Hi,

First of all, I am quite new to INAV, but I have about 7 years of building experience with Pixhawk and BF. 

So, because of many reasons, I decided to build a 4" "LR" quadcopter to be my mini camera cruiser. The selected components:

FCU: MATEK F722 mini 

FW: INAV 2.6

ESC: iFlight SucceX Mini 40A ESC (20x20mm, BL_Heli32, DShot1200 capable, with ESC telemtry output)

GPS/Compass: Matek GPS & Compass Module M8Q-5883 (UART+I2C)

Optical flow/lidar: Matek Micro Optic Flow and Lidar Sensor (UART)

Camera: Hawkeye Firefly 4K split mini 19*19mm camera + DVR

VTx: Rush Tank mini 0-800mW

Rx: Jumper R1+ (Sbus + S.port)

Port setup in INAV:

UART1 : - (MSP active)

UART2: Serial RX (Sbus) (USB/Bat powered)

UART3: MSP (Optical flow+lidar) (Bat powered)

UART4: GPS (115200) (USB/Bat powered)

UART 6: ESC telemetry (115200)

Softserial1: S.port

Frame:  Explorer LR4 178mm 4inch Carbon Fiber Micro Long Range Frame Kits Fits 16/20/25.5mm

Motors: GEPRC 1404 KV 2750

Battery: 4S 2500MAh (4*18650)

Since the frame is small, the build is very packed... I chose to mount the FCU over the ESC as a stack to the middle of the frame, the camera board over the VTx as a stack to the back, and the Rx under the camera in the front. The GPS was planned to be mounted to the 3D printed frame at the back, under the VTx antenna.

After soldering everything together I tried to powered it up first from USB, and I set up the ports in the INAV. Till now, everything was looking good. OK, switch to battery, even the Lidar-opt flow was blue in the status bar, so far so good. 

But, during the setup process ( calibration of Acc, compass, etc), I noticed 3 things:

1., the GPS did not find 3D fix (or any fix...) during about half an hour...

2., the compass direction was drifting randomly

3., there was no ESC data received (no V/current sensor data if it was set to "ESC", no debug ERPM on motor runs)

So, I have read a separate thread about the compass issues, so forget about that right now...

About the ESC telemetry issue: I watched the video of Paweł Spychalski about setting up the ESC telemetry in INAV (thank you Pawel for your great videos!!!), and did the setup accordingly (UART 6 set to ESC telem, motor protocol DShot600), but no luck. If I set up the voltage and current sensor to ADC, I get reasonable reading on the voltage, current is strange, I need to set the divider to ~30 and the offset to -5 to have any non-zero reading (it is more or less comparable to the motor's load, but needs to be tuned with my power meter). If I set these sensors to "ESC", both readings show 0.00. Also, no data on the Sensors screen at the Debug set to ERPM. I do not know, what I did wrong. I tried a strange thing, I connected the TX lead from the ECS to the Curr pin on the FCU, and I had "reading" at the current tab, (stable 65.4 A with default divider settings), so there is signal output from the ESC, but no reception in the FCU...
[url=https://www.youtube.com/channel/UCmX3OXToMBKTppgRskDzpsw][/url]

About the GPS: so, in short, the GPS was not able to get any fix during the build. The FCU was reading the messages, but no fix (0 Sats found). I disconnected the module and connected it to the PC via FTDI and tried it in the u-blox center software. Miraculously, the module was up and found the first sat in 20 seconds, and had 2D fix in one minute, and 3D fix right after. It was receiving steadily about 10-12 sats in the room, and it was using about the half of them for navigation. The default baudrate was set to 9600, so I set it to 115200, because INAV was not able to communicate with it on 9600... Get back to the craft, no luck... 0 sat found in 10 minutes... Maybe it is too close to the VTx-camera board stack... I made a long (~15cm) cable and started again... So, I found out by connecting the GPS to the PC and powering up the craft alone, that there is no GPS reception next to the VTx stack, but, about 6-8cms away, alone the GPS is getting a 3D fix. Back to the craft, same position, no GPS reception... More farther from the craft, nothing...

Last chance I downgraded the FW to INAV 2.5.1 and 2.5.2. Result: the GPS was firing up and found 3D fix about 6cm away from the powered up craft in 2 minutes. Blue LED blinking, INAV's GPS window showed the correct coordinates, height, sat count was 4-6 in the room. FrSky telemetry showed the same data in my Jumper TX. Problem: FW2.5 does not recognize the barometer in my FCU. I upgraded to the latest (and previously used...) 2.6 FW (during the process, the GPS was in 3D fix with LED blinking...). Result: no GPS fix... When I disabled the "ground assistance" thing, it started to work, but it is quite uncertain... After powering down the whole thing, and powering up it again this morning, again, no fix was found, and the FCU' main LED was blinkig fast showing hardware health issue...

So, I am out of ideas... I can live without compass, but not without the GPS in a long range mini quad... The ESC telemetry would be good for RPM filtering to make the whole thing more smooth thus energy efficient, so I would be pleased if we can find the solution!

Sorry for the long post I wanted to give all the info I could.

Thanks for any help in advance!

I'm surprised no one responded to this. There is a possible issue with external barometers ie bmp280 with the Matek gps compass combo in Inav. Sometimes you can have luck if you try adding the barometer first and then the mag. I don't know if it's an address problem or what, but it has driven me crazy in the past and I know other people that have had the same problem. They can get one or the other working but not both together. Strange problem.