Upcoming altimeter measurements : nadir altimetry from Ku/C to Ka-band, SAR mode, interferometric SAR

Transcription

1 Upcoming altimeter measurements : nadir altimetry from Ku/C to Ka-band, SAR mode, interferometric SAR Jean-Claude Souyris CNES, Service Altimétrie & Radar Acknowledgments : Nathalie Steunou, Roger Fjortoft, Alain Mallet

2 Nadir Ku/C altimetry Orbit GPS Ionosphere Ku + C altimeter Troposphere Ku+K+ka Radiometer (JMR) Geoïde Dynamical topography hauteur mer DORIS

3 Nadir Altimetry : from Ku/C to Ka (35 GHZ) Reduced inospheric effects using one single frequency ALTI-KA on SARAL Ka-band altimeter Developed by Thales Alenia Space- FRANCE (delivery and full validation : 12/09) Improved spatial resolution ( 30 km 8 km) Limitations : atmospheric water content (Data loss between 3% and 10%) A CNES ISRO cooperation

4 From nadir to wide swath altimetry Speed (azimuth along-track) Off-Nadir illumination range 1 4 ground resolution swath Improvement of range resolution ground resolution swath

5 ALONG TRACK Azimuth resolution Wide swath altimetry : SAR mode Improvement of azimuth resolution (Cryosat-2, SRAL/S3) ALONG TRACK Azimuth resolution

6 SAR mode in action RADAR IMAGES Radar echoes Azimuth synthesis

7 Wide swath altimetry : interferometry for HEIGHT extraction Baseline : B (10 m) Iso-altitude line (r1-r2) B A 2 A 1 r 1 r 2 M M Height measurement

8 Wide swath altimetry : interferometry for height extraction Baseline : B (20 m) B A 2 A 1 r 2 r 1 M M Iso-altitude line (r 1 -r 2 ) Height measurement

9 Wide swath altimetry : interferometry for height extraction Interferometry allows Height measurement Topographic sensitivity when B/λ B/λ when λ (easiest than to increase B because of mast length requiered ) Transition from Ku to Ka (Gain in height sensitivity : 2.6)

10 SWOT JPL concept NASA/CNES cooperation speed A 1 A 2 Across-track 4 M 1 50 km 20 km 50 km Cf. "KARIN/SWOT performance analysis tool - application to near nadir swath extension, A. MALLET

11 SWOT : geometry A 1 B A 2 Interferometric Phase (r 1 -r 2 ) Baseline H θ r 1 r 2 h = f ( H, B, r1, Φ) M h Orbit (DORIS, GPS) Range (on-board clock)

12 Height knowledge : absolute or relative? B baseline r 1 range B A 1 A 2 Φ phase r 2 r 1 Radar measurement M only gives the RELATIVE knowledge of the triangle (A 1, A 2, M)

13 From relative to absolute height : roll correction B A 1 A 2 α A 1 B A 2 r 2 r 2 r 1 r 1 M M Roll impact : α = 1 arcsecond height error : 30 cm

14 From relative to absolute height : tying the triangle to the reference frame A 1 B A 2 θ HOW? r 2 H r 1 M h

15 From relative to absolute height : tying the triangle OPTION 1 : A 1 B A 2 Reference : A1 (satellite) Precise Orbit Determination (POD) Tropospheric correction (r1 knowledge) θ r 1 r 2 Processing Pixel by Pixel H M Reference frame h

16 From relative to absolute height : tying the triangle OPTION 2 : A 1 B A 2 Reference : Ground Control Point (e.g. GPS) «Phase unwrapping» from GCP («continental» approach) θ r 1 r 2 Tropospheric correction not necessary H M M GCP h Reference frame GCP

17 SWOT radiometry : not nadir, not classic SAR, not Ku! Nadir altimetry SAR imagery 0.6 KaRiN Swath θ

18 AltiKa Ka band water signature Varying surfaces conditions ( : smooth : rough) Sigma0 (db) SWOT Incidence angle ( )

19 Ka band water / land contrast water F.R. Strong contrast (> 10 db) Parameter dependant Sigma0 (db) land Incidence angle ( )

20 Between geometry and radiometry : Layover θ = 45 θ << 45 Water response mixed with forest response Water response mixed with forest response LAYOVER when θ

21 SWOT Layover (4 1 ) Lay-over impact is important due to weak Water/Land contrast 4 DEM -2m 56m LOS Layover zones rg az

22 SWOT Layover (4 1 ) 3 DEM -2m 56m LOS Layover zones rg az

23 SWOT Layover (4 1 ) 2 DEM -2m 56m LOS Layover zones rg az

24 SWOT Layover (4 1 ) Lay-over impact is reduced thanks to strong Water / Land contrast 1 DEM -2m 56m LOS Layover zones rg az

25 Incidence variability effects 1 4! Wide relative incidence range 10 km Near Range (NR) Far Range (FR) ground range ground range RESOLUTION Near Range Interferometric performance reduced 20 km blind zone around nadir Land / water CONTRAST Far Range

26 SUMMARY Several challenges attached to Ka band : AltiKa / SARAL and the upcoming Ka band nadir altimetry (ISRO / CNES) SWOT : Wide Swath altimetry inc. SAR + Interferometry (NASA / CNES) Measurement concept understood and concurrent issues identified Physics of measurement needs clarification Importance for airborne experiments (AIRSWOT, BUSARD, )