Primary Surveillance Radar (PSR)
PSR is classical radar — pulse out, echo back, plot range and bearing. It sees any aircraft whether the aircraft cooperates or not. The downside: PSR has no idea who the aircraft is or how high it's flying. The famous green sweep on a controller's screen mostly comes from PSR.
Secondary Surveillance Radar (SSR)
SSR is the civilian descendant of military IFF. The radar transmits an interrogation pulse. The aircraft's transponder replies with a four-digit squawk code and (in Mode C) its altitude. The controller now knows the aircraft is American Airlines 1187 at flight level 350 — without having to read it off the radar return, which only shows a dot.
ADS-B — the next replacement
Automatic Dependent Surveillance-Broadcast lets each aircraft transmit its own GPS-derived position, altitude, velocity and identity twice per second. Ground stations and other aircraft receive it directly — no radar required. ADS-B has been mandatory in most controlled airspace since 2020 and gives the controller a much richer picture than radar alone, especially for low-altitude flight where radar coverage is poor.
Why the radar still spins
ADS-B depends on the aircraft choosing to broadcast accurate data. Primary radar doesn't. As long as some aircraft are old, lying about position, or hostile, the rotating dish at every major airport stays a non-negotiable backup. Modern ATC uses all three — PSR, SSR, ADS-B — fused into a single track per aircraft.
The Precision Approach Radar (PAR) Exception
While standard surveillance radars sweep the horizon to manage high-altitude traffic, Precision Approach Radar (PAR) operates at short ranges to assist during the most critical phase of flight: landing. Developed during World War II and refined through the 1950s, PAR systems utilize two separate antennas to track an aircraft's precise glide path and centerline alignment. Unlike the broad rotation of terminal radars, PAR scanners oscillate rapidly over a narrow sector of the approach path, providing high-resolution data update rates of up to one per second. This allows controllers to provide 'talk-down' guidance to pilots when visibility is near zero, effectively steering the aircraft onto the runway without automated landing systems.
The technical superiority of PAR lies in its use of X-band frequencies (8 to 12 GHz), which provide significantly higher resolution than the L-band or S-band frequencies used for en-route surveillance. However, these higher frequencies are more susceptible to atmospheric attenuation and rain fade, which is why they are reserved for short-range applications. In the modern era, while GPS and ILS have become the standard for commercial aviation, PAR remains a vital fallback in military aviation and specialized civilian airports. It serves as a ground-based verification system that does not rely on the aircraft's onboard navigation sensors, offering a redundant layer of safety when electronic interference or equipment failure compromises standard cockpit instruments.
Moving Target Indication and Clutter Suppression
The greatest challenge for Primary Surveillance Radar is not finding the aircraft, but filtering out everything else. Ground clutter—returns from buildings, hills, and even waves—can saturate a display, making a small aircraft invisible. To solve this, engineers utilize Moving Target Indication (MTI) based on the Doppler effect. By measuring the phase shift between successive pulses, the radar processor distinguishes between stationary objects and moving aircraft. If the radial velocity of an object is zero relative to the radar, the return is discarded as noise. This signal processing occurs at the nanosecond level, ensuring the controller sees a clean screen rather than a chaotic map of geological features and urban infrastructure.
A common misconception is that weather and terrain are the only obstacles to radar clarity. In reality, the advent of wind farms has created a modern technical hurdle for ATC systems. The rotating blades of a wind turbine create their own Doppler shift, which can mimic the signature of a moving aircraft or create 'blind zones' where MTI filters cannot differentiate between the turbine and a plane. Current mitigation strategies involve 'gap filler' radars and advanced software algorithms that use multi-static processing—analyzing signals from multiple angles—to determine if a moving return has a flight profile or is simply a stationary mast with spinning blades. This technical evolution ensures that renewable energy projects do not compromise the integrity of the national airspace.
The Beacon Delay and the Slant Range Constraint
A critical technical detail in SSR operation is the transponder turnaround time. When an SSR ground station interrogates an aircraft, the onboard transponder does not reply instantly; it introduces a standardized delay of 3 microseconds. Radar processing computers must subtract this constant from the total round-trip time to calculate the true distance. Without this correction, every aircraft would appear approximately 450 meters further away than its actual position, a discrepancy that could prove disastrous during tight terminal sequencing where separation standards are strictly enforced by the controller.
Furthermore, radar measures 'slant range'—the direct line-of-sight distance—rather than the ground distance. For an aircraft flying at 35,000 feet directly over or near a radar site, the reported range can be significantly greater than its horizontal position on a map. Modern digital processing systems use Pythagorean geometry to reconcile the Mode C altitude data with the slant range, projecting the result onto a two-dimensional grid. This 'range-offset' correction ensures that controllers see a flat, accurate map of the sky regardless of an aircraft's vertical proximity to the antenna site.
SSR Interrogation Modes and Fruit Interference
The evolution of SSR is defined by its Interrogation Modes. Standard Mode A provides identity via the squawk code, while Mode C adds pressure altitude. However, as airspace density increased, these modes suffered from 'Fruit' (False Replies Unsynchronized In Time). Fruit occurs when one radar station receives a reply triggered by a different station's interrogation. This creates 'phantom' targets or visual noise on the display. To combat this, Mode S (Select) was introduced, giving every airframe a unique 24-bit address. This allows the radar to interrogate specific aircraft individually, drastically reducing signal congestion and improving data integrity in busy corridors.
Before the digital transition of the 1990s, controllers relied on 'Analog Video' processing, where raw pulses were displayed directly on cathode ray tubes. This required the use of 'Passive Decoders'—hardware units that would physically highlight a specific pulse train if it matched the squawk code the controller was looking for. Today, this is entirely automated. The 'monopulse' technique used in modern SSR antennas can determine an aircraft's bearing from a single reply with incredible precision, whereas older systems required several hits over a 3-degree beamwidth to estimate the center of the target's position.