MadSci Network: Engineering |
A ground trace or ground track is the path you would have to follow on Earth, i.e. on the ground, in order to be as close to a satellite or spacecraft as possible. Imagine you're trying to listen to a very weak broadcast from a particular satellite -- the ground track would be the path you should follow to get the best reception. Such a ground track is usually plotted on a mercator projection, see liftoff.msfc.nasa.gov/academy/rocket_sci/orbmech/mercator.html you can find many of these plots at eol.jsc.nasa.gov/map_images/ It may also help you to get out a map of the world or, even better, a globe if you have one. An excellent web-based tool for understanding this is at liftoff.msfc.nasa.gov/RealTime/JTrack/3D/JTrack3D.html For the sake of argument, let us suppose that Earth does not spin. And, let's begin with a satellite that has a circular orbit directly above the equator and that the satellite passes over, say, Ecuador (in South America) every ninety minutes. The ground track for our imaginary satellite will be the equator (zero degrees latitude). In other words, the closest point to the satellite would always be on the equator. Now, let's take that orbit and tilt it a little so that it no longer passes over Ecuador, but Uruguay (about 33 degrees south of the equator on the east coast of South America). Our satellite's orbit is still a circle and gravity requires that the center of that circle must still pass through the center of Earth. This means that on the other side of Earth, our satellite must pass over South Korea (about 33 degrees north of the equator). This means that the ground trace of our satellite would pass through Uruguay, then go around Earth and pass through South Korea. Remember, the ground trace follows a path on Earth for which you are always as close to the satellite as you can be. When you draw a nice smooth line under this orbit you'll find that it makes that same sort of wavy pattern you've seen before. Let us no longer suppose that Earth does not spin. If our satellite is in a relatively low orbit, like the space shuttle's typical orbit, then our satellite will make its way once around the world about every 90 minutes. This means that after every 4 loops around Earth, 6 hours have gone by -- Earth has made a quarter-turn. Our satellite's orbit does not rotate with Earth (gravity pushes and pulls to and fro, never sideways) so after these 4 loops we'll find that our ground track doesn't go through Uruguay or South Korea, anymore! So, over the course of one loop, our satellite's ground trace shifts about 22.5 degrees (90 minutes is to 24 hours as 22.5 degrees is to 360 degrees) An excellent web-based tool for understanding this is at liftoff.msfc.nasa.gov/RealTime/JTrack/3D/JTrack3D.html When the J-Track 3D window opens, click on "View" and select "Ground Trace". Click on "Satellite" and select "Select" -- this will give you another window where you may choose a satellite to view, under the "Types" menu, choose "Human Crew" and select "STATION" At this point, you should see a red path indicating the International Space Station's orbit around Earth and an orange path on Earth. The orange path is the ground trace. You can also view ground traces with J-Track at liftoff.msfc.nasa.gov/RealTime/JTrack/Spacecraft.html
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