How the GPS system works
10 May 2015
This document explains the inner workings of the GPS system. Ever wanted to learn more about the C/A code or the P/Y code.Want to learn more about SA (Selective Availability). How does the GPS communicate with a PC?
The GPS system is described in details here.
NEMA
NMEA is a standard protocol, use by GPS receivers to transmit data. NMEA output is EIA-422A but for most purposes you can consider it RS-232 compatible. Use 4800 bps, 8 data bits, no parity and one stop bit ( 8N1 ). NMEA 0183 sentences are all ASCII. Each sentence begins with a dollarsign ($) and ends with a carriage return linefeed (<CR><LF>). Data is comma delimited. All commas must be included as they act as markers. Some GPS do not send some of the fields. A checksum is optionally added (in a few cases it is manatory). Following the $ is the address field aaccc. aa is the device id. GP is used to identify GPS data. Transmission of the device ID is usually optional. ccc is the sentence formatter, otherwise known as the sentence name. (Read more in the FAQ.) Read more from the source.