. Mispointing 4.3. Mispointing • Historically, due to the nature of the past altimeters, mispointing have only been evaluated irrespectively of the direction (pitch and roll). • We have now the opportunity of measuring mispointing as a function of the angle of arrival. • Considerations: • along-track mispointing can be misinterpreted as a datation error; • across-track mispointing can be misinterpreted as interferometric baseline error. • We will therefore estimate the mispointing with the traditional way: slope of the trailing edge of 10 minutes averaged waveform over ocean (LRM). 4. SIRAL calibration using transponder data

24. 4.4. Datation 4.4. Datation: • TRP’s can be used to measure the datation bias. • The expected time of minimum range (black) is compared with the actual time of minimum range seen by the altimeter over the TRP. • We can use a stack of echoes or single multi-looked echo. 4. SIRAL calibration using transponder data

25. 4.5. Angle of arrival 4.5. Angle of arrival • The across track angle of an incoming ray, Fmeas, can be inferred by determining the phase difference between the 2 received signals (one from each antenna). where B is the distance between the 2 antenna phase centres. • The theoretically computed angle of arrival, Ftheo, can be compared with the angle of arrival retrieved from the data. where r(t) is a hyperbola and h is the satellite height. • If using Level 1b data we compare a single angle measured against theoretical angle. If using the stack we compare the 2 equations. 4. SIRAL calibration using transponder data

26. r1 h1 r0 h0 d1 r -1 h -1 d0 TRP 4.5. Angle of arrival d-1 4. SIRAL calibration using transponder data

27. 5. Conclusions and way forward

28. 5. Conclusions and way forward • Transponders have demonstrated through the years the suitability of certain type of calibrations. • Doppler beam formation processing can be verified over the TRP. • We will use the transponder deployed in Svalvard for SIRAL calibration of angle of arrival and: • range bias, • sigma-0 bias, • datation. • A transponder RCS characterisation is recommended for Sigma-0 bias determination. • We will compare the retrieved biases using the stack beams (before multi-looked) and using the single multi-looked echo. 5. Conclusions and way forward

29. PiLDo Labs C. Llacuna, 162 Barcelona 08018 T +34 93 401 97 82 F +34 93 401 97 83 PiLDo@pildo.com Monica.Roca@pildo.comMercedes.Reche@pildo.com tel.: +34 93 401 9707 tel.: +34 93 401 9755