Development of a direct -sampling digital correlation radiometer for earth remote sensing applications.

Abstract

Synthetic thinned array radiometry, or STAR, has emerged as an attractive technique for high spatial resolution satellite imaging at L-band frequencies (1.4 GHz), especially for recovering soil moisture information. However, the implementation of aperture synthesis is limited by the complexity of controlling and synchronizing over 100 microwave heterodyne receivers in the array. In this dissertation, a 1.4 GHz direct-sampling digital radiometer (DSDR) is investigated as an alternative receiver architecture which simplifies the circuitry at each element and leads toward single-chip integration. A discrete-time statistical model of the direct-sampling radiometer is developed for the two constituent parts of aperture synthesis: the total power DSDR and the two-element correlation DSDR. General expressions for noise-equivalent sensitivity (NEDeltaT) and phase stability are derived in terms of quantization resolution, converter bias error, sampling rate, and rms timing jitter. Theoretical results show that a 3-bit L-band DSDR could attain a sensitivity within 4% of the figure for an ideal analog radiometer, and that sampling jitter has a negligible impact on the phase coherence between receivers. To accommodate large baseline STAR, which may suffer from fringe washing effects, a novel band division correlation (BDC) processor is proposed. Numerical simulations of a 27 meter L-band STAR sensor show that BDC improves spatial resolution by 40% at the swath edge. An L-band correlation DSDR prototype was designed and evaluated in a series of lab and field experiments. From noise floor tests, the observed sensitivity of the correlation DSDR fell within +/-0.4 dB of the theoretical NEDeltaT limit. Measurement of partially correlated noise sources demonstrated less than 0.1 dB loss in the cross-correlation output, implying a high level of phase stability in the samplers. However, an excess loss in fringe washing was discovered due to the non-linear nature of A/D conversion; as a remedy, coherence loss may be alleviated by applying the BDC technique. The DSDR hardware has served as a test bed for several important technologies, including wideband flash A/D conversion, field programmable logic, embedded systems, and thermoelectric temperature control.