Again, I understand your concern about only getting postion updates every 20 minutes, but the way I look at it is this:

On land, once you've activated a PLB you shouldn't be moving anyway. If for some reason you HAVE to move, then after 21-30 minutes searchers will have two locations on record and will have an indication of your direction of travel. I'm not sure if the locations have a time-stamp on them. If so, they'll have a sense of speed too.

If in big water, you will almost certainly be moving due to wind and currents. Even then the same thing holds. In 30 minutes they'll know the direction of travel and may have an approximate speed.

It seems that SAR crews will take some time be put into action and to get to your location, and getting a fresh location 20 minutes after the initial location seems about right. They should be able to get regular updates on location changes via radio - I would guess. Plus if so equipped and trained they can also use the 121.5 MHz homing beacon from the PLB to find my exact positon.

I don't expect my PLB to get me rescued in 10 minutes. I figure an hour or more is highly likely, which is why I still need to be "equipped".

Ken