RP-21 ONBOARD INTERCEPTION RADAR (NATO CODE NAME: SPIN SPAN)
The MiG-21PF aircraft was the first MiG-21 version to employ airborne radar. It required the nose cone that houses the antenna to be enlarged. Another modification was the incorporation of a continuously adjusting air inlet. The MiG-21F as well as all trainer versions (21U/US/UM) had a nose cone control with only three positions (in-below M=1,5; half out between M=1.5 ...1.9, full out M=1.9+). The new nose cone control used the pressure differential within the engine to continuously and smoothly provide the best operating conditions for the turbojet engine. The RP-21 radar has the NATO code name "Spin Span".
The working principle of the RP-21 relies on a continuously transmitting antenna that is move mechanically in horizontal lines +/-30 degrees from the centerline as well as vertically +/- 10 degrees. The antenna is gyro-stabilized between +/-60 degrees of bank and +/- 40 degrees of pitch. The maximum detection range is 20km with a maximum of 10km for locking on.
Real world data against a target of the size of a MiG-21 are rather lower, i.e. 13 km and 7 km respectively. The dead range is below 900 m due to the relatively low frequency. Both, the narrow detection sector as well as the range made MiG-21 pilots rely on ground based radar guidance. Minimum target altitude was around 1200 m. Even with the antenna raised to +1.5 degrees above the horizon and reduced sensitivity (decreasing range further) ground clutter usually makes a detection of low flying targets very difficult.