Geoscience Reference
In-Depth Information
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Figure 20.3
Scaled model of a PMT smoothed pixel at the nadir composed of 25 fine-resolution pixel footprints.
The overlap between IFOVs of adjacent fine-resolution pixels results in the possibility that lights
present on the Earth's surface will be averaged into the smoothed pixels multiple times. The number
labels marked on the polygons indicate the number of times lights present in the polygons would
be averaged into the resulting smoothed pixel.
20.2.2
OLS Data Preparation
Nighttime DMSP-OLS data from 2210 orbits acquired between April 26, 1992, and April 4,
2001, were processed to produce georeferenced images of lights and clouds of the southern
California region. The data were initially processed for the NOAA National Marine Fisheries
Service to determine the locations and temporal patterns of squid fishing activities conducted using
heavily lit boats offshore from the Channel Islands. Data were included from four day-night DMSP
satellites: F-10, F-12, F-14, and F-15. DMSP data deliveries to the archive were irregular during
1992, resulting in gaps in the early part of our time series.
Orbits were selected from the archive based on their acquisition time to include nighttime data
over California. The orbits were automatically suborbited based on the nadir track to 32˚-42˚ north
latitude. Lights and clouds were identified using the basic algorithms described in Elvidge et al.
(1997). The next step in the processing was to geographically locate (geolocate) the suborbits. The
geolocated images covered the area from 32˚-36˚ north latitude and 117˚-122˚ west longitude. The
OLS geolocation algorithm uses satellite ephemeris (latitude, longitude, and altitude at nadir)
generated by the SPEPH (Special Ephemeris) orbital model developed by the U.S. Air Force
specifically for the DMSP platforms. The orbital model was parameterized by bevel vectors derived
from daily RADAR sightings of each DMSP satellite. Ephemeris data were calculated for each
