# Lesson Ideas: Satellite Communications

## Lesson Resources

 Satellite Communications Title Suggested Audience Kepler's 1st Law 5 - 12 5 - 12 Kepler's 3rd Law 8 - College Newton's 1st Law 5 - 12 Newton's 2nd Law 5 - 12 Newton's 3rd Law 5 - 12

### Learning Activities

• How does a GPS tracker identify your position?

Students can triangulate their position on Earth using the NEMA (National Marine Electronics Association) information from a GPS tracker.

The NEMA information from the GPS can be very enlightening for students. Just the sheer volume of information that is packed into those 1 second bursts is impressive. And we only see a fraction of the information being transmitted.  I think most interesting is the number of satellites in view and received at any given moment, the time stamp, signal quality information, the geometry needed for 2D and 3D position. While there are probably 12 or more satellites in view at any given time, you only need 3 for position, and a 4th for altitude. It is all based on distance from the satellites and some great circle mathematics.   The whole system is based on time of arrival of the signals from the individual satellites, therefore accurate clock timing is essential, probably down to the micro second-- that is why the time is reported so often. Calculate the time it takes for a radio wave to travel from the satellite to the receiver. The next data point that determines accuracy is the position of the satellites. Knowing exactly where they are at any given moment determines the resolution of the position on Earth. So between the accuracy of the time and the accuracy of the satellite position moving at over 17,000 mph, to get 14 foot resolution is eye watering.   A good instructional tool is to have the students do a manual exercise to triangulate their position using a globe and some fake satellites. Use a globe and some Styrofoam balls for satellites suspended around the globe. Have some strings attached to the satellites that represent the time of arrival of the signal from that satellite (D=R*T). The students pull the strings taught and draw a great circle on the globe surface. Do that three times with the different satellites to triangulate where they are on the globe. Then add the fourth to see what the best geometry is to determine the altitude positon. (That satellite should be as close to directly overhead as possible, altitude is most inaccurate position from the GPS.) After the students go through all of that, tell them that their hand held technology does all that better and more accurately and all within 1 second.   from Mark Spencer, WA8SME

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