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What does the dish "see"? Technology has begun to come up with some very cool stuff. Check out this video for the latest in satellite location.
"What does the dish see?" is an often asked question and somewhat difficult to describe with words. Here are a couple of pictures that help illustrate the actual vs. perceived location of satellites when viewing an "offset LNB" or "reflector" type satellite dish. This type of dish is commonly used in DBS (Direct Broadcast Satellite) operations. Offset = 22.5° When observing a ODU (outdoor dish unit) it is often mistakenly assumed that the "face" of the reflector is pointing at the satellite locations. Due to the design of a reflector dish, the signal is "bounced", or reflected, into the LNB so that the LNB doesn't interfere or create a shadow in the path of the signal. Communication satellites like those used in DBS, orbit the earth in a line approximately 22,000 miles above the equator. This "line" or belt of satellites has been named the Clarke Belt after Arthur C. Clarke, the inventor of the communication satellite. If you were to stand on the equator and looked straight up, the satellites would be directly above you. As you move north (or south) of the equator, from your perception, the satellites would appear to drop in elevation, from directly overhead at the equator, to near the horizon at the poles. They appear lower in the sky the farther you move from the equator. Or, the further north latitude you move, the lower in the sky are the satellites.Note: The elevations given by location calculators are the actual elevation of the satellite in the sky. The design of the dish accounts for the offset LNB so you can use the elevation gradient without additional calculations. Any angle reference tool that gives elevation from horizontal will give you an idea at what point in the sky that you would find the satellites. Ideally, an inclinometer, or clinometer, should be used for the most accurate determination of location of satellites. It combines the use of a compass and an elevation finder into one handy tool! A clinometer like this one, used correctly, will give a very accurate location of known satellites. It allows sighting through the device while seeing the indicators. You first sight through it holding it horizontally, looking through the clinometer with one eye and viewing the horizon with the other. You will see compass readings. Here you find your azimuth or compass heading. When you've found the correct azimuth, use the vertical line in the meter to pick a reference object on the horizon as a landmark. Once you've found a landmark at that azimuth, you then turn the clinometer vertical and sight through the elevation gauge. With both eyes open, line up on your landmark and then look upward until the elevation meter reaches the elevation of the satellite you are looking for. At this point you will be able to see if there is anything in the way (trees, etc.) and accurately determine line of sight.
Click below for larger pics and notes
Note: Satellite 61.5 (not pictured above) is to the left (Southwest) as the Clarke Belt arc curves back down. To see NASA's actual tracking of the Clarke Belt look here: |
What do you want to know? Ask highdefjeff
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