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

The le_gnss_SetNmeaSentences() and le_gnss_GetNmeaSentences() functions configure the enabled NMEA sentences in the NMEA flow. 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.

References

See GNSS API

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