Split range calibration in pressure measurement of re-entry flush air data sensing system (FADS) for overall system accuracy enhancement

Flush Air Data Sensing System (FADS) forms mission critical sub system in 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 such as angle of attack, angle of sideslip, Mach number, 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 overall system accuracy of FADS is mainly dictated by the accuracy of the surface pressure measurements. The demanded accuracies are of the order of 100 Pascal in a full scale of 140000 Pascal (0.07% FS). This paper addresses the four split range pressure calibration scheme that is used for achieving the above accuracy. Trade off studies carried out on the number of split ranges and the order of least squares curve fit to be used are presented. The scheme for voltage based split range selection to reduce the errors at range boundary crossover points and during recovery after a port blockage is presented. Details of the hysteresis logic used for real time implementation for preventing back and forth switching of scale factors at the split range crossover points are also presented. The use of split range calibration and hysteresis logic for pressure sensors in FADS has not been reported in earlier literature. Experimental results are presented which establishes the demanded pressure measurement accuracy.