NAL Research Corporation (451-93156-002A)
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F.8 Iridium Geo-Location
The Iridium network makes calculations of the geographical location (geo-location) of an ISU each time
a call is placed. The technique employed to determine the geo-location of an ISU is based on measurements
of the ISU and satellite propagation delay and Doppler frequency shift. These measurements are used to
estimate cosines of spherical angles that identify the ISU’s location relative to the satellite by the gateway.
The Iridium network can locate an ISU to within 10 km only about 78% of the time. The so-called error
ellipse can have a large eccentricity with the major axis oriented in the azimuth dimension and the minor
axis oriented in the radial dimension. The position of the ISU in the radial dimension relative to the satellite
can almost always be determined to within 10 km with just one measurement. Errors in the azimuth
dimension relative to the satellite are largest along the satellite’s ground path and tend to increase with
distance from the satellite. Geo-location errors in the east-west dimension, therefore, are sometimes more
than 100 times greater than in the north-south dimension.
