Radiolocation using AM broadcast signals: The role of signal propagation irregularities

We have previously demonstrated that carrier phase measurements of the ubiquitous AM broadcast signals can be used for instantaneous radiolocation. The positioning accuracy, however, depends on how well the signal propagation characteristics can be modeled. In this paper, the signal propagation issues relevant to radiolocation are reviewed and empirical data on phase perturbation are presented. An AM radiolocation system, though not nearly as susceptible as GPS to tropospheric, ionospheric, and multipath errors, has its own set of challenges that include: ground property fluctuation, skywave interference, phase perturbation caused by nearby conductors, and phase variation caused by directional transmitter antennas. In this paper, the effects of these error sources are quantified using experimental results and two models are developed and evaluated: a ground model and a directional transmitter antenna model. Many AM stations employ multi-element directional antennas to maximize SNR to a market while minimizing interference with distant stations. Because even moderate baseline AM navigation can involve significantly varying lines-of-sight to some transmitters, a model is developed to account for the phase variation caused by this effect. The effectiveness of this model is demonstrated using experimental results. AM broadcast signals have wavelengths between 175 and 550 meters. For radiolocation purposes, the useful mode of propagation for signals with these wavelengths is groundwave, the speed of which depends on the electrical properties of the ground. Ground properties vary with time at a given place mainly due to groundwater fluctuations. In most areas, however, ground properties are constant enough to make modeling productive. A groundwave propagation model is developed and shown to significantly improve positioning performance over a wide area. At night when the D-layer of the ionosphere recombines, signals in the AM band reflect off the ionosphere enabling so-called skywave propagation. Because of the uncertain geometry of the reflection, the skywave component of a received signal is difficult to account for. To quantify this effect, the results of similar experiments, conducted at noon and at midnight, are compared. Many things we encounter in our modern world perturb measurements of AM carrier phase. Overhead and underground utilities are particularly insidious because they consist of vast interconnected conductor networks. The phase perturbations caused by the collection of conductors that are typically hung from power poles are experimentally quantified.