Surveillance Systems (Radar, ADS-B & TCAS)

Before an aircraft can safely share the skies with thousands of others, it must be able to detect nearby traffic, monitor weather, determine its height above terrain, and cooperate with air traffic surveillance systems. Modern surveillance systems provide this situational awareness, dramatically reducing collision risk and improving the safety and efficiency of global aviation. Surveillance Systems (Radar, ADS-B & TCAS) Understanding how modern aircraft detect weather, terrain, and other aircraft while supporting safe separation through advanced surveillance technologies.

Introduction

Every day, tens of thousands of aircraft share the world’s airspace simultaneously. To operate safely, pilots and air traffic controllers must continuously know:

Where aircraft are located.
How high they are flying.
Which direction they are moving.
Whether dangerous weather lies ahead.
How close they are to terrain.
Whether a collision risk exists.

These capabilities are provided by: Aircraft Surveillance Systems. Together, they create a comprehensive picture of the aircraft’s surroundings and significantly enhance flight safety.

What Is Aircraft Surveillance?

Aircraft Surveillance is the process of observing, tracking, identifying, and monitoring aircraft, terrain, weather, and other hazards using onboard and ground-based systems. Simply stated: Surveillance systems allow aircraft to “see” beyond the pilot’s direct field of view.

Why Surveillance Matters

Surveillance systems help:

Prevent mid-air collisions.
Detect hazardous weather.
Maintain aircraft separation.
Improve air traffic management.
Enhance situational awareness.

Support instrument operations.
Improve operational efficiency.

Modern aviation depends on multiple independent surveillance technologies.

Types of Surveillance

Aircraft surveillance generally includes:

Primary Radar.
Secondary Surveillance Radar (SSR).
Airborne Weather Radar.
Radar Altimeter.
ADS-B.
TCAS.
Terrain Awareness Systems.

Each provides different information.

Radar

The word RADAR stands for: Radio Detection And Ranging. Radar transmits radio waves, which reflect from objects. By measuring the reflected signal, the system estimates:

Distance.
Direction.
Relative location.

Primary Surveillance Radar (PSR)

Primary radar operates by:

Transmitting radio pulses.
Receiving reflections from aircraft.

Advantages:

Does not require onboard equipment.
Detects most sufficiently reflective objects.

Limitations:

Does not identify aircraft.
Does not directly determine altitude.
Performance depends on target reflectivity and environmental conditions.

Secondary Surveillance Radar (SSR)

Secondary radar operates differently. Instead of relying on reflections, it interrogates aircraft transponders. The aircraft responds with information such as:

Identification code.
Pressure altitude.
Additional data depending on transponder capability.

SSR provides more information than primary radar.

Aircraft Transponder

Every transport aircraft carries a: Transponder. The transponder automatically replies to SSR interrogations. Typical information includes:

Squawk code.
Pressure altitude.
Aircraft identity (for compatible systems).

Modern Mode S transponders support enhanced data exchange.

Squawk Code

A: Squawk Code is a four-digit octal code assigned by Air Traffic Control or selected according to published procedures. It allows controllers to identify aircraft on surveillance displays. Certain standardized codes indicate special situations, such as radio failure or emergencies.

Mode A, Mode C & Mode S

Transponders support different operating modes. Mode A

Transmits aircraft identification code.

Mode C

Transmits identification plus pressure altitude.

Mode S

Provides selective interrogation and supports additional data exchange, including unique aircraft identification and compatibility with systems such as ADS-B.

Airborne Weather Radar

Aircraft carry onboard: Weather Radar. It detects precipitation ahead of the aircraft, allowing pilots to avoid:

Thunderstorms.
Heavy rain.
Hail.
Regions of intense turbulence associated with convective weather.

Weather radar is a strategic weather-avoidance tool.

Weather Radar Principle

The radar antenna emits microwave energy. Rain, snow, and hail reflect the signal. The returning echoes are processed to estimate:

Location.
Intensity.
Shape of precipitation.

Modern weather radar can also assist with turbulence detection under appropriate conditions.

Radar Altimeter

(or Radio Altimeter) measures: Actual height above the terrain directly beneath the aircraft. Unlike a pressure altimeter, it measures true height above ground level, typically at relatively low altitudes. Radar altimeters are especially important during approach and landing.

Automatic Dependent Surveillance–Broadcast (ADS-B)

One of aviation’s most significant modern surveillance technologies is: Automatic Dependent Surveillance–Broadcast (ADS-B). ADS-B allows aircraft to automatically broadcast:

Position.
Velocity.
Altitude.
Identification.

No radar interrogation is required.

Why “Automatic Dependent Surveillance–Broadcast”?

Each part of the name has meaning. Automatic Operates continuously without pilot action.

Dependent

Relies on navigation information (typically from GNSS).

Surveillance

Provides aircraft position information.

Broadcast

Transmits information for reception by other aircraft and ground stations.

ADS-B Out

ADS-B Out Automatically transmits the aircraft’s:

Position.
Altitude.
Ground speed.
Identification.

Many airspace regions require ADS-B Out for compliant operations.

ADS-B In

ADS-B In Allows an aircraft to receive surveillance information transmitted by:

Other aircraft.
Ground stations (where supported).

This can enhance situational awareness when integrated with cockpit displays.

Traffic Collision Avoidance System (TCAS)

Another major safety system is: Traffic Collision Avoidance System (TCAS). TCAS independently monitors nearby transponder-equipped aircraft, helping prevent mid-air collisions. Importantly, TCAS operates independently of ground-based air traffic control.

How TCAS Works

TCAS interrogates nearby aircraft transponders, calculating:

Relative position.
Relative altitude.
Closure rate.

The system predicts potential collision risks.

Traffic Advisory (TA)

If another aircraft approaches too closely, TCAS first issues a: Traffic Advisory (TA). Typical guidance is: “Traffic, Traffic.” The purpose is to alert the flight crew to visually acquire the conflicting aircraft when possible.

Resolution Advisory (RA)

If collision risk continues to increase, TCAS may issue a: Resolution Advisory (RA). Examples include commands such as:

Climb.
Descend.
Adjust vertical speed.

When two TCAS-equipped aircraft are involved, their systems coordinate complementary advisories.

Terrain Awareness and Warning System (TAWS)

Aircraft also monitor terrain using: Terrain Awareness and Warning System (TAWS). TAWS compares aircraft position, altitude, and terrain databases, warning crews of potential controlled flight into terrain (CFIT). Many modern systems incorporate enhanced terrain databases (often referred to as EGPWS or Enhanced GPWS, depending on implementation).

Ground Proximity Warning System (GPWS)

Earlier aircraft primarily used:

Ground Proximity Warning System (GPWS). GPWS relied mainly on radar altitude and aircraft configuration to detect hazardous proximity to terrain. Enhanced systems later added terrain databases and GNSS integration.

Surface Surveillance

Many airports use advanced surveillance systems to monitor:

Taxiways.
Runways.
Ground vehicles.
Aircraft movements.

These systems reduce the risk of runway incursions and improve airport safety.

Sensor Fusion

Modern surveillance integrates information from:

Radar.
ADS-B.
TCAS.
TAWS.
GNSS.
INS.

Combining multiple sources produces a more complete and reliable picture of the surrounding environment.

Cockpit Displays

Surveillance information is presented on:

Navigation Displays.
Traffic Displays.
Weather Displays.
Terrain Displays.

Pilots can quickly assess nearby hazards.

Redundancy

Modern surveillance systems employ:

Multiple sensors.
Independent processors.
Redundant communication paths.

This architecture improves reliability and availability.

Future Surveillance Systems

Emerging technologies include:

Space-based ADS-B.
AI-assisted conflict prediction.
Four-dimensional traffic management.
Cooperative autonomous surveillance.
Enhanced sensor fusion.

Future systems will further improve safety and airspace capacity.

Common Misconceptions

Myth 1 “Radar is the only way to detect aircraft.” Modern surveillance also uses ADS-B, TCAS, satellite-based systems, multilateration, and other technologies in addition to radar.

Myth 2

“TCAS replaces Air Traffic Control.” TCAS is a last-resort airborne collision-avoidance system. Air traffic control remains responsible for providing separation services where applicable.

Myth 3

“Weather radar detects all turbulence.” Weather radar primarily detects precipitation. Some turbulence, particularly clear-air turbulence, may not produce detectable radar echoes.

Why Engineers Study Surveillance Systems

Understanding surveillance systems enables engineers to:

Improve collision avoidance.
Enhance weather detection.
Increase airspace capacity.
Design autonomous aircraft.
Improve flight safety.
Develop future air traffic systems.

Surveillance technology is a cornerstone of modern aviation safety.

Real-World Examples

Surveillance systems are essential for:

Commercial airliners.
Cargo aircraft.
Business jets.
Military aircraft.
Helicopters.
Air traffic control.
Search-and-rescue aircraft.
Future autonomous air vehicles.

Every modern transport aircraft uses multiple surveillance systems simultaneously.

Summary Table

Surveillance System Primary Purpose Primary Surveillance Radar (PSR) Detects aircraft by reflected radio waves Secondary Surveillance Radar (SSR) Identifies aircraft using transponder replies Weather Radar Detects precipitation and hazardous weather ahead Radar Altimeter Measures height above terrain ADS-B Broadcasts aircraft position and motion TCAS Detects nearby aircraft and provides collision-avoidance advisories TAWS / EGPWS Warns of terrain hazards Ground Surveillance Monitors aircraft and vehicles on airport surfaces

Major Surveillance Technologies

Technology Primary Role Radar Detects objects using radio waves Transponder Responds to SSR interrogations ADS-B Cooperative position broadcasting TCAS Airborne collision avoidance Weather Radar Hazardous weather avoidance Radar Altimeter Precision height measurement Terrain Awareness Prevents controlled flight into terrain Sensor Fusion Integrates multiple surveillance sources

Key Takeaways

Aircraft surveillance systems combine radar, transponders, ADS-B, TCAS, weather radar, radar altimeters, and terrain awareness technologies to provide a comprehensive understanding of the aircraft’s operating environment. These systems enable air traffic management, weather avoidance, collision prevention, and terrain awareness while significantly improving aviation safety. Through redundancy, sensor fusion, and increasingly advanced digital technologies, modern surveillance systems have become one of the most critical components of today’s highly reliable global air transportation network.

Module 10 Progress

Avionics, Flight Control & Aircraft Automation

✅ Introduction to Avionics
✅ Flight Instruments
✅ Air Data Systems
✅ Inertial Navigation Systems (INS)
✅ Global Navigation Satellite Systems (GNSS/GPS)
✅ Flight Management System (FMS)
✅ Autopilot Systems
✅ Fly-by-Wire Flight Control
✅ Sensors & Data Acquisition
✅ Aircraft Communication Systems
✅ Surveillance Systems (Radar, ADS-B & TCAS)
Electronic Flight Displays & Glass Cockpit
Aircraft Health Monitoring
Integrated Modular Avionics (IMA)
Future Autonomous Aircraft & AI

Looking Ahead

Next Article: Electronic Flight Displays & Glass Cockpit You’ll explore how modern aircraft present vast amounts of flight information through integrated digital displays. We’ll examine the Primary Flight Display (PFD), Navigation Display (ND), Engine Indicating and Crew Alerting System (EICAS), Electronic Centralized Aircraft Monitor (ECAM), Multifunction Displays (MFDs), Head-Up Displays (HUDs), Synthetic Vision Systems (SVS), Enhanced Vision Systems (EVS), and the human-machine interface principles that define today’s digital cockpit.