Mode S Technology
Sidelobe SuppressionSidelobe suppression has been implemented in many radar systems, including ATCRBS, to solve a common problem. This problem occurs due to signal leakage in the directional antenna. When interrogation signals are transmitted with a directional antenna, the signal often leaks through the sides of the antenna. These signals are called sidelobe signals. Aircraft flying close to the antenna respond to the sidelobe signals, causing interference on the channel.
The general solution to this problem uses an additional pulse to provide additional information to receiving aircraft. Under this system, the interrogation signal consists of two pulses, P1 and P2. The first pulse P1 is sent by the directional antenna. A second, weaker pulse P2 is immediately sent out after P1 by the omnidirectional antenna.
If a plane within the interrogation signal coverage receives the signal, P1 is significantly stronger than P2. Transponders receiving signals compare the first two pulses in the transmission, and reply only if P1 was significantly stronger than P2. Transponders in aircraft flying near the antenna in the sidelobes receive signals with P1 approximately equal to P2. The transponder is thus temporarily disabled, or ``suppressed'' for approximately 35 microseconds, preventing them from replying to the leaked interrogation signal.
The Mode S waveform was designed so that it began with a signal of two equivalent pulses, the same signal that caused ATCRBS transponders to be suppressed. While the transponders are temporarily disabled, the remainder of the Mode S data can be transmitted with the knowledge that any ATCRBS transponders would be suppressed and hence would not hear the signal.