Results of measurements performed on HF backscatter for evaluating the influence on short range HF data links

On short range HF links, where the receiving station is relatively close to the transmitting station, reception of long delayed echoes (up to 20 ms) may occur due to backscatter. These backscatter signals will interfere with signals received through the legitimate path, which is either groundwave of nearly vertically incident skywave. Even on low data rate links this time dispersion of signals may cause intersymbol interference. In order to assess the influence of HF backscatter on short range HF data links, measurements were performed. Path delays were measured using a direct-sequence spread-spectrum technique. A carrier, BPSK (Binary Phase Shift Keying) modulated with a pseudo-noise (PN) sequence, was transmitted and the received signal was correlated with the same PN sequence. Measurements were taken in the Netherlands and Norway. The measurement results show that especially on the short range link, long delayed echoes appear. The backscatter effect depends on propagation conditions, thus on frequency, time of day, season, and solar activity.