There was an interesting news story over on space.com about a new optical detector for use in laser communication systems. Higher frequencies give you higher bandwidth per given amount of signal power, but so far optical receivers have been inefficient enough that the required laser for communication would be fairly heavy compared to radio communication systems. Supposedly a canceled NASA Mars probe would have been able to get 1-30 Mbps with a laser system (depending on the time of the year) compared to about 128kbps that is the norm these days with radio. I’m not familiar enough with the various communication technologies to know if this advancement is as major as MIT would like everyone to think it is, but it definitely sounds interesting. Having high bandwidth communications to various locations in the solar system would make things a lot easier for future space settlements. Now if only there was a way to fix that latency problem…
Jonathan Goff
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Now if only there was a way to fix that latency problem…
We are just going to have to get used to the idea that after you leave cis-lunar space the internets (outside of your local net) are going to resemble Usenet more than anything else.
Which (to my mind) raises all kinds of interesting notions – the first of which is what will a ‘local’ internet look like when it only has to conform to local conditions save for what leaves the net for ‘other’ nets.
Isn’t there also an aiming problem with lasers? ISTR that this has dragged down milsat lasercomm efforts.
Or is that not an issue over interplanetary space because of beam spread?
Big D,
I think you answered your own question. At any sort of interplanetary distance, I think the beam diverges enough to make aiming pretty easy. You’re probably right about close range applications though.
~Jon
Vinton Cerf talked about this when he and Bob Kahn accepted the Turing Award last summer. I know the lecture was videotaped; maybe someday the ACM will acquire the technology to put it online… 🙁
http://www.acm.org/sigs/
sigcomm/sigcomm2005/
turinglecture.html
Isn’t beam spread a solid angle thing? So how does increasing distance make the pointing accuracy requirement any easier?
Usually, if you want to get really high accuracy, you need some sort of feedback system. The long latency makes that a bit of a problem. Feedback is going to be limited to updates to the calibration data.
Usually, if you want to get really high accuracy, you need some sort of feedback system. The long latency makes that a bit of a problem. Feedback is going to be limited to updates to the calibration data.
Maybe I’m missing something. We know where Mars is, and where it will be – this should simplfy aiming.
Space craft will (I imagine) have a well known trajectory – as long as your pointer knows where it is, you’re set. Make a course correction, feed this back to mission control and they’ll still know where you are.
This falls out the window if you’re a military craft and don’t want to advertise your location – but then you won’t want big honking lasers lighting up the sky pointing to you then either.
Brian,
The bit you’re missing (and Jon is missing too) is the pointing accuracy problem. Remember the spacecraft has six degrees of freedom: where it is, and which way it points. You have to point the antenna or laser or whatnot at Earth. Lasers, especially infrared lasers, are desireable because you get more antenna gain — the beam diverges less. This means you have to point more accurately.
Getting farther away from your target does not make your pointing accuracy requirements get any easier. If your beam divergence is 1 degree, you’ll need to point that beam within half a degree of the receiver, or you’ll miss entirely.
As I point out in my post, however, lasers and pointing are just the start of the story.