Customer Service Center

• PosApp (SimGEN)

• The effect of Spirent's example GPS Tx antenna pattern is not always “exactly” like the real GPS satellite Tx patterns
• The following key reasons are:-
      1) The Spirent pattern is symmetrical about the boresight (the line between the TX antenna and the centre of the earth) 
              - Spirent don’t currently model satellite rotation about this axis.
      2) The Spirent pattern is created with a 5 degree pattern cell resolution.
      3) The pattern is normalised to ensure the received signal level is close to the nominal -130dBm (GPS L1 C/A minimum received power)
               - See SOL10137 for further details on calculation of simulated satellite power level.
      4) The data Spirent have used for this example GPS Tx antenna pattern is derived from an old ION paper
               - “Description and Performance of the GPS Block I and II L-Band Antenna and Link Budget”, Francis M. Czopek, Scott Shollenberger, (ION GPS 1993).
      5) It is likely that later block satellites have slightly different patterns

NOTES FOR SPACE VEHICLE SIMULATION

1. The Spirent pattern does have a rearward-facing lobe and this may enable GEO satellites to “see” the GPS satellites on the same side of the earth.
       - The pattern may be edited to remove this lobe if it is felt that doing so makes the pattern effect more realistic or more recent data on the Tx antenna patterns has been accessed.
2. When simulating space vehicle motion at large distances from the Earth the number of visible satellites will tend to be much larger.
       - This may exceed the number of available simulator channels.
       - One way to influence which satellites are simulated is to use the "Satellite Selection Interval"
               - Go to the scenario tree [GPS constellation-->GPS signal sources file-->Satellite selection]
               - If set to "DOP" satellites with lower power levels may be included in preference to those with stronger signals.
               - Use “Power level” to favour satellites with stronger signals.