MultiNav Pro+ GNSS - fix rate drops when sensors module is active?

Seen this exact thing before - pretty sure what's happening is the sensors module is competing for SPI/I2C bus time and the GNSS can't service its buffer fast enough, so it misses fix cycles. Try bumping the GNSS to a dedicated bus if your config allows it, or throttle the accel/gyro stream rate just a little and see if the drops get less frequent. Also worth checking if you have DMA enabled for the sensor reads because that helped a ton in my setup.

@Kent Dawson yeah that diagnosis tracks — bus contention is almost certainly your culprit. The inconsistency makes total sense too, because it's going to depend on exactly when the accel/gyro transaction lands relative to the GNSS buffer window, so you'll see it come and go rather than a hard floor.

 

DMA is the move if you haven't enabled it yet — it basically takes the CPU out of the loop for those sensor reads so the GNSS processing doesn't get starved waiting for a blocking transfer to finish. If you're already on DMA and still seeing it, I'd look at your interrupt priority config next, because if the sensor interrupt is preempting the GNSS handler you're in the same boat through a different door.

Omar's on the right track with the bus contention thing - I'd also check if you have any interrupt-driven sensor reads happening because those can absolutely stomp on GNSS buffer servicing at the worst times. In my drone build I ended up just moving the IMU to a dedicated I2C bus and the GNSS fix rate stabilized immediately, hasn't dipped since.

Both of those fixes are solid but also worth double-checking your GNSS module's internal buffer size config - if it's set too small it'll overflow faster under bus contention and you'll see exactly the kind of inconsistent drops you're describing. I had a similar issue and bumping the buffer up bought me enough headroom that even with the IMU competing for bus time the fix rate stayed stable.

All three of those are worth trying but I'd actually start with Jason's buffer tip first since it's the quickest config change and will tell you pretty fast whether that's the culprit before you go rewiring busses - in my experience the default buffer size on the MultiNav GNSS module is pretty conservative and wasn't tuned with simultaneous high-rate IMU streaming in mind. If the drops are still inconsistent after bumping the buffer then yeah, dedicate a bus to the IMU and you should be good.

Gail's right that the buffer tweak is the fastest first step, but in my experience with the MultiNav Pro+ specifically the default GNSS IRQ priority is also pretty low out of the box - worth bumping that up in your config at the same time so sensor interrupts aren't preempting it. Did that alongside the buffer increase and my fix rate basically stopped fluctuating entirely even with the IMU screaming at full rate.