Propagation Measurements over Geographically Diverse Paths

Abstract

High frequency communication systems have long been in widespread use in military communications. The major limitations of HF have been the frequent outage due to the dynamic characteristics of the ionosphere and the noise generated by other spectrum users and natural sources. Links employing a communication frequency assigned on a fixed schedule based on long-term predictions could expect only about 50 percent availability on the average. Newer HF systems use techniques which sense propagation conditions and adapt to them by automatically selecting a frequency which provides communications. Link availability can be increased to above 90 percent with automatic frequency selection and can be further improved through path diversity which allows the relaying of messages through an alternate set of paths if the direct ionospheric path does not provide adequate propagation. At present no quantitative data exist which allow determination of the communications reliability available to a user over a long period of time as a function of the number and geographic distribution of network members and the number of communication frequencies assigned to them. This paper describes efforts to obtain such data and presents some sample results. Initially a receive-only system was developed from low cost, readily obtainable components. Transmissions of opportunity provided the channel sounding signals, and ionospheric loss data was obtained at a number of frequencies and over a number of diverse paths in near real-time. Experimental results obtained from this system demonstrate a marked reduction in the power required to communicate when frequency and path flexibility exist.