Not able to decode NMEA messages

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

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
does not help.

The initial baud rate is 115200.

Is there somebody able to help me?

Kind regards


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.



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

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.



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?



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

You have any other idea what we can do?



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.




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)
Step 2: Configures the module sentence type NMEA mode
Step 3: PMTK104 Full Cold Start

The response to PGCMD,218,1 commands is
-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)