Aircraft Surveillance

In Part 1 of our surveillance series, we looked into why classic Mode S transport aircraft do not broadcast their own GPS coordinates, and how ground networks rely on multilateration (MLAT) to figure out where they are.

That brings up a natural question for avionics engineers, technicians, and flight crews alike:

If an intruding aircraft isn’t broadcasting any GPS coordinates, how on earth does an onboard Traffic Alert and Collision Avoidance System (TCAS II) pinpoint its exact position in 3D space—and issue split-second, life-saving Resolution Advisories?

The answer is one of the coolest pieces of RF engineering in modern aviation: TCAS never asks for GPS coordinates. Instead, it acts as its own autonomous airborne radar, using clever antenna physics and nanosecond pulse timing to map out the surrounding sky.

The Architecture: Active Airborne Interrogation

TCAS doesn’t sit silently waiting to hear traffic report its position. Instead, the onboard TCAS transmitter-receiver acts like a miniature, airborne secondary radar:

1. The Call (1030 MHz): The TCAS computer reaches out into the surrounding airspace by sending directed interrogation pulses on 1030 MHz.
2. The Answer (1090 MHz): When the ATC transponder on a nearby aircraft hears that pulse, it instantly fires back a reply on 1090 MHz.
3. The Payload: That reply contains only two simple pieces of information:
   • Pressure Altitude: Sent straight from the target’s Air Data Computer (ADC) via Mode C or Mode S.
   • Identity: Its 4-digit squawk code or unique 24-bit Mode S ICAO address.

Notice what is completely absent: no latitude, no longitude, and no satellite fix.

How TCAS Builds a 3D Picture Out of Thin Air

Without a single coordinate in the data stream, the host TCAS computer builds a full 3D spatial target using three clever physical measurements:

                  [Target Aircraft]
                     /          ^
                    /            \  Mode C/S Altitude Reply
   1030 MHz Pulse  /              \ (1090 MHz)
                  v                \
        [Host TCAS Directional Antenna]
         - Distance: Measured by stopwatch (signal transit delay)
         - Bearing: Measured by 4-element antenna phase shifts
         - Relative Altitude: Target ADC altitude minus Own ADC altitude

1. Distance: The Speed-of-Light Stopwatch

Distance is pure time-of-flight physics. The TCAS processor starts an internal electronic stopwatch the instant it sends out the 1030 MHz pulse, and stops it the microsecond the 1090 MHz reply hits its antenna. Because radio waves travel at the speed of light, halving the round-trip travel time gives the precise slant range to the target.

2. Direction: The Directional Antenna Array

To draw that little intruder diamond on the flight deck display, TCAS has to know which direction the signal came from. It does this using a distinctive, low-profile directional antenna mounted on the crown of the fuselage.

Under the radome, this antenna houses four independent receiver elements arranged in a compact square or circle. When an incoming reply wave sweeps across the fuselage, it strikes each of the four antenna elements a fraction of a nanosecond apart.

The TCAS computer analyzes the tiny phase and amplitude differences between those four elements. By comparing how the radio wave washed over the array, the processor calculates the target’s relative bearing to within a few degrees.

3. Height: Barometric Subtraction

TCAS doesn’t care about ground elevation or GPS geometric height. It simply reads the barometric pressure altitude inside the target’s reply and subtracts its own ADC pressure altitude. If the other aircraft is reporting 24,000 feet and you are at 23,200 feet, your flight deck display shows +08 (800 feet above you). If its altitude is changing at 500 feet per minute or more, it slaps an up or down arrow next to the number.

Why TCAS Thinks in Time, Not Miles

Ask most people how collision avoidance works, and they assume TCAS draws a fixed virtual bubble around the plane—say, a 3-mile ring.

In real flight, fixed distance rings don’t work. Imagine two regional turboprops plodding toward each other at 150 knots; crossing a 3-mile gap takes plenty of time to see and avoid. Now imagine two airliners closing head-on at high altitude at a combined 900+ knots—they will devour that same 3-mile gap in less than 12 seconds.

Instead of fixed miles, TCAS thinks strictly in Tau ($\tau$): the predicted time remaining until the Closest Point of Approach.

By constantly measuring how fast the distance between the two planes is shrinking (the closure rate), TCAS sounds the alarm based on the clock, not the map:

• Traffic Advisory (TA): Triggered roughly 35 to 48 seconds before closest approach. The target turns amber, and the cockpit fills with the synthetic voice warning: “TRAFFIC, TRAFFIC.”
• Resolution Advisory (RA): Triggered roughly 20 to 35 seconds before impact. The intruder turns into a solid red square, red and green avoidance arcs light up on the VSI, and the system commands immediate vertical action: “CLIMB, CLIMB” or “MONITOR VERTICAL SPEED.”

Air-to-Air Negotiation: The Secret Coordination Loop

What prevents two approaching TCAS II-equipped aircraft from both choosing to dive, or both choosing to climb straight into each other?

When an RA is triggered between two Mode S-equipped aircraft, the pilots aren’t the only ones reacting—the two aircraft hold an automated negotiation over their 1030/1090 MHz data link:

1. The aircraft with the numerically higher 24-bit ICAO address wins priority and picks its escape maneuver first (for example, Climb).
2. It immediately fires an interrogation packet containing a Resolution Advisory Complement (RAC) across to the other plane’s transponder.
3. The opposing TCAS receives that complement message and instantly locks out the climb option; its computer commands its crew to Descend.

This machine-speed handshake happens in milliseconds, ensuring that both cockpits receive opposite, perfectly coordinated escape maneuvers without anyone touching a radio.

The Engineer’s Perspective: Line Maintenance Realities

Because TCAS relies on nanosecond timing and antenna phase angles, seemingly simple mechanical or wiring oversights can play havoc with the system:

• Matched Coaxial Cables: The four coax lines connecting the directional antenna to the TCAS computer must have strictly matched propagation delays. You can’t just splice in a random off-the-reel piece of coaxial wire during a repair; if one cable is electrically longer than the others, the phase angles get distorted, and intruder aircraft will display on the wrong bearing.
• The Mutual Suppression Bus: Transponders, TCAS units, and DMEs all yell loudly on the same frequencies. They are tied together by a mutual suppression bus that momentarily mutes other onboard receivers whenever one system transmits. A faulty suppression diode or damaged coax pulse line can cause systems to deafen each other, leading to mysterious target dropouts or spurious fault flags during takeoff roll.

Master Modern & Legacy Avionics Architectures

From classic matched-delay directional antenna feeds and Gillham altitude encoding to modern, glass-cockpit digital surveillance architectures, having solid foundational systems knowledge is what sets top technicians and maintenance managers apart:

• Working on classic regional turboprops? Explore our ATR 42/72 Series General Familiarisation Course for complete insights into classic avionics rack layouts, Air Data Computer interfaces, and legacy transponder integration.
• Maintaining next-generation commercial fleets? Enroll in our Boeing 737 MAX General Familiarisation Course to master modern DO-260B ADS-B Out architectures, integrated ADIRU data buses, and modern TCAS II v7.1 hybrid surveillance.

👈 Missed Part 1? Read Part 1: Transponders, Surveillance Evolution, and Understanding Flight Tracking Discrepancies to learn how Mode S ELS works and why ground tracking networks experience MLAT ghost tracks.