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 Figure 58: Schematic of the orbits of Earth and Mars. It is assumed that the same communication terminals will be used during an entire Mars mission, during which typically Mars will pass through conjunction and opposition several times. Since that means that wavelength, bandwidth, and power are fixed, the maximum data rate and order of the PPM modulation will need to be adjusted to close the link at the best possible data rate in all phases of the mission. 3.7.5.1 Downlink Budget Below are sample link budgets for a near-range (around opposition, see Figure 59) and a farrange (around conjunction, see Figure 60) scenario. At far range, the space loss is more than 15 dB greater than at near range. For this reason, in the far-range link budget calculation a lower data rate is considered with a higher PPM modulation order. In addition, when Mars is near conjunction, the presence of copious amounts of solar stray light in the atmosphere during daytime communication opportunities increases the number of signal photons per pulse required to achieve an adequate signal-to-noise ratio for detection. In the calculation, this effect is not taken into account. However, the far-range budget shows a good margin, which could accommodate a 4.4 dB increase to mitigate this effect. Page | 98
Optical Link Study Group (OLSG) Final Report IOAG.T.OLSG.2012.V1 M ARS (NEAR RANGE) DOW NLINK BUDGET INPUT PARAM ETERS LINK BUDGET Slant Range 6.88E+07 km Tx Ave Power 36.02 dBm Elevation 30 deg Tx Photons / Pulse 2.40E+11 TRANSMITTER Tx Antenna Gain 112.98 dBi Modulation Type 16-PPM Tx Transmission Loss -5.19 dB Tx Pointing Loss -0.05 dB Tx Wavelength 1.55 µm Tx Ave Power 4.0 W Isotropic Space Loss -354.93 dB Tx Data Rate 260.0E+06 Hz Atmospheric Loss -0.64 dB Tx Aperture Diam 0.22 m Rx Antenna Gain 147.72 dBi Tx Angular Diam 1.85 arcsec Array Gain 0.00 dB Tx Footprint Diam 6.17E+05 m Rx Transmission Loss -4.89 dB Tx Optical Transmission 30.3 % Rx Pointing Loss 0.00 dB Tx Depointing 0.10 arcsec Total Optical Path Loss -105.00 dB Uncoded Slot Rate 2.1E+09 s -1 Ave Power at Rx Detector -68.98 dBm Uncoded Bits Per Word 2.00 Photons / Pulse at Rx Detector 7.59 ATMOSPHERIC LOSSES Required Photons / Pulse 1.89 Atm Zenith Transmittance 95.0 % Link M argin 6.05 dB Relative Airmass 1.99 Atm Transmission Along LOS 90.3 % Scintillation Loss -0.2 dB RECEIVER Rx Aperture Diam 12.00 m Rx FOV 5.00 arcsec Rx Depointing 0.00 arcsec Rx Optical Transmission 32.4 % Rx Array Size 1 apertures Required Photons / Pulse 1.89 Code Rate 0.50 Figure 59: Example downlink budget for a Mars (near-range) Deep Space scenario. Page | 99
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Appendix A. List of Acronyms 2-PSK
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