Air traffic control radar beacon system

An ATC ground station consists of two radar systems and their associated support components.

The second system is the secondary surveillance radar, or SSR, which depends on a cooperating transponder installed on the aircraft being tracked.

In a transponder based system signals drop off as the inverse square of the distance to the target, instead of the fourth power in primary radars.

Surveillance aircraft, e.g. AWACS, have only the sum and difference antennas, but can also be space stabilized by phase shifting the beam down or up when pitched or rolled.

Mode-S interrogators require the sum and difference channels to provide the monopulse capability to measure the off-boresight angle of the transponder reply.

The SSR repetitively transmits interrogations as the rotating radar antenna scans the sky.

Mode S is a discrete selective interrogation, rather than a general broadcast, that facilitates TCAS for civilian aircraft.

Mode S transponders ignore interrogations not addressed with their unique identity code, reducing channel congestion.

At a typical SSR radar installation, ATCRBS, IFF, and mode S interrogations will all be transmitted in an interlaced fashion.

Typical installations also include an altitude encoder, which is a small device connected to both the transponder and the aircraft's static system.

This is done through a rotating or scanning antenna at the radar's assigned Pulse Repetition Frequency (PRF).

Once an interrogation has been transmitted, it travels through space (at the speed of light) in the direction the antenna is pointing until an aircraft is reached.

The jitter problem makes software tracking algorithms problematic, and is the reason why monopulse was implemented.

Mode 3/A uses a P1 to P3 spacing of 8.0 μs, and is used to request the beacon code, which was assigned to the aircraft by the controller to identify it.

Replies to interrogations consist of 15 time slots, each 1.45 μs in width, encoding 12 + 1 bits of information.

When aircraft are close to the ground station, the side lobe signals are often strong enough to elicit a reply from their transponders when the antenna is not pointing at them.

By comparing the relative strengths of P2 and P1, airborne transponders can determine whether or not the antenna is pointing at the aircraft when the interrogation was received.

The new and improved SLS employs a third pulse, spaced 2μs either before P3 (a new P2 position) or after P3 (which should be called P4 and appears in the Mode S radar and TCAS specifications).

The transponder must be suppressed with a 99 percent efficiency over a received signal amplitude range between 3 db above minimum triggering level and 50 db above that level and upon receipt of properly spaced interrogations when the received amplitude of P2 is equal to or in excess of the received amplitude of P1 and spaced 2.0 ±0.15 microsecond from P3.Any requirement at the transponder to detect and act upon a P2 pulse 2μs after P1 has been removed from the new and improved TSO C74c specification.

Most "modern" transponders (manufactured since 1973) have an "SLS" circuit which suppresses reply on receipt of any two pulses in any interrogation spaced 2.0 microseconds apart that are above the MTL Minimum Triggering Level threshold of the receiver amplitude discriminator (P1->P2 or P2->P3 or P3->P4).

The FAA refers to the non-responsiveness of new and improved TSO C74c compliant transponders to Mode S compatible radars and TCAS as "The Terra Problem", and has issued Airworthiness Directives (ADs) against various transponder manufacturers, over the years, at various times on no predictable schedule.

It is highly likely that one of those software algorithms was the proximate cause of a mid-air collision recently, as one airplane was reported at showing its altitude as the pre-flight paper filed flight plan, and not the altitude assigned by the ATC controller (see the reports and observations contained in the below reference ATC Controlled Airplane Passenger Study of how radar worked).

The half split method is computed by recording the azimuth of the first and last replies from the aircraft, as the radar beam sweeps past its position.

This is calculated based on the RF phase of the aircraft reply, as determined by the sum and difference antenna elements, and is called monopulse.

This monopulse method results in superior azimuth resolution, and removes target jitter from the display.

The Mode S system also includes a more robust communications protocol, for a wider variety of information exchange.

Diversity Mode S transponders may be implemented for the purpose of improving air-to-air surveillance and communications.

This problem has worsened with the increasing prevalence of technologies like TCAS, in which individual aircraft interrogate one another to avoid collisions.

Finally, technology improvements have made transponders increasingly affordable such that today almost all aircraft are equipped with them.

[citation needed] This aspect of mode S makes it a building block for many other technologies, such as TCAS 2, Traffic Information Service (TIS), and Automatic Dependent Surveillance-Broadcast.