OLSG Report_Final_06_05_12 - Interagency Operations Advisory ...

Optical Link Study Group (OLSG) Final Report
IOAG.T.OLSG.2012.V1
2.3.2 Uplink (Beacon)
2.3.2.1 Uplink Budget
In the scenarios considered, the optical communications ground terminal is required to
transmit a powerful laser beacon as a pointing and tracking aid to allow the space terminal
to track the position of the ground terminal with the necessary accuracy. The beacon power
ranges between 500 mW (LEO) and 5 kW (Mars).
To successfully conclude the acquisition procedure, the uplink beacon must reach the
aperture at the space terminal with sufficient irradiance, even under stressing atmospheric
conditions. Therefore, the performance of the link can be evaluated in terms of irradiance
margin.
A design of such uplink/beacon transmission may include the following considerations
reflected in the corresponding link budget calculations:
1. To mitigate the effect of atmospheric turbulence leading to significant
irradiance fluctuations/fading at the space terminal, the beacon would be
transmitted as an incoherent superposition of several beams emitted from
corresponding separate sub-apertures from mutually incoherent sources.
2. Given the characteristics of the local atmospheric turbulence at any given
time—which is conveniently parameterized by the Fried parameter r 0 (the
spatial scale over which the phase perturbations can be considered as
negligible)—the most suitable (sub-)aperture size of the transmitters would
be chosen to be equal to this parameter (r 0 ). Since r 0 varies with time (of day)
and local conditions (good sites have a large r 0 over longer periods), a
transmitter design would account for the local statistically worst case. At sites
considered adequate for optical communications, r 0 ranges from several cm
to few tens of cm.
3. The mutually incoherent sub-apertures should be spaced by a distance larger
than r 0 in order to experience statistically independent turbulence effects and
reduce the overall signal fading at target. When a single telescope is used for
uplink and downlink, the spacing of the transmitting sub-apertures is taken
such that they fit into the receiving telescope’s aperture as their envelope.
Other configurations may consider use of a separate uplink and downlink
ground terminal. Our scenarios correspond to an implementation where the
incoherent beams of smaller diameter beams are emitted through the main
telescope. For our considerations, the actual design is irrelevant.
4. For the following eye-safety calculations, which also consider near-field
propagation, each sub-beam is taken to be Gaussian with its beam waist
equal to the sub-aperture size.
Figure 4 is an example uplink beacon budget for a lunar mission. Most of the terms and
quantities appearing in the uplink budget are the same as in the downlink case, and are
explained in Table 1. Table 2 describes the additional terms.
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