Meteor-Burst Link Supports Radar Net.
Indeed, aside from some criticaly important intelligence-gathering missions, the military's primary reason for existence in Alaska is related to a refinement of this theory. For more than three decades, lonely and desolate aircraft control and warning radar sites, scattered throughout this immense state, have trained their high-power energy out over the northernmost reaches of our continent and contiguous seas.
It is important to remember the substantial size of Alaska. It is one-fifth the size of the continental United States and, if superimposed over the US, would stretch from coast to coast. (See Figure 1.) Why is this important? Because of the sheer distance involved. When the factors of remote mountainous terrain and temperature extremes are added, communications challenges start to become clear.
In years past, distances were always a communicator's nemesis--that is, until tropospheric scatter came along and the problem began to ease a bit. Then came satelites, and the nemesis fell away. No longer did the military communications planner need to concern himself with distances and controlling terrain between sites.
Alaska's evolution has clearly been typical, then. It all began in May of 1900 when Congress appropriated the astronomical sum of $450,500 to build the Washington-Alaska Military Cable and Telegraph System (sometimes affectionately called WAMCATS). Alaska Links with Washington
Using technology of the day, landline (open-wire) telegraph and submarine cable systems were combined to link Alaskan military garrisons with Washington, DC, and the rest of the world.
Things didn't stop there. With the advent of wireless radio, new opportunities were opened that had not been imagined in 1900. Telegraph landlines hung on for awhile, but they soon were replaced by the "newfangled" radio business. Radio totally changed in the years prior to World War II, when spark radio telegraph gave way to high-frequency radio using vacuum-tube technology. And Congress officially renamed WAMCATS the Alaska Communications Systems (ACS).
The threat of war brought fortifications, more people, and advances in technology to ACS and the vital Territory of Alaska. An open-wire pole line was built through Canada along the Alaska Highway, reaching Tok Junction, Fairbanks and Anchorage. Between 1940 and 1944, ACS grew to a force of 2,000 men, as radio and openwire lines spread across the state. DoD Initiates White Alice
The early 1950s brought construction of the aircraft control and warning (AC&W) radar system, which required an extensive long-line communications network. To fill this function, the Department of Defense initiated the White Alice Communications System in 1955.
White Alice provided the long-haul communications to and from the remote AC&W radar sites. The network used a mix of tropo-scatter and microwave radio to serve the remote, sparsely populated areas of Alaska. The system, as could be expected, presented difficult problems of huge distances, high costs, 24-hour manning, mountainous terrain and extremely harsh climate. As satellite technology developed, the tropo-scatter ultimately became too expensive to maintain.
Faced with these substantial costs and the desire to transfer this responsibility to industry--a normal job for a public utility, certainly not a logical one for the US Air Force--the Government negotiated a contract to sell the White Alice system to Alascom (then a subsidiary of RCA). One of the stipulations of this contract was that Alascom would install satellite terminals at each of the 13 AC&W sites. Satellite communications has proven to be a better answer, and has been the final evolutionary step in Alaska's military communications history.
Although the system has been quite reliable, and certainly less costly, it is not a panacea, due to its single-threaded aspects. There's only one link into each AC&W site. This has never been a desirable situation, but recent events have caused it to become untenable. Regional Center Controls All
Up until late-September 1983, there had always been Air Force weapons controllers at each of the AC&W sites who could perform the air-defense mission on an autonomous basis, should that be necessary. With the activation of the Regional Operations Control Center (ROCC) at Elmendorf Air Force Base (located near Anchorage), this isno longer possible. All of the surveillance, identification and intercept functions are now done centrally in this new facility. This, of course, intensifies the long-standing requirement for an alternate communications system to move radar track data between the AC&W sites and the Elmendorf ROCC.
Given the remoteness of all the radar sites and the extreme disances involved, the options for solving this problem were limited. An alternative satellite system, using the Defense Satellite Communications System, was expensive, plus the availability of terminals would have meant too long a wait. Metero-burst communications technology, which has been in use for relatively low data rate applications in Alaska, seemed like a logical candidate.
Consequently, Alaskan Air Command (AAC), through the Defense Commercial Communications Office, contracted with Alaska-based Metero Data Incorporated for testing of the concept. The company believed that by providing enough power and antenna gain, a metero-burst system could utilize a sufficiently large number of the weaker meteor-created radio paths, and could successfully tansfer the Air Force's radar target data from the remote AC&W sites to the ROCC at Elmendorf AFB. The meteor-burst system could replace a failed satellite circuit, and the controllers at the ROCC would then at least be able to observe the remote northern radar presentations, in real time. Controller Directs Pilots
The Air Force had a further need, however. How would controllers communicate with interceptor aircraft and direct their pilots toward intruders? Normally, a satellite circuit carries the controller's voice signals to radio outlets that are collocated with the remote radars. But, the nature of the proposed high-powered meteor-burst communications system would not support carrying ordinary voice in real time.
As the needed vocabulary to vector the interceptors is relatively small, a short sequence of code could be transferred by meteor-burst communications to the remote radar facilities. This code could then be interpreted at the remote location and, via voice-synthesization techniques, directions could be broadcast by ground-to-air radio to the interceptors.
Under Air Force sponsorships, Meteor Data proposed a two-phase test. The first phase would prove out the metoer-burst medium and the capability of the equipment to carry data at an adequate rate for real-time radar data display.
The second phase would be a live test to include not only the metor-burst communications link and the radar equipment, but also a controller at the ROCC and actual aircraft--for a true simulation of an intercept. This phase would provide assurance that the meteor-burst concept could perform in harmony with the people and machinery of the radar defense system.
The Tin City, Alaska AC&W radar was chosen as the remote test site. In January 1983, after installation of antenna arrays, radio equipment and a controlling computer, hourly testing of the meteor-burst communications system began.
Figure 2 shows the system arrangement. At Tin City and at Anchorage, pairs of antenna arrays, for receiving and transmitting, were aimed at an area of common sky visible from both locations. The Anchorage base station transmitted "probe" signals toward Tin City for periods of 10 minutes each hour. During the short times that ionized trails in the upper atmosphere created radio paths, the controlling computer at Tin City recognized the received probe signals and responded by returning a test message to the Anchorage base station. Transmissions took place at 4,000 baud. Message Consists of 128 bits
Each test message from Tin City consisted of a short packet of 128 information bits. As shown in Figure 3, each message carried an identifying sequential number and a count of the number of seconds the message had waited for transmission. By observing the distribution of waiting times found in the sequences of received test messages, the channel capacities of the medium were assessed for various transmitted powers and times of day.
The controlling computer at Tin City generated the text field of the message--an English representation of the message number--which was used to confirm the accuracy of the received test messages. Transmitted messages were enveloped within additional sychronizing bits and a nine-bit cyclic redundancy code (CRC).
The CRC not only enabled the base and remote stations to distinguish valid messages from noise, but virtually assured the accuracy of the received messages. The Tin City equipment required an acknowledgement from the base station before sending a new message.
As designed, the test system could not attain throughout greater than five test messages per second because of the time required by the computers to complete various housekeeping tasks. The test results, however, clearly indicated that the Air Force objective of remotely displaying tracks of 10 or more aircraft in a timely manner was quite feasible. Group Conducts Second Phase
On DEcember 1, 1983, specialists from the 1931st Communications Group and Meteor Data met at the Elmendorf ROCC to conduct the second phase of the test. Instead of transferring test messages, as in the first-phase, the meteor-burst communications system now carried the live remote radar target information that was being displayed at the ROCC. The RF power at the Anchorage base station was set to 10 kilowatts; and at Tin City, to five kilowatts.
A voice synthesizer had been added to the Tin City facility. Through a keyboard, the controller at the ROCC could enter a sequence such as "80/154/10," and within seconds an interceptor pilot would hear in clear English, "direction eight-zero, range one-five-four, altitude one-zero, repeat. . . ." On that morning, two T-33 aircraft of the Alaskan Air Command's 5021st Tactical Operations Squadron were flown to within the 200-mile range of Tin City's FPS-93 radar.
Controllers and pilots practiced vectoring the aircraft by using the synthesized-voice communications feature. The military test aircraft and several incidental civil aircraft were tracked using the meteor-burst communications system.
For comparison, satellite-supported radar and telecommunications were available at an adjacent console. To those observing the practice session, it was clear that with the synthesized-voice communications and radar tracking over the meteor-burst circuit, a pilot and controller could work with nearly the same agility as with the satellite circuit.
At approximately 1300 hours, a live intercept began. The "intruder" aircraft was located approximately 100 miles east-southeast of Tin City and was flying a course of approximately 260 degrees at 210 knots at 26,000 feet. The "defender" was approximately 100 miles to the south, south-southeast of the AC&W radar. Only the "defender" could hear the synthesized-voice commands of the controller.
Following calculations performed by weapons controllers, instructions were entered via keyboard at the ROCC, and a few seconds later--using the voice synthesizer--were transmitted over the ground-to-air transmitter to the "defender" aircraft. At the ROCC display console, the defender aircraft could be seen changing course direction in response to the instructions. After several more vectoring commands, the targets merged and the defender reported visual contact with the target aircraft over the normal communications channel, and thus was credited with a completely successful intercept.
Without satellite communications, aircraft were tracked and controlled with sufficient accuracy to achieve the defense mission, and the area of single-thread communications was at an end. Operational Command Does It
Innovation has always been commonplace to pioneers of Alaska. It is an expected norm in "the last frontier." The meteor-burst test was not all that atypical under this setting, perhaps. It was quite unusual, however, for this test to be conducted by an "operational" command, as opposed to one of the more-traditional development commands.
The overwhelming success of this history-making test has paved the way for an expanded operational application of this economical transmission medium. Meteor burst is not a new technology, but the Alaskan application, with its higher powers and increased throughput, was a first-ever accomplishment.
Work is under way now to lease a meteor-burst system connecting the ROCC at Elmendorf with all 13 radar sites. The inherent characteristics of meteor burst offer some distinct advantages over the existing single-threaded and vulnerable satellite system for radar track data. Alaskan Air Command is hopeful of exploiting these advantages in the very near future.
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|Author:||Heacock, P.; Price, F.|
|Date:||Jul 1, 1984|
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