Some information :
MiG-31 Foxhound
The MiG-31 long-range interceptor was developed from the MiG-25. The two-seat MiG-31 fighter has more capable equipment, including the powerful 'Zaslon' phased array radar with a range of 200 km. It is claimed that an unit of MiG-31 can link their radars together, to establish a search pattern -- covering a width of 800-900 km with four aircraft, spaced at 200 km.
MiG-31M Foxhound B
Under development since 1984, the MiG-31M, a substantially improved MiG-31, will most likely not see service. Only six prototypes have been built, and none have been ordered, although it was originally scheduled to enter service in the mid-1990s. The MiG-31M can carry six under-fuselage missiles in three columns. It can carry the R-37, a development of the R-33 (AA-9 'Amos'), and the R-77 (AA-12 'Adder'). A fully-retractable IRST and a new 1.4m-diameter Phazotron phased-array radar, which can simultaneously engage six targets, are used. The redesigned rear cockpit has three color CRT MFDs. The MiG-31M also has a one-piece canopy and windscreen, a bulged, wider dorsal spine for more fuel, no gun, and uprated engines. Aerodynamic refinements include redesigned LERXes for better high AoA handling, larger curved fin root fillets, and smaller wing fences. It also has a larger brake parachute housing, a retractable IFR probe on the starbord side (as opposed to on the port side like on the standatd MiG-31), rounder wingtips with front and rear dielectric panels, and the nosewheel landing gear is redesigned. Its maximum takeoff weight is raised to 52,000 kg (114,537 lb). One has been observed with large finned wingtip ESM pods. The MiG-31D is a version with the "Foxhound-A's" 1.1m-diameter radar. Converted MiG-31Ds are called MiG-31BS
(info from : http://home.iae.nl/users/wbergmns/jets.htm)
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