ESA Document - Emits - ESA

s
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 208 of 422
A trade-off ont he convenience of a Mars relay satellite for communications with Earth G/S is
done in SHM report (see section 4.3.7). In brief, it is used to relay TV – MEV – G/S.
3.3.7.2.5 Communications during solar superior conjunction
On 19 August 2034, there will be a superior Earth-Sun-Mars conjunction that will affect the
communications with Earth G/S. The minimum angular distance S-E-M will be 1.145 degrees,
and in that situation, communications will be feasible but with a reduced data rate and only in
Ka-band. laser link is not usable below 10 degrees of SEM angle, so for about 2 months, the
mission will have to rely only on the Ka-band link, the TV-G/S or the relay satellite-G/S one.
Therefore, during 2 months, the downlink data rate will be reduced (see section 3.3.7.5.4).
The effects of the solar conjunction on the received signal are amplitude scintillation, spectral
broadening and phase scintillation due the fluctuating columnar electron density. Phase
scintillation causes changes in the frequency of the signal, creating problems in ranging signal
(orbit determination), and phase locking loop. The solution is to not plan anything critical during
this time, such as orbit changes, and to increase the PLL bandwidth, respectively.
Using data obtained from Cassini mission [RD43], the study report concludes that in Ka-band a
link degradation of 7 dB will be obtained. However, the real value of this degradation will
depend directly from solar activity and solar transient events. Due to the solar maximum
expected in 2033, close to the conjunction date, this figure should be taken cautiously.
Data rates during superior conjunction are shown in Figure 3-62.
3.3.7.3 Laser link
The maximum data rate achievable from the maximum distance Mars-Earth (2.7 AU) using only
Ka-band and a reasonable antenna size, is less than 2 Mbps. For example, to obtain 9 Mbps for
downlink, a Ka-band 8 metre antenna would be needed. Weight, complexity and cost make it an
unrealistic option. The alternative is laser communications, which could increase the data rate to
10 Mbps at 2.7 AU, and even 250 Mbps at 0.7 AU. Despite the fact that these values are not
feasible nowadays, future technology should be able to support them.
In a laser link, one of the main problems is the atmospheric distortion. To reduce the influence of
the atmosphere, adaptative optics (AO) could be used in the G/S (basically optical telescopes).
For the uplink as well as for the downlink, AO would significantly improve the link
performances. Especially for uplink, AO are considered necessary. To limit the complexity and
hence the cost of the optical G/S, and considering that nowadays much more work goes into
downlink laser communications research and development, it has been decided to use laser only
for downlink, while Ka-band will be used for uplink.
The technical problems to be solved to make the optical link from Mars competitive respect RF
link are:
1 High power space qualified lasers of at least 5 W of transmitted power. Nowadays 1.5 W.
2 Reduce pulse length to 2 ns with high power lasers (at least 5W).
3 Adaptative optics in the G/S, to reduce atmospheric distortion.
4 System mass (around 45 Kg). It is not a serious constraint for this mission.