With a GNSS receiver attached, the robot passes satellite positioning data through as raw NMEA sentences. The robot's GPS program reads the sentences from the receiver and republishes them unchanged on a ROS 2 topic.
NOTE
GNSS needs robot software that includes the GPS program and a GNSS receiver. For receivers and the LTE/GPS module see Accessories.
| Item | Value |
|---|---|
| Topic | /rbq/gps/nmea |
| Type | nmea_msgs/Sentence |
| Direction | Pub (robot → client) |
The message has just two fields.
| Field | Contents |
|---|---|
header.stamp | The time the robot read the sentence (not the UTC inside it) |
header.frame_id | gps — the receiver (antenna) frame |
sentence | One raw NMEA 0183 line (starts with $, checksum included) |
Sentences are not processed. Parsing is left to the subscriber; NMEA is a standard format with parsers in most languages.
ros2 topic echo /rbq/gps/nmea
Of everything the receiver emits, only these three types are published; the rest are dropped. The GN prefix means the fix combines several constellations (GPS, GLONASS, Galileo, BeiDou).
$GNRMC — position, speed, time $GNRMC,<UTC time>,<status>,<latitude>,<N/S>,<longitude>,<E/W>,<speed>,<course>,<UTC date>,<magnetic variation>,<E/W>,<mode>,<nav status>*<checksum>| Field | Contents |
|---|---|
| UTC time · date | When the fix was taken |
| Status | Data valid (A) or not (V) |
| Latitude · longitude | Position |
| Speed | Speed over ground (knots) |
| Course | Heading, relative to true north |
| Magnetic variation | Difference between true and magnetic north |
| Mode · nav status | How the position was determined, and the navigation status |
$GNGGA — fix quality and altitude $GNGGA,<UTC time>,<latitude>,<N/S>,<longitude>,<E/W>,<fix quality>,<satellites>,<HDOP>,<altitude>,M,<geoid separation>,M,<DGPS age>,<station ID>*<checksum>| Field | Contents |
|---|---|
| Fix quality | This is where you check the RTK state — see the table below |
| Satellites | Number of satellites used for the fix |
| HDOP | Horizontal dilution of precision — lower is better |
| Altitude | Above mean sea level |
| Geoid separation | Difference between the ellipsoid and the geoid |
| DGPS age · station ID | How long since the last correction (RTCM) and which base station sent it |
The fix-quality values are NMEA 0183 standard.
| Value | Meaning |
|---|---|
0 | No fix |
1 | Standalone GPS fix |
2 | DGPS — corrections applied |
4 | RTK fixed — integer ambiguities resolved, the most accurate |
5 | RTK float — ambiguities not resolved |
$GNGST — position error estimates $GNGST,<UTC time>,<RMS>,<semi-major σ>,<semi-minor σ>,<orientation>,<latitude σ>,<longitude σ>,<altitude σ>*<checksum>| Field | Contents |
|---|---|
| RMS | RMS of the range residuals |
| Semi-major · semi-minor σ · orientation | Standard deviations and bearing of the error ellipse |
| Latitude · longitude · altitude σ | Per-axis position error standard deviation (m) |
This sentence is how you judge what RTK actually bought you. Once the latitude and longitude σ drop to centimetre level, RTK has locked in.
ros2 topic echo /rbq/gps/nmea and watch for sentences.0 in $GNGGA means no satellites yet. Move somewhere with open sky and wait a moment.4 (fixed) or 5 (float), and that the latitude and longitude σ in $GNGST are small enough.In Logs → Real Time, filtering down to the GPS process shows the connect and disconnect messages directly.
| Symptom | Cause and fix |
|---|---|
| Nothing arrives on the topic | Check that the receiver is attached and powered, and that the robot can reach its address |
| It connects but no sentences appear | Anything outside GNRMC, GNGGA and GNGST is dropped. Enable those sentences in the receiver's own configuration |
Fix quality never rises above 1 | No corrections (RTCM) are arriving. Check the DGPS age and station ID in $GNGGA |
| No fix indoors | GNSS needs a view of the sky. Indoors or between tall buildings there may be no fix, or a large error |
Tell us the receiver model and a few lines of ros2 topic echo /rbq/gps/nmea output.
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