Lesson Notes By Weeks and Term v4 - SHS 2

Lesson Video

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Performance objectives

• Define ground-based surveillance systems and their importance.
• Identify and describe different types of ground-based surveillance systems.
• Summarize the functions of these systems in aviation safety.
• Discuss their relevance to aviation in Ghana.

Lesson summary

This lesson introduces the critical "eyes" of Air Traffic Control (ATC) – the ground-based systems that allow us to see and track aircraft in the sky. Imagine the airspace over Ghana, especially around Kotoka International Airport (KIA), as a busy, multi-lane road in the sky. Without a way to see every vehicle, chaos and accidents would be inevitable. These surveillance systems are the tools the Ghana Civil Aviation Authority (GCAA) uses to ensure every flight—from Accra to Tamale, or from London to Accra—moves safely and efficiently. Understanding these systems is fundamental to appreciating the safety and complexity of modern aviation.

Lesson notes

What is Air Traffic Surveillance? Air Traffic Surveillance is the process of determining the position of aircraft in the airspace. Think of it as the "seeing" part of Air Traffic Control. The goal is to build a complete picture of the traffic situation for air traffic controllers (ATCOs) on the ground so they can maintain safe separation between aircraft.

There are two main philosophies for surveillance: Independent Surveillance: The ground system works independently of the aircraft's navigation equipment. It finds the aircraft on its own. (e.g., Radar). Dependent Surveillance: The ground system *depends* on the aircraft broadcasting its own position, which it calculates using its own equipment (like GPS). (e.g., ADS-B).

We will focus on four key ground-based systems. System 1: Primary Surveillance Radar (PSR) Principle: PSR works like a simple echo. A large rotating antenna on the ground sends out a powerful pulse of radio energy into the sky. This energy travels outwards at the speed of light. If the pulse hits an object (like an aeroplane), a tiny fraction of that energy is reflected back towards the antenna. The antenna "listens" for this reflected energy (the "echo"). Information Provided: Range (Distance): Calculated by measuring the time it takes for the pulse to travel to the aircraft and back. *Formula:* Distance = (Time × Speed of Light) / 2. We divide by 2 because the time measured is for a two-way trip. Bearing (Direction/Azimuth): Determined by the direction the antenna was pointing when it received the echo. Analogy: Imagine you are in a large, dark hall. You shout "Hello!" and listen for the echo. The time it takes for the echo to return tells you how far away the wall is. The direction you were facing when you heard the echo tells you where the wall is. Key Feature: It is a non-cooperative system. It does not require any special equipment on the aircraft. It can see anything that reflects radio waves, including birds, storm clouds, or an aircraft with total electrical failure. Limitations: It cannot identify the aircraft. It's just a "blip" on the screen. It does not know the aircraft's altitude. It requires very powerful (and expensive) transmitters. System 2: Secondary Surveillance Radar (SSR) Principle: SSR works like a conversation or a question-and-answer session. The SSR ground antenna sends out a coded "interrogation" signal. An aircraft equipped with a device called a transponder receives this interrogation. The transponder automatically sends back a coded "reply" signal. Information Provided: The reply from the transponder contains much more information than a simple echo: Identity: A unique 4-digit code (called a "squawk code") assigned by ATC. For example, a flight might be told to "squawk 7341". Altitude: The aircraft's pressure altitude is included in the reply (this is called Mode C). More advanced transponders (Mode S) can provide even more data like aircraft callsign. Analogy: Instead of just shouting into the dark hall, you now have a walkie-talkie. You ask, "Who is at position X, identify yourself and state your height!" The person on the other side replies, "This is Kofi, I am at a height of 100 metres." Key Feature: It is a cooperative system. It only works if the aircraft has a functioning transponder. Advantages over PSR: Provides identity and altitude. Requires much less power from the ground station. Reduces clutter on the controller's screen, as it only shows aircraft with transponders. System 3: Automatic Dependent Surveillance-Broadcast (ADS-B) Principle: With ADS-B, the aircraft determines its own position and then tells everyone where it is. The aircraft uses its onboard Global Navigation Satellite System (GNSS), like GPS, to determine its precise position, altitude, speed, and direction. This information is automatically packaged into a digital message. The aircraft's ADS-B equipment broadcasts this message once or twice per second. Ground stations (and other ADS-B equipped aircraft) receive this broadcast. Breakdown of the Name: Automatic: No action needed from the pilot or ATC. Dependent: It *depends* on the aircraft's own navigation system (GPS). If the GPS fails, ADS-B fails. Surveillance: It provides "eyes on" information. Broadcast: It sends its information out for anyone equipped to receive it. Advantages: Extremely accurate and updates very frequently. Cheaper to implement on the ground than radar (needs small receivers, not large rotating antennas). Can provide coverage in remote areas where radar is impossible (e.g., over the ocean or mountainous terrain like the Afadjato area, if receivers are placed strategically). Allows aircraft to "see" each other (ADS-B In), improving pilot awareness. System 4: Multilateration (MLAT) Principle: MLAT pinpoints an aircraft's position by listening to its transponder signals from multiple locations. An aircraft's SSR transponder sends out signals (either replies to SSR or periodic ADS-B signals). A network of at least four simple ground-based receivers, all synchronised to the same clock, listens for these signals. The signal will arrive at each receiver at a slightly different time. This is called the Time Difference of Arrival (TDOA). A central computer analyses these tiny time differences to calculate the exact 3D position of the aircraft through hyperbolic geometry. Analogy: Imagine a lightning strike during a storm. You and three friends are standing in different places in a large field. You will all see the flash at roughly the same time, but you will hear the thunder at different times depending on your distance from the strike. By comparing when each of you heard the sound, you could calculate exactly where the lightning struck. Advantages: Very high accuracy, often better than radar. Can be a cost-effective way to provide surveillance where radar is not feasible. Can track aircraft and vehicles on the airport surface (runways and taxiways). Summary Comparison Table

| Feature | Primary Surveillance Radar (PSR) | Secondary Surveillance Radar (SSR) | Automatic Dependent Surveillance-Broadcast (ADS-B) | Multilateration (MLAT) | | :--- | :--- | :--- | :--- | :--- | | Principle | Echo-location (Reflection) | Interrogation & Reply | Self-reporting (Broadcast) | Time Difference of Arrival (TDOA) | | Cooperative? | No (Non-cooperative) | Yes (Requires Transponder) | Yes (Requires GPS & ADS-B Out) | Yes (Requires Transponder) | | Info Provided | Range, Bearing | Range, Bearing, Identity, Altitude | Precise Position, Altitude, Speed, Identity, etc. | Precise Position, Altitude, Identity | | Main Advantage | Sees everything, even without a transponder. | Provides identity and altitude data. | High accuracy, low cost for ground infrastructure. | High accuracy, works well on the ground and in difficult terrain. | | Main Disadvantage | No identity/altitude info. High power consumption. | Blind to aircraft without a working transponder. | Fails if aircraft's GPS fails. | Requires a network of receivers; complex calculations. |

Evaluation guide

• Present a summary of ground -based surveillance systems .
• Talk for Learning: Through a question and answer session , learners share views on ground -based
• conceptual
• understanding