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193
"Smarter Timing Solutions"
Appendix
J
Operation with a GPS Simulator
This appendix describes several commands that are intended for use before and after operation with
a GPS simulator. In addition, recommendations for the simulator setup are given so that the ad-
vanced integrity-checking and GPS week number ambiguity resolution algorithms operating in the
GPS Subsystem will not cause unexpected behaviors during and after running on the simulator.
Background
GPS-based timing systems supporting critical infrastructure functions could be vulnerable to mal-
function due to weak signals, jamming, spoofing or accidental GPS control system errors. EndRun
Technologies’ GPS timing receiver technology has evolved to be highly robust against these threats.
Because of this, operation with a GPS simulator requires careful attention when setting up the simula-
tor. Careful consideration must also be given in configuring the GPS Subsystem before and after
simulator operation. Failure to understand and implement the information that follows could result in
confusing simulation results and/or improper operation when the Meridian II is subsequently recon-
nected to live GPS signals.
Console Port Commands
clearalmanac
The GPS Subsystem maintains a record of the GPS almanac data received from the satellites in its
non-volatile memory. These almanacs are used to calculate the elevations and doppler frequency
offsets of GPS satellites so that they may be more quickly acquired and tracked by the signal proces-
sor. When operating on a GPS simulator, any previously stored almanacs gathered from the live GPS
may not contain valid data, so they should be cleared prior to operation with a simulator. Likewise,
following operation with a simulator, they should also be cleared before reconnecting to live GPS
signals. The
clearalmanac
command provides this capability.
This command clears all GPS almanac data from both non-volatile and working memory in the GPS
Subsystem. (It does not clear the copy of the Yuma-formatted almanac that is retrieved with the
dumpalmanac
command. That data will be updated on reception of new almanac data.)
The
clearalmanac
command can be used at other times, but is typically used before and after op-
eration with a GPS simulator.
SUMMARY
The
clearalmanac
command MUST be executed both before and after use with a GPS simulator.
Summary of Contents for Meridian II
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