Over the last couple of months I've ported the Plan13 satellite tracking algorithm to a library for the Arduino platform. To inspire Arduino hobbyists and fellow amsaters, I've made a website to explain and explore this material, http://sites.google.com/site/qrptracker/ There you'll find a 4 min. introductory video. Around 1:33, the Arduino demo starts.
Though I was originally thinking of making a sort of doppler tuning dongle for my FT-817 -- a sort of LVBTracker for the trail -- along the way I've become aware that there are many uses of a $4 device that can track 13 tles all alone, and leave about 14kB for other programming, all with the possibility of average power use in the tens or hundreds of microwatts and SMD dimensions of 8mm^2.
For instance, equipped with a low-power tracker like this, an amateur satellite could track its own location over the earth by means of a reliable real time clock, and it could change its mode depending on its calculated location, say turning off its beacon over the poles and oceans. (Plan13 *in* outer space!)
I've implemented this idea in the demo http://code.google.com/p/qrptracker/source/browse/trunk/modeSwitcher_powersa... Regions of earth, defined with latitude and longitude limits, are stored as an array, and when Plan13 finds the satellite is within one of these regions, it raises one of its pins to a 'high' state, otherwise, this pin is 'low'. When not calculating position, the AtMega328 can drop into a low-power sleep, consuming around 20 microWatts, but retaining the state of its signaling pin. The satellite uses the state of the pin to turn off its beacon, or to switch from a linear to an FM mode. I plan to illustrate this in a future screencast.
Of course, the same idea could be implemented in a multitasking IHU of any platform. qrpTracker is offered as a simple means by which people can play around with satellite tracking in microcontrollers, and I welcome collaborators and bug reports at its open-source code repository at http://code.google.com/p/qrptracker/
Finally, if anyone has use of a chip that spits out AZ/EL and doppler info, either through serial-line requests or off of the I2C bus, I'd be happy to send off a chip loaded with the keps you need. Your application would have to supply the real time clock, and eventually the loaded keps would get stale, but by that time you'll be hooked and have the equipment to update the keps yourself :-)
73, Bruce VE9QRP