Statistical Analysis and Compensation of I/q Imbalances in Satellite Altimeter Demodulators

Satellite radar altimeter plays an important role in ocean remote sensing. In China an altimeter will be embarked on the forthcoming HY-2 satellite, which can bring significant benefits to international altimetry society. Modern altimeters adopt I/Q demodulators to detector sea echoes, so the imbalances between the in-phase (I) and quadrature (Q) channels introduce errors on altimeter parameters estimation. This paper begins with the analysis of statistical characteristics of I/Q signals. Based on several assumptions, the PDF of the phase, amplitude and power of altimeter waveforms with I/Q imbalances are deduced. It is shown that in altimeter the amplitude imbalance has more stringent specification. Then the influences of I/Q imbalances on altimeter waveforms are evaluated. The PDF of the echo power spectrum deviates from exponential distribution, and side-lobes appear at the imaginary frequency. Finally, several innovative approaches to measure and/or compensate the imbalances are presented. The theories and results presented in this paper are applicable not only to conventional altimeters, but also to other ocean-observing radar systems.