Equatorial spread F effects on an HF path: Doppier spread, spatial coherence, and frequency coherence

Abstract

In August 1990 we participated in the Equatorial Ionospheric Studies sounding rocket campaign near Kwajalein Atoll in the equatorial Pacific region. The campaign included measurements of plasma density using rocket probes and coherent and incoherent scatter radar. During the campaign we fielded high‐frequency ionospheric sounders over a bistatic path between Maloelap Atoll and Bikini Atoll in the Marshall Islands. The distance between the transmitters and receivers was 700 km; the ionospheric‐reflection region was at 10.18°N, 168.40°E, near the magnetic equator. We made three types of measurements: Doppler spread and spatial coherence for a single‐frequency CW path; frequency coherence of multiple CW paths; and Doppler spread and time‐delay spread for a 60‐kHz bandwidth path. We obtained such data over a period of 2 weeks for approximately 2 hours each evening; during this period spread F was common. Fifty percent of the evenings showed Doppler spread of greater than 6 Hz at the −10 dB level (relative to the peak signal power) and greater than 15 Hz at the −30 dB level. Forty percent of the evenings showed spatial coherence distance of less than 180 m in the direction normal to the bistatic path; 40% of the evenings showed spatial coherence of less than 75 m in the direction parallel to the path. Seventy‐five percent of the evenings showed coherence bandwidths of less than 1.5 kHz.