ESA Document - Emits - ESA

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Basic RF link
Four 70-m Ka-band stations with more than 20 kW RF uplink power are set up.
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 117 of 422
Four Ka-band stations are taken, because the Ka-band link is very vulnerable at low elevations.
For the required availability four stations distributed over the Earth are required.
The technology is not yet available, neither for the precision pointing nor for the uplink power,
but is expected to be available in 2030.
High performance optical down link
Six 10-m optical terminals are set up.
The telescopes are assumed to be of photon bucket design. A photon bucket has a large photon
gathering area but only a limited optical quality. (The signal reception is limited by Poisson
statistics because of the limited number of photons received.) These telescopes are only used for
data reception. Modulation will be based on pulse length. Coherent modulation schemes may be
not feasible.
Choosing good sites (such as the Tenerife mountaintop site), means availabilities of 84% are
achievable for a single station. To achieve above 90% availability, at least two telescopes have to
be visible from the spacecraft at any time. The telescopes have to be at distances of more than
2000 km from each other to be in areas of different weather patterns.
Weather is permanently monitored. The spacecraft switches beam pointing to a redundant station
if weather conditions are bad at one station (beamwidth on Earth is only 350 km to 1250 km).
The performance at low Sun-Earth-S/C angles is unclear. Buffers of for example 5 to 10 times
the telescope diameter seem impractical. Heating of primary mirror will require design similar to
Sun observation telescopes with forced cooling. (Night operations only is not acceptable.)
The technology at this scale is unproven. Current ESTEC technology studies reveal technical
performance limits. One of the problems is the on-board pointing. The pointing device has to be
decoupled from microvibrations and astronaut movements.
2.10.2.5 Communication network
A dedicated worldwide communications network (see Figure 2-60) needs to be set up. It serves
all the parties involved in the ground and flight operations of the mission.
The network consists of two separated networks: the Ground Station Network and the
Operational Network. Both are connected via the Mars Operations Control Centre.
Only a single control centre (MARS OCC) is shown in Figure 2-60. In the case of a distributed
concept, control centres dedicated to a major task can be introduced into the network concept.