The History of Radar | Radar History: Isle of Wight Radar During The Second World War | Radar: The Basic Principle
Radar Technology: Main Components | Radar Technology: Side Lobe Suppression | Radar Technology: Airborne Collision Avoidance
Radar Technology: Antennas | Radar Technology: Antenna Beam Shapes | Radar Technology: Monopulse Antennas | Radar Technology: Phased Array Antennas | Radar Technology: Continuous Wave Radar | Theoretical Basics: The Radar Equation
Theoretical Basics: Ambiguous Measurements | Theoretical Basics: Signals and Range Resolution
Theoretical Basics: Ambiguity And The Influence of PRFs | Theoretical Basics: Signal Processing | Civilian Radars: Police Radar | Civilian Radars: Automotive Radar | Civilian Radars: Primary and Secondary Radar
Civilian Radars: Synthetic Aperture Radar (SAR) | Military Applications: Overview | Military Radars: Over The Horizon (OTH) Radar
How a Bat's Sensor Works | Low Probability of Intercept (LPI) Radar | Electronic Combat: Overview | Electronic Combat in Wildlife
Radar Countermeasures: Range Gate Pull-Off | Radar Countermeasures: Inverse Gain Jamming | Advanced Electronic Countermeasures

With the development of SSR¹ in the mid-1990s, the ability to transmit data on radar and the reduced workload on transponders gave rise to the Airborne Collision Avoidance System. It is known as ACAS is Europe but in America where it was largely developed it is known as TCAS (Traffic-alert and Collision Avoidance System). It does not mean Air Traffic Controllers (whose job it is to keep aircraft apart) are redundant, but is a last resort if all else fails.

The Way it Works

Mode S transponders transmit their aircraft's identity at regular intervals. When this is received by a Mode S transponder on another aircraft, it logs the identity and signal strength. If subsequent transmissions from the same aircraft are weaker it takes no action, but if the signal is getting stronger, it starts to take an interest in that aircraft. It will then send out an 'interrogation' in order to glean more information. The reply will contain height information, which the home transponder compares with its own height. If there is enough vertical separation, again it takes no action.

If the aircraft is at a similar altitude the system becomes very interested. By measuring the time interval between transmission and reply, it will determine the distance between the two aircraft. From subsequent interrogations its closing speed can be determined. When the two aircraft are just 40 seconds apart, the system will issue a traffic alert to warn the pilots that there is another aircraft in close proximity.

By this time the two aircrafts' ACAS units will be 'talking' to each other at very frequent intervals to decide on a coordinated course of action. At 25 seconds apart the ACAS units will issue instructions to both pilots, telling one to climb and the other to decend. If there is any height separation at all, the lower one will be instructed to descend and the other, higher aircraft, to climb. In the unlikely event that the two aircraft are at exactly the same level the ACAS units will still nominate one to descend and one to climb, thus avoiding collision (if at least one of the pilots follow the instructions).

History: The History of Radar | Isle of Wight Radar During WWII
Technology: Basic Principle | Main Components | Signal Processing | Antennae | Side Lobe Suppression | Phased Array Antennae | Antenna Beam Shapes | Monopulse Antennae | Continuous Wave Radar
Theoretical Basics: The Radar Equation | Ambiguous Measurements | Signals and Range Resolution | Ambiguity and PRFs
Civilian Applications: Police Radar | Automotive Radar | Primary and Secondary Radar | Airborne Collision Avoidance | Synthetic Aperture Radar
Military Applications: Overview | Over The Horizon | Low Probability of Intercept | How a Bat's Sensor Works
Electronic Combat: Overview | Electronic Combat in Wildlife | Range Gate Pull-Off | Inverse Gain Jamming | Advanced ECM | How Stealth Works | Stealth Aircraft