Exposure Notifications more detailed design

More detail on the “Exposure Notifications” part of this page:

Hashing of GPS points - Design

That article originally showed this image as an example of how an Exposure Duration is calculated.

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However, there could be multiple disjoint such ocurrences in a day - which does not fit with the current App display capabilities.

The current pre-MVP1 App function only supports display of a single Exposure per-day.

Therefore, rather than worrying about durations of individual bursts of exposures, for MVP1 the App simply adds up how many 5 minute time intervals fall within a block of 6, of which at least 66% have matches (these 6 & 66% numbers are adjustable by the Health Authority, as explained elsewhere in the design).

For each such 5 minute time interval, we add 5 minutes to the total exposure for the day and report that. In MVP1 the minimum exposure duration that can be reported is 30 mins. There will only be one exposure notification per-day, which could in theory be up to 24 hours long.

In this illustration, the brackets on the left indicate groupd of 6 time intervals with 4+ matches. And the Xs indicate the intervals that fall within these groups. In total there are 15 time intervals shown, so 15 x 5 = 75 minutes of exposure. If there were further exposure in the same day it would be added to this total, not reported as a separate exposure - in MVP1 we only ever report one exposure notification per day.

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In future releases (after MVP1) we may move to a model that allows us to report multiple exposures within a single day.