Trimble Outdoors AcutimeTM2000 Synchronization Kit Satellite Acquisition and Time to First Fix

F Theory of Operation

F.2 Satellite Acquisition and Time to First Fix

This section describes satellite acquisition times for different start conditions.

F.2.1 Cold Start

The term cold start describes the performance of a GPS receiver at power-on when no navigation data is available. Cold signifies that the receiver does not have a current almanac, satellite ephemeris, initial position, or time. The cold start search algorithm applies to a smart antenna that is powered on without the memory backup circuit connected to a source of DC power. Since the Acutime 2000 has no provision for external backup power, it always operates from a cold start. This is the condition of the GPS module as received from the factory.

In a cold start condition, the receiver automatically selects a set of satellites and dedicates an individual tracking channel to search the Doppler frequency for each satellite in the set. If none of the selected satellites are acquired after a pre-determined period of time (time-out), the receiver selects a new search set of satellites and repeats the process until the first satellite is acquired. As satellites are acquired, the receiver automatically collects ephemeris and almanac data. The smart antenna uses the information gained from acquiring a specific satellite to eliminate other satellites (those below the horizon) from the search set. This strategy speeds the acquisition of additional satellites required to achieve the first position fix.

The cold start search sets are established to ensure that at least three satellites are acquired within the first two time-out periods. As soon as four satellites are acquired, the receiver computes an initial position fix. The Acutime 2000 typically achieves a cold start in under two minutes.

A complete system almanac is not required to achieve a first position fix. The almanac is used in subsequent warm starts, and to aid in acquiring GPS satellites that come into view.

Acutime 2000 Synchronization Kit User Guide F-3