Manual Title:
LD7 Operations Manual
Ref.:
AB-V-MA-00555
Rev No:
A9
Page 53
NMEA GRS Sentence
GNSS Range Residuals
This sentence is used to support Receiver Autonomous Integrity Monitoring (RAIM). Range residuals
can be computed in two ways for this process. The basic measurement integration cycle of most
navigation filters generates a set of residuals and uses these to update the position state of the
receiver.
These residuals can be reported with GRS, but because of the fact that these were used to generate
the navigation solution they should be recomputed using the new solution in order to reflect the
residuals for the position solution in the GGA or GNS sentence.
The MODE field should indicate which computation method was used. An integrity process that uses
these range residuals would also require GGA or GNS, GSA, and GSV sentences to be sent.
If only GPS, GLONASS, etc. is used for the reported position solution the talker ID is GP, GL, etc.
and the range residuals pertain to the individual system. If GPS, GLONASS, etc. are combined to
obtain the position solution multiple GRS sentences are produced, one with the GPS satellites,
another with the GLONASS satellites, etc. Each of these GRS sentences shall have talker ID “GN”,
to indicate that the satellites are used in a combined solution. It is important to distinguish the
residuals from those that would be produced by a GPS-only, GLONASS-only, etc. position solution.
In general the residuals for a combined solution will be different from the residual for a GPS-only,
GLONASS-only, etc. solution.
$--GRS,hhmmss.ss,x,x.x,x.x,x.x,x.x,x.x,x.x,x.x,x.x,x.x,x.x,x.x,x.x*hh<CR><LF>
Range residuals in meters for satellites used in the navigation solution1,2.
Order must match order of the satellite ID3 numbers in GSA.
When GRS is used GSA and GSV are generally required.
Mode: 0 = residuals were used to calculate the position given in the matching GGA or GNS
sentence
1 = residuals were recomputed after the GGA or GNS position was computed
UTC time of the GGA or GNS fix associated with this sentence
Notes:
1) If the range residual e99.9 meters, then the decimal part is dropped, resulting in an
integer (-103.7 becomes -103). The maximum value for this field is +999.
2) The sense or sign of the range residual is determined by the order of parameters used in the
calculation.
The expected order is as follows: range residual = calculated range - measured range.
3) When multiple GRS sentences are being sent then their order of transmission must match the
order of corresponding GSA sentences.
Listeners shall keep track of pairs of GSA and GRS sentences and discard data if pairs are
incomplete.
