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It is quite a common phrase, usually without having anything to do with radar or aviation. But. Is there such a thing as 'flying under the radar'? If so, why is this (and how low is that)?

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I was expecting somebody to have mentioned Mathias Rust by now... – Greg Hewgill Apr 1 '14 at 20:07
Rust was picked up by Soviet air defenses, but they chose not to shoot him down. His stunt was soon after the KAL 007 shootdown, and Moscow was not willing to get in hot water so soon again, especially if this was simply a lost pilot. – Phil Perry Apr 2 '14 at 17:01
Yes. That's why some airplanes have terrain following radar. – Martin Schröder Apr 3 '14 at 10:33
Nice question and nice avatar. – n00b Apr 3 '14 at 18:25
The same way you can fly in the shadow of the Sun early in the morning. That's why airborne radars exist, they are like the Sun at noon. Note that you cannot fly under the radar if the radar is just above you... and the phrase is more accurately: to fly below the radar horizon. – mins Mar 9 at 8:24

In many cases, yes you can fly under the radar because typical civilian radar is line of sight, meaning that it has to have a straight unobstructed path to an object in order to "see" it.

This is because it works by sending a radar signal out and waiting for it to reflect off of something. If it reflects off of another object, like the ground, a mountain, a building, a thunderstorm, etc. then it won't see what is behind it.

Since the earth is round, flying "under" the radar is flying beneath the coverage area that the radar can "see" directly from where it is, and the height of radar coverage depends on the distance from the site as well as the terrain.

An example is here:

Radar Line of Sight

The top half shows how the curvature of the earth affects it and the bottom half shows how other objects can shadow radar (even though is is using aircraft radar, the same principle applies).

That being said, there are other types of radar and other ways of utilizing radar that minimize this problem and make it nearly impossible to fly under it.

One common type in use is the Tethered Aerostat Radar System:

Radar balloon

This is downward looking radar attached to a 25,000 ft. tether. From 15,000 ft. it can detect aircraft and even vessels all of the way down to the surface of the ocean out to 200 miles.

Another type is Over-The-Horizon Radar which can see further by reflecting radar off of the ionisphere like this:


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Note that doing so maybe be difficult for commercial and military planes, but helicopters(choppers) can do this very easily due to there small size and ability to fly at very low heights like 10 ft. – Registered User Apr 3 '14 at 7:30
Private airplanes routinely drop off radar during approaches and sometimes even enroute. I've been told by ATC that radar contact was lost during an emergency approach to an airport once and another time while enroute over the Northern part of Michigan's lower peninsula. – Brian Knoblauch Apr 3 '14 at 16:27
sweet graphics. thank you for that simple explanation – n00b Apr 3 '14 at 18:26
One should note, though, that flying too low in certain atmospheric conditions will make radar detection easier (because of surface ducting). – Deer Hunter Sep 26 '15 at 11:28

It is certainly possible to fly "under the radar." Military pilots practice a type of flying called nap-of-the-earth for exactly this purpose. This tactic is primarily used by smaller fighters and attack aircraft, but larger planes like the B-1 are also capable of this. Here is why this tactic may be useful:

In civil aviation, mountains or other terrain can block radar signals. Going too low in a mountainous area will take you out of their radar coverage. For this reason, controllers have Minimum Vectoring Altitudes. These are based both on the radar coverage and terrain so that ATC can ensure aircraft will be at a safe altitude.

Here is a cross section of how radar coverage may look. Obstructions can block lower portions of the coverage as it radiates outward from the radar facility. The red indicates the normal radar used by ATC (the green is called "over the horizon" radar, generally used for military early warning radar).

Diagram showing radar view vs. horizon

Even military radars have a limit of the lowest altitude at which they can track aircraft. This is because the radar signal will also reflect off of the ground and objects on the ground, causing interference. This is called clutter and can be avoided by Doppler radars that detect velocity. The concept of the radar horizon determines the point at which clutter will no longer affect the radar.

The SA-6 air defense system can engage targets using radar down to 100m, and the SA-8 can engage down to 10m. These are both short range systems, though (less than 30km).

This report has some references about areas on ATC radar that are difficult to cover because of obstacles such as wind farms. Because the radar doesn't distinguish altitude, controllers may not be able to distinguish an aircraft within these areas of interference.

Radar image with areas of interference labeled

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Excellent answer! +1! Actually, the B-1 was specifically DESIGNED to fly nap-of-the-earth because it eventually became clear that bombers were never going to be able to fly higher and faster than air defenses, so the only way to avoid them was to avoid being seen by them before it is too late. That of course is the idea behind more modern stealth aircraft as well – rotard Apr 3 '14 at 16:37
Great answer overall, but one minor error. Doppler radars were actually originally built to overcome ground clutter. The velocity information provided by the doppler portion of the radar is used to discriminate between moving targets (aircraft) and the ground. This is why a lot of early airborne Doppler radars emphasized their "look-down" capability. They were able to see enemy aircraft against the ground by filtering on velocity. – Joel M. Dec 16 '15 at 22:10

Secondary Surveillance Radars work on 1030 MHz and 1090 MHz. Most primary radars work at higher frequencies. Signals at these frequencies do not follow the curvature of the earth very well. They work best in line of sight. Aircraft far away from the radar must be at high altitude to be above the horizon, otherwise they are "under the radar"

In flat terrain at 1 000ft, the horizon is about 33 nautical mile away. Aircraft further away will be shielded below the horizon. For 40 000ft, the horizon is about 220 nautical mile away.

L-Band signal horizon

L-Band signal horizon

Due to a slight refraction of the signal the practical range is somewhat further than the visual line of sight.

Keep in mind that the signal is also blocked by terrain; nearby hills and mountains obstruct views.

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@RalphJ Just noticed you changing my radar answer. It's fine when you add a comma for clarity (although confusing for me since in my country we use the comma as a decimal separator), but please be careful not to change the magnitude of the number (you increased 1000ft to 10,000ft). – DeltaLima Dec 15 '15 at 20:52
Sorry - I thought I was reading it as 10 000 feet the first time thru. Thus the value of separators! – Ralph J Dec 15 '15 at 20:56
@RalphJ You clearly have a point there, or should I say comma? :-) – DeltaLima Dec 15 '15 at 20:57

It also depends on the premise of the radar system. The civilian ATC radar system is based on the targets being cooperative (transponders) and they actually want certain aircraft to be invisible: private aircraft not using controlled airports, for example. The system also deliberately removes slow-moving targets like flocks of birds.

Military radars, of course, assume the opposite. They deal with the line-of-sight problem with remote installations (e.g. DEW line), over-the-horizon systems that can boil water at short ranges, and during a battle will put the radar station overhead (AWACS). Mounting your radar antenna at 30,000 feet will extend its range a long way, and make it both harder to blow up and easier to replace.

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"they actually want certain aircraft to be invisible: private aircraft not using controlled airports, for example" - Who are 'they'? This isn't true in the U.S. where private aircraft pilots are encouraged to use a transponder and talk to ATC, even if flying in airspace where not required by the FARs. – CJBS Jan 20 '15 at 7:13
A large number of GA aircraft don't have transponders. I don't think it's true that civilian radar utilises almost exclusively secondary radar. They have to use primary radar to see all the traffic. Secondary is then a bonus and relies on cooperation, as you say. – PriceChild Apr 1 at 13:12

ATC radar is line of sight and doesn't follow the curvature of the earth. It does a bit but not enough to stay parallel to the ground.

The radar is also blocked by terrain (read: mountains).

This is one of the limiting factors of radar.

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First, yes, it is entirely possible to fly under the coverage area of most civil radar systems. Other have already explained that in detail.

However, there are radar systems that are not so limited by line of sight, and are able to see Over the Horizon using various techniques. So just because you're out of or below the range of the far-away radar does not mean that no one is watching you.

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Retired F-111 A-G and EF models, and existing B-1B Lancer aircraft use(d) Terrain Following Radar (TFR) to fly under radar coverage.

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To your question of "how low": In the US, radar coverage is generally pretty good, but there is a distinct floor below which an aircraft cannot be picked up by a radar system. The figure below shows this radar floor in the US: Lowest Altitude with Radar Coverage in US based on ETMS data

The figure was taken from here, page 73.

With the introduction of ADS-B as a surveillance source, this will change quickly. Areas where radar coverage is poor can be improved by the installation of an ADS-B receiver, which is significantly cheaper than a radar installation (~25X cheaper).

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