One Comms station on Phobos and two or three other dedicated sats in the same orbit means that there will always be at least one sat above the horizon for all the equatorial and mid latitude areas where the earliest landings and activities are likely to be.
There was a FISO telecon recently on geostationary Mars telecom orbiters:http://spirit.as.utexas.edu/%7Efiso/telecon/Edwards-Lock_8-2-17/
And that would be the end of the story if the planets were spherical and homogeneous and there were no other perturbing effects. But of course the real world isn't as simple as that. At Earth, the major effects on satellites trying to stay in one position are the presence of the Moon and Earth's polar flattening. These combine to tilt the plane of the satellite's orbit. Also, Earth's nonspherical shape -- the lumps and bumps that make its equator noncircular -- causes the satellite to migrate in longitude over time. There are two stable points (at 75.3E and 104.7W, corresponding roughly to the longitudes of India and Mexico) and two unstable points (at 165.3E and 14.7W, corresponding to longitudes of the Solomon Islands and the western edge of Africa), where the drift rate is negligible; but satellites not located at these points tend to shift in longitude away from the unstable toward the stable point over time. The stable points are where Earth's gravity has a local low, the unstable ones where it has a local high.To stay in the intended position, geostationary satellites currently have to use thrusters to counteract these forces. Using thrusters means using up a limited resource -- fuel -- so stationkeeping is one thing that sharply limits a geostationary spacecraft's lifetime. To discuss fuel budgets, space navigators talk of "delta v," which is kind of a measure of how much change in velocity a spacecraft can accomplish, measured in units of speed. It takes about 50 meters per second of delta v per year to keep a geostationary satellite in Earth orbit, and almost all of that has to do with counteracting the tendency to tilt north and south in latitude rather than the tendency to drift east or west in longitude. The maximum delta v needed to counteract longitude drift for a satellite located right in between the stable and unstable points is about 2 meters per second. Since it's so small, the choice of longitude doesn't have a major effect on the lifetime of your geostationary satellite.Silva and Romero show in their paper that the story is quite different at Mars, because Mars is much less spherical than Earth. It has a monstrous large pile of dense volcanic material deposited on one side (the Tharsis volcanoes and Olympus Mons), balanced on the opposite side by a broad gravity rise, that give its gravitational field much larger deviations from ideal smoothness than Earth has. Like at Earth, there are stable points over the two gravity lows and unstable points over the two gravity highs. Unlike at Earth, there is a huge cost if you want to put your satellite at a longitude between the stable and unstable points: Silva and Romero calculated that it can cost up to 22 meters per second of delta v per year in order to put a geostationary spacecraft at one of these spots. You would also have to perform stationkeeping maneuvers much more frequently at Mars than you do at Earth: approximately once every few days, rather than once every few weeks.
You'd probably want to put them into one of these four areostationary orbits to minimise stationkeeping. Three might cover most of the planet longitudinally, but there might be a gap.Stationkeeping in Mars orbit
But one thing that works against us is that since Curiosity and Opportunity are located nearly on opposite sides of the planet, no one geostationary satellite could serve both spacecraft. And another problem, that I haven't even gotten into, is that it's costly in terms of delta v to get a spacecraft into such an orbit.So don't expect to see a geostationary orbiter at Mars anytime soon. What has been proposed (but canceled) for Mars is a telecommunications orbiter that might circle at an intermediate altitude of around 5000 kilometers, giving it nice long communications passes over landers and a view of Earth that's only rarely obstructed by Mars.
It might be worth waiting to see what SpaceX comes up with as standardized systems mass produced for its Constellation. How might they be adapted for Mars. It ought to be possible to have high bandwidth low latency internet everywhere on Mars before the first human footprint. It's what would make serious robotic exploration and work possible. The same satellites ought to be able to provide constant detailed surveillance of the whole planet. The Constellation would need a high powered laser relay to earth.If a SpaceX Mars internet isn't funded at all by government or NGOs it might make sense as a spec project. Just build it and charge service fees.
Quote from: Ludus on 08/30/2017 01:41 amIt might be worth waiting to see what SpaceX comes up with as standardized systems mass produced for its Constellation. How might they be adapted for Mars. It ought to be possible to have high bandwidth low latency internet everywhere on Mars before the first human footprint. It's what would make serious robotic exploration and work possible. The same satellites ought to be able to provide constant detailed surveillance of the whole planet. The Constellation would need a high powered laser relay to earth.If a SpaceX Mars internet isn't funded at all by government or NGOs it might make sense as a spec project. Just build it and charge service fees.The sats around Mars is only half the problem. The other half is the bottle neck with the DSTN on Earth. An alternate communications system besides the DSTN is needed if higher data rates on a near constant basis is desired. For this a Laser communication package on the Mars orbiters with the corollary of a hosted Laser comm package on Earth GEO sats preferably HTS ones would provide high data rates at near constant communication without needing the DSTN. This freeing up the very expensive DSTN assets for other deep space missions is a big plus.