A Global Positioning System, also known as GPS, is a system based on satellite navigation and designed to help navigate on the Earth, in the air, and into water. It is designed to provide location and time information in all weather conditions if there is at least four GPS satellite data available in line of sight.
GPS receiver can be used worldwide to find three-dimensional location (latitude, longitude, and altitude) navigation and precise timings using many channel satellite broadcast signals from space. The GPS receiver’s tracking sensitivity allows continuous position coverage in nearly all application environments. NMEA 0183 v3.0 protocol supports baud rate of 9600 or 115200 bps.
This GPS receiver Modem is based on SIMCOM Sim28M/Sim28 ML GPS Module. SIM28 ML is stand alone or a GPS receiver with built in LNA. SIM28M can track as low as -165dBi signal even without network assistance. SIM28ML has excellent low power consumption characteristics and it supports various location and navigation applications.
NMEA Messages and Protocol
The serial interface protocol is based on the National Marine Electronics Association’s NMEA 0183 that defines communication interface and the data format. The GPS receiver supports the latest release of NMEA 0183, Version 3.0.
All NMEA-0183 begin with $ and end with a carriage return and a line feed. Data fields follow comma (,) delimiters and are variable in length. Null fields still follow comma (,) delimiters, but contain no information. An asterisk (*) delimiter and checksum value follow the last field of data contained in an NMEA-0183 message. The checksum is the 8-bit exclusive of all characters in the message, including the commas among fields, but not including the $ and asterisk delimiters. The hexadecimal result is converted to two ASCII characters (0–9, A–F). The most significant character appears first.
Some of standard NMEA 0183 protocol used are:
1. $GPGGA – Global Positioning System Fix Data
2. $GPGLL – Geographic position, latitude / longitude
3. $GPRMC – Recommended minimum specific GPS/Transit data
4. $GPVTG – Track made good and ground speed
NMEA 0183 Message Format Description
|“$”||The “$” signifies the start of a message.|
|ID||The identification is a two letter mnemonic which describes the source of the navigation information. The GP identification signifies a GPS source.|
|MSG||The message identification is a three letter mnemonic which describes the message content its number and order of the data fields|
|“,”||Commas serve as delimiters for the data fields.|
|Dn||Each message contains multiple data fields (Dn) which are delimited by commas. The length of the fields can be variable.|
|“*”||The asterisk serves as a checksum delimiter.|
|CS||The checksum field contains two ASCII characters which indicate the hexadecimal value of the checksum.|
|[CR][LF]||The carriage return [CR] and line feed [LF] combination terminate the message.|
NMEA 0183 standard messages vary in length, but each message is limited to 79 characters or less. This length limitation excludes the “$” and the [CR][LF]. The standard message data field block, including delimiters, is limited to 74 characters or less.
Example of NEMA message format
GGA – Global Positioning System Fix Data, Time, position and fix related data for a GPS receiver.
Structure: $GPGGA,hhmmss.sss,ddmm.mmmm,a,dddmm.mmmm,a,x,xx,x.x,x.x,M,,,,xxxx*hh<CR><LF>Field 1 2 3 4 5 6 7 8 9 10 11
General GPS Receiver User’s Tips
If the satellite signals cannot be locked or experiencing receiving problem (while in urban area), following steps are used:
a) Relocate the GPS receiver near window or open sky for better receiving performance.
b) Move to another open space or reposition GPS receiver toward the direction with least blockage
c) Move the GPS receiver away from the interference sources.
d) Wait until the weather condition is improved.
- Some vehicles having heavy metallic sun protecting coating on windshields may affect signal receptions
- Driving in and around high buildings, and tunnels may affect signal reception.
- When GPS receiver is moving, it will take longer time to get its position fixed. Wait for satellite signals to be locked at a fixed point when first power-on the GPS receiver to ensure quick GPS position fix
- Weather will affect GPS reception – rain & snow contribute to worsen sensitivity.