I am a member of the team developing BikeAngel. My implications are for the development of the device itself (the hardware) and part of the embedded software.
The project can be found here:
https://www.kickstarter.com/projects/bike-angel/bike-angel
It's difference from the rest of the devices that have similar specifications is the fact that it is placed inside the frame of the bicycle and it is not visible and it is very hard to access.
The device consists of a GSM module, a GPS module. The positioning is determined from the GSM cell and the GPS position and the GPS coordinates are transmitted through GSM or GPRS.
Some say that, being inside the bicycle frame means that it is shielded - it's not. Try putting you phone in the microwave oven(with the oven turned off , don't start it...you can even let it unplugged) and call it(don't turn on the oven).
The phone will ring without a problem.
Put your phone inside a metal case(a STR8 parfume metal casing). It will still work. Shielding depends on materials, thickness, and other stuff, it's not so easy to understand.
And remember, there is an opening on the top.
The device will have an autonomy of 30 days and it will have rechargeable batteries.
I hope I raised your interest for this device. I am opened for suggestions and questions.
