M. Younis 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) International Geoscience and Remote Sensing Symposium July 24-29, 2011 – Vancouver, Canada

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 2 SAR Instrument Requirements ParameterValue frequency9.65 GHz (X-Band) coverage> 300 km resolution≤ 1 x 1 m ambiguity-to-signal ratio≤ -20 dB noise-equivalent sigma zero≤ -20 dB 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

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 3 Operation of Transmit in Elevation swath Tx illumination ground range reflector flight direction slant range transmit with all feed elements narrow beam of feed array illuminate small portion of reflector wide and low gain beam illuminating complete swath transmit with all feed elements narrow beam of feed array illuminate small portion of reflector wide and low gain beam illuminating complete swath Transmit in Elevation

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 4 reflector swath ground range Rx window Operation of Receive in Elevation flight direction slant range SCan-On-REceive (SCORE) follow the pulse echo on the ground by activating corresponding elements cycle through all elements within on PRI SCan-On-REceive (SCORE) follow the pulse echo on the ground by activating corresponding elements cycle through all elements within on PRI Rx element activation matrix energy from a small portion of the ground illuminates complete reflector focused on individual elements of feed narrow and high gain beam energy from a small portion of the ground illuminates complete reflector focused on individual elements of feed narrow and high gain beam Receive in Elevation

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 5 Azimuth Operation flight direction Transmit in Azimuth transmit with all feed elements narrow beam of feed array illuminate small portion of reflector wide and low gain beam transmit with all feed elements narrow beam of feed array illuminate small portion of reflector wide and low gain beam swath width

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 6 Azimuth Operation flight direction Doppler span 4 beam 3 beam 1 Doppler span 1 Doppler span 2 Doppler span 3 beam 2 beam 4 Receive in Azimuth each azimuth channel is sampled each azimuth channel covers a narrow Doppler spectrum low PRF combining the azimuth channels yields a wide Doppler bandwidth high resolution each azimuth channel is sampled each azimuth channel covers a narrow Doppler spectrum low PRF combining the azimuth channels yields a wide Doppler bandwidth high resolution

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 7 single azimuth channel T/R-Module N el 1 2 ADC feed elements AMP Digital Beamforming Hardware Functional Block Diagram flight direction slant range memory signal gen. reflector digital feed array in elevation direction SCan-On-REceive (SCORE) digital feed array in elevation direction SCan-On-REceive (SCORE)

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 8 Hardware Functional Block Diagram reflector flight direction slant range memory signal gen. N el 1 2 ADC feed elements AMP Digital Beamforming T/R-Module single azimuth channel digital feed array in elevation direction SCan-On-REceive (SCORE) digital feed array in azimuth direction good azimuth resolution digital feed array in elevation direction SCan-On-REceive (SCORE) digital feed array in azimuth direction good azimuth resolution 2D Digital Feed Array

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 9 deployable reflector are mature technology flight heritage in space telecommunications satellites Lightweight mesh reflectors spanning diameters > 20 m exist deployable reflector are mature technology flight heritage in space telecommunications satellites Lightweight mesh reflectors spanning diameters > 20 m exist Deployable Reflector Antennas X-Band Reflector System ParameterValue reflector diameter (elevation x azimuth) 12 x 12 m focal length12 m elevation offset0.5 m feed patches & TRMs114 x 10 digital feeds38 x 5 approx. size3.5 x 0.3 m

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 10 image any single swath within access range conventional stripmap processing image any single swath within access range conventional stripmap processing swath 1 swath 2 swath 3 swath 4 Operation Mode and Timing 95 km 82 km 70 km 75 km access range315 km orbit height745 km receive window Tx time PRI = 1/PRF PRI·dc PRIpulse repetition interval PRFpulse repetition frequency dcduty cycle sswsub-swath

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 11 Range-Ambiguity-to-Signal Ratio range-ambiguity-to-signal ratio signal ambig Tx Rx elevation patterns 2-way good range ambiguity suppression due to narrow Rx pattern increase of PRF is possible But: timing issues limit the swath width good range ambiguity suppression due to narrow Rx pattern increase of PRF is possible But: timing issues limit the swath width

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 12 Azimuth-Ambiguity-to-Signal Ratio azimuth-ambiguity-to-signal ratio AASR shows degradation at swath edges due to degraded azimuth patterns improvement through: higher PRF, antenna optimization, azimuth beamforming, or waveform encoding AASR shows degradation at swath edges due to degraded azimuth patterns improvement through: higher PRF, antenna optimization, azimuth beamforming, or waveform encoding proc. Doppler595x10 Hz oversampling3.8 azimuth resolution10.3/10 m signal ambig Tx Rx azimuth patterns 2-way near range mid range NESZ does not meet requirement

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 13 Noise-Equivalent Sigma Zero (NESZ) Noise-Equivalent Sigma-Zero 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 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 P av = 900 W 720 W 600 W 540 W 2-way loss2 dB sys. noise temp.450 K duty cycle10% Av. power per TRM2 W NESZ performance does not meet requirement

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 14 3 Rx elements active pulse extension on ground nadir receive beam reflector pattern steering pulse Pulse Extension Loss (PEL) The pulse extension loss (PEL) is the integral effect over multiple points simultaneously illuminated by the pulse. pulse extension loss

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 15 Pulse Extension Loss (PEL) near range 3 Rx active elements 4 Rx active elements far range wide beam: low PEL but low gain wide beam: low PEL but low gain PEL not critical at far range

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 16 SCORE beam 1  feed 1 ADC On Off On Off On Off On Off feed 2feed 3feed 4 reflector swath 1 On/Off Beamforming in Elevation On/Off : switch element On or Off

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 17 SCORE beam 1 SCORE beam 2   feed 1 ADC On Off On Off On Off On Off On Off On Off On Off feed 2feed 3feed 4feed 5feed 6feed 7 reflector swath 1 swath 2 Two-Swath On/Off Beamforming On/Off : switch element On or Off

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 18 SCORE beam 1 w w w w w w w SCORE beam 2   i i i i i i i ADC reflector feed 1feed 2feed 3feed 4feed 5feed 6feed 7 Time Varying Beamforming i : range sample (discrete time) : complex time-varying weight i : range sample (discrete time) : complex time-varying weight w i

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 19 SCORE beam 1 w4w4 w3w3 w2w2 w1w1 w4w4 w3w3 w2w2 w1w1 w4w4 w3w3 w2w2 w1w1 w4w4 w3w3 w2w2 w1w1 w4w4 w3w3 w2w2 w1w1 w4w4 w3w3 w2w2 w1w1 w4w4 w3w3 w2w2 w1w1 SCORE beam 2        i+3ii+2i+1 i+3ii+2i+1 i+3ii+2i+1 i+3ii+2i+1 i+3ii+2i+1 i+3ii+2i+1 i+3ii+2i+1 reflector swath 1 ADC feed 1feed 2feed 3feed 4feed 5feed 6feed 7 FIR Filter Beamforming i : range sample (discrete time) : complex time-varying weight i : range sample (discrete time) : complex time-varying weight w i

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 20 elevation angle in degree pattern gain [dB] Noise-Equivalent Sigma-Zero elevation beamforming gain ground range in km NESZ [dB] 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 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 3 dB 5 dB 3 dB 5 dB 3 dB Elevation Beamforming to Increase Antenna Gain MVDR: Minimum Variance Distortionless Response LCMV: Linear Constraint Minimum Variance

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 21 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. 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. X-Band Reflector System diameter6 x 12 m focal length12 m elevation offset0.5 m center elements edge elements Reflector Illumination 6 x 2 Active Patches Reflector Illumination Efficiency

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 22 Noise-Equivalent Sigma-Zero Azimuth Beamforming Gain ground range in km NESZ [dB] SNR gain [dB] PRF [kHz] far range 0.8 dB.8 dB 2.2 dB.8 dB near range 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 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 PRF range Azimuth Beamforming for SNR Improvement LCMV: Linear Constraint Minimum Variance

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 23 AASR without Beamforming ground range in km far range near range PRF range PRF [kHz] AASR [dB] AASR with LCMV Beamforming -28 dB -40 dB -28 dB 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 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 Azimuth Beamforming for AASR Improvement

Microwaves and Radar Institute M. Younis – IGARSS’11 – Viewgraph 24 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. 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. ConclusionConclusion