Monopulse SSR

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The Monopulse Secondary Surveillance Radar or MSSR is an improved version of the classic SSR of the 50s. In the middle of the 70s engineers tried to avoid some standard problems of SSR. In particular Garbling and the False Replies Unsynchronized with the Interrogation Transmissions or simply FRUIT. Garbling was happening when flights close to each other were sending very narrowly spaced replies to the SSR and its decoder was unable to detect them separately. The FRUIT was the result of many SSRs working in the same area where the reply from a flight due to the interrogation of one was also received by another SSR that has not yet sent out an interrogation for this flight. Both problems resulted in loss of the aircraft position producing inaccuracies.

At the end of the 80s the SSR antenna was modified to the LVA (Large Vertical Aperture) type were a series of many dipoles independently read the reply from a flight and the radar was then calculating the received difference in strength and phase (delay) of each one. The mathematical result was able to calculate and resolve simultaneous replies from various flights with a directional angle difference of some 0,5 degrees whereas the classic SSR could not see the difference within an angle of some 3 degrees. In actual fact the mathematicam model was able to calculate the flight position using a single pulse of the many (maximum 15 possible) in the reply signal. This provided the term Mono.

The first effect was that Garbling was minimized to a very low level until it and became insignificant in multi-radar environments as no garbling can appear at the same moment to many different and distant SSRs. FRUIT was also reduced because the need for very few reply pulses to detect a flight allowed now for a lower rate of interrogations. Once the interrogations were limited however, FRUIT was diminished on its turn as less replies were produced together with the probability for FRUIT to happen. The MSSR replaced most of the existing SSRs by the 90s and its accuracy provided for a reduction of separation minima in en-route ATC from 10 nautical miles (19 km) to 5 nautical miles (9.3 km).

Additionally the MSSR is the only radar that can be used for the Mode-S channel (S stands for Selective Interrogations)