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2
where T is the time of averaging of I. Note that mean(I
) - mean (I) is the dispersion
of the mean value of I. If DBH is selected, SNR is presented in dH*Hz units in all
messages that report SNR.
The AMP method computes the SNR in actual amplitude, and this value is dependent
upon hardware. In the receiver, an internal scale coefficient is chosen such that under
usual circumstances, AMP is approximately equal to satellite elevation in degrees.
The range is from 1 to 99. If AMP is selected, SNR is presented in AMP units in all
messages that report SNR.
Satellite Search Algorithm
When the GG Surveyor operates for the first time after receipt from Ashtech, no
almanac or ephemeris data are loaded. The GG Surveyor always assigns the first 12
elements of a 32-element table of satellite PRN numbers to its first 12 channels and
the first 12 elements of a 24-element table of the GLONASS frequency numbers to its
last 12 channels. If no ephemeris data is available in the memory, or if the data is
older than ten hours, 30 to 60 seconds will be needed to collect data. After locking
onto four or five satellites and collecting almanac/ephemeris data, the GG Surveyor
computes its first position. The GG Surveyor continuously collects in its on-board
battery-backed-up memory (no external battery is required for memory) almanac and
ephemeris data as well as the most recent position. The time to the first position
computation, if no almanac/ephemeris data are available, is typically two minutes
(this is called a cold start).
At the next power up, if the almanac/ephemeris data from battery-backed-up memory
are available, the GG Surveyor uses the almanac data, the last computed position, and
the time from the on-board real-time clock to search only the visible satellites; under
these conditions, the GG Surveyor recomputes a position in 10 to 20 seconds (this is
called a warm start).
Ionospheric and Tropospheric Models
The GG Surveyor can be set to use an ionospheric and tropospheric model in its
position computation using the $PASHS,ION,Y/N command. The ionospheric and
tropospheric models are based on the models defined in ICD-GPS-200, Revision B.
Typically this function is used to improve the accuracy of stand-alone position by
minimizing the influence of ionosphere and troposphere on the code phase. In
differential mode, however, the model should not be applied since differential
corrections already contain the errors induced by ionosphere and troposphere. Both
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GG Surveyor GPS+GLONASS Reference Manual
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