Interferometer Signal Processing
In this chapter, the fundamental signal processing required to implement an interferometer is introduced. The processing required for an interferometer is similar to the processing required for radars in general with the exception that angle estimation is implemented using phase comparison. Because phase is defined only over an interval [0 2p], the phase can wrap over many 2p intervals, creating an ambiguity in the phase measurement. For the interferometer, that relative phase ambiguity between two antennas must be resolved in order to estimate angle-of-arrival. In this chapter, we discuss methods to resolve interferometer phase ambiguities. In particular, all receive interferometer antennas or arrays must be calibrated to ensure that relative phase is consistent across the radar field of regard. The orthogonal interferometer has unique processing requirements due to its implementation of pseudoorthogonal waveforms. Calibration of an orthogonal interferometer requires that all antennas be calibrated for transmit as well as receive. The derivation of angle-ofarrival from relative phase measurements is presented for various far-field conditions. The first-order derivation assumes that the constant phase contours that emanate from a target are essentially linear across the interferometer distributed antennas. This linear far-field condition is satisfied when the target is sufficiently far in range from the interferometer but depends on the interferometer baseline and radar operating frequency. Second-order derivations are also presented for quadratic phase behavior across the interferometer antenna.
Interferometer Signal Processing, Page 1 of 2
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