Boeing is currently using a real-time Microwave Airplane Position System MAPS for noise certification testing. The system measures ranges and range rates from several ground transponders to an airplane, and computes the airplane position using a Kalman Filter algorithm. Airplane position relative to a fixed earth coordinate system is available for recording and cockpit display 5 times per sec. Comparisons of position data from MAPS with data from Boeing photo-theodolite systems show that MAPS accuracy is better than two meters when the airplane is within the design envelope of the ground transponder array. Unfortunately, the minimum practical altitude of a design envelope is 50 meters, so MAPS cannot be used foor takeoff and landing tests. To bridge the gap from the ground to 50 meters altitude, an alternate source of altitude information is required. The problems of obtaining absolute altitude information from other sources are summarized. Radio altimeters give excellent altitude information when over a runway, pressure altimeters give an excellent indication of a local change in altitude, and an IRU is an excellent source of vertical speed data. This report examines the inclusion of these alternate sources of altitude data in the Kalman Filter used by MAPS.