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

In August 1990 we participated in the Equatorial Ionospheric Studies soundi ng rocket campaign near Kwajalein Atoll in the equatorial Pacific region. T he 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 transmi tters and receivers was 700 km; the ionospheric-reflection region was at 10 .18 degrees N, 168.40 degrees E, near the magnetic equator. We made three t ypes of measurements: Doppler spread and spatial coherence for a single-fre quency CW path; frequency coherence of multiple CW paths; and Doppler sprea d 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 thi s 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 signa l power) and greater than 15 Hz at the -30 dB level. Forty percent of the e venings showed spatial coherence distance of less than 180 m in the directi on normal to the bistatic path; 40% of the evenings showed spatial coherenc e of less than 75 m in the direction parallel to the path. Seventy-five per cent of the evenings showed coherence bandwidths of less than 1.5 kHz.