Design for a Private Covid19 Tracing App

The features of a private, effective, tracing app are:

• The time and location of a phone remains on the individual’s mobile phone and is never transmitted elsewhere except if the individual permits it.
• Phones are identified by a group code.
• Phones put themselves into a group.
• An individual has to do nothing except download the app and start it going.
• Optionally if a phone is detected without the app operational and is in your vicinity for any length of time your phone alerts you to the fact.

Starting the App Operation

1. Download and start the app.
2. Give permission for the app to access location, blue tooth, and wifi
3. Allow/disallow alerts of nearby phones without the app
4. The app accesses a website and gets an identifier for the post/zip code where the phone is currently located.
5. The app checks the website to see if there are messages for the members of the same post/zip code. Process any messages that give the location history of an infected person.

Keeping track of location, time and contacts

1. While ever the phone is on and the app is running in the background it guesses where it is located and records it. It looks for other phones in the vicinity that are broadcasting their location with a copy of the same app.
2. The app takes those locations and estimates the distance of the other phone from the strength of the signal.
3. The app recalculates where it thinks it is from the other phone locations and estimated distances. The other phones do the same and they soon converge on a ‘solution’.
4. Every app does this and records its location over time. It does this at periodic intervals. Its rate of calculation varies dynamically depending on how many phones are near and if they are changing their position. It records the time it is “near” other phones and the post/zip codes to which the phones belong.

Tracing contacts

If a person is diagnosed with Covid19 they send their position at different times when they were near another broadcasting phone. The message goes to the broadcasting post/zip code website. The app on other people’s phones periodically checks their post/zip code website to see if they were ever near a Covid19 carrier. If they are near a Covid19 carrier they tell their app and it sends their information to a Tracing Team.

Alerting for phones without the app

If a phone detects, for a period of two minutes, that there is another phone that does not have the app on it then all the phones in the area that have the app are notified and an alert raised on those phones. Those with the app may choose to move to a safer area or towards people who have the app alert.

Some venues may decide to restrict entrance only to people who have the app installed.