How Often Do Airlines Fly Into Areas Without Radar Coverage? (Ask Cranky)

Air Traffic Control, Ask Cranky

Here’s a very timely Ask Cranky that was spurred by the Air France accident earlier this week. I’m sure many of you heard that the airplane was flying in an area where there was no radar coverage, and that might be surprising. You may have had these same questions . . .

How often do commercial airline flights fly into areas without any radar coverage?

Where are these areas that lack radar coverage?


It may surprise you to know that most of the world is not covered by radar. Part of that is a technical challenge. Our air traffic system runs on ground-based radar, and most of the world is covered in water. It’s kind of hard to plant something that’s ground-based into the ocean. Ask CrankyFor that reason, once you’re more than a few miles off the coast (as was the Air France aircraft), you’re going to be out of radar range.

But even over land, radar coverage isn’t always stellar. For example, there has been much discussion about Brazil’s gaps in radar coverage over the Amazon over the last few years.

So is this a huge problem? Well it’s certainly not ideal, but it’s not dangerous either as long as proper procedures are being followed.

Look at the North Atlantic, for example. That is one incredibly busy area every single day with tons of traffic going between the US and Europe. So how do they handle all that traffic without radar? They introduce inefficiencies to keep planes far apart.

First of all, the North Atlantic operates under a track system. So every night, winds are taken into account and certain tracks are used by all airplanes. Eastbound and westbound airplanes fly different tracks at different altitudes – it used to be 2,000 ft differences but now it’s been reduced to 1,000 ft. (Those vertical separations are used over land as well.) They also have started to fly a mile or two off-center of the track to provide even more protection from a mid-air collision. There is also greater separation introduced between airplanes on the same track to give them some leeway.

So as you can see, it’s not dangerous but just slightly inefficient. It is important, however, to note that weather radar is a different story. Every commercial jet flying has weather radar to help it avoid storms regardless of whether it’s over land or ocean.

One of these days, we’ll finally have GPS systems throughout the fleet that will fix this issue once and for all. But we’ll talk about “NextGen” in another post.

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24 comments on “How Often Do Airlines Fly Into Areas Without Radar Coverage? (Ask Cranky)

  1. Out of curiosity, since all long-haul flights seem to have that little flight map on the IFE based on GPS, wouldn’t it make sense that automatic signals are sent to the airline’s maintenance computers with the location and other stats of the flight? Or is there just too little utility to invest in obtaining and storing information that is 99.99999% unnecessary?

  2. Shane – I imagine it would take a lot of bandwidth to send exact position reports for every single airplane on a constant basis. Many airplanes do now automatically report in when they’re out of radar contact, but it only happens periodically at certain reporting points. I’m not sure if there’s any move to make this a reality, but maybe some pilot readers can comment.

  3. Also out of curiosity – if an aircraft encounters turbulant air then it will want to fly around it which means it would have to leave its booked route. I assume in radar-covered areas this is coordinated by ATC who make sure that everyone goes the same way around the weather and maintains separation. But what happens if a plane wants to leave its North Atlantic track? Or are they chosen each day to take routes which are as turbulance-free as possible?

  4. Simon – The good news is that while they’re out of radar contact, they aren’t out of radio contact. So pilots will call in to report if they are deviating around weather and other airplanes will take that into account. If there is an emergency, airplanes can step off the track so they’ll have plenty of space to maneuver as necessary.

  5. Please clarify….

    All aircraft have weather radar to detect adverse conditions.
    All aircraft have hard contact radar to detect other aircraft in the vicinity.
    All aircraft have INS (Inertial Navigation Systems) to plot flight paths and pinpoint their own exact locations on that track.

    If all of the above are true, ground based radar isn’t particularly necessary on overwater segments. In normal flight any aircraft and others around it know where it is. In the case of an accident, as Cranky pointed out yesterday, I’m curious why other aircraft ostensibly in that same area and equipped with hard-contact radar did not report a sudden disappearence and relay the coordinates.

    Pilots are chatty cathies. They share radio frequencies for safety reasons and use them during down time to gossip and send alerts regarding good or bad weather. They also would be in a great position to hear early warnings of distress unless the flight in trouble is too busy to transmit.

    Please correct any of the above if it is untrue or incomplete. My basic question is why only one visual reference from the area where AF-447’s transponder would have pinged the hard-contact radar of any other aircraft in the vicinity?

  6. Am I the last queasy flyer who still feels a little uneasy listening in on UA channel 9 on a flight to Europe when I hear that Canadian controller saying to my flight…”…radar coverage terminated. Good Night!”?

    I look out the window and all I see is dark? And then turbulence! How many more miles ’til Shannon picks us up?

  7. I remembered something that provided a “faux radar” view of the ocean provided via data tramissions from the plane.

    I haven’t had time to fully dig into it, but I think this is generated off of the Automatic dependent surveillance-broadcast system..

    Basically the plane figures out where it is using GPS then it transmits it via satellite to the ground stations, and a computer generates the radar-like view from this….

  8. How about TCAS? Does that work over the ocean? I suppose it also relies on radar interrogation. It really is a dark sky out there. Thank goodness it’s also big.

  9. Optimist – I don’t think the aircraft-based radar (which is used for TCAS, Nimitz, and it does work over the ocean) goes out all that far. According to one article, the range is about 40 miles from the aircraft. So while other aircraft were in the general vicinity, we don’t know that they were within 40 miles. And even if they were, would the pilots have even noticed if a plane fell off the radar if it wasn’t an imminent threat to that aircraft?

    JK – Hah, I’ve had that feeling before. It seems awfully lonely out there, though being over the north Atlantic at night is one of the busiest places around!

    US Travel – Dude, what the heck are you talking about?

    Nicholas – That’s interesting – I wasn’t aware of that.

  10. It may look and feel lonely but knowing the volume of traffic on any given night over the North Atlantic I’d know we weren’t the only ones out there.

    If you want lonely, consider the half-dozen or so flights between California and Australia every night. Six thousand miles of open water with only Tahiti, Fiji and the Samoas sprinkled along the way. “Radar contact terminated” for sure!

  11. Am I the last queasy flyer who still feels a little uneasy listening in on UA channel 9 on a flight to Europe when I hear that Canadian controller saying to my flight…”…radar coverage terminated. Good Night!”?

    I look out the window and all I see is dark? And then turbulence! How many more miles ’til Shannon picks us up?

    Well, of course, if everything goes to pot….it’s probably not going to matter that much if there are people “around” or not.

    Just saying…..

  12. Well Cranky you should save NextGen for another time and upgrade the radar coverage discussion to more accurate and objectives levels.

    The good news first:
    Ground based radars in the developed countries have just about line-of-sight range because of transponders. This means that an airliner can be located within a mile or two over 100 nautical miles from the ground radar.

    There are navigation aids, satellite phones, sometimes HF radios and other means for an aircraft to transmit its position and ETA to landfall. Commercial airliners have laser-gyroscope inertia systems good to about a nautical mile.

    Airliners equipped with cheaper and more accurate GPS navigation aids can transmit their position to 10 metersa or so.

    Now the bad news:
    Air traffic control is wedded to ground radars. NextGen will use the same radars and transponders. Between the runways and gates and over the oceans and other areas without radar coverage, airlines are in charge.

  13. Jim Spensley – I’m not sure I’m following you completely here. You said, “Air traffic control is wedded to ground radars. NextGen will use the same radars and transponders.”

    But the FAA would beg to differ. From their website – “At its most basic level, NextGen represents an evolution from a ground-based system of air traffic control to a satellite-based system of air traffic management.”

  14. to Nimitz
    tcas uses mode s atc transponders. the mode s transponders on your airplane is searching for mode s transponders on other airplanes then the tcas processor processes what altitude heading airspeed and if it see,s a airplane to close to you it will warn the flight crew plus the crew can see all the aircrafts that tcas see on there nd are dedicated tcas display. so yes tcas will work over the ocean. it docent need ground base radar. all the system tcas needs are all equipment of the aircraft

  15. CF and interested others. In the NextGen concept, every aircraft would have a GPS receiver and a computer component that stores its GPS-determined positions every so often. The NextGen ADS-B transceiver on each aircraft would send its position plus the plane’s identity to other aircraft and ground stations within 150 miles.

    There may be 450 or more aircraft and ground stations within 150 miles. The GPS data transmissions must be synchronized, broadcast one at a time. This is when the ground radar beam illuminates the aircraft, just as in the current transceivers. As I said, just about the same for air traffic control, except the geocentric coordinates of the aircraft are not, like current systems, direction and distance (azimuth and range) from the ground station. The possibly more-accurate GPS positions in latitude/longitude are converted for use by the controllers, who view their radar vector displays and radio instructions to the cockpits one aircraft at a time, same as now.

    Each aircraft has a computer that converts the many labeled positions of other aircraft and displays nearby aircraft tracks relative to the aircraft, a kind of collision-avoidance idea but not yet fleshed out in the designs. FAA propoganda and a Popular Mechanics article a few months ago assert that “now pilots and controllers see the same display …” I say this is not true and useless if it were true.

    Finally, if any aircraft in a sector lacks a working GPS/ADS-B subsystem, air traffic control shifts to a back-up mode (same as now). Think about this in the context of security.

  16. Brett,

    I was recently on a turbulent Cape Air flight that spent much of the time in the clouds. I asked the pilot how they avoided collisions in the clouds with GA aircraft. He said that the pilots talk to each other and ATC would see if there was a problem. However, what contact do they have under VFR? Are they always on someone’s screen and at someone’s direction?


  17. The discussion is getting pretty far afield if the topic is over the ocean navigation and re-entering air traffic controlled airspace.

    Air Traffic Collision Avoidance (ATAC), Aircraft Digital Position – Broadcast (ADS-B), and transponder radar returns are related but not compatible ways to deal with relative positions of aircraft in flight. Position data or navigation “fixes” are used to adjust direction and speed to reach a fixed point at a planned time.

    The realtime application of position to collision avoidance or air traffic control separations depends on forecasting future positions from a time-history of positions — tracks. In general, decisions about adjusting tracks are best made from data displayed relative to the decider.

    So the “when” is right now for collision avoidance, minutes from now for air traffic control, and eventually for navigation. One size will not fit all.

  18. I guess I hadn’t given the radar coverage much thought. I sort of presumed (incorrectly) that GPS transponders filled in the gaps in ground-based radar coverage. Much of the world’s shipping is fitted with AIS (Automatic Identification System) and transmits position, course and speed at regular intervals. This is a VHF system… AIS is required on all ships over a certain tonnage and for ALL passenger ships. Wikipedia (everyone’s best source!) indicates that as many as 40,000 ships are fitted with AIS. Would this be similar to the “NextGen” technology you said you’d discuss in a future post?

  19. I’m sure I could come up with a ballpark figure myself – but roughly how many flights are over the N Atlantic at once each night?

  20. Perhaps we should wait to look at NextGen and address what technology is available for tracking aircraft over the oceans.
    A. NOT ground -based FAA radars or EU or Asian or Austrailian air traffic control radars.
    B. Aircraft that do fly in US or EU airspace use radar transponders to extend the range of ground radars and to tag tracks with aircraft identification codes. The coverage includes various over-ocean jetways.
    C. In the US and the EU, enroute air traffic control coverage is thorough for jetways but incomplete in some places, including some fairly large areas in the western US and the Canadian northwest. En route aircraft fly at designated altitudes, depending on headings, separated vertically. These conventions are kept over the ocean generally.
    D. Near airports, control tower radars — still using transponders for identification — allow controllers to guide aircraft in departure or approach and keep them separated horizontally. Coastal airports may acquire tracks from inbound aircraft that were not tracked by en route radars.
    E. So flight plans are filed for ocean crossings and changes are radioed to airline systems operation centers. Generally speaking, all the long-distance jets fly at about the speed and one would not usually overtake another.
    F. Various navigation aids are available: inertial, GPS, a few radio beacons, etc.But if the aircraft doesn’t report its position, only the aircrew knows or thinks they know where they are.
    G. Because of altitude/direction separations, aircrew can see any other critical aircraft or exchange positions and planned flight paths using various communivcations (NOT radar transponders, no protocol).
    H. Aircraft can send and receive short-range position/ID data if equipped with ATCA or in some cases aircraft systems can detact a radiator is close enough to .look for. Geocentric positions can be displayed or more usefully, converted to aircraft-centered coordinates.

  21. Graham – If we’re talking about small airplanes flying VFR over land, then they will show up on radar. If they have their transponders on, it should broadcast information about the actual aircraft. (I think.) If not, they would just show up as a blip, but they would still show up. (Again, I think.)

    Jeremy – I don’t think this will be similar to the AIS, but I honestly don’t know. This is a good link for learning more about NextGen:

    Simon – It’s a good question, and I don’t know the answer. Anyone else know?

  22. The link given by CF says:”NextGen is an umbrella term for the ongoing, wide-ranging transformation of the United States’ national airspace system (NAS). At its most basic level, NextGen represents an evolution from a ground-based system of air traffic control to a satellite-based system of air traffic management. This evolution is vital to meeting future demand, and avoid to gridlock in the sky and at our nation’s airports.

    The second sentence is misleading. Also, the “transponders will broadcast if on ” statement by CF. The radar transponders transmit when stimulated by a ground radar signal, and this carries the direction (azimuth) and diatance (range) information (time the signal was received from the ground radar). The aircraft ID is added in and the transmission is at the radar frequency.

    If GPS geocentric position was also sent, the air traffic control use of the information would be limited by the resolution of the artifical radar displays. Given the right systems and equipment, various ground stations or aircraft could receive the GPS position and time data, compute histories, and convert tracks to relative coordinates for a cockpit display. The computation is not trivial, and numerous transmitters at random distances from an aircraft can overlap and garble each other. I think to be sure of limited garbles, the interval for fixes from all aircrat within 150 miles might be something like 400! and the current NextGen requirement is 449! (factorial).

    I doubt this many signals are needed or that power is such that a 150 mile range is possible to an aircraft in flight. Some kind of filtering or selection would be used. The ground stations sort out the aircraft by azimuth because they transmit a narrow beam and a short pulse.

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