Knotlog calibration

[Ross]

When I want to check the speed of the

current I shift into neutral and wait for the bubbles to keep up with the boat when the wind is calm. The gps then give me speed by current only and also give me direction of the current. As long as you aren't expecting triple digit accuracy you can make a run in both directions across the current. you should be good to one decimal place.

 

[Rick]

GPS vs. Knotlog

The GPS consistantly is higher than the Signet knotlog. While the knotlog registers approx 5.5 knots, the GPS will display 7-8 knots. The knot log is new this year and calibrated at the factory.

 

[Bill Roosa]

GPS units are adjustable

You can set the refresh rate just like on a knotmeter. Increase the time between refreshes of position and you get a more accurate "average speed" and less frequent reports of the speed. Basicly you are going from "instantaneous" once a second to once every 5, 10, 15,... till you have a "trip speed" GPS. The latter being not very useful as GPS functions go.Scott asked about "doppler shift". This is an internal measurement made by the GPS receiver and the basis for some of the position calculation. A GPS gets a very accurate time signal (3-5 usec) and then compares the expected frequency of the transmitting satelite with the actual frequency of the satelite (doppler shift). This tells the GPS how fast the satelite is moving relative to the GPS and since it knows the orbital parameters of the satelite (where it is, how fast it is moving and in what direction) it can calculate a circle of position for it. Just like you do with a sextant. Given 3 satelites in view you get 3 CoP and a two solution answer for your position. One of these is above the satelites and the GPS unit presumes it is not in space and displays the other. With 4+ satelites in view you get one solution and height above the GPS geoid datum. Unfortunately that datum is not the same surface as MSL which is why you don't get your tidal height. Now if you want to talk something interesting, ionospheric refraction or the government "diluting" the accuracy of the signal.....

 

[Bill Roosa]

Rick,

While the factory may have calibrated it you have to account for the placement on your boat. None of the water flowing past the hull is going your boat's speed. It is all going various shades of faster as the boat is pushing it out of the way.