Apple and Google have announced a hugely ambitious — and potentially controversial —contact tracing system designed to help users prevent spreading the novel coronavirus by figuring out who has had contact with infected patients.

While details on the system are still slim — given how early in development the project is, not even Apple and Google have figured everything out here yet — we do know a few things, including that the companies are planning to use Bluetooth Low Energy (Bluetooth LE) radio technology as the core of the system.

As the name suggests, Bluetooth LE is a low-powered alternative to standard Bluetooth technology, with a focus on shorter bursts of connectivity that use less power than a traditional, constantly transmitting Bluetooth connection. A good illustration of the difference is in how Apple’s AirPods work: a short-burst Bluetooth LE connection is used to prompt the initial connect and pairing of the headphones before switching over to regular Bluetooth for streaming the music you want to listen to.

The choice for Bluetooth LE for the contact tracing system is interesting for a couple of reasons. On the surface, there’s a lot about Bluetooth LE that makes it ideal for this sort of application: it’s supported by nearly every single modern smartphone on the market, from the cheapest budget Android handset to the most expensive iPhone or Samsung flagship. And it’s a very flexible specification, one that works with extremely low power levels so as to not impact battery life.

There’s a lot about Bluetooth LE that makes it ideal for this

One of the keys here is Bluetooth LE’s proximity profile (PXP), the core technology that Bluetooth relies on for device locating and tracking purposes. By measuring how much power is received from a Bluetooth radio signal (the RSSI value), we can estimate how far away it is. It’s how AirPods know to pop up the “Would you like to connect” dialogue on your phone that’s nearby and not your roommate’s phone across the room, or how Tile trackers can help you locate your missing keys.

There have also been similar applications of Bluetooth LE technology in this vein with products like iBeacons, hardware transmitters that can track local devices and trigger location-based notifications when you’re close by. While iBeacons are typically used to track customers in retail stores, offer deals, and serve advertising, the basic technology is actually extremely similar to Apple and Google’s proposed system.

It also highlights the flaws, though: to go back to our Tile comparison, anyone who’s used one of the trackers knows that they’re only good for a certain level of precision. Bluetooth can help you find the rough area of your missing object, but discerning the exact physical location isn’t something Bluetooth LE tech excels at. Interference can also be a big issue. The more obstacles and obstructions between devices — like backpacks, pockets, walls, or windows — the worse Bluetooth LE is at accurately tracking something since those obstructions will degrade the radio signal strength used to measure distance.

It’s why Apple is starting to use a wholly different technology (Ultra Wideband radio, or UWB) in its recent iPhone 11 line for short-distance location of other devices. (Apple is also expected to use the technology in its own upcoming AirTag trackers for similar purposes.) The issue is that while other technologies may be more suited to the task at hand, very few phones have UWB radios. Meanwhile, everything has Bluetooth, making it the only real option for local device-to-device communication like this that doesn’t rely on an external network like Wi-Fi, GPS, or cellular data.

Achieving that level of precision and range is what Apple and Google are presumably going to be working on over the coming weeks in order for the system to be useful. Notifying everyone who’s been in contact with someone who is infected with the novel coronavirus within 100 feet won’t be useful information, but only alerting you after your phones are just inches apart won’t be helpful either.

Achieving that level of precision and range is the issue to solve

That said, some of Bluetooth LE’s limitations when it comes to range may actually be advantageous when it comes specifically to helping warn about exposure to the virus. Given that the virus can’t travel through walls (one of the reasons barriers like personal protective equipment are so crucial in preventing transmission), the fact that Bluetooth operates the best in similar open-air situations means that — in theory — it’ll be able to help notify users when they’re most at risk.

Bluetooth also — at least on paper — offers a better solution than QR codes, which have already been used in countries like South Korea since Bluetooth works at a farther range than the optical scanners needed for QR or barcode technology. Given that distancing is still a key part of preventing the spread of the virus, technology that doesn’t require users to get close to patients (or their phones) is a plus.