Aviation Topic of the Week
Original by Michael Oxner, May 11, 2003
Updated 2022-05-08


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This week's topic:
(Radar) Identification of Aircraft

Introduction
Brief Radar History
9 Main Ways of Identifying Aircraft
    1 Mile of Departure Runway
    Report Over a Fix
    Report of DME Fix
    Identifying Turn
    "Squawk Ident"
    Code Change
    Standby
    Tower Observation
    Hand-off
Identification Doubtful
Loss of Target
Phraseology
Aircraft Already Identified

Introduction

In previous articles, I briefly introduced the idea of radar and procedural control by ATC, specifically as they are related to the requirement for pilots to make position reports. This week, I'll start with a (brief) history of radar. After that, I'll introduce the methods available to ATC to "identify" an aircraft using ATC surveilance systems and at what ATC expects from pilots when each are employed. It's worth noting, at this point, this article's title has the word 'radar' in parentheses. When originally written, the term was "radar identification". ATC direction in Canada has removed the term 'radar' from the concept, since radar isn't the only form of surveillance used these days. In addition to the traditional radars, MLat (short for Multi-Lateration) and ADS-B are also used. This means that the term "surveillance" and the systems included are more broad than they were back then. This article is one of many in this series that had several large portions rewritten to include the new terms. Buckle in.

Radar in ATC

Air Traffic Control radars were, in the past, like most others: They consisted of a rotating antenna with a large "dish-like" structure. As the antenna rotated, pulses were sent out on sepcific wavelengths -- tuned for reflecting off the skin of aircraft. When reflected radiation was received, the azimuth of the antenna combined with the round-trip time from emission to reception of reflection allowed a calculation of a position of the aircraft relative to the radar antenna.

Use of detection of reflected radation took on the name "Primary Radar". Because it relied on reflected radiation, smaller aircraft and those further from the antenna could become difficult to detect, their signals getting lost in clutter, sometimes. Enter the Transponder. Another antenna, shaped like a bar, could be strapped to the top of the big dish. It would emit an "interrogation pulse", and the transponder on board the aircraft could "reply". It could even have a limited amount of data coded with it, such as a discrete code (a Mode 1 (military)/Mode A (civilian) reply was a 4-digit code, each digit an octal, ranging from 0 to 7), allowing easier tracaking of aircraft for ATC. With an active transmitter in the aircraft replying to the radar atenna, the range was greatly increased, and the size and covering of an aircraft (metal vs fabric) no longer mattered. Mode 3/C replies, later, would return a pressure altitude from the transponder (not the aircraft's altimeter) for use by controllers instead of asking pilots to report their altitudes.

You have seen raster displays (every TV and computer monitor uses them, and they draw from left to right, from top to bottom in successive lines), and may have heard of older vector displays (lines would be drawn straight from point to point). These older radar displays drew their image by starting in the center going toward the edges along the azimuth the dish was pointing, "painting" targets as it received them. In the dark days of yore, ATC radar screens were "black radars", in that they needed a cone-shaped shade over them to reduce glare from outside light sources to be able to be seen -- the controller would literally "bury his head in the radar". Later, "scan converted displays" made radar visible in daylight without the shades, but it was still primitive by today's standards.

The map was limited, aircraft were shown as "double slashes" if they were yours and "single slashes" if they were someone elses, all determined by transponder codes (typically defined in blocks like 4300, meaning 4301-4377). Mode C altitude readouts were not displayed on the older radars, even if aircraft had them. Even more limiting was the fact that these systems were analog not digital, meaning the signal couldn't be sent very far before degradarion would render it too low to be usable. They, of course, had strong points, too, in that they showed what they saw: A knowledgable radar operator could interpret the data, determining the difference between aircraft, flocks of birds, and even precipitation, and the display could be adjusted on the fly to see just what was needed. Either way, the radar screen was nothing more than a television; one would watch it, and that was the limit of interaction.

Later, digital radars were built that could process radar data electronically and send the data through modern transmission lines, vastly extending both the tracking capabilities and the range that a facility could see. Instead of a radar antenna only serving one ATC unit (or a pair of co-located units like a Tower and a Terminal Control Unit at an airport), now one Area Control Centre (ACC, as they are known in Canada), could scatter a dozen of these radars across the countryside and "see" the entire area from hundreds of miles away. But the benifits of the digital age didn't stop there.

The workstations ATC used also got a facelift. Instead of a static display, digital data processing led to real-time flight data handling. Aircraft flight plans could be entered into the radar "situation display" and the data could be interacted with, updating aircraft routing, altitudes, and other details, all while automating the process of handing off aircraft from one controller to another, which was all manual and done by voice on ATC hotlines and adding a variety of display tools and much-needed detail on the screens.

But wait! There's more! As technology advanced, so did avionics and surveillance systems. MLat, short for "Multi Lateration", sprung up. One transmitting antenna sending an interrogation pulse in all directions simulatneously could interrogate all the transponders in an area at once, and an array of antennas could be arranged to receive the transponder replies. The "Time of Arrival" difference between the same replies at different stations could be used to accurately triangulate aircraft positions. Using existing aircraft equipment and existing radar data processors, these became cheaper, lower-maintenance (though range-limited) radar antennas.

This is all great, but in order to use radar data, ATC has to be absolutely sure the "target" they see on their screen is the aircraft they think it is. With the current limitation on the number of discrete codes (4,096 are available in total, not counting non-discrete codes such as 1200, 2000, and emergency codes), seeing code repetition is a common thing. For this reason, controllers need methods of identifying which target is which aircraft.

Of course, ADS-B and the Mode S Transponders increase capabilities and coverage even more. That's a topic for another article, though.

The 9 Main Ways

There may be 50 ways to leave your lover, but there are 9 main ways of radar identifying an aircraft. And, finally, I have reached the main topic of this article. ATC's Manual of Air Traffic Services (MATS) defines these methods. There is a long history of why this has become important, but enough history for now. Let's get on with it.

You may consider an aircraft identified, provided one of the following conditions is met:

Method
Notes for Controllers
Pilot Action
The aircraft is observed on radar to be in a position, within one mile of the end of the runway used for take-off, that is consistent with the time of take-off and the route of flight or assigned heading of a departing aircraft. A controller must have adequate radar coverage to make use of this method. VATSIM is blessed with the best coverage, since it's always right to the ground as long as the aircraft is within the selected visual range. Real-world airports are sometimes lucky, depending on the placement of radars. Simply call in on departure, stating your altitude so ATC can verify your Mode C.
The aircraft is observed on radar to be over a fix, which is indicated on the radar display, that is consistent with a position report received directly from the aircraft, provided the track is consistent with the route of flight or reported heading of the aircraft. The fix must be depicted accurately on the screen to use this. A position report relayed by another controller or FSS, or by another pilot, renders this method unavailable. You must be talking directly with the aircraft. If the pilot reports southbound over the fix and your target is northbound, you better confirm the track, since it might lead to miss-identification. Someone else might be there, too, hopefully at a different altitude. Report over any compulsory reporting points until ATC identifies you. ATC may ask you to report over a non-compulsory reporting point, or simply to verify that you are over one.
The aircraft is observed on radar to be in a position, relative to OMNI and DME NAVAIDs which are indicated on the radar display, that is consistent with a position report received directly from the aircraft in the form of a DME fix, provided that the track is observed to be consistent with the route of flight or reported heading of the aircraft. The facility used as a reference must be shown on the radar screen. Again, Direct Controller-Pilot Communication (DCPC) must be used. The format referred to as a DME fix must include a radial from a VOR facility, and a DME distance. For example, a report like, "Moncton, CVA116 is on the YHZ 082 radial at 10 DME"

Additionally, aircraft at high altitude or at considerable distance from the facility may not be able to accurately identify their position. Angular errors at long range and DME slant range may affect the pilot's readings. For example, an aircraft at FL300 directly over a DME facility will read 5 DME.

 ATC may ask you to provide such a position report. If so, use your NAV1 or NAV2 radio to tune up the requested facility, and use your VOR1 or VOR2 (respectively) to figure out which radial you're on and read the DME in conjunction with the radial from the facility. To determine the radial on "old-school" gauges, find where bearing where the VOR needle is centered and the To/From flag reads "From". An example of such a report is in the cell immediately to the left of this one in Italics.
The aircraft is observed to have carried out a specified turn of at least 30 degrees, provided:
  1. except in the case of a lost aircraft, a position report received directly from the aircraft indicates that the aircraft is within radar coverage of the area displayed;
  2. only one aircraft is observed to have carried out the specified turn; and
  3. the track is observed to be consistent with the heading or track of the aircraft both before and after completion of the turn.
 You must have a position report from the aircraft that ensures the aircraft is within radar coverage. If the pilot is gives you any indication that he doesn't know exaclty where he is, you should use another method to identify him. I once had a pilot report his position over the Bay of Fundy but it turned out he was actually lost and over the Bras D'Or lakes, 200 NM miles away. You should have an idea of the aircraft's heading before the turn, and what it should be after. Also, before issuing a turn, ensure the aircraft is at a safe altitude for vectoring if it is IFR. Remember when you vector an IFR aircraft, you're responsible for terrain and obstacle clearance. Turning an aircraft onto an airway or other commonly used track should be avoided, since another aircraft may turn to intercept the track coincidentally, thereby leading to miss-identification. (This rule stands, though few aircraft track airways in comparison to old days)

When you are calling to ask for radar nagivation assistance, or when you are checking in with ATC, give a report that gives the controller an idea of where you are. "Edmonton Center, CVA670 30 West of Medicine Hat through 4,500."

Pilots should never take an ATC requested turn that is meant for another aircraft. In the past, ATC has issued turns, attempting to identify one aircraft, only to see two or three aircraft make the requested turn. This means ATC must take additional steps, and time, to identify an aircraft. It could also lead to miss-identification of an aircraft.
The appropriate change in the PPS is observed after the aircraft is instructed to operate the "Ident" feature of its transponder. I'll bet the phrase, "Squawk Ident" sounds familiar. That's what this method is actually saying. The PPS, or Present Position Symbol, is what the targets on Canadian controllers' screens are officially called. In our case, the target turns white and flashes. On VATSIM, it depends on the display you're using.

For VATSIM pilots, this "Ident" feature may be activated in a number of ways. There is often a button in the virtual cockpit of your simulator somewhere, and this depends on the aircraft and how it was modeled. There may also be a button marked Ident on the software you use to connect to VATSIM's network.

For real world pilots, this means to press (and release) the button maked "Ident" on the Transponder.

Note for all pilots: Do not operate the Ident feature unless specifically requested.
The appropriate change in the PPS is observed after the aircraft is instructed to change from one code to another. Simply by assigning a new code to an aircraft and watching the target change will provide ATC with positive identification. On Canadian screens, there are several different kinds of targets, based on what the radar is seeing. This may vary depending on the display being used as a controller. Simply change to the new code assigned by ATC. For both VATSIM and the real world, don't turn off your transponder or squawk "Standby" while changing codes. This may cause total loss of the target from ATC's radar screen during the change, and controllers would rather see a target than nothing at all. Just roll through the codes to the new one. Pilots have come up with ways of doing this to avoid inadvertently dialing up emergency codes on older transponders such as changing the last digit first, but newer equipment is less likely to have such an issue to compensate for.
The PPS disappears or changes to a PSR symbol after the aircraft is instructed to change its transponder to "standby" and the PPS reappears or changes back to an SSR symbol after the aircraft is instructed to return the transponder to normal operation. This sounds like a lot. Here's what it means: Ask the aircraft to "squawk standby", watch the symbol change (or disappear if outside Primary Radar Coverage, which is not always accounted for very well in VATSIM) and then have the aircraft "squawk normal" and watch it change again. This one isn't used as often as squawking ident because of the issue mentioned above -- target loss -- but could be used if the aircraft's "Ident" feature isn't working. "Off" can be used instead of "standby", as well. In VATSIM, turning the transponder off and back on depends on either the model you're using in your flight simulation or the software being used to connect ot the network, similar to the Ident method discussed above. You'll have to be familiar with the software.
The position of the PPS on the tower display is consistent with the position observed visually by the airport controller. Tough to simulate this in VATSIM. For use by tower controllers only, this simply means that the target on radar is where it should be, according to visual observation through the windows of the tower cab. Well, unless Microsoft Windows count, this one is out for us in VATSIM. No action required on the part of pilots, real world or VATSIM.
Identification is transferred by a hand-off. One of the most used methods, this is simply the radar hand-off we're all familiar with. This can be the "automated" hand-off, where one controller hands-off an aircraft to another by using the hand-off feature associated with the software, or the old-fashioned verbal method of stating the position of the aircraft relative to a fix displayed on both screens. "CVA822 is 15 East of TAFFY at FL350" Again, no action required for pilots.

Those are the 9 main ways to identify an aircraft. There are a few more notes, too.

Identification Doubtful

Because identification of aircraft targets is so important, and because, like most rules in aviation, incidents are the typical genesis of them, there are a few paragraphs regarding what a controller should do if, "identification becomes doubtful." They really do come down to common sense, though. Things like, "If identification becomes doubtful, take action to re-identify the aircraft," and to use more than one method of identifying an aircraft if deemed necessary. These really aren't specific enough to of much use, but they're almost undoubtedly there because someone didn't.

Loss of Target

A slightly different issued falls in the same lines: If you lose identification of a target, take action to regain it or terminate surveillance services and provide procedural separation. I mean, it makes sense: If you lose identification, you simply can't continue to provide surveillance services, including vectors around traffic or even surveillance-based traffic information, and navigation assistance. Losing identification could mean that more than one target is located in a given position (perhaps two targets have passed one over the top of another and you're no longer sure which target is the one you're tracking), or it could also mean a complete loss of any target assocaited with the aircraft. Transponder failure outside of Primary Radar coverage and the aircraft simply flying outside of your radar coverage, perhaps while still in your airspace, would mean the complete loss of a target.

Phraseology

More common sense is contained in ATC direction, but I've seen a few individuals who should have abided by it. It says to "Inform the aircraft if identification is established or lost." This is accomplished by using the phrases:

All this is meant to do is inform the pilot whether the flight is still being monitored by ATC. For example, a pilot may, in an emergency situation, thinks the aircraft is still being monitored might try to save some critical time in the cockpit by just changing the transponder to 7700 instead of also attempting to make radio calls. If the aircraft is not visible to ATC surveillance, controllers will not get an indication of an emergency. The pilot should be aware of such a limitation. Similarly, while in surveillance coverage, workload permitting, ATC can provide information on other radar observed traffic. If an aircraft's progress is no longer being watched on radar, the pilot knows there is no chance of a traffic alert from ATC. It is a fact that, regardless of whether IFR or VFR when operating in visual conditions, the pilot is primarily repsonsible for seeing other traffic, but a pilot should know there is no "safety net" when it comes to someone else possibly looking for traffic on that pilot's behalf.

Already Identified

A final note for use in VATSIM is one that we will tend to make more use of, and modify a little, too. Controller direction allows one to consider an aircraft radar identified, even without a hand-off, if it has flown in the airspace of an adjacent unit where radar service is normally provided, if identification is initially established in accordance with the methods in the table above. In real life, Canadian ATC can only apply this between other Canadian facilities. For VATSIM's purposes, I'll leave it to you and your regional staff to decide how far to take this. Note that this would not apply in some areas, such as Gander Center's domestic airspace with respect to traffic coming out of the oceanic airspace. Surveillance services are not provided there, so Gander Domestic must take steps according to the table above to identify each aircraft if he wants to provide surveillancae services, including the great reduction in separation standards afforded by use of surveillance. I know, I know, some are thinking, "What about Aerion and space-based ADS-B? Things are changing." They sure are.

So be sure the target you're looking at is the aircraft you think it is. Remember the similarity between pilots and controllers:




Questions? Comments? By all means, send them to me at mo@xlii.ca. Thanks for your support!