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I'm trying to parse gps nmea output from a modem connected to serial port of a cubietruck board (ARM® Cortex™-A7 Dual-Core). The configuration is done succesfully I get 4g network form modem but when I try to see nmea outout of the gps module, I get the following : 

$GPRMC,,V,,,,,,,,,,N*53
$GPGSV,2,1,08,07,49.9,43.6,28.2,28,49.9,158.9,29.2,02,3.5,239.1,,05,49.9,286.9,E
$GPGSV,2,2,08,06,,,,08,9.8,68.9,,09,31.6,104.1,,13,20.4,299.5,,1*5E
$GNGNS,112218.9,,,,,NNN,,,,,,*03
$GPVTG,,T,,M,,N,,K,N*2C
$GPGSA,A,1,,,,,,,,,,,,,,,*1E
$GNGSA,A,1,,,,,,,,,,,,,,,*00
$GPGGA,,,,,,0,,,,,,,,*66
$GPRMC,,V,,,,,,,,,,N*53
$GPGSV,2,1,08,07,49.9,43.6,27.9,28,49.9,158.9,29.7,02,3.5,239.1,,05,49.9,286.9,F
$GPGSV,2,2,08,06,,,,08,9.8,68.9,,09,31.6,104.1,,13,20.4,299.5,,1*5E
$GNGNS,112219.9,,,,,NNN,,,,,,*02

Looking in http://aprs.gids.nl/nmea/ I found that $GPGSV is GPS Satellites in view. So what I understand is that I get 2 satellites. Is this perhaps the case that I don't get valid $GPGGA $GPRMC or should I check something else?

dk13
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2 Answers2

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You don't have a positional "fix" and therefore get empty values in some of the telegrams. As you probably know, there should be latitude, longitude and other data instead of empty values between the commas. Two satellites are not enough to get a fix, you need at least 3 or 4 to get the position.

The NMEA 0183 standard is weakly defined and doesn't mention what GPS units should do when they don't have a positional fix. Sending empty values is pretty common, but some units might act differently. You can easily tell whether the data is valid or not from the A and V letters. V means void and A means active. In the RMC telegram, you can see a V, meaning the entire thing is void.

The reason you get values in the GSV telegrams is because the GPS is able to see two satellites and therefore calculate the values needed in the telegram.

The third field, 08, is the theoretical number of satellites you would be able to see in good conditions.

So what you have to do is to take the GPS outdoors or connect it to a proper antenna. It will start sending proper values when it's able to see the satellites.

Mika Sundland
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Actually by your two GPGSV sentences you can see 08 satellites - eight of them. You have 2 'sentences' of GSV 2,1,08 means 1 of 2 sentences describing 08 satellites, and 2,2,08 means 2 of 2 sentences describing 08 satellites

Gerry B
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