Design Goals

• 100% Coverage - Continuous coverage of the entire Martian surface.
• Convenient Terminal - Mars is a long ways a way, and as exploration begins, there will not be a lot of resources on the surface.  Therefore the surface terminal should be as small as possible.  It should also use a minimum amount of power.
• Economical - To reduce cost, the required number of satellites was minimized.  The transmitted power was kept as low as possible.

System Design

The heart of the Mars network consists of a constellation of satellites around Mars.  The satellites will be interconnected with RF links and can relay signals between terminals on any part of the planet.  Some of the satellites will be equipped with high gain antennas to relay signals to Earth.  The NASA Deep Space Network (DSN) will be used as the Earth terminal in the network.  As the DSN is connected to the internet, the Mars network is available form anywhere on Earth. 

Orbital Geometry

The satellite orbits are the primary design consideration.  A good balance must be found between two competing objectives: number of satellites and distance from the Martian surface.  Satellites are very expensive, so the minimum number of satellites will be used.  At the same time, we want the satellites as close to the surface as possible.  Closer satellites mean less propagation loss and smaller power requirements.  However, as the satellites get closer to the surface, they can see less of the planet, meaning that more satellites will be required to cover the entire planet.  There are satellite constellations containing four satellites that can cover the entire planet, but they require very high orbits (much higher than synchronous) [1].  Another option is to use satellites very close to the surface (800km) [2].  The trade-off is that many satellites must be used or the whole planet can not have continuous coverage.  Neither of these options is acceptable.  A good compromise is to use six satellites as described in [3] and pictured below.  This constellation was designed to provide global coverage with minimum satellite height.

Mars Network Satellite Constellation

Satellites S1 and S2 are in a circular equatorial orbit.  The other satellites are also in circular orbits but inclined by 58.9 degrees.

The image below is the satellite constellation as viewed from a planet's surface that is spinning at the same rate as the satellites. The outline of the Earth's continents is just for spatial reference.  Notice that satellites S1 and S2 are stationary relative to the other satellites.  These two satellites will be used to communicate with Earth.  At all times, one of them will be in site of Earth.

Satellites as viewed from a planet that is rotating at the same rate as the satellites.

This constellation requires that Θ = 66.42 degrees.  With a minimum elevation angle (ε) of 10 degrees, the orbital radius (h) should be 12,120 km.  This gives a maximum distance from a terminal to a satellite (d) of 11,636 km.  The geometry is pictured in the figure below.

Orbital Height Calculation

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