TRMM TMI Level IC Development
 |
Sensor Description:
The TMI is a nine-channel passive microwave radiometer based upon the Special Sensor Microwave/Imager (SSM/I), which has been flying aboard the U.S. Defense Meteorological Satellite Program (DMSP) satellites since 1987. The key differences are the addition of a pair of 10.7-GHz channels with horizontal and vertical polarizations and a frequency change of the water vapor channel from 22.235 to 21.3 GHz. This change off the center of the water vapor line was made in order to avoid saturation in the tropical orbit of TRMM. Table 1 presents the performance characteristics of the nine TMI channels. The increased spatial resolution evident in Table 1 is due to the lower orbit of the TRMM satellite with respect to the DMSP rather than sensor differences.
The TMI antenna is an offset parabola, with an aperture size of 61 cm (projected along the propagation direction) and a focal length of 50.8 cm. The antenna beam views the earth surface with a nadir angle of 498, which results in an incident angle of 52.88 at the earth's surface. The TMI antenna rotates about a nadir axis at a constant speed of 31.6 rpm. The rotation draws a circle on the earth's surface. Only 1308 of the forward sector of the complete circle is used for taking data. The rest is used for calibrations and other instrument housekeeping purposes. From the TRMM orbit, the 130 scanned sector yields a swath width of 758.5 km shown in Fig. 1. During each complete revolution (i.e., a scan period of about 1.9 s), the subsatellite point advances a distance d of 13.9 km. Since the smallest footprint (85.5-GHz channels) size is only 6.9 km (down-track direction) by 4.6 km (cross-track direction), there is a gap of 7.0 km between successive scans. However, this is the only frequency where there is a small gap. For all higher-frequency channels, footprints from successive scans overlap the previous scans.
Table 1. TMI characteristics ::
| Center Frequencies(GHz) | 10.65 | 10.65 | 19.35 | 19.35 | 21.3 | 37.0 | 37.0 | 85.5 | 85.5 |
| Polarization | V | H | V | H | V | V | H | V | H |
| Bandwidth (MHz) | 100 | 100 | 500 | 500 | 200 | 2000 | 2000 | 3000 | 3000 |
| Sensitivity (K) | 0.63 | 0.54 | 0.50 | 0.47 | 0.71 | 0.36 | 0.31 | 0.52 | 0.93 |
| IFOV (km x km) | 63 x 37 | 63 x 37 | 30 x 18 | 30 x 18 | 23 x 18 | 16 x 9 | 16 x 9 | 7 x 5 | 7 x 5 |
| Sampling Interval (km x km) | 13.9x9.1 | 13.9x9.1 | 13.9x9.1 | 13.9x9.1 | 13.9x9.1 | 13.9x9.1 | 13.9x9.1 | 13.9x4.6 | 13.9x4.6 |
| Integration Time (msec) | 6.6 | 6.6 | 6.6 | 6.6 | 6.6 | 6.6 | 6.6 | 3.3 | 3.3 |
| Main Beam Efficiency (%) | 93 | 93 | 96 | 96 | 98 | 91 | 92 | 82 | 85 |
| Beamwidth (half-power, degrees) | 3.68 | 3.75 | 1.90 | 1.88 | 1.70 | 1.00 | 1.00 | 0.42 | 0.43 |
Figure 1 is provided by Kummerow et al.[1998]
Source Data:
The Level 1C data are derived from the TRMM TMI 1B11 data. Currently no changes are made to the 1B11 data to the Level 1C data with the exception of the data format. The data is archived and publicly available from Goddard Earth Sciences (GES) Data and Information Services Center (DISC) Distributed Active Archive Center (DAAC).
Satellite Intercalibration
The TRMM TMI brightness temperatures have been defined as the calibration reference for the current radiometer constellation. As a result, no changes were made to the 1B11 TBs.
Additional Documentation/References:
| – | |
| – |