Litton Industries' Aero Products division has received a $35 million contract to retrofit Northwest Airlines' inertial navigation systems installed on 33 Northwest B747 and 21 Northwest DC-10-40 aircraft. Litton will install its strapdown Ring Laser Gyro LTN-92 INS, which replaces spinning mass gyro systems installed in more than 10,000 commercial and military aircraft. It features the inertial navigation unit, the control/display unit and the mode select unit. Three 28-cm ring laser gyros and three force-balanced accelerometers make up the instrument cluster. Northwest Airlines is currently flying Litton's LTN-92 on its DC-10-30 fleet of commercial aircraft.