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Tropical Rainfall Measuring Mission
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  TRMM @ JPL Rain Q&A Global Change TRMM combined algorithm GPM core reference algorithm DSD
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(last update: August 25, 2003)

In support of NASA's rain measuring missions, the radar group at JPL designed and built the Airborne Rain MApping Radar (ARMAR) in the early 90's, and in 2001 we completed the 2nd-generation Airborne Precipitation Radar (APR-2). While ARMAR was a single-frequency system designed to emulate TRMM's PR, APR-2 is a dual-frequency radar designed to emulate the GPM core satellite's Ku- and Ka-band radars. The following table summarizes APR-2's characteristics:

Parameters Ku-bandKa-band
Frequency: 13.4 GHz 35.6 GHz
Polarization: HH, HV HH, HV
Antenna effective diameter: 0.4 m 0.14 m
Antenna gain: 34 dBi 33 dBi
Antenna sidelobe: -30 dB -30 dB
Antenna scan angle: ± 25 o ± 25 o
Polarization isolation: -25 dB -25 dB
Peak power: 200 W 200 W
Bandwidth: 4 MHz 4 MHz
Pulse width: 10 - 40 microsecs 10 - 40 microsecs
Pulse Repetition Frequency: 5 kHz 5 kHz
Vertical resolution: 37 m 37 m
Horizontal resolution
  • at 9 km (30,000 ft) altitude:
600 m 600 m
  • at 6 km (30,000 ft) altitude:
400 m 400 m
Ground swath
  • at 9 km (30,000 ft) altitude:
8.5 km 8.5 km
  • at 6 km (30,000 ft) altitude:
5.6 km 5.6 km
Sensitivity
  • at 7 km range:
5 dBZ 5 dBZ
Doppler precision: 0.3 m/s 1 m/s

APR-2 participated in the CAMEX-4 campaign in August and September of 2001 on board NASA's DC-8, marking the first time a dual-frequency polarimetric doppler radar was ever flown over precipitating systems. While the 14 GHz channel performed very well, several glitches had to be overcome with the 35 GHz channel. This limited quite severely the amount of dual-frequency data that was gathered during the campaign. Both frequencies share the same antenna, shown here before deployment:

Nevertheless, the problems with the 35-GHz TWT were partially overcome before the end of the experiment, and several minutes of dual-frequency data were acquired over Tropical Storm Gabrielle on September 15, and again over Hurricane Humberto on September 25. The following six panels illustrate the nadir data measured over Humberto:

Notice the area of strong convection to the right of the panels where the 35-GHz signal is rapidly attenuated into the noise as one drops below the melting layer. An extensive analysis of these data and their implications as to the effects of the drop-size variability on dual-frequency retrievals are summarized on our DSD page.

In January and February 2003, APR-2 participated in the "Wakasa Bay" AMSR validation campaign over the sea of Japan (shown in this GMS-5 Vis/IR composite from 29 January 2003, 0200Z) on board a NASA P-3 aircraft. The experiment was designed to validate AMSR's shallow rain and snow retrievals, and to extend the precipitation database that is needed to implement a physical validation strategy. In addition to APR-2, the aircraft carried the 94-GHz Airborne Cloud Radar, a passive microwave sensor emulating the AMSR observations, a high-frequency passive radiometer (MIR) covering from 90 to 340 GHZ, and an upward-looking 21-GHZ and 37-GHZ radiometer (AMMR). The data gathered during the campaign are currently being analyzed. The following panels show two examples of the measurements that were collected:

Of course, the main reason to build and deploy these airborne radars is to gain knowledge that can subsequently be applied to the design and use of spaceborne rain radars. As the banner at the top of the page suggests, our proposal for an advanced high-resolution wide-swath dual-frequency Doppler spaceborne rain radar was funded by NASA. We have now completed the design of the system, and built and integrated much of the electronics, shown in the following image:

A half-scale breadboard model of the antenna is currently being built and tested (see image below), and the instrument should be ready in 2005.


To download the PR-2 Wakasa-Bay data file from January 27, 2003, click here.
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