There are three ways that yellowcog provide a "proximity" stream that can be logged or transmitted. Firstly, the Pilot devices collect data only from devices that are in range; for most sport (racing, yachting) this is enough of a measure of proximity. Secondly, a device can be configured to collect the serial number of devices that are near enough to provide data. Since each device has a unique serial number, and if the allocation of devices is known (e.g. each driver in a rally has their own wearable device) then not only is the dataset automatically assignable to each device (e.g. and therefore a driver) but also it provides a record of what (or who) was nearby. In situations where you want to know physically where someone is (say, on a yacht) then there is also a measure of signal strength which allows a relative indication of distance or proximity. Using one or more of these methods, datasets can be automatically annotated with user activity or the location of assets.
See product listing for Proximity Sensor or help on proximity sensors.