RP - 21 Sapfir The RP - 21 " Sapfir " ( NATO codename : " Spin Scan ) " was a radar system , developed by the Soviet Union for use in the updated MiG - 21PF fighter , and later versions of the MiG - 21 . The early MiG - 21 fighter variants , MiG - 21F and MiG - 21F - 13 , were clear - weather daylight - only fighters . Although they were in use with the Soviet Air Force Frontal Aviation ( VVS FA ) , due to their limitations they were unsuited for the interception tasks of the Soviet Air Defence Forces ( PVO ) . The Mikoyan OKB started development of a more sophisticated interceptor , based on the MiG - 21F - 13 in the late 1950s . The MiG - 21P and MiG - 21PF were the first MiG - 21 ' s to be equipped with a real radar that would enable them to search , track and intercept targets by night and in foul weather : the RP - 21 " Sapfir " ( ' Sapphire ' ) radar , which was given the NATO codename of " Spin Scan - A . " The RP - 21 " Sapfir " replaced the MiG - 21F - 13 ' s SRD - 5M " Kvantum " ( ' Quantum ' ) ranging - and gun - radar . RP - 21 featured a mechanically steered and gyroscopic stabilised radar dish , and had a thermionic valve ( vacuum tube ) circuit . In theory it was able to detect fighter - sized targets from a range of 20 km , and lock on to them at a range of 10 km though in practice this got reduced to 13 km and 7 km respectively . The associated weapon was the Vympel K - 13 infrared guided air - to - air missile , also known as R - 3S , object 310 or AA - 2 " Atoll - A " by NATO , already in use with the older SRD - 5M radar . Later on , newer versions of the aircraft ( namely , MiG - 21 PFS ) used the further upgraded RP - 21M radar ( NATO codename " Spin Scan - B " ) which allowed the semi - active radar homing ( SARH ) missile , the new R - 3R ( NATO codename AA - 2 " Atoll - B " ) . Though it was a great improvement over the older SRD - 5M , the new intercepting capabilities were still limited , mostly due to an inherent design flaw of the MiG - 21 ; the nose inlet limited the size of the small radar cone , so the radar field was limited to 20° vertically and 60° horizontally . Even in the latest Chinese F - 7MG copies of the MiG - 21 ( equipped with more advanced Israeli and Italian radars * source ? ) this is still said to be a significant limiting factor for radar use . Also , because of the vacuum tube electronics , the radar system ( like most other early generation radars ) had a low mean time between failures ( MTBF ) . The radar lacked look - down / shoot - down capabilities ( though this was not at all a common feature of radar systems at the time ) , meaning that it couldn ' t intercept targets that were flying under the MiG - 21 , because the radar system was unable to filter out " ground clutter " return . Another disadvantage of the new radar ( and other avionics ) was the weight increase in the MiG - 21 PF , which reduced the baseline MiG - 21 ' s otherwise impressive flight characteristics . In its defence , the radar was simple to use and repair and , because of vacuum tube technology , was able to withstand a powerful electro - magnetic pulse ( EMP ) , a side - effect from nuclear explosion , radiation harmless to men but very damaging to modern solid state ( silicon transistor ) electronics . Its capabilities , as limited as they may seem , were offset by the standard PVO doctrine : pilots were tied to a ground - controlled interception ( GCI ) system , which , through ground - based radars and data links , provided interceptors more extensive and more precise information . Over the years , the West became more acquainted the RP - 21 Sapfir during the Vietnam war in Indochina during the Six - Day and the Yom Kippur War s in the Middle East . Its combat record is not impressive , mostly due to the primitive and overestimated early generation R - 3R radar guided missiles . Also , the radar was rather quickly compromised because of defections , and its weaknesses were well known to the west , giving way for the creation of effective electronic countermeasures ( ECM ) . Western pilots learned to distinguish the RP - 21 ' s rhythmic three tone pulse . Still , the RP - 21 and its developments , like the RP - 22 radar ( NATO codename " Jay Bird " ) of the later MiG - 21bis series , had a long career and the latter was even used for export purposes in downgraded MiG - 23S and MiG - 23MS interceptors .