GPS Example with supl: AGPS data is getting dropped from GPGSV NMEA in

[Stratosphere]

Sdk: 1.6

Modem fw: 1.3

Hi,

I am testing the GPS + supl example on our custom board. I am sitting indoor close to a window so satellite reception is affect.

the example is downloading AGPS data and injecting it..

---------------------------------
Seconds since last fix: 91
Searching [\]

NMEA strings:

$GPGGA,140122.22,,,,,0,,99.99,,M,,M,,*62

$GPGLL,,,,,140122.22,V,N*4E

$GPGSA,A,1,,,,,,,,,,,,,99.99,99.99,99.99,1*2D

$GPGSV,4,1,14,1,45,073,,3,04,101,,10,04,346,,13,07,238,,1*6F

$GPGSV,4,2,14,14,63,132,,15,08,267,,17,71,195,,19,44,222,,1*6F

$GPGSV,4,3,14,21,30,051,,22,15,080,,24,31,306,,28,79,168,,1*64

$GPGSV,4,4,14,30,12,173,,32,00,012,,1*66

$GPRMC,140122.22,V,,,,,,,070721,,,N,V*00

Tracking: 0 Using: 0 Unhealthy: 0

and then the next second, all the GPGSV data gets dropped and stays like that:

Seconds since last fix: 92
Searching [|]

NMEA strings:

$GPGGA,140123.22,,,,,0,,99.99,,M,,M,,*63

$GPGLL,,,,,140123.22,V,N*4F

$GPGSA,A,1,,,,,,,,,,,,,99.99,99.99,99.99,1*2D

$GPGSV,1,1,0,,,,,,,,,,,,,,,,,1*54

$GPRMC,140123.22,V,,,,,,,070721,,,N,V*01

Tracking: 0 Using: 0 Unhealthy: 0

and then after a while it starts to get satellites again:

Seconds since last fix: 188
Searching [|]

NMEA strings:

$GPGGA,140259.22,,,,,0,,99.99,,M,,M,,*6D

$GPGLL,,,,,140259.22,V,N*41

$GPGSA,A,1,,,,,,,,,,,,,99.99,99.99,99.99,1*2D

$GPGSV,1,1,1,17,,,24,1*55

$GPRMC,140259.22,V,,,,,,,070721,,,N,V*0F

Tracking: 1 Using: 0 Unhealthy: 0

something seems to be broken or causing the AGPS injected data to be lost. Can the AGPS data expire and be deleted in a few seconds?

Regards

Noaman

[Håkon Alseth]

Hi,

 

Thank you for posting the NMEA strings, these helps to debug the process.

 

I am sitting indoor close to a window so satellite reception is affect.

 This is highly likely your issue. The reception is not great when you're seated in-door, and windows might have reflective coating that dampens RF.

 

If you look at the GPGSV that you have reported:

 

$GPGSV,4,1,14,1,45,073,,3,04,101,,10,04,346,,13,07,238,,1*6F
$GPGSV,4,2,14,14,63,132,,15,08,267,,17,71,195,,19,44,222,,1*6F
$GPGSV,4,3,14,21,30,051,,22,15,080,,24,31,306,,28,79,168,,1*64

 

And we decode the CN0 (SNR):

https://www.trimble.com/OEM_ReceiverHelp/V4.44/en/NMEA-0183messages_GSV.html

 

You will see that the signal levels are not that high. You would need >= 4 satellites with 30 dB/Hz or higher to be able to track properly.

This is also reflected in the tracking/using statistics that is printed.

 

Kind regards,

Håkon 

[Stratosphere]

Haken, thanks for your response.

I think you missed the point. Bad signal can cause bad SNR but it does not explain why the AGPS data is being dropped. Regardless of where I am, indoors or outdoors, the AGPS data in my view should not get dropped.

Is there a memory problem or bug in the library that it drops that data?

[Håkon Alseth]

Hi,

 

AGPS can not fix bad coverage in this magnitude, but it can kick-start the progress by injecting data for your location (ephemerides, almanac, etc); but the GPS must still receive data from the satellites.

Can you please explain what you mean? What are you expecting that the NMEA should show in this scenario?

 

Kind regards,

Håkon

[Didrik Rokhaug]

Hi, Håkon is on summer vacation, so I have taken over this ticket.

I believe what you are seeing is expected.

The modem is probably still using most of the AGPS data it downloaded, but as it didn't find the satellites (due to bad signal), it no longer reports their expected position.

Best regards,

Didrik