Received data and calculations

The essentials

NavimetriX reads NMEA 0183 sentences for position, heading, speed, wind, depth, rudder, XDR sensors and AIS. It calculates the true wind, leeway, current, VMG and waypoint data itself.

Two advanced mode settings on the “GPS & NMEA” page change these calculations:

• “Speed reference”: “Surface” (the log speed, the default) or “Ground” (the GPS speed, when the log is fouled);
• “True wind”: “Computed” by NavimetriX (the default) or taken from the “Instruments” when they send it.

The “NMEA barometer offset” in the “Barometer” block corrects the error of an NMEA barometer.

To see what is really arriving, look at the “Incoming NMEA sentences” console at the bottom of the page (see NMEA 0183 connection).

Going further

Sentences read

The magnetic declination is calculated at the boat’s position with the World Magnetic Model (WMM), every 10 seconds.

For DPT depth, the offset sent by the depth sounder is added, as on the onboard display: positive, it gives the depth below the surface; negative, the depth below the keel. A depth of 0.0 is read as “no bottom” (many depth sounders send it when they lose the bottom).

Not read: RMB, WPL, APB, XTE, BOD and BWC (the distance, bearing, VMC and time to the waypoint are calculated by NavimetriX from your route or your “Goto”), VLW (trip log), ZDA (time only) and VDO (your own boat’s AIS position).

When a value arrives in several sentences

Every 10 seconds, NavimetriX checks which sentences are actually arriving and, for some data, keeps only one, in this order:

If the selected sentence stops arriving, the next one takes over at the next check. Position sentences (RMC, GGA, GLL, VTG) and AIS sentences are always read.

With several links (advanced mode), position and time come only from the link chosen in “GPS source” (see Multiple links and position source). For other data, if two links send the same sentence, the instruments win, then the GPS, the AIS and additional links 1 to 3. AIS targets and XDR and MDA measurements are taken from all links, measurement by measurement: a barometer sent in XDR on an additional link is not masked by an instrument system that sends the rudder angle in XDR.

XDR sensors

The XDR sentence carries measurements from various sensors. NavimetriX recognizes:

Names are read without regard to case, except PTCH and ROLL, which must be exact. A pressure in bars outside 0.8 to 1.1 bar (oil, hydraulic) is ignored. Detailed formats and examples are in the FAQ below.

What NavimetriX calculates

• True wind (speed, angle, direction) from the apparent wind corrected for heel, the reference speed corrected for leeway, and the heading.
• Leeway (the angle between the boat’s centerline and her course through the water), estimated from heel and speed.
• Current (speed and direction), from the difference between the movement over the ground (GPS) and the movement through the water (log and heading).
• VMG and performance relative to the polar.
• Waypoint data (distance, bearing, VMC, time remaining) from the route being sailed or the “Goto”.

The displayed values are damped according to the “Instruments”, “Damping” settings (see Instrument types).

Speed reference

The speed reference feeds the calculation of true wind, leeway, VMG and efficiencies relative to the polar. “Surface” (the log speed) is the physical choice. “Ground” (the GPS speed) is a stopgap when the log is fouled: true wind and performance then remain usable, at the cost of the effect of the current. The measured speed through water is still displayed as such.

True wind

By default, NavimetriX recalculates the true wind itself. With “Instruments”, it displays the true wind that your instruments send (MWV reference T, VWT, MWD), with the correction tables they apply: you then see the same wind as on their displays. NavimetriX uses the true wind values sent by the instruments as long as they are fresh (received less than 5 seconds ago) and falls back to the calculation for missing or stale quantities. This row only appears in advanced mode.

These two settings only exist in advanced mode. Going back to simple mode resets them to “Surface” and “Computed”.

Barometer correction

The correction is added to the pressure received over NMEA. It is saved with your account and applies on all your devices. On a device that has its own barometer, two more rows appear: “Use the device sensor” and “Device sensor offset”.

“Calibrate online” sets the correction of the source in use from the sea-level pressure forecast by the weather model at your position. It needs a real GPS position (not the approximate position from the IP address) and Internet. The correction takes the sensor’s height into account: it only applies where it was taken, and a note appears when you have changed location. “Erase history” empties the pressure history, after confirmation.

The barometer and barograph are described in Barometer and barograph.

See also

• NMEA 0183 connection
• Multiple links and position source
• NMEA outputs
• Instrument types
• FAQ: Which NMEA 0183 sentences are supported?
• FAQ: Which NMEA0183 XDR sensors can I use?