An Earth-Station Planner's Primer.
The planning required to send and receive these satellite signals properly is detailed and intricate. The concept of planning an earth station, simply put, is to transmit and receive signals to and from specific satellites while avoiding interfering with, or receiving signal-degrading interference from, another communications facility.
The intent of this article is to relate some of the typical phases that the planning of an earth station goes through, whether by my firm or another. At with other technologies, once the planning process becomes clear, one gains a greater appreciation of the effort required to make an earth station operational.
The planning of an earth station starts with determining that there's need for a communications facility between two or more points. When the need is best satisfied by the use of satellite technology (instead of terrestrial microwave, fiber optics or telco leased lines), a potential site is located.
The site location can be done in one of two fashions. The company that intends to use the earth station can specify a site location to an engineering consultant and see if that site "clears" during a preliminary computer analysis (discussed below). Or, the company can simply state the need for a site somewhere within the general area of its facility and request that the consultant locate a workable site.
It's Imperative to Refer to a Data Base
In order to properly plan to avoid interfering with (or receiving interference from) other communications facilities that share the radio spectrum with satellite transmissions, it's imperative to know where these other facilities eixst, what radio channels they're utilzing, and other critical data.
All engineers who plan earth-station or terrestrial facilities must reference information about other users of the radio spectrum from a comprehensive data base. Such a data base is typically constructed so that a computer can detect, by geodetic coordinates, which other systems are within the potential-interference radius of the planned site and also share the frequency the earth station intends to use.
In C-band installations (6-GHz uplink, 4-GHz downlink), the problem of potential interference is highly accentuated as a result of sharing these frequencies with terrestrial users. In other words, where a transmitting earth station is placed could cause interference with already in-place terrestrial microwave systems. Or, conversely, a planned earth station could be susceptible to interference from a terrestrial station transmitting in the same portion of the spectrum.
In order to determine the extent of the potential-interference problem, a preliminary computer analysis is performed, in which the computer simply lists which terrestrial systems are within a specified radius of the planned earth station. This radius varies based on a variety of parameters.
In reviewing the output of the computer-generated search, it's fairly simple for an experienced engineer to determine, on a preliminary basis, whether the site is workable or not. Quite often, an inexpensive preliminary analysis can save the intended owner of an earth station thousands of dollars if it's determined that the planned site is unacceptable. Rather than installing a dish and later finding that it doesn't work at an acceptable level, this preliminary analysis offers a quick overview of the level and number of potential-interference cases that would require resolution, consequently avoiding the choice of poor sites.
It's Necessary to Clear Interference Cases
(One will quite often hear of "case resolution" or "clearing an interference case" in regard to planning an earth station or terrestrial microwave facility. What these refer to is actually corresponding with the other users of the same frequencies in the area and developing solutions satisfactory to all.)
After reviewing the results of the preliminary analysis to determine cases of potential interference, a detailed evaluation of these conflicts is made by a project manager. Methods of resolving these conflicts to the satisfaction of an existing licensee and a new applicant are then discussed by both parties.
The most-common method of resolving these potential-interference cases is to perform calculations that determine the amount of signal loss as the signal crosses the terrain and travels over the horizon. The calculations factor how much signal loss will occur at various distances from the potential source or receiver of interference based on the variations in the terrain and the curvature of the earth. This calculation, commonly referred to as OH (over-the-horizon) loss, demonstrates to other parties that interference will be at a tolerable level by showing that the interfering signal would dissipate to an acceptable level when it reaches their facilities.
Other methods of resolving the interference cases can include such alternatives as physically measuring the strength of the interfering signal to verify or refute a computer-predicted signal level. Prior to these on-site measurements, recommended changes in the antenna other equipment, analysis of potential shielding (either artificial or man-made), physical separation of the currently used frequencies from the applied-for frequencies (referred to as frequency offset) are usually made by the project manager to the client.
These analyses and recommendations require skill, experience and, quite often, a good imagination. This analysis phase is likely the most-critical segment of the satellite planning process in that the creativity and knowledge of the engineer who's attempting to resolve these interference cases will largely determine the possibility of installing an earth station at its desired location.
When the planned site is defined as workable, the process of informing the licensees who will be receiving or generating interference begins. Written correspondence between one's project manager and the frequency coordinator for the licensee (typically an engineering firm, but sometimes performed internally in large, active organizations) is then initiated.
Projected levels of interference are outlined--along with any available resolutions or means of verifying that the interference won't be harmful--and are included in the correspondence. Response from the frequency coordinator for the licensee states either acceptance of the projected interference or requests further attempts at resolution. If the projected interference is unacceptably high, negotiations continue to determine the means of reducing this interference to an acceptable level. These negotiations can also entail requesting shielding, higher-performance equipment, reduced power levels or changes in the proposed frequencies from the earth-station applicant.
It should be noted here, however, that typically when an existing licensee is finally contacted by letter, methods of minimizing or eliminating interference have already been agreed upon. Only in unusual cases will this step require additional negotiation for case resolution.
The project manager planning the earth station will assimilate this information and discuss recommendations with the client. Quite often, an engineer's experience can help clearly evaluate tradeoffs between increased costs associated with means of resolution and expedited approval by the protesting coordinator. This process is often lengthly and complex, requiring both skill and patience on the part of the project manager.
The ultimate result of this process is the preparation of a document used to support the license application to the FCC. This document, or exhibit, contains the technical data about the planned site and a description of how all interference cases were resolved, with a list of common carriers that the earth station was coordinated against.
These documents, along with other supporting material usually prepared by the client, are then forwarded to the FCC with a license application. Sometimes, other services are required of the engineer or consultant one is using. These can include, but are not limited to the following:
* Land Acquisition and Negotiation--To act as the interface between a client and landowner.
* Field Survey--To accurately verify levels of interference by physical measurement; to identify natural and man-made obstructions that might serve as shielding, and to blueprint suggested construction, placement and other planning data.
* Frequency Protection--To monitor the client's site for potential sources of interference after it's been licensed or coordinated.
* Marketing Research--To determine levels and locations of demand for various satellite services prior to planning.
The planning of an earth station requires accurate and comprehensive data bases, experienced and knowledgeable engineering and a fair degree of creativity to solving potential-interference cases before they actually arise. It may appear as if a dish was simply planted on a concrete pad and pointed at the sky. Clearly, it involves a great deal more than that.
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|Date:||Mar 1, 1985|
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