SSR Polar Diagram RNAV
For the SSRSSR —Secondary Surveillance Radar interrogator to determine where the aircraft is, it must know which direction it is sending the signal that the aircraft is responding to. Therefore, the P1 and P3 interrogations are directional, the same as primary radar. The P2 or SLSSLS —Side Lobe Suppression pulse is omnidirectional but used for a different purpose other than tracking the aircraft.
However, the aircraft needs to be able to receive from all directions as well as reply in all directions (as it does not know where the SSRSSR —Secondary Surveillance Radar beacon is!) Therefore, the aircraft receives and transmits omnidirectionally.