Primary target: An aircraft not reporting mode-C, the only thing the controller has is the return on the radar.

When a controller reports a primary target as traffic to other aircraft, the controller does not have the altitude of the target. Given this, I conclude that ATC radar does not have the altitude (angle-up) to the target, and only provides azimuth.

So then without the altitude, how does the radar-system know where to put the target laterally on the screen?

Example, a radar picks up a target that is 10 miles from the station. If the target is 0 AGL, the proper position would be 10 miles out. However, if the target is at 15,000ft, the proper position would be 9.5 miles out.

Since the difference is so small, does the radar just put the target at 10 miles, and the FAA separation guidelines factor in the discrepancy?

  • $\begingroup$ If the target is at 15000 and the station is at 0, then the slant range will be LONGER. $\endgroup$
    – mongo
    Apr 17, 2017 at 17:11

2 Answers 2


Primary radar, even modern digital systems, do not give any possibility to the controller to determine altitude. The digital system has that capability, but civil ATC does not.

In primary radar displays usually just the azimuth and range are plotted. So an aircraft at 15.000 ft 9.5 NM away will be plotted on 10 NM distance, just like an aircraft at 0 ft 10NM away. Usually the secondary radar is collocated with the primary so secondary targets can be displayed using the same projection (disregarding the altitude).

If you now apply horizontal separation based on this projection, you will never bust the separation minima. It is a conservative way of providing separation, but when there is no altitude information it is the best possible way.

Note that this does not only apply to primary targets, but also to Mode-A only targets

  • 1
    $\begingroup$ Do you have a primary source/reference for this? I do know that some radar systems are capable of determining altitude (e.g., military). I'm wondering if ATC systems can to some extent, and could use it to adjust slant range, even if it's not accurate enough to display or use for separation purposes. I'd like to read more from an authoritative source. I don't doubt your answer though; reading your bio it sounds as if you likely know what you're talking about in this area :) $\endgroup$
    – TypeIA
    Apr 9, 2014 at 20:01
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    $\begingroup$ @dvnrrs I am not sure where to find an official reference quickly, but this might help. Modern primary radar systems determine elevation angle as well, wich combined with a meteo model and corrections for the curvature of earth's surface can help to determine the altitude of an aircraft. This helps especially if you are fusing radar data from multiple radars and other surveillance systems (multisensor tracking). In such cases simple projections don't work. $\endgroup$
    – DeltaLima
    Apr 9, 2014 at 20:22
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    $\begingroup$ Good enough... thanks for the link and good answer! $\endgroup$
    – TypeIA
    Apr 9, 2014 at 21:22
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    $\begingroup$ @dvnrrs another way to determine altitude is with another radar pass doing RHI scans (constant azimuth, varying elevation). ATC radar doesn't have the motors to do this though (and no electronic beam steering either). $\endgroup$
    – casey
    Sep 17, 2014 at 19:43

Primary radar returns, in complete absence of secondary radar data (transponder A, C, S, etc. and ADS-B type data), will display slant range. Some military radar can provide elevation data, for example PAR radar, and using range and elevation, the height of a return can be inferred.

Secondary systems, such as Mode-C and Mode-S transponders, and ADS-B report pressure altitude of the transponder's altimeter, and are used to determine altitude. These schemes do not really rely on radar, but rather on barometric data, GPS data or both, to provide an altitude. In civilian ATC it is generally considered that pressure altitude is adequate.

So the answer is: 1. Using only primary radar, the range on the radar screen is based upon slant range to the target. 2. There may be range errors relative to a planar datum due to the fact that slant range of a target at the datum may be accurate, but the slant range of a target at altitude will be greater. [Simple trig may be employed to calculate that error. Eg 10nm ~= 60000 feet, and would be the slant range at the same elevation as the radar station. A plane at 60,000 feet, directly above a point 10 nm from the station, will show a slant range of 1.414 * 60,000 feet or about 14 nm.]


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