Propagation of an electromagnetic groundwave over a plane surface in which the ar.gument of the surface impedance is greater than -rr/4 but less than -rr/2 is considered in some detail. The numerical distance, p, over such a surface is characterized by 0,;; arg p,;; -rr/2. The groundwave behaves in a peculiar manner, and this is attributed to the interaction of two phasor components representing a trapped wave and a Norton surface wave. Approximate expressions are derived which determine the magnitude and phase of the groundwave attenuation function when these two waves are inphase and antiphase. A. method is also given for estimating the asymptotic phase for large jpj which was previously not possible except through detailed calculations. Finally, detailed curves are presented which show the amplitude and phase of the groundwave attenuation function versus p.

The objective is to calculate the electromagnetic field of a dipole located over a flat inhomogeneous ground whose upper layer has a variable thickness. Although a formally exact derivation for a lossy dielectric wedge model is available, its complexity prohibits immediate application. Instead, using a geometrical-optical technique, an approximate expression is developed for the effective surface impedance at the ground surface. This method takes full account of multiple reflections within the wedge region but neglects scattering from the wedge apex. Thus, the resulting surface impedance formula is valid everywhere except near the apex of the wedge. With this point kept in mind, the compensation theorem is employed to derive an integral equation for the electromagnetic field produced by a dipole located above this model of a nonparallel stratified ground. Using an appropriate reference field, the resulting integral equation of the Volterra type is solved by an iterative method. The excellent convergence of this procedure is demonstrated for a particular model. This solution yields quantitative results for the ground-wave attenuation function. In the case of a parallel stratified ground, it reduces to earlier known results.

The directional dependence of the calculated surface impedance on a nonparallel stratified conductor is pointed out. In spite of this dependence, the reciprocity theorem is not violated when the mutual coupling between two dipoles is considered.

The ability to access very large data sets in seconds rather than hours or days is key to an efficient analysis procedure. We have developed a high performance robotic storage and retrieval system plus an innovative data management technique which provides this capability and is now in operation at user sites. Our EMASS (E-Systems Modular Automated Storage System) achieves this high performance with a combination of state-of-the-art technology. Helical-scan digital recorders, each having a sustained transfer rate of 1 5 megabytes/second and a burst transfer rate of 20 megabytes/second are used to store data, and many recorders can be accessed concurrently to achieve the desired transfer rate. Commercially available, outof-box D2 high density magnetic tape is used for archiving data, and this 19 mm tape is available in three cassette sizes having capacities of 25, 75 or 165 gigabytes, respectively. Cassettes are stored in modular robotic systems whose capacities can be expanded from a few terabytes to thousands of terabytes. EMASS software controls all aspects of data management including commanding the robots to move cassettes between their storage locations and the recorders. Performance is matched to each user' s needs by tailoring the number of recorders, the size and configuration of the robotic archive, and the number of robots for the user's environment.1 . AUTOMATED STORAGE MANAGEMENT EMASS provides storage management at two basic levels, volume serving and file serving. When the user has an existing file management system and only desires to automate the volume retrieval process, we provide volume serving. The request for a volume is sent to a volume server processor which is responsible for executing the commands required to locate, retrieve, and mount the proper tape volume in a tape transport where it is then read from or written to by the user's application. When the user desires the complete system, we supply file serving which incorporates volume serving as a part of the overall process. File serving supports user's requests for data on a file name basis. The user need only request the desired file, and the file serving application handles all activities associated with the file retrieval such as a volume mount request, copying a file from tape to disk, and data recovery processes.The EMASS product line addresses the high performance requirements of both file and volume serving for Concurrent, Convex, and Cray computing environments. EMASS accomplishes this through a combination of advancements in UNIX software applications, magnetic tape media, tape drives, and robotic technology. Figure 1 depicts the architecture in which an EMASS volume server supports main line computing environments. Our volume server software allows the client to control any combination of the three robotic archives shown in Figure 1 , and it also supports manual archiving. Figure 2 shows a high performance file storage server which can support many users. The file server software lets all users store or access their files via...

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