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Signal K aims to be the next generation solution for marine data exchange. It is intended to be used not only for communication between instruments and sensors on board a single vessel, but also to allow for sharing of data between multiple boats, aids to navigation, bridges, and marinas. |
Signal K aims to be the next generation solution for marine data exchange. It is intended to be used not only for communication between instruments and sensors on board a single vessel, but also to allow for sharing of data between multiple boats, aids to navigation, bridges, and marinas. It is designed to be easily consumable by Web and Mobile applications and to connect modern boats to the Internet of Things.
There is a need for a new communications protocol for the marine industry to address the needs of a changing and ever more interconnected world. The currently most deployed mechanism for connecting instrumentation together has been with us for more than thirty years. Unlike some other ubiquitous communication protocols of similar age, NMEA 0183 has not aged so well. This was recognized as early as 1994 when development began on a replacement, the NMEA 2000 protocol. Unfortunately, by the time this replacement arrived on the market in early 2001, it was already showing its age. Fourteen years later, it is clear that a new forward-thinking approach is required.
There are several major problems with the existing standards. Breaking this down, the first problems are technical. Both standards are fundamentally local area serial networks designed for limited deployments in simple environments. NMEA 0183 is limited to 4800 baud (34,800bps in high speed mode, which has seen limited deployment for AIS instruments) and one transmitting device. NMEA 2000 works at a significantly higher bit-rate (250kbps) and allows multiple devices to transmit on one shared bus, but it is limited to 50 devices on a bus. These decisions may have been perfectly rational twenty or thirty years ago, but today we need something more capable and extensible.
The next major problem isn't technical, but rather a legal one. These standards are published and available for use by developers. However, while they are technically open standards and anyone can develop a product using them, both are non-free, in that their use is governed by a license agreement which restricts how they can be used and in the case of NMEA 2000, a non-disclosure agreement. In essence, this prevents the release of an open source application developed using these standards.
Finally, and most importantly, both NMEA standards and the other proprietary protocols in the industry were developed when the instruments on the average boat were much simpler and much less capable. In a world where your home thermostat is connected to the Internet and the number of Internet connected devices in the United States alone is projected to exceed a billion by 2016, it seems archaic that your boat is still an island to itself.
After all, your boat is not the only one out on the water. There are lots of other boats out there, and lots of other sources of information. In the modern connected world, shouldn't you be able to access that information from your boat where ever you are and on whatever device you want?
To move forward we need to think in a connected way, a way fundamentally different from NMEA protocols. Sun Microsystems famously coined the phrase The network is the computer, and Signal K reflects that global peer-to-peer concept.
Signal K is broken down into three components. The first is the data model. The Signal K data model describes a large hierarchy of data points organized into topics and subtopics. It can contain data from your boat, other boats in the vicinity, aids to navigation (AToNs), marinas, local points of interest, online cruising guides, in fact any manner of data sources. It has one important overarching rule and that is every piece of data has a defined location — a path in Signal K terminology — which is immutable. These paths can be thought of as a form of Uniform Resource Identifier (URI). We specifically do not call them Uniform Resource Locators (URLs) because, well, they're not.
In addition to dynamic data such as wind speed or GPS coordinates, the Signal K data model is capable of storing static data such as cruising notes, charts, weather, notices, port data, etc.
The second component of Signal K is the security model. The Signal K security model should be familiar to anyone who has spent some amount of time using a Unix or Unix derivative (such as Linux or OS X). For those less familiar, the Unix file system security system specifies three permissions (read, write, execute) for three security principals (user, group, other). In Signal K, things are somewhat simplified by the fact that execute (as in cause an application to run) doesn't really apply. Therefore, we only have read and write permissions. The classifications of user, group, or other still apply.
The third component is the Signal K protocol. This specifies how interactions between various devices which speak Signal
K should be carried out. It consists of five basic commands: list, get, put, subscribe, and unsubscribe.
For the examples which follow, we will be using JSON to display the data model, security model, and sample exchanges of commands between devices. This is not meant to be taken as a proscription that Signal K data must be transmitted using JSON or that it somehow relies on JavaScript or JSON to operate. It is merely a convenient way to display the information. However, one should note that Signal K is very much a work in progress and there has not yet been any decision made on the format of the initial handshake exchange which would set up between two communicating devices the data serialization format to use. Caveat implementor.
Let us begin with a simplified example of the Signal K data model. The complete reference for the currently implemented model is here.
{
"self": "123456789",
"vessels": {
"123456789": {
"name": "motu",
"mmsi": 123456789,
"navigation": {
"headingTrue": {
"value": 23,
"source": "self",
"timestamp": "2014-03-24T00:15:41Z"
},
"headingMagnetic": {
"value": 43,
"source": "self",
"timestamp": "2014-03-24T00:15:41Z"
}
}
}
}
}Since the data model is consistent, your instruments can show your data, or others data, and vice-versa. Each instrument and sensor holds a partial copy of the model, containing just the data it is interested in. Since instruments supporting Signal K can always read the data, they are compatible between all boats. Since all sensors store their data in the same model, all sensors are compatible with all instruments.
So we have a clean extensible model of data, capable of holding information from many boats, many sources. Now we need to transfer that data efficiently between interested devices. The model is a hierarchical tree, with meta-data on the structure and definition. Its transferred as a JSON string, which is a filtered image of the full tree containing just the information you want. So a device may just require a vessels speed
vessels.localBoat.navigation.speedThroughWateror maybe full navigation data
vessels.localBoat.navigationor your chartplotter might display wind information from every location that was offering it
var windVectors = Array.prototype.slice(vessels)
.map(function(node) {
return node.environment.wind
});Signal K allows easy replication and merging of data between devices. It enables universal interoperability between devices, and it provides a common base for existing and new marine applications.
Signal K data is transmitted as a JSON string. JSON is native to the Internet, and to browsers, so transmission is supported on any medium that can access the Internet. That includes mobile data, WiFi, Ethernet, USB, Bluetooth, and pretty much anything coming. But importantly we are just sending a string of characters &endash; so we can send over serial cables (RS232/442/etc.), Onewire, CAN, in fact almost anything.
Over the years the speed of data transmissions has increased enormously. NMEA 0183 is 4800 bits/s, the highspeed version is 38.4 kbits/s and NMEA 2000 is 250 kbit/s, these are all fine for their respective uses, 38.4k is quite enough for AIS messages for example. Most WiFi is now over 54Mbits/s, as are cellphones, and Ethernet is over 1000Mbits/sec. By utilizing these commodity media Signal K avoids bandwidth traps, and allows you to use a transmission medium that suits the immediate need.
As a bonus it's a string format, with full meaningful names, so it's also easy to read with human eyes. This helps enormously when developing or debugging.
For existing instruments and sensors, Signal K supports converters for legacy protocols (NMEA0183, NMEA2000, SeaTalk) and others as they are required. And it's extensible for future needs, just submit a change on GitHub.