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Author Topic:   NMEA Satellite ID Numbers for GPS, SBAS, and GLONASS
jimh posted 12-22-2014 08:37 AM ET (US)   Profile for jimh   Send Email to jimh  
Now that GNSS receivers are available that track satellites in global navigation systems in addition to the USA NAVSTAR GPS, it appears that NMEA have augmented their satellite ID numbers to include the GLONASS system. Based on some documentation from Simrad for one of their GPS and GLONASS receivers, it appear the NMEA Satellite ID number that will be seen in various NMEA messages and, most commonly, in the usual display of satellite status showing which satellites are being tracked, will use a new numbering convention, as follows:

FOR GPS Satellites:

The NMEA ID will be the same as the satellite PRN designator. The GPS PRN designators range from 1 to 32. The NMEA ID numbers are in the same range, 1 to 32.

For Satellites in a Space-Based Augmentation System (SBAS)

The NMEA ID will be the satellite PRN designator minus 87. The PRN designators for SBAS satellites range from 120 to 151. The NMEA ID numbers are thus in the range of 33 to 64.

For GLONASS Satellites:

The NMEA ID will be the GLONASS slot number plus 64. GLONASS slot numbers range from 1 to 24. The NMEA ID numbers are thus in the range of 65 to 96.

You can deduce which satellites are being used in a position solution for a GNSS receiver by noting the NMEA Satellite ID numbers as follows:

NMEA ID 1 to 32: a GPS satellite with corresponding PRN.

NMEA ID 33 to 64: most likely a WAAS satellite with PRN equal to NMEA ID plus 87. The current WAAS PRNs in use are

PRN - - - - NMEA ID
133 - - - - 46
135 - - - - 48
138 - - - - 51

NMEA ID 65 to 96: a GLONASS satellite with slot number corresponding to NMEA ID minus 64.

EXAMPLE: A modern GNSS receiver shows a satellite status page with the following satellite NMEA ID's:

12, 14, 15, 18, 21, 22, 24 27, 66, 74, 75, 76

The satellites being tracked were:

GPS: PRN 12, 14, 15, 18, 21, 24, 27

GLONASS: Slot ID 2, 10, 11, 12

jimh posted 12-22-2014 08:49 AM ET (US)     Profile for jimh  Send Email to jimh     
The usual satellite status display looks something like this:

Satellite status screen.

The most commonly used SBAS is the FAA's WAAS. Note that two of the FAA's WAAS SBAS satellites (PRN 135 and 138, NMEA ID 48 and 51) also transmit a navigation message, and a GPS receiver can use those signals in a pseudo-range measurement for a position solution. These PRN (135, 138) or NMEA ID (48, 51) are sometimes seen among the satellites being tracked or used in the position solution of a receiver.

The WAAS PRN 133 (NMEA ID 46) does not transmit a navigation message; it only sends augmentation data. It cannot be used in a position solution, so it typically is not seen in the status displays. Some receivers do not identify which SBAS source they are using. In the eastern USA, the best signal is usually from PRN 133 (due to its location), but this PRN may not show up in status displays because it is not sending a navigation message.

See a prior discussion for more information about WAAS satellites, their location, and their coverage of North America.

jimh posted 12-22-2014 09:02 AM ET (US)     Profile for jimh  Send Email to jimh     
For a brief overview of GLONASS satellites in orbit and their slot numbers, see

jimh posted 12-22-2014 01:22 PM ET (US)     Profile for jimh  Send Email to jimh     
If any readers have a Simrad chart plotter with a GPS and GLONASS receiver, I would like to get a nice screen capture (not a photgraph of a screen) that shows some of the GLONASS satellites being tracked. If you have a screen capture like that, please send it to me via email and I will include it in this thread. You will have my thanks.
saumon posted 12-23-2014 08:48 AM ET (US)     Profile for saumon  Send Email to saumon     
A good ressource about the NMEA IDs for the various satellites systems:
jimh posted 12-23-2014 11:22 PM ET (US)     Profile for jimh  Send Email to jimh     
SAUMON--Excellent data at the site you pointed to.

Re the other GNSS constellations:

QZSS is not much use outside of Japan, due to the orbits of its satellites.

Beidou will eventually expand to more medium Earth orbits, but right now it has only a few. Its coverage is primarily aimed at Asia.

When GALILEO gets more satellites in orbit, it may become more common, but right now it is not very useful. There are only a couple of working satellites in orbit.

IRNSS will be good for the Indian Ocean region, but no where else.

For the immediate future of recreational boating in North American, only GPS and GLONASS are likely to be seen on a marine chart plotter.

If you have a recent model iPhone you can use GPS, GLONASS, and QZSS. If you have certain recent Android phones you can use GPS and GLONASS on all, and on some add QZSS and/or BeiDou.

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