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The Cloud Profiling Radar (CPR)

The Cloud Profiling Radar (CPR) is a 94-GHz nadir-looking radar which measures the power backscattered by clouds as a function of distance from the radar. The CPR was developed jointly by NASA/JPL and the Canadian Space Agency (CSA). The overall design of the CPR is simple, well understood, and has strong heritage from many cloud radars already in operation in ground-based and airborne applications.

The design of the CPR is driven by the science objectives. The original requirements on CPR were: sensitivity defined by a minimum detectable reflectivity factor of -30 dBZ, along-track sampling of 2 km, a dynamic range of 70 dB, 500 m vertical resolution and calibration accuracy of 1.5 dB. The minimum detectable reflectivity factor requirement was reduced to -26 dBZ when the mission was changed to put CloudSat into a higher orbit for formation flying.

To achieve sufficient cloud detection sensitivity, a relatively low frequency (i.e. <94 GHz) radar would require an enormous antenna and high peak power. At frequencies much greater than 100 GHz, a large antenna and high peak power are also needed due to rapid signal attenuation through cloud absorption. Furthermore, technologies at such high frequencies are less well developed. The 94-GHz frequency chosen by CPR offers the best compromise, meeting performance within the spacecraft resources. In fact, most existing airborne cloud radars operate at 94 GHz. These airborne radars provide extensive heritage for CPR on instrument design and technology, data processing, and retrieval algorithms. A primary frequency allocation of 94 GHz for spaceborne cloud radar sensing has been formally approved at the 1997 World Radio Conference.

CPR System Characteristics

Nominal Frequency 94 GHz
Pulse Width 3.3 µsec
PRF 4300 Hz
Minimum Detectable Z* < -29 dBZ
Data Window 0-25 km
Antenna Size 1.85 m
Dynamic Range 70 dB
Integration Time 0.16 sec
Nadir Angle (since 15 Aug 2006***) 0.16°
Vertical Resolution 500 m
Cross-track Resolution 1.4 km
Along-track Resolution** 1.7 km
Data Rate 20 kbps

*Equivalent radar reflectivity that gives a mean power equal to the standard deviation after integration and noise subtraction. Atmospheric attenuation is not included.

**The along-track resolution is based on averaging the instantaneous footprint over the integration time. Based on purely geometric arguments, the along-track resolution would be approximately 2.5 km. However, a more rigorous convolution calculation gives an along-track resolution of 1.7 km, as shown in the table.

***Nadir angles were changed from approx. 1.71° to 0.0° on 7 July 2006 and from 0.0° to 0.16° on 15 August 2006

The CPR capitalizes on existing radar expertise and experience at JPL. Other radars already flown successfully or being developed by JPL include the SEASAT SAR, SIR-A, SIR-B, SIR-C, SRTM, Cassini Radar, NSCAT, QuickScat, and SeaWinds.