The Global Positioning System (GPS)

The data presented by a GPS serial interface can take several forms, though the NMEA 0183 standard is by far the most common, specified by the US National Marine Electronics Association. This organoscopy provides standards for echo sounder, sonars, anemometer, gyrocompass, autopilot, GPS receivers as well as other instruments

The electrical standard specified by the NMEA is EIA-422 (differential balanced), although most hardware with NMEA-0183 outputs are also able to drive a single EIA-232 port (an unbalanced line interface). A TTL interface is also common. This interface differs in the voltage levels and polarities, but is otherwise identical and can be used interchangeably with simple line drivers.

Example of a GPS Receiver Module

These interfaces differ in the length of cable they may support:

The NMEA 0183 link layer uses a simple ASCII, serial link protocol that defines how data are transmitted in a "sentence" from one "talker" to multiple "listeners" at a time. Through the use of intermediate expanders, a talker can have a unidirectional conversation with a nearly unlimited number of listeners, and using multiplexers, multiple sensors can talk to a single computer port. Transmission is at 4800 baud (8 bits, no parity, 1 stop bit).

USB was introduced in 1996, and offers advantages for PC-connected GPS devices - it provides electrical power, supports higher rates (but over shorter distances), and allows devices to automatically identify themselves to the system. Electrically, it's still a serial protocol, but with much higher signalling rates, and while RS-232 simply defines how bits get from one device to another, the USB standard describes a far more complicated system.

Unfortunately, USB complicates things for small devices. There's no standard defining how to carry GPS data over USB. Most USB GPS receivers require a matching driver to be loaded on the host, and it's the driver that knows how to communicate with the GPS receiver.

See also:

Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland.(2014)