Detection of low-level water clouds from space is one of the outstanding challenges in radar remote sensing. Spaceborne remote sensing is the only means of assessing the distribution and variability of cloud cover on a global basis. Uncertainties in models of the Earth’s heating budget will persist until CloudSat and follow-on missions such as ACE (Advanced Composition Explorer), with enhanced radar capabilities, complete their missions. Detecting weak scatters at lower altitudes presents significant challenges. Millimeter-wave radars offer the only chance to measure these scatters from space. Unfortunately, the peak power available at Ka and W-band — desirable wavelengths for cloud remote sensing — does not provide adequate sensitivity at the resolution required. For many spaceborne radars, pulse compression techniques are used to overcome the limitations in peak power and take advantage of the average power available. But the backscatter from clouds, even at W-band, can be 7 to 8 orders of magnitude weaker than the surface backscatter. In order to use pulse compression techniques, peak range sidelobes need to be suppressed by upwards of 80 dB.

This work addresses this problem by developing a unique processing and system architecture that can accurately measure the distortions caused by the radar hardware, and through pre-distorting the transmit waveform, cancelling these distortions to less than 0.001 dB in amplitude and 0.01 degree in phase. As a result, the radar system can then achieve peak range sidelobe suppression on the order of 80 to 90 dB.

The pulse compression processing architecture can achieve ultra-low peak sidelobe performance of better than –75 dB with a goal of –90 dB. Over the past several decades, development of waveforms and windowing techniques have promised such performance levels, but distortions in transmit and receive signals introduced by the radar hardware have prevented this from being achieved.

This approach uses a high-fidelity internal calibration and embedded distortion processing in an FPGA-based digital receiver to measure the distortions for the ideal waveform, and then pre-distort the transmit waveform to compensate and effectively remove these distortions.

This work was done by James Carswell of Goddard Space Flight Center. GSC-16482-1

NASA Tech Briefs Magazine

This article first appeared in the November, 2014 issue of NASA Tech Briefs Magazine.

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