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INDIVIDUAL SUBSCRIBER UNIT REREGISTERS WITH OPTIMUM GEOMETRY TO REDUCE TRAFFIC CHANNEL OCCUPATION

IP.com Disclosure Number: IPCOM000008276D
Original Publication Date: 1997-Sep-01
Included in the Prior Art Database: 2002-Jun-03
Document File: 4 page(s) / 175K

Publishing Venue

Motorola

Related People

Dan Tayloe: AUTHOR [+2]

Abstract

The IRIDIUMa system provides global cellular telephony coverage via a constellation of 66 satellites positioned in lo~w 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 to perform this location process make use of both the round trip time delay and the Doppler frequency shift of the signals between the individual subscriber unit and the satellite. Directly to the right or left 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 contours 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 the left or the right of the path of satellite. This is called the left- right ambiguity, which further adds to the inaccuracy of the position measurement for beams directly under the path of the satellite.

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MO-LA Technical Developments

INDIVIDUAL SUBSCRIBER UNIT REREGISTERS WITH OPTIMUM GEOMETRY TO REDUCE TRAFFIC CHANNEL OCCUPATION

by Dan Tayloe and Johanna Wild

PROBLEM DESCRIPTION

  The IRIDIUMa system provides global cellular telephony coverage via a constellation of 66 satellites positioned in lo~w 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 to perform this location process make use of both the round trip time delay and the Doppler frequency shift of the signals between the individual subscriber unit and the satellite. Directly to the right or left 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 contours 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 the left or the right of the path of satellite. This is called the left- right ambiguity, which further adds to the inaccuracy of the position measurement for beams directly under the path of the satellite.

  The user ground position information is used for two main purposes. The first is for the access decision process, The normal case is that an individ- ual subscriber unit is not near a country boundary, and the ground location measurement needs only to be good enough to locate which country or SCA (Service Control Area) the individual subscriber unit is in. Since SCAs are normally the size of a country, the position resolution for this purpose needs not be very fine.

On the other hand, if the ground position infor- mation is going to be used for registration purposes,

the accuracy needs to be much tighter as the system tries to locate the individual subscriber unit to the best LAC (Location Area Code). Since LAC's tend to be much smaller than SCAs (SCAs are made up of a number of LACs), the ground position accuracy required for registration is tighter than that used for mere aticess decision determination. The ground accuracy currently required for registration purposes is 10 Km accuracy or 4 geolocation measurements (spread in time across 15 seconds), whichever comes first. Along a wide band near the satellite ground track, the geolocation process for registration will almost always go to 4 geolocation measurements (15 seconds), and still not resolve the individual subscriber unit's location to 10 Km.

  The system will force the location accuracy to be tighter for an autoregistration location determi- nation than for most other purposes. However, th...