SSR is based on the military identification friend or foe(IFF) technology …
Mode S was the obvious choice, with the S standing for select. It comprises pulses P1 and P2 from the antenna main beam to ensure that Mode-A and Mode-C transponders do not reply, followed by a long phase-modulated pulse.The ground antenna is highly directional but cannot be designed without sidelobes. It can thus give the distance and radial speed of the target with good precision but requires often one or more radars to obtain the vertical position and the actual speed. This form of All-Call interrogation is now not much used as it will continue to obtain replies from aircraft already known and give rise to unnecessary interference. Primary radar (PSR, 2.8 GHz) works usually on higher frequencies than secondary radar (SSR, 1.030 GHz). Precise knowledge of the positions of aircraft would permit a reduction in the normal procedural separation standards, which in turn promised considerable increases in the efficiency of the airways system. The process is self-defeating as increasing the reply rate only increases the interference to other users and vice versa.If two aircraft paths cross within about two miles slant range from the ground interrogator, their replies will overlap and the interference caused will make their detection difficult. The term thus refers to a radar system used to detect and localize potentially non-cooperative targets. The final 24 bits in each case is combined aircraft address and parity. It is specific to the field of air traffic control where it is opposed to the secondary radar which receives additional information from the target's transponder. The ambiguity about boresight can be resolved as there is a 180° phase change in the difference signal either side of boresight. A test is made by inverting the state of some or all of these bits (a 0 changed to a 1 or vice versa) and if the parity check now succeeds the changes are made permanent and the reply accepted. Secondary surveillance radar (SSR) is a radar system used in air traffic control (ATC), that not only detects and measures the position of aircraft, i.e. A slight change of a few feet could cross a threshold and be indicated as the next increment up and a change of 100 feet. Unlike primary radar sy However, as airspace became increasingly congested, it became important to monitor whether aircraft were not moving out of their assigned flight level. The accompanying diagram shows a conventional main or "sum" beam of an SSR antenna to which has been added a "difference" beam. The existing transponders installed in aircraft were unaffected. A third pulse, P2, is transmitted from this second beam 2 µs after P1. This is reflected by the aircraft and then picked up again by the rotating antenna on its own axis. The rapid wartime development of radar had obvious applications for air traffic control (ATC) as a means of providing continuous surveillance of air traffic disposition. The contents of these registers are filled and maintained from on-board data sources. The primary radar unit has a major quality: It works with passive echoes.
A mode-C interrogation produces an 11-pulse response (pulse D1 is not used), indicating aircraft altitude as indicated by its altimeter in 100-foot increments. It undoubtedly resulted in the delay of Mode S.A more detailed description of Mode S is given in the Eurocontrol publication A mode S interrogation comprises two 0.8 µs wide pulses,The reply also has parity and address in the final 24 bits. A five-foot vertical dimension was found to be optimum and this has become the international standard.The new Mode S system was intended to operate with just a single reply from an aircraft, a system known as monopulse. Eng., Part F, 128(1), 43–53, 1981Bowes R.C., Drouilhet P.R., Weiss H.G. In addition, it requires powerful emissions which limits its scope. Upon reaching an aircraft (or other object) the wave is reflected and some of the energy is returned to the antenna. The primary radar functions, therefore, results in detection and measurements of position if there is the presence of a target by the recognition of the useful signal.