Old dishes for the newest satellites
The groundswell of cubesat projects underway at universities, government labs and private companies worldwide promises to generate more data than the ad hoc communications systems originally devised for the tiny birds can handle. But just as the former graduate students who pioneered cubesats a couple of decades ago are finding ways to advance their small-space technology as entrepreneurs, teachers and corporate engineers (see p. 37), the community is starting to grapple with the flow of data expected to be generated as short-lived cubesats give way to swarms of tiny spacecraft carrying cameras, telescopes and other high-data sensors.
When cubesats were getting started as relatively inexpensive teaching tools for engineering professors canny enough to see the lure for prospective students of hands-on experience with real spacecraft, communication with the ground was almost secondary. Typically, each student mission devised its own communications link, usually with a one-off transmitter designed to work on an amateur-radio frequency. That held the cost down at both ends, and it met the relatively simple needs of the day.
Now many see small satellites as the wave of the future for science and military applications, with cubesats at the crest. There seems to be no limit to the applications the undergraduates are dreaming up, after cutting their teeth on simple 1-U cubesats, and the ham-radio links are no longer adequate. One solution proposed at the annual small-satellite conference at Utah State University in Logan looks back to when satellites were small—the beginning of the Space Age—for the infrastructure needed to handle the growing bandwidth needs of cubesats and their slightly larger kin.
“It was built in 1960 like a battleship; in fact, I think the gears came off a battleship,” says's Scott Schaire of an 18-meter (59-ft.) tracking dish at the in Virginia.
The old antenna has been communicating with the DICE (Direct Interestablishment Communications Experiment) mission—a trailblazing two-cubesat science mission measuring ionospheric plasma densities, electric fields and magnetic fields—in the UHF frequency, and Schaire believes it is ready for more and bigger things. Unlike amateur-radio frequencies, which are theoretically off limits for government-backed missions, transmissions through wide-diameter ground stations like the one at Wallops dramatically increase bandwidth—from 9.6 kbps in amateur UHF bands to 1.5 mbps, according to the abstract of Schaire's conference presentation.
, which is making wide use of smallsat capability in its open-ended space technology development effort, plans to refurbish the Wallops dish “in the next few months,” says Schaire.
Interference proved a problem at Wallops, which is home to civil and military government radio emitters, and the dish there works with cubesats only because engineers were able to find a clear band. A backup 21-meter tracking dish at Morehead State University in Kentucky (photo) does not have the same interference problem, and there are dishes in Boulder, Colo., that may also offer cubesat utility. Ultimately, though, the solution for cubesat connectivity will be dedicated frequencies that can carry high-bandwidth links to constellations of the tiny spacecraft.
“There should be a band where we can say a few years from now, 'that's where we're going to go with cubesats,'” Schaire says, noting that he and his colleagues at Wallops andalready are working with NASA headquarters on standardizing cubesat communications.
Given the contentious nature of spectrum allocation, that work could take some time. The Wallops group believes X-band is the way to go for government-backed smallsat com, in part because a lot of the infrastructure is in place.
But while the ground end is ready for X-band, cubesat designers would have some spacecraft development to complete before an operational network can be established.
“People have stayed away from X-band for whatever reason—its high power need, accurate pointing,” the Wallops engineer says. “But if you could get an X-band cubesat radio, you could get hundreds of megabits per second with a large aperture dish, and you can get megabits per second with a small aperture dish.”
The Wallops group has also studied communications with cubesats orbiting the Moon. Schaire says with a large tracking dish, the data rate could be bits or kilobits per second “depending on the antenna on the cubesat.”
A ground network of old tracking dishes could greatly enhance cubesat utility for government-backed missions, and be available for commercial missions at “very reasonable” rates, Schaire says. And with applicable dishes like the ones in Boulder scattered around the world, using them to expand the ground network makes sense, because “if swarms get large and start to use this network, we could get overwhelmed very shortly.”