Interview Question for Associate Product Manager at Google:

First of all, it should be attached in the center of the handle area, for better visibility and reception. Should not be bigger than half the size of a hand. Firmware and user settings should be recorded on a flash memory inside the GPS unit.

Energy source depends on bike dynamo, with small Lithium battery.

When running the algorithm (namely A*, or any other) it should give priority to bike paths, previously loaded in the device.

For usage afterwards, GPS unit records bike position every 10 seconds and interpolates between positions to establish that day's route. User can load data into the computer and upload it to the website of choice.