Communications Strategies, How can the Deep-Space Network be improved on? Options

[Greg Hullender]

Given the recent news that Cassini will lose some data during the Phoenix landing due to contention for DNS access:

http://www.unmannedspaceflight.com/index.p...ic=4831&hl=

It seemed reasonable to create a topic to discuss where the DNS might be going.

Jasedm suggested a relay station at the Earth-Sun L2 point, which I think might be discussing (despite the expense) assuming the relay station could use laser communication with space probes, while beaming data to Earth via microwaves.

I know that experiments with laser communications were part of the now-abandoned Mars Telecommunications Orbiter, and I know that there was even a recent laser experiment involving Mercury Messenger.

http://www.space.com/missionlaunches/060104_laser_comm.html

These seem intended to support laser communications with ground-based receivers, though -- something that seems much too risky if remote space probes can't store days of data allowing for multiple retries.

Obviously a relay station would have its own risks -- you'd probably need two for redundancy -- and it'd be fabulously expensive, but it might also include quite a few savings as well. I've looked for figures for what the DSN costs to run, but I haven't found them yet. Not sure how large a part of a given mission (if any) is charged to the DSN, but it seems to me that a relay satellite ought to be a good bit cheaper to operate, and for far higher bandwidth.

Also, a laser transmitter ought to be lighter and consume less power than an equivalent microwave transmitter. That alone could result in huge savings for outer-system missions.

The only time I saw a serious proposal for a laser-to-satellite communications system was in a description of options for the "Grand Tour" that later became Voyager. (Assuming that counts as serious.) :-) With DSN needing more and more maintenance, I wonder if anyone is seriously considering laser for the future.

--Greg

[Greg Hullender]

Need maintenance be such a problem? Commerical communications satellites in geosynchronous orbit seem to sit there for years and years without it. And, as I mentioned earlier, Geosync is a much better place than the L2 point. (More sunlight there too.)

I do agree that putting radio dishes in space seems pointless, but I do still wonder if laser might not work much better. Dish size for sender and receiver is proportional to wavelength (if I recall correctly) so the size of what you have to orbit ought to be a great deal smaller than what you need on the ground.

I'm still thinking the power requirements for laser vs. microwave should be lower, not higher, since (again, if I recall correctly) it's a lot easier to keep a shorter-wavelength beam narrow. One of the articles on the topic even mentioned that targeting was a problem for laser, since the beam might still be just a few thousand miles across, as opposed to microwaves which are spread so widely that you only need to be roughly accurate.

Something that could reduce the weight and power requirements for all future probes would justify a good bit of expense, I'd think. Are my assumptions just wrong, or is there an additional factor that makes this impractical?

--Greg