# Laser Satellite Communications

Laser Satellite Communication: The Third Generation |
Mott & Sheldon |
2000 |

P149-P160: Chapter 7, Regulatory Issues |

Describes limits, bandwidth, regulation by the ITU and the nations composing it.

P167-P168: STRV-2, second Space Technology Research Vehicle, launched 1998 |

LSC system by AstroTerra:

- 1.2Gbps over 1800 km, 1000 cubic inches, 14.3 kg
single 5.4" Schmidt-Cassegrain telescope ± 40μradian pointing error

book suggests 1μradian possible with high precision bearings, sophisticated resolvers, and high accuracy micro-positioners

- piezoelectric drivers, flexible telescope lenses, or dual-beam tracking techniques

- eight lasers
125mW each, 810 nm, divergence 80 μradian, power switched modulation

- four lasers form 1st 600 Mbps channel right hand circular polarised
- four lasers form 2nd 600 Mbps channel left hand circular polarised
- Gaussian radial dispersion

- avalanche photodiode receivers
2 acquisition and tracking lasers, 852 nm, 500/1500 μradian divergence

receiving telescope must point within ± 30% of the center of the beam

Let's calculate. At 1800 km, 80 μradian is 144 meters; the telescope must converge this somehow. Let's presume the telescope sets the actual convergence; 5.4 inch = 0.37 meter, spot size estimate = 1.24 * 1.8E6m * 810E-9m / 0.37m = 5 meter spot. 1240eVnm/810nm -> 1.53 eV photons, 500mW = 3e18 photons per second, telescope captures about (.37/5)^{2} = 5.5e-3 of those photons, assume 30% commbined path loss and pointing loss and avalanche diode quantum efficiency, 3e18*5.5E-3*0.3 = 5e15 photons per second received, 8E6 photons per symbol. 40μradian pointing error is 70 meters, 1μradian pointing error is 2 meters.

At 10,000 km distance (M288 to 45N), the spot size would be 27m, 1.9E-4 of the photons would be captured, 1.7e-4 photons received, 3E5 photons per symbol. 40μradian pointing error is 400 meters, 1μradian pointing error is 10 meters.

Obviously, this is not agile enough for server sky communications. I presume there are better recent laser experiments.