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Acutime 2000 Synchronization Kit User Guide F-3
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.