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 ft, the horizon is about 33 nautical mile away. Aircraft further away will be shielded below the horizon. For 40 ft, the horizon is about nautical mile away. 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.
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. 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, feet will extend its range a long way, and make it both harder to blow up and easier to replace. 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.
This is one of the limiting factors of radar. 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:.
The figure was taken from here , page With the introduction of ADS-B as a surveillance source, this will change quickly. 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.
Yes, staying at low altitudes can mask your presence to ATC primary radar systems as well as all but the most advanced military radar systems. Low azimuth scanning by a radar antenna introduced unwanted electromagnetic interference with its signals also known as ground clutter.
It becomes much more difficult for a radar set to detect a single target flying in this clutter until the aircraft is much closer to the transmitter, thereby greatly reducing the detection range.
Military pilots in non stealth combat aircraft will often opt to fly at low altitudes to hide in clutter. While this does offer a measure of protection against command guided surface to air missiles as well as fighters, it makes the aircraft vulnerable to short range surface threats like close in radar guided AAA or MANPADS. Drug smugglers perfected an art of low flying to avoiding detection in the late s and early s by flying mule aircraft low on the deck over the ocean.
See Mickey Munday's or Barry Seal's story from the documentary Cocaine Cowboys where they talk about flying an aircraft into the Bahamas at night 50 feet above the ocean. They claimed that as long as they couldn't see the glow of lights from Tampa, FL, they were too low to be detected by ATC. In addition to the other answers, radars need to account for moving objects on the ground. To deal with this, most radars don't want to pick up a moving truck or a tree swayed by the wind at all.
There's just too much stuff moving on the ground already, so even if the radar waves do reach you, if you're low enough you're part of the noise at ground level. As already well explained, it is possible to hide in the shadow of obstacles when the radar is on the ground. But if the radar antenna is high enough and the angle of view is greater than the slope of the obstacle envelope, hiding is unpractical:.
The extend of the radar shadow depends on the radar height. Airborne radars are not required to directly overfly the targeted area not desirable for side-looking radars using the Doppler shift to detect moving objects.
Moreover with an airborne radar:. Remaining in the shadow during a significant time would require matching the terrain configuration and the trajectory, which may not be tactically useful.
If such match is possible, it is possible to have a second airborne radar to illuminate the shadow side of the obstacles. Boeing E-3 Sentry with its characteristic shape is a well known aircraft, there are other airborne surveillance radars with fixed or rotating aerials, manned or UAV.
E-3A, source. E-3 operational range is about km. From NATO :. An E-3A can detect low flying targets within km or nautical miles; and at medium altitude targets within km or nautical miles. Three E-3As in overlapping orbits can provide complete coverage of Central Europe. E-3 embed a synthetic aperture radar using the distance traveled by the radar during the pulse roundtrip. The principle is to detect moving targets against the stationary background using Doppler spatial filtering see theory here.
E-3 can stay in flight for about 10 hours without refueling, and are all able to be refueled in flight. Source Chosun. The user must decide whether the threat from enemy air vehicles outweighs the threat from ground forces. Compared to Stealth , it allows helicopters to be more effective at close ground support.
Low-flying aircraft are, however, more vulnerable to ground fire, including unguided or wire guided rockets, as well as fire from armored vehicles. Because of this, pilots of aircraft and encouraged to open, heavy-traffic areas when going below radar to avoid ground fire. Activation of Below Radar can be identified in mainly 3 ways: Below Radar will activate once the aircraft is within 30 meters or feet whether altimeter measurement in metric or imperial units is currently unknown, not even mentioned by the developers from surface level altimeter measurement is based on sea level, not surface level.
Pilots equipped with Air Radar will also witness the HUD symbol of their aircraft disappear when they are below the minimum altitude of activation and re-appear again when above the minimum altitude of activation. For Attack and Scout Helicopters, the specialization is activated at 25 or below, which also turns the aircraft's altimeter red, signifying both a dangerously low altitude, and Below Radar active. Another peculiar, but more inconsistent way of identifying whether Below Radar is activated, is to fly within Air-Radar-Range of enemy aircraft, particularly other hostile Jet Aircraft.
More experienced, dog-fighting professional pilots may appear to be oblivious to the enemy aircraft's presence when below radar, especially when trailing them from behind at extremely low altitudes where Below Radar automatically activates.
This can be identified somewhat accurately when the pilot of the targeted aircraft fails to initiate evasive combat manoeuvres to counter the aircraft trailing them from behind with Below Radar activated, therefore revealing the targeted aircraft's pilot ignorance of the enemy aircraft's presence.
However, the targeted aircraft may also simply be pre-occupied with other tasks such as ground-attack and fails to acknowledge the trailing aircraft anyway, sometimes leading to confusion. This is further compounded with the fact that there is no rear-facing aircraft camera in Battlefield 3 , making it very hard to tell if another jet is following the player.
Below Radar is a permanent vehicle specialization introduced to Battlefield 4 with the Community Operations expansion, and is enabled by default for all air vehicles.
It functions almost identically to its Battlefield 3 counterpart, but it is available by default and does not hinder Spotting. Below Radar will protect against weapon locks from all anti-air vehicle missiles, including those from Mobile Anti-Air vehicles. Connect and share knowledge within a single location that is structured and easy to search. When flying at low altitudes your heat signature is decreased, hiding you from enemy radar and preventing enemy air-to-air missile systems from locking on to you.
I can't find it anywhere in my loadout screen even though I've unlocked it, and it doesn't seem to be a passive perk that's always on, as I can always be locked onto by enemy aircraft. It's altitude based, not terrain based. Just because you're skimming low over a mountain cliff doesn't necessarily mean you're under the radar barrier altitude. Keep the map's geography in mind. When you want to be below radar you must be near sea level.
Well, I have it unlocked, and even way outside the range of all ground based anti-air, with only a jet on my tail and flying so close to the ground a slight bump could've caused me to crash, I'm still being locked on to. So, it's not a passive unlock that needs to be equipped to work. If so, then Air Radar, which is also classified as a "Permanent Upgrade", would also have to be passive — but it still has to be equipped in order to work.
It does work, but only for air-based AA. Ground-based AA both infantry and vehicles are not affected. I can't offer a definitive answer, but FWIW here are the two current schools of thought. So far I've seen only assertions, not evidence.
Below Radar is considered an Upgrade, and in general that should mean it needs to be equipped to function, so I lean toward the former interpretation. I hope someone can provide an actual, unequivocal answer—with supporting evidence since there are already lots of assertions flying around no pun intended. But it may be hard to confirm empirically. Going by the description, ground-to-air can still lock onto you.
So if you're buzzing the treetops and someone locks onto you despite the fact that you have Below Radar unlocked, that might not tell you whether the unlock is working: it could be a ground-based missile lock. Permanent Upgrade: Maintenance - what this does is passively increase the rate at which your jet fixes itself. If you equip Maintenance as a perk, the speed of fixing itself is increased further. Note that after being damaged, there is around 5 second delay before your jet will fix itself, this is not affected by either upgrade.
Permanent Upgrade: Below Radar - this is absolutely useless on a jet, but it works regardless. Note that if you're locked on before entering "below radar" it won't loose your lock on, you'll still need to pop your flares.
As for the helicopters all of these do the exact same things with the only difference being "below radar" is incredibly useful for the ZW and the AH "Little Bird". When flying low and mowing down infantry, they won't be able to lock on to you with the Igla or Stingers. These are all my observations, I might be wrong, I am yet to confirm everything in real terms. Hope it helped! Below radar works at altitudes of 20 meters or less above the ground directly under you so 20 meters above a mountain will still work.
It works by not allowing anyone to lock onto you, and any locks currently in progress will be broken. It doesn't work to stop missiles that are already in the air, and the lock won't break for about. It's very easy to tell when you're below radar in a helicopter, as the altimeter on the right hand side of the screen will be red when you're at 20 or below.
In a jet however, there's no indication, so the best way for you to learn is to fly helicopters until you're comfortable enough with knowing instinctively how low 20 meters is. Before the latest patch, it wouldn't work on Surface-Air Missiles, but as of the latest patch it works on everything in fact it's a little bit OP sometimes.
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