| Summary: | The Federal Aviation Administration has recently mandated the installation of transponders that provide position reporting (Extended Mode S) in aircraft operating in most types of domestic controlled airspace by January 1, 2020. The resulting proliferation of aircraft transponder data has accelerated the potential for the use of such data in measuring operations counts at nontowered airports, as it may be easily collected with an inexpensive receiver and analyzed with appropriate algorithms. While many of the data (Basic Mode S and Mode C) do not include aircraft position information, the portion of Extended Mode S data that do may be used to directly compute the distance of the corresponding aircraft from a receiver located at an airport of interest. This article describes a method by which these computed distances may be utilized to calibrate an adaptive digital filter that can subsequently estimate distances for the remainder of the transponder records that do not provide position information. The digital filter is a combined first-order Butterworth low-pass filter and a Rayleigh maximum likelihood estimator for the signal variance. The resulting distance estimates from two different antenna installations exhibited median absolute deviations of 0.92 and 1.12 nm per transponder record, respectively, within 5.0 nm of the receiver. These accuracies are sufficient for the estimation of aircraft operations counts at nontowered airports.
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