Writing/formulating a story in which a large spaceship (≈1km/0.6 miles long, about a few hundred meters wide) spontaneously appears above land and (until authorities are noted of its presence) stays stationary where it appeared previously. Would nearby air traffic control radars be able to pick it up, and if so how would it appear on one?
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1$\begingroup$ While aviation radars might not pick it up, ground-based weather radar systems almost certainly will. $\endgroup$– Michael SeifertCommented Nov 22 at 4:16
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1$\begingroup$ Radar may or may not pick it up, but an ancient technology (Mk 1 Eyeball) most certainly will. It's likely that the local police will know all about it within a minute or two of it's arrival. $\endgroup$– CatchAsCatchCanCommented Nov 22 at 4:48
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$\begingroup$ shrimps might be able to detect it ;-) $\endgroup$– PatrickTCommented Nov 22 at 8:59
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1$\begingroup$ Radars will pick many airballons, or also large flock of birds. And more interesting: weather radar will see them (and they covers more larger area). $\endgroup$– Giacomo CatenazziCommented Nov 25 at 15:58
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3 Answers
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Summary
Your UFO will be detected, but:
Only by primary radars used by military. ATC centers usually use secondary radars, but they can also receive primary feeds in some cases. So ensure your ATC center has a primary radar source. In that case the size won't appear on ATC display, only a standard track, like for any aircraft.
ATC secondary radar detects only cooperative targets, i.e. targets with a system aboard (ATC transponder) replying to interrogations from the ground, or from other aircraft for collision avoidance.
Many tracking/detection systems in aviation work only with cooperative targets and don't see the other. Tracking sites like FR24 display only cooperative aircraft.
Military with their primary radars can see the actual size, but a target is visible on primary radars only if it reflects radar waves, it must be somehow conductive.
Stationary targets at low elevation over the horizon are difficult to extract from echos created by ground, buildings, etc. Most (primary) radars discard echos from stationary targets at low elevation.
Details follow. You can also create a chat room for additional discussion about details. This way anyone interested can freely provide information without having to meet the rules and format of a formal answer.
Primary vs. secondary radar
ATC radars are secondary surveillance radars (SSR). These radars actually interrogate a system, the ATC transponder, aboard the target. Without entering into details SSR work based with transponders of two compatible kinds: the older mode A/C and the modern mode S. If you need more information, we can provide it.
If the target isn't equipped with a transponder, or if the transponder is off, ATC SSR won't see anything. This is how MH370 disappeared from ATC radars soon after takeoff, while still in the coverage of land ATC centers. The transponder system became inoperative for some reason (unit switched off, or unit/antenna deteriorated).
However military have to detect uncooperative targets, and they use primary surveillance radars (PSR), the good old echo radar working by reflection on the target. In that case the radar station just sends a radio pulse and listen for any echo.
PSR stations may also be available at some ATC centers.
PSR stations use the Doppler shift to assess the target velocity. Targets stationary relatively to the radar station have no Doppler shift, the echo wave has the same frequency as the wave sent by the station. PSR stations may discard echos from stationary targets at low elevation, since most of them are just echos from buildings, trees, lakes, ground, towers, etc. So your stationary UFO might be discarded and not visible.
Not all materials are able to reflect waves of a certain frequency. Usually this is limited to conductive materials which can react to the wave as if the material was a transmitting antenna efficient for that frequency (technically it must be an electric dipole). If your UFO is made of plastic or stealth material, it will be transparent for waves and will reflect nothing significant.
SSR and PSR data can be mixed
In some countries (notoriously the US, as stated in other answers and comments), PSR and SSR stations are part of a network with a hub where all data from all stations are centralized, processed, correlated and redistributed to all ATC and military centers. In that case ATC operators will see an unidentified target detected by a PSR, without the exact altitude information (a PSR system is usually able to detect the direction, the velocity and the range).
PSR data may be discarded
Military are highly interested in uncooperative targets, ATC is not. It is possible ATC operators choose to not display PSR targets, only SSR targets. A reason is collision avoidance is based on altitude, and a target without an altitude cannot be managed, hence aircraft without a transponder able to indicate the aircraft altitude are usually not allowed to enter a crowded airspace controlled by ATC. There are ATC operators on this stack, they may provide additional details about that.
ATC operator may not see the unidentified target. If they see it, the size won't be indicated, a target being basically a line segment (track) with SSR data (transponder id, altitude, speed, vertical speed) displayed as a label close to the track. Stationary targets may be indicated using a specific symbol. As the operator who assigns the transponder id ("squawk code" in mode A/C) can associate this id with the aircraft flight plan and the flight number, often this flight number replaces the transponder id on ATC displays.
STARS ATC symbol for an aircraft, source.
Whatever, if military detect an unidentified target, they will inform related civil ATC units. Actually it's possible a military controller will be present in the civil ATC unit, working with a civil controller to coordinate military and civil traffics.
Questionable targets in the US are managed according to §5.3.2 of FAA Order JO 7110.65AA.
Weather radar
Many civil aircraft have a weather radar onboard, they may detect the uncooperative target, as these radars work by reflection on water droplets (rain in the cloud and below them).
These radars show somehow the actual horizontal shape of the target by reflecting on droplets at any distance. However a solid target may be impermeable to waves, echos will be limited to the envelope portion facing the aircraft, the target will appear as a silhouette.
Weather radar pulses are sent close to the horizon, they don't illuminate targets in the ground direction. The weather radar of an aircraft flying at a high altitude won't see a UFO hovering close to the ground .
Collision avoidance system
Many aircraft (incl. all commercial aircraft) have a collision avoidance system aboard. This system works like a SSR station, it interrogates the transponder of other aircraft in the vicinity, to know their position and altitude. Such systems won't see any target not equipped with a transponder.
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2$\begingroup$ An air traffic control unit using only a single secondary surveillance radar, may still "detect" the UFO if it blocks the radar signal. When the UFO is between the radar and the aircraft, the aircraft is no longer visible. The sudden lack of aircraft in a sector would certainly be noticed. A 1km long object could take quite a chunk out of the radar coverage if it is not too far away from the radar. However, many ATC systems use multiple radars and multi sensor data fusion, so as long as those missing aircraft are seen by other radars, the air traffic controllers will not notice anything. $\endgroup$– DeltaLima ♦Commented Nov 21 at 9:45
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$\begingroup$ "If your UFO is made of plastic with organic creatures inside, it will be transparent for waves and will reflect nothing." This and other definitive statements about being unable to detect via radar are stated too strongly. Yes, conductive (metallic) materials are dramatically better at reflecting radar, but organic materials do so too, just much less. How radar observable something will be is greatly dependent on many factors. As to organic material showing up on radar, a search for "flock of birds on radar" shows many examples. $\endgroup$– MakyenCommented Nov 22 at 19:55
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$\begingroup$ @DeltaLima Yeah, I was going to say I suspect such an object would be seen by ATC as their radar set detecting an error. It probably depends on the specific software package, but that's likely to show up as bad data that might indicate damage to the set. $\endgroup$ Commented Nov 22 at 20:10
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$\begingroup$ @mins That really depends on how you define "organic matter". I'm not aware of any organism above a virus that doesn't have water as a substantial portion of its mass. Thus, I'd assume that "organic creatures" (and their support) are highly likely to include water (or at least some other substance that's similarly reflective). That doesn't mean that the UFO couldn't have been intentionally constructed to not show up on radar, have shields, or cloaking, etc., but that's not part of the question. $\endgroup$– MakyenCommented Nov 23 at 0:56
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Such a spaceship would be very easily "painted" by any radar system in use today—unless it was designed specifically with radar stealth in mind, and even then its immense size would make it very difficult to stealth-ify. However, as @RetiredATC mentions, if it were stationary that may well prevent it from appearing on radar scopes because of the Moving Target Indication feature.
In the usa, considering only the software used by Terminal (low-altitude) controllers that I am familiar with, there are two possibilities: the ship could be depicted as a primary-only radar target or as an area of presumed precipitation. (It is not uncommon for the software to interpret larger-than-aircraft targets, such as a windmill farm or a large flock of geese, as an area of precipitation.)
A primary-only target is shown as a green diamond shape within a blue circle. Coincidentally the blue circle is approximately 0.6 NM wide on the scope, although the exact size does vary a little based on the range setting.
Precipitation is shown in 0.25 NM-by-0.25 NM square chunks. The ship would probably be "light" precipitation, which is a light blue color, of perhaps 3x1 or 3x2 chunks.
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$\begingroup$ Does the Moving Target Indication filter out hovering helicopters? If a helicopter stops squawking and starts hovering, does it become "invisible" to ATC? $\endgroup$– vszCommented Nov 21 at 5:39
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2$\begingroup$ Haven't seen that before @vsz, that I've noticed anyway. If the target is squawking it will stay on the scope of course, as you alluded to. And if it was squawking but it stops squawking the system is almost always able to still track it and still associate it with the existing flight data. If it was never squawking in the first place then the MTI likely would end up excluding it, at least after a little while. $\endgroup$ Commented Nov 21 at 6:31
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$\begingroup$ @vsz and if it stops squawking while being tracked it'd most likely also be flagged as there is potentially a serious problem with the aircraft (electrical failure, crash, etc.). $\endgroup$– jwentingCommented Nov 21 at 9:15
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If the object reflects at least as much radiation as a Cessna 150, traffic control radar can see it with a "skin paint". Unfortunately, if they're in normal operating mode, they look almost exclusively for transponder squawks, and even a huge reflection might be filtered out by the radar's computers.
Weather radar would be even worse, as it looks primarily for Doppler effect indicating motion toward or away from the antenna.
Military radar (especially threat detection sites) would easily pick up anything even as large as a good sized drone, however, and is far less likely to be dependent on transponder identification to put a blip on the operator's screen.
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1$\begingroup$ Really? I hear ATC give traffic alerts with altitude unknown pretty frequently. I guess those could be people with just a mode A transponder, but I'd think that's probably even less common than no transponder at all. $\endgroup$– ChrisCommented Nov 20 at 19:32
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3$\begingroup$ It depends on the altitude of the object and the distance from the radar antenna. The further away from the antenna and the lower the altitude the harder it is for the radar to detect. Additionally, if the radar did receive a return from the object, it would likely not be shown on the controller's display. Air traffic computers try to filter out objects that are not moving because it's looking for airplanes, which move (known as moving target indicator in some installations). $\endgroup$ Commented Nov 20 at 19:32
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$\begingroup$ @Chris (and @ZI): in the USA, ATC radars are almost invariably combined primary-and-secondary units so they will detect both transponder-equipped and non-transponder-equipped targets. Only in very rare cases are secondary-only radars used. (Of course ADS-B-only receiving sites have become quite common but they are mostly used to supplement traditional radar.) In other countries secondary-only radar may be much more common. $\endgroup$ Commented Nov 20 at 19:55
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$\begingroup$ What is true in the USA is that controllers whose airspace consists is entirely Class A are allowed to disable the display of primary-only targets on their scopes, because the assumption is that anyone who may be a factor for Class A traffic is transponder-equipped. $\endgroup$ Commented Nov 20 at 19:57
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$\begingroup$ I'm skeptical of the assertion that ground-based weather radars will filter out returns that aren't moving towards or away from the antenna. If nothing else, that would lead to the forecasters completely ignoring weather systems that are moving perpendicularly to their line of sight. $\endgroup$ Commented Nov 22 at 4:15