Track and Trace Apps— How do they work?

Alexander Miller
4 min readSep 29, 2020

As of last week, it seems that every location that you go to wants you to check in with the new NHS track and trace app.

Having downloaded the app, and being a curious sole, I wanted to know a bit more about how it worked. Especially now that I might get a notification that I need to self isolate due to being in contact with an anonymous covid infected person, and what’s more if I don’t follow this advice, there could be a significant fine.

Will I get notified if I have walked past someone that had covid, even if I passed them in the street some meters away?

~ this could end up becoming a real pain if I get notified when I was not at real risk of catching the virus from them.

Is bluetooth and the app capable of measuring the two key factors in aerosol transmission of the virus?

The current medical position on this is that you would need to be within 2 meters of an infected person for at least 15 minutes, and the NHS has factored this in to their design using an algorithm developed by NHSx which measures the proximity to the other persons phone and categorises these as Close, Medium and Far, as well as the time spent — but how does that work?

Photo by Arthur Edelman on Unsplash

So how can a phone know the distance it is from another phone, it uses a bluetooth feature called bluetooth beaconing rather than GPS. There are two Bluetooth Low Energy chip standards out there in phones BLE 4 and BLE 5. BLE 4 has a range of 100m and 5 upto 400m. However the signal distances are affected by environmental factors, location of the phone around your body and the type of phone.

There are two parts in the Bluetooth radio signal that help with this there is a transmit signal strength which is broadcast in the packet and the received strength measured in db, using a calculation to find the difference between these it can classify in to the Close, Medium and Far categories.

This is not an exact science, as well as the environmental conditions for the signal — whether you are indoors or outdoors — EMF interference and antenna quality can play a part in reducing the signal strength measured and estimated distance from the other person.

So what about the time you have been exposed for and cumulative exposure (humans move around) so according to apple the API that is used to feed data to the app it reads bluetooth broadcasts every 3.6 minutes to check what phones are around each of these instances are logged, the algorithm calculates the time you were near the other person based on the logged data.

So when will I be notified i’ve been in contact with a Covid carrier?

When a user of the NHS Track & Trace app confirms that they have Covid the phone uploads the last 14 days exposure log to the server and it then shares the information to other people you have come in contact with, your phone receives these updates of infected users through the app based on this information it works out your exposure risk value for the matches.

Notifications, are based on an algorithm that works out your Exposure Risk Value based on a variety of factors; not just the proximity and time to the identified person but also how the infected user has identified they have covid ie self identified, as a result of a test or by clinical diagnosis and also how long ago you were in contact with them.

So what happens when you check in to a location — you don’t have the option to check out. will you be notified about anyone that has been there for that day with covid?

No - this feature is just for if the location has been at the centre of an outbreak ie multiple people at the same location have covid then it may be deemed to be an infected location in which case there would be a notification to all people who have visited within a certain time span.

Why you should download the app

Image OpenCovid Trace

Whilst not perfect its a good solution using existing technologies and available resources to help us as a society to manage and hopefully reduce the impact of COVID 19 on the population at large.

For a good general description of what the app is about have a look at this recent BBC article

Technical Overview of google / Apple contact tracing app — Article with more info on the topic including Privicy concerns and the centralised vs decentralised debate.

I’m no expert in any of these technologies so if you want to find out more I would recommend going to the sources linked in the article or please comment below if you have and questions or corrections.



Alexander Miller

I’m a UX Designer with a background in Business Process Analysis. A Lean Six Sigma Blackbelt, Agile coach & ScrumMaster. Im a big fan of self organising teams.