Introduction
This is the documentation for the Signal K Specification master (latest) version.
What is Signal K?
Signal K is a modern and open data format for marine use. It is an Internet friendly standard built on standard web technologies, such as JSON and WebSockets. Signal K is Free and Open Source software. This document is licensed under the Creative Commons CC-BY-SA license. All Signal K source code is licensed under the Apache License, Version 2.0. Signal K is developed in the open with help from the marine community. Your ideas and feedback are valuable and welcome.
Signal K consists of the Signal K Data Model and the Signal K Message Format.
Signal K Data Model (A.K.A. The Schema)
The Signal K Data Model or schema defines a universal model for marine related information and it is specified as a JSON schema. See the Signal K Data Model section for details.
In traditional marine standards there are many tightly defined messages, each with a specific purpose, but there is no data model to relate them. Furthermore, any device which needs to decode those messages must have a copy of the data dictionary in order to do so. By defining a data model in JSON we can make the messaging layer simpler, and easily extensible. We define consistent units and meta data for each data point in the model. This means that a specific data point (e.g. COG) will always be found at a predictable address.
It also means that a display device such as a chartplotter implementing Signal K does not need to know about the data model beforehand. It can query the central Signal K server on the boat to get all the information it needs to display any data point. This metadata may include information such as the unit of measure, minimum and maximum permissible values, alarm thresholds and localized display name for every data point in the model.
Signal K Message Format
Signal K defines methods for combining arbitrary data from the Data Model into valid messages. These messages are in UTF-8 JSON format.
Rather than define hundreds of specific messages, Signal K has a few common message formats which can contain any combination of data from the Data Model. This means that we don’t need new messages for every new case, just extra data in the payload. It means that any device can read any message and a device can introduce a new data point which can be understood by existing devices without the need for firmware upgrades.
Signal K Transport Layer
Signal K does not define the transport or wire protocol. Signal K messages are JSON text and can be sent over almost any transport layer. However, we do provide guidance on how to make an initial connection, handle negotiation, subscription, and disconnection for a given transport (e.g. TCP/IP or serial).
Where possible we use well established standards like HTTPS, REST, WebSockets, MQTT, STOMP, etc. But within each method there are always many options in message addressing, formating, or transfer (GET, POST), etc.
The goal is to try to establish sensible conventions for each protocol, to make development and interconnection more predictable.
Signal K Implementations
The Signal K project has many implementations of servers, clients, and utilities. These are both proof of concept tests and reference code. The goal is to provide a range of software and utilities to simplify development.