1. A KU-BAND ATI SAR MISSION FOR TOTAL OCEAN SURFACE CURRENT VECTOR RETRIEVAL: CHALLENGES, CONCEPT, AND ERROR ANALYSIS. Paco López-Dekker, Francesco De Zan, Steffen Wollstadt, and Marwan Younis [German Aerospace Center (DLR)] Volker Tesmer and Robert Ernst [OHB System AG] Rick Danielson [Nansen Environmental and Remote Sensing Center (NERSC)] Giovanni D'Aliesio and Luis Martins Camelo [MacDonald, Dettwiler and Associates (MDA)]

2. Effective line-of-sight Doppler velocity Effective along-track Doppler velocity ATI Phase

3. OSCM Paco López-Dekker 28.10.2014 Slide 3 Effective Doppler velocities Land

4. OSCM Paco López-Dekker 28.10.2014 Slide 4 Surface current (and wind) retrieval errors Total error Measurement noise Geophysical noise Instrument related systematic errors Geophysical biases Extremely challenging Limiting factor???

5. OSCM Paco López-Dekker 28.10.2014 Slide 5 Ocean surface velocity error (sensitivity related) July 28th, 2011 5 Ocean Backscatter: Sigma0 (back, HH) Sigma0 (back, VV) Sigma0 (fore, HH) Sigma0 (fore, VV) NESZ: NESZ_HH NESZ_VV SNR: SNR (back, HH) … Coherence: Gamma (back, HH) … Number of looks (N) Temporal baseline 12m/2/vs ~ 0.8ms  4 velocity error contributions (HH&VV / backward & forward looking)  Combination of 2 lines of sight (fore & aft) to obtain total velocity error Interferometric error/performance - ocean current velocity error

6. OSCM Paco López-Dekker 28.10.2014 Slide 6 Resolution vs SNR trade-off More bandwidth means more looks, and more noise The key are the other sources of decorrelation (temporal) We want looks to fight non-thermal decorrelation, until thermal noise becomes relevant For our case of rather high temporal coherence the optimum SNR is around 0-10 dB. July 28th, 2011 6 Optimum bandwidth for each system  Upper limit for bandwidth! Optimum bandwidth for each system  Upper limit for bandwidth!

7. OSCM Paco López-Dekker 28.10.2014 Slide 7 Derivation of Instrument Requirements July 28th, 2011 7 SAR performance inputs result in TSCV performance  system optimization process very work intensive, due to large variety of instrument possibilities/adjustable parameters Given science requirement:  max. TSCV error = 3 cm/s TSCV requirement results in required SAR performance  3 cm/s traslated in corresponding instrument requirements, like NESZ and 2D-resolution instrument to product product to instrument  No End-to-end simulation  But clearer instrument design due to better trading possibilities with (well-known) SAR parameters

8. OSCM Paco López-Dekker 28.10.2014 Slide 8 Derivation of Instrument Requirements: example July 28th, 2011 8 TSCV error Product resolution4 x 4 km 2 Pulse Bandwidth10 MHz Azimuth resolution (nominal)2 m Surface wind2 m/s 2-D SLC resolution 120 to 60 m 2 Requirement

9. OSCM Paco López-Dekker 28.10.2014 Slide 9 Instrument requirements: no unique solution July 28th, 2011 9 Product resolution4 x 4 km 2 Surface wind2 m/s TSCV error3 cm/s Required NESZ

10. OSCM Paco López-Dekker 28.10.2014 Slide 10 Near range vs Far range solutions NESZ Requirements July 28th, 2011 10 Near range Incident angle16° to 27° Antenna squint13.5° Far range Incident angle26° to 36.5° Antenna squint18.5° 2D-resolution [m 2 ] [dB] Incidence angle [deg] Easier

11. OSCM Paco López-Dekker 28.10.2014 Slide 11 Near range vs Far range solutions Scatterometry July 28th, 2011 11 Near range Incident angle20° Far range Incident angle30° much clearer signature better inversion

12. OSCM Paco López-Dekker 28.10.2014 Slide 12 Systematic phase error budget July 28th, 2011 12 AT velocity error LoS error AT Interferometric error Baseline length error Hardware calibration 5 cm/s1.4 cm/s3 cm/s Phase error 0.18 deg 0.1 deg XT velocity error LoS error XT Interferometric error Pointing error Hardware calibration 5 cm/s1.4 cm/s3 cm/s Phase error 0.18 deg 0.1 deg Near range Incident angle16° Antenna squint13.5° Far range Incident angle26° Antenna squint18.5° AT velocity error LoS error AT Interferometric error Baseline length error Hardware calibration 5 cm/s2.2 cm/s3 cm/s Phase error 0.28 deg 0.16 deg XT velocity error LoS error XT Interferometric error Pointing error Hardware calibration 5 cm/s2.2 cm/s3 cm/s Phase error 0.28 deg 0.16 deg Requirement

13. OSCM Paco López-Dekker 28.10.2014 Slide 13 Hybrid Polarimetry July 28th, 2011 13 Two single polarized beams Insufficient for wind and current retrieval Add Polarization diversity Cross-pol terms usually small Hybrid polarization: transmit circular and receive two orthogonal polarizations. Polarization diversity requires larguish incident angles. p: Polarization br: Two-scale Bragg sp: Specular reflection wb: Wave breaking Polarization independent

14. OSCM Paco López-Dekker 28.10.2014 Slide 14 Polarimetry July 28th, 2011 14 Single Pol (VV)Simple, bad inversionDual Pol (HH, VV) Switching (HH, VV, HH…) High PRF, no good scientific motivation Alternating (ASAR style) Tx switch, better NESZ, worse resolution, requires good HH and VV polarizations. Hybrid pol Assumes very low cross pol, better resolution, relaxed antenna polarizations. 14 Dual Pol pushes us towards larger incident angles!

15. OSCM Paco López-Dekker 28.10.2014 Slide 15 Near range vs Far range solutions July 28th, 2011 15 Near range 16° - 27° Far range 26° - 36° Better geometry reduces impact of systematic errors More polarization diversity Better scatterometry

16. OSCM Paco López-Dekker 28.10.2014 Slide 16 Instrument Concept 16 Rx-1 Rx-2 Tx H-pol. aft V-pol. aft H-pol. fore V-pol. fore H-pol. aft V-pol. aft H-pol. fore V-pol. fore C-pol. aft C-pol. fore

17. OSCM Paco López-Dekker 28.10.2014 Slide 17 Antenna Configuration 17 Polarization: single circular (may be generated by H- and V-pol with 90° phase shift) Beam: two beams electrically squinted to for and aft direction. Mechanical squint and tilt: no mechanical squint (0 Doppler) but a fixed tilt (swath center) Elevation Beams: switching between multiple elevation beams (only one beam at any time) Total Simultaneous Antenna Beams: 1) circular polarization fore 2) circular polarization aft Tx Antenna Polarization: dual vertical and horizontal Mechanical squint and tilt: same as Tx antenna Elevation Beams: scanning beam in elevation (SCORE) Total Simultaneous Antenna Beams: 1) H-pol fore2) H-pol aft3) V-pol fore4) V-pol aft Rx Antenna

18. OSCM Paco López-Dekker 28.10.2014 Slide 18 Instrument concept (798km Orbit, Far) ParameterValue orbit height798 km min/max elevation angles 23.1° - 26.6° 26.5° - 29.3° 29.2° - 31.7° Doppler angles (all sub-swaths) 71.5° swath width210 km timing diagram imaging geometry & swath ParameterValue Tx length3.6 m height0.2167 m Rx length3.6 m height0.8 m single antenna parameter ParameterValue signal bandwidth (ssw 1, 2, 3)10 ; 10 ; 10 MHz processed Doppler bandwidth168 ; 346 ; 840 Hz total average Tx power2.97 kWatt total data rate598.4 MBit/s mode/power parameter

19. OSCM Paco López-Dekker 28.10.2014 Slide 19 Instrument concept (798km Orbit, Far) Resolution and NESZ 2D resolution noise-equivalent sigma-zero We put more resolution where it is most needed Requirement

20. OSCM Paco López-Dekker 28.10.2014 Slide 20 Instrument concept (798km Orbit, Far) Ambiguities range-ambiguity-to-signal ratio azimuth-ambiguity-to-signal ratio

21. OSCM Paco López-Dekker 28.10.2014 Slide 21 Possible calibration strategies 1.Specify error budget in spectral domain Do not require very small relative errors over very long scales 2.Make system as good a reasonably possible, characterize system error budget spectrally. 3.Use land where available Coastal areas! 4.Understand and introduce geophysical constraints. Systematic errors take the form of along-track undulations and range ramps. Explore if something can be done at mosaicking level (like TanDEM-X, but also SWOT) 5.Explore the use of spectral diversity techniques used successfully in somewhat similar problems Usually it is assumed that surface is not moving: refined concept exists, but performance needs to be investigated (where are not too optimistic) May suppress high frequency errors System errors Spectral region of intererst (submesoscale, mesoscale)

22. OSCM Paco López-Dekker 28.10.2014 Slide 22 Outlook Main achievement: instrument concept that has the required sensitivity. Main departures from Wavemill concept: Single-side looking for larger contiguous swath Larger indicent angles for better scatterometry Hybrid polarimetry Main engineering challenges Characterize systematic errors Provide appropriate calibration scheme Scientific questions Inversion model that exploits polarization diversity Model (achievable) inversion performance July 28th, 2011 22

23. OSCM Paco López-Dekker 28.10.2014 Slide 23 Outlook high resolution (precision) low resolution (absolute accuracy)