NASA Future In-Space Operations: What does the future hold for the Deep Space Network
NASA FISO Presentation: The Deep Space Network – The Next 50 Years
“Dr. Leslie J. Deutsch is the Deputy Director of the Interplanetary Network Directorate at NASA’s Jet Propulsion Laboratory. This Directorate provides information services to spacecraft exploring the solar system and beyond. The Directorate’s facilities include NASA’s Deep Space Network, the giant antennas used to communicate with these spacecraft.”
Note: The audio file and presentation are available online and to download.
DSN: An underappreciated jewel
Most of the regular users of the DSN appreciate it, and its people, very much. Unfortunately, they have an infrastructure problem. It’s hard for anyone to get adequate funding for improvements, or even maintenance to sustain existing capabilities, when you are “just” supporting other projects. Funding a new planetary mission sounds good and exciting. Funding the guys running the radio which will get that new mission’s data back can look dull and unexciting. Somehow, the dull but vital jobs rarely get the attention and funding they actually need.
Good points. I was thinking of underappreciated in the context of the broader public, perhaps even other scientists, who haven’t thought about the problems of finding and talking with such far flung instruments decades after launch. I also agree with your broader point about funders wanting the flash. Once the ribbon is cut, they move on.
Same thing is true for many tasks supported by institutional funds, like in-house research and development. They had to change the name of “cross-agency support” because management could not understand why it was needed.Everything has to be charged to “projects” that support the “NASA Misison”. But in reality NASA does not have such a unitary mission and cannot do usefil R&D without the ability to flexibly allocate resources.
And it’s not just planetary missions that rely on it. All the astrophysics spacecraft significantly beyond earth orbit use the network, for example, Spitzer. This will be increasingly common, as getting away from the Earth has a lot of advantages for high sensitivity missions.
They want to migrate to optical for higher data rate. I wonder what aperture this will need? It seems to m that optical would be very sensitive to weather and the earth stations would be better located in space, maybe on the TDRSS satellites, which already have a high bandwidth groundlink.
One slide mentions 12-meter stations, and from the context, that seems to mean the optical stations. I hope I’m misunderstanding, since that’s huge. But I suspect the optical station would need a larger mirror than you could put on a relay satellite (JWST will have a 6.5 meter mirror.)
Weather is an issue I’ve been thinking about for some time. The current plan, well underway, calls for shifting from X to Ka band radio. That gives you an estimated factor of four increase in data rates, but it’s also more sensitive to weather. So, in effect, the shift to Ka has the same issues as the shift to optical, just of a smaller magnitude.
I don’t think this is too much of a problem for routine operations. Normally, missions are not in continual contact with the Earth; they store data for a day or two and then have a downlink session to empty the recorders. With improvements in on-board storage, and some changes in operational practices (e.g. no immediate over-writing after playback), you could just replay any data lost to bad weather at the ground station. That means the improvement from shifting to Ka band won’t be the claimed factor of four. Weather and rebroadcasting would add inefficiency to the process. I won’t be surprised if a were only a factor of three. The same approach, on a larger scale, would probably apply to an optical link.
That would not work for critical events, such as an orbital insertion. But I don’t think anyone would use an optical link for critical events and safings. That can be done with a low gain radio antenna and a low data rate.
I also worry about the viability of the optical system at large distances. It’s never been demonstrated beyond lunar orbit, and I don’t think the presentation’s claim that this will be done on the next Discovery mission isn’t quite true. The AO encouraged but did not require proposals to carry a optical communications demonstration. Of the five finalists, I believe some do and some do not. So the demonstration depends on which mission is selected.
My biggest concern is the pointing stability requirements. For the sort of science I do for a living, the measurements place very forgiving requirements on spacecraft pointing. A magnetometer, for example, might only require after-the-fact knowledge of pointing to +-0.1 deg., and no need to control the spacecraft’s pointing. Other measurements strongly benefit from a spinning spacecraft (which places limits on how closesly an antenna/telescope can be pointed at the Earth.) For current X or Ka band radio, that isn’t too bad. But if we have to use optical communications, keeping the antenna/telescope pointed at Earth could drive the spacecraft’s attitude control requirements and/or preclude spinning spacecraft.
The presentation is correct.
That is a vague statement. Are they planning on multiple, 12 meter telescopes to support optical communication? The presentation was not clear on that point.
MESSENGER’s ranging laser was detected from Earth while in transit to Mercury. Considerably beyond lunar orbit. It was modulated but I don’t recall if by data.
A detection of an optical signal is not a demonstration of a viable communications link. A viable link has been demonstrated by a lunar mission. A link, rather than a detection has not been demonstrated at greater distances.
What is the attraction of developing a light-based tech? Bandwidth, yes; anything else?
The narrow beam and high data rate mean that power requirements are lower, if you can achieve to pointing accuracy needed. Personally I feel the term “bandwidth” is not exactly accurate, since by its nature a laser has an extremely narrow bandwidth in terms of its actual carrier frequency.
In practice, transmitted power, antenna/mirror size (beam width at a given wavelength) and data rate are all tied together. Going to higher frequency radio or to optical communication would let you either increase data rate, reduce antenna size or reduce transmitted power. Or combinations of the three. But I think the DSN is thinking in terms of higher data rate. Antenna size and power are spacecraft issues, and the DSN isn’t really in a position to dictate spacecraft design trade-offs. So they probably think in term of the bit rate similar spacecraft could send down.
Along those lines, there are ways to improve downlink without changes to the DSN. Data compression, for example, is not as good as it could be. Some instruments on planetary missions don’t compress data at all. For the rest, techniques like Rice compression are typical. That is far less effective than modern techniques used by, for example, the smartphone I’m typing on. Partially that’s a lack of on-spacecraft CPU power. Partially, I think there is a reluctance to have non-deterministic data production. Similar issues apply to on-spacecraft editing or processing. But, with more CPU power, I think you could manage a factor of five or more increase in effective data rates.
Ah, once again the lack of computing power aboard spacecraft and rovers rears its ugly head. We don’t have a single billion-dollar spacecraft with the computing power of your cell phone. What happened to that new program called, what was it…. space technology?
From the presentation it appears that for optical as for radio a larger aperture on the ground allows a smaller aperture or lower power on the spacecraft so if laser receivers are groundbased they would be as large as practical, possibly either one 4-meter or (more likely) two or more 2-meter telescopes.
here’s some info http://www.osa-opn.org/home… At some point int he future Earth stations could be small enough for space-based installation, particularly if a receiver array is used. The ISS might be a good test site for a 1-meter test receiver. As you note, pointing accuracy is a significant challenge. Weather problems might be overcome by joining forces with European and other networks with ground stations in other locations to provide redundancy.