Re: Laser communications idea

ChuckKuecker (
Sun, 10 May 1998 09:00:33 -0500

At 10:47 PM 5/9/98 -0700, you wrote:
>I had an idea for long-range (6000+ miles) laser communication system using
>(more or less) off the shelf technologies. I am unable to find any obvious
>flaw in my idea, so I was hoping for some feedback from people who may be
>more knowledgeable about this than I.
>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..

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?

>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 worked.

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..

Neat idea..

Chuck Kuecker