The nmea package is a for reading nmea standard data from a serial port or file. It supports logging serial data to a file and playing it back at a variable rate. There are no gps-specific functions in this module, it is more of a support library for an eventual gps module. The nmea protocol supports many devices besides gps although that is the most well known one.
Documentation is at http://tcllib.sourceforge.net/doc/nmea.html
For more information on the nmea protocol see