GPGST

Estimated error in position solution

Platform:

OEM719, OEM729, OEM7500, OEM7600, OEM7700, OEM7720, PwrPak7, CPT7, CPT7700, SMART7, SMART2

This log contains pseudorange measurement noise statistics are translated in the position domain in order to give statistical measures of the quality of the position solution.

This log reflects the accuracy of the solution type used in the BESTPOS log and GPGGA log, except for the RMS field. The RMS field, since it specifically relates to pseudorange inputs, does not represent carrier-phase based positions. Instead, it reflects the accuracy of the pseudorange position which is given in the PSRPOS log.

The GPGST log outputs these messages without waiting for a valid almanac. Instead, it uses a UTC time, calculated with default parameters. In this case, the UTC time status (see the TIME log) is set to WARNING since it may not be one hundred percent accurate. When a valid almanac is available, the receiver uses the real parameters. Then the UTC time status is set to VALID.

If the NMEATALKER command is set to AUTO, the talker (the first 2 characters after the $ sign in the log header) is set to GP (GPS satellites only), GL (GLONASS satellites only) or GN (satellites from multiple systems) or GA (Galileo satellites only).

Message ID: 222

Log Type: Synch

Recommended Input:

log gpgst ontime 1

Example 1 (GPS only):

$GPGST,141451.00,1.18,0.00,0.00,0.0000,0.00,0.00,0.00*6B

Example 2 (Combined GPS and GLONASS):

$GNGST,143333.00,7.38,1.49,1.30,68.1409,1.47,1.33,2.07*4A

  1. See the Note in the GPGGA log that applies to all NMEA logs.

  2. Accuracy is based on statistics, reliability is measured in percent. When a receiver can measure height to one metre, this is an accuracy. Usually this is a one sigma value (one SD). A one sigma value for height has a reliability of 68%, that is, the error is less than one metre 68% of the time. For a more realistic accuracy, double the one sigma value (1 m) and the result is 95% reliability (error is less than 2 m 95% of the time). Generally, GNSS heights are 1.5 times poorer than horizontal positions.

    As examples of statistics, the GPGST message and NovAtel performance specifications use Root Mean Square (RMS). Specifications may be quoted in CEP:

    • RMS - root mean square (a probability level of 68%)

    • CEP - circular error probable (the radius of a circle such that 50% of a set of events occur inside the boundary)

Field

Structure

Description

Symbol

Example

1

$GPGST

Log header. See Messages for more information.

 

$GPGST

2

utc

UTC time status of position
(hours/minutes/seconds/ decimal seconds)

hhmmss.ss

173653.00

3

rms

RMS value of the standard deviation of the range inputs to the navigation process. Range inputs include pseudoranges and DGPS corrections

x.x

2.73

4

smjr std

Standard deviation of semi-major axis of error ellipse (m)

x.x

2.55

5

smnr std

Standard deviation of semi-minor axis of error ellipse (m)

x.x

1.88

6

orient

Orientation of semi-major axis of error ellipse (degrees from true north)

x.x

15.2525

7

lat std

Standard deviation of latitude error (m)

x.x

2.51

8

lon std

Standard deviation of longitude error (m)

x.x

1.94

9

alt std

Standard deviation of altitude error (m)

x.x

4.30

10

*xx

Check sum

*hh

*6E

11

[CR][LF]

Sentence terminator

 

[CR][LF]