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1) Transportation over distances further than 1500 kilometers
2) Failure of the on-board memory battery
3) Stored date/time off by more than 30 minutes
See Section 4 for more information on initializing the GPS 25LP.
2.3 Navigation
After the acquisition process is complete, the GPS 25LP will begin sending valid
navigation information over its output channels. These data include:
1) Latitude/longitude/altitude
2) Velocity
3) Date/time
4) Error estimates
5) Satellite and receiver status
The GPS 25LP sensor boards will select the optimal navigation mode (2D or 3D) based
on available satellites and geometry considerations. When navigating in the 2D mode
the board set utilizes the last computed altitude or the last altitude supplied by the host
system, whichever is newer. The host system must ensure that the altitude used for 2D
navigation is accurate since the resulting position error may be as large as the altitude
error. See Section 4 for information altitude initialization.
The GPS 25LP will default to automatic differential mode -- "looking" for real-time
differential corrections in RTCM SC-104 standard format using message types 1,2,3 or
9 and attempting to apply them to the satellite data in order to produce a differential
(DGPS) solution. The host system, at its option, may also command the board set to
choose differential only mode. When navigating in the differential only mode, the board
set will output a position only when a differential solution is available.
2.4 Satellite Data Collection
The GPS 25LP sensor boards will automatically update satellite orbital data as they
operate. The intelligence of the board set combined with its hardware capability allows
these data to be collected and stored without intervention from the host system. A few
key points should be considered regarding this process:
1) If the sensor board is not operated for a period of six (6) months or more, the unit
will "search the sky" in order to collect satellite orbital information. This process is
fully automatic and, under normal circumstances, will take 3-4 minutes to achieve
8
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