iSXblue/SXBlue II GNSS Series Technical Reference Manual
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probFALSE
Maximum allowed probability that there is a false alarm (That the position error is
reported outside the of the HPR, but it is really within the HPR). The acceptable range
for this value is from 0.001% to 50%. The default is 1%.
The purpose of the probability of false alarm (probFALSE)
is to help make a decision as to whether to
declare a fault or warning in an uncertain situation. As an example, you have several satellites which may
have failed, all giving similar probabilities of failure. One would put you outside of the horizontal
protection radius (HPR) and the others would not. You always choose the one that puts you outside of the
radius to be safe, but in this situation, you have likely made an error (the failed satellite may actually be
another satellite). So here, there is a high probability of false alarm. The high probability of false alarm
can be used to downgrade a fault to a warning. If the probability of false alarm (probFALSE) is > 5%,
(assuming this is the chosen threshold) a fault is downgraded to a warning.
There are other situations where the probability of false alarm is high due to geometry. Because of the
geometry, you may get excessive faults, even though you are actually within the protection radius. Again,
downgrading to a warning prevents excessive faults when the conditions are highly uncertain.
The philosophy is to only issue a fault if you are relatively certain (to within the probability of a false
alarm) that the protection radius has been exceeded, else issue a warning.
5.10.2
$PSAT,GBS Data Message
The GBS message is used to support Receiver Autonomous Integrity Monitoring (RAIM). In the table
below, the GBS data message is broken down into its components. This message takes the following
form:
$PSAT,GBS,hhmmss.ss,ll.l,LL.L,aa.a,ID,p.ppppp,b.b,s.s,flag*cc<CR><LF>
Table 5-24 $PSAT,GBS Data Message Defined
Field
Description
hhmmss.ss
UTC time in hours, minutes, seconds of the GGA or GNS fix associated with this sentence
ll.l
Expected error in latitude
LL.L
Expected error in longitude
aa.a
Expected error in altitude
ID
ID number of most likely failed satellite
p.ppppp
Probability of HPR fault
b.b
Estimate of range bias, in meters, on most likely failed satellite
s.s
Standard deviation of range bias estimate
flag
Good (0), Warning (1), Bad or Fault (2), (based on horizontal radius)
*cc
Checksum
<CR><LF>
Carriage return and line feed
To start outputting the $PSAT,GBS message once per second (the only output rate available), enter the
following:
$JASC,GPGBS,1<CR><LF>
Or, to turn the $PSAT,GBS message off, send the following command.
$JASC,GPGBS,0<CR><LF
