Viavi 87339, Operation Manual

Page: 99 / 198

Testing
4 -18
4.1.8 RAIM Testing and SV Geometry
The orbital characteristics of SV’s within a GPS constellation are contained in the
transmitted almanac and ephemeris data. The GPSG-1000 does not permit the user to
directly change individual SV orbital parameters, therefore individual SV Elevation and
Azimuth, at any UTC within the simulation, are determined by almanac and the ephemeris
generated from the almanac.
RAIM requires at least 5 SV’s to be in view and monitors specific accuracy boundaries
which if crossed, will initiate specific alerts. Positional accuracy is dependent on SV
geometry, examples of poor geometry are groups of SV’s with azimuth angles close
together and/or groups of SV’s with low elevation angles. Positions with low elevation
SV ‘s will also exhibit low relative RF Levels, due to the longer pseudo ranges. Due to the
SV orbital plane inclination of 55 deg, positions in temperate latitudes, the equator and
polar regions.
Temperate latitudes exhibit SV azimuths that range in an arc either side of 90 and 270
degs.
Equator exhibits SV azimuths through a full 360 degs and elevations up to 90 deg.
Polar regions at 0 and 180 deg. Polar regions are devoid of satellites. Polar positions
exhibit SV elevations that are low on the horizon.
Fig. 4-5 Global SV Tracks
The simulated position and Almanac/Ephemeris UTC, will determine the individual SV’s
elevation, azimuth and RF level. The SV PRN Elevation and Azimuth indications are used
to select combinations of SV’s that exhibit poor geometry for RAIM testing.
A bad geometry scenario may be setup in the SV PRN table by selecting only low
elevation SV’s, only SV’s with closely spaced azimuths or only high elevation SV’s.
Alternatively polar positions may be simulated, which inherently have low elevation SV’s.
SV health may be set to BAD to ensure the receiver does not use that SV.