GPS telemetry

[ZZ-Cat]

Overview

After my disastrous first attempt at doing telemetry, I have decided to split adding full telemetry support up into smaller & easier to manage modules, based on the types of telemetry being added. These are:

• Attitude/Orientation
• Altitude
• Battery
• Current/Power Consumption
• GPS

Each module will have its own Issue, Development Branch & Pull Request associated with it, with the end goal of CRSF for Arduino eventually having full telemetry support. Over time, I will make my way through each module, starting with GPS (because this is what I already have on-hand).

What's needed

This Issue is to remind myself to add GPS telemetry to CRSF for Arduino.

Therefore, I need to write code that reads GPS NMEA 0183 sentences & converts that into meaningful data that the CRSF protocol can work with. I will need to write a new GPS hardware driver as well.

The new driver will have similar features to my previous attempt, plus a few extra goodies:

• Selectable update rates up to 10Hz (subject to GPS hardware availability)
  • I have an idea that expands on this, where the update rate is dynamic, based on whether-or-not a connected Receiver is sending an Armed/Disarmed field in the RC Channels packet. For ExpressLRS, this is Channel 5 or "AUX1".
  • Additionally, I would like to trial a low-power mode that kicks in when that same Channel 5 value is "Disarmed", & the GPS goes to its full update rates & full power when Channel 5 is "Armed".
  • Both of these two things may not happen, at this stage. Because I don't see how much of a benefit this functionality could bring to CRSF for Arduino. So, no guarantees, here.
• Automatic baud rate negotiation (subject to the selected update rate)
  • Previously, I had this setup based on a pre-set baud rate (EG 115200) & the microcontroller would poll the GPS at incrementing baud rates until valid data came back & the baud rate matched that of the set baud rate. I may want to revise this, & have the baud rate set automatically based on the chosen set of NMEA sentences coming in from the GPS & the chosen update rate, as both of these things directly influence the final baud rate.
• DMA (subject to hardware capabilities)
  • This will be enabled globally at compile time, by default.
• Hardware Serial
  • For SAMD21 & SAMD51 microcontrollers, I will be using a hardware SERCOMx port & I will do my best to put it adjacent to the pins where you connect your ExpressLRS receiver. Some smaller development boards may not allow this.
  • Another reason why older generation boards like the Arduino UNO R3 aren't compatible with CRSF for Arduino.

Once I have this working, you will be able to view Location, Ground Speed, Heading, Altitude & Number of Satellites in View on your EdgeTX controller.

Additional

Gear list

The gear that I am using for this is as follows:

• Controller: RadioMaster TX16S Max Edition w/EdgeTX version 2.9.0 Nightly
  • Lua script: iNav telemetry widget
• Transmitter Module: RadioMaster Ranger w/ExpressLRS version 3.2.1
• Receiver: RadioMaster RP3 diversity w/ExpressLRS version 3.2.1
• Development board: Adafruit Metro M4 Express
• GPS: Adafruit Ultimate GPS Breakout V3
  • Yea, yea. I know it's outdated. But, it's what I currently have on-hand & getting things shipped to New Zealand takes way too long.

Also...

Despite what #3 & #5 will have you believe, I somehow managed to totally obliterate all existence of that code & I managed to do a "clean commit". This means that no code relating to any sensor hardware drivers exist, despite things like Pull Requests & Commits alike referencing them being added.

Still to this day, I have no idea how I even managed to do that, but it totally yeeted all of my work regarding telemetry. Because of that, I am having to start this portion of CRSF for Arduino from scratch all over again. Yay. =-/.-= ...& it has taken me this long to get back into it & convince myself that it's definitely worth doing. Especially now that I know that other people are benefiting from my project.

[ZZ-Cat]

I have decided against writing my own hardware GPS driver for this. There is no need for me to reinvent the wheel, & create unnecessary work for myself.

To allow for greater flexibility, I will bring up all relevant GPS telemetry fields to the API. That way, anyone that wants to use CRSF for Arduino, they can provide their own GPS drivers & parsing libraries, then pass that into CRSF for Arduino in order to use GPS telemetry.

What I will do is, I will include an example firmware that demonstrates how a third party GPS driver can be used to provide data for CRSF for Arduino's GPS telemetry.

This will be the format with other telemetry modules going forward.