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Chapter 19:
Putting It All Together: Link Budgets
OK, this is where you become a satellite telecommunication engineer. (Well, not quite: we have left out a lot of nasty mathematics and physics and other details, but you have the general idea.) In previous chapters we have assembled all of the components that go into a link budget: we have compiled all of the things that either contribute to or detract from the quality of a signal sent between an earthstation and a satellite. While most of the other formulas mentioned in the book have been banished to Appendix E, the link budget is defined and discussed in detail in this chapter due to its importance. When a customer has specified a desired quality of the final received signal, a satellite engineer can then calculate the necessary operational parameters to achieve that goal. This is the purpose of a link budget. It is the computation that tells us how well we can accomplish a link. We are now going to calculate a link budget. While there are a lot of numbers in this chapter, don’t panic. They are almost all simple additions and subtractions of items that have already been explained. The main purpose of this chapter is to show you what a link budget is rather than expecting you to do one yourself. Most importantly, you should take away from this chapter an understanding of what items in the link have the largest effects on the user’s signal quality, and what you can and cannot adjust to make it better. The analogy is the common household budget: add what comes in, subtract everything that goes out, and see what is left. That “bottom line” is a number, but you cannot say whether that number is good enough without knowing what you need. You’ll need a better “bottom line” if you want to vacation on the Riviera than if you want to go camping at a nearby park, so knowing the user requirement is absolutely necessary to determining if the budget is sufficient for your purposes. If it is not, you have to make choices about increasing income or decreasing outflows.
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