Holographic Radar Signal Processing

ABSTRACT: Chiaro Technologies and the University of Colorado will prove the feasibility and investigate the capabilites, limitations, and extensions of the HOlographic Range-doppler processor for Ubiquitous radar Systems (HORUS) in this Phase II STTR. HORUS encodes the signals from a 1-dimensional RF antenna array onto the broad optical bandwidth of a femtosecond comb laser as pulse compressed optical modulations using a multichannel acousto-optic tunable filter, and then uses these modulated waveforms to encode the pulse-by-pulse range correlations with a similarly encoded reference in the volume of a photorefractive crystal as a 2-dimensional array of reflection holograms. A swept frequency laser reads out this volume reflection hologram and Fourier transformation by a lens onto a detector array provides angle-of-arrival (eg multiple beamforming) and Doppler focusing, with Fourier transformation across frames yielding high dynamic-range complex range profiles for each AOA/Doppler bin. HORUS can operate with arbitrary radar waveforms with agile pulse-to-pulse variations, can switch between active and passive radar modes using transmitters of opportunity, and has the capability for MIMO processing using multiple tranmitters in combination with the 1-D receive array. Volume holographic radar correlation processing combined with AOA/Doppler focusing will provide an enabling technology for ubiquitous multibeam radars on platforms like UAVs. BENEFIT: Unlike any previous holographic radar processors, HORUS can be used in either a multibeam ubiquitous radar system or a passive multistatic radar system. A radar system equipped with HORUS could use a stealthy passive multistatic radar when operating in densely populated areas with many illuminators of opportunity or jammers, and it could use an active, monostatic radar when operating in remote areas with no illuminators of opportunity. A fully-functional HORUS system could offer all of these properties within a single implementation using COTS equipment. Other range-Doppler processors, including previous holographic and CCD-based interferometric approaches can produce 1000 range bins and 1000 Doppler bins. To this capability, the HORUS system adds the capability for parallel multi-beam TTD beamforming with up to 1000 parallel beams and an additional 30 dB of beamformer processing gain. This is enabled by harnessing the third dimension of a volume hologram for signal processing, and not just for multiplexing, as in holographic optical data storage. This range-Doppler processing across 1000 simultaneous AOA beams allows an active radar system to monitor up to 1000 regions on the ground with enormous dwell times and sensitivities. In a multistatic radar system, the system can perform parallel Doppler correlations for up to 1000 different transmitters, a feature which can have a dramatic impact on the detection and resolution of the system. A third feature is that these correlations are performed on arbitrary optically-upconverted RF signals without an RF receiver. Arbitrary waveform processing allows monostatic radars to transmit dynamic radar pulses without reconfiguring the receiver, and even more significantly it allows passive multistatic radars to use whatever signals are available without reconfiguring the receiver. Thus, the passive radar could conceivably work with the entire spectrum of FM broadcasters, TV broadcasts, and cellular phone transmitters simultaneously if the RF signals could be collected. And finally, the HORUS system can process RF bandwidths of 100 MHz or more using detectors and digitizers of 1 MHz or less not the 200 MHz digitizers required by DSP systems. Slower digitizers are available with higher dynamic range, so the HORUS system could use 16-bit, or even 2024-bit, digitizers instead of 812-bit digitizers; this improves the detector dynamic range to 96 dB or more. Moreover, the HORUS system performs the Doppler signal processing and beamforming before it digitizes the signal on a high dynamic range detector. This dramatically enhances the clutter-limited dynamic range and the direct-transmitters-limited dynamic range of the active and passive radars, respectively.