The deficiencies in modes A and C were recognised quite early in the use of SSR and in 1967 Ullyatt published a paper and in 1969 an expanded paper, which proposed improvements to SSR to address the problems. The essence of the proposals was new interrogation and reply formats. Aircraft identity and altitude were to be included in the one reply so collation of the two data items would not be needed. To protect against errors a simple parity system was proposed – see "Secondary Surveillance Radar – Today and Tomorrow". Monopulse would be used to determine the bearing of the aircraft thereby reducing to one the number of interrogations/replies per aircraft on each scan of the antenna. Further each interrogation would be preceded by main beam pulses P1 and P2 separated by 2 µs so that transponders operating on modes A and C would take it as coming from the antenna sidelobe and not reply and not cause unnecessary FRUIT.