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The new M.I.T. Microwave Temperature Sounder (MTS), which is designated NAST-M when flown as the microwave component of the NPOESS Aircraft Sounding Testbed, is a complete upgrade of the MTS instrument which has been flown by M.I.T. on NASA ER- 2 aircraft since 1988. (ref Gasiewski) This package consists of two radiometers: the first with eight single-sideband channels between 50 and 57 GHz and the second with nine double-sideband channels within 4 GHz of the 118.75 oxygen line. The instrument block diagram is shown in Figure 1. Channel passbands are listed in table 1. Both radiometers have scalar feedhorns with 7.5 °   3-dB beamwidths and a shared mirror scans pattern of 18 spots from -65 ° to +65 ° from nadir, two black-body calibration loads and a chimney-view of zenith during a nominal 6.5-second scan. This scan pattern provides abutting beams across track and beams also overlap along track at distances greater than 10 km from the aircraft at the ER-2's flight velocity of 210 m/s. Scan speeds can be adjusted to achieve other overlaps in beam coverage, but the nominal pattern has an integration time on the order of 100 ms per spot and yields data at a rate of less than 2 kB per second or 7.2 MB per hour.

A video camera mounted in the package will provide digitized stereoscopic images (using scan lines forward and aft) from which cloud-top altitudes may be estimated. A GPS receiver has also been included in the package as a backup to other navigational data streams.

Atmospheric opacity in the MTS passbands is primarily due to diatomic oxygen, and thermal emission from this well-mixed species is the primary source of signal in MTS views of clear air. Temperature weighting functions for nadirial view from 20 km though a U.S. Standard 1976 atmosphere over a black surface are shown in Figure 2. Clear-air temperature retrievals which are being developed based upon MTS data are expected to have vertical resolutions on the order of 2.5 km and RMS temperature errors on the order of 1 K. Such soundings are only moderately perturbed by nonprecipitating clouds. Strong, frequency-dependent scattering of millimeter-wave radiation by large graupel particles in convective storms will perturb radiances, providing imagery of storm cores through enshrouding clouds. (Schwartz, et al., Spina, et al.)

Key personnel for the MTS from the Research Laboratory of Electronics of the Massachusetts Institute of Technology are Dr. Philip W. Rosenkranz, Principal Investigator, Mr. John W. Barrett, Principal Research Engineer, Mr. William J. Blackwell, Principal Research Assistant, Dr. Michael J. Schwartz, Assistant Principal. Prof. David H. Staelin will participate in his capacity at Lincoln Laboratories.

TABLE 1

50 - 57 GHz Radiometer Channel Number Center (MHz) Width (MHz) A1 50300 180 A2 51760 400 A3 52800 400 A4 53750 240 A5 54400 400 A6 54940 400 A7 55500 330 A8 56020 270

4 GHz Radiometer Channel Number LO (MHz) Sideband Center (MHz) Width (MHz) B1 118750 3500 1000 B2 118750 2550 500 B3 118750 2050 500 B4 118750 1600 400 B5 118750 1200 400 B6 118750 800 400 B7 118750 450 300 B8 118750 235 130 B9 118750 120 100

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