In-flight pitot-static calibration

#1. Field of the Invention

#This invention relates to in-flight calibration of aircraft pitot-static systems. More specifically, the invention models pressure error as a continuous function of airspeed rather than computing error for discrete airspeeds. High data-rate measurements of static and differential pressure and Global Positioning System (GPS)-based ground speed measurements are utilized for computing pressure errors over a range of airspeed.

#2. Description of the Related Art

#FIG. 1 is a schematic view of the calibration process;

#FIG. 2 is a flowchart providing further details of the pressure error modeling and output-error;

#FIG. 3 illustrates various pressure error models;

#Referring now to the drawings, wherein like reference numerals refer to like components, and more particularly to FIG. 1, a calibration system 10 in accordance with an embodiment of the present invention is shown. High rate measurements of impact pressure 102 and static pressure 104 are acquired and recorded continuously for a continuous flight pattern within set range boundaries throughout the speed range of interest. The test aircraft is outfitted with pitot-static probes 106 that are plumbed to onboard differential pressure transducers 108 to acquire static and differential pressure measurements. A suitable sample rate, such as 50 Hz, is used. The sample rate is determined by the data system, generally ranging from about 10 Hz to about 200 Hz. It is to be noted that 50 Hz is not a limitation of the present invention, as other suitable rates can be used. The transducer output is converted 110 from analog to digital signal for calibration processing 112.

#The calibration process 112, as illustrated in FIG. 2 and further illustrated via the block diagram 20 of FIG. 2, involves integration of direct measurements of ground speed received from a global positioning system 114, atmospheric conditions 116 (such as air temperature and density), and differential pressure from the pitot-static probe 106. The algorithm 112 computes the pressure error model 118 as a function of indicated airspeed and optimizes 120 the model parameters based on comparisons 124 of the measured impact pressure with the estimated impact pressure. Using Output-Error Optimization 120, the model parameters are automatically adjusted 122 to minimize the error between the measured impact pressure and the estimated impact, pressure from the model 118. Pressure error model 118 output is computed 126 with statistical boundaries.

#Referring to FIG. 2, block diagram 20 further illustrates the algorithm used for the calibration. Input, comprising the differential pressure (qci) of the pitot-static probe 106, GPS 114 produced ground speed, and atmospheric conditions is received by the selected pressure error model 118 and its defined equations 206. The error of predicted qci is then minimized 208. System identification methods, using output-error optimization 120, are used to generate the continuous pressure error function over the range of airspeed tested. An optimization algorithm from the System Identification Programs for AirCraft (SIDPAC) software can be used, such as the output error routine (OE) described in Klein, Vladislav and Morelli, Eugene A; Aircraft System Identification, Theory and Practice, AIAA Education Series, 2006, incorporated herein by reference in its entirety. The present invention is not limited to this output error routine, as other suitable routines may also be used. Pressure error versus airspeed is computed 210 and calibrated with statistical boundaries. Δp/qc, wind speed Vw and wind direction ψw are output 212.