An Interplanetary Laser Communications System

caffiend666 writes "A news article at Yahoo states NASA is planning on testing the first laser-based interplanetary communications system on the Mars Telecommunications Orbiter to be launched in 2009. 'Unlike radio frequency signals that wash over the entire Earth, Fitzgerald and his colleagues will be shooting for a much smaller target - the southwestern corner of the United States.' Does this mean we will soon have telescopes outside of our homes soon to pick up high definition TV signals instead of our current 18 inch dishes?"

Comments

Dishes ARE Telescopes!

[CyberBill]

I always wondered why they would want to use the visible spectrum...

We *CAN* make Laser-Radio waves! They go through atmosphere and trees and buildings....

Very specific uses

[Chairboy]

It's unlikely you'd use lasers for wide scale signal distribution. A laser must be aimed, and to provide a signal to a thousand receivers you would need to fire a thousand beams, or have some intricate device that actively retargets thousands of times per second, squirting packets off to each receiver. Moving parts, complicated, no clear advantage.

Lasers for interplanetary communication is another thing. It's one sender to one receiver, and then you can go radio for inside planetary systems. Eg, you could set up a Mars Relay Station that takes low power local radio transmissions and beams the info back to Earth via laser, and vice versa. You get the advantage of cheap, small radio technology plus the range and bandwidth of laser.

Re:Very specific uses

[Naikrovek]

when i was a kid (early 80s) my dad set up a thing kinda like that. he used a focusable flashlight, hooked it up to an amplifier, and pointed at a sensor he had in the window of our detached garage.

whenever he'd go out there to work, he'd turn on a microphone in the house, and turn the reciever in the garage on. he originally built it when cordless phones were a high-priced luxury, and didn't want to wire a phone just for the garage, but he still wanted to hear the phone ring from in there. later he used it to listen to the TV while he worked outside.

he used a cadmium-sulfide cell on the recieving end. those change resistance according to light. conveniently, they ignore the signal bias (ambient light) and only respond to changes in light intensity. the amplifier inside the house changed the amount of current to the flashlight, and thus the brightness. that variable-intensity light got sent to the CdS cell and the variation in light was reproduced into sound. it sounded surprisingly clear. i don't remember a muffled sound at all.

you could update the design by using polarized light going in two directions. horizontal polarization for transmission, vertical for reception, or simply seperate them a little. our seperated garage had a window adjacent to our home, and light shined into the garage would bounce off the glass and back into the house. if we tried to do two-way then we would have had some signals bouncing off windows in weird ways, and probably some weird sound->light->sound->light feedback loop.

wonder what that would have sounded like...

anyway the setup worked great, and my dad used it until the day he died. good designs last.

I recently tried it again with a laser pointer, but it seems that they have voltage regulators in them that smooth out the variations far too much.

4.3 Gigabytes

[morcheeba]

a little math...

344 million km / (0.3 million km/sec) = 1147 seconds travel time
1147 seconds * 30 megabits/sec peak rate = 4.3 Gigabytes in transit at any instant.

Re:That's really cool, but....why?

[jfengel]

The advantage is that lasers are collimated, which means that the light doesn't spread out in a cone. Since you're concentrating the energy on a few hundred square miles rather than a few million square miles, you can broadcast with a lot less power. You can also make much more reliable communications, which means your bandwidth is higher.

In theory you can do this with any wavelength of light; if you do it with microwaves it's called a maser rather than a laser. Higher frequencies mean more bits, which is a good reason to choose light over microwaves, but the light is absorbed by clouds. I'm not sure about microwave frequencies, and I'm not sure if anybody's ever built a laser-type thing for radio frequencies (raser? I find people joking about it on the Internet but it doesn't seem unreasonable to me).

Eventually you might want a relay system: Mars to earth-orbiting satellite via laser, which then amplifies it and relays it to the earth on a frequency which cuts through coulds better, or just saves it up for a time when it can get through. But the first step is to see if you can get light accurately aimed at the Earth.