Friday, May 15, 2009
296.8 MHz, 259.7 MHz and 279.0 MHz are the three UHF simplex frequencies that are used by the EVA astronauts while outside the shuttle. The shuttle communications system relays these frequencies down to earth via S-band, Ku-band or TDRSS links.
To the best of my knowledge, these are not encrypted and are AM. There was some reserch being done a few years ago about replacing the EVA radios with something new and possibly digital in the 400 MHz band, but I don't know where that project is currently (my NASA sources have moved on to other careers).
Here are some good links to the NASA web site that deal with shuttle EVA and flight communications: