The UK may replace ankle-mounted GPS trackers with portable fingerprint readers that the person is required to carry at all times.

I think the two systems are equally intrusive, more or less, but they need to be compared with being in an immigration prison. Overall, I am more frightened by spread of "voluntarity" GPS tracking that is imposed implicitly, by apps and web sites used via snoop-phones, and that hardly anyone has the stubbornness to refuse.

I think it should be illegal for those devices to track the user except under special conditions — and the snoop-phone should never be allowed to send a GPS location directly except in an emergency call. Other than that, the user should be able to send the current location by requesting to display it, then cutting and pasting that location into an app. This gives the user the opportunity to decide what location to tell to the app. This flexibility is what shields users from tracking.