A Day in the Life of a Navigator » Dead Reckoning Plot
A DR plot is started from a previous established position. For our example, the last known electronic fix was at 04:00:00 UT, at location 38° 08.0' N, 053° 52.2' W. Ordered course and speed are 315°, 6 knots. Here, Chief Sheedy plots the last known electronic fix and the DR position up to 0800 UT.
A Navy navigator’s major concern is the accurate determination of the ship’s position. But it is not enough to know only the present position; equally important is calculating the ship’s position for a desired time in the future. For this, the navigator obtains the ship’s approximate present or future position by using a Dead Reckoning Plot.
In the U.S. Navy, a DR plot is always started from an established position; that is, an estimated position or a fix. In the U.S. Navy, only ordered courses and ordered speed are used to determine a DR plot. The effect of current is not considered in determining a DR position.
Although some of the details of dead reckoning are beyond the scope of this module, let’s look at our navigator making a DR plot.
Chief Sheedy: I’m now going to take you through a day in the life of a navigator. The last electronic fix we have is at 0400. The position was 31 degrees 08 North. I’m now going to plot that on the Universal Plotting Sheet. The longitude was 053 degrees 52 decimal 2 minutes West. I’m now going to put a triangle around my fixed position, since it was an electronic source fix GPS.
In the Navy, we label our fixes based on the type. A triangle is an electronic fix, a square is an estimated position, a circle is celestial or visual. A half circle is dead reckoning. In the Navy we have six rules to dead reckoning. We do one every course change, every speed change, every fix or running fix, every line of position, DR out two fix intervals each fix, and plot label the course and speed. I’m now going to show you how we do that.
In our scenario, our course is 315 degrees, so we’re going to go 315 degrees true. Our speed in 6 knots, or 6 minutes latitude. Now I’ll get my 6 minutes. That’s where we’ll be at 0500, 06, 07, 08. Add the half circles. The DR times go diagonally: 0500, 0600, 0700, 0800.
