1. Alternating Polarization ´Single´ Look Complex Product Status
Nominal Product Characteristics
Impulse Response Measurements
Ambiguity Measurements
Noise Equivalent RCS
Scalloping Measurements
Localization Accuracy
Conclusions
Josep Closa
ESA-ESRIN
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002

2. APS Product Characteristics
Slant range complex products
One product contains two images in two possible polarization combinations: HH-VV HH-HV VV-VH
Elevation antenna pattern and range spreading loss corrections are not applied during processing
Product size of up to 1.2 Gbytes: 4 bytes per complex sample (2I + 2Q)
Swath widths of 100 km (IS1) to 56 km (IS7) with azimuth extents of ~100 km
Range resolution fixed to ~ 8.3 m (16 MHz chirp bandwidth used for all swaths)
Natural pixel spacing (sampling frequency and PRF)
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002

3. APS Product Characteristics
Products processed with the modified range-Doppler phase preserving algorithm
Total azimuth bandwidth is kept to 4 looks (2 azimuth looks per image)
Azimuth resolution depends on number of pulses per burst + modulation of the IRF due to scansar acquisition ~ 6.7 m
Doppler is estimated using data from both polarization and the same valued is used to process both images
No window is applied during range processing
Inverse of the azimuth antenna pattern is applied to the azimuth spectrum to descallop the images
Range compression performed with nominal chirp
Image normalization performed using constant reference values due to AP calibration pulse corruption
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002

4. ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002
APS Product Examples
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002
VV and VH Polarization image orbit 3712
HH and HV Polarization image orbit 3579

5. ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002
APS Product Examples
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002

6. ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002
Ceos Offset Test
Ceos Offset Test performed on orbit 3579 over Resolute Bay (HV polarization) introducing a shift of 200 samples in range and azimuth
Mean Phase difference: degrees. Mean absolute difference: degrees Std. Dev: (requirements mean <0.1, stdev <5.5)
No artefact detected in range. Azimuth processing blocks visible
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002

7. ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002
IRF Analysis
Analysis based on acquisitions over ESA transponders
First analysis on the optimization of product digital values dynamic range
I/Q peak values below optimum value
Amplitude statistics on backscatter
Edam in VV polarization orbit IS2
Processing Gain too low
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002

8. ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002
IRF Function
Response is visible in both polarization images
Modulation in the azimuth response
Azimuth Resolution on modulated response: 5.6 m
Azimuth resolution on envelope: 27.6 m
Range Resolution: 8.35 m
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002

9. Interpolated IRF function
Edam VV resampled Orbit IS2
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002

10. Range and Azimuth Resolution
Range and azimuth resolution within requirements for most cases (theoretical value ± 10%)
Range resolution: 8.35 ± 0.1 m
Azimuth resolution: 6.11 ± 0.5 m
Additional modulation detected on some azimuth responses
Range resolution
Azimuth resolution
Theoretical range resolution: 8.31 m
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002

11. Strange azimuth responses
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002

12. ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002
Range and Azimuth PSLR
Range and azimuth PSLR within requirements (theoretical value 5 dB!!)
Low azimuth PSLR values detected on the degradated azimuth IRF
Theoretical range PSLR: dB
Theoretical azimuth PSLR: -3.5 dB
Mean: dB
Stdev: 0.2
Mean: dB
Stdev: 0.3
Range PSLR
Azimuth PSLR
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002

13. ISLR and Spurious Sidelobe Levels
ISLR values slightly different from requirements (-0.64 dB) except for the cases with degradated azimuth IRF
Spurious SLR significantly different from requirements (-25 dB)
Mean: dB
Mean: dB
ISLR
Spurious Sidelobe Levels
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002

14. ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002
Azimuth Ambiguities
Azimuth ambiguities from the transponder response can be observed on Edam, Swifterband and Aalsmeer transponders
Results in the range of -31.0dB to dB
Average PT azimuth ambiguity ratio: dB < Requirement: –25 dB
Edam ambiguity response 3828 IS4 HV
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002

15. Calibration Constant and Radiometric Stability
Very few measurements to derive calibration constant
Different calibration constants per beam to be compensated with updated nominal reference energy values
High stability within the beam
Almost no variation with polarization
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002

16. ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002
Cross- Talk
Transponder response cal always be detected on both polarization images
High variation of peak to peak intensity difference
Peak Intensity Differences
Edam HV on the resampled image
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002

17. ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002
Noise Equivalent RCS
Azimuth modulation similar to scalloping detected on the cross-polar images for ALL the acquisitions
Effect due to the descalloping function applied on thermal noise
Noise measured on the sea areas (using previous calibration constants) for all the beams agrees with predicted estimates
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002

18. ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002
Scalloping
Scalloping detected only on one image and over the sea area
Scalloping visible on both polarization images
Radiometric error due to scalloping 0.2 dB on both images
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002

19. Localization Accuracy / Channel co-registration
Localization accuracy derived from the difference between the estimated transponder position and the detected one
Cross-talk on the transponders allows to give an estimate of the co-registration of the two images. Values within requirements <0.25*pixel
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002

20. ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002
Conclusions
Modulated azimuth response
APS characterization: two azimuth looks
Internal calibration not used to perform product normalization
Processing gains need to be increased
Transponder responses can be detected on both polarization images
Range and azimuth geometric resolution agree with expected values
ISLR and Spurious Side Lobe Ratio different from expected values
Very few transponder measurements per swath to derive calibration constant
Different calibration constant per swath
NES0 slightly better than predictions
Almost no scalloping. Descalloping function can be detected on noise images
Localization accuracy within 3 pixels except for some outliers
Channel co-registration within requirements
ENVISAT Cal/Val Workshop – ESRIN – 9/13 December 2002