Viavi 87339, Руководство по эксплуатации

Страница: 88 / 198

Testing
4 -7
STEP PROCEDURE
11. Press Stop Recording button to stop the recording process.
12. To load a data file for playback, select File function key from the Launch Bar.
13. Select the Trajectory tab and press the Load button.
14. On the Load Trajectory popup, select the desired file from the list and press the
Load button.
15. Select Simulation function key from the Launch Bar.
16. Select RUN key to start simulation.
17. Select STOP key to stop simulation.
NOTE:
When a field containing NMEA sentences is imported to the GPSG - 1000 for
playback, at a minimum the file must contain sentence GGA or RMC. If the file
contains only GGA messages the GPSG-1000 will use the date specified on the
Setup page of the unit. If sentence RMC is available in the imported file, then the
date of the simulation will match that of the data within the file. In either of the
previous two cases, the correctly dated almanac file must be loaded to match the
date of the simulation if the user intends to match the SV constellation simulated
by the GPSG - 1000 to that of the SV constellation in view by the receiver at the
time of the recording.
NOTE:
Most GPS receivers expect active antennas. This means they supply a DC voltage
to the antenna connector. The GPSG-1000 has built in DC voltage blocking on the
GPSG - 1000 TX Port.
NOTE:
Some receivers ‘sense’ current draw on the DC supply to their active antenna. If
there is no current drawn, they may assume that no antenna is connected. In such
cases, the current draw must be simulated by some resistive load and perhaps a
series inductor between the signal line and the ground. Such a device may need
to be custom built, depending on the receiver requirements.
NOTE:
It is recommended that the receiver under test is ‘cold started’ and a fresh
almanac obtained from the GPSG-1000 simulation. This process may take several
minutes, dependent on the specific receiver.
NOTE:
Some receivers will not obtain a stable fix when using a simulation time earlier
than the selected almanac. Generally it is best to utilize an almanac that is dated
at least one day before the simulation date.