The quickest and easiest solution to precision piloting

Three Arm Protractor

Although the 3-body fix is not often used these days of GPS, it remains one of the best and quickest ways to find out where you are.

$44.99   ...item# 1861

The piloting technique with the highest accuracy is a "3-body fix" using a sextant to measure the two angles subtended by the three bodies. The simple and inexpensive Davis Mark 3 sextant is more than accurate enough to obtain the data. It is easy to use, and the sights are quick.

With three charted bodies (A, B, C) in sight that can be identified on the chart, use the sextant horizontally to measure the angles between A and B and then B and C.

The center arm of the device is the reference. Set the left arm at angle AB and the right arm at angle BC, and then holding the set angles in place, align the three arms to cross the three bodies sighted. The center of the protractor then marks your position.

For a brief explanation of three body piloting see article

The Starpath StarPilot-PC program includes an analytical solution to the problem, which offers a very convenient way to confirm your position when using electronic charting and GPS... always a good thing to do, once in a while. With it, you just read the lat/lon of the three targets with the chart program, measure the angles with a sextant, and then enter the data in the program and it instantly computes your lat/lon. It is a slick way to use the old and the new to check the new. It may seem a bit circular, but it is indeed an independent confirmation of your GPS position. The computer is just doing some computations and plotting for you.

To see the high precision instrument that forms the basis for this device click US Navy Station Pointer, which is an instrument of ours on display at the Institute of Navigation's Virtual Museum.


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