Viavi 87339, Operation Manual

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6 -1
Chapter 6 - Principles of Operation
6.1 PRINCIPLES OF OPERATION
6.1.1 GPSG-1000
6.1.1.A Satellite Simulation
The orbit parameters for each satellite are configurable in terms of the standard
Keplerian (almanac) elements. The almanac used for simulation may selected from a list,
maintained by a file management system. The files may obtained from the built in GPS
receiver, as a current almanac load or may be loaded from a USB flash drive in
Yuma .alm format.
The standard Keplerian elements (almanac) are used to calculate the enhanced Keplerian
Elements (ephemeris), which are in turn used to generate the satellite motion, and are
broadcast as part of the navigation message.
As almanac data is typically valid for several days, the GPSG-1000 does not update the
standard Keplerian (almanac) elements during the course of the simulation.
The GPSG-1000 computes the enhanced Keplerian (ephemeris) elements by
extrapolation of the configured standard Keplerian (almanac) elements. Any enhanced
Keplerian elements not included in the standard Keplerian elements are set to zero. The
GPSG-1000 generates updated enhanced Keplerian (ephemeris) elements at a fixed time
interval of 4 hours, this being the update rate supported by both GPS and Galileo.The
GPSG-1000 generates in real time the positions of all simulated Satellites i.e. positions
are not be pre-computed for the entire simulation run.
6.1.1.B Satellite Selection
For each simulated GNSS system the GPSG-1000 will perform an independent satellite
selection process and if the total number of visible satellites exceeds the maximum
number of hardware channels allocated to that system, then the GPSG-1000 will select
those satellites which approximate the minimum DOP for that system. The GPSG-1000
updates its satellite selection every 60 minutes.
NOTE:
In the case of the 6 channel GPSG-1000, with large numbers of simulated satellites it
becomes possible for many more satellites to be visible at the user location than
there are available hardware satellite channels. If this happens the GPSG-1000
allocates channels on a 'best effort' basis but does allow the operator to override the
selection manually.
The GPSG-1000 allows the operator to manually replace any selected satellite with
another visible satellite from the same GNSS system, with the manually selected satellite
remaining selected until it is no longer visible, or until it is manually replaced. The
manually de-selected satellite will remain eligible for re-selection at the next automatic
update.