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ASAR APP & APM Image Quality Peter Meadows & Trish Wright

ASAR APP & APM Image Quality Peter Meadows & Trish Wright. Properties of APP & APM Products Example APP & APM Products Analysis Approach Format Verification Visual Inspection Impulse Response Function Measurements AP Cross-Polarisation Ratio. AP Channel Co-Registration

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ASAR APP & APM Image Quality Peter Meadows & Trish Wright

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  1. ASAR APP & APM Image Quality Peter Meadows & Trish Wright • Properties of APP & APM Products • Example APP & APM Products • Analysis Approach • Format Verification • Visual Inspection • Impulse Response Function Measurements • AP Cross-Polarisation Ratio • AP Channel Co-Registration • Equivalent Number of Looks and Radiometric Resolution • Azimuth Ambiguities • Localisation Accuracy • Preliminary Radiometric Calibration • Noise Equivalent Radar Cross-Section • Summary

  2. Ground range detected alternating polarisation imagery Dual polarisation (HH & VV, HH and HV or VV & VH) Elevation antenna pattern and range spreading loss corrections applied Size up to 300Mbytes with 2 byte (16bit) amplitude pixel values Swath widths of 100 km (IS1) to 56 km (IS7) with azimuth extents of ~100 km Azimuth resolution of 27.6m (2 looks of ~250 Hz each) Range resolution from 21m (IS2 far range) to 37m (IS1 near range) and ~26m for IS3 to IS7 (1 look) 12.5m by 12.5m pixels (hence under-sampling for spatial resolutions less than 25m) Properties of APP Imagery

  3. Ground range detected alternating polarisation medium resolution imagery Dual polarisation (HH & VV, HH and HV or VV & VH) Elevation antenna pattern and range spreading loss corrections applied Size up to few Mbytes with 2 byte (16bit) amplitude pixel values Swath widths of 100 km (IS1) to 56 km (IS7) with azimuth extents of ~100 km up to 4000 km Azimuth resolution of 135m (10 looks of ~50 Hz each) Range resolution from 109m (IS1 far range) to 163m (IS1 near range) and ~130m for IS3 to IS7 (1 look) 75m by 75m pixels Properties of APM Imagery

  4. The blue orbit numbers are of The Netherlands and the purple are of Resolute, Canada. All products are processed with v3.03 No IS1 data Two APP products had saturated ASAR transponders (orbits 3812 & 3855) APP & APM Products

  5. Example APP Products 14 July 2002 IS2 HV & HH Ottawa, Canada

  6. 6 November 2002 IS2 HV & HH Resolute, Canada

  7. 6 November 2002 IS2 HV & HH The Netherlands

  8. Example APM Products 9 November 2002 IS3 HV & HH

  9. 12 November 2002 IS3 VV & VH

  10. Format verification using EnviView & the ESA SAR Product Control Software (developed by DLR & BAE Systems) Image analysis performed using SAR Control Software (point target & calibration modules) and following the ESA document on Quality Measurement Definitions for ASAR products Analysis Approach Format Verification • No problems identified with APP & APM format or header parameters, however many of the APP products had no Chirp Parameter ADSR (hence no annotated Chirp Powers) Visual Inspection • No problems found with any post Orbit 3661 v3.03 APP or APM products

  11. IRF parameters have been derived using the ESA transponders in The Netherlands and the Radarsat transponders in Canada (images 1.6 by 1.6 km). Impulse Response Function Measurements (APP) Aalsmeer (HV) Edam (HH) Edam (HV) Fredericton (HH) Fredericton (HV) Swifterbant (HV) Edam (VV) Zwolle (VH) Resolute (HH) Resolute (HV)

  12. Example APP Impulse Response Functions Radarsat transponder, i = 15.81°, azimuth resolution = 27.49m, range resolution = 34.70m ASAR transponder, i = 23.85°, azimuth resolution = 27.83m, range resolution = 24.14m

  13. Azimuth resolution (y): 28.09±1.97m (c.f. ~27.6m theoretical value, +10% limit & 30m requirement) Range resolution (x): (c.f. theoretical values, +10% limit & <38m requirement for IS1 and <30m requirement for IS2 - IS7) Spatial Resolution (3dB width of IRF): Note that the IS1 swath extends to 22.2° and that the IS2 swath starts at 19.2°.

  14. -12.87±1.44dB (c.f. -12.4dB theoretical value, +5dB limit & <-12dB requirement) Integrated Sidelobe Ratio (ratio of energy in the sidelobes up to a box 20x by 20y to the energy in the mainlobe(2x by 2y)): Peak Sidelobe Ratio (ratio of the intensity of the most intense peak outside the main lobe up to 10x by 10y to the energy in the mainlobe): • -19.07±0.94dB (c.f. -21.2dB theoretical value, +5dB limit & <-20dB requirement) Spurious Sidelobe Ratio (ratio of the intensity of the most intense peak outside 10x by 10y up to 20x by 20y to the energy in the mainlobe): • -25.98±2.20dB (c.f. <-25dB requirement)

  15. IRF parameters have been derived using the ESA transponders in The Netherlands and the Radarsat transponders in Canada (images 4.8 by 4.8km): Impulse Response Function Measurements (APM) Aalsmeer (VH) Edam (VV) Edam (HV) Swifterbant (HV) Resolute (HH) Aalsmeer (HV) Edam (VH) Swifterbant (VH) Zwolle (VH) Resolute (HV)

  16. Example APM Impulse Response Functions ASAR transponder, i = 25.36°, azimuth resolution = 145.9m, range resolution = 122.1m Radarsat transponder, i = 44.22°, azimuth resolution = 144.0m (HV) & 143.7m (HH), range resolution = 132.0m (HV) & 133.9m (HH)

  17. Azimuth resolution (y): 143.9±5.9m (c.f. ~135m theoretical value & +10% limit) Range resolution (x): (c.f. theoretical values & +10% limit) Spatial Resolution (3dB width of IRF): Note that the measurements are to 1/8 pixel (9.4m) and that APM products are undersampled if the resolution is < 150m.

  18. -12.37±1.55dB (c.f. -12.4dB theoretical value, +5dB limit & <-12dB requirement) Integrated Sidelobe Ratio (ratio of energy in the sidelobes up to a box 20x by 20y to the energy in the mainlobe(2x by 2y)): Peak Sidelobe Ratio (ratio of the intensity of the most intense peak outside the main lobe up to 10x by 10y to the energy in the mainlobe): • -16.62±2.57dB (c.f. -21.2dB theoretical value, +5dB limit & <-20dB requirement) Spurious Sidelobe Ratio (ratio of the intensity of the most intense peak outside 10x by 10y up to 20x by 20y to the energy in the mainlobe): • -21.34±2.55dB (c.f. <-25dB requirement)

  19. AP Cross-Polarisation Ratio The ratio of the total power in the IRF of an ASAR transponder in both channels. Examples of IRF in second polarisation: Aalsmeer (HH) Edam (HV) Swifterbant (HH) • Average APP cross-pol ratio (17 measurements): -32.1±4.2dB (c.f. predicted value of < -35dB). Caused by ASAR and/or transponders

  20. AP Channel Co-registration The mis-registration between the two channels computed from the difference in the location of a point target IRF peak in both channels. As the Resolute transponders give strong IRF’s in all polarisations, they have been used for the co-registration: • APP products (2 measurements): 0.0m & 0.0m • APM products (2 measurements): 9.4m & 0.0m

  21. Mean equivalent number of looks: 1.99±0.05 (c.f. >1.9 requirement, -10% limit) Mean radiometric resolution: 2.32±0.03dB (c.f. <2.37dB requirement) Equivalent Number of Looks and Radiometric Resolution Equivalent number of looks and radiometric resolution are derived using uniform distributed targets. APP co-polarisationmeasurements (4 measurements): APM co-polarisationmeasurements (16 measurements): • Mean equivalent number of looks: 56.1±16.8 (c.f. > 30 requirement, -10% limit) • Mean radiometric resolution: 0.56±0.09dB (c.f. < 0.7dB requirement)

  22. ASAR Transponders: -27.9±2.6dB Azimuth Ambiguities As Doppler frequencies can only be distinguished modulo the PRF, azimuth ambiguities occur within the azimuth antenna pattern sidelobes. Measurement requires either a very bright point target or a bright point target with a low ambiguity background radar cross-section. Average APP ambiguity ratio: Average APM ambiguity ratio: • ASAR Transponders: -28.7±1.9dB The requirement is -25dB while the worst case prediction is ~ -27.9dB. APM Edam (VH)

  23. Mean range displacement: -3.8±26.3m Mean azimuth displacement: -12.9±60.1m Mean displacement: 57.8±30.9m Localisation Accuracy The difference between the measured and predicted positions of the ASAR transponder. The predicted positions are based on image header parameters, the known location of the transponders and their time delay. The ASAR transponders have a small terrain height and hence a small range terrain displacement. APP measurements:

  24. The localisation accuracy requirement is <900m while the worst case prediction is ~75m in azimuth and between ~125m (IS1) and ~50m (IS7) in range.

  25. Preliminary Radiometric Calibration The ASAR APP transponders have been used to give a preliminary radiometric calibration for swaths IS2 to IS7. As the v3.03 AP products seem to have been processed with IM nominal chirp powers, corrections have been applied to all AP radar cross-section measurements.

  26. APP K for swaths IS2 to IS4 is: 57.05±0.51dB APP K for swaths IS5 to IS7 is: 60.59±0.47dB (based on non-saturated ASAR transponders) Different K values due to different product scaling factors

  27. The derived APM calibration K is: 69.47±0.50dB (based on ASAR transponders with acceptable ISLR’s) Further measurements required before definitive calibration constants can be derived

  28. Noise Equivalent Radar Cross-section NESigma0 estimated using low radar cross-section regions (ocean, open water or lakes). This gives an upper limit to NESigma0. HV APP polarisation measurements:

  29. 10 v3.03 APP and APM IS2 to IS7 products analysed No format problems identified but many of the APP products had no Chirp Parameter ADSR (hence no annotated Chirp Powers) No visualisation problems found with any post Orbit 3661 v3.03 products APP azimuth & range resolutions, ISLR, PSLR & SSLR acceptable. Some range under-sampling APM Azimuth & range resolutions and ISLR acceptable. PSLR & SSLR outside expected range due to APM under-sampling in both azimuth and at almost all ground ranges Acceptable APP cross-polarisation ratio Sub-pixel AP channel co-registration Summary

  30. APP & APM equivalent number of looks and radiometric resolution acceptable APP & APM azimuth ambiguities acceptable APP Localisation accuracy good Preliminary calibration constants derived - more work required before definitive K values can be calculated. Noise equivalent radar cross-sections lower than predicted NESigma0

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