The US DoD is studying whether to continue with its’ current broadband satellite systems or to move on to a global space laser com relay network. According to Undersecretary of the Air Force Peter Teets at a DoD News Briefing on Tuesday, Sept. 3, 2002, they hope to be ready for a decision by December 2004:
“Just exactly that way. We will progress in the development of the laser comm. technology between now and 2004. In 2004, we will decide whether or not we have confidence enough to deploy — whether we have confidence enough to not procure AEHFs 4 and 5 and, rather, rely upon a high bandwidth relay network of some kind using some form of laser comm.”
They seem primarily interested in space-space links, but I predict usefulness for space-ground links as well. Laser links have many admirable characteristics for this if you can get the pointing right. They do not have the extensive sidelobes or wide footprint of radio signals1; they are difficult to jam2; they can carry enormously more data3; and left entirely unsaid at this briefing… they are amenable to quantum cryptography4.
Oh I just love the future!
1 = This makes it very difficult to intercept. Even tightly beamed microwaves have enough off axis signal to be read miles away as the Russians did in New England in the 80’s. They purchased an old country house as a diplomatic site, stuck up a bunch of antennas and started picking off White House and other phone calls. At that time the exchange number was part of a clear text header, easily filtered for out of the massive volume of long distance voice traffic. It goes without saying US ELINT sats can pick up the faint leakage of microwave links from orbit.
2 = Someone will certainly comment about the effect of fog, clouds etc. It is not as much of a problem as you think, and most especially for point to point orbital communications. Even on ground links, much depends on the frequency in use. Water vapour does not absorb at all frequencies.
3 = Think of live two way hiresolution video links between pilots in theatre and control centres elsewhere in the world; perhaps even holographic 3D heads up data displays. The possibilities are staggering.
4 = Even without encryption, quantum tricks lets them make sure undetected “man in the middle” attacks are literally impossible.
Well, just because air and water don’t absorb all frequencies, doesn’t mean that there isn’t refraction and scattering (when the beam passes through a cloud, or air of a different temperature). How do they deal with this?
Modern cryptography technology is essentially unbreakable in a lot of important ways. It’s based around problems that are genuinely hard, for example the discrete logarithm problem on elliptic curves or factorization. The only known ways to break these are through some very difficult computational work, and better methods are unlikely (at least for the moment).
Quantum cryptography would be a pointless diversion for the military at the moment, as classical methods of cryptography are so good.
As for the quantum trick mentioned in (4) (which I’m guessing means using the laser beam to couple two small particles), I would think this would be made nearly impossible by atmospheric effects. Any distortion in the laser would cause the two particles to decouple, losing the signal.
How soon till we can have holographic 3D blogs? The Samizdata logo in 3D would look cool. With animation and sound, the Rottweiler blog’s logo would be quite menacing…
Offtopic, kinda:
Reminds me of the LISA project the European Space Agency and NASA are planning. More lasers into space. In this case a trio of satellites to orbit around the sun to measure gravitational waves. What lasers got to do with it would be the distance measurement. Basicly the satellites are 5 million kilometers apart, and the distance needs to be measured with 20 picometer accuracy, or so. Takes fairly accurate work, in other words. Of course the NASA website calls that a “coarse” measurement. NASA project website.
Lucas is right, there are a host of other factors that disperse the beam, particularly atmospheric despersion even in bands of near-zero absorption. Refractive effects, too, are a bitch because the atmosphere has a widely varying refractive index from layer to layer, and temporally varying with changing atmospheric and weather conditions. Besides the fact that this makes it hard to point right on, an orthogonally variant refraction index could distort the gaussian cross-section of the laser, and then dispersion just goes all to hell real quick.
Which is not to say this is impossible, but it is to say that we will likely never see the tight beamwidths that most imagine, and never be able to use quantum effects for cryptography or information, especially since most of that depends on the polarization of light, which also gets all screwy as it goes through the atmosphere.
Course, they said we’d never put a man on the moon, either.
— Brendan
BSEE Applied Electrophysics (a long time ago)