Not able to decode NMEA messages

IoT Modules XS Series (GNSS)
1

Dear All,

we have some GPS chips form GlobalTop on our boards, that do not send NMEA messages in text mode.
The response to $PMTK605 is
$PMTK705,AXN_5.1.1_3333_17081700,8518,FireFly1,1.0*74

The messages that are received seem to be binary. But they start with 0xde 0xae in contrast to the binary mode description in “AirPrime GMM-G3, XA11xx and XM11xx Software User Guide Rev 3”. In “Software User Guide AirPrime XM/XA Series” there is no reference of the binary format.

Can someone help me to decode this format?

Trying to change the sentence type with
$PGCMD,380,7
$PGCMD,218,1
$PMTK104
does not help.

The initial baud rate is 115200.

Is there somebody able to help me?

Kind regards
Bernhard

2

@bseiz,

Those are pretty old units if they identify themselves as Firefly’s. Couple of questions.

  • Is the response to the 218 command ‘No_Change_data’?
  • Where these units delivered from the factory configured as binary output by default?
  • Have you tried setting 218 and then resetting the unit by pulling the power? Not necessarily recommended but just for a test.

Regards

Matt

3

Dear Matt,

thanky you very much for your reply!

Yes, I am aware that these are very old chips. Therefore I appreciate any type
of information and help.

We have an external circuit board production and they had difficulties to get
these chips. I can’t say anything about the production and reasons of the chips
firmware and their configuration.

The response to PGCMD,218,1 commands is
$PGACK,001,-1*73

I quess -1 means rejected or error?

If having a look to the binary data I can not see a correspondence to the binary
format discribed in “AirPrime GMM-G3, XA11xx and XM11xx Software User Guide”
pp 76, 77.

Bernhard

4

@bseiz,

Its obviously not documented but I think it is probably a good assumption that -1 is an error.

Can you confirm the full 218 string you are sending to the unit i.e. with the $ and the checksum?

Regards

Matt

5

Yes teh $ * and checksum is sent when issuing the command.

You have any other idea what we can do?

Bernhard

6

@bseiz,

Not really I send the command to my unit which is not as old as yours (but that should not make any difference) and it comes back with the No_change’ response and is outputting NMEA correctly. I am also confused by the non verbose response (so the -1 vs the Error/No_change) as it is not referred to anywhere.

Need to check with some other people.

Regards

Matt

7

Hi,

If You want to send NMEA messages in NMEA mode, You can try to change the sentence type as below:
Step 1: Disable; switches to NMEA mode. Please refer to AirPrime_GMM_G3_XA11xx_and_XM11xx_GNSS_Aiding_Application_Note_Rev2_2 document for detail)
$PMTK253,02B
Step 2: Configures the module sentence type NMEA mode
$PGCMD,218,157
Step 3: PMTK104 Full Cold Start
$PMTK104*37

The response to PGCMD,218,1 commands is
$PGACK,001,-1*73
-1 is mode switch fail ( Please refer to GTop One Sentence Specification-v13 document)

Please find the document as in attachment for your detail.AirPrime_GMM_G3_XA11xx_and_XM11xx_GNSS_Aiding_Application_Note_Rev2_2.pdf (459.9 KB) AirPrime_GMM_G3_XA11xx_and_XM11xx_Software_User_Guide_Rev4_0.pdf (717.0 KB) GTop One Sentence Specification-v13.pdf (206.6 KB)

Thanks,