I just noticed this excellent review from March 2013 at OutdoorGearLab.com:

The Best Personal Locator Beacon and Satellite Messenger Review

They compare and contrast these Personal Locator Beacons (PLB)s, Satellite Emergency Notification Devices ( SEND)s, and an iPhone app (while emphasizing the extreme coverage limitations of the app). In the bulleted list below, I've hyperlinked Doug Ritter's articles where available.

At the time of the review, their "Editor's Choice" for a SEND was the DeLorme InReach Satellite Messenger, and for a PLB, the ACR ResQlink 406 Personal Locator Beacon, but of course your priorities may vary, and the market continues to evolve - the new Ocean Signal RescueMe PLB, which was unavailable at the time of their review, seems to competetive with the ResQlink.
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A signal mirror should backup a radio distress signal, like a 406 MHz PLB (ACR PLB) (Ocean Signal PLB)