2. What is a communications satellite? As you know, the surface of the Earth is curved. As look out onto an open plain or over the open ocean, the Earth drops away from you 32 feet for every 5 miles you can see. Most communications must be made by line-of-sight That means that the antenna sending the information and the antenna receiving the information must be able to “see” each other If you have two 32 foot antennas five miles from each other the curve of the Earth will not allow them to see each other. You are going to have to have a taller antenna. Could you get antennas tall enough to send a message from coast to coast?
3. How do you get a tall enough antenna? Instead of making it tall, why not make it high? The higher you can put an antenna in the sky the more it can see. You can see however that there is an upper limit
4. Why would a satellite work? You can see that the satellite can see points A and B So even if the points can’t see each other the satellite can bounce or retransmit the communication to the other station. A B
5. How much area can they cover? You will notice that each satellite can only see just over 1/3 of the Earth. Therefore you would need at least three satellites to cover the equator. Also note that the satellites can see each other allowing for easier coverage
6. Geostationary Orbit Communications satellites are placed in a geostationary orbit. At the altitude they orbit they appear to stay in the same place in the sky the all the time. They are traveling around the Earth at the same speed that the Earth is rotating. This magic orbit is 22,500 miles (36,000 km) above the Earth’s equator. Ever notice that all television satellite antennas point south toward the equator? Go here to see a graphic of geostationary orbits. http://en.wikipedia.org/wiki/File:Geostat.gif
8. How Does GPS Work? GPS Satellites send out a coded radio signal. The signal includes information from the satellite like the name and the time of the signal. It takes three satellites to be accurate about the position of your GPS receiver. Why?
9. Why 3? A GPS satellite sends out a signal that looks like a bubble.
10. Why 3? A GPS satellite sends out a signal that looks like a bubble. A second satellite does the same thing. You could be anywhere along that center line.
11. Why 3? Notice that if we bring the three circles together they fit only one point.
12. GPS vs Communications GPS satellites are not at the magic Geostationary Orbit where they seem to be in the same place in the sky all the time. There are several that circle the Earth. Would their orbits be around the equator or over the poles? Why?
13. In this Unit In this unit we will be discussing communications satellites and how they work. In the beginning we will be discussing GPS satellites. Your cell phone and maybe even your car has a GPS unit in it.
