According to this article from Jet Propulsion Labs, http://descanso.jpl.nasa.gov/Propagation/mars/MarsPub_sec2.pdf
, Mars does indeed have a usable ionosphere for over the horizon HF propagation. It is nominally 1/10 the density of the F2 layer on earth.. It is much less stable, and has much lower MUF values than we are used to on Earth. But according to the JPL scientists, it would be usable for communications.
Phobos and Demios are both close to the Martian surface and orbit very quickly. They would only be visible for a few hours at a time, not long enough for reliable communications. They are also small, around 11km in diameter, meaning that your antenna aiming problems are pretty hairy, you have to track a small, fast moving, and not very reflective rock to bounce your radio waves from. HF may be the only way initially to communicate over the horizon...
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