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

[jasedm]

" We've lost high-value encounter data at times due to the lack of a north-south redundant DSN pair during hardware failures or severe rain events, etc.

Not trying to put you on the spot here Ed, but can you give examples? I'm trying to build up a picture of the percentage (cost) of a mission which could conceivably be written off due to deficiencies in the DSN.

[edstrick]

My crusting over memory cells fail me at the moment. I recall some Cassini data lost due to outage and tracking conflicts. There was some significant data, but I don't recall what, lost from some mission a few years ago due to rain over Australia (I think) that an imaginary Japanese DSN station might have caught. But in general, a southern DSN station gets fewer high-quality tracking hours with a spacecraft high northerly ecliptic latitudes and vice versa, and rain events that muck up X-Band will RARELY occur at 2 different DSN stations at similar longitude but opposing latitude.

Mariner 10 did lose it's third encounter high rate data, planned around high resolution mosaics in the vicinity of "targets of interest" near closest approach, I believe due to a DSN breakdown. They had to switch to lower data rates and transmit only a vertical strip corresponding to the central 25% of each image which obliterated any contiguous coverage the spacecraft's wobbling attitude control would have permitted. The third encounter images are mostly forgettible, otherwise, it would have been as good as the other 2 encounters. The real science value was in the close in high latitude nightside flyby which confirmed the magnetic field was internal in origin, but that's partly due to the loss of 3/4 of the imaging data.

Putting a $$$ on it is worse than hard with arm-waving vague recollections, but we're stretched thin and the current 3 spacecraft in safe mode and one doing an encounter status point out just how thin is thin.

[nprev]

Probably the only argument that really has a chance of working is financial. "We spend $X on these missions, and lose critical data due to DSN shortfalls and aging equipment, which grows more expensive to maintain each year. Therefore, we need to spend $X on upgrading the DSN in order to maximize the return on investment from these missions."

Gotta speak in beancounter in order to persuade beancounters...

[helvick]

I've wondered before if the technologies and techniques that will be needed to build and manage the Square Kilometer Array would make it easier (and cheaper) to build array based additional DSN capability. I'm also curious about how the SKA is being funded as it's not immediately clear from their web site where the €1.5bn that it is currently estimated it will cost to build will come from. Obviously there are many different engineering issues between building a science telescope and a deep space tx\rx ground station but it seems to me that there are many more similarities than differences and some sort of partnership between the SKA project and a global project to extend the DSN in terms of capability and redundancy would be very financially beneficial to both.

[Greg Hullender]

I'm still thinking we just want to lobby for a "NASA Attachment" to ordinary commercial communications satellites. Something to sit on top, collect laser images from remote spacecraft, and send the data to NASA in the same Earth-bound datastream the satellite already manages.

Uh, I'm not proposing trying to add this to existing satellites. ;-)

--Greg

[djellison]

A reception facility for optical comms isn't a 'bolt on' sort of resource - I don't have an exact figure, but we're talking metre+ scale light buckets.

Doug

[Greg Hullender]

Didn't mean to trivialize the problem. Obviously there are a number of complications, including the fact that the optical part needs to be able to swivel to target different parts of the sky while the comm satellite always points "down". On the other hand, the comm satellite is very large -- maybe the side of a bus -- so we're probably not adding a large percentage to its mass. Also, the back part of the satellite (facing away from Earth) doesn't currently do much of anything, so an optical receiver on a boom extended away from the rest of the satellite shouldn't be in the way.

Best place would probably be on some successor to the TDRS L&K Satellites that NASA just hired Boeing to build. Given NASA is already paying to put up its own comm satellites in Geosync orbit, that would seem to be the logical place to put something like this. And the lifetime of the new satellites is probably 15 to 20 years, which is probably the right timeframe to evaluate optical comm systems.

--Greg

[dvandorn]

I still have this image in my head of an antenna farm consisting of 10,000 old DirecTV and/or Dish Network 50cm dishes, all attached to altazimuth drives and being pointed by a central computer complex. Just lay the thing out on any convenient piece of desert or tundra, and run the multitude of signals received through rectifiers to pull out the weak signals coming from far away.

I'm sure there are good reasons why this would never work, but it seems like a good, cheap way to quickly put together a temporary addition to the DSN.

-the other Doug

[mcaplinger]

I've wondered before if the technologies and techniques that will be needed to build and manage the Square Kilometer Array would make it easier (and cheaper) to build array based additional DSN capability.

Antenna arrays for the DSN have been considered for quite a while and I believe are now in active development.
For example, http://www.jb.man.ac.uk/ska/workshop/Weinreb.pdf

This certainly makes more sense that naive ideas of orbital facilities.

[edstrick]

"...still have this image in my head of an antenna farm consisting of 10,000 old DirecTV and/or Dish Network 50cm dishes..."

The idea is not bad, but there's an optimum antenna size that balances antenna size, tracking mount size, and microwave components <feed horn, amp, whatever>. As pointed out above, antenna farm arrays are viable, but more in the scale of C-Band satellite dish, or small commercial dishes for communicating with synchronous sats.

[helvick]

The idea is not bad, but there's an optimum antenna size ..

The expense that would result from the multiplication of the tracking, control, cooling and low noise tx\rx circuitry (for a start) would make an array of domestic satellite TV dishes very expensive very quickly. To get into the range of the signal to noise levels from a single 35m antenna you would need something like 4000 of the small home satellite TV dishes.

The SKA seems to be opting for 12-15m component dishes as that appears to be the economic sweet spot - you still need 34 of those to give you the equivalent of a single 70m dish (in terms of S/N ratio).

[edstrick]

Exactly. And 34 of those -- with sufficient smarts -- can re-deploy to give lower aperture equivalents to 2 or 3 vehicles at once and provide more capability to handle more than one task at once.