Indirect atmospheric measurements utilizing rake tropospheric scatter techniques—Part I: The rake tropospheric scatter technique

Abstract

Unusually detailed multipath resolution of the tropospheric-scatter medium has been obtained by incorporating Rake instrumentation into tropospheric transhorizon microwave experiments. Radio-frequency carriers at approximately 900 MHz, each binary phase-shift keyed at 10 megabauds by a pseudorandom sequence of digits and subsequently filtered to a 10-MHz bandwidth, were transmitted during separate experiments in February and in November and December, 1965, over two paths--a 480-km overland path near the eastern coast of the United States and a 614-km over-water path in the Caribbean. At the receiver an identical binary stream was used in a series of 10 cross-correlators (Rake taps) where each stream was delayed 0.1 µs with respect to the stream in the preceding tap. This provided a multipath resolution of 0.1 µs that, depending upon the path length and altitude, corresponded to a separation of the common volume into ellipsoidal shells varying in thickness from about 200 to 550 meters, The principal goal of the experiments was to record the randomly varying in-phase and quadrature components of each tap output and thus investigate the fluctuations in signal phase, as well as in amplitude, caused by fading. The test procedures also permitted multipath structure and Doppler shifts to be directly observed. Scattering functions, which clearly illustrate the distribution of received signal energy as a function of multipath time delay and Doppler frequency shift, were calculated. In contrast to a multipath time-delay spread that was ordinarily less than 1 µs for the overwater path, spreads of over 3 µs were common for the shorter, overland path. Typical fading rates and Doppler spreads were also markedly different--several fades per second over land versus one fade per several seconds over water. Although some of the observed variations can be attributed to differences in antenna size (8.5-meter diameter for over land, 18-meter diameter for over water) and pointing angle, others apparently result from different typical refractive structures (meteorological conditions) within the atmospheric common volume of the respective radio links. Doppler shifts of as much as 15 Hz, which probably resulted from off-great-circle antenna alignment, were observed on the overland but not the overwater path.