28 G12 OEM Board & Sensor Reference Manual
To maintain communication with the G12 while changing the baud rate, issue the
$PASHS,SPD (set port speed) to change the baud rate of the G12 port, then
change the baud rate of the command device to match the new baud rate setting
on the G12 port.
When the G12 is powered on for the first time, or when the power and back-up
battery have been disconnected, there is no almanac or ephemeris data in
memory. In these cases, the G12 assigns the first 12 elements of a 32-element
table of SV PRN numbers to its 12 channels as it begins searching for satellites. If
no ephemeris data are in memory, or if the data are older than ten hours, 30 to 60
seconds are needed to collect data. The G12 synchronizes its clock to GPS time
within six seconds of locking an satellite. After three or four satellites are locked
and the almanac and ephemeris data are collected, the G12 computes its first
position. The G12 continuously updates almanac, ephemeris, and position data in
its battery-backed memory to help optimize satellite reacquisition and time to first
fix when the unit is next powered on.
At the next power-up, if the almanac and ephemeris data are available in battery-
backed memory, and if the ephemeris data are less than ten hours old, the G12
restricts its satellite search to those satellites that should be visible based on this
information. Under these conditions, the G12 on average recomputes position in
10 to 15 seconds (hot start). If the almanac and ephemeris data are available in
battery-backed memory, but the ephemeris data are more than ten hours old, the
G12 needs 30 to 40 seconds on average to compute a position (warm start). If
almanac and ephemeris data and a valid position are not available at power-up,
the G12 computes position in less than one minute on average (cold start).
On initial power-up or after issuing the $PASHS,RST (restore defaults) command,
the G12 reverts to its default parameter settings. Enter the following three
commands to query the G12 for the current parameter status:
1. $PASHQ,PAR (general parameters)
G12RevD.book Page 28 Tuesday, April 2, 2002 4:33 PM
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