Description of NMEA Protocol
The National Marine Electronics Association developed a set of specifications and standards to define the interfaces used to connect various pieces of equipment. The NMEA Protocol, first released in March 1983 and known as NMEA 0183, focuses on GPS data transmissions, and all computer programs that work with GPS data are built on the NMEA 0183 protocol.
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Connectivity
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The hardware connection between the GPS unit and the computer is done via computer serial ports. The cable used is a two-wire cable, one wire is used for data received from the GPS unit and the other wire is used for grounding. However, some GPS units use a three-wire cable, with the added wire for data transmission to the GPS unit, used for waypoint or map uploading.
Features
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The NMEA Protocol uses data strings called "sentences" to transmit data between devices. Sentences were designed to be self-dependent and contain essential data packets, which include position, velocity and time.
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Format
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Each sentence starts with a "$" and can contain a total of 80 characters. There are standard sentences available for different device categories, but hardware manufacturers can also use their own proprietary sentence formats. Standard sentences use the "GP" prefix to define GPS data transmission. Hardware manufacturers need to start their proprietary sentences with the letter "P", marking the sentence as being proprietary, followed by a three character code that indicates the name of the manufacturer.
Speed
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The standard speed for the NMEA Protocol is 4800 bits/second. Units usually send data every second or every two seconds to constantly stay updated. However, this speed may be less for some systems that need real-time updates.
Limitation
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At 4800 bits, only 480 characters can be sent in one second, which means you are limited to only 6 sentences of full length, or more if they're shorter. This may be a disadvantage if you require real-time updates with a large quantity of data. However, some newer devices support faster speeds, although those are not officially supported and may have incompatibility issues.
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