Understanding NMEA 0183
NMEA 0183 is a serial communication standard published by the National Marine Electronics Association that defines how marine navigation devices — GPS receivers, chartplotters, autopilots, depth sounders, wind instruments, AIS transponders — exchange data with each other and with computers. Published in 1983 and revised several times since, it remains the dominant protocol for marine electronics despite being technically superseded by NMEA 2000 for newer installations.
Every NMEA 0183 message follows the same basic structure: a dollar sign, a talker identifier, a sentence type, comma-separated data fields, an asterisk, and a two-character hexadecimal checksum. The simplicity of the format — plain ASCII text at low baud rates — makes it easy to capture with a serial adapter and decode manually, which is why it has proven so durable despite its age.
Talker identifiers
The two-character prefix after the dollar sign identifies which device produced the sentence. Understanding the talker tells you the source before you even look at the data fields.
GP — GPS
GPS receiver, typically single-constellation. Most common talker in older equipment. A sentence like $GPRMC means the RMC position data came from a GPS-only receiver.
GN — GNSS
Multi-constellation receiver using GPS, GLONASS, Galileo, or BeiDou simultaneously. Modern receivers typically use GN as they can receive from multiple constellations at once.
GL — GLONASS
Russian GLONASS satellite system. Seen on receivers that can separate position data by constellation — e.g. $GLGSV for GLONASS satellites in view.
AI — AIS
Automatic Identification System transponder. AIS sentences use a more complex encoding for vessel traffic data. $AIVDM carries received AIS messages from other vessels.
II — Integrated Instrumentation
Integrated navigation system combining multiple sensors. Common on chartplotters that aggregate data from several instruments before retransmitting on a network.
WI — Weather Instrument
Wind and weather instruments. $WIMWV carries wind speed and angle data from an anemometer. The wind direction can be relative to the vessel or true depending on whether apparent or true wind is computed.
Common NMEA Sentence Types
| Sentence | Full Name | Key Data | Typical Source |
|---|
| RMC |
Recommended Minimum Navigation Information |
Position, speed over ground, course over ground, date, time, magnetic variation. The most information-dense single sentence — contains everything needed for basic navigation. |
GPS/GNSS receiver |
| GGA |
Global Positioning System Fix Data |
Position, fix quality, number of satellites, horizontal dilution of precision (HDOP), altitude, geoid separation. The primary sentence for position with quality metrics. |
GPS/GNSS receiver |
| GSA |
GPS DOP and Active Satellites |
Fix mode (2D/3D), satellite PRN numbers used in fix, PDOP, HDOP, VDOP. Useful for diagnosing poor fix quality by examining which satellites are being used. |
GPS/GNSS receiver |
| GSV |
Satellites In View |
Number of satellites visible, satellite PRN, elevation, azimuth, and signal-to-noise ratio. Multiple GSV sentences required when more than 4 satellites are visible. |
GPS/GNSS receiver |
| VTG |
Course Over Ground and Ground Speed |
Course over ground (true and magnetic), speed in knots and km/h. Redundant with RMC but useful when separate sentence streams are more convenient. |
GPS/GNSS receiver |
| DBT |
Depth Below Transducer |
Water depth in feet, meters, and fathoms as measured from the transducer face. Does not include transducer depth below waterline — add vessel draft for true depth under keel. |
Depth sounder / fishfinder |
| MWV |
Wind Speed and Angle |
Wind angle (0–360°), reference (relative or true), wind speed, speed units, status. Apparent wind from the instrument; true wind requires vessel speed and course calculations. |
Anemometer / wind instrument |
| VDM / VDO |
VHF Data-link Message |
Encoded AIS payload containing vessel MMSI, position, speed, course, vessel name, dimensions, and voyage data. VDM = other vessels, VDO = own vessel. |
AIS transponder / receiver |
| HDT / HDG |
Heading True / Heading with Deviation |
Vessel compass heading. HDT gives true heading; HDG includes magnetic variation and deviation for correction to true. Critical for autopilot and radar overlay. |
Compass / heading sensor |
Checksum Validation
Every NMEA 0183 sentence includes a two-character hexadecimal checksum after the asterisk that allows receiving devices to detect data corruption. The checksum is calculated by XOR-ing all bytes between the dollar sign and the asterisk — the delimiters themselves are not included.
For the sentence $GPRMC,092751.000,A,5321.6802,N,00630.3371,W,0.06,31.66,280511,,,A*45, the checksum 45 is the hex result of XOR-ing all characters from G through A (the last character before the asterisk). A receiving device recomputes this value and compares it to the transmitted checksum — if they differ, the sentence is discarded. This decoder validates checksums and flags mismatches so you can identify corrupted sentences in your data stream.
Frequently Asked Questions
What is the difference between NMEA 0183 and NMEA 2000?
NMEA 0183 is the older standard using simple ASCII text over RS-232 serial connections at 4800 or 38400 baud. It uses a point-to-point or point-to-multipoint wiring approach where one talker transmits to one or more listeners. NMEA 2000 (also known as N2K) is the modern successor using a CAN bus network where up to 50 devices share a single backbone cable. NMEA 2000 supports much higher data rates, bidirectional communication, device addressing, and standardized binary message formats called PGNs. Most modern marine installations use NMEA 2000 for the core network with NMEA 0183 bridges for compatibility with older equipment. Many chartplotters output both standards simultaneously.
Why do my GPS sentences show empty fields?
Empty fields in NMEA sentences — indicated by consecutive commas with nothing between them — mean that particular data is not available at the time the sentence was transmitted. Common causes include: no valid fix yet (RMC and GGA fields will be empty until the receiver acquires enough satellites), magnetic variation database not loaded (the variation fields in RMC will be empty), or the sentence type does not apply to this device's capabilities. A fix validity flag of V (void) instead of A (active) in an RMC sentence tells you the position data is not reliable even if coordinates are present — this happens during the brief period after power-on while the receiver is acquiring satellites.
How do I capture NMEA data from my instruments?
NMEA 0183 devices output serial data that you can capture in several ways. A USB-to-serial adapter connected to the instrument's NMEA output wires is the simplest approach — the data appears as a COM port in Windows or a /dev/ttyUSB device on Linux. Many chartplotters and MFDs expose NMEA data over TCP/IP on port 10110 of their WiFi network, which lets you log sentences wirelessly using netcat or a navigation application. OpenCPN, Expedition, and most other navigation software can both receive and log raw NMEA streams. For permanent installations, dedicated NMEA multiplexers from Actisense or Digital Yacht aggregate multiple instrument streams onto a single output and typically include data logging capability.
What does HDOP mean and what values are acceptable?
Horizontal Dilution of Precision (HDOP) is a dimensionless number that indicates the geometric quality of the satellite constellation being used for your position fix. When satellites are spread across the sky, the geometry is good and errors in individual measurements have less impact on the computed position — this gives a low HDOP. When satellites are clustered in one part of the sky, a small measurement error causes a larger position error — high HDOP. As a practical guide: HDOP below 2.0 is excellent and suitable for all navigation, 2–5 is good for most purposes, 5–10 is moderate and suitable for general guidance, above 10 indicates poor geometry and unreliable position data. Most modern GNSS receivers with sky view rarely see HDOP above 2.0.