How satellite domes work


Under the Dome


Satcoms and Satellite TV deliver the world. But how does the signal reach us?

Using satellites for communicating or watching TV is great and worth delving into, but before we take the top off a satellite dome, let’s take a look at why we go to the complexity and expense of satellites in the first place. Why not just use good ol’ traditional land-based radio and television signals?

The answer, in a nutshell, is that the characteristics of radio waves depend very much on their frequency. Radio waves that are at a low enough frequency in order to bend around the surface of the earth (a few 100,000 cycles per second, usually measured in kilohertz) are not very good at carrying large amounts of information and are prone to interference. Radio waves that are at high enough frequencies to carry lots of information (millions or billions of hertz—megahertz or gigahertz) can only travel in straight lines.

In order to spread these very high frequencies over large areas without having them blocked by the curvature of the earth, their transmitting antennas need to be mounted very high above the surface. That presents two solutions: either using a lot of exceptionally tall masts or a much smaller number of satellites.

Almost all satellite TV and communication services (Iridium is a significant exception) rely on satellites that orbit around the earth just over 22,000 miles above the equator. If they were any lower, they would have to “fly” faster in order to avoid crashing back to earth, and if they were any higher, they would have to go slower to avoid hurtling off into space. But at that specific distance, each satellite can complete its orbit in exactly 24 hours so that it seems to hang in the sky, hovering over one particular spot.

Being so high means that the area each satellite can “see” (its footprint) covers a huge area—more than a third of the earth’s surface. Unfortunately, it also means that signals have to travel a long way, so by the time they reach the surface of the earth, they are very weak. The satellites help by focusing their transmissions into “spot beams” with each beam covering an area roughly equivalent to two or three states. But even that isn’t enough. We have to help ourselves by using dish antennas to catch enough of the very weak signals for our satellite receivers and decoders to be able to use.

At home that just means mounting a satellite dish on the roof or in the backyard and making sure it is pointing at the satellite that is transmitting the programs you want (and have paid for). The satellite is stationary, relative to the earth, and your house is stationary. That’s all there is to it. But boats move, and that’s where the dome comes in.

The dome itself, of course, is just a cover for what’s inside. In the early days of satcom, some superyachts had three domes—one for satellite TV, one for satcom, and one for storing mops, buckets, and polish. Now, it’s quite common to see relatively modest boats with a pair of matching domes. Owners and crews need to find somewhere else to keep the cleaning materials!

The size of the dome depends mainly on the size of the dish. And the size of the dish depends largely on the strength of the signal it intends to receive. The power of a satellite signal is usually quantified by comparing it with the power of a one-watt transmitter broadcasting equally in all directions, known as its EIRP or Equivalent Isotropically Radiated Power and measured in dBW. Many people find the dBW scale a little confusing because it is logarithmic rather than linear. That means that every increase or reduction of three points on the dBW scale corresponds to a doubling or halving of the power. So a 42-dBW signal is only one-eighth as powerful as a 51-dBW signal. If the 51-dBW signal can be received by a dish with a 13-inch diameter, it will need a dish with eight times the surface area (and a diameter of about 37 inches) to receive the 42-dBW signal.

Fortunately, as customers, we don’t need to worry about this too much: Satellite-service providers supply coverage maps showing how the signal strength varies over large areas, and the satellite-dish manufacturers provide specification sheets showing the minimum signal strength each model can handle. All we have to do is make sure that we buy a dome that is able to receive the weakest signal we expect to experience. Since the main service providers tend to concentrate most of their spot beams on the contiguous states, the farther offshore you go, the bigger the dome you are likely to need if you want to receive domestic U.S. programs.

This article originally appeared in the June 2011 issue of Power & Motoryacht magazine.

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