ESA Document - Emits - ESA

s
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 210 of 422
The TV communications direct link availabilities, considering just the direct visibility in the
worst case, with the different mission elements are:
• G/S: maximum blackout 41 min= 65 % availability.
• MEV: average visibility 15 min = 12 % total time.
• Relay: availability around 65 % of the time.
From relay satellite to the G/S, the maximum blackout duration is 77.5 minutes, so 95% of total
time, the relay will have contact with G/S. The exact figures will depend on the season.
To increase the communications availability between MEV and TV, apart from a direct link, the
solution is to use a relay satellite. With this solution, the availability will be around 65% of total
time in the worst case.
In LEO (400 km, 62 degrees), each of the three ground stations will have three or a maximum of
five passes per day with a maximum duration of around 8 minutes. However, in LEO data relay
satellites should be used.
3.3.7.5 Baseline design
3.3.7.5.1 Band and frequency design
Laser communications has been used only for downlink while Ka-band has been used for uplink.
The reason is a compromise between cost and data rate. In case of using laser communications
for uplink as well, just the double of G/S would be needed in principle, so it is a too expensive
option. Additionally, Ka-band data rates are higher for uplink than for downlink, mainly the
higher transmitted power by the G/S.
For contingencies, X-band has been used because it has less weather dependence than Ka-band,
so in case of contingency the link availability will be higher.
For the TV-relay satellite link, X-band has been choosen, since the pointing requirement is lower
than Ka-band and with an small dish antenna diameter (40 cm) a high enough data rate, for the
project needs, is achieved.
For the TV-MEV/MAV/SHM link, UHF is used because its low propagation losses and very low
pointing requirements, make it ideal for docking-undocking manoeuvres.
The bands and frequencies used are consistent with the Space Frequency Coordination Group
[RD48].
3.3.7.5.2 Ground station assumptions
Ground stations with Ka- and X-band capability and 70 m of diameter are used. See Table 3-40.