INDIVIDUAL SUBSCRIBER UNIT REREGISTERS WITH OPTIMUM GEOMETRY IF REGISTRATION CEP IS TOO LARGE
Original Publication Date: 1997-Sep-01
Included in the Prior Art Database: 2002-Jun-04
The IRIDIUM" system provides global cellular telephony coverage via a constellation of 66 satellites positioned in low earth orbit. During call setup, it is important to determine the location of the user for both billing and system access determination purposes. The geolocation measurements that are used by the system JO perform this location process make use of both the round trip time delay and the Doppler frequency shift of the signals between the individual subsc,riber unit and the satellite. Directly to the right or let3 of the satellite, the contours of constant delay and of constant Doppler shift cross at right angles, providing for quick resolution of the individual subscriber unit's position to well under 10 Km with a single geolocation measurement. See Figure 1. However, directly fore or aft of the satellite, these c:ontours tend to be more parallel at their intersections, causing much poorer positional resolution. In addition to this, along the beams under the ground track of the satellite's travel, there is the added inaccuracy of determining if the indi- vidual subscriber unit's location is to tbe left or the right of the path of satellite. This is called the let?- right ambiguity, which further adds to the inaccuracy of the position measurement for beams directly under the path of the satellite.