"PR-2" denotes a proposed spaceborne radar system for measuring three-dimensional rainfall profiles and rainfall velocities. The PR-2 system would incorporate a number of improvements over the precipitation radar (PR) system now operating aboard the satellite of the Tropical Rainfall Measuring Mission (a joint project of the United States and Japan).

The Same Scene Can Look Different in radar returns at different polarizations, as shown in this example of like- and cross-polarization reflectivity profiles generated by NASA's Airborne Rain Mapping Radar. Corresponding differences in radar returns from regions of precipitation yield information on the shapes of precipitation particles.

The improvements and their rationale are the following:

  • Dual-Frequency Radar: The PR system operates at a carrier frequency of 13.8 GHz, for which the system is less sensitive to back-scattering from smaller raindrops. The PR-2 system would operate at frequencies of 13.6 and 35 GHz, so that it would be sensitive over a greater range of drop sizes and rainfall rates. The 35-GHz channel would significantly increase the sensitivity of the system to light rain and drizzle. The use of the two frequencies would also reduce errors caused by uncertainties in drop-size distributions.
  • Larger Antenna to Increase Horizontal Resolution: At an altitude of 400 km, the horizontal resolution of the PR is about 4 km - about twice the horizontal dimensions of typical convective cells in the tropics and subtropics. Thus, there is a need for at least double the horizontal resolution of the PR. In the PR-2 system, a horizontal resolution of about 2 km would be obtained by use of a larger antenna. Avoiding the increase in weight that would be occasioned by a larger rigid antenna, the antenna in the PR-2 system would be of an advanced, lightweight, deployable type.
  • Doppler Measurements: By use of Doppler-signal-processing techniques, it should be possible to measure vertical velocities of raindrops to within ≈ 1 m/s.
  • Like-Polarized and Cross-Polarized Measurements: The PR-2 system would be made capable of measuring like- and cross-polarized radar back scatter (see figure) in order to exploit polarimetric techniques for distinguishing among precipitation particles of different types (e.g., raindrops versus graupel).
  • Chirp and Pulse Compression: Because raindrops scatter radar signals weakly in comparison with land and ocean surfaces, rain mapping demands relatively high peak radar-transmitter power; equivalently, it is difficult to obtain enough rain-detection sensitivity at a given power level. In the PR-2 system, increased sensitivity (without need for increased power) would be afforded by use of relatively long-duration pulses that would be chirped (frequency-modulated) so that most of radar echoes would be redirected into range bins of the target of interest (rain).
  • Wider Ground Swath and Adaptive Scanning: The PR-2 system would cover a ground swath about 600 km wide (it is about 220 km wide in the PR system). In the absence of other provisions, observation over a wider swath would entail a significant reduction in the time available to dwell on any spots of interest. However, scanning over the whole swath would be wasteful because rainfall usually occurs over a small fraction of the observed area. Therefore, the PR-2 system would be designed to implement an adaptive-scanning technique, in which it would largely ignore regions of no rainfall and thus have more time to dwell on regions that contain rainfall.
  • Flexible Design: The PR-2 system would be designed so that its timing and pulse parameters could be adjusted for use on different spacecraft moving in different orbits.

This work was done by Eastwood Im, Stephen Durden, and Ziad Haddad of Caltech and by Eric Smith of FSU for NASA's Jet Propulsion Laboratory.