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Ambiguity Suppression by Azimuth Phase Coding in Multichannel SAR Systems

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

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Ambiguity Suppression by Azimuth Phase Coding in Multichannel SAR Systems

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

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

  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. 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] [Dal04] J. Dall, A. Kusk, “Azimuth Phase Coding for Range Ambiguity Suppression in SAR”,IGARSS2004. APC is based on three main steps: 1) Azimuth, i.e. pulse to pulse, phase modulation on Tx APC modulation phase Tx pulse number 2) Azimuth phase demodulation on Rx APC demodulation phase @ round-trip delay APC residual phase azimuth sample number, order of range ambiguity, APC shift-factor 3) Azimuth filtering over the processing bandwidth

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

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

  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 (uniform PRF*) PRF , ,  The residual phase a “stair” shape (<≠> Doppler shift):  The ambiguity spectrum: *PRF matched to the antenna length and No. of apertures > regular sampling in azimuth results

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

  9. APC Performance Analysis Reference Multichannel Planar Systems The systems have the same azimuth patterns Processing bandwidth 2316 Hz ≤ Bp≤ 4168 Hz • Behavior of APC versus the number of Rx channels, N Investigation: • Effect of the Doppler oversampling • The effect of the pattern shape is not evident

  10. Numerical Results: Gapc APC Gain v.s. oversampling factor For the considered systems, for M=2: • 0.1dB ≤ Gapc ≤ 3.13dB • for a given N, the Gapcincreases with the oversampling factor,  • the Gapcdecreases for increasing number of channels, N • the sensitivity of Gapc to  decreases with increasing N

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

  12. HRWS SAR Multichannel Systems 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 Planar system: currently adopted design Reflector system: alternative design option, studied in DLR

  13. Peculiarities HRWS Systems Planar system Reflector system The pattern of each Rx channel covers Bp Multichannel processing: Multi-Aperture Reconstr. The patters do not change along the swath The pattern of each Rx channel covers 1/N of Bp Multichannel processing: Spectral decomposition The patters change along the swath • Evidence of the dependence of the APC performance on the pattern shape

  14. Numerical Results: Planar HRWS System Normalized PSD 1st range ambiguity used to compute the Gapc (after multichannel reconstruction) without APC with APC For M=2, Gapc = 0.69 dB • The high number of channels (7) and the small oversampling (1.96) associated lowGapc

  15. Numerical Results: Reflector HRWS System Normalized PSD 1st range ambiguity used to compute the Gapc (before multichannel reconstruction, single Rx channel) without APC with APC For M=2,3.2 dB ≤ Gapc ≤ 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

  16. Conclusions • In multichannel systems, the APC effect is no more a frequency shift of the range ambiguity. • Also in multichannel systems, the APC allows for improved ambiguity suppression. • The azimuth pattern strongly affects the APC performance. • For a given azimuth pattern, the suppression is directly proportional to the oversampling factor and inversely proportional to the number of receive channels. • In a conventional SAR system with g = 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. • In the planar and reflector based HRWS systems the APC suppression is about 0.7 dB and between 3 and 8 dB, respectively.

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