Radar changes the grammar of control: instead of building separation from reports and estimates, the controller watches the traffic and steers it. This lesson covers the working rules of that service: how an aircraft becomes identified, when and in what forms it is told its own position, what vectoring is for and how it must be done, and the surveillance radar approach that can talk a crew down to two miles from touchdown. It ends at the other end of the service spectrum: the air traffic advisory service, which watches over class F airspace with suggestions where a control service would issue clearances.
Seeing and being seen: identification
Two families of sensor feed the situation display. Primary surveillance radar (PSR) uses reflected radio signals: the ground station's own energy bounced off the airframe, needing nothing from the aircraft. Secondary surveillance radar interrogates the aircraft's transponder and displays what it replies, which is why codes and Mode S identification exist. Before any of it can be used to separate traffic, the blip must become a known aircraft: identification must be established, and Doc 4444 prescribes how.
With SSR (or MLAT) — by recognising the aircraft identification in the label; by recognising an assigned discrete code, its setting verified, in the label; by direct recognition of a Mode S aircraft identification; by transfer of identification from another controller; by observing compliance with an instruction to set a specific code; or by observing compliance with an instruction to squawk IDENT. A newly assigned discrete code is checked against what the pilot actually set before it is used as a basis for identification.
With PSR — by correlating a radar position with an aircraft reporting its position over, or bearing and distance from, a displayed point, the track consistent with the aircraft's path; by correlating a position with an aircraft known to have just departed, within 1 NM of the runway end; by transfer of identification; or, after a period of track observation, by instructing one or more heading changes of 30 degrees or more and correlating the observed movement with the acknowledged execution (or with manoeuvres the aircraft reports it is currently executing), checking that only one position indication moves accordingly, and that the manoeuvre will not carry the aircraft out of coverage.
Direction-finding bearings may assist, but shall not be the sole means of identification unless the ATS authority has prescribed it for particular cases. And when two position indications sit close together or move alike, or any other doubt exists, heading changes are prescribed or repeated, or additional methods employed, until all risk of error is eliminated.
Telling you where you are
An aircraft under ATS surveillance service should be told its position in five circumstances:
Upon identification — except when the identification came from the pilot's own position report; from a departure correlation within 1 NM of the runway, the observed position consistent with the time of departure; from ADS-B or Mode S identification or a verified discrete code with the position consistent with the flight plan; or by transfer of identification.
When the pilot requests it.
When the pilot's estimate differs significantly from the controller's estimate based on the observed position.
When the pilot is instructed to resume own navigation after vectoring, if the vectors had diverted the aircraft from its previously assigned route.
Immediately before termination of the surveillance service, if the aircraft is observed deviating from its intended route.
And it is passed in one of five forms:
A well-known geographical position.
Magnetic track and distance to a significant point, an en-route navigation aid, or an approach aid.
Direction, using points of the compass, and distance from a known position.
Distance to touchdown, for an aircraft on final approach.
Distance and direction from the centre line of an ATS route.
Vectoring
Vectoring is the provision of navigational guidance to aircraft in the form of specific headings, based on the use of an ATS surveillance system: headings which will enable the aircraft to maintain the desired track. The document names what it is for. Vectoring is applied:
To departing aircraft — to build an expeditious and efficient departure flow and expedite the climb to cruising level.
To resolve potential conflicts.
To arriving aircraft — to establish an expeditious and efficient approach sequence.
To assist pilots in their navigation — to or from a radio navigation aid, or away from and around areas of adverse weather.
The rules of execution are their own learning objective:
Vector along tracks the pilot can monitor with pilot-interpreted aids whenever practicable: it minimises the assistance needed and softens the consequences of a surveillance failure.
On the initial vector off an assigned route, tell the pilot what the vector is to accomplish, and specify its limit where a loss of communications on that heading would create a safety risk.
Not closer than 2.5 NM to the boundary of the controller's airspace — or half the applicable minimum, where that minimum exceeds 5 NM, unless transfer of control is being effected or arrangements guarantee separation from the neighbours' traffic.
Not into uncontrolled airspace, except in emergency, to circumnavigate adverse weather (the pilot then informed), or at the pilot's specific request.
Unreliable directional instruments reported: agree the turn rate first, and have every instruction executed immediately on receipt.
Underneath every vector runs the obligation the clearances lesson introduced: while an IFR flight is vectored, or sent direct off an ATS route, the controller's clearances must keep the prescribed obstacle clearance in existence at all times until the pilot resumes own navigation, with the minimum vectoring altitude corrected for low temperature when necessary. And the ending is prescribed too: in terminating vectoring, the controller instructs the pilot to resume own navigation and, where the vectors had taken the aircraft off its previously assigned route, gives the aircraft's position with appropriate instructions in the track-and-distance form: you are released, here is where you are, and here is how to rejoin.
The surveillance radar approach
Where no ILS serves the runway and no precision approach radar is available, the controller's picture can substitute: the surveillance radar approach (SRA), flown entirely on the controller's talk-down. It is conducted only with suitably sited, specifically approved equipment whose display is marked with the extended runway centre line and distance from touchdown, by a controller doing nothing else; and a final approach on surveillance radar alone should not be flown at all where precision approach radar is available, unless the weather gives reasonable certainty of success. The procedure:
At or before the commencement of final approach, the aircraft is told the point at which the SRA will be terminated.
Approaching the computed descent point, the aircraft is told the obstacle clearance altitude/height and instructed to descend and check the applicable minima.
Azimuth corrections follow the precision approach technique; distance from touchdown is passed each NM, together with the pre-computed level the aircraft should be passing to hold the glide path.
The SRA terminates at 2 NM from touchdown, before the aircraft enters continuous radar clutter, or when the pilot reports able to complete visually — whichever comes earliest.