A method for accurate estimation of altitude in re-entry vehicles using flush air data sensing system (FADS)

Flush Air Data sensing System (FADS) forms a mission critical sub system for future reusable space transportation systems. FADS makes use of surface pressure measurements from the nose cap of the vehicle for deriving the air data parameters of the vehicle like angle of attack, angle of sideslip, Mach number, Altitude, etc. These parameters are used by the control and guidance system of the vehicle for reentry flight management as well as for subsequent mission phases upto landing. The altitude information is used in re-entry vehicles for gain scheduling, terminal area energy management and for homing the vehicle to the runway for landing. The FADS algorithm computes free stream static pressure from the surface pressure measurements made at the nose-cap. This paper addresses the computation of altitude from the free stream static pressure computed by FADS algorithm. Using the International Standard Atmosphere, hydrostatic equation, perfect gas law and temperature lapse rate equation, estimate of the vehicle altitude is obtained. The accuracy of the estimated altitude is of the order of 55 meter upto an altitude of 2 km. For higher altitudes of 5 km 10 km and 15 km, the accuracy of 235 meters, 565 meters and 695 meters is achieved respectively. Simulation results show that the proposed method is suitable for real time implementation.