Linear acoustic theory predicts that a large ground-level explosion--several hundred tons of TNT should produce a pressure wave amounting to several per cent of ambient in the atmosphere 200 km above the explosion. Analysis of the interaction of such a pressure disturbance with the ionosphere implies that the passage of the wave should be readily measurable by means of vertical-incidence radio phase-path soundings. Radio measurements made at the time of a 500-ton explosion have confirmed the hypothesis. The shape of the observed phase disturbance agrees well with that predicted, and the observed onset times check to within 10 seconds with acoustic ground-to-ionosphere travel times. The amplitude and duration of the disturbances were approximately 100 radians and 2 minutes, respectively. The amplitude is slightly smaller than predicted; however, the observed duration is approximately twice that calculated. Refining and fitting the model to obtain better agreement--especially in disturbance onset time, shape, and duration--may represent a technique for improving present estimates of the upper-atmosphere density molecular weight and temperature.
Scatter from man‐made, field‐aligned, F‐layer irregularities offers a means of over‐the‐horizon communication at VHF frequencies. Experimental data collected in 44 days of field operation in the autumns of 1972 and 1973 defines representative characteristics of signals transmitted over such a scatter circuit. A cloud of field‐aligned scatterers was created by the Platteville, Colorado, ionospheric modification facility; test transmitting sites were established in Texas and receiving sites in California. The data describe mean and peak cloud radar cross section, the dependence of cross section on heater power, heater modulation, time of day, and signal frequency, the circuit coherent and incoherent bandwidths, the signal amplitude statistics and spatial correlation, and the signal spectral spreading. The experiment also included the transmission of representative types of modulated signals including SSB voice, 100‐word/min FSK teleprinter data, 2400‐bit/sec data, 30‐kHz FM voice, and 3‐kHz FSK facsimile.
The aspect sensitivity of man‐made, field‐aligned ionospheric irregularities implies that the geometry of VHF communications circuits using these scatterers must be carefully designed. The design problem is sufficiently complex that it is best approached with the aid of a computerized model. Examples are presented showing the results of path modeling for man‐made clouds located in Colorado and in Italy. The effects of variations in cloud height and communications circuit frequency are evident from these examples. It is concluded that geometric restrictions are sufficiently strong as to limit the application of the technique to certain specialized point‐to‐point or point‐to‐zone uses.
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