4 Operation and Performance
5 0 Lassen SQ GPS Receiver
4.3
Satellite Acquisition and Time to First Fix
4.3.1
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
Lassen SQ GPS receiver which has no memory of its previous session
(i.e., is powered on without the memory backup circuit connected to a
source of DC power). This is the “out of the box” condition of the GPS
module as received from the factory.
In a cold-start condition the receiver automatically selects a set of
eight satellites and dedicates an individual tracking channel to each
satellite, to search the Doppler range frequency for each satellite in the
set. If none of the eight selected satellites is acquired after a pre-
determined period of time (time-out), the receiver will select a new
search set of eight satellites and will repeat the process, until the first
satellite is acquired. As satellites are acquired, the receiver
automatically collects ephemeris and almanac data. The Lassen SQ
GPS receiver uses the knowledge 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
three satellites are found, the receiver will compute an initial position
fix. The typical time to first fix is less than 2 minutes.
A complete system almanac is not required to achieve a first position
fix. However, the availability and accuracy of the satellite ephemeris
data and the availability of a GPS almanac can substantially shorten
the time to first fix.
Summary of Contents for SQ
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Page 14: ...About this Manual xiv Lassen SQ GPS Receiver...
Page 46: ...2 Hardware Integration 32 Lassen SQ GPS Receiver...
Page 150: ...B TSIP Tool kit User s Guide 136 Lassen SQ GPS Receiver...
Page 166: ...C NMEA 0183 152 Lassen SQ GPS Receiver...
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