Avoiding Traditional Radar Systems
Yes, you can actually fly below the radar. Since the earth is round and radar uses radio waves, which travel in a straight line, “flying under the radar” means flying beneath the area that the radar can directly “see.” The curvature of the earth will block incoming radar signal.
Because radar emits a radio signal in a straight line and waits for it to reflect off of something, if the reflection comes from another object, such as the ground or a mountain, trees or even an approaching thunderstorm, it will not detect what is behind it.
In civil aviation, mountains or other terrain can sometimes block radar signals from reaching airplanes. If an airplane strays too low in a mountainous area, it will be out of the controller’s coverage range. To prevent this from happening, controllers have Minimum Vectoring Altitudes.
Air traffic control utilizes both radar coverage and terrain to establish safe altitudes for aircraft.
Although military radars can track planes at extremely low altitudes, they have a limit. This is due to the radar signal reflecting off of the ground and objects on the ground, which causes interference.
This is known as clutter and can be prevented by Doppler radars that detect speed.
The radar horizon concept determines when clutter will no longer impact the radar. The SA-6 air defense system can engage targets up to 100 meters down using radar, while the SA-8 can engage targets up to 10 meters down. These systems are both very short-range systems (less than 30 kilometers).
Flying Techniques to Avoid Radar Detection
Military pilots actually train to avoid radar detection in a maneuver called ‘nap-of-the-earth'(NOE). NOE flight uses geographical features for cover by flying into valleys and folds in the terrain instead of over them. This puts the aircraft below radar coverage, so it’s harder to spot against the sky.
This is mostly used by small fighter jets but is also viable to larger aircraft like the B-1.
Newly built aircraft, on the other hand, will be equipped with defensive aids such as chaff and flares to obscure their radar signature. Furthermore, military pilots in non-stealth combat planes will frequently fly at low altitudes to hide in the ‘clutter’, other objects getting in the way of the radar signal.
While this does provide some protection against command guided surface-to-air missiles and fighters, it exposes the aircraft to short range surface dangers like close in radar guided anti-aircraft artillery or MANPADS.
Smugglers refined their low-flying skills in the late 1970s and early 1980s, flying mule planes low over the water. They said that as long as they couldn’t see the glow of lights from Tampa, FL, they were too low to be spotted by ATC.
While these are anecdotal claims, the facts show that drugs were coming into the country in records amount so they did discover some kind of way to reliable hide from US radars.
How Flying Under the Radar is Prevented
The radars straight line of sight problem is well know and many other radars systems have been invented that minimize this problem and make is next to impossible to fly under it.
Tethered Aerostat Radar System
The aerostats are helium-filled fabric envelopes that can reach a height of 15,000 feet (4,600 m) when tethered by a single rope. The largest lifts a 1000 kg payload to an operational altitude and provides low-level downward-looking radar coverage.
This technology can detect aircraft and vessels up to 200 miles away, even if they are just below the surface of the ocean.
These are operational and the their main objective is to support federal agencies involved in the United States’ drug interdiction program by providing low-level radar surveillance along the international border between the United States and Mexico, as well as the Straits of Florida and Caribbean.
Over-The-Horizon Radar
The detection of targets beyond the radar horizon, which is the distance limit for regular radars, is known as over-the-horizon (OTH) or beyond the horizon (BTH) radar. It’s a kind of radar system that may detect things at hundreds to thousands of kilometers distant beyond the radar horizon, which is the distance restriction for standard radars.
A number of technologies are utilized by OTH radars to see beyond the horizon. Shortwave systems that refract their signals off the ionosphere for long-range detection and surface wave systems, which utilize low frequency radio waves that follow the curvature of the Earth due to diffraction, are two techniques that are most often used.
Shortwave radio signals are refracted off an ionized layer in the atmosphere, commonly known as the skywave or “skip” propagation, after being refracted off of a tropospheric reflection surface. Given particular atmospheric conditions, radio transmissions angled into space will be refracted towards the ground by the ionosphere, allowing them to return to earth beyond the horizon.
This signal is then scattered off intended targets back into the sky, refracted through the ionosphere, and re-emerges at the receiving antenna via the same route.
Airplanes Outfitted with Transponders
All current practical application of radar is what is known as secondary radar today. Aircraft are now outfitted with a transponder, which is a tiny electronic gadget coded with the “registration plate” of the aircraft and occasionally information on speed, altitude, and other data, in place of a emitter/receiver and passive signal reflection.
What happens is that the radar sends out a pulse of radio waves, and when it hits the transponder, it “interrogates” it. The advantage of this is that because the radar station does not need to receive its own signals, it can be much smaller and transmit a lot more information.
Another function of the transponder is to send out a variety of signals, which may be used to notify radar controllers of certain calamities like radio failure, emergency, unlawful interference (highjack) and so on without requiring anyone to touch the radio.
Of course, a transponder can simply be disabled, in which case the system reverts to primary radar.
Can You Fly Above a Radar?
The majority of air traffic control radars can detect aircrafts flying way above 10,000 meters; however, there is a location directly positioned above the antenna called the “cone of silence”. Radar frequencies don’t reach as high at these altitudes, therefore creating a larger area that the radar won’t be able to see.
Inverted cone of silence or gap is formed by the rotating antenna as a result of the antenna back angle being less than 90 degrees. As a result, the back angle is an essential antenna parameter. Aircraft will fall outside radar coverage if they fly over the radar site with a shallow back angle.
The limitations of the antenna and radar equipment prevent it from determining the azimuth of the target, which in turn causes clutter and display errors. However, these problems can be alleviated by using data from another nearby sensor.