Friday, November 22, 2013

Communication with CubeSats

The final frontier is becoming more open.  Humans have gone from two players in the space game in the 1960's to a world where universities and small groups can launch their own mini-satellites into orbit by piggybacking on the delivery of much larger satellites into orbit.   These mini-satellites are generally designed to meet the Cube-Sat design specification, where the shape of the satellite must be cube like and each component cube has an edge length of 10 cm.  The consistency of the dimensions of a Cub-Sat make it easier for companies to accommodate for a known shape and volume, this is the same rational behind flat-rate boxes and shipping containers, while you may not always need the full volume you have a very clear idea of how much space you are allowed and the delivery company knows how to mail boxes that size.

Like many design choices the dimensions of a Cube-Sat have their trade-offs, while Cube-Sats can be easily launched in relatively large numbers, the small volume limits the available space for communication equipment, the less space for communication hardware, generally speaking, the lower the maximum bandwidth of the satellite.  As Cube-Sats become a more common tool of scientific exploration there is always a desire for more data from these small boxes.  One potential solution being investigated at MIT is to re-purpose an old anti-aircraft gun mount as a platform for 6 meter diameter satellite dish that will be large enough to concentrate the signal from research satellites and effectively communicate.  The challenges of overhauling this system has led to the question, instead of using a single massive communication array to contact orbiting Cub-Sats, why can't researchers use dozens, if not hundreds, of smaller dishes to communicate?  
SETI, the search for extre terrestrial intelligence has already done something similar to listen to the stars in the hopes of finding signs of alien life.  Instead of  a single massive radio antennae like the Aricebo Observatory SETI aims to use 350 antennae to create the equivalent signal capturing ability as a 100 meter radio telescope.  I would like to propose something similar, but on a smaller scale.  Instead of a single 6 meter diameter dish, with an effective collecting area of roughly 30 meters, network roughly 100 tracking dishes with a diameter of 1 meter.  Achieving the same operational surface area with a much more granular control on the effective collecting area and more critically, greater flexibility on where the platform can be deployed.  Now instead of a single use dish design solution, amateurs and professionals around the world could build their own radio arrays capable of creating data links with research satellites.


Now for the caveats, and there are many, the first is technical, would the mini-dish solution actually make sense for both uplink and downlinks.  As a data down link I am reasonably certain that this array concept could work, if the SETI design works I don't see why a smaller diameter dish array for closer signals couldn't work.  As to uplinks that is more dubious, I don't know enough about the physics of radio communication to make any real comments beyond the fact that I like this idea.  The second limitation is legal, which is also a big one, assuming the physics works out, the international community would need to establish some part of the EM band that could be used by these mini-arrays without interfering with other established orbital infrastructure.  The third is economics, while I would generally suggest that this design be open-source regardless, the scope of the market is hard to judge.  While the intent of smaller dishes is intended to lower the barrier of entry, the number of separate moving components might actually not make financial sense.  While the cost of an individual dish would be less than that of a larger tracking system, cost to own and operate the system over time is a greater concern.  Hundreds of dishes with at least 2 moving parts and sensitive electronics would require quite the maintenance effort.   Hopefully a lower cost of installing less massive infrastructure would mitigate the overall cost, the lower installation cost and the potential for economies of scale might make a technology like this viable.


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