The NASA/Jet Propulsion Laboratory (JPL) Airborne Rain Mapping Radar (ARMAR) was developed to support spaceborne rain radar systems, including the Tropical Rainfall Measuring Mission (TRMM) mission. ARMAR flies on the NASA DC-8 aircraft and operates at 13.8 GHz (Ku-band); it has Doppler and multi-polarization capabilities. It normally scans its antenna across track +/- 20 degrees but can also operate with its antenna pointing at a fixed angle. 

In addition to acquisition of radar parameters, ARMAR also spends a small fraction of its time operating as a radiometer, providing the 13.8 GHz brightness temperature. ARMAR is a pulse compression radar, meaning that it transmits an FM chirp signal of relatively long duration. The raw data is recorded directly to a high speed tape recorder. 

Post-processing occurs in two steps; first, the raw data is compressed by correlating it with the transmitted chirp, giving data comparable to a conventional short pulse radar. These data are used to form various second-order statistics, which are averaged over at least 100 (often several hundred) pulses. 

The second processing step takes the pulse-compressed and averaged data and performs calibration. This step uses data acquired by the system calibration loop during flight to convert the measured power to the equivalent radar reflectivity factor Ze. It also produces Doppler velocity and polarization observables, depending on the mode of operation during data collection.