Radar is an object-detection system which uses electromagnetic waves — specifically radiowaves — to determine the range, altitude, direction, or speed of both moving and fixed
objects such as aircraft, ships, spacecraft, guided missiles, motor vehicles, weather
formations, and terrain. The radar dish, or antenna, transmits pulses of radio waves or
microwaves which bounce off any object in their path. The object returns a tiny part of the
wave's energy to a dish or antenna which is usually located at the same site as the transmitter.
3.1 Principles of radar
It mainly works on the two principles:
1. ECHO and
2. DOPPLER SHIFT
Echo is used to detect time and distance of target
Doppler shift is used to detect the speed of target approaching
3.2 Echo and Doppler Shift
Echo is something you experience all the time. If you shout into a well or a canyon,
the echo comes back a moment later. The echo occurs because some of the sound waves in
your shout reflect off of a surface (either the water at the bottom of the well or the canyon
wall on the far side) and travel back to your ears. The length of time between the moment you
shout and the moment that you hear the echo is determined by the distance between you and
the surface that creates the echo.
Doppler shift is also common. You probably experience it daily (often without
realizing it). Doppler shift occurs when sound is generated by, or reflected off of, a moving
object. Doppler shift in the extreme creates sonic booms (see below). Here's how to
understand Doppler shift (you may also want to try this experiment in an empty parking lot).4
One of the important factors is the internal construction. Behind the skin of some
aircraft are structures known as re-entrant triangles. Radar waves penetrating the skin of the
aircraft get trapped in these structures, bouncing off the internal faces and losing energy.
The most efficient way to reflect radar waves back to the transmitting radar is with
orthogonal metal plates, forming a corner reflector consisting of either a dihedral (two plates)
or a trihedral (three orthogonal plates). This configuration occurs in the tail of a conventionalaircraft, where the vertical and horizontal components of the tail are set at right angles.
Stealth aircrafts use a different arrangement, tilting the tail surfaces to reduce corner
reflections formed between them.
Stealth design must also bury the engines within the wing or fuselage, or in some cases where
stealth is applied to an existing aircraft, install baffles in the air intakes, so that the turbine
blades are not visible to radar. A stealthy shape must be devoid of complex bumps or
protrusions of any kind; meaning that – weapons, fuel tanks, and other stores must not be
carried externally. Any stealthy vehicle becomes un-stealthy when a door or hatch is opened.
a. Propulsion subsystem shaping:
Fluidic nozzles for thrust vectoring with aircraft jet engines, and ships, will have lower RCS,
due to being less complex, mechanically simpler, with no moving parts or surfaces, and less
massive (up to 50% less). Fluidic nozzles divert thrust via fluid effects. Tests show that air
forced into a jet engine exhaust stream can deflect thrust up to 15 degrees.
b. Non-metallic airframe:
Dielectric composites are relatively transparent to radar, whereas electrically conductive
materials such as metals and carbon fibers reflect electromagnetic energy incident on the
material's surface. Composites used may contain ferrites to optimize the dielectric and
magnetic properties of the material for its application.
The Stealth fighter/bomber detects its targets via the forward looking infrared turret, called
FLIR, embedded in its nose. This provides a good picture of the target from several miles
away, on even the darkest of nights.
Bombing from medium altitude, the F-117's fire-control computer calculates the proper
release point for the weapons to reach the general target vicinity. Weapons release will
generally be a range of one or two miles.
Closer to the target, control is switched to the downward looking infrared turret, or DLIR.
This is equipped with a laser designator.
As the weapon approaches the target, the laser designator is fired. Sensors in the nose of the
weapon now steer it toward to radar reflection, where it detonates with devastating accuracy.
5.5 Retirement:
Despite its successes in the Kosovo and Iraq Wars and its high mission-capable rate, the F-
117 was nevertheless designed with late 1970s technologies. Its stealth technology, while still
more advanced than that of any other aircraft except the B-2 Spirit, F-22 and F-35, is
maintenance heavy.
Furthermore, the facet-based stealth design has been surpassed by newer technology.Program Budget Decision 720 (PBD 720), dated 28 December 2005, proposed retiring the
entire fleet by October 2008 to allow for buying more F-22As. PBD 720 called for 10 aircraft
to be retired in financial year (FY) 2007 and the remaining 42 aircraft in FY 2008 and stated
there were other more capable Air Force assets that could provide low observable, precision
penetrating weapons capability including the B-2, F-22 and JASSM.
By late 2006, the Air Force had closed the F-117 pilot school, and announced the retirement
of the F-117. The first six aircraft to be retired made the last flight on 12 March 2007 after a
ceremony at Holloman Air Force Base (AFB) to commemorate the aircraft's storied career.
