BLE Tracker for specific case: Hearing aid using RPA address rotation

[yerpj]

I would like to develop a handheld Espruino device locator (estimator, indeed) capable of telling me, using RSSI, how far I am from BLE devices. Those devices are my children's hearing aids (2 children, 4 devices in total), which are using address rotation RPA.
I read some posts related to RPA, especially for Bangle watches but I was unable to fully understand what's implemented in Espruino, what is usable or not.

I understand that my handheld locator needs to first pair/bond with the hearing aids, to obtain their IRK. In practice, I don't know how to retrieve the IRK (I didn't see anything in the doc) and how to use it to recover the device's unique address. Is it done entirely in the SoftDevice ?
If anyone could share some experience using RPA/IRK, I would be happy to read about it :-)

[gfwilliams]

As I understand it, address rotation in Bangle.js is more for the Bangle itself - so making sure that it rotates its MAC addresses.

I could be wrong but I think using it to identify devices that are advertising could be tricky.

However do you want to be able to find specific hearing aids, or just find whatever is in range? Because I believe normal bluetooth scanning should be able to determine that there's a hearing aid nearby - it's just that you won't know which one because of the address randomisation?

[yerpj]

Unfortunately, the hearing aid themselves advertise with metadata only for about 2-3 minutes after power-up, after that, a new random address is given and advertisement payload is more or less empty, which prevents me from being able to identify them as soon as address has changed. I did try to identify them based on their (hopefully) kinda-unique adv interval (~215ms) but in reality it is not that stable and can go from 200 up to 300ms, which prevents me from using this technique to isolate them from the rest of advertising data. Only solution IMO is to deal with RPA....

[enaon]

I understand that my handheld locator needs to first pair/bond with the hearing aids, to obtain their IRK. In practice, I don't know how to retrieve the IRK (I didn't see anything in the doc) and how to use it to recover the device's unique address.

I am getting the IRK for google tags, so that I can identify them by name. It works fine. You need to store the resolved address, and the check device you found against it. Have a look at NRF.resolveAddress(id);

I would like to develop a handheld Espruino device locator (estimator, indeed) capable of telling me, using RSSI, how far I am from BLE devices.

I have an app like this, you can very easily change it to support you children's hearing aids, but it is is based on my eucWatch system, not pure bangle.

It does run on stock bangle, have a look in this discussion, if you care I can explain how to install it.
https://github.com/orgs/espruino/discussions/7887

[yerpj]

That's interesting, especially the Google Find My case, I will give it a try soon, thank you!