Radar 101

As a backseater who has actually "flown low to beat the radar", I think that I can offer a few contributions to the understanding of radar. Aviators (we're not all pilots dammit!) usually use the phrase "fly low and avoid the radar", although the semantics are neither here nor there. While radar (RAdio Detection And Ranging) functions on the principle of reflected radio waves, there are a few steps that can be taken to detect low flying aircraft.

Most radar systems operate on the principle of pulse modulation, which is to say that RF at a given frequency is released in bursts at determined lengths, known as pulse width, giving range and azimuth (if the radar is a rotating sort) information. The process is simple, the antenna is pointed in a certain direction when the pulse goes out (azimuth) and the time it takes to get back is calculated based on the speed of light (162,000 NM/sec or 186,000 mi/sec).

RADAR ANTENNA transmit >>> >>> >>> TARGET

RADAR ANTENNA receive <<< <<< <<<

A somewhat different variation is the CW (continuous wave) radar, which broadcasts a steady stream of RF energy, giving only doppler speed and azimuth. The difficulty with this is that range cannot be estimated due to the fact that range is measured from the return time of individual pulses. If the RF is always going out, there is never a chance to measure the time that it is gone. The nice thing is that it does give the ability to determine relative speed to the operator. This speed is determined by the equation:

(Fh-Fl)/2 (3000/Fo) = speed in knots

Fh = highest returning frequency
Fl = lowest returning frequency
Fo = frequency emmitted by the radar
(this can also be used for acoustic applications)

PW RADAR ANTENNA transmit >>> >>> >>> TARGET
PW RADAR ANTENNA receive  <<< <<< <<< 

Advanced radars can infer doppler shift (frequency change due to relative velocity, like the sound of a car shifting from high to low when it passes you) for a single pulse and really advanced radars can perform NCTR (Non-Compliant Target Recognition) using a phenomenon known as JEM (it's on yahoo news so don't freak out CIA). While most details remain classified, JEM involves unique analog signal return traits to identify specific aircraft.

Stealth aircraft do no rely on terrain masking, the process of trying to mix with the confused reflections of stuff behind them. Instead, stealth is reliant on design features to produce a low target cross section, i.e. the amount of RF that will reflect off the aircraft from a given angle. Primarily, faceted, or angled, surfaces bounce the RF energy in a different angle from the radar receiver antenna. This is why stealth aircraft try to avoid having any 90 degree surfaces. As you will remember from geometry class, if something goes into a right angle, it will reflect out in the direction it came from. Try it yourself: throw a tennis ball in a square corner and marvel as it whacks you in the forehead. Add to all that radar absorbant materials and you've got yourself a hard to detect aircraft (and a sore head if you tried the geometry experiment).

A last consideration to take into mind is that the height of the radar antenna above the ground affects the distance it will be able to see it's target. To figure that out, the radar horizon is determined by the equation:

1.23 (square root of antenna in feet) = distance in nautical miles.

For a target above the surface of the Earth, the equation is:

1.23 (square root of antenna + square root of target) = distance in nautical miles.

With the above information, you can easily build an IADS (Integrated Air Defense System) for an emerging third world nation, but don't expect to shoot down any cruise missiles or stealth fighters anytime soon. That takes far more money, equipment, and classified documents than you will find on E2. Good hunting.