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.


Wednesday, November 13, 2013

A Powerful Wallet

One of the challenges for smart phone users is limited battery life.  While there are external batteries available to allow for on the go phone charging, these external chargers have two flaws aesthetic and clutter.  An external battery system is difficult to make aesthetically pleasing.  Beyond the aesthetic consideration many consumers are left with the challenge of finding space for another device to keep track of.  Personally as a male consumer, who isn't interested in getting a men's purse and has been told that cargo pants are no longer acceptable attire, I would rather limit the number of things I bring with me for a night (please note that I am simply referring to my personal experience, I feel that both men and women would appreciate a new solution).
To solve the concern of sufficient battery life in a convenient package, I would like to propose the "Power Wallet"  The Power Wallet aims to be a smart wallet for modern consumer, shaped like a standard bi-fold wallet, it would users to recharge their smart phone via an inbuilt USB power connector, all while still serving as a traditional wallet.

 For the Power Wallet to make sense it must achieve these design parameters, the volume should not exceed that of a standard wallet, it should be able to contain an assortment of currency and credit cards, all wiring elements should be able to be stored conveniently, and the wallet should be robust enough to survive regular daily use.  Ideally the design would provide minimal interference with RFID/NFC based communication standards used in many card systems.  Additional features might include, solar panels on the back of the wallet, a mini-flashlight, e-ink display (for reasons), the ability for wireless charging, and outlet prongs for the internal DC adapter(this might be pushing things).

 
The hard science limit of the design is the energy density of battery technology, for Li-Ion batteries this  value currently ranges between 250 Wh/L to 730 Wh/L. If we assume a middle value of 500 Wh/L= 500mAh/cm^3  we can then start to estimate how large of a battery we can add into the wallet.  For reasons of safety I am also going to imagine that the battery requires 2 mm of some kind of external protection, before the material of the wallet is considered.


Credit Cards have the dimension of 53.98 mm x 85.6 mm x 0.76 mm 
Starting from these baselines and a quick test of simply placing my cellphone's battery in my wallet we can assume that the Smart Wallet's internal battery could have the external dimensions of 6.4cm x 9.4 cm x 0.6 cm, making the actual battery volume 6.0cm x 9.0cm x 0.2 cm or 10.8 cm^3 continuing with the energy density assumption of 500mAh/cm^3 we can estimate the ability to store up to 5400mAh of power, or enough to fully charge a smart phone one and a half times (there are some efficiency issues to consider).  

While these numbers do sound appealing they should be taken with a grain of salt, to the best of my knowledge Li-Ion battery manufacture doesn't generally allow for company's to order batteries to volume, most of the sub-elements of the battery, referred to as cells, are cylindrical in nature, consequently the actual storage capacity of the battery system will be less.  

April 17 2014 It turns out someone was already looking into this idea, here is an article on gizmag that highlights a credit card sized back up battery.