Design Optimization Aspects for Reflector Base Synthetic Aperture Radar

1. International Geoscience and Remote Sensing Symposium July 24-29, 2011 – Vancouver, Canada Design Optimization Aspects for Reflector Base Synthetic Aperture Radar Marwan Younis, Anton Patyuchenko, Sigurd Huber, and Gerhard Krieger, Microwaves and Radar Institute, German Aerospace Center (DLR)

2. System and Requirement Parameters • Reflector based SAR System architecture and operation • System Performance range- & azimuth-ambiguity-to-signal ratio, noise-equivalent sigma zero, pulse extension loss • Performance Optimization beamforming in elevation and azimuth SAR Instrument Requirements

3. Operation of Transmit in Elevation Transmit in Elevation flight direction • transmit with all feed elements • narrow beam of feed array • illuminate small portion of reflector wide and low gain beam illuminating complete swath reflector slant range Tx illumination swath ground range

4. Operation of Receive in Elevation Rx element activation matrix Receive in Elevation flight direction reflector slant range • energy from a small portion of the ground illuminates complete reflector • focused on individual elements of feed narrow and high gain beam SCan-On-REceive (SCORE) • follow the pulse echo on the ground by activating corresponding elements • cycle through all elements within on PRI Rx window swath ground range

5. Azimuth Operation Transmit in Azimuth • transmit with all feed elements • narrow beam of feed array • illuminate small portion of reflector wide and low gain beam flight direction swath width

6. Azimuth Operation Receive in Azimuth • each azimuth channel is sampled • each azimuth channel covers a narrow Doppler spectrum low PRF • combiningthe azimuth channels yields a wide Doppler bandwidth high resolution flight direction beam 4 Doppler span 4 beam 3 Doppler span 3 beam 2 Doppler span 2 beam 1 Doppler span 1

7. Hardware Functional Block Diagram T/R-Module reflector flight direction slant range • digital feed array in elevation direction • SCan-On-REceive (SCORE) single azimuth channel Nel ADC AMP memory Digital Beamforming 2 ADC AMP 1 ADC AMP feed elements signal gen.

8. Hardware Functional Block Diagram 2D Digital Feed Array reflector flight direction slant range • digital feed array in elevation direction • SCan-On-REceive (SCORE) • digital feed array in azimuth direction good azimuth resolution single azimuth channel single azimuth channel single azimuth channel T/R-Module Nel ADC AMP memory Digital Beamforming 2 ADC AMP 1 ADC AMP feed elements signal gen.

9. Deployable Reflector Antennas • deployable reflector are mature technology • flight heritage in space telecommunications satellites • Lightweight mesh reflectors spanning diameters > 20 m exist X-Band Reflector System

10. Operation Mode and Timing 75km swath1 70km 82km swath2 swath3 95 km swath4 • image any single swath within access range • conventional stripmap processing PRI pulse repetition interval PRF pulse repetition frequency dc duty cycle ssw sub-swath PRI·dc receive window Tx Tx time PRI = 1/PRF

11. Range-Ambiguity-to-Signal Ratio range-ambiguity-to-signal ratio elevation patterns ambig Rx Tx signal 2-way good range ambiguity suppression due to narrow Rx pattern increase of PRF is possible But: timing issues limit the swath width

12. Azimuth-Ambiguity-to-Signal Ratio azimuth-ambiguity-to-signal ratio azimuth patterns Rx Tx 2-way mid range NESZ does not meet requirement • AASR shows degradation at swath edges due to degraded azimuth patterns • improvement through: higher PRF, antenna optimization, azimuth beamforming, or waveform encoding ambig signal near range

13. Noise-Equivalent Sigma Zero (NESZ) Noise-Equivalent Sigma-Zero 540W 600W NESZ performance does not meet requirement Pav=900W 720W • lower average power per swath than planar antenna systems • a sub-set of the TRMs are activated for each swath • the number of TRMs determine the total power • reducing the swath width does not improve the NESZ

14. Pulse Extension Loss (PEL) pulse extension loss reflector 3Rx elements active receive beam pattern steering nadir pulse pulse extension on ground The pulse extension loss (PEL) is the integral effect over multiple points simultaneously illuminated by the pulse.

15. Pulse Extension Loss (PEL) near range far range wide beam: low PEL but low gain PEL not critical at far range 4 Rx active elements 3 Rx active elements

16. On/Off Beamforming in Elevation SCORE beam 1 feed 1 feed 4 feed 3 feed 2 ADC S ADC On/Off : switch element On or Off ADC On On On On ADC Off Off Off Off reflector swath 1

17. Two-Swath On/Off Beamforming feed 7 feed 1 feed 6 feed 5 feed 4 feed 3 feed 2 SCORE beam 1 ADC S SCORE beam 2 ADC On/Off : switch element On or Off ADC On On On On On On On ADC Off Off Off Off Off Off Off reflector ADC S swath 2 ADC swath 1 ADC

18. Time Varying Beamforming feed 7 feed 6 feed 5 feed 4 feed 3 feed 2 feed 1 w SCORE beam 1 ADC i w i S SCORE beam 2 w ADC i i : range sample (discrete time) : complex time-varying weight w ADC i w ADC i reflector w ADC i S w ADC i w ADC i

19. FIR Filter Beamforming feed 7 feed 6 feed 5 feed 4 feed 3 feed 2 feed 1 w1 w2 w3 w4 SCORE beam 1 ADC i i i i i i i i+1 i+1 i+1 i+1 i+1 i+1 i+1 i+2 i+2 i+2 i+2 i+2 i+2 i+2 i+3 i+3 i+3 i+3 i+3 i+3 i+3 w i S SCORE beam 2 w1 w2 w3 w4 ADC S i : range sample (discrete time) : complex time-varying weight w1 w2 w3 w4 ADC S w1 w2 w3 w4 ADC S reflector w1 w2 w3 w4 ADC S w1 w2 w3 w4 ADC S w1 w2 w3 w4 ADC S swath 1

20. Elevation Beamforming to Increase Antenna Gain elevation beamforming gain Noise-Equivalent Sigma-Zero 5dB 3dB 5dB 3dB pattern gain [dB] NESZ [dB] 3dB 3dB 3dB elevation angle in degree ground range in km • Use elevation beamforming to increase antenna gain • Most effective at large scan angel, where beams overlap (defocus) • In best case increase the gain (NESZ) by 3dB to 5dB MVDR: Minimum VarianceDistortionlessResponse LCMV: Linear Constraint Minimum Variance

21. Reflector Illumination Efficiency • The reflector is only partially illuminated in elevation • The illumination is a function of pulse duty cycle • reflector height reduction • Although all azimuth elements are active on receive no sub-illumination occurs. Reflector Illumination6 x 2 Active Patches center elements X-Band Reflector System edge elements

22. Azimuth Beamforming for SNR Improvement Azimuth Beamforming Gain Noise-Equivalent Sigma-Zero 2.2dB far range SNR gain [dB] NESZ [dB] PRF range .8dB 0.8dB .8dB near range PRF [kHz] ground range in km • Due to wide azimuth beams, several elements share common Doppler spectra. • Combine azimuth channels to increase signal engery • Increase the gain (NESZ) by .8dB to 2.2dB LCMV: Linear Constraint Minimum Variance

23. Azimuth Beamforming for AASR Improvement AASR without Beamforming AASR with LCMV Beamforming PRF range AASR [dB] AASR [dB] near range far range -28dB -28dB -28dB -40dB PRF [kHz] ground range in km • The LCMV algorithm uses overlapping beams to place nulls at the ambiguity positions • However the azimuth channels are sampled adequatly, i.e. no reconstruction needed. • Azimuth-ambiguity suppression better than -38dB

24. Conclusion Reflector based systems allow for high-resolution wide-swath operation using digital beamforming • High performance reflector SAR is feasible at X-band. • The power consumption per swath is less than for planar systems. • Time varying digital beamforming is required in elevation to reach full antenna gain. • On-Ground digital beamforming is required in azimuth to suppress ambiguities.