From the instrument home page at JPL:
The NGV (formerly HIAPER) MTP is a passive microwave radiometer, which measures the natural thermal emission from oxygen molecules in the earth’s atmosphere for a selection of elevation angles between zenith and nadir by scanning through an arc in the flight direction. The NGV observing frequencies are 56.363, 57.612 and 58.363 GHz. These frequencies are different from our other instruments because the NGV MTP makes measurements centered on the oxygen absorption lines rather than being centered between oxygen absorption lines. We expect this to improve the overall performance. The measured "brightness temperatures" versus elevation angle are converted to air temperature versus altitude using a quasi-Bayesian statistical retrieval procedure. An altitude temperature profile (ATP) is produced in this manner every 15 seconds (about 3 km) along the flight track.Home page for this instrument.