Surveillance, identification and position
ATS surveillance lets a controller observe an aircraft's position, identify its return and then provide control, advisory or information service. Identification must come first. Seeing a target is not, by itself, the provision of a radar service.
What surveillance adds
Primary radar detects reflected energy and does not depend on an aircraft reply. Secondary surveillance radar, called SSR, interrogates the transponder and receives a coded reply. Mode A supplies an identity code. Mode C adds pressure altitude. Mode S supports selective addressing and an aircraft identification entry. ADS-B broadcasts aircraft information for ground stations and suitably equipped aircraft, while ADS-C sends reports under a communications contract, commonly on long-range routes.
The word RADAR may appear in the unit callsign or in a service statement. Under radar control, headings supplied by the controller are normally magnetic. The word DEGREES is omitted for radar headings: TURN LEFT HEADING ZERO FIVE ZERO.
Identification of aircraft
| Purpose | Controller phrase | Pilot action or meaning |
|---|---|---|
| Obtain data | REPORT HEADING AND FLIGHT LEVEL (or ALTITUDE) | State present heading and level. |
| Observed turn | FOR IDENTIFICATION TURN LEFT (or RIGHT) HEADING (three digits) | Make and read back the instructed turn. |
| Use transponder | TRANSMIT FOR IDENTIFICATION AND REPORT HEADING | Operate the requested identification feature and report heading. |
| Identification achieved | RADAR CONTACT (position), or IDENTIFIED (position) | The controller identifies the return and normally gives its position. |
| Not achieved | NOT IDENTIFIED (reason), RESUME (or CONTINUE) OWN NAVIGATION | Do not assume that radar service exists. |
Position information uses POSITION (distance) MILES (direction) OF a significant point, or position OVER or ABEAM the point. Example: IDENTIFIED ONE EIGHT MILES NORTHWEST OF BENGALURU, CONTINUE PRESENT HEADING.
Vectoring, manoeuvres and speed
A vector is a heading instruction used to establish separation, position an aircraft for an approach, avoid traffic or create delay. The pilot flies it until ATC changes it or returns navigation responsibility.
Vectoring phrase set
| Task | Phraseology |
|---|---|
| Start or continue | LEAVE (point) HEADING (three digits); CONTINUE HEADING (three digits); CONTINUE PRESENT HEADING; FLY HEADING (three digits) |
| Change direction | TURN LEFT (or RIGHT) HEADING (three digits) (reason); TURN LEFT (or RIGHT) (number) DEGREES (reason) |
| Stop a turn | STOP TURN HEADING (three digits); HEADING IS GOOD |
| Direct when able | FLY HEADING (three digits), WHEN ABLE PROCEED DIRECT (significant point) |
| End vectoring | RESUME OWN NAVIGATION (position) (specific instructions); RESUME OWN NAVIGATION DIRECT (point) MAGNETIC TRACK (three digits) DISTANCE (number) MILES |
| Delay manoeuvre | MAKE A THREE SIXTY TURN LEFT (or RIGHT) (reason); ORBIT LEFT (or RIGHT) (reason) |
If an aircraft has unreliable directional instruments, ATC may replace precise heading commands with rate and timing instructions: MAKE ALL TURNS RATE ONE, START AND STOP ALL TURNS ON THE COMMAND NOW, TURN LEFT NOW, or STOP TURN NOW. A reason may be stated, such as due traffic, for spacing, for delay, or for downwind, base or final.
Speed control
ATC may ask REPORT SPEED or REPORT MACH NUMBER. It may use SPEED (number) KNOTS, MAINTAIN (number) KNOTS OR GREATER, MAINTAIN MACH (number), DO NOT EXCEED (number) KNOTS, MAINTAIN PRESENT SPEED, INCREASE or REDUCE SPEED TO a value, or change speed BY a stated number of knots.
Approach sequencing may require REDUCE TO MINIMUM APPROACH SPEED or REDUCE TO MINIMUM CLEAN SPEED. Restrictions end with RESUME NORMAL SPEED, RESUME PUBLISHED SPEED, or NO ATC SPEED RESTRICTION.
Position-report control
Surveillance can reduce voice workload. ATC may say OMIT POSITION REPORTS UNTIL a specified point, NEXT REPORT AT a significant point, REPORTS REQUIRED ONLY AT named points, or RESUME POSITION REPORTING.
Traffic, communications and safety alerts
Surveillance traffic information builds a relative picture in a fixed order. Avoiding action then gives an immediate manoeuvre, while safety alerts demand prompt confirmation and correction.
Traffic information and avoiding action
| Element | Content |
|---|---|
| Relative bearing | Clock position, such as TRAFFIC TEN O'CLOCK |
| Distance | Range in miles |
| Direction | Northwest, south or another cardinal direction when established |
| Movement | Unknown, slow moving, fast moving, closing, opposite, same direction, overtaking, crossing left to right or right to left |
| Known detail | Aircraft type, level, and CLIMBING or DESCENDING |
The controller may ask DO YOU WANT VECTORS?, and the pilot may REQUEST VECTORS. The urgent form is AVOIDING ACTION TURN LEFT or RIGHT IMMEDIATELY HEADING (three digits) TO AVOID TRAFFIC, followed by bearing and distance. A turn by a stated number of degrees may also be used. When the conflict has passed, ATC says CLEAR OF TRAFFIC followed by appropriate instructions.
Communications lost or suspected
If contact is lost, ATC may issue RADIO CONTACT LOST with instructions, IF NO TRANSMISSIONS RECEIVED FOR a stated time followed by instructions, or REPLY NOT RECEIVED. If the controller suspects that the aircraft can receive but cannot transmit, an instruction is passed slowly, clearly and repeatedly: IF YOU READ followed by a manoeuvre or SQUAWK code or IDENT. ATC confirms with TURN OBSERVED, SQUAWK OBSERVED or IDENT OBSERVED, then gives position and instructions. A radio-failure aircraft is expected to select 7600.
Minimum safe altitude warning and terrain alert
MSAW is generated by ATC radar processing to warn of possible infringement of a minimum safe altitude. The urgent call is LOW ALTITUDE WARNING, CHECK YOUR ALTITUDE IMMEDIATELY, QNH IS (number) and may add THE MINIMUM FLIGHT ALTITUDE IS (altitude). A direct warning uses the aircraft callsign followed by TERRAIN ALERT and suggested pilot action where possible.
SSR, Mode S, ADS-B and ADS-C
Surveillance equipment is controlled by precise verbs. The pilot reads back specific settings, confirms what is selected and reports when the equipment cannot comply.
Capabilities and identification
| Controller phrase | Required response or purpose |
|---|---|
| ADVISE TRANSPONDER CAPABILITY | State the transponder capability shown in the flight plan, or NEGATIVE TRANSPONDER. |
| ADVISE ADS-B CAPABILITY | State ADS-B TRANSMITTER, ADS-B RECEIVER or NEGATIVE ADS-B, with the data link when required. |
| FOR DEPARTURE SQUAWK (code); SQUAWK (code) | Select and read back the assigned code. |
| RESET SQUAWK (mode) (code) | Reselect the assigned mode and code; reply RESETTING. |
| RE-ENTER ADS-B or MODE S AIRCRAFT IDENTIFICATION | Correct and reselect the aircraft identification entry. |
| CONFIRM SQUAWK (code) | Check and report the code actually selected. |
| SQUAWK (code) AND IDENT; SQUAWK IDENT | Set the code if stated and operate the IDENT feature. |
| SQUAWK LOW; SQUAWK NORMAL; TRANSMIT ADS-B IDENT | Use the requested identification power or ADS-B identification function. |
Standby, altitude and termination
| Function | Phrase |
|---|---|
| Temporary suspension | SQUAWK STANDBY |
| Emergency code | SQUAWK MAYDAY (CODE SEVEN SEVEN ZERO ZERO); SQUAWK SEVEN SEVEN ZERO ZERO |
| Stop a transmitter | STOP SQUAWK (TRANSMIT ADS-B ONLY); STOP ADS-B TRANSMISSION (SQUAWK code ONLY) |
| Pressure altitude | SQUAWK CHARLIE; SQUAWK ALTITUDE; TRANSMIT ADS-B ALTITUDE |
| Check level | CHECK ALTIMETER SETTING AND CONFIRM LEVEL; CONFIRM LEVEL; VERIFY LEVEL |
| Faulty altitude | STOP SQUAWK CHARLIE WRONG INDICATION; STOP ADS-B ALTITUDE TRANSMISSION (WRONG INDICATION or reason) |
Mode S and ADS-B functions can be independent. If ADS-B comes from the transponder's 1090 MHz extended squitter, it may not be possible to stop one function while retaining the other. The crew must say when it cannot comply.
ADS-C is contract based rather than a continuous public broadcast. If the contract or system degrades, ATC may say ADS-C or ADS-CONTRACT OUT OF SERVICE with appropriate instructions. ADS-B or ADS-C may also convey an emergency, communication failure, unlawful interference, minimum fuel or medical condition.
Codes, faults and service termination
A transponder code is operational information, not a decoration on the display. The code, selected mode and altitude source must all be correct, and failures must be declared promptly.
Special-purpose codes
| Code | Use stated in the chapter |
|---|---|
| 0000 | Available as a general-purpose code for domestic use by a State. |
| 2000 | Pilot-selected recognition when no ATC squawk instruction has been received. |
| 7500 | Unlawful interference. |
| 7600 | Radiotelephony communication failure. |
| 7700 | Emergency. |
Transponder failure
If a transponder failure is detected in airspace where carriage of a functioning transponder is mandatory, ATC should try to let the flight continue to the aerodrome of first intended landing in accordance with the flight plan. Traffic may make continuation impossible, especially if the fault is found shortly after take-off. ATC may require a return to the departure aerodrome or landing at the nearest suitable aerodrome acceptable to the operator and ATC. If failure is known before departure and repair is impracticable, the aircraft should be permitted to proceed as directly as possible to the nearest suitable aerodrome where repair can be made.
Identification and equipment degradation
Service status must be explicit. ATC may say RADAR SERVICE TERMINATED, CONTACT (unit), or RADAR SERVICE or IDENTIFICATION TERMINATED DUE a reason and add instructions. Advance warning is WILL SHORTLY LOSE IDENTIFICATION. Actual loss is IDENTIFICATION LOST with the reason and instructions.
Equipment degradation is announced as SECONDARY RADAR OUT OF SERVICE, PRIMARY RADAR OUT OF SERVICE, or ADS-B OUT OF SERVICE, with operational information as necessary. Example: IDENTIFICATION LOST DUE RADAR FAILURE, CONTACT DELHI CONTROL ON ONE TWO FOUR DECIMAL FIVE FIVE.
Vectors to final and direction finding
Radar vectors can form an orderly path from the initial approach area to the final approach track. Direction finding is different: it derives a bearing from the aircraft's transmission and may provide a QDM to steer.
Vectoring to an instrument final
An identified inbound aircraft may receive a sequence of headings to join final. A worked ILS pattern uses an initial heading 120, a base-leg heading 190, then heading 240 to intercept the localiser for runway 28. Speed reduction keeps spacing between successive arrivals. If speed adjustment is insufficient, the controller may issue further vectors or deliberately take the aircraft through the localiser for spacing.
| Stage | Worked phrase |
|---|---|
| Initial vector | RADAR CONTACT ONE SIX MILES NORTH OF DELHI, TURN LEFT HEADING ONE TWO ZERO, VECTORING FOR ILS APPROACH RUNWAY TWO EIGHT, QNH ONE ZERO ONE ONE |
| Speed and descent | REPORT SPEED; REDUCE TO MINIMUM CLEAN SPEED; DESCEND TO TWO THOUSAND FIVE HUNDRED FEET |
| Base leg | TURN RIGHT HEADING ONE NINE ZERO FOR BASE LEG |
| Intercept | ONE TWO MILES FROM TOUCHDOWN, REDUCE TO MINIMUM APPROACH SPEED, TURN RIGHT HEADING TWO FOUR ZERO, CLEARED FOR ILS APPROACH RUNWAY TWO EIGHT, REPORT ESTABLISHED LOCALISER |
| After establishment | DESCEND ON THE ILS, QNH ONE ZERO ONE ONE; NO ATC SPEED RESTRICTION, CONTACT TOWER ONE ONE EIGHT DECIMAL ONE |
Direction finding and QDM
A direction-finding station measures the direction from which an aircraft transmission arrives. QDM is the magnetic bearing to the station, the heading to steer in still air. A pilot can request QDM; the station replies with the three-digit bearing and may use STEER (three digits). A QDM is not a wind-corrected track guarantee, so the pilot monitors drift and requests another bearing when required.
Worked service statements
Where the service is not self-evident, the controller informs the pilot whether the aircraft is under RADAR CONTROL, RADAR ADVISORY or RADAR INFORMATION. At the end, the controller states RADAR SERVICE TERMINATED. A pilot reads back headings and acknowledges transfers in the normal way.
SRA and PAR approaches
A surveillance radar approach gives azimuth instructions, distance from touchdown and advisory height information. A precision radar approach adds continuous elevation guidance, producing a full talkdown.
Surveillance radar approach
Where local step-down fixes do not exist, approval for a modified procedure may be obtained. A pilot wishing to fly by altitude requests the QNH. ATC adds touchdown elevation to advisory heights and all level references become altitudes. With QFE, the altimeter indicates height above the touchdown reference. The book's example notes that setting QFE 947.6 on an altimeter set to 1013.2 would produce approximately 1,842 feet at touchdown.
The controller tells the aircraft the runway, the point at which the SRA terminates, the QNH, the level to maintain, and to check minima. Descent begins when instructed so that the aircraft joins the intended glide path. If the SRA ends at 2 miles, intermediate half-mile checks may be omitted and the pilot transmits from the ground station. If it continues inside 2 miles, distance and advisory-altitude checks are normally passed every half mile.
The aircraft replies to transmissions until instructed DO NOT ACKNOWLEDGE FURTHER TRANSMISSIONS. Inside 4 miles, controller transmissions should not be interrupted for more than about five seconds. Height checks below Category A obstacle clearance height are omitted. If visual early, the pilot may be asked whether the SRA should continue. If visual contact is lost or landing cannot be assured, the missed-approach action applies.
| Distance from touchdown | Book's advisory height example |
|---|---|
| 6 NM | 1,850 ft |
| 5.5 NM | 1,700 ft |
| 5 NM | 1,550 ft |
| 4.5 NM | 1,400 ft |
| 4 NM | 1,250 ft |
| 3.5 NM | 1,100 ft |
| 3 NM | 950 ft |
| 2.5 NM | 800 ft |
| 2 NM | 650 ft |
| 1.5 NM | 500 ft |
| 1 NM | 350 ft |
Precision radar approach
PAR supplies azimuth and elevation guidance continuously. Typical calls are ON TRACK, SLIGHTLY LEFT OF TRACK, TURN RIGHT HEADING, APPROACHING GLIDE PATH, ON GLIDE PATH, SLIGHTLY ABOVE GLIDE PATH, SLIGHTLY BELOW GLIDE PATH, COMING BACK TO THE GLIDE PATH, and rate-of-descent corrections. The controller includes CHECK GEAR DOWN AND LOCKED, distance calls and, near completion, landing or go-around information. The pilot may be told DO NOT ACKNOWLEDGE FURTHER TRANSMISSIONS. If the aircraft moves progressively above the glide path, the controller may say GOING FURTHER ABOVE GLIDE PATH and then YOU ARE GOING AROUND.