Trimble Outdoors SK8 - Manual (Page 54)

4-2 Lassen-SK8 Embedded GPS Module
Operation and Performance
4.2 Satellite Acquisition and Time to First Fix
4.2.1 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 Lassen-SK8 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-SK8 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.
*
Note – When installed in the interface unit, the Lassen-SK8 receives back-up power from
a lithium battery. This battery enables the Lassen-SK8 to always start from either a warm
or hot start. To force a cold start, issue the 1E TSIP command (
[Control] + [K] in the TSIP
chat program on the GPS toolkit diskette in the Starter Kit).
4.2.2 Warm Start
In a warm start condition, the receiver has been powered down for a period of 1-6 hours
but has a current almanac and an initial position and time stored in memory.
When connected to an external backup battery and power is applied, the Lassen-SK8
retains the almanac, approximate position, and time to aid in satellite acquisition and
reduce the time to first fix. When an external back-up battery is not used, the TSIP
protocol allows the almanac, an initial position, and time to be uploaded to the receiver via
the serial port, to initiate a warm start.
During a warm start, the Lassen-SK8 identifies the satellites which are expected to be in
view, given the system almanac, the initial position and the approximate time. The
receiver calculates the elevation and expected Doppler shift for each satellite in this
expected set and directs the eight tracking channels in a parallel search for these satellites.