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The DMSP satellites
The first satellite, F8, was launched on June 19, 1987, aboard the Defense Meteorological Satellite Program (DMSP). For the first time, it takes on board a new type of Special Sensor Microwave Imager (SSM/I) and a Special Sensor Microwave Temperature (SSM/T-I).
This satellite orbit is near-circular , sun-synchronous, and near-polar. The F08 orbital altitude is near 833 km, with a 98.8 ° inclination, and 102 mn orbital periods. It executes 14.1 revolutions per day and the local time for the ascending equatorial crossing is 6:12 am. The swath is about 1400 km wide.
The SSM/I instrument
Goals
As it was defined by N.S.S.A. and A.F.S.D., the SSM/I program consists in obtaining a world wide athmospheric parameters which have a high importance for the meteorology and climatology.
- athmospheric water vapor content
- athmospheric water liquid content (cloud) and rain rates
- wind speed on ocean surfaces
- ground humidity
- snow surfaces detection and water content analysis
- sea-ice detection and concentration
- sea-ice characterisation
Characteristics
The SSM/I was built by Hughes Aircraft Company under the management of N.S.S.A (Naval Space System Activity) and A.F.S.D (Air Force Space Division).
The SSM/I consists of 7 separate total-power radiometers, each simultaneously measuring the microwave emission coming from the Earth and the intervening atmosphere. Table 1 gives the frequencies, polarizations, and temporal and spatial resolution for the 7 channels. Dual-polarization measurements are taken at 19;35, 37.0, and 85.5 GHz, and vertical polarization is observed at the 22.235 GHz [water vapor channel] (Figures 1 & 2).
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| Figure 1 : F08 SSM/I orbit and scangeometry (from Hollinger 1989) |
Figure 2 : Geometric representation ofSSM/I scan (from Spencer et al., 1989) |
A two step scanning :
- A-scan : during the 102.4 arc of an A-scan, 64 samples of each of the lower channels are taken, with the integration period being 7.95 ms. During each scan, the 85 GHz channels are sampled 128 times over the 102.4° arc. This sampling scheme results in 128 pol footprints, each having an effective 3dB spatial resolution of 15 km.
- B-scan : Only the 85 GHz channels are realised giving again 128 pol footprints (Figure 1).
There are 1579 A or B-type pairs per orbit.
| Channels |
1,2 |
3 |
4,5 |
6,7 |
| Central Frequency Pol. (GHz) |
19.35 |
22.24 |
37.0 |
85.5 |
| Radiometric Pol. (V/H) (*) |
V ,H |
V |
V ,H |
V, H |
| Thermal resolution (K) |
0.8 |
0.8 |
0.6 |
1.1 |
| Integrated FOV (Km) |
70 x 45 |
60 x 40 |
38 x 30 |
16 x 14 |
| Spatial sampling (Km) |
25 |
25 |
25 |
12.5 |
| Scan angle |
102.4 ° |
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| Sweep periodicity |
1.9s |
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| Ground incidence |
53.1° |
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| Swath width |
1394 Km |
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| Antenna diameter |
65 cm |
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| Weight |
120 Kg |
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| Power |
70 W |