gps-wizard

GSA - GNSS Active Satellites

Summary

This is one of the sentences always supported by GPS / GNSS receivers.

The GSA sentence includes GPS / GNSS receiver operating mode, satellites used for navigation, and DOP values.

In a multi-GNSS receiver the GSA message will be output multiple times, once for each GNSS.

The number of unique SV IDs (or PRNs) should correspond with the number of satellites reported by GGA (possibly limited to 12) and GNS.

In a multi-band receiver the GSA message will be output multiple times, once for each GNSS / signal.

Structure

NMEA 4.10 onwards:
        1 2 3                        14 15  16  17 18 19
        | | |                         |  |   |   |  |  |
 $--GSA,a,a,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x.x,x.x,x.x,x*hh<CR><LF>

Prior to NMEA 4.10:
        1 2 3                        14 15  16  17  18
        | | |                         |  |   |   |   |
 $--GSA,a,a,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x.x,x.x,x.x*hh<CR><LF>

#	Field	Format	Example	Description
0	Sentence ID	string	$GPGSA	Talker ID (GP) + message ID (GSA)
1	Op mode	character	A	2D / 3D operation mode; M = manual, A = automatic
2	Fix mode	digit	3	Fix mode; 1 = no fix, 2 = 2D, 3 = 3D
-	# Start of block	-	-	Start of repeated block (12 times)
3 + idx	SV ID	numeric	12	Satellite ID or PRN number (leading zeros sent)
-	# End of block	-	-	End of repeated block (12 times)
15	PDOP	numeric	1.83	Position dilution of precision (PDOP), typically 1 or 2 dp
16	HDOP	numeric	1.09	Horizontal dilution of precision (HDOP), typically 1 or 2 dp
17	VDOP	numeric	1.47	Vertical dilution of precision (VDOP), typically 1 or 2 dp
18	System ID	hexadecimal	1	GNSS system ID (NMEA 4.10 and later), or maybe signal ID
19	Checksum	hexadecimal	*7B	Checksum

Notes:

The talker ID may be BD, GA, GB, GI, GL, GN, GP, GQ, IM, NC or QZ.
If less than 12 SVs are used for navigation, the remaining fields are left empty.
If more than 12 SVs are used for navigation, only the IDs of the first 12 are output.
The system ID (field 18) was added in NMEA 4.10.
Multi-band receivers may use field 18 to represent the signal ID, using the talker ID to identify the GNSS constellation.

Examples

NMEA Revealed

$GNGSA,A,3,80,71,73,79,69,,,,,,,,1.83,1.09,1.47*17

Wikipedia

$GPGSA,A,3,10,07,05,02,29,04,08,13,,,,,1.72,1.03,1.38*0A

Locosys GT-31

$GPGSA,M,3,15,13,14,05,23,24,17,10,,,,,1.7,0.9,1.4*32

Interpretation

The GSA sentence is being used in a number of different ways in modern GPS / GNSS receivers. When parsing the GSA sentence, code should be able to handle all of the scenarios described below.

Broadcom BCM4774

This example for the BCM4774 was captured from a Samsung Galaxy S7.

$GPGSA,A,3,10,12,13,14,15,17,19,24,32,,,,1.5,0.8,1.3*34
$GNGSA,A,3,10,12,13,14,15,17,19,24,32,,,,1.5,0.8,1.3*2A
$GNGSA,A,3,66,67,68,77,85,,,,,,,,1.5,0.8,1.3*20

Notes:

Talker ID does not distinguish the different GNSS constellations:
- $GPGSA reports the GPS satellites in use.
- $GNGSA reports the GPS and GLONASS satellites in use.
It is therefore necessary to de-duplicate the PRNs for each GNSS.

Samsung Exynos 1280

This example for the Exynos 1280 was captured from a Samsung Galaxy A53.

$GPGSA,A,3,02,03,04,08,14,17,19,21,22,32,,,0.6,0.4,0.6*32
$GNGSA,A,3,65,72,79,80,86,87,88,,,,,,0.6,0.4,0.6*29
$GAGSA,A,3,103,105,108,113,115,121,127,,,,,,0.6,0.4,0.6*18
$BDGSA,A,3,205,209,211,214,216,233,,,,,,,0.6,0.4,0.6*29

Notes:

Talker ID distinguishes the different GNSS constellations.
The GSA sentences from the Exynos 1280 are reporting PRNs.

Samsung Exynos 1380

This example for the Exynos 1380 was captured from a Samsung Galaxy A54.

$GNGSA,A,3,05,13,14,15,20,22,23,24,30,,,,0.8,0.4,0.6,1*3B
$GNGSA,A,3,65,72,75,76,,,,,,,,,0.8,0.4,0.6,2*3D
$GNGSA,A,3,104,109,111,119,121,136,,,,,,,0.8,0.4,0.6,3*3A
$GNGSA,A,3,207,210,223,225,232,237,240,241,,,,,0.8,0.4,0.6,4*3A

Notes:

$GNGSA is used for all of the GNSS constellations.
- Field 18 specifies the system ID.
The GSA sentences from the Exynos 1380 are reporting PRNs.

Qualcomm SDM845

This example for the SDM845 was captured from a Samsung Galaxy S10 Lite.

$GNGSA,A,3,65,71,72,79,86,87,88,,,,,,0.7,0.5,0.6,2*39
$GNGSA,A,3,03,05,08,13,15,21,27,34,,,,,0.7,0.5,0.6,3*3E
$GNGSA,A,3,09,11,14,16,27,28,,,,,,,0.7,0.5,0.6,4*34
$GNGSA,A,3,02,03,04,08,14,17,19,21,32,,,,0.7,0.5,0.6,1*31

Notes:

$GNGSA is used for all of the GNSS constellations.
- Field 18 specifies the system ID.
The GSA sentences for Galileo and BeiDou are reporting satellite IDs, not PRNs.

Broadcom BCM4776

This example for the BCM4776 was captured from a Google Pixel 7.

$GPGSA,A,3,04,06,07,09,11,20,30,,,,,,1.8,0.9,1.5*3B
$GPGSA,A,3,04,06,09,11,30,,,,,,,,1.8,0.9,1.5,8*2A
$GAGSA,A,3,103,107,108,113,115,125,126,,,,,,1.8,0.9,1.5,1*02
$GNGSA,A,3,04,06,07,09,11,20,30,,,,,,1.8,0.9,1.5*25
$GNGSA,A,3,71,72,,,,,,,,,,,1.8,0.9,1.5*2B
$BDGSA,A,3,219,221,222,234,236,244,245,246,,,,,1.8,0.9,1.5,5*34
$GAGSA,A,3,103,107,108,113,115,125,126,,,,,,1.8,0.9,1.5*1F
$BDGSA,A,3,211,219,221,222,234,236,244,245,246,,,,1.8,0.9,1.5*1F

Notes:

Talker ID distinguishes the different GNSS constellations.
Field 18 is only present for some rows but when present it indicates the signal ID.
- The signal ID is only included when it is not the default signal.
To determine the actual number of satellites being used, some de-duplication will be required.
The BCM4776 also outputs $PSSGR,GSA which is like $GPGSA, but includes system ID and signal ID.

Summary

Determining the satellites in use for each system:

When available use the talker ID to distinguish between different GNSS.
- Different vendors may use $GNGSA to report GNSS or GLONASS satellites that are active.
- Must cope with $GPGSA and $GNGSA from chipsets such as the Broadcom BCM4774.
  - GPS satellites are reported in both on the BCM4774, so de-duplication is required.
When available use the system ID (field 18) to distinguish between different GNSS.
- Field 18 will probably be the system ID when all GSA sentences are $GNGSA.
- Need to de-duplicate the SV IDs (or PRNs) on multi-band receivers.
When available use the signal ID (field 18) to distinguish between signals - e.g. Broadcom BCM4776
- Field 18 will probably be the signal ID when GSA sentences are present for different talker IDs.
- Sometimes $PSSGR,GSA will also be available which is like GSA but includes system ID and signal ID.
In addition to the above, consider the possibility of system ID and signal ID both being present in GSA.
- It is not inconceivable that some vendors may use fields 18 and 19 for system ID and signal ID.
Vendors may use either SV IDs or PRNs in the GSA output, so code needs to be aware of this subtlety.

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