Coordination Critical for Temporary Earth Stations.
While the overwhelming majority of temporary satellite links are for video coverage of sporting events, more and more news events are reported via temporary satellite transmissions. Temporary earth station suppliers have risen to the demands of responding to unscheduled, fast-breaking news stories.
Nor are video coverage of news and sports events the only temporary uses for satellites. There's a steady demand for temporary uplinks for live audio coverage of concerts and other happenings. Temporary video teleconferencing remains a small but fast-growing area. Temporary teleconferencing generally employs one-way video and two-way audio, appropriate for speeches and similar events. Full two-way temporary video (with a temporary earth station at bath ends) can be achieved, but the complexity of setting up two coordinated paths on short notice limits its use.
A surprising aspect of this booming industry is that many temporary uplinks share the C-band frequencies used by the telephone common carriers. To prevent interference, every temporary uplink must be coordinated in advance with existing microwave users. Frequency-coordination service companies such as Comsearch have succeeded in shortening coordination lead times from months to days. In part, this is possible because the earth stations are not licensed for permanent transmission: frequencies reserved for future expansion of systems are open for temporary use. Another reason is the system of short-term coordination among the common carriers and the temporary operators' frequency coordinators.
Two Types of Authorization
Under current FCC practice, there are two ways that temporary satellite links are authorized. The first is under Developmental Authority (Section 25.390 of the FCC rules). In 1981, Western Tele-Communications Incorporated (WTCI) applied for and received authority to operate its transportable 4.5-meter C-band transmitters using a streamlined approval procedure without prior formal notification and coordination with common carriers using the same frequencies. Instead, WTCI conducts an in-house interference analysis and then notifies the FCC engineer in charge and the local carriers in the area of operations.
The company received its original grant of authority for a year, and it has been renewed yearly thereafter. A few other carriers received Developmental Authority grants, but after the first few such approvals, the FCC became swamped with applications and consequently has made few additional grants.
The alternative is Special Temporary Authority (STA). This is obtained by a letter application to the FCC stating the purpose of the link and including a frequency-coordination package, a supplemental showing of carriers contacted and an adjacent satellite analysis. The FCC requests three days to process such an STA request. If this procedure sounds time-consuming, it is. What has evolved instead is a Blanket STA request, which in practice works much the same way as Developmental Authority, in that the same information is provided, but the term of the blanket STA authority is limited to six months or less.
An additional complication in the coordination process is that the vendors of temporary services are also negotiating for transponder time while they're trying to find a transmission site. The choice of transponder directly affects the potential interference cases. What has evolved is an interactive process that begins with the temporary vendor's selection of a tentative site and a transponder. If the site can be cleared by changing transponders (changing frequency, in other words) that's most often the easiest way to go.
If frequency shift does not clear all the cases of potential interference (as often happens in major cities), the next approach is to find natural or man-made shielding for the transmitter. Here we have the fortunate coincidence that crowded downtown areas also tend to have plenty of nearby buildings for shielding. For sporting events, the stadiums themselves or their parking garages provide the needed protection. If additional protection is needed, it can sometimes be found in a semi-underground loading dock area or other area that shields the interference without blocking the line of sight to the required satellite. In rare cases where time permits, a metal fence can be erected.
Lest the process be made to sound overly straightforward, note that frequency coordination requires latitude and longitude of the site to the nearest second (approximately 100 feet). It is not enough to specify, for instance, "the parking lot of RFK Stadium": the frequency coordinator needs to identify a specific point in the parking lot, often by translating information from a street map to a topographic map. Once the site is chosen, it may not be easy for the crew of the transportable truck to determine the coordinated location to the required one-second precision. Moreover, the site selected as suitable from an interference standpoint may not have available three-phase power, ordinary phone lines for control purposes and maneuvering from fro the truck. Additional coordination may be required to establish a site that meets all of these requirements.
In a typical example, a temporary uplink was ordered for the Chicago Bears' training camp in Santa Rosa, California, prior to the NFC Championship game. The order was called in on December 31, for installation on January 2. The order included a site and a transponder--in this case, transponder 10X on Westar V at 6.325 GHz. A run of the proposed site against the computerized data base at Comsearch turned up nine interference cases, of which five could be eliminated immediately because of frequency offset. On the other hand, three of the remaining four were on high bills in three different directions. Unfortunately, only one building was available for shielding, making the proposed link doubtful.
A call to the temporary supplier generated an alternative transponder choice--transponder 3H on Telstar 301 at 6.045 GHz. all of the five potential interference cases at this frequency were eliminated: one had enough frequency separation, one was not operating, and three had sufficient terrain blockage, as determined by digital terrain data in the Comsearch data base. Once the site and transponder choices were confirmed, the engineer coordinated the temporary site with the terrestrial microwave carriers (in this case, AT&T, Pacific Bell and GTE) and made courtesy calls to Western Union and the other major carriers' representatives. The temporary uplink was approved and on the air by January 2.
Obviously, the cooperation of AT&T and local telephone companies is an essential part of the process. These companies have an understandable concern about maintaining the integrity of their microwave paths. Yet they've very cooperative, particularly where news developments call for fast response. In a few instances, they may agree simply to monitor potential interference indicated "by the numbers," knowing that the equations used for predicting interference have a conservative bias built in. Such a response is not the norm; but overall, the telcos' tough-but-fair attitude makes the temporary business possible.
The increasing use of temporary satellites is creating some need for further guidelines. The newly formed National Spectrum Managers Association, which had its organizational meeting last August, set up a working group to address the problems raised by "short-fuse" temporary uplinks. William McDaniel, frequency coordinator for Southern Bell and chairman of the working group, sees the problem as one cutting across industry lines. "We need to involve the major interest groups--the broadcasters, the transportable earth station operators, the frequency-coordination companies, the telephone carriers and the satellite owners."
He sees the problem as one requiring cooperation from all groups to minimize last-minute coordinations. Part of the problem is institutional: "By the time a program director tells the communications manager; the manager calls the temporary supplier, the supplier calls the frequency coordinator and the coordinator calls one of the carriers, there may be little or no lead time left," he explains.
McDaniel notes that there have been many cases of suspected interference, as when several microwave paths fail simultaneously at a time when a temporary uplink is known to be operating. However, before such a case can be confirmed by field measurement, the temporary is usually off the air.
One of the Suggested Solutions
One of the solutions suggested is that of coordinating "envelopes" near popular earth stations sites, such as stadiums. This would not be a universal solution, but it might reduce the number of rush requests for coverage of sporting events--the major use of temporary uplinks.
As the use of temporaries continues to grow along with the use of common carrier microwave systems, technological solutions are emerging to provide additional siting flexibility. One of these is using short microwave links from a site back to an uplink. Videocom Incorporated (Dedham, Massachusetts) operates two trucks with 13-GHz microwave links for use in tight locations (see photo above). The microwave transmitters operate on slightly offset frequencies so that both trucks can operate at the same location if required. While the signal will carry up to six miles, Frank Cavallo, director of telecommunications for Videocom, says that ordinarily a quarter-mile or less is far enough to find a quite site.
Interference with terrestrial microwave systems is only half the challenge of temporaries. Because of the high power and small diameter of the temporary dish, adjacent satellite interference is an additional concern. The frequency-coordinating firm supplies as azimuth and elevation to the temporary operator, who must then accurately point to dish under less than ideal conditions.
This already difficult job will become more difficult with the introduction of two-degree spacing of C-band satellites. Several antenna manufacturers, such as Scientific Atlanta, Comtech and Andrew, have earth stations suitable for temporary use that meet the two-degree rules of the FCC. Nevertheless, the use of these antennas will require more time and care on the part of temporary operators to achieve accurate pointing that avoids adjacent satellite interference.
As with many features of satellite communication, it would have been difficult to foresee the present scope of diversity of temporary satellite installations as little as 10 years ago. Yet they continue to operate successfully while sharing one of the most critical of terrestrial microwave frequencies. This is due in large measure to the cooperation of the engineers among the common carriers and the coordination companies who maintain the frequency-coordination system.
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|Author:||Crutcher, T.; Prives, D.|
|Date:||Mar 1, 1985|
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