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Banning location data from contact tracing apps makes them less effective and people less safe...

Banning location data from contact tracing apps makes them less effective and people less safe - here’s what we should be doing

Apple and Google have announced that the technology that they are making available to fight COVID-19 will ban the use of location data in any contact tracing apps that are built on their platform. We think this is surprising, counterintuitive and will possibly be ineffective - here’s why:

Apple and Google suggest that they are doing it because of privacy fears

The success of contact tracing apps depends on a sufficient number of people having them installed, which in turn depends on people trusting that their data is safe and won’t be abused by those who hold it. This is right and important - just as people should feel safe when being vaccinated against a disease, so they should equally feel safe when using a contact tracing app to quell an epidemic for the greater good. It goes without saying that location data and privacy should be well protected.

But by denying the ability to use of location data, Apple and Google are reducing the efficacy of these apps

Effective contact tracing depends on location data - while the proposed Bluetooth-based model helps to identify the moments when two people are near to each other, that is only one part of contact tracing. Another important aspect in the case of COVID-19 is to identify when someone touches a surface that was recently touched by an infected person - as the World Health Organisation has stated, transmission of the COVID-19 virus can occur by direct contact with infected people and indirect contact with surfaces in the immediate environment or with objects used on the infected person (our emphasis). The virus is known to survive on a variety of surfaces for many hours, and in some circumstances days.

Contact tracing needs good people and good data

South Korea showed the way in mobilising an army of contact tracers who would interview potential carriers to identify all of the places that they had been, then track down others who may have also been in the same location by looking at CCTV, card payments, interviews, and so on.

The digital augmentation of this therefore has two key parts to it: use smartphone proximity apps to catch as many proximity events as automagically as possible, AND provide traditional contact tracers with a detailed breakdown of all the times and locations of other potential exposures too. It is obvious that if only 10% of people go on to use contact tracing apps, then the vast majority of the potential transmissions from a given smartphone-app-enabled spreader will not be recorded in the Bluetooth records, but can still be tracked down by traditional contact tracing methods using the smartphone location histories as a guide.

The real reason Apple and Google are not allowing location data MAY be because current location data is simply not accurate enough

While we like to believe that the blue dot we see on our smartphone maps is an accurate reflection of where we are, the truth is that it can at times be out by over 100m, and may not correctly estimate which floor of a multi-story building you are on.

The biggest challenge for contact tracing is knowing that smartphone location estimates can be overconfident and wrong. The size of the blue dot on your map shows the uncertainty in the position fix but at times this dot can be suggesting a 5 metre confidence while actually being 50m away from truth. This could result in contact tracers interviewing completely the wrong shopkeepers, or restaurant owners, or even forcing thousands of people into quarantine who were never anywhere near a potentially infected surface.

What Apple and Google should do now

Despite these inaccuracies, access to this data with a rich understanding of its limitations and likely inaccuracies would still be extremely powerful in helping to trace potential contacts and stem the rate of infections.

We recommend that users of the apps themselves should be able to choose what they want to enable and disable. Rather than preventing contact tracing apps from accessing location at all, Apple and Google should mandate these features are selectable in the app itself and defaulted to “off”.

There are plenty of workarounds already available

Denying access to location for contact tracing apps makes no sense when so many other apps on the same phone use location data without restriction while the contact tracing apps are running. A simple look at one of our staffer’s phones revealed 35% of the apps on the phone wanted access to location data including a file management app, the Amazon TV controller and the BBC News app.

In reality there are so many workarounds that might be thought of - a contact tracing app just asking you to share your strava ID for example, or showing you how to upload your Google Location History traces - that it makes much more sense to just provide contact tracers with the data they need directly rather than waste precious time.

The apps could also cross-check GPS position fixes with WiFi position fixes to check that they match where both are available, in order to provide more robust location estimates. This will help a little with removing overconfident position fixes but won’t solve the problem completely

What we should do next

Solving the problem of inaccurate and unreliable GPS data is a matter of solving the problem of signals bouncing between buildings, and being blocked by people and objects. With a combination of S-GPS, a technology that we have developed that can filter out reflected GPS signals to increase the accuracy of GPS by a factor of 10, alongside a human motion modelling tool such as D-Tail, we can get much more accurate understanding of where people have been, indoors and outdoors, and even confirm whether they have been just inside or just outside - useful information for contact tracers!

What’s clear is that the key to our short term goals of getting out of lockdown AND our long term goal of building resilience for the next epidemic will be to have the best possible data, used in the most secure way, for the benefit of entire societies.

We believe that responsible use of the right location data is a critical part of that equation.