EUVE Satellite Dataflow Demonstration

By Marlene Wilson and Dennis Biroscak, with contributions from Bill Hammerman and Dan Reimer.

©1995,1996 The Regents of the University of California

PURPOSE

This is a hands-on demonstration of the communications path between scientists and NASA's Extreme Ultraviolet Explorer (EUVE) Satellite, showing how scientific data are downloaded from the satellite. The same demonstration can also be used to illustrate how the instruments on the satellite are commanded by scientists on Earth. It gives students a feel for the dynamics of satellite communications and orbital motion around the Earth.

INTRODUCTION

Students should be shown the viewgraph which illustrates the data flow in one page. This demonstration originated from an attempt to provide a grasp of the dynamics involved.

Walk the students through the diagram, following the data path as shown: Star, EUVE, TDRSS (pronounced "Tee'-driss"), White Sands, DOMSAT, Goddard, DOMSAT, UCB's CEA, Scientist.

Explain that the EUVE satellite receives, stores, and relays starlight that is detected by its science instruments.

Explain that TDRSS is operated by NASA and only talks to one ground station at White Sands, New Mexico. TDRSS is in a Geosynchronous orbit, meaning it is synchronized with the Earth's rotation (GEO = Earth). This means that TDRSS revolves about the Earth in 24 hours and always is above one spot on the Earth, in this case, White Sands, New Mexico, which will always be able to see and communicate with TDRSS. TDRSS is approximately 26,000 miles from the center of the Earth (all Geosynchronous satellites are positioned this far out so that their period of revolution is the same as the time it takes for the Earth to rotate once around its axis). For the curious: NASA actually operates two geostationary satellites, TDRSS East and TDRSS West, but only one TDRSS communicates with a satellite at a time. Both TDRSS East and TDRSS West talk to the same ground station at White Sands, New Mexico and together the whole Earth is covered by the two TDRSS.

DEMO REQUIREMENTS

Plenty of space - a large room is ideal (outdoors is problematic - since the demo requires throwing and catching a ball, weather conditions, particularly the wind, can seriously affect the viability of the demonstration). Everyone must be free to move - no tables or chairs in the way.

These signs (minus the cardboard and ribbon) are provided as part of this online lesson. Clicking on each one will open a file that you can download and print.

ARRANGEMENT OF PARTICIPANTS:

Earth Locations:

Five students are placed back to back in the center of the room forming a circle that will represent the Earth. (This is done for ease of the demonstration although in reality, all of the ground stations are on the continental United States and therefore the satellites would actually only send signals to a portion of the Earth rather than to the whole globe.)
Each of the five students is given a sign to wear - in consecutive order from left to right: Scientist, UCB's CEA, Goddard, White Sands. See drawing with top view (looking down on north pole) of Earth locations with arrows specifying rotation direction. Or watch this Java applet animation of the EUVE Satellite Communications.

Satellite Locations:

TDRSS:

One student is placed at the edge of the room exactly opposite White Sands, NM. and wears the sign TDRSS.

DOMSATs:

Another student, wearing DOMSAT, is placed just as far away from the Earth as TDRSS, but is positioned between White Sands and Goddard. Again: explain that DOMSAT is another Geosynchronous satellite that NASA uses, but it also relays TV programs and other communications.

Another student, wearing another DOMSAT, is again placed just as far away from the Earth and positioned between Goddard and Onizuka Air Station.

At this point, you could start the Earth rotating SLOWLY and let the geosynchronous satellites try to keep up with the Earth. They have to remain over the same spot on the Earth. You could also wait until everyone gets assigned a position before you start the rotation.

EUVE:

Another student is given the EUVE sign to wear. Explain that EUVE is only 320 miles above the Earth. Position the student very close to the Earth students. EUVE travels around the Earth 15 times for every once that the Earth turns (15 times per day), so it moves around the Earth much faster than the rate at which the Earth rotates.

STAR:

Finally, from a corner of the room or anywhere well outside of the outer satellites, a student is positioned with the STAR sign. The student is given the small, slightly-deflated ball that will be representing star light. The STAR does not move for the purpose of this demonstration, so the student stands still.

The path of data from the star to the scientist is as follows: Star, EUVE, TDRSS, White Sands, DOMSAT, Goddard, DOMSAT, UCB's CEA, Scientist.

MOTION IN SPACE:

Make sure students understand the difference between rotation and revolution. Within the revolution category are two subcategories, geosynchronous (or geostationary) revolution and non-geosynchronous revolution. For this demonstration, the only satellite that is non-geosynchronous is the EUVE. At this point, you may want to do a small "sub-demo" of having one student stand at the center of the room representing Earth who will then rotate while one person on the outside (a satellite) follows the face of the "Earth." Students will see that the Earth needs to rotate very slowly to allow time for the satellite to follow (around the circumference of the room). Then start the MOTION IN SPACE with everyone except the STAR as described above. Now call out the path as the students are throwing the ball. Have them say who they are as the ball is caught by each.

Explain that the data travel by land lines (Pacific Bell) from Onizuka to U.C. Berkeley ("hand" rather than "throw"). At this point you may want to take another look at our movie, to pull the above steps together into a visual demo.

Video of Demonstration with kids! We know this file is large, but it is worth it. Your Netscape disk cache (Options Menu) should be set for 2 Megs.

VARIATIONS:

Data Drop-out:

If someone drops the ball, it is called a data "drop-out" and the ball is given back to the star again. Explain that a data drop-out can occur at any point in the communication path.

Commanding the EUVE:

Another ball (tennis ball) could be used to represent a command from the UCB scientist to EUVE. The path of the command is exactly the same but in the opposite direction (Scientist, UCB's CEA, Onizuka AS, DOMSAT, Goddard, DOMSAT, White Sands, TDRSS and EUVE). If your students are up to the challenge, this can be done simultaneously with the incoming data from the star, as in real life.

Supplemental Information

By using two geostationary satellites, TDRSS East and TDRSS West, NASA can "see" a satellite like EUVE for about 85% of its orbit. Commands or data can be sent or received from a satellite almost all of the time by scheduling TDRSS to contact the satellite. If a satellite has a problem, NASA can quickly get in contact with it.

Now that you have read all this, and it is quite a lot to read, and you are asking yourself if this is worth it, consider the following:

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