The surveillance picture
ATS surveillance is the general name for systems that let a controller determine and follow an aircraft's position. Radar, ADS and multilateration do not all obtain that position in the same way, so their capabilities and failure modes must not be treated as interchangeable.
Primary surveillance radar
Primary surveillance radar, or PSR, transmits radio energy and listens for energy reflected from an object. Range comes from the elapsed time between transmission and echo. Bearing comes from the direction of the antenna beam. The aircraft need not carry a transponder, but the return is only a target or blip until the controller correlates it with a particular aircraft.
Secondary surveillance radar
Secondary surveillance radar, or SSR, interrogates an airborne transponder. The transponder sends a coded reply that can provide identity and pressure-altitude information. SSR therefore gives a cleaner, information-rich label, but it depends on serviceable airborne equipment and compatible ground processing.
| System | How position is obtained | Aircraft equipment | Important result |
|---|---|---|---|
| PSR | Ground radar measures reflected energy. | No cooperative equipment required. | Detects an object but does not itself provide identity. |
| SSR | Ground station interrogates a transponder reply. | Transponder. | Can provide identity, code and pressure altitude. |
| ADS-B | Aircraft repeatedly broadcasts derived position and other data. | Position source and ADS-B transmitter. | Several suitably equipped users can receive the broadcast. |
| MLAT | Several receivers compare reply arrival times. | Compatible transponder or ADS-B signal. | Ground processing calculates a precise position. |
SSR modes
| Mode | What the reply supplies | Operational use |
|---|---|---|
| Mode A | A four-digit identity code using digits 0 to 7. | Associates a displayed return with an assigned code or special condition. |
| Mode C | Pressure altitude referenced to 1013.2 hPa. | Adds level information to the surveillance label. |
| Mode S | A selective address and enhanced data capability. | Supports individual interrogation, improved identification and data exchange. |
ADS-B, ADS-C and multilateration
The similar names ADS-B and ADS-C hide a major operational difference. One broadcasts openly and repeatedly; the other reports to a contracted ground system under agreed conditions.
Automatic dependent surveillance broadcast
ADS-B is automatic because a report does not require a separate pilot action, dependent because the report depends on the aircraft's navigation and data sources, surveillance because the data supports traffic monitoring, and broadcast because the aircraft transmits it for every suitable receiver within range. A report may include identity, position, altitude, velocity and other status data.
Advantages of ADS-B
ADS-B can extend coverage where installing radar is difficult, improve tracking accuracy and update rate, support more direct routing, and show equipped aircraft or vehicles on airport surfaces. Its quality still depends on the airborne position source, transmitted data, receiver coverage and system integrity.
Automatic dependent surveillance contract
ADS-C sends reports to one or more specified ATS units or aeronautical operational-control facilities under a contract held by the ground system. The contract defines what information is sent and what causes a report. A periodic contract reports at agreed intervals. An event contract reports when a defined event occurs. A demand contract produces a report when requested. The aircraft may also send an emergency report to an ATS unit that holds a contract.
| Feature | ADS-B | ADS-C |
|---|---|---|
| Distribution | Broadcast to suitable receivers within range. | Addressed to contracted ground systems. |
| Trigger | Repeated automatic broadcasts. | Contract conditions, demand, event or emergency. |
| Typical coverage value | Dense terrestrial or surface surveillance. | Remote and oceanic route conformance monitoring. |
MLAT applications
MLAT uses time difference of arrival. Several ground stations receive the same transponder or ADS-B transmission at slightly different times because each receiver is a different distance from the aircraft. Ground processing turns those time differences into intersecting range differences and calculates the aircraft position. No separate MLAT box is normally required aboard an aircraft already transmitting a compatible signal.
Surveillance services and operational uses
A surveillance return is not itself a service. ATC first establishes identification, then uses the information within the limits of coverage, accuracy, communications and the service being provided.
Radar applications
Radar procedures are preferred to procedural control whenever the ATS unit and the aircraft gain an operational advantage. Vectors are expressed as magnetic headings.
Services described by Bali
| Service | Where or to whom | Purpose |
|---|---|---|
| Radar control service | Aircraft operating in controlled airspace. | Apply control, separation and vectoring as authorised. |
| Radar advisory service | Aircraft operating in Class F airspace. | Give advice using surveillance information. |
| Radar flight information service | Identified aircraft in any part of an FIR. | Provide useful traffic and flight information. |
Direct two-way communication with the unit providing the service is normally required. Separation between two radar-identified aircraft may still be provided when only one is in direct communication with the radar unit, but the communication and coordination arrangement must support the service.
Use in air traffic control
Surveillance improves airspace use, reduces delay and enhances safety. Controllers vector departures for an efficient flow and an expeditious climb, vector arrivals for an orderly descent and approach sequence, resolve potential conflicts and assist navigation. It can help maintain separation and normal traffic flow when an aircraft with communication failure remains inside coverage.
Monitoring supplies improved position information on controlled aircraft, supplementary information about other traffic, and early detection of significant deviations from an assigned route or level.
Use of radar in approach control
Approach control uses surveillance to sequence traffic, vector aircraft to pilot-interpreted approach aids and conduct surveillance radar approaches.
Use of radar in aerodrome control
Aerodrome control may use a surveillance display as an approach-monitor aid. Surface movement radar supports control of traffic on the movement area, especially where visibility prevents an adequate visual watch.
Position displays
The display may show individual or combined PSR, SSR, ADS-B and MLAT position symbols, primary blips and secondary replies. A controller must understand which sensor supports a label because its identity, altitude and continuity depend on that source.
Use of SSR without primary radar
SSR information may be used by itself to separate aircraft when carriage of SSR transponders is mandatory in the area and each aircraft is identified and remains identified by a discrete code. Procedural separation is applied between a transponder-equipped aircraft and one without a transponder or with a non-functioning transponder.
How an aircraft is identified
Identification is the positive association of one displayed position indication with one known aircraft.
ATS surveillance service identification
Identification must be established by an approved method and the pilot informed. If identification is lost, the pilot is told and instructions restore procedural separation.
ADS-B identification
An aircraft may be identified by direct recognition of its identification in the ADS-B label, transfer of ADS-B identification, observation of compliance with an instruction to TRANSMIT ADS-B IDENT, or another approved correlation method.
SSR and MLAT identification
Approved methods include recognition of the aircraft identity in the label, recognition of an assigned and verified discrete code, direct recognition of a Mode S aircraft identity, transfer of identification, observation of compliance with an instruction to set a specified code, and observation of SQUAWK IDENT. The IDENT function is selected only when instructed because an unsolicited highlight can confuse the controller's identification process.
PSR identification
| Method | Required correlation | Protection against error |
|---|---|---|
| Position report | A displayed position agrees with the aircraft's reported position over, or bearing and distance from, a displayed point. | The observed track agrees with the aircraft's path or reported heading. |
| Departing aircraft | A target is correlated with an aircraft known to have just departed. | Identification is established within 1 NM of the runway end and nearby traffic cannot be confused with it. |
| Turn method | The controller observes a target make instructed heading changes of 30 degrees or more, or correlates reported manoeuvres. | Only one return responds and the manoeuvre remains inside coverage. |
| Transfer | Another controller transfers a previously established identification. | Transfer conditions ensure the same target is accepted. |
Before using the turn method, the controller may first ascertain the aircraft heading. Any instructed manoeuvre must not take the aircraft outside surveillance coverage or the displayed area.
Separation, vectoring and speed control
Surveillance permits measured horizontal spacing, but the minimum is attached to approved capability and local conditions. It is not a universal permission to use the smallest number shown on a display.
Surveillance separation minima
| Minimum | Where it may apply | Condition |
|---|---|---|
| 5.0 NM, 9.3 km | General ICAO surveillance minimum unless otherwise prescribed. | Identification is established and maintained. |
| 3.0 NM, 5.6 km | A location where radar, ADS-B or MLAT capability permits. | The appropriate ATS authority prescribes it. |
| 2.5 NM, 4.6 km | Successive aircraft established on the same final approach track within 10 NM of the runway threshold. | Local factors, runway occupancy and wake requirements permit the reduction. |
Distance is applied between the centres of position symbols or primary blips representing the aircraft. Procedural separation replaces surveillance separation during a surveillance-system failure, outside coverage, or when an aircraft leaves coverage or enters an adjacent FIR without an effective surveillance transfer.
Indian and wake-turbulence minima
Bali directs the reader to the separation chapter for the surveillance minima prescribed in India and to the operational-hazards chapter for wake-turbulence radar minima. Both remain additional constraints: a smaller display distance cannot displace the published Indian or wake minimum.
Vectoring
Radar vectors are magnetic headings. Vectoring may expedite departure or arrival, establish an approach sequence, resolve a conflict or assist navigation. The controller keeps the aircraft within adequate surveillance coverage and protects an IFR aircraft from terrain while it is being radar vectored. When vectoring ends, RESUME OWN NAVIGATION means the pilot resumes navigation using the aircraft's own equipment from the stated position or track.
Speed control in India
| Traffic condition | Maximum stated by Bali |
|---|---|
| All aircraft operating below 10,000 ft | 250 kt IAS. |
| Arriving aircraft below 10,000 ft within 15 NM of a VOR or DME serving the aerodrome | 220 kt IAS. |
| Additional ATC restriction | May be imposed when traffic conditions require. |
| Restriction suspended | NO SPEED RESTRICTION when traffic permits. |
Transponder codes and Indian operation
A Mode A code is four octal digits, so each digit can be only 0 through 7. The code identifies a flight or declares a special condition. It does not by itself guarantee a control service.
Special and conspicuity codes
| Code | Meaning | Use |
|---|---|---|
| 7500 | Unlawful interference | Indicates unlawful interference when circumstances permit its use. |
| 7600 | Radio communication failure | Alerts ATS to loss of two-way radio communication. |
| 7700 | Emergency | General emergency code when another assigned code is not more useful. |
| 7000 | VFR conspicuity | Used under the applicable regional or national procedure when no discrete code is assigned. |
| 2000 | No code assigned | Bali requires Mode A3 code 2000 before entry to an Indian FIR when no SSR code has been assigned. |
| 0000 | Reserved or ATC-directed use | Not selected merely as a normal conspicuity code. |
Four octal digits provide 4,096 possible Mode A codes. If an aircraft has already been identified by an assigned discrete code and then develops an emergency, the pilot normally retains that code unless ATC directs otherwise. It may preserve a positive identity more reliably than changing to 7700.
Operation in Indian FIRs
Aircraft carrying a serviceable transponder operate it at all times during flight within the Chennai, Delhi, Guwahati, Kolkata and Mumbai FIRs, whether inside or outside airspace where SSR is used for ATS.
| Situation | Required code action |
|---|---|
| Departing from an aerodrome in one of the five FIRs | Use the appropriate SSR code assigned on departure and retain it until instructed otherwise. |
| International flight entering an Indian FIR | Continue the code being used in the adjacent FIR until instructed otherwise. |
| Domestic flight | Operate on the last assigned code. |
| No SSR code assigned before FIR entry | Select Mode A3 code 2000 and maintain it until instructed otherwise. |
| On the ground under Bali's stated procedure | Do not operate the transponder merely on the ground, except from entering the runway for take-off until vacating it after landing. |
Controller instructions
SQUAWK followed by a code requires selection and read-back of the code. SQUAWK IDENT requires momentary use of the IDENT feature. CONFIRM SQUAWK checks the displayed setting. RESET SQUAWK followed by a code requires reselection. SQUAWK CHARLIE requests pressure-altitude transmission. SQUAWK STANDBY suppresses normal replies as instructed.
Altitude verification and equipment failure
Mode C and ADS-B altitude make vertical monitoring possible only when the transmitted pressure altitude agrees sufficiently with the level reported by the pilot.
Accuracy tolerances
| Airspace | Approved tolerance used by Bali | Controller action |
|---|---|---|
| RVSM airspace | Plus or minus 200 ft. | Compare the displayed pressure altitude with the pilot's reported level. |
| Other airspace | Plus or minus 300 ft, unless an authority specifies a practical criterion no smaller than 200 ft. | Verify on initial contact or as soon as possible. |
On first contact with a radar unit, every aircraft reports the level or altitude being maintained or passed. A suitably equipped ATC unit compares that radio report with the Mode C or ADS-B altitude data. If the data is within tolerance, the pilot need not be advised that verification succeeded.
If the discrepancy exceeds tolerance, the controller asks the pilot to check the pressure setting and confirm the level. If the discrepancy remains after the correct pressure setting and level are confirmed, the controller may instruct STOP SQUAWK CHARLIE WRONG INDICATION or STOP ADS-B ALTITUDE TRANSMISSION WRONG INDICATION. Geometric height is not used for separation in place of the verified pressure-altitude value.
Using Mode C to judge level change
| Displayed relation to a level | Operational interpretation |
|---|---|
| Within 300 ft of the allocated level | The aircraft is treated as occupying or maintaining that level. |
| More than 300 ft from the previously allocated level in the required direction | The aircraft is treated as having vacated that level. |
| Within 300 ft of a specified level during climb or descent | The aircraft is treated as passing that level. |
| Within 300 ft of the allocated level at the end of climb or descent | The aircraft is treated as reaching the level. |
Transponder failure
When a mandatory transponder fails in flight, the flight is normally permitted to continue to the next point of intended landing, subject to coordination and any restriction needed to preserve safety. An aircraft without functioning SSR equipment is separated procedurally from transponder-equipped aircraft where necessary.
If failure is known before departure, ATC may specifically authorise a flight without a serviceable transponder. The request must be included in the flight plan and the aircraft may be directed to the nearest suitable aerodrome where repair can be made.
Surveillance failure and termination
If the surveillance system fails or identification cannot be maintained, procedural separation is restored and the pilot is informed. A pilot receiving a surveillance service is told immediately when the service is interrupted or terminated. For an arrival, the service ends automatically when the aircraft is instructed to contact Tower, but its position from touchdown should be passed before transfer.
ACAS, TCAS and surveillance safety nets
Airborne collision avoidance and ground surveillance safety nets are separate layers. ACAS protects the aircraft independently of ground ATC, while controller tools warn of developing conflicts or terrain risk on the surveillance display.
ACAS and TCAS II
ACAS interrogates compatible transponders and assesses collision risk independently of ground equipment. TCAS II is an implementation that can issue both traffic advisories and vertical resolution advisories. Carriage and use requirements apply in the operational categories prescribed by the authority, including the relevant RVSM operations.
| Alert or report | Meaning | Pilot action |
|---|---|---|
| Traffic advisory, TA | Traffic may become a collision threat. | Look for the traffic, prepare for a possible RA and do not manoeuvre solely in response to the TA. |
| Resolution advisory, RA | A vertical manoeuvre or vertical-rate limit is required. | Respond immediately and follow the RA unless doing so would jeopardise the aircraft. |
| TCAS RA | Pilot report that an RA is being followed. | ATC acknowledges and does not issue a conflicting vertical instruction. |
| CLEAR OF CONFLICT | ACAS indicates the threat has ended. | Return promptly to the cleared flight path, inform ATC and comply with the amended clearance. |
An RA takes precedence over a conflicting ATC instruction. The pilot must not manoeuvre in the opposite sense to an RA. As soon as flight-deck workload permits, the pilot reports the RA and its direction to ATC. ATC responsibility for separation between aircraft directly involved in the RA is limited until the pilot reports returning to the current clearance or CLEAR OF CONFLICT. Stall warning, wind-shear and ground-proximity warnings have priority when following an RA would jeopardise the aircraft.
STCA and MSAW
| Safety net | What it detects | Controller response |
|---|---|---|
| Short-term conflict alert, STCA | Predicted loss of separation between tracked aircraft. | Assess immediately and issue avoiding action or traffic information as appropriate. |
| Minimum safe altitude warning, MSAW | An aircraft at or predicted to be at an unsafe altitude relative to terrain or obstacles. | Alert the aircraft and issue appropriate terrain-avoidance instruction or advice. |
These alerts support controller judgement. They do not make ATC generally responsible for terrain clearance when an aircraft navigates on its own. During radar vectoring of an IFR flight, however, the controller must provide the prescribed terrain clearance.