Decoding NMEA Identifiers in navigation apps
What are NMEA strings
I work with a number of different boats and boat types to setup navigation packages such as Expedition, TimeZero, Adrena, QTVLM and with each there is the ability to retrieve transducer data from a NMEA bus through some sort of bridge between wifi or usb and the N2K bus itself.
The bridges I most commonly come across are:
Actisense USB - usually wired to the nav station and plugged into the USB port.
Actisense W2K1 - NMEA2k to wifi bridge
Maretron NMEA2K bridges - usb/cat5/wifi
This NMEA data is then used to enhance the display and routing software packages using that data regardless of which package you are using.
A few questions are quite common:
Why can’t I see my wind angles/gps location/magnetic heading in the software logs?
Why isn’t my position being updated on the route/shouldn’t this track my boat position?
When we go into the settings screens to determine why, we generally have to tell the nav software what data to receive, in most packages, the NMEA strings list is a pretty cryptic set of strings.
Why does the screen say NMEA 0183 when I have a NMEA 2k bus?
Most of us have NMEA 2000 devices and bus on our boats, so quite often people ask why are we receiving NMEA 0183 strings. The short answer is that, while all the devices are talking NMEA 2000, most bridges convert the data to NMEA sentence strings before sending them out over the wifi or usb network.
Also, most nav packages can receive data over those bridge technologies using the NMEA string of a given data value, so configuration is often referred to as NMEA 0183 values, but they are strings using common abbreviation character codes. Those abbreviation codes are what we are discussing.
This does not diminish the network bridge technologies technology or the advantages of having a NMEA 2000 system on the boat.
In the case of network bridge technologies, the transport is still NMEA 2000 and presentation is NMEA sentence strings.
A NMEA strings guide
Last week I was working with four different boats on data analysis and setup, and had to keep researching different NMEA sentences. Sentences varied because of differences in the boats sensor and sensor to network bridge setups.
About midweek, after doing the pick and poke method of searching the NMEA site over and over for PGN definitions and mappings, I started to wonder if somebody had published a definitive guide for how to interpret the sentences in a single document.
I was very happy to have come across Eric S. Raymond’s article titled NMEA Revealed in which he breaks down how to parse logged information we commonly see in tools where we are looking at data being received by let’s say Expedition, as the article photo indicates. The best part, it has a table for the NMEA 0183 string values and their names as they appear in the configuration screen of different apps.
