C-Nav 2050, User Manual

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2050 User Guide
D.5 GST NMEA Sentence
GPS Satellites Position Error Statistics (RAIM compliance)
The GST sentence is used to support Receiver Autonomous Integrity Monitoring
(RAIM). The data is determined from ‘statistical analysis’ of the residuals following the
C-Nav GPS Receiver computation of a position fix.
Example:
$GPGST,124214.0,0.0736,0.0879,0.0723,-07.3718,0.0877,0.0726,0.3067*4B
FIELD Description
1 UTC time of GGA position fix in HHMMSS.ss format
2 RMS value of the standard deviation of the corrected pseudorange
inputs to the navigation position process.
3 Standard deviation of the semi-major axis of error ellipse from the
navigation position process residuals analysis in meters.
4 Standard deviation of the semi-minor axis of error ellipse from the
navigation position process residuals analysis in meters.
5 Orientation of semi-major axis of error ellipse, in degrees of rotation
from True North
6 Standard Deviation of latitude error, in meters
7 Standard Deviation of longitude error, in meters
8 Standard Deviation of altitude error, in meters
9 Check Sum (hexadecimal representation) followed by CRLF
terminator pair
Table D. 7 GST Sentence Fields
The C-Nav[2050] GPS Receiver uses a conventional weighted least squares
algorithm for its position solution. The results of the weighted least squares solution
are reported directly to the external interfaces with no additional filtering or
smoothing. Each corrected pseudorange measurement is assigned a weight
proportional to the sine of the elevation angle of the associated GPS satellite. The
weights are normalized such that their mean is unity. The weighted least squares
solution fixes are achieved in a North, East, and Up local tangent plane coordinates
system (NEU). This facilitates altitude aiding (“2D mode’). The position fix results for
each epoch are kept internally as both latitude, longitude, ellipsoidal height and
ECEF x,y,z.
The byproduct of the NEU weighted least squares solution fix are the inverted least
squares solution matrix and the vector of post-fix residuals. Both of these play a key
role in the computation of the uncertainty estimates and horizontal error ellipse
parameters contained in the GST message.
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