The method of passive beam formation using a four-element Butler matrix to improve the signal availability of meteor scatter communication systems is investigated. Signal availability, defined as the integrated time that the signal-to-noise ratio (snr) exceeds some sf•r threshold, serves as an important indicator of system performance. Butler matrix signal availability is compared with the performance of a single four-element Yagi reference system using --6.5 hours of data from a 720 km north-south temperate latitude link. The signal availability improvement factor of the Butler matrix is found to range between 1.6-1.8 over the snr threshold range of 20-30 dB in a 300-Hz bandwidth. Experimental values of the Butler matrix signal availability improvement factor are compared with analytical predictions. The experimental values show an expected snr threshold dependency with a dramatic increase at high snr. A theoretical analysis is developed to describe this increase. The signal availability can be further improved by --10-20% in a system employing two four-element Butler matrices with squinted beams so as to illuminate the sky with eight high-gain beams. Space diversity is found to increase the signal availability of a single antenna system by 15%, but the technique has very little advantage in a system already employing passive beam formation.One strategy to overcome the low data throughput focuses on improved error coding. Fixed rate forward error correction (FEC) coding offers an improvement of--2•% in the data throughput over uncoded systems [Miller and Milsrein, 1990]. Further slight improvements can be made by using more complex variable-rate coding schemes [Pursley and Sandber•, 1989], but significant advances are constrained by the limited bandwidth of most M$C systems. An alternative technique is •o employ variable data transmission rates, whereby the data rate profile mimics, in some fashion, the decay profile of the trail snr. Such a scheme takes advantage of the Published in 1996 by the American Geophysical Union. Paper number 95RS03258. relatively high snr at the beginning of the trail [e.g., Weitzen et al., 1984; Davidovici and Kanterakis, 1993]. Shukla et al. [1992] and Cannon et al.[1993] investigated increasing the trail duration, and thereby the data throughput, by using antenna space diversity. This technique takes advantage of the fact that toward the end of a trail, and particularly in the case of long-duration trails, the incoming wavefront is no longer planar because of interference between decorrelated signals. Shukla et al. [1992] found that -40% of all trails with duration >0.75 s showed significant signs of spatial decorrelation over antenna separations of 10A. Further breakdown of trail statistics into underdense, overdense, and not-known categories showed that the underdense and not-known categories did not benefit from increasing the antenna separation above 5A. The overdense trails, however, required a separation of 20A before signal decorrelation was apparent. Cannon et al. [1993] ...

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Abstract. A computer model to predict the underdense meteor arrival rate over a forward meteor scatter communications link is presented. The model incorporates important effects such as major shower streams, a nonuniform radiant distribution, and antenna polarization coupling. A particularly useful aspect of the model is its capacity to predict the passage of sporadic and shower hotspot regions across the sky and thereby provide directional information to drive an adaptive beam steering system. Directional data from a phased array reception system in the United Kingdom has been used to determine the diurnal arrival distribution of sporadic meteors. This diurnal variation is broadly reproduced by the model and suggests that just two daily changes in the direction of a high-gain beam would offer considerable advantage over a fixed beam system. This paper shows that further improvements in system performance can be achieved with accurate predictions of the time of appearance and location of shower streams. A comparative study between monthly predictions and experimental data from a highlatitude link in Greenland over a year shows reasonable agreement but highlights the need for higher-resolution radiant data.