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A technique that enhances the performance of meteor-burst communications is described. The technique, the feedback adaptive variable rate (FAVR) system, maintains a feedback channel that allows the transmitted bit rate to mimic the time behavior of the received power so that a constant bit energy is maintained. This results in a constant probability of bit error in each transmitted bit. Experimentally determined meteor-burst channel characteristics and FAVR system simulation results are presented. >

Modern meteor burst communications systems do not make maximum use of all available trails. Moreover, throughput drops when the interarrival time is minimum due to overlapping trails. By combining high resolution direction finding techniques with classical beamforming techniques, it is possible to prevent loss of throughput during overlapping trail periods and raise the average bit rate, resulting in a net gain in throughput. The general problems that must be solved to implement this approach are discussed, and a few of the specific combinations under investigation are examined. >

Data acquired with the Geophysics Laboratory's high-latitude meteor scatter testbed between Sondrestrom Air Force Base (AFB) and Thule AFB, Greenland, during the solar disturbances of March and August 1989 are presented. These events provided an opportunity to observe the effects of a number of naturally occurring disturbances on meteor scatter links operating in the frequency range 35 to 147 MHz. The two events presented are very different. The August event was dominated by ionospheric absorption, which affected meteor arrival rates and duty cycles primarily at the lower frequencies (35 and 45 MHz), although some effects could also be seen at the higher frequencies (65 to 147 MHz). The March event combined weak ionospheric absorption with large solar noise bursts. The effects of this event on the testbed were dominated by increased solar noise at all frequencies. The relative influence of solar noise and ionospheric absorption during the events is discussed. >

The performance of a high rate projection code used with a trellis coded FM system operating in a meteor burst channel is described. Meteor burst communication has wide ranging applications, usually limited to low bit rates. The system described has been developed for the USAF Alaskan Air Command and can transmit at bit rates up to 90 kbit/s in a 40 kHz bandwidth. The projection code is a geometrical code which yields very large coding gain with relatively low complexity. Results are presented to illustrate the benefits derived by the use of the projection code. >

High resolution measurements have been carried out to characterize the time spread properties of overdense meteor trails. A number of other parameters of interest for adaptive communication systems have been examined for both underdense and overdense trails. It has been found that the time spread in the overdense trails is not severe for data rates up to 250-500 kbit/s. For the underdense trails, the exponential decay constant shows great variations, and a great number of the trails have significant risetime. >

The use of adaptive data rates in the meteor-burst communications environment is investigated. Measured results obtained from a number of meteor links are presented and compared with previous theoretical predictions. The contribution of various meteor trail families to throughput capacity are also investigated. The results show that the use of adaptive data rates can significantly increase the throughput capacity of meteor-burst communication systems. The greatest rate of increase in throughput with increase in operating rate is found at low operating rates. This finding has been confirmed for a variety of links and days. Reasonable correspondence is obtained between the predicted modified overdense model and the observed results. Overdense trails, in particular two trail types within the overdense family, are shown to dominate adaptive data throughput. >

In recent years a renewed interest has been shown in the possibility of using meteor burst links in tactical communications, both for networking and covert operations. Some of the applications that recent performance improvements would permit are evaluated. In evaluating the feasibility of a meteor burst implementation, certain technical and physical limitations are addressed. For the success of these applications, interoperability with other communication systems is necessary. The level of interoperability with other media, and the standards necessary to assure this interoperability are examined. Methods of minimizing and combating jamming are proposed. Meteor burst systems can be used in a large number of applications within a tactical environment. The principal disadvantage of the meteor burst medium is the problem of interference to other spectrum users from the probe end, and the interference from other users at the receiver end. The low throughput characteristic of meteor burst compares with some of the channel capacities used in other systems. Interoperability with other networks or communications links is relatively easy if certain straightforward protocols and standards are established. >

A meteor burst link/network simulator that has been developed to facilitate the testing of the next-generation meteor scatter communication equipment and terminals is described. With the capability to interface to 15 terminals configured into an arbitrary network with up to 30 independent links, the simulator will have significant flexibility to test a variety of network architectures. In addition, the 400 kHz simulator bandwidth provides transparency to different modulations at data rates from 75 b/s to 128 kb/s independently of waveform design. The design of the simulator, including the channel modeling, the multiplexing of the hardware to serve all desired links, and the simulator control, is discussed. A number of the potential applications of the simulator in future testing of modems, protocols, terminals. and networks are presented. >