Constellations of low-cost, small instruments provide global, distributed, and frequent coverage, enabling unique science observations. However, radars are active instruments with size, mass, and power requirements that are often not compatible with small satellite platforms such as CubeSats or SmallSats.
Radars traditionally use offset video modulation, which requires multiple stages of mixing, filtering, and amplification (see figure, a). Pulse compression (also known as matched filtering) is often used in radar systems because it allows the use of long pulses for high signal-to-noise ratio without sacrificing resolution. IQ (in-phase quadrature) modulation simplifies the up and down conversion processing chain by converting the signal directly from/to baseband to/from the output transmit frequency (figure, b). The largest problem with IQ mixers is that gain and phase imbalances create unwanted spurious signals, such as single sideband image and local oscillator (LO) leakage. This is particularly detrimental for pulse compression, because the sidelobe suppression is approximately given by the timebandwidth product plus the level of image/LO leakage suppression. This can significantly degrade the performance due to clutter contamination. A common approach to improve the image suppression and LO leakage rejection of IQ mixers is digitally pre- and/or post-distortion of the signal to compensate for the mixer imbalance. However, the imbalance drifts with temperature and aging, and complete suppression would require complex feedback loops that are impractical.
A novel modulation scheme was developed for Ka-band precipitation profiling radar that reduces the size, mass, and power of the electronics by about an order of magnitude compared to similar radars. It is based on pulse compression and direct IQ up-conversion to reduce the number of components. Through an optimum selection of transmit signal and digital signal processing, spurs are suppressed, the system is not highly sensitive to LO leakage and image suppression, and can achieve high-purity signals with exceptional sidelobe suppression.
This work was done by Eva Peral, Simone Tanelli, Chaitali R. Parashare, Douglas L. Price, and Ninoslav Majurec of Caltech for NASA’s Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Technology Transfer at JPL
Mail Stop 321-123
4800 Oak Grove Drive
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Refer to NPO-49670.