1. Folie 1 Ambiguity Suppression by Azimuth Phase Coding in Multichannel SAR Systems DLR - Institut für Hochfrequenztechnik und Radarsysteme F. Bordoni, M. Younis, G. Krieger IGARSS 2011, 24-29 July, Vancouver, Canada

2. Microwaves and Radar Institute 2 Outline o Introduction o APC (Azimuth Phase Coding) technique o APC in multichannel SAR (Synthetic Aperture Radar) systems o Figure of merit o Numerical analysis o APC performance versus system parameters o Example: two multichannel systems for high resolution wide swath imaging o Conclusions

3. Microwaves and Radar Institute 3 Introduction Current spaceborne SAR systems limitation: trade-off spatial resolution v.s. swath width Research in two main directions: Processing methods for removing the ambiguities APC - low implementation complexity - effectiveness for point and distributed ambiguities New, more flexible SAR systems - Multichannel systems -Digital Beamforming (DBF) on receive - Multichannel processing APC is conceived for conventional SAR systems:  APC in multichannel systems based on DBF on receive?

4. Microwaves and Radar Institute 4 Review of the APC Technique APC is a technique for range ambiguity suppression, conceived for conventional (1 Tx and 1 Rx) SAR systems [Dall, Kusk 2004] azimuth sample number, order of range ambiguity, APC shift-factor APC residual phase [Dal04] J. Dall, A. Kusk, “Azimuth Phase Coding for Range Ambiguity Suppression in SAR”, IGARSS 2004. @ round-trip delay APC modulation phase Tx pulse number 3) Azimuth filtering over the processing bandwidth APC demodulation phase APC is based on three main steps: 1) Azimuth, i.e. pulse to pulse, phase modulation on Tx 2) Azimuth phase demodulation on Rx

5. Microwaves and Radar Institute 5 APC residual phase  Doppler shift order of range ambiguity (0 useful signal)  M =2  maximum Doppler shift of the 1 st order range ambiguity  Larger oversampling  Larger ambiguity suppression Time domain: linear phaseFrequency domain: Doppler shift Az. FILTER

6. Microwaves and Radar Institute 6 Application to Multichannel Systems MULTICHANNEL PROCESSING N Rx az. signals sampled at PRF APC residual phase: 1 2 N Multichannel SAR system: 1 transmitter, N receivers  The behavior of the APC changes when applied to a multichannel system reconstructed multichannel signal sampled at PRF eff =N PRF: PRF << B p

7. Microwaves and Radar Institute 7 APC & Reconstructed Multichannel Signal The APC residual phase has no more a linear trend versus the azimuth sample (pulse) number  no shift of the Spectrum  The residual phase a “stair” shape ( Doppler shift):  The ambiguity spectrum:,, PRF (uniform PRF*) *PRF matched to the antenna length and No. of apertures > regular sampling in azimuth results

8. Microwaves and Radar Institute 8 Figure of Merit Measurement of the ambiguity suppression induced by APC PSD (Power Spectral Density) range ambiguity of 1st order if APC is not applied processed bandwidth PSD range ambiguity of 1st order if APC is applied APC Gain: useful signal after multichannel reconstruction (neglect. elev.) Computed on the SAR signal after multichannel processing  Note: the G apc depends on the azimuth pattern shape

9. Microwaves and Radar Institute 9 ParameterSystem # 123(Ref.)4 Orbit height [km]520 Carrier frequency [GHz]9.600 Rx antenna total length [m]361224 Tx antenna length [m] (and Rx subapert. length) 3 No. of az. Rx channels1248 PRF [Hz] (uniform) 506825341267633.5 PRF eff [Hz]5068 APC Performance Analysis Reference Multichannel Planar Systems The systems have the same azimuth patterns  Effect of the Doppler oversampling  The effect of the pattern shape is not evident  Behavior of APC versus the number of Rx channels, N Processing bandwidth 2316 Hz ≤ B p ≤ 4168 Hz Investigation:

10. Microwaves and Radar Institute 10 Numerical Results: G apc  0.1dB ≤ G apc ≤ 3.13dB  for a given N, the G apc increases with the oversampling factor,   the G apc decreases for increasing number of channels, N  the sensitivity of G apc to  decreases with increasing N APC Gain v.s. oversampling factor For the considered systems, for M =2:

11. Microwaves and Radar Institute 11 Numerical Results: PSD v.s. N  larger N, the upper profile PSD with or without APC are similar and G apc reduces Normalized PSD 1 st range ambiguity after multichannel reconstruction N = 8 with APC N = 2N = 1 without APC N = 1, 2, 8 The thickness of the curves is a fast variation of the spectrum, due to aliasing

12. Microwaves and Radar Institute 12 HRWS (High-Resolution Wide-Swath) SAR System promoted by the German Aerospace Centre (DLR) conceived to obtain high resolution and wide swaths (1 m resolution, 70 km swath width in stripmap mode) Different Rx azimuth patterns & multichannel reconstruction HRWS SAR Multichannel Systems ParameterPlanarReflector Orbit height [km]520745 Carrier frequency [GHz]9.6009.650 Tx/Rx antenna total length [m]8.75 Paraboloid diameter (elev., az.) [m]10, 12 Total number of feeds (elev., az.)60, 10 No. of az. Rx channels710 PRF [Hz]17502792 Processed bandwidth [Hz]62525946 Oversampling factor1.9604.696 Planar system: currently adopted design Reflector system: alternative design option, studied in DLR

13. Microwaves and Radar Institute 13 Peculiarities HRWS Systems Reflector systemPlanar system The pattern of each Rx channel covers B p Multichannel processing: Multi-Aperture Reconstr. The patters do not change along the swath The pattern of each Rx channel covers 1/N of B p Multichannel processing: Spectral decomposition The patters change along the swath  Evidence of the dependence of the APC performance on the pattern shape

14. Microwaves and Radar Institute 14 Numerical Results: Planar HRWS System For M =2, G apc = 0.69 dB  The high number of channels (7) and the small oversampling (1.96) associated low G apc with APCwithout APC Normalized PSD 1 st range ambiguity used to compute the G apc (after multichannel reconstruction)

15. Microwaves and Radar Institute 15 Numerical Results: Reflector HRWS System For M=2, 3.2 dB ≤ G apc ≤ 8.6 dB over the swath, depending on the azimuth pattern  The azimuth pattern strongly affects the APC performance  The reflector based system, characterized by a higher oversampling factor (4), takes better advantage from the application of APC Normalized PSD 1 st range ambiguity used to compute the G apc (before multichannel reconstruction, single Rx channel) without APCwith APC

16. Microwaves and Radar Institute 16 Conclusions o In multichannel systems, the APC effect is no more a frequency shift of the range ambiguity. o Also in multichannel systems, the APC allows for improved ambiguity suppression. o The azimuth pattern strongly affects the APC performance. o For a given azimuth pattern, the suppression is directly proportional to the oversampling factor and inversely proportional to the number of receive channels. o In a conventional SAR system with  = 2, the achievable suppression of each ambiguity of odd order is about 3 dB. In multichannel systems based on planar antenna architectures, the suppression is generally poorer. Reflector based systems reach better performance, because of the higher oversampling. o In the planar and reflector based HRWS systems the APC suppression is about 0.7 dB and between 3 and 8 dB, respectively.