14 August 2016
The distance is how far Points A and B are apart measured
in miles or kilometers. The direction is the angular
measurement between Points A and B, and a reference
such as Magnetic North as is used in maps.
If a real navigation problem were as simple as Figure
4, we would have no problem navigating anywhere we
wanted to go. In the real world, however, things are not
quite so simple. A boat moving on the water or an aircraft
moving in the air make things a bit more complicated.
Take a look at Figures 5 and 6 that show the boat and
aircraft navigation problems, respectively. The wind and
water currents (neither of which is predictable) cause the
vehicle to drift from the course it would take with no wind
or current which is the course we want to travel. I am an
aviator, not a sailor, so some of our readers may need to
keep me straight on the nautical aspects of navigation.
Wind and tides affect the course of a boat but for this
discussion, let’s not add those complications.
Dead reckoning is a means of navigation in which
the speed of the vehicle, the desired direction of travel
(the course), and the wind/current effects are used to
calculate the actual direction the vehicle should be pointed
(heading) and the time required to reach the destination
(very important in managing fuel on board requirements).
Dead reckoning would be used in all forms of navigation if
the wind/current, vehicle speed, and magnetic course were
constant. Unfortunately, the wind/current changes, often
dramatically over the route of travel. The vehicle speed can
change due to wind/current, vehicle weight, and power
plant variations which further render dead reckoning not as
accurate as desired.
Figure 7 shows a dead reckoning course as the
brown line and another form of navigation – pilotage -- as
an orange line. Pilotage involves finding a course from
the start to the destination by moving between easily
recognizable landmarks such as road crossings, railroads,
etc., for aircraft. Cruise missiles use a form of pilotage using
digitized terrain maps combined with GPS and radar/
atmospheric altimeters to navigate accurately to their
Radio signals greatly improve navigation when
implemented in VHF Omni-Range (VOR), non-directional
beacons/automatic direction finders, and Loran and Global
Positioning Systems (GPS). Figures 8, 9, and 10 show the
basics of the VOR, LORAN, and GPS navigation. The VOR
operates on a phase variable signal transmitted by the
n FIGURE 5. Effect of Currents on a Boat’s Path.
set = direction the current flows
drift = velocity of the current
n FIGURE 6. Effects of Wind on Aircraft Flight Path.
n FIGURE 7. Aeronautical Navigation by Dead Reckoning (Brown Line) and Pilotage (Orange Line).