DATA DESCRIPTION
Visible and infrared imagery from DMSP Operational Linescan System
(OLS) instruments are used to monitor the global distribution of clouds
and cloud top temperatures twice each day.
The archive data set consists of low resolution global and high
resolution regional, imagery recorded along a 3,000 km scan,
satellite ephemeris and solar and lunar information. Infrared pixel values
correspond to a temperature range of
190 to 310 Kelvins in 256 equally spaced steps. Onboard calibration is
performed during each scan. Visible pixels are relative values
ranging from 0 to 63 rather than absolute values in Watts per m^2. Instrumental
gain levels are adjusted to maintain constant cloud reference values under
varying conditions of solar and lunar illumination. Telescope pixel values
are replaced by Photo Multiplier Tube (PMT) values at night. A telescope
pixel is 0.55 km at high resolution (fine mode) and 2.7 km at low resolution(smooth mode). Low resolution
values are the mean of the appropriate 25 high resolution values.
INSTRUMENT DESCRIPTION
The OLS instrument consists of two telescopes and a photo multiplier tube
(PMT). The visible telescope is sensitive to radiation from 0.40 - 1.10
um (0.58 - 0.91 um FWHM) and 10-3 - 10-5 Watts per cm2 per sterradian. The
infrared telescope is sensitive to radiation from 10.0 - 13.4 um (10.3
- 12.9 um FWHM) and 190 to 310 Kelvins. The PMT is sensitive to radiation
from 0.47 - 0.95 um (0.51 - 0.86 um FWHM) at 10-5 - 10-9 Watts per cm2 per
sterradian. The detectors sweep back and forth in a "whisk broom" or pendulum-type
motion. The continuous analog signal is sampled at a constant rate so the
Earth-located centers of each pixel are roughly equidistant, i.e., 0.5
km apart. 7,325 pixels are digitized across the 1080 swath from limb to
limb. The instruments are built by Westinghouse Corporation. DMSP
satellites are in a sun-synchronous, low altitude polar orbit.
PROCESSING
OLS data are decompressed, reordered, restructured and deinterleaved. Satellite ephemeris
are computed using a four body orbital mechanics program with observed,
rather than predicted, orbital elements used as input. Missing scan lines
are correctly positioned. Quality assessments are made of each pixel and
characterized for the each complete scan line.
NGDC has archived analog images of the aurora from DMSP OLS sensors since 1972. The National Snow and Ice Data Center (NSIDC) maintained the archives of other analog OLS data 1979 - 1992. An archive file contains an orbit header, and data records organized by scan line. A "smooth" resolution scan line consists of 1,465 visible pixels, 1,465 IR pixels, satellite ephemeris, a quality assessment and other satellite and astronomical parameters. A high resolution scan line contains 7,325 pixels and the same supporting information.
APPLICATIONS
OLS visible data record visible and near-Infrared emissions from the sun or the
moon reflected off clouds and other features. Ground-based sources such
as fires and upper atmospheric sources like the northern lights are
seen. OLS thermal infrared data record thermal emissions by the Earth's surface and atmosphere.
OLS imagery are the primary input to AFWA's cloud analysis program. In
addition, nighttime imagery record the aurora [Snyder et al., 1973], city
lights [Akasofu et al., 1975], manmade and natural fires [Cahoon et al.,
1993] and natural gas flaring [Croft, 1975].
