ashtech GG Surveyor, Справочное руководство

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Advanced Operation 39
Advanced Operation
models are simultaneously turned on or off with the $PASHS,ION command. See
“$PASHS,ION,x” on page 89 for more information.
RAIM
GG Surveyor RAIM (Receiver Autonomous Integrity Monitoring) provides the
detection of anomalous satellite pseudorange error with miss detection probability
0.999 and false alarm probability 0.002 per hour (requirements from RTCA/DO-208)
under given horizontal alarm limit in range 200 m to 2 nautical miles. In addition GG
Surveyor RAIM isolates wrong satellite and correct position and velocity errors.
GG Surveyor RAIM includes three procedures which are called every epoch. The first
one is Availability Check which checks current satellites constellation available to
determine the possibility of anomalous error detection with given alarm threshold,
false alarm and miss detection probabilities. Availability percentage depends on
alarm threshold value, satellites number and their position. The less alarm threshold
is, the less availability percentage will be. For example, if 7 satellites of the same
system or 8 ones of two different systems (GPS/GLONASS) with good PDOP are in
view and alarm threshold is the one nautical mile (terminal mode) detection is always
available. If only 4 satellites of the same system or 5 satellites of the two different
systems are visible, detection is impossible.
If detection is available then Detection procedure is called. Detection algorithm
compares the residuals with threshold depending on number of redundant satellites in
view. If the threshold is exceeded then anomalous error is detected. RAIM is a
snapshot type algorithm, so detection usually takes place at the first epoch after alarm
limit being exceeded.
If error is detected and at least 6 satellites of the same system or 7 ones of the two
different systems with good PDOP are in view, then Exclusion And Correction
algorithm is called. Exclusion And Correction algorithm determines the number of
"wrong" satellites by maximal normalized residual, after that the position and velocity
are corrected by exclusion of that "wrong” satellite. To avoid possible incorrect
isolation, the rest of satellites' set is tested by Availability Check and Detection
algorithm. If the rest of satellites' set is available and no error is detected, it means the
successful correction of position and velocity. The procedures above can be executed
recursively. It provides the possibility of more than one simultaneously wrong
satellites exclusion. However, in some cases where not enough satellites are available
or too many errors are detected, the probability requirement can not be met because of
statistical limitations.