Simple BLE Broadcast Message

I'll have to use an ANT dongle with the rapsberry pi, really whatever can pick up a signal broadcasted from the watch.

That's a good idea. Is there a way I can program a simple app to display that ANT ID? I intend to make the app as easy to use as possible.

Unfortunately I don't think so. It's even worse than that: I don't think it's own ANT ID is displayed anywhere in the watch. Not even in the System > About menu (where you do see it's own MAC addresses for BT for example).

Now it's usually not a problem, when you're alone, but at a race where there will be many people enabling HR broadcasting, that'll be a race condition (pun intended) between the different watches. Because the only thing I can think of is to ask them one by one to enable broadcasting next to your device, and then your device will display the ANT ID of the "closest ANT HR monitor", but if there are 3 people close to it then it can be a bit challenging (you'll also need to display the HR next to the ANT ID and use that as a "pin code" to make sure you're looking at the ANT ID of the person who you are talking to...)

For watches you could build a "Faraday cage" with your device in it and ask them to put their arm in it (maybe this style: https://en.wikipedia.org/wiki/Bocca_della_Verit%C3%A0 :) but for people running with a chest strap it'll be harder (on the other hand they'll be able to look at the ANT ID in their watch's menu and tell you)

Keep us updated, I like this project!

Garmin has some info on how to set up an ANT stick with Linux, so I'll probably start there.

As far as I remember it's plug and play. But I don't see why would you need it? The HR broadcast isn't gonna work? It works on some non Garmin watches too.

If you make a cup app to broadcast some data then it'll only work on Garmin and people will need to install it from the store

Since this is a prototype, I plan on supporting a max of 8 runners, 1 for each track lane. The idea is that anyone with a Garmin can download my app from the IQ store and save their name to the app as well as lane number or race ID. That way the system can read each runner's name. I've transitioned the project from a race timing system to more of a workout and time trial tracker for coaches managing athletes. If there is little interference, however I can increase the number of runners. I want to have each runner ID act like a seed for determining the channel to reduce interference, so each runner ID will have its own channel that the Raspberry Pi can scan for. Before I realized BLE doesn't work, I was initially planning on having each watch have a UUID with the same ending bytes but vary in the starting bytes (Encoding or distinguishing different runners) so the rpi can scan for all UUIDs only in the workout or race. Now, I need to do the same but with ANT. ANT+ is not viable as it only has a package send rate of 4Hz, whereas ANT has a 200Hz data send rate.
The reason I don't want to use the HR broadcasting is because it would be annoying to have to track each athlete with some obscure number. The entire point of my project is to ease coaching management and give both the coaches and athletes immediate results, so it would be ideal if each user could input a username (probably 8 chars long), then have that broadcast over ANT, then the system can record and perform other metrics on it.

Also, thank you! I'll keep sharing updates. I have a website under development that's going to show off the project as well. I'll post it later.

Ideally even that would not be necessary. The "pairing" of a watch/ANT ID with the lane could be done automatically, kind of like with a touch of an NFC device (the watch) to the running block before the start. Your computer already should know the names based on the lane. Though you could even have a small button on the block for pairing. They would need to push it for "pairing" and then it would connect to the nearest ANT ID.

Regarding the frequency: I'm not sure every device is able to do 200Hz, and certainly can be problematic if they also run the fastest of their life :) so you'll have just a few messages to capture.

I think you'll need to look at GenericChannel. Technically I haven't used it for sending data, but actually I kind of did, because for the HR strap you can send configuration to the strap to change how it behaves, which does work for me. But you still have the problem that you need to establish the connection. Though this might be doable if you do the "other way": the app/data field will need to download the configuration from a web service. In the configuration you can send the ANT ID of each lane, and the user will set the lane they run in, and then you can connect to the ANT ID of the lane. I think you'll have to have a device in each lane, otherwise you'll have different problems, for example that the further you are from the device the worse the connection will be.

Curious: as a runner why would anyone bother doing all this? Downloading an app, go through the setup, probably do some pairing with the lane. If I ran in a race probably I wouldn't care less.

My current design is going to only support forerunner models 245 and better. I plan on using GenericChannel and sendBroadcast() to broadcast the runner's proximity, without any pairing, in fact, I want it to have zero pairing so setup can go faster. I have also published my code to GitHub (what I have so far) https://github.com/kadenjones1212/PTC. I currently have the ability to edit your runner username or race name in the garmin connect app, and the ability to display that username, but I need a way to always broadcast whenever the data field is open.

Then everything need to be packed into 1 message that you probably burst.

Later IMHO you'll want to do some optimization. The watch could look for a signal from the "gate" and only do the burst when it's close, to save battery. Imagine running 10k and bursting 100 times a second for 30 minutes even though you only need it to burst for 10m before and 10m after the gate.