The West Ford belt as a communications medium

A complete description of the communications properties of a dipole belt is given by its scattering function σ(τ, f), the scattering cross section of a differential volume located at propagation delay τ and Doppler shift f, evaluated over the entire common volume of the belt. A coarser description, but one which is sufficiently accurate if σ(τ, f) is well-behaved, is given by the two parameters, multipath spread L (the width of ∫σ(τ,f)df), and Doppler spread B (the width of ∫σ(τ, f)dτ). This paper describes two experiments which were performed concurrently on the test belt: 1) a propagation experiment in which σ(τ, f) was measured, and 2) a communications experiment in which digital signals designed using estimates of B and L were transmitted and the performance of the system measured. Both experiments were performed using the West Ford sites described in Nichols, et al. [4]. The results may be divided into two phases. During the first 50 days after ejection, corresponding roughly to the period of belt formation, the scattering function was measured and the communications experiment performed. In general, the scattering function was well-behaved. Digital data were transmitted at rates from 20,000 bits/sec initially, down to around 100 bits/sec. PCM voice was transmitted during the first week. The performance of the digital communications was in good agreement with the expected theoretical performance [14] of diversity signaling systems. After this first period, the belt density was generally too low to permit measurement of σ(τ,f). However, the spectrum of the signal received from the entire common volume ∫σ(τ,f)dτ was measured with results most applicable to physical belt studies [21]. During this latter period the only communications signals have been teletype.