HF (High Frequency) Absolute Time of Arrival Sensing

In late 1980 questions arose concerning whether the ionosphere was sufficiently stable to allow precisely measured time of arrival of skywave signals to be used for geolocation in the high frequency (HF) band between 2 and 32 MHz. The chief limitation in the accuracy of this type of systems is the amount of uncertainty in the ionospheric height estimation and its temporal stability. Traditional ionospheric research resources did not address the issue in sufficient detail and time resolution to be of any assistance. In order to understand the exact nature of the ionospheric uncertainties and to quantify their extent, experimentation was proposed to sense the variation in the refraction height of the ionosphere as it relates to the time of arrival of the HF signal. The objective of this work was to determine the range of environmentally induced errors in a skywave Time Difference of Arrival (TDOA) measurement, thereby bounding the ultimate geolocation accuracy one could expect from this technique. The first experimental measurement system started operation in early 1981. This effort involved establishing a continuous absolute Time of Arrival (TOA) experiment over the one-hop midlatitude path between San Diego, California and Fort Collins, Colorado. The system is fully digital and stabilized with a cesium beam standard. This work was supplemented with vertical incidence sounder data at both ends of the path, a collateral Doppler sensing system, and coincident satellite solar data. A fully annotated database was prepared and is maintained by the Naval Ocean Systems Center Advanced Propagation Forecasting System (PROPHET).