Just a quick comment. (1) As you said, your test setup, including Mode 2, are all OK except coming to Mode 3 (AT+CGPS=1,3), my first wild guess is that it is not USB/UART problem, because it is all simple serial UART interface, at which level only the level at AT commands are interpreted. (2) If I was doing troubleshooting, I would try the following: (a) Use Windows 7/10 USB to serial adaptor cable, and not Widnows USB to SIM7600 USB, because SIM firmware might not entertain all USB to SUB stuff, / to continue, ...

(b) If you are using Rpi python, you might like to compare and contrast my basic SIM7600 test program with sample outputs: (3) Reference: Rpi3B+ Python ... SIM7600 Problem, Asked 1 year, 5 months ago, Viewed 642 times: raspberrypi.stackexchange.com/questions/113992/….

(3) Ah, you remind me one thing: I played with Neo-6/7/8 GPS Modules before trying SIM7600 GPS. When I first tried SIM7600 GPS, my first feeling was that its GPS hardware is a bit outdated, comparing to Neo. So I decided to switch back to Neo, with their very user friendly Windows based GUI interface. (4) References: Rpi Neo-6M / Neo-8M GPS Module Setup / Fix / Update Problems (a) electronics.stackexchange.com/questions/557417/… / to continue, ...

(b) raspberrypi.stackexchange.com/questions/113544/…. So my suggestion is to separately test Neo6/7/8/9M first, then come back to SIM7600 with integrated GPS hardware.

(4) Ah, I also remember another thing. SIM800 "autobaud" function actually did not work as specified in their user manual. And as the saying goes: "there isn't only one cockroach in the kitchen", I am not surprised that your problem is another SIM hardware/firmware bug.

@tlfong01 Thank you very much for your replies! I don't think it's necessarily a USB/serial communication issue (but tell me if I'm wrong), as all other GPS modes (1 and 2) both work via USB with the "AT Command Tester for Simcom Modules" app, as well as via UART with my Arduino and the serial interface. Given that I am able to relatively quickly obtain GPS coordinates with mode 1 (AT+CGPS=1,1), I imagine that my standalone GPS portion is alright, would you agree? As for autobaud, I made sure that it has been fixed on 115200 in both environments that I tested.

@tlfong01 What other possibilities do you think I should consider as to why only mode 3 returns 0, indicating that the GPS is now turned off? Please let me know if I misunderstood any advice you gave me. Thanks in advance!

Thank your for your confirmation and advice. (5) I am glad to hear that the "autobaud" thing seems to be fixed. (6) I agreed with all you said in your second last comment. I now think that I might have over worried on the USB to USB interface, and suggested you to use UART to USB instead. Please ignore my earlier suggestions.

(7) I have the feeling that you have almost thoroughly tested all the rest, except the Mode 3 thing. So now I think it is not a good idea to go to test using Neo7/8/9M, because even Neo tests OK, you still need go back the SIM7600 to solve the original problem.

(9) Ah, it is bed time for me. So good luck and see your tomorrow. Cheers.

Hi @tlfong01, thanks for the reply! Your document that you shared does use the same commands I am using, as seen on PDF page 36, section 3.2.1 which discusses A-GPS with mode 3. Another manual with maybe more details can be found here: waveshare.net/w/upload/6/68/…. Let me know if you come up with any other ideas, and thanks again for the replies!

(10) OK. So we should be using the same AT command set. (11) I am reading Section 2 of the App note V1.00 I referred earlier. (12) This Section 2 is on "A-GPS Quick Start – Setting" with two sections "2.1. Set A-GPS Server Address", and "2.2. Set Certificate". I did not go that far when I played with SIM7600 because, as I said, I found the GPS module in SIM7600 a bit old, comparing to Neo-8M, which is Windows GUI is very newbie friendly, and I don't remember I needed to set manually,

(not very sure, because it was some two year ago.) (13) Anyway, I am curious to know if you did try these sections: "2.1. Set A-GPS Server Address", and "2.2. Set Certificate"? (14) You seem to have discussed your problem in another forum. Perhaps you might like to link to your previous forum discussion, so I have more background info and how you have been setting your A-GPS server and the Certificate thing. (15) No hurry, I am going to bed again, see you tomorrow.

@tlfong01 Thanks for the reply! I've... definitely asked this in a few places, simply because it seems like no one has successfully used mode 3. :) So far, zero responses except for yours, so I appreciate that! To answer your question though, I have indeed set it up that way before, both with and without using a certificate as per section 2.1 and 2.2.

My original post was on a Raspberry Pi forum, where I follow-up and say that mode 2 does indeed work with the Google SUPL server, but not with mode 3 (mode 2 is very inaccurate, and unfortunately not exactly what I need). Here is the link: forums.raspberrypi.com/viewtopic.php?p=1950452#p1950452