Arduino GPS — not exactly pocket-sized, but cool!
http://www.seancarney.ca/projects/arduino-gps-receiver/arduino-gps-system
This is a sketch to interface an Arduino, SparkFun GPS Shield and Nuelectronics LCD Keypad Shield together in a useful way, providing all the functionality of older commercial GPS receivers.
The sketch provides four modes of operation: current location, heading and speed; distance traveled, average and maximum speed; bearing, distance and ETA to a way-point; and the date and time. All settings can be configured on the fly from a configuration menu, and the settings are saved in EEPROM so they are persistent between power cycles.
The sketch uses Mikal Hart's excellent TinyGPS library and includes code from the ArduPilot Projectfor navigation.
This is the power of Arduino shields — it's possible to integrate a few shields to add all the functionality you need for an Arduino-based navigation system. It's also the power of open source: By getting started with open source Arduino code, you can build projects fast!
Arduino Projects FTW 
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