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I'm writing a story about the pilot of the Austrian Air Force, who uses his Eurofighter Typhoon aircraft for unintended purposes.

I've read stories in the internets that civil pilots allegedly can make their planes invisible for the flight controllers by turning off some device (transponder?). I'm not sure if it is true, but I saw with my own eyes a pilot of a small aircraft (Cessna Skyhawk), who took off without a transponder (the antenna was broken), was told by the ATC to fix the transponder, landed, and then took off again -- now with a temporary fix that made the antenna work for a while. In other words: Pilots of small aircraft do have the option to fly with or without a transponder (at least on small Austrian airfields). I'm not sure if it's 100 % legal, but sometimes pilots do it.

But a military Eurofighter is different from a little Cessna plane.

Is it technically possible for a pilot of Eurofighter Typhoon to become invisible for the air traffic controllers by switching off the transponder?

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    $\begingroup$ Possible duplicate of Is "secondary radar" a misnomer? $\endgroup$ – Simon Apr 2 '17 at 10:49
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    $\begingroup$ You don't need to switch off the transponder if you are flying at an altitude of 200ft above the floor of a valley through the mountains ;) In fact I've seen a couple of Tornados "hide" behind a 200ft slope at the edge of an air base during a "war games" exercise - they just stooged in quietly below the crest of the slope with no radio communication, hopped over the slope at the last minute, and beat up the airfield at 50 ft AGL with the throttles set to "reheat". If that had been "for real" there would have been a few hundred cluster bombs to dig out of the runway. $\endgroup$ – alephzero Apr 2 '17 at 14:32
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    $\begingroup$ @mins: ATC (at least in the US) does use primary radar. It's not unusual to have them give a bearing to a target with no altitude info, which may turn out to be a sailplane, hang glider, flock of birds, &c. $\endgroup$ – jamesqf Apr 2 '17 at 19:18
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    $\begingroup$ If the Eurofighter is armed, it could turn also off ATC radars in a more permanent way. $\endgroup$ – Steve V. Apr 3 '17 at 1:57
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    $\begingroup$ @alephzero: Very nice anecdote, and yes, hugging the terrain was standard tactics for the Tornado. Only one nitpic: There's nothing "quiet" about a Tornado at that flight level. ;-) $\endgroup$ – DevSolar Apr 3 '17 at 15:14
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Radar systems have 2 aspects, the primary and secondary systems, or the Radar and the interrogator. The radar sends out a RF signal and receives everything that is returned on that same frequency, and performs calculations to determine the azimuth and range of everything it receives. The interrogator sends out radio pulses which transponders in aircraft receive and decode. The transponders then transmit data on a different frequency than what was received. The interrogator then receives that signal and decodes it and correlates it with the primary radar data to tell the operator that 'yes, that is definitely a valid target' and gives the info on the flight to the operator.

You can turn off the transponder, but all that will do is make it so that the ATC won't be able to see a correlated flight, (one that is definitely an aircraft and not just noise coming through the primary radar). The screen would still show a blip where the radar picks up a signature though. If the plane has radar absorbent material or is close enough to the ground, that would get filtered out as noise.

An ATC, who is used to only working with aircraft who want to be seen, and thus keep their transponders on, might be fooled by this. But a military radar operator might not be so easily fooled by this.

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    $\begingroup$ @FranzDrollig I think this is a quite new / different question. Care to ask this as separate one? $\endgroup$ – trejder Apr 2 '17 at 13:03
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    $\begingroup$ @trejder Right. Here's the new question: aviation.stackexchange.com/questions/36789/… $\endgroup$ – Franz Drollig Apr 2 '17 at 15:49
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    $\begingroup$ @FranzDrollig Flying over Austria is a bit different than flying over the South Indian Ocean 1,500 miles from the nearest inhabited land mass, though. While MH370 was near inhabited land masses, it was indeed tracked on primary radar (as it crossed the Malay Peninsula and turned South.) It was a while after the initial disappearance before Malaysia released this information, though, as they were concerned about releasing information on the capabilities of their primary radar. I don't remember if Indonesia ever released theirs or not (I'm thinking they didn't.) $\endgroup$ – reirab Apr 4 '17 at 15:18
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    $\begingroup$ It should also be noted that civil aviation authorities can and often do utilize only a secondary surveillance radar (SSR) as a standalone system. This is done as a cost saving measure, as a primary surveillance radar (PSR) initial acquisition and reoccurring operational and maintenance costs can be prohibitive. If an aircraft is operating SSR only airspace, ave assuming there are no military integrated air defense system ASV radars being used, an aircraft could go undetected if the transponder was deactivated. $\endgroup$ – BigNutz Apr 5 '17 at 5:45
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    $\begingroup$ A military operator will sound the alarm. Unidentified flying object coming in with the speed and overall signature of a fighter plane is a possible incoming attack. $\endgroup$ – Mast Apr 5 '17 at 12:12
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The Eurofighter has a transponder that can be switched off, similar to a Cessna. By switching off the transponder, the aircraft becomes invisible to cooperative surveillance. This means the aircraft will not be detected by secondary radar and active multilateration. However, the aircraft can still be detected by primary radar because that does not rely on the transponder.

Civil ATC centres sometimes use only secondary radar, so in such a case the aircraft can become invisible when the transponder is switched off.

Military air traffic control and air defence systems do not rely so much on cooperative surveillance, mainly because you can't expect the enemy to cooperate. Therefore hiding a Eurofighter from military ATC by switching off the transponder is doomed to fail.

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    $\begingroup$ While hiding a Eurofighter from military ATC by switching off the transponder is doomed to fail, you could hide a Eurofighter by switching on a transponder. One that claims the Eurofighter isn't a fighter plane at all. $\endgroup$ – Peter Apr 2 '17 at 22:07
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    $\begingroup$ @Peter that won't hide it, merely masquerade it as something else. And if the primary radar return conflicts with what the transponder tells the secondary, I'd guess a military radar would instantly flag the return for extra attention. A civilian radar indeed would probably accept the transponder reading at face value. $\endgroup$ – jwenting Apr 3 '17 at 6:57
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    $\begingroup$ @Peter The transponder sends information such as altitude, bearing, and velocity with a 4 digit code. That code references a filed flight plan which has the airplane type listed. Military planes have had fight plans generated which do not match their actual type as a stealth measure. However the pilot should be told that so that they don't exceed the flight performance of the type they are pretending to be. There is a rather Infamous story of a SR-71 over Canada doing 150MPH over the KC-135 that they are pretending to be. $\endgroup$ – Rowan Hawkins Apr 3 '17 at 7:43
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    $\begingroup$ airspacemag.com/flight-today/blackbird-diaries-180953373 the anecdote mentioned by @RowanHawkins - at least with 1970s tech the radar picks it up but the operator can't tell what plane it is exactly. $\endgroup$ – Pranab Apr 3 '17 at 9:56
  • $\begingroup$ It still functions the same way. ADS-B has the aircraft's callsign embedded in each packet, but the transponder is just a 4 digit code reference for the flight plan. $\endgroup$ – Rowan Hawkins Apr 4 '17 at 23:43
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ATC radar has two modes: Primary and Secondary.

Primary shows radar echo returns. Normally this is from metal aircraft, precipitation, birds and anything which reflects radio frequency energy at the frequencies of the radar transmitter.

Secondary radar shows the replies from transponders. A transponder responds to an "interrogation" signal transmitted along with the primary radar transmission, but at a different frequency. The transponder replies with a return, which has some binary data encoded with it. That data could include a 12 bit squawk code, pressure altitude of the aircraft binary data, aircraft identification data (mode S), as well as a single bit "ident" data which is set temporarily by pressing a button on the control panel of the transponder inside the aircraft.

Some military aircraft have coatings and designs to minimize primary radar returns, so that they can operate without being recognized by common ATC radars and some military defense radars. Additionally, they may have active systems designed to fool or jam the expected operation of primary radars.

Any aircraft can operate with their transponder turned off. If that aircraft has a low radar cross section (RCS), perhaps due to it's size or radar reflectivity, it may not be seen by primary radar.

Normally ATC is run with secondary radar, and the controller will "turn down" the display of primary radar, to reduce clutter on their screen.

So if a military aircraft with primary radar supression technology turns off their transponder, they may not be seen by ATC. If they then engage active measures to fool or jam primary radar, they may not be visible to most primary radar systems.

So, to answer your question, yes, it is possible for a military aircraft to become "invisible" to ATC.

If you have further questions, ask. I have spent 28 years writing radar processing algorithms, and will share what I can.

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    $\begingroup$ If the target uses active jamming the ATC will know it. And most likely would not be able to see anyting on primary radar. $\endgroup$ – TomMcW Apr 2 '17 at 20:35
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    $\begingroup$ I worked on the TPS-75 in the air force, it used frequency staggering. So it would send a pulse at 2.9 GHz then one at 3.1 and some more in between. It did this in a certain order that might be classified, I'm not sure. I think the reason for this was to make jamming a bit more difficult. It also had some feature that would show which azimuth jamming was coming from if jamming indeed was being attempted. $\endgroup$ – alex Apr 3 '17 at 7:53
  • $\begingroup$ There are active techniques which are not clearly observable by ATC primary radar and effectively prevent the display (and detection) by primary radar. $\endgroup$ – mongo Apr 3 '17 at 16:18
  • $\begingroup$ Methods varying the spectral and temporal signals are good methods, as they tend to block simplistic counter-measures. $\endgroup$ – mongo Apr 3 '17 at 16:20
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Since your story is set in Austria, you may also consider the fact that the airport of Innsbruck (ICAO: LOWI) has no primary radar for aircraft flying lower than 3.000m because it's impossible due to the topology of the Inntal.

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The DASS ("Defensive Aids Sub System") of the Eurofighter Typhoon can "delay/hamper/prevent aquisition by enemy RADAR", using powerful jamming transmitters in the wing tips. It also has towed jammer transmitters ("Towed Decoy", being towed behind the airplane by 100m of Kevlar line). Via Cross-Eye-Jamming, it can also make Monopulse-RADARs blieve that the plane is at a different position.

The Eurofighter Typhoon can dispende chaff, which can block simple RADARs, but is mostly useless against Doppler RADAR.

All these counter measurements are primarily targeted as a defense against RADAR-guided missiles. These counter-measurements show up on the RADAR displays. Chaff will produce a large RADAR echo (trying to convice the simple brain of an approaching missile that the chaff is a better target than the airplane). Jamming will show up as noise on the receiving RADAR. The noise will will occupy a sector of the RADAR image. Depending on the strength of the jammer and the quality of the RADAR, the sector may be wider or rather tight, but it will always point into the general direction of the jammer. The RADAR operator will know that there is a RADAR jammer in a certain direction, but he will not know altitude and distance. Obviously, networked RADARs, which combine data from RADARs at different locations can triangulate the position of the jammer.

But there's always the classical approach: low altitude flight, preferably in a hill/mountain region.

In any case, it will be the last flight the pilot ever did as a licensed pilot.

Edit: Oh, yes, Anti Radiation Missiles. Most probably the AGM-88 HARM ("Highspeed Anti Radiation Missile") in case of the Eurofighter Typhoon. These will definitely make ALL aircraft invisible to a RADAR. Useful in a range between 25km (if launched at low altitude) and 80km (if launched in the stratopause, a height of 50km or so), it will approach a RADAR at a speed of Mach 2.9 or above. Of course, the RADAR operator will, most likely, notice that his RADAR just got destroyed by a missile. The NATO Counter-Surprise alert state will about immediate jump to SCARLET. Military RADAR (like the AWACS ("Airborne Warning and Control System")) will, most likely, have already detected the missile attack. Anti aircraft weapons will be manned, fighter and bomber aircraft will be readied and awaiting combat orders. The USA will, most probably, also go to DEFCON 2, with a stand-by for a possible increase to DEFCON 1.

What exactly is the "unintened purpose" of the Austrian pilot. Trying to impress his girlfriend, maybe? Hm.

More plausible is what Peter mentioned: impersonating another flight by switching transponder codes. The ATC will, most probably, notice when a flight does not behave as expected, is in two places at the same time, or is present at the wrong time.

Also note that there a different ATCs: Eurocontrol for the Upper Air Space. Austro Control for general Austria, Deutsche Flugsicherung for general Germany, plus the individual ATCs for the airspace near the individual airports.

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    $\begingroup$ "a height of 50km" - that is way outside the envelope of any airplane capable of launching an AGM-88, and consequently outside the design limits. The rocket engine will fire at that height (doesn't need atmospheric oxygen) but the guidance fins need sufficient airflow to work. Did you perhaps mean 50.000 feet? That's somewhat high, but not for an EuroFighter Typhoon (ceiling ~65.000 ft/20 km) $\endgroup$ – MSalters Apr 3 '17 at 20:04
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    $\begingroup$ While I'm definitely glad to see someone mentioning that fighter planes and fighter pilots have a vested interest in being able to evade radar detections through a variety of means, I do have to politely suggest that deploying a HARM against the ground radar is most likely not in the CONOPS scope the OP was intending. It would certainly make it so the radar is incapable of seeing the plane, but it also is a very... visible action. =) $\endgroup$ – Cort Ammon Apr 3 '17 at 21:19
  • $\begingroup$ "A height of 50km": good point about the airplane. I do not know which airplane is capable of launching an AGM-88 at that height, but the idea of firing this missile into the stratopause is real. The diagrams I have seen suggest that AGM-88 is launched at some lower altitude, then the AGM-88 ascends into the stratopause...and then waits for an enemy RADAR to appear. Not sure about details there, maybe it could perform the final approach as a glide bomb? $\endgroup$ – Klaws Jun 14 '17 at 14:36
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A pilot cannot make a plane invisible to radar by switching off the transponder.

The transponder simply makes the plane easier to see and transmits a code. ATC sees aircraft with transponders as bright dots with a code next to them. They see unidentified objects as a dull dot with no code. Uncoded aircraft are a great concern to controllers because they do not know what it is. It could be a flock of birds, a foreign military aircraft, a balloon, a glider, a puff of dust, etc. Gliders are spooky because they have no transponders, but can go in excess of 100 MPH and fly at high altitudes, just like powered aircraft.

Normally, ATC can recognize an aircraft because they move fast in a straight line. If a controllers sees a dot moving 600 MPH in a straight line at 30,000 feet, there is little question what it is. To hide from radar a plane has to go slow at low altitude, but even the best sneak will usually show up on radar sooner or later, so in practice it is hard to stay hidden forever, especially for a fast mover like a military jet.

Sometimes, aircraft accidentally leave their transponder off, or in rare cases it stops working (usually due to a disconnected power line). In these cases the controller will call out on the radio for the aircraft to identify itself.

ATC: Aircraft 5 miles west of Fitchburg YOUR TRANSPONDER APPEARS TO BE MALFUNCTIONING. Please sqawk one two zero zero and ident.

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The aircraft specified is a military Eurofighter Typhoon. The IFF transponder in the Typhoon might be different, but in all military IFF systems I know of, the military modes are not changeable from the cockpit. While it would be possible to change the Mode 3 responses, that would create a conflict with the Mode 1 and 2 responses. Again, I'm not familiar enough with civilian versions to be sure, but it would seem that such a conflict would cause the transponder responses to be marked as invalid, and the aircraft as unidentified. As someone else noted, this would not stop the aircraft from being seen by RADAR, but would merely fail to identify it, generating radio queries by ATC.

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