OLSG Report_Final_06_05_12 - Interagency Operations Advisory ...

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Optical Link Study Group (OLSG) Final Report IOAG.T.OLSG.2012.V1 Table 4: Estimated Uplink Laser Scattered Flux from Uplink Beacon (Rayleigh scattering calculated at 1550 nm) Mission Type Average Uplink Power (W) Mean Irradiance (W/m 2 ) at telescope due to scattered uplink laser. Engagement zone is 1 km away Mean Irradiance (W/m 2 ) at telescope due to scattered uplink laser. Engagement zone is 3-km away LEO 0.5 1 E-14 2.5 E-15 GEO 10 0.2 E-12 0.45 E-13 Lunar 40 0.8 E-12 1.8 E-12 L1 560 1.1 E-11 0.9 E-13 L2 400 8 E-12 0.9 E-13 Mars 5000 0.8 E-10 2 E-11 Furthermore, the uplink beacon must not interfere with the observations of astrophysics missions in space that employ highly sensitive science instruments (imaging/spectroscopy) observing in the visible and far infrared. Such missions are likely to be located in the L2 region, and care will have to be taken to ensure that the uplink beacon does not interfere (including scattering and diffraction on the satellite itself) with science observations. In most cases this issue can be mitigated by choosing the beacon wavelength and incorporating corresponding filtering in the instruments. However, in extreme cases this issue could become an impediment to beacon-aided pointing, leading to a corresponding data rate reduction due to pointing/jitter loss. On the other hand, as long as science observations and optical data downlink are not simultaneous, much of the interference issue will be avoided. 2.3.3 Modulation and Detection An important factor in the system and link design is the choice of modulation and detection. Examples include: On-Off Keying (OOK) with non-coherent detection Serially Concatenated Pulse Position Modulation (SCPPM) with non-coherent photon counting detection Binary Phase Shift Keying (BPSK) with coherent/homodyne detection Differential Phase Shift Keying (DPSK) with differentially coherent detection 2.3.4 Operating With Small Sun Angles Another major area of concern for optical communications is the need to operate very close to the Sun. An optical communications terminal attempting to communicate with a terminal in Earth orbit may find it impossible to acquire when the terminal in space is directly in front Page | 28
Optical Link Study Group (OLSG) Final Report IOAG.T.OLSG.2012.V1 of the Sun; however, with the right modulation and coding, it is possible to maintain communications with a previously-acquired terminal passing in front of the Sun. This process can be particularly difficult for a deep space optical communications system, because of the very low photon flux, the relatively large apertures (which are harder to protect from heating), and the modulation generally proposed. Such circumstances are easily envisioned when considering the outer planets; for example, from Pluto, the Earth is always very close to the Sun. The OLSG evaluated this problem for a Mars scenario, where there is an optical communications terminal in orbit around Mars and an optical communications terminal at Earth. Figure 6 illustrates what is happening during solar conjunction in this scenario. SEP is the Sun-Earth-Probe angle, while SPE is the Sun-Probe- Earth angle. Small SEP angles interfere with the Earth terminal’s ability to acquire and track the laser communication signal. Small SPE angles interfere with the Mars terminal’s ability to acquire and track the uplink beacon laser from Earth. During solar opposition, small SPE angles also affect the Mars terminal, as the Earth is again very close to the Sun. SPE Angle (3) SEP Angle (5) SPE Angle (3) Figure 6: Sun-Earth-Probe and Sun-Probe-Earth angles. Each day there is a time of sunrise, Mars rise, sunset, and Mars set. Most of the time, both the Sun and Mars will be in the sky simultaneously. From an optical communication systems engineering perspective, a critical design driver is the fact that Mars is simultaneously at its farthest distance and at its smallest SEP angle. The communication outages that arise when either the Earth or Mars is too close to the Sun have been evaluated for various SPE angles (and the corresponding SEP angles during solar conjunction) and are shown in Table 5. The objective of ground terminal design will be to minimize the number of outage days. For example, the Lunar Laser Communications Demonstration (LLCD) will operate with an SPE of two degrees. Page | 29
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3.7.6 Ground Station Cost Optical L
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5.3 Standardization Considerations
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Functional breakdown Cross support
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7.3 Astronomical Site Survey Optica
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Appendix A. List of Acronyms 2-PSK
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