Links

Option 1- Bluetooth

Frsky Bluetooth Trainer
You can use this Head Tracker directly with radios that support a Bluetooth Trainer Module. There are 3 choices in Bluetooth modules that will work with the Head Tracker.
Note: Option 3 - Bridged also transmits over Bluetooth but doesn't connect to the radio using the same interface. It uses PPM or SBUS. Please see Option 3 - Bridged for more information.

1) FrSky - PARA Bluetooth

This is FrSky's latest Bluetooth module. It is shipped with their new radios. It is based on the Texas Instruments CC2650. (As best we can tell, the part number is laser etched off)

2) FrSky - Bluetooth Trainer

This is the older version of the Bluetooth module available. It is based on the Texas Instruments CC2540 IC.
FrSky Transmitter Bluetooth Module For Horus X10, X10S & X12S
English
Both PARA and older "Bluetooth Trainer" will work.

3) BTWifiModule - Open Source Option - ESP Module

To overcome the requirement to buy a FrSky module I have started this side project as well. It uses an easy to find and cheap ESP module to work with the existing Bluetooth interface. There are future plans for it as well. It is still a work in progress but will work for the Head Tracker at the time of this writing.

Radio Firmware OpenTX or EdgeTX

To use any of the above modules you will need the Bluetooth option compiled into your radios firmware. You can check if you have the Bluetooth option already by going to the Radio Hardware Page, if you see a Bluetooth entry you already have it enabled.

OpenTX

To enable Bluetooth in OpenTX you need to select the Bluetooth option in companion and flash the new firmware.

EdgeTX

To enable Bluetooth in EdgeTX you will need to compile it with the -DBLUETOOTH=y option.
There are pre-compiled versions available as well for the major releases. Search the Issues to find them.

How to Use

Head Tracker Setup

In the GUI under the Bluetooth Tab, select Wireless Transmitter (Head). Also see Bluetooth in the GUI documentation for extra information. Make a note of the Address shown here (It will be used below)

OpenTX / EdgeTX Setup

First you need to enable Bluetooth as a Trainer. Long press System button. Then go to Hardware tab.
The color Open/EdgeTX screens are very similar setup.

Under Hardware Tab->Bluetooth choose Trainer

The Name field will set what the Bluetooth module is advertising as. It is not important what it is set to.

Under Model Setup -> Trainer choose Mode Master/Bluetooth

A button will appear below Master/Bluetooth that shows [Discover] click this button to find all the Bluetooth devices. Pick the Nano 33 BLE from the list using the address noted from above. The Blue light on the Nano 33 BLE will come on when a connection has been made.
If you only see [Init] below Master/Bluetooth the Bluetooth module wasn't discovered.