Image Processing Reference
In-Depth Information
17.2
Overview of the Nighttime Satellite Image Data Products
The DMSP platform was designed as a meteorological satellite for the United
States Air Force. The DMSP system consists of two sun-synchronous polar orbiting
satellites at an average elevation of 865 km above the earth. The swath width of a
typical DMSP image is 3,000 km. One satellite observes
the earth at dawn and dusk, the second observes the earth
at approximately noon and midnight. The sensor on the
system has two bands: a panchromatic visible-near infra-
red (VNIR) band and a thermal infrared band. For a more
detailed description of the DMSP platform see Dickinson's
DMSP user's guide (Dickinson et al.
1974
). Nighttime
imagery provided by the DMSP OLS has been available
since the early 1970s. The DMSP sensors are more than
four orders of magnitude more sensitive to visible-near-
infrared radiances than traditional satellite sensors optimized for daytime observa-
tion (Elvidge et al. 1995). However, the high sensitivity of the sensor was not
implemented to see city lights. It was implemented to see reflected lunar radiation
from the clouds of the earth at night. The variability of lunar intensity as a function
of the lunar cycle is one of the reasons why the satellite system's sensors were
designed with such a large sensitivity range.
DMSP data was not available in digital format until 1992. Prior to that the
images were produced on mylar film, interpreted by Air Force meteorologists, and
archived or thrown away. This kind of data did not lend itself easily to analysis.
Nonetheless, the imagery was so striking and potentially useful, several people
conducted the tedious and difficult research necessary to quantify relationships
between the DMSP OLS imagery and various other variables such as population
and energy consumption (Welch
1980a, b
; Foster
1983
). The subsequent develop-
ment of the digital DMSP archive has dramatically improved
the access to, and utility of, the DMSP data.
The digital archive of the DMSP OLS data is housed at
the National Geophysical Data Center in Boulder, Colorado
(the NGDC is a subsidiary of NOAA). Algorithms devel-
oped by Elvidge et al. have produced a 1 km
2
resolution
dataset of the city lights of the continental United States
(Elvidge et al. 1995). Elvidge et al. (1995,
1997
) developed
algorithms to identify spatio-temporally stable VNIR emis-
sion sources utilizing images from hundreds of orbits of the
DMSP OLS platform. The resulting hyper-temporal dataset
is cloud-free because the infrared band of the system was
used to screen out cloud impacted data. Later, a global ver-
sion was prepared. Despite the cloud screening performed
in these 'hyper-temporal' mosaics there is some difficulty
separating the cold clouds from the cold snow-capped tops
DMSP OLS data
is a good example
of military
technology being
put to peaceful
scientific and
commercial
applications
DMSP OLS data
products are not
'snapshot'
satellite images,
they are
sophisticated
compilations of
many
DMSP
OLS 'snaphots'
or orbits that
meet many
criteria such
as being
'cloud-free' or
non-ephemeral
