ESA Document - Emits - ESA

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5 Reduce 2dB pointing precision to 2µrad.
6 Problems when angular distance between Sun- G/S – Spacecraft is low.
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 209 of 422
The development of space-qualified cavity dump lasers [RD51] is expected to solve the two first
problems. As regards the third problem, adaptative optics for downlink in G/S are expensive, but
are considered state of the art.
One of the main problems for laser links is the high pointing accuracy needed (around 2 µrad in
this case). The approach is based on using high bandwidth inertial sensors to compensate for
jitter excursions caused by spacecraft vibrations. This use of high bandwidth inertial sensors,
together with a fine pointing mirror (see Figure 3-57), would enable the implementation of laser
communication links by achieving submicro radian-pointing precision [RD45]. As a reference,
ESA-Artemis is obtaining 3 µrad of pointing accuracy.
In this report, a minimum SEM angle of 10° for laser communications feasibility was taken,
which is a conservative value. A lower minimum SEM angle (3°) is considered in [RD47].
Two laser transmitter options have been considered, the first is a 30.5 cm telescope transmitting
5W, and the second is a 50cm telescope transmitting 20W. See Table 3-41 for details.
The 30.5 cm telescope with a 5W laser is enough from the point of view of data rate (it covers
the average downlink data rate shown in Table 3-40) and the pointing precision needed is lower.
Nowadays, a maximum power of approximately 1.5W is achievable, 5W is a more realistic
option than the 20W. Therefore, the 30.5 cm and 5W option has been selected.
Mirror diameter Transmitted power Expected data rate Pointing accuracy
2.7 A.U. - 0.7 A.U. (3dB beamwidth)
30.5cm 5W 10 Mbps-250Mbps 6 µrad.
50cm 20W 100 Mbps-1Gbps 2.5 µrad
Table 3-41: Laser transmitter considered options.
Figure 3-57: Pointing detector system. It is used together an inertial sensor.
3.3.7.4 Communications continuity
An analysis of communications during solar superior conjunction is done in section 3.3.7.2.5