Comment by sliken

A friend is in involved in the sat based version. They did manage incredible optimizations for power efficiency.

With a clock, pressure sensor, light sensor, a low power radio you can, and a bit of compute you can:

  * Use sunrise/sunset to get an idea of your longitude
  * Use the length of the day get an idea of your latitude 
  * Then use the above and some orbital calculations to find a 1-2 minute window when the sat will be visible.
  * Use the sat communications to upload a few 100 bytes and update your local clock, and get a better idea of your longitude.

Such optimizations would allow a 1-2 gram transmitter to last for months to a year.