The second system is the secondary surveillance radar, or SSR. The SSR is equipped with a main antenna, and an omnidirectional "Omni" antenna at many older sites. Newer antennas (like the picture to the right), are grouped as a left and right antenna, and each side connects to a hybrid device which combines the signals into sum and difference channels. Still other sites have both the sum and difference antenna, and an Omni antenna. 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 banking. The SSR beam antenna is typically fitted to the PSR antenna, so that they point in the same direction as the antennas rotate. The omnidirectional antenna is mounted near and high, and usually on top of the radome if equipped. 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 scans the sky. The interrogation specifies what type of information a replying transponder should send by using a system of modes. There have been a number of modes used historically, but four are in common use today: mode 1 mode 2 mode 3/A, and mode C. Mode 1 is used by sort military targets during phases of a mission. Mode 2 is used to identify military aircraft missions. Mode 3/A is used to identify each aircraft in the radar's coverage area. Mode C is used to request an aircraft's altitude.
Neither Mode 4 nor mode S are part of the ATCRBS system but use the same transmit and receive hardware. Mode 4 is used by military aircraft for the Identification Friend or Foe (IFF) system. Mode S is a discrete selective interrogation rather than a general broadcast, that facilitates TCAS for civil 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.
Returns from both radars at the ground station are transmitted to the ATC facility using a microwave link, a coaxial link, or (with newer radars) a digitizer and a modem. Once received at the ATC facility, a computer system known as a radar data processor associates the reply information with the proper primary target and displays it next to the target on the radar scope.
Typical installations also include an altitude encoder, which is a small device connected to both the transponder and the aircraft's static system. It provides the aircraft's pressure altitude to the transponder, so that it may relay the information to the ATC facility.
The transponder has a small required set of controls and is simple to operate. It has a method to enter the four-digit transponder code, also known as a beacon code or squawk code, and a control to transmit an ident, which is done at the controller's request (see SPI pulse below). Transponders typically have 4 operating modes: Off, Standby, On (Mode-A), and Alt (Mode-C). On and Alt mode differ only in that the On mode inhibits transmitting any altitude information. Standby mode allows the unit to remain powered and warmed up but inhibits any replies, since many transponders incorporate transmitters which must be warmed up before they will function.
Mode 3/A uses a P1 to P3 spacing of 8.0μs, and is used to request the beacon code, which was assigned to the aircraft by the controller to identify it. Mode C uses a spacing of 21μs, and requests the aircraft's pressure altitude, provided by the altitude encoder. Mode 2 uses a spacing of 5μs and requests the aircraft to transmit its Military identification code. The latter is only assigned to Military aircraft and so only a small percentage of aircraft actually reply to a mode 2 interrogation.
F1 C1 A1 C2 A2 C4 A4 X B1 D1 B2 D2 B4 D4 F2 SPI
The F1 and F2 pulses are framing pulses, and are always transmitted by the aircraft transponder. They are used by the interrogator to identify legitimate replies. These are spaced 20.3μs apart.
The A4, A2, A1, B4, B2, B1, C4, C2, C1, D4, D2, D1 pulses constitute the "information" contained in the reply. These bits are used in different ways for each interrogation mode.
For mode A, each digit in the transponder code (A, B, C, or D) may be a number from zero to seven. These octal digits are transmitted as groups of three pulses each, the A slots reserved for the first digit, B for the second, and so on.
In a mode C reply, the altitude is encoded by a Gillham interface, which uses gray code. The Gillham interface is capable of representing a wide range of altitudes, in 100-foot increments. The altitude transmitted is pressure altitude, and corrected for altimeter setting at the ATC facility. If no encoder is attached, the transponder may optionally transmit only framing pulses (most modern transponders do).
In a mode 2 reply, the information is similar to the mode A reply in that there are 4 digits transmitted between 0 and 7. The mode 2 reply differs from the mode A reply in that the transmitted code is assigned by a military air traffic controller, not a civilian air traffic controller.
The X bit is currently only used for test targets. This bit was originally transmitted by BOMARC missiles that were used as air launched test targets. This bit may be used by drone aircraft.
The SPI pulse is positioned 4.35μs past the F2 pulse (3 time slots) and is used as a "Special Identification Pulse". The SPI pulse is turned on by the "identity control" on the transponder in the aircraft cockpit when requested by air traffic control. The air traffic controller can request the pilot to ident, and when the identity control is activated, the SPI bit will be added to the reply for about 20 seconds (two to four rotations of the interrogator antenna) thereby highlighting the track on the controllers display.
To combat these effects, side lobe suppression (SLS) is used. SLS employs a third pulse, P2, spaced 2μs after P1. This pulse is transmitted from the omnidirectional antenna (or the antenna difference channel) by the ground station, rather than from the directional antenna (or the sum channel). The power output from the omnidirectional antenna is calibrated so that, when received by an aircraft, the P2 pulse is stronger than either P1 or P3, except when the directional antenna is pointing directly at the aircraft. 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 power to the difference antenna pattern (for systems so equipped) is not adjusted from that of the P1 and P3 pulses. Algorithms are used in the ground receivers to delete replies on the edge of the two beam patterns.
To combat these effects more recently, side lobe suppression (SLS) is still used, but differently. 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). This pulse is transmitted from the directional antenna at the ground station, and the power output of this pulse is the same strength as the P1 and P3 pulses. The action to be taken is specified in the new and improved C74c as:
2.6 Decoding Performance. c. Side-lobe Suppression. The transponder must be suppressed for a period of 35 ±10 microseconds following receipt of a pulse pair of proper spacing and suppression action must be capable of being reinitiated for the full duration within 2 microseconds after the end of any suppression period. 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 descriminator (P1->P2 or P2->P3 or P3->P4). This approach was used to comply with the original C74c and but also complies with the provisions of the new and improved C74c.
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. The ghosting and ring around problems have recurred on the more modern radars.
To combat these effects most recently, great emphasis is placed upon software solutions. 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). See the reference section below for errors in performance standards for ATCRBS transponders in the US.
See the reference section below for FAA Technician Study of in-situ transponders.
The beacon code and altitude were historically displayed verbatim on the radar scope next to the target, however modernization has extended the radar data processor with a flight data processor, or FDP. The FDP automatically assigns beacon codes to flight plans, and when that beacon code is received from an aircraft, the computer can associate it with flight plan information to display immediately useful data, such as aircraft callsign, the aircraft's next navigational fix, assigned and current altitude, etc. near the target in a data block.
Mode S, despite being called a replacement transponder system for ATCRBS, is actually a data packet protocol which can be used to augment ATCRBS transponder positioning equipment (radar and TCAS). Mode S is designed to fully interface with ATCRBS systems: mode S SSRs can interrogate ATCRBS transponders, and AIS-P transponders will also reply to older ATCRBS and TCAS interrogations.
The system also has provisions for transferring arbitrary data both to and from a transponder. 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.