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I have read somewhere that if a helicopter is traveling under 100 km/h and below 5 meters AGL then most radars will ignore it. From what I understand of this process, the radars can pick up things like cars and vehicles driving on the ground and thus need to ignore certain signals if they are traveling below a certain speed and height.

Is this actually the case, though? Does this also apply to military-grade radars?

Edit: This is not asking if a helicopter can fly under the radars coverage, but whether the helicopter can using techniques to notch its signal from being displayed as a contact on a radar operator's screen.

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    $\begingroup$ Possible duplicate of Can someone actually "fly under the radar"? $\endgroup$
    – Pondlife
    Commented May 28, 2019 at 3:40
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    $\begingroup$ Not a dupe - this question is about seen-but-ignored-as-ground-clutter, while that question is about being unseen by the radar. $\endgroup$
    – Ralph J ♦
    Commented May 28, 2019 at 6:38
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    $\begingroup$ @Carl Witthoft: But it's a lot easier for a helicopter to stay out of LOS, especially in hilly/mountainous terrain. $\endgroup$
    – jamesqf
    Commented May 28, 2019 at 18:23
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    $\begingroup$ Ever read Tom Clancy Debt of Honor? AWACS operator saw a fast moving blip right where a high speed train was expected. He told the system to ignore the blip as a friendly. The high speed train headed out to sea and put a cap in his AWACS. Also the first couple chapters of Command Authority where a Kiowa drives around on the streets... $\endgroup$
    – Harper - Reinstate Monica
    Commented May 28, 2019 at 20:03
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    $\begingroup$ The first shots of Desert Storm were fired by helicopters at radar stations as part of the Suppression of Enemy Air Defenses. For military helicopters, it's their one job, prior to established air superiority. Ironically, the first time the Apache attack helicopter was used in the role it was designed for, it didn't go so well; 2003 attack on Karbala $\endgroup$
    – Mazura
    Commented May 29, 2019 at 1:53

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Note that for advanced radar systems, moving helicopters might always be distinguishable from cars and other objects, due to the fact that helicopters have moving rotors.

While I have no information on military systems, I know the systems by robinradar can separate drones from birds by using doppler shift techniques.

Doing this on an actual, full-size helicopter without rotor guards should be a lot easier than on a drone.

This means avoiding detection by moving slow and staying low might not work, unless you can stay out of the line of sight of the radar system.

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  • $\begingroup$ Wow, that's a good point and I didn't think of that. If the rotor has a unique signature that can be distinguished from other vehicles like cars or birds, then perhaps this masking technique is wrong? $\endgroup$
    – Igneous01
    Commented May 28, 2019 at 15:18
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    $\begingroup$ As I understand it, it's not hard to distinguish rotors because the tips move at extraordinarily high speeds, and that causes an evident doppler shift in the rotor reflections. You do need a system that actively looks for it, has a high-enough resolution (= limited distance), and doesn't dismiss it as noise, but I assume the military does, since commercial companies also use it (and for the military it can also be useful to identify dual/coaxial rotors to narrow down model, which is also possible using this technique afaik). $\endgroup$
    – Erik A
    Commented May 28, 2019 at 15:27
  • $\begingroup$ +1 for the robinradar reference. I didn't know that was available commercial. $\endgroup$
    – J. Chris Compton
    Commented May 28, 2019 at 18:07
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    $\begingroup$ @ErikA yeah, there should be an identifiable "doppler spread" on the return. A car going +20m/s radially will have almost all of its return at a +20m/s doppler, but a helicopter with the same groundspeed has components moving anywhere between -180 and +220 m/s. $\endgroup$
    – hobbs
    Commented May 28, 2019 at 20:15
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The answer is yes (in most cases) but it has little to do with the helicopter itself. 5 meters (~16.5 feet) AGL is quite low to the ground. Chances are you are simply below the radar horizon for whatever the local radar facility is. At that height you are even below trees, buildings and the like. This of course is affected by the distance to the radar unit as well as having a clear line of sight to it. So being on an airfield or very close to a radar facility you may very well be visible. Military grade radars are bound by the same laws of physics as civilian ones, so yes it applies.

Like any electronic system, radar units are susceptible to noise and do implement filters but they tend to be for general "noise" exclusively filtering out helicopters flying <100km/h and under 5 meters is a somewhat specific constraint.

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    $\begingroup$ I think the point is not that there's a specific filter for "helicopters doing less than 100km/h under 5m AGL" but, rather, that there might be a filter for anything doing less than 100km/h under 5m AGL because, otherwise, every vehicle on the road would be causing clutter. $\endgroup$
    – David Richerby
    Commented May 28, 2019 at 12:54
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    $\begingroup$ I would expect a large factor to be whether it's ground-based radar or an airborne system. $\endgroup$
    – Acccumulation
    Commented May 28, 2019 at 15:33
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    $\begingroup$ This is just "flying under the radar" and doesn't count. $\endgroup$
    – Carl Witthoft
    Commented May 28, 2019 at 17:52
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The JSTARS (Joint Surveillance Target Attack Radar System) military radar developed and used from 1991 was particularly good at identifying helicopters.

JSTARS was designed to identify moving objects on the ground, such as tank columns and supply convoys used by the Iraqi forces during Operation Desert Storm. It used a doppler radar suspended below a large aircraft and, as such, it had generally better line-of-sight capabilities compared to ground based radars. Using doppler, any movement was highlighted, especially the rotational movement of the rotors, which meant that even if a helicopter were sitting on the ground with its rotors idling, it was still seen by JSTARS operators.

Helicopters trying to fly slowly in order to avoid air tracking radar systems would make it far more likely that they'd be identified by a GMTI (Ground Moving Target Indicator) system.

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A helicopter is no different from anything else. And radar doesn't care whether you're in a helicopter or whatever.

All the radar cares about is whether you're reflecting energy back to its reception antenna. A helicopter will do that, IF it's in line of sight to the antenna (and to the sending antenna, which may be somewhere else).

At 5m above the ground, a helicopter or indeed anything would have to be pretty close to the radar installation to do that, not only due to the curvature of the earth but the transmission angle of the radar installation, terrain features, etc..

For example even a low rise in the terrain, like an earthen berm protecting the radar installation, or some trees standing around it, a shack or house in the vicinity, would be enough to hide something from detection.

And yes, many radar installations can be set up to ignore things that don't have at least a certain speed and/or are below a certain altitude. This to prevent cluttering the displays with radar returns from birds, cars, motor cycles, and things like that. But that's a function of the display unit, not the radar receiver. It still gets all those returns, there's just a software filter between it and the screen that declutters the data for easier interpretation by the radar operator, who should have an option to tweak or even turn off that decluttering as needed.

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    $\begingroup$ This is a good answer, I just wanted to clarify that in this case I was referring to being displayed as a contact on the users screen (or being engaged by radar guided weaponry). $\endgroup$
    – Igneous01
    Commented May 28, 2019 at 15:15

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