Interesting . . . Checking the Cospas/Sarsat spec, it turns out that the third of a mile figure isn't all that far off from their requirements for units with a built in GPS. While it looks like the location encoding scheme accomodates greater precision than that, it only requires the following:

"4.5.5.3 Internal Navigation Device Performance

An internal navigation device shall be capable of global operation and shall conform to an applicable international standard. An internal navigation device shall incorporate selfcheck features to ensure that erroneous position data is not encoded into the beacon message. The self-check features shall prevent position data from being encoded into the beacon message unless minimum performance criteria are met. These criteria could include the proper internal functioning of the device, the presence of a sufficient number of navigation signals, sufficient quality of the signals, and sufficiently low geometric dilution of precision.

The distance between the position provided by the navigation device, at the time of the position update, and the true beacon position shall not exceed 500 m for beacons transmitting the Standard or National location protocols, or 5.25 km for beacons transmitting the User-Location protocol. The encoded position data shall be provided in the WGS 84 or GTRF geodetic reference systems.

The internal navigation device shall provide valid data within 10 minutes after its
activation. Internal navigation device cold start shall be forced at every beacon activation. Cold start refers to the absence of time dependent or position dependent data in memory, which might affect the acquisition of the GNSS position. "