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GPS Receivers and NMEA Standards

Table of Contents:

  1. GPS Receivers and NMEA Standards
  2. GPS Sentences
  3. Proprietary Sentences

GPS receivers receive almanac data from the satellite and also calculate their position by calculating its distance from then visible satellites and then by using triangulation method to calculate its position.
After the data has been received and position has been calculated, the data is configured according to standards set up by NMEA (National Marine Electronics Association) and is serially transmitted at a baud rate of 4800 bps.
The National Marine Electronics Association (NMEA) has developed standards that describe the interface between various marine electronic equipments. The standards allow marine electronics to send information to computers and to other marine equipments.
GPS receivers also work on these NMEA Standards. Most of the computer programs and devices which provide position and other related information expect the data to be in NMEA format.
The data given by the GPS receiver includes many information like position (latitude and longitude), altitude, speed, time etc. In its standards, NMEA has specified to send a series of data in a sentence. A particular sentence is totally self-reliant and is independent from other sentences. There are standard sentences for particular type of data and for various categories of devices. NMEA has also provided the functionality for individual companies to write their own sentences.

All standard devices have a two letter prefix that defines the device for which it is being used, for GPS receivers the prefix is GP. The two letter prefix is then followed by three letters which represent the content of the sentence. The proprietor sentences allowed by the NMEA always start with P and are followed by a three letter sequence identifying manufacturer code and additional characters to define sentence type. For example a Garmin sentence would start with PGRM and Sony would begin with PSNY.
Every sentence begins with a ‘$’ sign, has about 80 characters and ends up with a carriage return/line feed sequence. Sentences are mostly framed in single lines (may run over to multiple lines sometimes) and the data items in each sentence are separated by commas.
The data received is just ASCII text and varies in precision. A sentence ends with checksum which consists of a ‘*’ and two hexadecimal digits. The checksum digits represent an 8 bit exclusive OR of all the characters between, but not including, the $ and *.
GPS units are made compatible to NMEA standards and are also compatible with serial ports using RS232 protocols. The serial configuration of a GPS receiver is summed as follows :
DATA bits
STOP bits
4800 bps