Abstract :
The results of a year´s observations in southern England of long-range back-scatter are analysed. A pulse transmitter coupled to alternative directional aerials was used, and transmissions were made at noon each day on a number of frequencies between 10 and 27 Mc/s. The echo patterns observed in winter were simple and were formed by echoes from the ground just beyond the skip distances for one- and two-hop F2-layer propagation. In summer, however, ground echoes returning by way of the Es, E and F1 layers arrived nearly simultaneously and were difficult to distinguish. The marked increase in Es ionization in summer could be seen clearly from the echo patterns. Examples of fixed frequency range-time recordings (p´t) of backscatter are given. On one record echoes can be seen at ranges corresponding to each of the ground reflection points for four-hop F2 propagation between England and Malaya. The application of the p´t technique to the direct measurement of maximum usable frequencies is described. To assess the accuracy of the back-scatter technique of skip-distance measurement, comparisons were made between measured scatter ranges and ranges calculated from vertical-incidence ionosphere measurements. The measured ranges were consistently shorter than those calculated. It is uncertain whether this discrepancy arises from approximations in the theory used for the calculations from vertical-incidence data or from inadequate directivity in the aerials used in the scatter measurements. Comparisons of scatter ranges with direct measurements of m.u.f. over the same path are considered desirable to resolve this uncertainty.
