Setting up the navigation system

Although Florian still goes about fixing the position with his sextant and a pencil, such a long cruise is made a little bit easier and safer by a good electronic navigation system – and recording all that data will greatly help for the analysis of biological samples. It is a little bit messier, but far less complicated than it sounds. Here is the idea.

Most navigation instruments around (from a radar to a handheld hiking GPS) use the “NMEA0183” language to deliver their measures. It’s a very simple language: at regular intervals, the instruments send out a “sentence” that looks like this:

$GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,3.1,M,46.9,M,,*47

Which means, “I’m a GPS unit” ($GPGGA), “it’s 12:35’19” in Greenwich” (123519), “our latitude is 48°07.038 North” (4807.038,N) “and our longitude 01°131.000 East” (01131.000,E). “that’s standard GPS precision” (1), “extracted from 8 satellite” (08), “with a bit of uncertainty” (0.9). “The antenna is at 3.1 meters above the sea level” (3.1,M), “and here there is a 46.9 meter difference between the sea level and the standard earth model” (46.9,M). “There was no error transmitting the data” (*47).

That sounds a little dense, but we don’t need to read it ourselves: the system will directly point our position on a map instead..! So, in our system, all instruments (the GPS, the weather station, the radar, the radio-location system…) will receive their measures using antennas. They will analyse these measures, and make up NMEA sentences like this one. Then they will all speak at the same time.

So, to avoid getting confused, we use a NMEA multiplexer. This little piece of electronics receives all these sentences at the same time through multiple ports, and puts them in order, to transmit them one by one through a single cable to the informatic “heart” of the system. This heart is a Raspberry Pi 2 nano-computer – a very little, very cheap Linux computer that runs on almost no electricity at all. On it, we installed Olivier Le Diouris’ excellent navigation system that takes a little bit of fiddling around to set up, but works like a charm afterward. This nano-computer receives all the navigation data, writes it down in a log file so that we can look back at any time, and broadcasts it on the wifi inside the ship: we can then use any laptop computer (for us, an old Toughbook with Linux Navigatrix installed) to follow our position on the map, using the great and free OpenCPN software.

Here’s the precise logical schmea of the whole thing..!

Navigation_scheme

One thought on “Setting up the navigation system

  • Whenever you have a chance, please let me know if you can think of any way to make the soft installation any smoother…, your insight would be much appreciated.
    Thank you!
    – Olivier

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