T h e usual method to show a line of
position from celestial observation consists of
(1) observation, (2) coordinate conversion, and
Modern radar can be a valuable aid to naviga-
(3) plotting. The navigator tries, whenever
tion. Some radars present a maplike display of
possible, to select three bodies about 1200 apart
the terrain around the aircraft on the screen of
in azimuth. This not only results in lines of
a cathode-ray tube (CRT). This allows pilotage
position that cross cleanly, it also minimizes the
to go beyond some of the limitations of visual
effects of a constant error in the observations.
Radar transponders are devices that do not
operate until interrogated or triggered into
action by a suitable signal from another radar
Inertial navigation systems (INS) are dead-
which the interrogating radar receives. These are
reckoning devices that are completely self-
used both for fixed navigational aids, such as
contained. They are independent of their
operating environment, such as wind, visibility,
radar beacon stations and for airborne identifica-
or aircraft attitude. They do not radiate or receive
tion friend or foe (IFF) systems.
RF energy; therefore, they are impervious to
Doppler radar can detect and show actual
ground speed and drift of an aircraft, regardless
laws of motion that Newton described three
of wind speed or direction.
centuries ago. Of course, you must enter the
Radar altimeters give the actual distance from
starting position into the system. When known
the aircraft to the surface below. The surface
below can be a body of water or land masses far
positions are available, you may correct or
above sea level.
update the system if an error exists.
Another input to the system is from accelera-
tion detectors that measure the rate of change in
the motion of the aircraft. The first integral of
acceleration is velocity. Velocity results when
Radio navigational aids vary from a fairly
simple direction finding receiver to complex
example, a body starts from rest and constantly
systems using special transmitting stations. These
accelerates at 8 feet per second per second for 11
special stations make it possible to fix the
seconds. The velocity at the end of this time would
position of an aircraft with considerable accuracy.
be 88 feet per second (60 miles per hour).
The usable range varies according to its intended
However, in actual practice, acceleration is not
use and also with weather and ionospheric
conditions. Beacon stations associated with an
tion is the process of summing all minute
instrument landing system (ILS) are usually of low
acceleration-time increments over a given amount
power. Long-range air navigation (loran) stations
have a range extending to 1,400 miles under
By integrating velocity with respect to time,
favorable conditions. Aviation Electronics
the result is displacement (distance). Therefore,
Technicians (ATs) maintain the airborne portions
the second integral of acceleration is displacement.
of radio and radar systems.
The inertial navigator's purpose is to keep track
of position and not the total distance traveled.
This causes the system to integrate all values of
acceleration (positive and negative) detected over
Celestial navigation is the method of fixing the
the time involved.
If the earth were flat and vehicles traveled only
position of the aircraft relative to celestial bodies.
on the earth's surface, a two-axes inertial naviga-
Since the earth is constantly revolving, an accurate
time device is necessary. You may use a sextant
tion system could plot the position using two
to measure the angle of the celestial bodies with
respect to the horizon. In marine navigation, the
sensitive along the x-axis (E-W) and the other
visible horizon is the reference. In air navigation,
sensitive along the y-axis (N-S). The important
point to note about detecting acceleration of a
you use an artificial horizon as the reference
point. Also, the navigator needs an almanac to
at the time of observation.
sensitive axis. They have no way of telling whether