the Technology Interface / Spring 97

Understanding and Performing Satellite
Link Budgets


by

Bill Sepmeier

bill@nsn.net
NSN Network Services
Avon, CO

The first step in designing a satellite network is performance of a satellite link budget analysis. The link budget will determine what size antennae to use, SSPA or TWTA PA power requirements, link availability and bit error rate, and in general, the overall customer satisfaction with your work.

What exactly is a link budget?

Like most RF work, it has long been considered by the uninitiated as one of the " Black Arts " involving the dreaded invocation of mathematics, uttering of formula, and skillful estimation of the unknown. The secrets of the successful link budget have long been treated as if they are Masonic in Origin; satellite service providers tend to guard the Holy SFD and Righteous IO Backoff Value to the death…. and beyond.

What a link budget actually involves is a relatively simple addition and subtraction of gains and losses within an RF link. When these gains and losses of various components are determined and summed, the result is an estimation of end-to-end system performance in the real world. To arrive at an accurate answer, factors such as the uplink power amplifier gain and noise factors, transmit antenna gain, slant angles and corresponding atmospheric loss over distance, satellite transponder noise levels and power gains, receive antenna and amplifier gains and noise factors, cable losses, adjacent satellite interference levels, and climatic attenuation factors must be taken into account.

Fortunately in this age of computers and spreadsheet programs, the link budget does not have to be all that difficult to compute.

Several companies now market quite sophisticated link budget calculation programs that contain large databases of information regarding satellite performance parameters, ground station antenna performance data, and other information vital to calculation. With one of these programs, all the user must do is fill in the blanks regarding earth station location, planned satellite(s) to use, required link availability, and "quick as a wink" the program generates a very good estimation of link performance. A very good example of this type of program is the link budget program included in the SatFinder CD Rom available (for both PC and Macintosh) from Design Publishers.

If your liquid assets of knowledge exceed those in the bank, you can design a spreadsheet to obtain similar results, assuming you have to time to drag out the IEEE Engineering Handbook and input all of the formulae. Prepare to spend some time interpolating terrestrial microwave data into your satellite application.

For those without either a lot of time or money, I have decided to risk the Holy Inquisition, and make my own Excel Version 7.0 Spreadsheet available. (It's also much easier to write an article covering this piece of work than to re-invent the wheel telling you, gentle reader, how to input all the math!) Since "The Half of Knowledge is to Know Where to Find Knowledge" CLICK HERE to download the program, and then read on.

When you run the sheet using Microsoft Excel 7.0 for Windows 95, you will notice that some fields are highlighted in BOLD type. These fields are important, as they must contain data relevant to your network. You must change the data from my defaults in order to obtain an accurate calculation of your network's performance. Fields that are italicized indicate end results.

As a former college botany professor of mine used to say: " Here's what you will need to know to pass the test. "

The above information can generally be obtained from the satellite operator, or from a good satellite database such as SATNEWS.COM. Other sources of this data include printed media, such as the International Satellite Directory also available at SATNEWS.

You will also need the following information that you and your customer can supply:

Once the above information is available, it is a simple matter to plug the data into the spreadsheet and calculate the link budget.

In Ku band networks, it is a good rule of thumb to allow 7 or 8 dB of margin above threshold at the receive site with clear sky conditions. This will generally provide a link availability in excess of 99.5%. C band networks require much less margin, typically about 3 dB, for the same performance expectation, since there is less atmospheric attenuation with the C band.

Most satellite space providers prefer to sell carrier power levels that are proportional to bandwidth used -- or balanced power/bandwidth service. If your link budget indicates that significantly more power is required than bandwidth indicates, you will want to use a larger receive antennae. This "free gain" will lower the power requirement from the satellite, and therefore, the continuing costs of network operation. " Bandwidth Limited " service, or the use of less power than would be used in balanced services, is sometimes available for even less cost. If you can oversize the network's receive antennas a bit, the difference in hardware costs will more than be paid for in lower bandwidth-limited spacetime fees.

Always approach satellite space segment as you would a sexually transmitted disease. Once you've gotten it, you're stuck with it. Anything that can be done to minimize the recurring costs of satellite access should be done before you sign the provider's agreement!

Most satellite operators limit satellite received EIRP to a specific maximum level of 6dBW/4kHz, or about minus140 dBW per square meter on the ground. If spectral density exceeds these limits, you should use better LNB's or larger receive antennas to lower the power requirements. You can also spread the signal over greater bandwidth; either by changing FEC rates, changing modulation formats from QPSK to BPSK, or by using some form of additional signal spreading. With today's newer satellites operating at 4 times traditional satellite power levels, the established limits can become a problem with even balanced power / bandwidth carriers.

The sample data included in the Excel spreadsheet provided is valid for NSN Network Services' transponder 13 on the new GE-1 satellite, at the locations specified. Performance has been field verified and is accurate to within 1 dB EB/No. As you begin to manipulate the variables, you will quickly "get a grip" on how the various system parameters effect overall network performance. Once you understand what is going on, you can plug in the variables required in your own network applications.

I hope you find the information useful, and welcome your comments and suggestions.

©1997 Bill Sepmeier All Rights Reserved

Bill Sepmeier is Vice President, Satellite Engineering, for NSN Network Services, Ltd. NSN is a Colorado company that provides worldwide satellite network systems and international Internet connectivity. For more information, contact Bill at bill@nsn.net or visit http://nsn.net on the World Wide Web.

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