ashtech GG Surveyor, Reference Manual

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GPS and GLONASS Concepts A-9
GPS and GLONASS
Test conditions: 10o elevation mask, correction rate: 90 seconds, HDOP less than or
equal to 4, number of GLONASS satellites used in position computation greater than
or equal to 4.
When a position is not differentially corrected, SA degrades the position accuracy
from the GPS constellation to about 100 meters (2-sigma, 95%). The GLONASS
constellation does not implement SA, so position accuracy improves as GLONASS
satellites are added to a mixed system. The attained accuracy is proportional to the
number of healthy GLONASS satellites above the elevation mask. When the number
of healthy GLONASS satellites is fewer than five, accuracy is degraded. The
approximate stand-alone position error attainable by a mixed GPS+GLONASS
system with an HDOP close to 1 is presented in Table A.4.
Figure A.1: GG Surveyor Code Differential Horizontal Position Decay
Table A.4: Approximate Position Error, Mixed GPS+GLONASS
Number of
GPS Satellites
Number of
GLONASS Satellites
50% 95%
≥0 ≥
5
~ 7 m ~16 m
≥1
4 ~10 m ~20 m
≥2
3 ~15 m ~30 m
≥3
2
~20 m ~40 m