How is aircraft separation maintained when there is no radar?

  • How is aircraft separation maintained when there is no radar? Lnafziger

    In remote areas that have no radar (oceanic airspace, etc.) how is aircraft separation maintained so that airplanes don't get too close?

    What do areas that normally have radar do when there is a sudden radar outage? There are airplanes going everywhere, not necessarily on standard airways, and a lot of them are being given radar vectors. What happens when the screens go dark?

  • For oceanic airspace, flights proceed along specified tracks and altitudes specified in their filed flight plan. From there, once they leave radar-controlled airspace, radio reports are made to ATC, who ensure that aircraft are appropriately separated if a flight needs to change its track or altitude. See the wiki article the North Atlantic tracks, which give a good overview on the track system. Specifically:

    Prior to departure, airline flight dispatchers/flight operations officers will determine the best track based on destination, aircraft weight, aircraft type, prevailing winds and Air Traffic Control route charges. The aircraft will then contact the Oceanic Center controller before entering oceanic airspace and request the track giving the estimated time of arrival at the entry point. The Oceanic Controllers then calculate the required separation distances between aircraft and issue clearances to the pilots. It may be that the track is not available at that altitude or time so an alternate track or altitude will be assigned. Planes cannot change assigned course or altitude without permission.


    Contingency plans exist within the North Atlantic Track system to account for any operational issues that occur. For example, if an aircraft can no longer maintain the speed or altitude it was assigned, the aircraft can move off the track route and fly parallel to its track, but well away from other aircraft. Also, pilots on North Atlantic Tracks are required to inform air traffic control of any deviations in altitude or speed necessitated by avoiding weather, such as thunderstorms or turbulence.

    I'm going to leave the "what happens when things break" to someone else.

  • When the screens go dark ATC breaks out the flight progress strips (and possibly the shrimp boats or other airplane-substitutes to lay on a map & push around) and uses brain power to substitute for the computer and radar. Check out one of my favorite "Say Again?" columns over on AvWeb which talks a little bit about it (and you can find lots more if you browse the archives).

    Broadly, there's a few implications to non-radar operations - both routine and "when stuff breaks" - most of which are concessions to safety. The one most pilots will feel, and the biggest safety item, is that "random RNAV routes can only be approved in a radar environment" (you'll find that gem in the AIM, but the translation is this: "GPS Direct? YOU NO CAN HAS!").

    To facilitate separation in a non-radar environment the controllers are going to chuck you (and every other IFR flight) on airways, at standard IFR altitudes, quite probably with a specified airspeed, and they're going to route you navaid-to-navaid.
    The idea here is simple: "You gotta put them together to keep them apart": By ensuring all flights are on known airways, at known altitudes, and at known speeds controllers have a good idea of who is where in their airspace. They will supplement this mental picture by asking pilots to report crossing certain fixes (intersections/radials), so that they know exactly where that flight is when they make their report (which will help them account for winds aloft, and give them an idea of what the flight's ground speed is in addition to the airspeed they're asking them to fly).

    All of the above provides IFR-to-IFR separation, which is what the controller is concerned with. IFR-to-VFR separation (which is normally supplemented by radar) becomes the responsibility of the pilots ("See and Avoid") - this means it's largely a factor of altitude (that 500-foot difference between IFR and VFR cruising altitudes and those cloud clearance requirements should start to seem pretty important right about now).

  • ATC aircraft separation is built upon the assumption of no radar by way position reports. At compulsory reporting points (black triangle on enroute charts), an IFR aircraft (unless radar identified) is required to report (see 5-3-2 of the AIM):

    (a) Identification;

    (b) Position;

    (c) Time;

    (d) Altitude or flight level (include actual altitude or flight level when operating on a clearance specifying VFR-on-top);

    (e) Type of flight plan (not required in IFR position reports made directly to ARTCCs or approach control);

    (f) ETA and name of next reporting point;

    (g) The name only of the next succeeding reporting point along the route of flight; and

    (h) Pertinent remarks.

    ATC uses the position and ETA to keep aircraft separated. Most of the US is covered by radar, but remote parts of Alaska and oceanic still use position reports. If ATC radar is not functioning (rare), then the system reverts to position reports.

    Moving forward, ADS-B allows aircraft to self broadcast its position. A network of ground and satellite ADS-B receivers can build up a picture of the traffic.

Related questions and answers
  • In remote areas that have no radar (oceanic airspace, etc.) how is aircraft separation maintained so that airplanes don't get too close? What do areas that normally have radar do when there is a sudden radar outage? There are airplanes going everywhere, not necessarily on standard airways, and a lot of them are being given radar vectors. What happens when the screens go dark?

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