WHAT IS NMEA 2000 and how to judge the communication quality of NMEA 2000 protocol?

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Definition
NMEA 2000 is a standard protocol for communication between ships and Marine electronics. Developed by the National Marine Electronics Association (NMEA-National Marine Electronics Association), it builds on the previous NMEA 0183 protocol to provide a more efficient and flexible communications standard. To adapt to the increasingly complex functional and data exchange requirements of modern Marine electronic equipment.
Protocol architecture
NMEA 2000 is based on the Controller Area Network (CAN) bus technology. The CAN bus was originally developed for communication between electronic control units (ECUs) in the automotive industry. In Marine applications, it provides a reliable, high-speed network connection. The network topology can be a linear bus, a tree structure, or a combination of both. This flexibility allows equipment to easily access the network and adapt to different ship layouts and equipment numbers.

For example, on a small yacht, GPS navigators, depth gauges, and engine monitoring devices CAN be simply connected to the CAN bus in a linear manner; In large merchant ships, a more complex tree-shaped network can be built to connect numerous sensors, navigation equipment, communication equipment, etc.

Data transmission characteristics
High speed: NMEA 2000 has a data transfer rate of up to 250 kbps, a huge improvement over the NMEA 0183 protocol (maximum baud rate of 4800bps). This allows it to quickly transmit a large amount of navigational data, such as the ship's position, speed, heading, depth, weather information, and the status information of various equipment. For example, when a ship is traveling at high speeds, rapidly updated position and speed data can be transmitted between navigation equipment and autopilot equipment in a timely manner through the NMEA 2000 protocol, ensuring the accuracy of navigation.

Multi-device communication: It supports multiple devices to communicate at the same time. In an NMEA 2000 network, devices from several different manufacturers can be connected, as long as these devices comply with the NMEA 2000 standard. This is because the protocol sets uniform rules for data formats and communication. For example, shipowners can choose from different brands of GPS, radar, sonar and autopilot equipment, all of which can exchange data and work together through the NMEA 2000 network.
Data format specification

The NMEA 2000 protocol has strict regulations on the format of the data. Data is transmitted across the network in the form of messages, each with a unique identifier called a PGN-Parameter Group Number. PGN identifies the type and content of a message and ranges from 0 to 65535. For example, PGN 129025 indicates the position information of the ship, including longitude, latitude, altitude, etc. PGN 129026 Indicates the speed information of the ship, including speed to the ground and speed to the water. The device can recognize and process the received data based on PGN, enabling accurate data sharing and interaction.
Application field

Marine navigation and autopilot: In Marine navigation, the NMEA 2000 protocol enables GPS navigators to transmit accurate position, speed, heading and other information to other devices on board, such as electronic chart displays, autopilot and so on. The autopilot can automatically adjust the course and speed of the ship according to this information, combined with the preset route and electronic chart, to achieve automatic navigation. For example, when the ship is sailing at sea, the navigator sends the real-time position of the ship to the autopilot through the NMEA 2000 protocol, and the autopilot automatically controls the rudder Angle of the ship according to this information and compares it with the preset course, so that the ship travels along the correct course.

How to judge the communication quality of NMEA 2000 protocol?

Data transmission accuracy
Position information verification: For nautical applications, position information (longitude and latitude) is one of the most critical data. This can be determined by comparing the location information provided by multiple NMEA 2000 compliant navigation devices, such as different brands of GPS. If the location data they display is within a reasonable error range (usually within a few meters to tens of meters, depending on equipment accuracy and environmental factors), the data transmission accuracy is high. For example, in an open and well-signaled environment such as a port, the error of the ship's position displayed by the two devices is less than 10 meters, indicating that the communication of the protocol is of good quality in transmitting location data.

Speed and heading data verification: By looking at the consistency of speed and heading data to determine. When the ship is sailing steadily, the speed and heading data obtained from different equipment (such as shipborne radar, log, etc.) should match. If the speed data differ by more than a certain range (such as more than 10% of the ship's normal speed fluctuation range) or the heading data differ significantly (more than 5-10 degrees), it may indicate a problem with the quality of communication.

Data transmission integrity
Message integrity Check: Each message of the NMEA 2000 protocol has a fixed format and content, including the message header, parameter group number (PGN), data fields, and message tail. You can use a professional network monitoring tool or the diagnostic function of the device to check whether the received message is complete. For example, if a message containing ship depth information (PGN 129029) is missing parts of the data field (such as the significant numeric part of the depth value), the data transmission is incomplete and the communication quality is poor.

Data loss rate assessment: Statistics of data loss during a long period of data transmission. Some advanced network monitoring equipment or software can record the number of data lost and the amount of data lost. In general, the data loss rate should be lower than a certain threshold, for example, in normal navigation, if the data loss rate exceeds 1%, the normal use of the equipment may be affected, and the communication quality needs to be further checked.

Data transfer rate and real-time
Rate test: Using professional network test equipment to measure the data transfer rate of the NMEA 2000 network. Its theoretical maximum speed is 250kbps, and in practical applications, although it may not reach this maximum due to factors such as device performance and network load, it should also be within a reasonable range. For example, in a medium-sized Marine electronic equipment network (including GPS, radar, engine monitoring and other equipment), the data transmission rate should be able to stabilize at about 100-200kbps to ensure that various data (real-time location, speed, equipment status, etc.) can be transmitted in a timely manner.

Real-time detection: Observe whether the data is updated in time. For example, for a ship's position data, when the ship is traveling at high speeds (such as 15-20 knots), the position information should be updated every few seconds (usually 1-5 seconds) to ensure that the navigation equipment and autopilot equipment can operate according to the latest information. If the data update delay is significant, more than twice the normal update period, it may be caused by insufficient communication rate or network congestion, which will affect the communication quality.

Network stability and anti-interference capability
Network connection stability: Check whether network connections between devices are frequently interrupted. You can check the connection status through the device's connection indicator or network management software. During normal navigation, if the connections between devices are frequently disconnected (for example, more than 3 to 5 times per hour), this may be due to network hardware faults (such as cable damage, loose interfaces) or electrical interference, indicating poor network stability.