Our objective with this post is to compile an objective analysis of the relative strengths and weaknesses of Bluetooth vs GPS for the purposes of contact tracing and how best to leverage them both for the purposes of contact tracing and social distance alarms.

Definitions

  • Location | GPS: Global Positioning System (GPS) refers to the blended location information provided by Apple and Google mobile OS SDKs that combines GPS, Wifi & Cell Tower, Accelerometer data using Kalman filters, ML to increase accuracy beyond just GPS
  • Direct Proximity Detection | Bluetooth: refers to the use of Bluetooth to detect proximity with other Bluetooth devices running in the background.

Basic Comparison

The question is do we use location history to infer when two people were in close proximity or do we use Bluetooth to actively detect proximity.

GPS vs. Bluetooth for contact tracing

GPS Negatives

Accuracy drops to unusable in subways. Can’t handle indoors and underground scenarios particularly well.

GPS collects exact location information and uses it to perform contact matching.  However, location data is highly sensitive and difficult to secure.  For instance, the use of GPS location data in fitness apps revealed the exact layout of US bases, and location data is so unique that 1000% anonymization is effectively impossible.  Deployment of location-based contact tracing at scale thus poses privacy and security risks if it can’t be completely secured.

GPS Battery Usage

The rate of battery usage is dependent on: accuracy, frequency, and latency. These parameters can be adjusted, but they trade off against one another.

The highest accuracy setting uses GPS and all other signals and inputs available, which significantly drains battery. The balanced accuracy setting uses less power, primarily relies on WIFI and cell signals, and only occasionally uses GPS.

https://developer.android.com/guide/topics/location/battery

Bluetooth Accuracy

Bluetooth based proximity detection “uses received signal strength indicator (RSSI) values to measure the signal strength between phones. Calibrated RSSI values are used to estimate the approximate distance between users during an encounter.” (Source)

Range: Up to 10 Meters

Bluetooth Negatives

iOS sometimes terminates the Bluetooth scanning when the app is running in the background. Does bluetooth need to be enabled during the entire time frame for it to work? (No, just bluetooth app permission enabled)

Foreground vs Background app

Medical Considerations

What % of cases are transmitted through the air vs. surfaces?

COVID-19 can survive on surfaces for potentially days, but if the % of cases that are spread through surfaces is insignificant, then the advantage of GPS+cell to track historical location becomes less relevant. I believe more research needs to be done on what proportion of cases are spread through surfaces.

What is a “safe distance” with respect to airborne transmission?

Maximum plausible distance to get infected so far: within 18ft of an infected person (in a closed air environment)

Passengers in a Wuhan bus got infected from 4.5m (15ft) away, within 30min:

COVID-19 spread through a bus

Aerosols dispersed at least 6ft away are still infectious (Bischoff).

Particles from a sneeze or cough can travel up to 2.5m (8ft) horizontally and up to 6m (20ft) vertically (which can reach air ducts & ventilation systems) (MIT).

This study analyzes the spread of SARS 1 on an aircraft (NEJM) (note for scale: 3 rows = 90 inches).

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