Operation 45
Operation
data at 20 Hz ($PASHS,POP,20), it uses no more than 10 satellites in the
navigation solution, although it continues tracking up to 12 satellites. During
periods in which a 20 Hz update rate is not required, you can revert to a 10 Hz
update rate and resume using up to 12 satellites in the position solution by issuing
the command $PASHS,POP,10.
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, RAIM
isolates wrong satellite and correct position and velocity errors.
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
with good PDOP are in view and alarm threshold is the one nautical mile
(terminal mode), detection is always available. If only 4 satellites are visible,
detection is impossible.
If detection is available then Detection procedure is called. Detection algorithm
compares the residuals with threshold depending upon the 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 the alarm limit being exceeded.
If error is detected and at least 6 satellites with good PDOP are in view, then the
Exclusion And Correction algorithm is called. The 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 the satellites' set
is tested by Availability Check and Detection algorithm. If the rest of the satellites'
set is available and no error is detected, it means the successful correction of
CAUTION
When collecting data at 10 or 20 Hz, a 486-33 MHz or Pentium computer with a fast
serial and parallel port card (i.e., 16550 serial and parallel card) is required
because of large amounts of data being output through the serial ports. The serial
port baud rate should be set to 115200.
G12RevD.book Page 45 Tuesday, April 2, 2002 4:33 PM
To Next Page
Loading...