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PMTC Multipath Film Creates Worldwide Attention in the Communications Field.

Most everyone, especially crows, knows that a straight line is the shortest distance between two points. But what do you do if you are transmitting microwave data great distances over water and the local climate does not cooperate?

This was the problem encountered at the Pacific Missile Test Center (PMTC), Point Mugu, in its communications with San Nicolas Island, Santa Criuz Island and Vandenberg Air Force Base a few years ago.

As a result of working with this problem and the methods used to overcome it, a film called "PMTC Multipath" was made which created worldwide attention in the communications field.

Jim Kaness (Ventura), formerly senior engineering project manager in Range Development, recalls how in 1978 it was decided to encrypt the microwave system handling these communications. Kaness comments, "Of course, we have had conventional analog microwave at PMTC for 20 years. We have known in the course of that time because of the long over-water paths involved and the way the islands are positioned, the microwave propagation was less than ideal. In fact it was pretty bad at times."

"In deciding how to implement a bulk-encrypted microwave system we had to build a digital microwave system. In terms of technology, state of the art was not as well developed to handle difficult propagation as it was for analog," says Kaness.

Kaness adds, "As a consequence of that project we decided to study the propagation to determine just what was happening and what we were dealing with in order to be better able to understand what techniques were necessary to overcome our problem. We hired Bob Hubbard from the Institute of Telecommunications Sciences in Boulder, Colorado, who is a nationally known expert in the field of radio propagation to come here and we feel we learned quite a bit."

According to Kaness, he learned that the whole problem in the Southern California coastal area is caused by the well-known temperature inversion layer that exists quite a bit of the time. This is one of the regions of the world where this phenomenon is pronounced, Kaness points out.

Kaness adds, "Among other things, this layer is responsible for the temperate climate. Besides making the coastal area overcast during the summer months, it is responsible for much of the Los Angeles smog, by putting a lid on the Los Angeles basin and preventing the pollutants from escaping. It is responsible for our poor radio signals at times, particularly in the summer months. It's a condition, of course, about which man can do nothing as far as we know. We are stuck with it and the name of the game is how do we live withit."

Now head of the Instrumentation Data Transmission System at Range Operations, Kaness produced the "PMTC Multipath" film. He says in any radio transmission system a transmitter talks to a receiver.

Kaness explains, "Classically, the path is a straight line between the transmitting antenna and the receiving antenna. Ideally, if that's all that happened, everything would work perfectly.

In the case here, we have some energy which is not sprayed in a straight line; it's being sprayed at different angles. Some of that energy will go up to this temperature inversion boundary in the atmosphere and get bounced back.

Multipath, as a term, means that there is more than just one path between the two antennas for the energy to travel. "It is known to be responsible for the fading in high frequency radio back in the 1920s. It is the phenomenon that is responsible for the ghosts in television pictures that we are familiar with and it is also responsible for the distortion in some FM broadcasts," explains Kaness.

According to Kaness, it is basically a case where you not only receive the desired signal, but you receive the signal delayed a bit in time. The signal has not only come the shortest path which is a straight line between the two antennas, but also arrives over the longer path and interferes with itself. When the signal bounces off buildings, mountains, atmospheric anomalies or aircraft, multipath can occur.

The results of Hubbard's testing indicated there was some hope the equipment could be made to work. Results of Data "On the Fence"

Kaness continues, "We went to Rockwell and bought two terminals of their digital radio equipment to test on the path. We had some conflicting opinions from them so we installed the terminals ourselves and gathered data on the path from Laguna Peak to San Nicola Island. The results of the data were sort of 'on the fence'. It wasn't good enough to stop worrying about and it wasn't bad enough to give up on. We proceeded to design and construct the rest of the system around the equipment with the understanding and expectation that we would do further work and figure out how to improve on the reliability."

At about this time, a number of visitors to Point Mugu trudged up to Laguna Peak to watch how the radio was behaving or misbehaving in the presence of the multipath. Some days people went up the hill and there was nothing to see. Everything was just classically perfect. "We were looking for a way to be able to show this to people without having to take them up the hill and risking their not having anything to see," states Kaness.

Kaness adds, "At that time, we were invited to talk with some people at Scott Air Force Base in Illinois. They were working with the Digital Radio and Multiplex Acquisition program of the Defense Communication Agency. The program's purpose is to provide encrypted microwave systems throughout Europe in support of NATO. In order to document the effect of multipath, we felt one way to do this would be to make a film of the effect that it had on the signal. We had been watching these effects with a spectrum analyzer attached to the radio for some time but it was always in real time and never recorded for anybody."

In October of 1980 Kaness took a motion picture cameraman up to the peak with all of his film equipment, including five boxes and a tripod. At the same time the road to Laguna Peak was closed due to a landslide, so everything had to be shipped up on the back road. According to Kaness, they picked a day at random to go up there and he had never seen such severe multipath effects going on in nearly two years of observations. Exposures Turn Out Absolutely Perfect

"Because of light meter problems involved in filming off an oscilloscope, there was no way to get a correct exposure for the film, but I realized that we would never get a better picture. The exposure turned out to be absolutely perfect," states Kaness.

The film was shown at Scott Air Force Base and several other places. As a result, Kaness was invited to present it at the International Telecommunications Conference in December 1981 and this exposure helped to stimulate worldwide interest which ensued.

"At the time of our studies the entire microwave communications industry was being rocked with the problem of where these distortions were coming from and what to do about them. We were not alone with the problem, but here at PMTC we had a job to do and we had to find a way to successfully do it, and we had the worst path to study," adds Kaness.

Kaness adds, "Although our original injunction from the command five years ago was to bulk encrypt the microwave system, it has resulted in our having a leadership position in this area. Another benefit to the Department of Defense, of course, is that there is a large quantity of this equipment throughout the world, so solving the problem here helps solve it for the rest of the Department of Defense."
COPYRIGHT 1984 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1984 Gale, Cengage Learning. All rights reserved.

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Author:Byrne, K.
Publication:Communications News
Date:Jan 1, 1984
Words:1311
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