There is a very in-depth 3 part discussion and review of devices on Backpacking light including some discussion of one way devices like SPOT:
SOTMR: Two-way Satellite Communications for Backpacking
I am thinking about renting the Yellowbrick to give it a try.
I don't think you need a subscription to get this article.
http://www.backpackinglight.com/cgi-bin/...ml#.U3TG-F6yhFw
