Defense Meteorological Satellite Program (DMSP) satellites are primarily intended to gather weather data for military purposes. We’ll look at the system as an example of low-light imaging. This system consists of platforms in sun-synchronous orbits with dayside equatorial crossings at 6:00 AM and 10:30 AM, local time. The purpose is to help forecasters identify and track developing weather patterns. Characteristics of some recent Block 5D-2 missions are given here.
The Operational Linescan System (OLS) instrument consists of two telescopes and a photomultiplier tube (PMT). The scanning telescope of the OLS has a 20-cm aperture with an effective collecting area of 239 cm2 and effective focal length of 1.22 m. Light is split into two channels by a beam splitter. The system is designed to produce constant high-resolution imaging rather than accurate radiometry. Some of the characteristics are given here. Note that the sensitivity of the low-light sensor is two to four orders of magnitude higher than that of the daylight sensor.
Table 3.4 DMSP satellite parameters.
Common name |
DMSP 5D-2-F14 |
International number |
1997-012A |
Launch date (YYYY/MM/DD) |
1997/04/04 |
Launch time |
1647 |
Mass |
850.0 kg |
Apogee |
855 km |
Perigee |
841 km |
Period |
101.8 min |
Inclination |
98.8° |
Table 3.5 DMSP sensors.
* stored data are resampled to 2.7 km
The DMSP detectors operate as whiskbroom scanners. The continuous analog signal is sampled at a constant rate making the Earth-located centers of each pixel roughly equidistant, i.e., 0.5 km apart. For high-resolution (fine mode) imaging, 7325 pixels are digitized across the 108° swath from limb to limb. The low-light-level sensor operates at lower spatial resolution. The low-light data are useful for a variety of purposes beyond weather; for example, Chris Elvidge at NOAA has been archiving DMSP data sets for tracking population trends.
Figure 3.45 shows U.S. light patterns, based on cloud-free imagery. The sensor saturates when viewing city lights, so there is no dynamic range for this data.