Iridium NAL SHOUT ns User Manual page 86

SHOUT ns/nsx User Guide Version D
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.
Geolocation errors in the east-west dimension, therefore, are sometimes more than 100 times
greater than in the north-south dimension.
Document Number: 451-01037-001D 86 of 91