ESA Document - Emits - ESA

s
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 312 of 422
To maximize the communications from the SHM and MAV to the Earth once on the Martian
surface, the visibility for communications with the TV and/or G/S has been studied. For
example, for Gusev crater the black out for communications is around 6 h (so no visibility of TV
or G/S). For other latitudes, the values change but are of the same order of magnitude. So, to
accomplish the requirement of a maximum of 1 hour without communications with the Earth, a
relay satellite must be included in the design.
Two kinds of relays have been considered: the first is satellites orbiting Mars and the second is a
relay in a Sun - Earth Lagrangian point. This second option has the problem of its high distance
to Mars, since basically the relay is in an Earth like orbit. So, data rate from Martian surface or
TV would be low. Additionally, there is experience with relay satellites in Mars orbit, but not in
Lagrangian points, so that option would be more complex. Therefore Mars relay has been
selected.
To maximize the communications availability from SHM, Mars aerostationary satellites are
needed, since they will be always visible from SHM. Two options have been considered: the first
is a constellation of three relay satellites and the second is the minimum configuration, just one.
The constellation would provide complete surface coverage and will avoid any eclipse of the
Earth, as would happen with a single aerostationary satellite, but would be more complex and
expensive. Since coverage is needed just in a small zone around the MEV landing point and the
communications availability for relay –Earth communications is high enough (95% of time there
is visibility), so the second option has been selected.
Due the situation of the SHM dish antennas and to avoid the occultation of the relay by the SHM
structure, which could happen with high elevations, the relay satellite should not be placed just
over SHM-MAV (in case the landing point is on the equator).
To select the link relay-G/S, Ka-band has been selected because it will give more link
availability and will be simpler and cheaper than an optical link.
In addition, communications are required from SHM-MAV to the TV. Due to the low link
availability for a direct transmission being about 12% of the time, to increase it TV could use the
relay satellite as well.
4.3.7.2.2 Requirements for EVA
During EVA, a communication link must be provided. For EVAs, high-quality video and high
quality voice is required within the astronauts’ walking distance, approximately 1 km (despite a
pessimistic value of 2 km will be used for the calculations). This distance is the maximum
distance that an astronaut would be able to walk back to the base in case of contingency.
EVA astronaut operations are done in groups of two, so communication between both astronauts
and with the SHM should be provided. Biomedical data, space suit data, high-quality voice and
high-quality video are basic requirements, and the design will be done to support them. The data
rate estimation for these requirements is shown in Table 4-17.