PPL(A) Theory

Radar services

Page ID
010-07-03
Syllabus (AMC1 FCL.210; FCL.215)
Radar services
Syllabus anchor
ICAO Annex 11; Doc 4444
Rule in force
Reg. (EU) 2017/373 (ATM/ANS); SERA
Regulatory baseline
SERA EAR August 2025; ATM/ANS EAR March 2025; ICAO Annex 11 (2018) and Doc 4444 (2016) for comparison
Reading time
27 min
Revising? Key concepts ↓ Test yourself ↓

Learning objectives

The idea

A controller watching a screen sees dots, not aeroplanes. Before the controller can help you, they must know which dot is you, and you must know they know. From then on they can point you in a direction, warn you about other dots, and tell you when they stop watching. The screen is only as good as what the sensors pick up, and the controller sees only some of the sky. The help is real, but your own eyes, your own map and your own judgement stay switched on.

What an ATS surveillance service is

"Radar service" is the older name. The modern term is air traffic services (ATS) surveillance service: any air traffic service given directly by means of an ATS surveillance system.

An ATS surveillance system is an umbrella term. It covers automatic dependent surveillance-broadcast (ADS-B), primary surveillance radar (PSR), secondary surveillance radar (SSR), or any comparable ground-based system that lets ATS identify aircraft.

The sensors and what each one gives

Primary radar asks "is something there?" It sees an aircraft by its reflected echo and needs nothing from the aircraft. It shows only a position, the so-called PSR blip.

Secondary radar asks "who are you?" SSR is a surveillance radar working with ground interrogators and airborne transponders. Mode S, ADS-B and multilateration all rely on the aircraft's own transmissions. They can add an identity, a code and a pressure level. The price is that an aircraft with no working transponder is invisible to them.

ADS-B is the broadcast version. Aircraft, vehicles and other objects automatically broadcast and/or receive data such as identification and position over a data link.

Multilateration (MLAT) is the "triangulation" version. Several ground receivers work out a position from transponder replies or squitters, mainly by time difference of arrival, and can also extract identification.

Four ways a controller sees an aircraft Four rows. PSR: a pulse goes out and an echo returns; the controller gets position only. SSR: an interrogation goes out and a reply returns; position plus reply data. ADS-B: the aircraft broadcasts identity and position. MLAT: several receivers compare arrival times of the aircraft's transmissions to find position. Without a working transponder the aircraft is invisible to SSR, ADS-B and MLAT. Low or small aircraft may be missed. PSR pulse echo Position only A blip: no identity, no code, no level. No help needed from aircraft. SSR interrogation reply Position + reply data The reply can add a code, a pressure level, identity. Needs a working transponder. ADS-B broadcast ID + position Data sent by the aircraft Identity and position are broadcast automatically. Nothing sent: nothing seen. MLAT Position by timing Receivers compare arrival times; identity can be read. Needs a transmission too. Limits No working transponder: SSR, ADS-B and MLAT cannot see it. Low or small aircraft may be missed.
Figure 1. PSR needs only an echo and gives a position. SSR, ADS-B and MLAT use the aircraft's own transmissions and can add identity, code or level, but cannot see an aircraft that transmits nothing. Original diagram
Describe this figure

Four rows show the sensor family. In the PSR row a pulse goes out from the antenna and an echo comes back, so the controller gets a position only, with no identity, code or level. In the SSR row an interrogation goes out and a reply returns, so the reply can add a code, a pressure level and identity, but the aircraft needs a working transponder. In the ADS-B row the aircraft itself broadcasts identity and position data. In the MLAT row several ground receivers compare the arrival times of the aircraft's transmissions to find its position, and identity can be read too. A box at the bottom warns that an aircraft with no working transponder cannot be seen by SSR, ADS-B or MLAT, and that low or small aircraft may be missed.

Four panels: a radar echo (position only), an interrogation and reply (position, code, level, identity), an ADS-B broadcast, and several receivers timing one transponder signal.

Coverage has gaps, and an adequate primary return depends on the aircraft type.

Who may use surveillance, and for what

Surveillance information can support area control, approach control and aerodrome control, and also flight information service (FIS). It is not limited to controlled airspace or to air traffic control (ATC).

In FIS, the situation display may be used to give an identified aircraft information on conflicting traffic, the position of significant weather, and help with navigation.

Identification: nothing starts without it

First the words. The situation display is the controller's electronic display of aircraft position and movement. A position indication is the blip on it that shows where an aircraft is.

Identification means a particular aircraft's position indication is seen on a situation display and positively identified. "Radar contact" is the radar-specific form of that.

Rule ATS.TR.155(d)

Before any surveillance service is given to an aircraft, the aircraft must be identified and the pilot told. Identification is kept until the service ends. If it is lost, the pilot is told.

A situation display with three kinds of target Mock-up of a controller's situation display with range rings. Target A is a filled square with a label showing CS-ABC and a level readout: an identified aircraft. Target B is an open diamond with no label: a primary-only return with no identity and no level. Target C is an open square with only a level readout: a Mode C target without identity, whose level may still be passed in traffic information. Situation display (mock-up) CS-ABC 045 A B 072 C A Identified: label shows CS-ABC + level Position indication seen and positively identified. B Primary only: a position, nothing else No identity, no level. Not identified. C Mode C level, no identity Pass the level in traffic information even if unverified; tell the pilot its accuracy is uncertain.
Figure 2. Original mock-up of a situation display. Only target A is identified; B is a primary-only position; C shows a Mode C level without identity. Original diagram
Describe this figure

A mock-up of a controller's situation display with range rings. Target A is a filled square with a label showing CS-ABC and a level readout: its position indication has been seen and positively identified. Target B is an open diamond with no label: a primary-only return, so a position and nothing else. Target C is an open dashed square with only a level readout, 072: a Mode C target without identity. Its level can still be passed in traffic information even if unverified, as long as the pilot is told its accuracy is uncertain.

The mock-up display shows three targets. One is your aircraft CS-ABC with a label showing its call sign and a level readout: an identified, transponding aircraft. Another is a bare primary blip with no label: something is there, but nobody knows what. The third is a target with a level readout but no identity: a transponding aircraft that has not been identified.

How ATC identifies you

With ADS-B, ATC should be able to read the aircraft identification in the label, take over the identification from another controller, or see the aircraft respond to "transmit ADS-B IDENT".

With SSR and/or MLAT, ATC should be able to do any of these: recognise the aircraft identification in the label, recognise an assigned discrete code whose setting has been verified, recognise the Mode S aircraft identification directly, take over the identification, see you set a code you were told to set, or see you respond to an instruction to squawk IDENT.

A discrete code is a four-digit SSR code whose last two digits are not "00". ATS checks at the earliest opportunity that the code you set is the one assigned. Only after that check may a discrete code be used to identify you.

Eligible flights whose Mode S downlinked identification matches their flight plan, in airspace volumes declared to the Network Manager, may be given the conspicuity code A1000 instead of a discrete code.

With primary radar only, ATC should be able to identify you in one of four ways. It can match a blip to your reported position over a point, or bearing and distance from it, with a consistent track. It can match a blip to an aircraft that has just departed, if identified within 1 NM of the runway end. It can take over the identification. Or, under ATC only, it can observe you make one or more heading changes of 30° or more when instructed, or manoeuvres you report.

Position information

Once you are identified, you should be told your position in some cases and not in others. You should be told on identification, unless identification came from your own position report, from a departure within 1 NM of the runway consistent with the departure time, from an ADS-B/Mode S identity or discrete code consistent with the flight plan, or from a transfer. You should also be told on request, when your estimate differs significantly from the controller's, when told to resume own navigation after a vector took you off your assigned route, and, under ATC, just before the service ends if you are off your intended route.

From first contact to the end of a surveillance service A flow in six steps. 1 Contact. 2 Identify, using methods that depend on the sensor: primary radar uses a just-departed aircraft within 1 NM of the runway end or a turn of 30 degrees or more; SSR and MLAT use the label, a verified discrete code, Mode S identity, transfer, a code set as instructed, or squawk IDENT. 3 The pilot is told identified, plus position. 4 The service is given and identification kept. 5 Either identification is lost, or the service is interrupted or ended; the pilot is told. 6 The pilot resumes own navigation and, if exempt from reporting points, position reports. 1 Contact The pilot calls the controller 2 Identify: the method depends on the sensor Primary radar (PSR) Just-departed aircraft within 1 NM of runway end Turn of 30° or more (ATC service only) SSR / MLAT Identity in the label Verified discrete code Mode S identity, transfer Code set as instructed Squawk IDENT seen 3 “Identified” + position The pilot is told before any service starts. Position is also given, except in listed cases. 4 Service given Identification is kept until the service ends 5 Identification lost Tell the pilot and, where applicable, give instructions Service interrupted or ended Tell the pilot immediately or 6 Pilot resumes own navigation A controlled flight exempt from compulsory reporting points resumes position reports.
Figure 3. Identification comes before any surveillance service and is kept until the service ends. If it is lost or the service stops, the pilot is told. Original diagram
Describe this figure

A flow in six steps starts with contact. Step 2 is identification, with methods that depend on the sensor: for primary radar, a just-departed aircraft within 1 NM of the runway end, or a turn of 30 degrees or more under an ATC service; for SSR and MLAT, the label, a verified discrete code, Mode S identity, transfer, a code set as instructed, or squawk IDENT. Step 3 tells the pilot it is identified and gives its position, except in listed cases, and step 4 is the service, with identification kept. Step 5 has two outcomes: identification lost, or service interrupted or ended; in both the pilot is told. Step 6: the pilot resumes own navigation, and a controlled flight exempt from compulsory reporting points resumes position reports.

The flow runs left to right. You make contact. ATC identifies you by the method that fits the sensor. ATC tells you "identified" and, where needed, your position. The service runs. Then it is either lost or terminated, you are told immediately, and you resume your own navigation.

Traffic information

Traffic information alerts a pilot to other known or observed traffic that may be near the aircraft's position or intended route, to help avoid a collision. Traffic avoidance advice is ATS advice that specifies manoeuvres to help the pilot avoid a collision.

Rule SERA.7002(a)

For an identified controlled flight on a conflicting path with an unknown aircraft: tell the pilot about it, offer suggested avoiding action on request or if warranted, and tell the pilot when the conflict is over, whenever practicable.

The standard call

Traffic on a conflicting path should, whenever practicable, be described by four things: the relative bearing as a 12-hour clock position, the distance in km or NM, the direction it appears to be going, and its level and type (or, if the type is unknown, its relative speed: slow or fast). In a turn, compass points may replace clock positions. The level may be a flight level, altitude, height or a relative figure such as "above" or "below".

Picture yourself at the centre of a clock face with 12 o'clock straight ahead. A call such as "traffic 2 o'clock, 4 NM, crossing left to right, 500 ft above, type unknown, slow" places the traffic to the right of your nose, four miles out, moving across your path from left to right, above you, and slow.

Traffic information on a 12-hour clock Own aircraft CS-ABC at the centre of a ring of twelve clock positions, nose towards 12. Traffic is shown at 2 o'clock. The traffic call is built in four parts: relative bearing as a clock position, distance, the direction the traffic appears to be going, and its level and type, or relative speed if the type is unknown. The sample call is: traffic 2 o'clock, 4 NM, crossing left to right, 500 ft above, type unknown, slow. CS-ABC 1 2 3 4 5 6 7 8 9 10 11 12 Build the call 1 Bearing 2 o'clock 2 Distance 4 NM 3 Direction crossing left to right 4 Level, type or speed 500 ft above type unknown, slow In a turn, compass points may replace clock positions. Full call Distance not to scale CS-ABC, traffic 2 o'clock, 4 NM, crossing left to right, 500 ft above, type unknown, slow
Figure 4. Pick the clock position and details of the traffic to build a standard traffic call in the four-part order. Original interactive
Describe this figure

Own aircraft CS-ABC sits at the centre of a ring of twelve clock positions, nose towards 12. The traffic call is built in four parts, always in this order: the relative bearing as a clock position, the distance, the direction the traffic appears to be going, and its level and type, or its relative speed if the type is unknown. The sample call is: traffic 2 o'clock, 4 NM, crossing left to right, 500 ft above, type unknown, slow. In a turn, compass points may replace clock positions. A level from an unverified Mode C is still useful, but the pilot is told its accuracy is uncertain.

The interactive clock shows your aircraft at the centre of a 12-hour ring. Pick a clock position and the call builds itself with distance, direction, level and type or speed. A static version shows the same clock with the sample call above printed beside it.

The pressure-altitude level from an unknown aircraft, for example a VFR flight's Mode C, should be used in traffic information even if unverified, because it helps the other pilot find the traffic. If unverified, you are told its accuracy is uncertain.

ATC gives this information where practicable, after its other priorities and within equipment and workload limits. ATS can pass on only the traffic it knows about. That picture can be incomplete, so ATS cannot accept responsibility for always issuing it or for its accuracy.

Good practice

Traffic information supplements see-and-avoid and never replaces it. Keep looking out, report "traffic in sight" or "negative contact", and remember that non-transponding traffic may be shown only by primary radar, or not at all.

For the exact words used on the radio, see page 010-07-05.

Vectoring

Vectoring is navigational guidance given as specific headings, based on an ATS surveillance system. The headings are chosen so that the aircraft makes good the track the controller wants.

When a first vector takes you off a previously assigned route, you should be told what the vector is for. The limit of the vector should be stated when a radio failure on that heading could create a safety risk.

ATC should not vector an aircraft closer than 2.5 NM to the boundary of its airspace, except at a transfer of control. Where the separation minimum is greater than 5 NM, the buffer is half that minimum. Local arrangements protecting separation with the traffic next door can override this. The limit protects separation from controlled flights in adjoining controlled airspace. It is not required next to uncontrolled airspace, although an authority may still set one.

A controlled flight should not be vectored into uncontrolled airspace except in an emergency, to avoid adverse weather (pilot told), or at the pilot's specific request.

Special VFR flights should not be vectored unless special circumstances, such as emergencies, require it.

When vectoring ends, you should be told to resume own navigation. If the vectors took you off a previously assigned route, you are also given your position and any needed instructions.

Vectoring around weather, inside the airspace buffer Plan view. Controlled airspace is bounded at the top by other controlled airspace, with a hatched 2.5 NM buffer just inside the boundary where vectors normally do not go. An aircraft on its assigned route is vectored in three legs around a weather cell, staying below the buffer, and rejoins at a point marked resume own navigation. At the bottom is uncontrolled airspace, entered only in an emergency, to avoid weather with the pilot told, or at the pilot's request. Adjoining controlled airspace Boundary 2.5 NM buffer 2 Own airspace Uncontrolled airspace 5 5 NM Assigned route Weather 4 1 Resume own navigation 3 1 First vector off the route: tell the pilot why, and give its limit if radio failure would be a risk. 2 Normally keep 2.5 NM inside the boundary, or half the separation minimum if over 5 NM (not at a transfer). 3 Vectoring ends: tell the pilot to resume own navigation, with position and instructions if off the route. 4 Warn early of weather so the pilot can decide. The surveillance display may not show it. 5 Uncontrolled airspace: vector in only for an emergency, weather (pilot told) or the pilot's request.
Figure 5. Vectors take the aircraft around the weather but not into the 2.5 NM buffer next to adjoining controlled airspace. The pilot is then told to resume own navigation. Original diagram
Describe this figure

A plan view with other controlled airspace at the top and a hatched buffer, 2.5 NM wide, just inside the boundary. An aircraft on its assigned route is vectored in three legs around a weather cell and stays clear of the buffer. The first vector takes it off the route, so the pilot is told what the vector is for. At the end, a marked point shows where the pilot is told to resume own navigation, with position and instructions if the vectors left the route. Uncontrolled airspace lies at the bottom: a controlled flight is vectored there only in an emergency, to avoid weather with the pilot told, or at the pilot's request. A warning about weather should come early, and the display may not show it.

The vectoring figure shows a controlled airspace outline with a shaded band just inside the boundary, representing the 2.5 NM buffer. Headings route an aircraft around a weather cell and keep it clear of the band. A marked point later on the new track is where ATC tells the pilot to resume own navigation.

Your VFR responsibility while vectored

Controllers should take care when vectoring VFR flights so that they do not inadvertently enter IMC (instrument meteorological conditions). Where practicable, vectors should follow tracks the pilot can monitor with navaids, which also limits the impact of a surveillance failure.

When a controller vectors an IFR flight off a published route, the controller must issue clearances that keep the prescribed obstacle clearance. That duty is written for IFR flights only.

Good practice

If a vector or instruction would take you into cloud or below a safe height, tell ATC at once ("unable", with the reason) and ask for an alternative. Do not follow the heading into danger.

Rule GM3 SERA.7002(a)(1)

Collision hazard information does not take away a VFR pilot's responsibility for terrain, obstacles and staying in VMC.

The ICAO procedure asks you to state your reason when you ask a surveillance-equipped unit for navigation assistance, for example to avoid adverse weather or because of unreliable navigation instruments, and to give as much information as possible. The EU text is silent on this.

Level information from the display

A transponder with pressure-altitude reporting sends your level to the display. ATC cannot use it blindly. Unless the competent authority says otherwise, each suitably equipped ATS unit must verify the displayed pressure-altitude level at least once, on initial contact or as soon as possible afterwards. The accepted method is to compare the displayed level with the altimeter level you report by radio at the same moment, and geometric (GNSS) height should not be used for this check.

The tolerance for treating displayed level as accurate is ±300 ft outside reduced vertical separation minimum (RVSM) airspace (the authority may tighten it but not below ±200 ft). In RVSM airspace it is ±200 ft.

Only verified pressure-altitude level information may be used to decide that an aircraft is maintaining, vacating, passing or reaching a level.

The criteria are these. An aircraft is maintaining its level while within the tolerance. It has vacated a level once the readout has moved more than 300 ft away in the expected direction, and passed a level once more than 300 ft beyond it. It has reached its cleared level after three display updates, three sensor updates or 15 seconds (whichever is greatest) within tolerance.

Level verification: tolerance, vacated and reached Two charts of displayed level against display updates, with the assigned level as a line and a tolerance band of 300 ft either side (200 ft in RVSM airspace). Top: a readout drifts down, stays inside the band for three updates, then moves more than 300 ft away; the level is vacated. Bottom: a readout descends from above, enters the band, and after three updates within tolerance the aircraft has reached its cleared level. Only verified level information is used. Leaving a level +300 ft assigned −300 ft assigned level maintaining vacated: over 300 ft away 1 2 3 4 5 display updates Reaching the cleared level +300 ft assigned −300 ft assigned level 1 2 3 reached 1 2 3 4 5 6 display updates Filled dot: within tolerance. Open square: outside it. Band: ±300 ft outside RVSM; dotted: ±200 ft in RVSM. Reached: 3 display updates, 3 sensor updates or 15 s, whichever is greatest, all within tolerance. Only verified pressure-altitude level is used here. Out of tolerance: ask the pilot to check the pressure setting and confirm the level.
Figure 6. Level readout against display updates. An aircraft has vacated a level when the readout is more than 300 ft away; it has reached its cleared level after three updates within tolerance. Original chart
Describe this figure

Two charts plot the displayed level against display updates, with the assigned level as a line and a tolerance band of 300 ft either side outside RVSM airspace, dotted at 200 ft for RVSM airspace. In the top chart the readout drifts down, stays inside the band for three updates, then moves more than 300 ft away: the level is vacated. In the bottom chart the readout descends from above, enters the band, and after three updates within tolerance the aircraft has reached its cleared level. The three updates can also be three sensor updates or 15 seconds, whichever is greatest. Only verified level information is used, and outside tolerance the pilot is asked to check the pressure setting and confirm the level.

A vertical scale: the assigned level with a shaded band of ±300 ft (maintaining), marks beyond 300 ft (vacated, passed), and a timeline for the reached criterion.

Ending the service

If you have been told a surveillance service is being given, you should be told immediately if for any reason it is interrupted or terminated.

Rule SERA.8025(a)(2)

A controlled flight exempted from compulsory position reports must resume them when instructed, when told the surveillance service is terminated, or when told identification is lost.

Separation minima and safety nets in brief

This is awareness level: numbers and names you may meet, not procedures you apply.

The normal horizontal separation minimum based on radar, ADS-B and/or MLAT is 5 NM. With the authority's approval the provider may set a smaller one, not below 3 NM, where the system's capability at that location permits. On the same final approach track within 10 NM of the runway threshold, a minimum of 2.5 NM between succeeding aircraft is possible, but only when a list of conditions is met and the authority approves. Position indications must never touch or overlap unless vertical separation exists. Whether ATC separates a VFR flight at all depends on the airspace class (see page 010-07-01).

Surveillance can drive automatic safety nets on the controller's display. Short-term conflict alert (STCA) warns of a potential or actual loss of separation. Minimum safe altitude warning (MSAW) warns that an aircraft may go below a minimum safe altitude, to help prevent controlled flight into terrain. Which flights trigger them is set locally.

If the surveillance system fails completely but radio still works, controllers plot the positions of identified aircraft and set up procedural separation. As a temporary emergency measure they may use levels spaced at half the normal vertical separation minimum.

A unit giving surveillance services uses the call sign suffix RADAR, and approach radar units use ARRIVAL and DEPARTURE (page 010-07-01). The provider must automatically record surveillance data for accident investigation, search and rescue, evaluation and training.

A radar approach is one whose final approach is directed by a controller using radar. In a surveillance radar approach the controller gives heading corrections and distance and level information. It normally ends 2 NM from touchdown, or earlier if the aircraft is about to enter continuous radar clutter or the pilot reports able to complete the approach visually. It is an IFR procedure; you need only recognise it.

This lesson is educational and does not replace ATO instruction or the aircraft's POH/AFM.

Worked examples

Worked example

Example 1: is the displayed level accurate?

Given: you report your altimeter reads 5,500 ft (pressure altitude) at the moment of the check. The display shows 5,780 ft. You are outside RVSM airspace and the authority has not tightened the tolerance.

Method: find the difference between displayed and reported level and compare it with the ±300 ft tolerance.

Working: 5,780 − 5,500 = 280 ft. 280 ft is within 300 ft.

Answer: the displayed level is within tolerance, so ATC treats it as accurate (280 ft difference).

Worked example

Example 2: has the aircraft vacated its level?

Given: you are cleared to leave 8,000 ft and descend. The verified readout is 7,650 ft.

Method: compare the distance moved from 8,000 ft with the criterion of more than 300 ft in the expected direction.

Working: 8,000 − 7,650 = 350 ft. 350 ft is more than 300 ft.

Answer: the aircraft has vacated 8,000 ft (350 ft away).

Worked example

Example 3: when has the aircraft reached its level?

Given: the display updates every 6 s and the sensor updates every 4 s. The aircraft has levelled within tolerance at its cleared level.

Method: take the greatest of three display updates, three sensor updates and 15 s.

Working: three display updates = 3 × 6 = 18 s. Three sensor updates = 3 × 4 = 12 s. Fixed value = 15 s. The greatest is 18 s.

Answer: the aircraft counts as having reached its level after 18 s within tolerance.

In the real world

Real world AAIB Bulletin 11/2018, EW/C2017/11/02 (https://www.gov.uk/aaib-reports/aaib-investigation-to-cessna-152-g-wacg-and-guimbal-cabri-g2-g-jamm)

On 17 November 2017, two training aircraft, Cessna 152 G-WACG and Guimbal Cabri G2 G-JAMM, collided near Waddesdon, Buckinghamshire, in uncontrolled airspace after leaving their aerodrome's frequency. Neither was receiving any ATC service. Both were squawking 7000 with pressure altitude, so both were visible to ground radar, but no controller was watching them for conflicts and neither had a cockpit display of electronic conspicuity data. They had been close on similar tracks for some time without seeing each other.

For this page: a transponder makes you visible to surveillance, but you get traffic information only if you are talking to a unit giving a surveillance-based service. Even then it supplements see-and-avoid and does not replace it.

Exam traps

Exam trap

"Identified means ATC keeps me clear of terrain and cloud." Wrong for a VFR flight. Terrain, obstacle and VMC responsibility stays with the pilot. The obstacle-clearance duty while vectoring is written for IFR flights.

Exam trap

"Under a radar service ATC sees all traffic." Wrong. ATC passes on only known or observed traffic. Non-transponding aircraft may show as primary-only blips or not at all, and coverage has gaps.

Exam trap

"A squawk code alone identifies me." Wrong. A discrete code is used for identification only after its setting has been checked.

Exam trap

"Unverified Mode C is useless to ATC." Wrong. It is still used in traffic information, flagged as unverified. Only verified level counts for deciding that you have vacated or reached a level.

Explain it back

Explain it back

You are flying VFR and a controller says your aircraft is identified and then gives you a heading that looks like it leads toward cloud. In your own words, explain what "identified" does and does not mean for you, what you say to the controller, and what you do when the controller later says "resume own navigation".

Self-check

  1. Your transponder has failed in flight. Which kind of surveillance can still show your position to a controller?
    mcq · easy
    Show answer

    Primary radar sees an aircraft by its reflected echo and needs nothing from the aircraft, although it shows only a position and a small aircraft may give a weak return. Secondary radar, ADS-B and multilateration all rely on the aircraft's own transmissions, so an aircraft with no working transponder is invisible to them.

    Reference: GM1 Annex IV (Part-ATS) terms PSR blip, SSR response, MLAT system; SERA Article 2(118)

  2. Scenario: you are flying VFR and receiving vectors from an approach unit. The latest heading would take you towards a cloud base you can see ahead. What is the best action?
    mcq · medium
    Show answer

    Collision hazard information and vectors do not relieve a VFR pilot of responsibility for terrain, obstacles and staying in VMC, so the first option is wrong. The good practice is to say 'unable' with the reason and ask for something else. Turning away silently leaves ATC working from a different picture, and flying on into cloud is the outcome you must avoid.

    Reference: GM3 SERA.7002(a)(1); GM1 to AMC1 ATS.TR.155(c)(3)

  3. Scenario: an approach controller is vectoring you, a controlled flight, next to the edge of controlled airspace. Which statement about the vectoring limits is correct?
    mcq · medium
    Show answer

    The buffer is 2.5 NM (or half the separation minimum where that is greater than 5 NM), with the stated exceptions. A controlled flight should not be vectored into uncontrolled airspace except in an emergency, to avoid adverse weather, or at the pilot's request, so the second option is wrong. Special VFR flights should not be vectored unless special circumstances such as emergencies require it, so the third is wrong. Ten nautical miles is not a vectoring limit.

    Reference: AMC1 ATS.TR.155(c)(3)

  4. Scenario: you are climbing away from 6,000 ft outside RVSM airspace, cleared to leave that level. The verified readout on the controller's display shows 6,250 ft. Using the standard criterion, what does the controller conclude?
    mcq · hard
    Show answer

    6,250 - 6,000 = 250 ft, which is not more than 300 ft, so the level is not yet counted as vacated. Being higher is not the test, so the first option is wrong. 'Reached' is judged against the new cleared level after the update criterion, so the third is wrong. Verified level is exactly what may be used to decide vacating, so the fourth is wrong.

    Reference: AMC1 ATS.TR.155(g); ATS.TR.155(g)

  5. Scenario: CS-ABC calls an approach unit that shows a target near its reported position. Which statement is correct about when the surveillance service starts and ends?
    mcq · medium
    Show answer

    Identification and telling the pilot come first, and identification is kept until the service ends. A pilot who has been told a service is being given is told immediately if it is interrupted or terminated. A target merely seen is not an identified aircraft, so the first option is wrong. A discrete code may identify an aircraft only after its setting has been checked, so the third is wrong. A service does not silently end, so the fourth is wrong.

    Reference: ATS.TR.155(d); AMC1 ATS.TR.155(c)(10); AMC1 ATS.TR.155(c)(1)

Key concepts

Written with ASD-STE100 writing principles: short sentences, one topic per sentence, active voice, one word for one meaning.

01

Identification before service

Before a surveillance service starts, a controller must identify the aircraft and tell the pilot. The controller keeps the identification until the service ends. If identification is lost, the controller tells the pilot.

Rule: ATS.TR.155(d)

02

Collision hazard information

The controller must, whenever practicable, tell the pilot about unknown traffic that is a collision hazard. The controller offers traffic avoidance advice when the pilot asks or the situation warrants it. The controller tells the pilot when the conflict no longer exists.

Rule: SERA.7002(a)(1)-(2); ATS.TR.155(e)(1)-(2)

03

VFR pilot responsibility

Collision hazard information does not relieve the pilot of a VFR flight of responsibility for terrain and obstacles. The pilot also remains responsible for the flight to stay in visual meteorological conditions (VMC).

Rule: GM3 SERA.7002(a)(1)

Do: Tell ATC at once if a vector can take the flight into cloud or below a safe height.

Do not: Follow a heading that leads into cloud or below a safe height.

04

Traffic information

A controller describes traffic in conflict with four items, whenever practicable:

  1. The relative bearing, as a 12-hour clock position.

  2. The distance, in km or NM.

  3. The direction in which the traffic appears to move.

  4. The level and type, or the relative speed (slow or fast) if the type is unknown.

In a turn, compass points can replace the clock positions.

Rule: AMC1 SERA.7002(a)(1) point (a)

05

Radar identification methods

With SSR or MLAT, a controller can identify an aircraft from its label, its verified code, or its response to an IDENT instruction. With primary radar only, a controller can match a blip to a reported position or to a departure within 1 NM of the runway end. Under ATC service only, a controller can also identify a target after heading changes of 30° or more.

Rule: AMC1 ATS.TR.155(c)(1)

06

Vectoring

Except at a transfer of control, a controller does not vector an aircraft closer than 2.5 NM to the boundary of controlled airspace. A controller does not normally vector a special VFR flight, unless special circumstances, such as emergencies, require it. When the vectors end, the controller tells the pilot to resume own navigation.

Rule: AMC1 ATS.TR.155(c)(3)

Do not: Vector a controlled flight into uncontrolled airspace, unless an emergency, adverse weather or a pilot request applies.

07

Service interruption

A controller tells the pilot immediately if a surveillance service is interrupted or terminated. A controlled flight exempt from compulsory reporting points must resume position reports when instructed, when told the service ends, or when told identification is lost.

Rule: AMC1 ATS.TR.155(c)(10); SERA.8025(a)(2)(i)-(iii)

08

Level verification

The ATS unit must verify the displayed pressure-altitude level at least once, on initial contact or as soon as possible afterwards. Displayed level information is accurate within ±300 ft outside RVSM airspace and within ±200 ft inside it. An aircraft vacates a level when its readout moves more than 300 ft away from it.

Rule: ATS.TR.155(f); AMC1 ATS.TR.155(g)

Do not: Use geometric height from GNSS for the level check.

Flashcards

Print them: A6 landscape cards (PDF, front and back) · 4 cards per A4 landscape sheet (PDF, cut lines) · Key figures deck (PDF, 21 figures)

Print double-sided, flip on the short edge.

  1. 010-07-03-01Before a surveillance service starts, what must a controller do with the aircraft?

    Identify the aircraft and tell the pilot. Keep the identification until the service ends.

  2. 010-07-03-02What must a controller do when identification is lost?

    Tell the pilot and, where applicable, give the instructions that apply.

  3. 010-07-03-03Unknown traffic is a collision hazard. What must the controller do, whenever practicable?

    Tell the identified pilot about the traffic, offer avoiding action if asked or warranted, and tell the pilot when the conflict ends.

  4. 010-07-03-04Does traffic information relieve a VFR pilot of responsibility for terrain and obstacles?

    No. The VFR pilot stays responsible for terrain, obstacles and staying in VMC.

  5. 010-07-03-05A vector would take a VFR flight into cloud. What is good practice?

    Tell ATC at once that you are unable, give the reason, and ask for an alternative. Do not follow the heading.

  6. 010-07-03-06What are the four items used to describe conflicting traffic?

    Relative bearing as a 12-hour clock position; distance in km or NM; direction of travel; level and type, or relative speed if the type is unknown.

  7. 010-07-03-07What replaces the clock positions when describing traffic during a turn?

    Compass points can replace the clock positions in a turn.

  8. 010-07-03-08Name the ways a controller can identify an aircraft with SSR or MLAT.

    Its label, a verified discrete code, its Mode S identification, a transfer, a code it was told to set, or its response to IDENT.

  9. 010-07-03-09With primary radar only, how can a controller match a blip to an aircraft?

    To the aircraft's reported position over a point, with a consistent track.

  10. 010-07-03-10Primary radar only: a departed aircraft matches a blip. How far from the runway end?

    Within 1 NM of the runway end.

  11. 010-07-03-11Primary radar only, under ATC: which heading change identifies the aircraft?

    A heading change of 30° or more, when instructed by ATC.

  12. 010-07-03-12Which conspicuity code is assigned when Mode S downlinked identification is used?

    A1000, instead of a discrete code, for eligible flights in declared airspace.

  13. 010-07-03-13What makes an SSR code a discrete code?

    Four digits, and the last two digits are not "00".

  14. 010-07-03-14After identifying a departing aircraft within 1 NM, is its position needed?

    No, when the observed position is consistent with the departure time.

  15. 010-07-03-15How close to the airspace boundary can a controller vector an aircraft?

    Not closer than 2.5 NM, except at a transfer of control or where local arrangements protect separation.

  16. 010-07-03-16When the separation minimum is greater than 5 NM, what is the vectoring limit?

    Half the separation minimum from the airspace boundary.

  17. 010-07-03-17Is a controlled flight normally vectored into uncontrolled airspace?

    No. Only in an emergency, to avoid adverse weather (pilot told), or at the pilot's specific request.

  18. 010-07-03-18Is a special VFR flight normally vectored?

    No. Only when special circumstances, such as emergencies, require it.

  19. 010-07-03-19When vectoring ends, what does the controller tell the pilot?

    To resume own navigation. If the vectors took the aircraft off route, also the position and any needed instructions.

  20. 010-07-03-20A surveillance service is interrupted. How soon must the pilot be told?

    Immediately.

  21. 010-07-03-21A flight exempt from compulsory reporting points: when must it resume position reports?

    When instructed, when told the service ends, or when told identification is lost.

  22. 010-07-03-22What is the default horizontal separation minimum with radar, ADS-B or MLAT?

    5 NM.

  23. 010-07-03-23What is the general reduced horizontal separation minimum with radar, ADS-B or MLAT?

    3 NM, the lowest general value, when the provider establishes it, the authority approves it and the system permits.

  24. 010-07-03-24Complete surveillance failure, radio working: what temporary level spacing is used?

    Half the applicable vertical separation minimum, as a temporary emergency measure.

  25. 010-07-03-25Outside RVSM airspace, what level tolerance applies?

    ±300 ft.

  26. 010-07-03-26How far can the competent authority tighten the outside-RVSM tolerance?

    Down to ±200 ft, the smallest value the authority can set.

  27. 010-07-03-27Inside RVSM airspace, what level tolerance applies?

    ±200 ft.

  28. 010-07-03-28After clearance to leave a level, when has the aircraft vacated it?

    When the readout moves more than 300 ft away from the level in the expected direction.

  29. 010-07-03-29When has a climbing or descending aircraft passed a level?

    When it is more than 300 ft beyond that level.

  30. 010-07-03-30How long within tolerance before an aircraft has reached its cleared level?

    3 display updates, 3 sensor updates or 15 s, whichever is greatest.

  31. 010-07-03-31How often must the displayed pressure-altitude level be verified?

    At least once, on initial contact or as soon as possible afterwards, unless the authority says otherwise.

  32. 010-07-03-32Which height is used to verify the displayed level?

    Not geometric (GNSS) height. The check compares the displayed pressure-altitude level with the altimeter level the pilot reports.

  33. 010-07-03-33Where does a normal surveillance radar approach end?

    2 NM from touchdown, unless the authority allows it closer, or clutter or a visual report comes first.

  34. 010-07-03-34What does a controller give during a surveillance radar approach?

    Heading corrections and distance or level information. It is an IFR procedure.

  35. 010-07-03-35When vectoring a VFR flight, what must the controller avoid, and which tracks are preferred?

    Avoid taking the flight inadvertently into IMC. Prefer tracks the pilot can monitor with navaids.

  36. 010-07-03-36On the same final approach track, what reduced minimum between succeeding aircraft is possible?

    2.5 NM, within 10 NM of the threshold, only when the listed conditions are met and the authority approves.

Sources

  1. Commission Implementing Regulation (EU) No 923/2012 (SERA) — via EASA Easy Access Rules for Standardised European Rules of the Air (SERA), August 2025, with the related AMC/GM.
  2. Commission Implementing Regulation (EU) 2017/373 (ATM/ANS), Annex I definitions and Annex IV (Part-ATS) — via EASA Easy Access Rules for ATM/ANS – Provision of Services, March 2025, with the related AMC/GM.
  3. ICAO Annex 11, Air Traffic Services, 15th edition (2018), and ICAO Doc 4444, PANS-ATM, 16th edition (2016): used for ICAO comparisons only.
  4. AAIB (UK) Bulletin 11/2018, EW/C2017/11/02, mid-air collision Cessna 152 G-WACG and Guimbal Cabri G2 G-JAMM near Waddesdon, 17 November 2017: gov.uk.

This page is educational. It does not replace instruction at your ATO or your aircraft's POH/AFM. Rules change: check the regulation in force.

Glossary

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