DMSP: Visible Sensor, Earth at Night (Visible Imagery) (Remote Sensing)

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.

Spectral channels

tmp1A-270 tmp1A-271

Sensitivity

Spatial resolution

OLS-L

0.40-1.10

0.58-0.91

10 3-10 5 Watts/cm2 per sterradian

0.56 km*

OLS-PMT

0.47-0.95

0.51-0.86

10 5-10 9 Watts/cm2 per sterradian

2.7 km

OLS-T

10. 0-13.4

10.3-12.9

190-310 K

5.4km

* 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.

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