GNSS platform constraints

Acquisition rate

The le_gnss_SetAcquisitionRate() function configures the GNSS device acquisition rate.

On Qualcomm-based platform the “tracking” mode is selected for the “continuous” navigation.

When a hot start is requested, it is possible that GPS engine can’t get fix within first 1s, hence NMEA message can possibly get empty frame outputs every second. Tracking mode is based on periodic hot start: periodically, low layer GPS engine starts a new session and updates GPS location until it gets a fix, from which NMEA output can be composed.

In other words, after first fix is gotten, GPS engine restart the GPS session to get the fix again, it will not output the last valid fix before we get the new fix, and NMEA message can get empty frame again.

The 'Acquisition Rate' value is used to set the interval between two fix requests.

Qualcomm-based platform hot start performance is about 1~3s, it’s an average value. So you may notice a void NMEA string (and the corresponding Legato position sample notification) during hot start and then the valid NMEA string on an AcquisitionRate minimum time interval. But it’s should not be so accurate, you may have 1~3s error-range.

Here below an example of NMEA traces in tracking mode with a 30-second acquisition rate. Note the GPGGA string sequence:

$GPVTG,,T,,M,,N,,K,N*2C
$GPGSA,A,1,,,,,,,,,,,,,,,*1E
        $GPGGA,,,,,,0,,,,,,,,*66                -> say it’s 1st second from logging start, no fix from beginning for 1st GPS session
$PQXFI,,,,,,,,,,*56
$GPRMC,,V,,,,,,,,,,N*53
$GPGSV,2,1,05,02,,,42,06,48,299,50,09,53,213,50,19,28,230,43*48
$GPGSV,2,2,05,31,09,029,*4C
        $GPGGA,100836.0,4849.944172,N,00216.094351,E,1,03,1.7,54.2,M,48.0,M,,*69  -> second 2 from start (100836), get the fix for 1st session
$PQXFI,100836.0,4849.944172,N,00216.094351,E,54.2,5.74,11.44,1.86*6A
$GPVTG,318.5,T,320.6,M,0.0,N,0.0,K,A*2B
$GPRMC,100836.0,A,4849.944172,N,00216.094351,E,0.0,318.5,220116,2.1,W,A*12 valid final NMEA
$GPGSA,A,2,06,09,19,,,,,,,,,,2.0,1.7,1.0*31
$GPVTG,,T,,M,,N,,K,N*2C
$GPGSA,A,1,,,,,,,,,,,,,,,*1E
        $GPGGA,,,,,,0,,,,,,,,*66               -> second 31, no fix for 2nd GPS session
$PQXFI,,,,,,,,,,*56
$GPRMC,,V,,,,,,,,,,N*53 non valid intermediate NMEA
$GPGSV,2,1,05,02,,,41,06,49,299,49,09,53,213,48,19,28,230,43*4B
$GPGSV,2,2,05,31,09,029,*4C
        $GPGGA,100906.0,4849.942986,N,00216.093882,E,1,03,1.7,48.3,M,48.0,M,,*60  -> second 32 from start (100906, 30s after 100836), get the fix for 2nd session
$PQXFI,100906.0,4849.942986,N,00216.093882,E,48.3,7.25,15.04,0.77*6A
$GPVTG,318.5,T,320.6,M,0.0,N,0.0,K,A*2B
$GPRMC,100906.0,A,4849.942986,N,00216.093882,E,0.0,318.5,220116,2.1,W,A*17
$GPGSA,A,2,06,09,19,,,,,,,,,,2.0,1.7,1.0*31
$GPVTG,,T,,M,,N,,K,N*2C
$GPGSA,A,1,,,,,,,,,,,,,,,*1E
$       GPGGA,,,,,,0,,,,,,,,*66               -> second 61, no fix for 3rd session
$PQXFI,,,,,,,,,,*56
$GPRMC,,V,,,,,,,,,,N*53
$GPGSV,2,1,05,02,,,43,06,49,298,50,09,53,213,47,19,27,230,44*47
$GPGSV,2,2,05,31,09,029,*4C
$GPGGA,100936.0,4849.941714,N,00216.095230,E,1,03,1.7,55.9,M,48.0,M,,*66  -> second 62 from start (100936, 30s after 100906), get the fix for 3rd session
$PQXFI,100936.0,4849.941714,N,00216.095230,E,55.9,15.01,27.89,0.34*5A
$GPVTG,318.5,T,320.6,M,0.0,N,0.0,K,A*2B
$GPRMC,100936.0,A,4849.941714,N,00216.095230,E,0.0,318.5,220116,2.1,W,A*17
$GPGSA,A,2,06,09,19,,,,,,,,,,2.0,1.7,1.0*31
$GPVTG,,T,,M,,N,,K,N*2C
$GPGSA,A,1,,,,,,,,,,,,,,,*1E
$GPGGA,,,,,,0,,,,,,,,*66

Enabled NMEA sentences

le_gnss_SetNmeaSentences() function configures the enabled NMEA sentences in the NMEA flow and le_gnss_GetNmeaSentences() function gets the enabled NMEA sentences.

Not all NMEA sentences are supported by the different platforms. The table below lists the supported NMEA sentences for each platform.

NMEA sentences AR7 AR758x AR759x AR8652 WP750x WP76xx WP77xx WP85
LE_GNSS_NMEA_MASK_GPGGA X X X X X X X X
LE_GNSS_NMEA_MASK_GPGSA X X X X X X X X
LE_GNSS_NMEA_MASK_GPGSV X X X X X X X X
LE_GNSS_NMEA_MASK_GPRMC X X X X X X X X
LE_GNSS_NMEA_MASK_GPVTG X X X X X X X X
LE_GNSS_NMEA_MASK_GLGSV X X X X X X X X
LE_GNSS_NMEA_MASK_GNGNS X X X X X X X X
LE_GNSS_NMEA_MASK_GNGSA X X X X X X X X
LE_GNSS_NMEA_MASK_GAGGA X X X X X X X X
LE_GNSS_NMEA_MASK_GAGSA X X X X X X X X
LE_GNSS_NMEA_MASK_GAGSV X X X X X X X X
LE_GNSS_NMEA_MASK_GARMC X X X X X X X X
LE_GNSS_NMEA_MASK_GAVTG X X X X X X X X
LE_GNSS_NMEA_MASK_PSTIS X X X X X X X X
LE_GNSS_NMEA_MASK_PQXFI X X X X X X X X
LE_GNSS_NMEA_MASK_PTYPE X X X X X X X X
LE_GNSS_NMEA_MASK_GPGRS X
LE_GNSS_NMEA_MASK_GPGLL X
LE_GNSS_NMEA_MASK_DEBUG X X X X
LE_GNSS_NMEA_MASK_GPDTM X
LE_GNSS_NMEA_MASK_GAGNS X

Please note that the Qualcomm-based firmware release SWI9X15A_07.10.15.00 (AR755x, AR8652) is subject to some limitations:

  • Not all NMEA sentences supported by the AT!GPSNMEASENTENCE can be (de)activated through the Legato API: the GPGSV_EXTENDED and GPGRS NMEA sentences are not available.
  • The Galileo NMEA sentences always appear as not enabled when retrieving their status, even if they are enabled. This concerns the GAGGA, GAGSA, GAGSV, GARMC, and GAVTG NMEA sentences.

Setting configuration

This section summarizes for each GNSS setting API, on Qualcomm-based platform, how the setting is performed.

Most of the APIs write the setting into a non volatile (NV) memory at each call, making it persistent to reset. Pay attention that in this case, a reboot of the device is required to take the change into account. For exemple, a new constellation set by le_gnss_SetConstellation() is only valid after a reboot of the module. Enabling/Disabling the 'Extended Ephemeris' file injection into the GNSS device set by le_gnss_EnableExtendedEphemerisFile()/le_gnss_DisableExtendedEphemerisFile() is only valid after a reboot of the module.

It is recommended to call these APIs, for which the setting takes effect after a reboot, only once in a GNSS session.

Other APIs don't write the setting into a non volatile memory making it lost after a device reboot. This setting is operational imediately in the next GNSS session (no reboot is needed). However, there are difference upon GNSS restart session and upon Legato restart.

The following APIs write the setting to NV, the setting is persistent and effective after reboot: le_gnss_SetConstellation() le_gnss_EnableExtendedEphemerisFile() le_gnss_DisableExtendedEphemerisFile() le_gnss_SetSuplServerUrl() le_gnss_SetNmeaSentences() le_gnss_SetMinElevation()

The following APIs don't write the setting to NV, the setting is thus not persistent to reboot but effective imediately in next GNSS session. The setting is persistent to GNSS factory/cold/warm/hot restart, but is lost after Legato restart: le_gnss_Enable() le_gnss_Disable() le_gnss_SetAcquisitionRate()

The following APIs don't write the setting to NV, the setting is thus not persistent to reboot but effective imediately in next GNSS session. The setting is persistent to GNSS factory/cold/warm/hot restart, but is not lost after Legato restart: le_gnss_SetSuplAssistedMode()

GNSS WARM restart

Qualcomm-based platform "WARM" restart GNSS session needs an addional 10 sec internal delay comparing to "COLD" or "FACTORY" restarts.

"WARM" restart le_gnss_ForceWarmRestart() API stops the current GNSS session then starts it again using the available GNSS data. In this case, the Ephemeris are being cleared but not the position data (they are cleared in "COLD" or "FACTORY" restarts). Reusing them after restarting the GNSS session too soon leads to a wrong time to First Fix (TTFF). For this reason a 10 sec delay is added before the restart session.

Note that this additional delay doesn't impact the measurement to get a fix given by le_gnss_GetTtff(). This API calculates the TTFF started from a succeed GNSS start session. In case of le_gnss_ForceWarmRestart(), the 10 sec delay is added before re-starting the GNSS session.

References

See GNSS API

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