This is freom the Braemar Mountain Rescue Facebook page re PLB location accuracy.
"Braemar Mountain Rescue
6 January at 09:31
PLBs (personal location beacons); they are not commonly used in the hills but there was on used by the casualties on the Callater call-out we had on Saturday evening. Worth noting that the reference returned was approximately 9km west of where the incident occurred; so if you do have one of these, and perhaps it has never been checked since you bought it, perhaps it is worth getting it checked out via the distributor/manufacturer."
That's really interesting and puzzling. I have a reasonable understanding of GNSS's and can't see how such a big error would occur with the technology.
First thought was maybe someone tried to convert from lat/long to OS grid without also changing from WGS84 to OSGM36 (*) but 9km, even if significant altitudes involved, no way. Of course there are other ways to get that wrong too.
Still I'd put my money on human error somewhere within or between the cospas-sarsat control centre and the mountain rescue team.
As for getting a PLB checked they all have internal diagnostics and come with instructions on how to run those. Folk should obviously do that at appropriate intervals.
It's actually a very worrying report. PLB's are almost idiot proof and very reliable tech that's been around a long time. If anyone has further information on this incident or a good theory as how this 9km error happened do share, please.
* Yes, I have done this myself once. Over ten years back I forgot to switch a GPSr from OS to WGS 84 when using it in the Ardenne. Instead of ending up at a small bridge by a stream it took my to the booze section of Vielsalm SPAR a few hundred metres away. Oops indeed. In my defence it was back when I was a map and compass user and woefully ignorant of GPS usage, etc. Trouble is GPS seems so simple to use, it just works, until it doesn't.