Re: Laser communications idea

James Rogers (
Sun, 10 May 1998 14:00:14 -0700

At 09:00 AM 5/10/98 -0500, ChuckKuecker wrote:
>At 10:47 PM 5/9/98 -0700, James Rogers wrote:
>>Laser targeting/ranging systems, such as those used by the military, mark
>>their targets by detecting the laser light which is reflected off the
>>object being illuminated. My idea was to use objects in geosynchronous
>>orbit as reflective bodies for the purpose of scattering laser light. The
>>ground stations would use laser pulses to send data by reflecting them off
>>the geosynchronous object. A passive sensor on the receiving end would
>>detect the laser light that scatters off the satellite and (hopefully)
>>discern the bits. In the simplest case scenario, the data communication
>>would be half-duplex, but more complex arrangements could be developed. I
>>am unsure of the type of baud rate that could be accomplished this way.
>Path loss looks to be the biggest problem here. Scattered laser light will
>be WAY down in amplitude by the time it makes it back to Earth from
>geosynchronous orbit..

The path loss shouldn't be insurmountable. I know that a 3 Joule laser is
sufficient to get a good return signal from the lunar reflectors. I would
expect most of the loss to occur at the satellite when the laser light is
reflected off an object that was not designed for that purpose. However,
depending on your choice of wavelengths, even a very low power return
signal may be adequate if you have decent contrast against the background.

>If you can detect the signal once it's back on Earth, I imagine you want an
>extremely robust error correction algorithm, as atmospheric distortion and
>stray light will be adding lots of noise.
>The theoretical maximum baud rate is 1/2 the carrier frequency, so you
>won't be seriously limited in speed with a laser signal. The actual data
>rate will probably be about 10% of the maximum to give a reasonable S/N
>number, and then the packeting and error correction overhead will be
>subtracted from that.
>I assume this would be a packet based system, otherwise you are just doing
>a single point to point link?

Point-to-point would be a good start. I was thinking of ways to set up a
relatively high-bandwidth mobile point-to-point communications link without
having to rely on RF bands or commercial channels. A more ambitious
project would be to set up a packet network. The system wouldn't be too
good for real-time communications, but should work very well for data. If
you used Ethernet type collision protocols, packet communications would
probably work pretty well. Every satellite would be the equivalent of a
hub. The biggest issue that I see is the high latency. Geosync and back
is not a short hop.

>>If this worked, it would be an interesting option for
>>covert/subversive/etc. communications. Lock onto your satellite of choice
>>and send the data. Encryption would be necessary, but it would probably be
>>difficult to locate the sender, especially if the laser didn't operate at
>>visible wavelengths. The problem I see is the possibility that you
>>wouldn't get a good scatter off the satellite. I know that the
>>ground-laser-to-geosync works because my mother used to work in a building
>>where data was transmitted that way, but I am unsure how the downlink
>It would be impossible to locate the sender from the ground. Surveillance
>satellites could be easily made to pinpoint such laser signals from orbit,
>however. The probability of a short transmission being seen is very low,
>unless there's hundreds of such satellites up there..

Visible wavelength laser uplinks are potentially visible up to 5-10 miles
or so. The blue-green laser uplinks (financial data) in San Diego, for
example, are visible several miles from the source. However, burst
transmissions should reduce this significantly.

My thought was that it wouldn't be too difficult to put together a
relatively mobile (vehicle-mounted?) secure communications system that does
not rely on commercial channels. Setting up such a communications system
is non-trivial, but doesn't rely on exotic technologies either (lasers,
detector optics, and maybe GPS if you are mobile) so it should be do-able,
assuming that there are no serious problems with the design.

-James Rogers