= 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.