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Estimation of Aircraft Operations at Airports Using Mode-C Signal Strength Information

An understanding of aircraft operations counts at airports is important due to the use of that information in the allocation of the Federal Airport Improvement Program funds. Operations at airports lacking full-time personnel are typically sampled over time. The sample sizes are generally small, due to the inherent difficulty and expense of deploying acoustic or pneumatic counting devices for extended periods of time, leading to inaccuracies in the estimation of long-term totals. A means of counting operations using Mode S extended squitter (ES) aircraft transponder data received with a 1090-MHz software-defined radio (SDR) system has been developed by the authors. A limitation of this approach, however, is the relatively small ratio of Mode S ES signals to Mode S short squit and Mode C signals, which do not contain aircraft position information. This article presents a method for estimating distance information from the latter two types of transponder signals, enabling them to be used by the SDR-based device, along with Mode S ES signals, in the operations counting process. The distance estimation method described here is based on an adaptive Kalman filter incorporating parameter optimization using known distance information from Mode S ES signals and results in an average error of 0.83 nm in measurements within a 5-nm radius of the receiver. This level of uncertainty enables the use of Mode S short squit and Mode C signals to count aircraft operations at airports without the additional overhead of multilateration.