Satellite Overpass Schedule and downloading calculated times
for GLOBE Aerosol Measurements

The Solar Photometers that we have may be used to collect Atmosphere Optical Thickness (AOT) measurements based upon the voltages generated by an op-amplifier connected to two light emmiting diodes (LED's) contained within the instrument. One measures "red" light and the other measures "green" light. These LED's are sensitive to those particular wavelengths of visible light. Our measurements can be most useful when compared to overpassing satellite measurements which monitor aerosols by remote sensing technologies. Remember, satellites look down upon the earth, thus "see" the surface of the earth as a background. Solar radiation is thus reflected (or re-radiated) back into the instruments aboard the satellites. Whereas our solar photometer is looking upward, and "sees" the sun with a dark background. Solar radiation in our case is directly from the sun and any scattering or absorbtion is due to atmospheric conditions which we are sensing directly from light passing through it.

In order to make these timely observations, we must know when to measure aerosols, or at exactly the same time that the satellite is passing somewhere above us. Since we know where we are on the earth's surface, by our latitude and longitude, we can use the NASA Satellite Overpass Predictor web page to display the calculated times that the satellite will be overhead, or nearly so. Click on the link above and enter the data below:

1. Select the "Terra / Aqua / Aura" satellites from the top pull down menu

2. Enter our school coordinates. . . . . . Latitude 36.1972 North
. . . . . . . . . . . . . . . . . . . . . . . . . . . . Longitude 92.2688 West
. . . . . . . . . . . . . in the Ground Location boxes

3. Click on "Set Predict Start Date to Today" button

4. Select the number of days you desire from the "Predict for . . . days" pull down menu

5. Click the "Predict" button at the bottom of the screen to view the results


Press the "back" button to return to the satellite data entry page to select another satellite, then press "Predict" to view its results. Record only those times during daylight overpasses since we are interested only in times when the sun is visible. Record only those times when the "peak angle" is over 30 degrees since we are interested in just those times when the least atmospheric scattering is present; i.e. the satellite is closest to us. Finally, record the satellite name, peak overpass time (UT), and peak angle for later use in the GLOBE Data Entry Screen.

If the current weather conditions permit an aerosol measurement, set up the instrument and begin measuring data just before the "peak overpass" time. Remember Universal Time (UT) is six hours ahead of our local time in the fall/winter (CST) and five hours ahead during central daylight savings time (CDT). The measurement time "window" is only a few minutes long and gets shorter for overpasses that are further away from a 90 degree peak angle. After measurements are complete, enter the GLOBE web site and access the Aerosols Protocol Data Entry web page.

Mathematics / Geography Extension Activity

Using a highway map of Arkansas and a circular proctractor ( a semi-circular one will also suffice) plot the angles for satellite Rise / Set for each overpass. Align the origin over our school site location and the zero degree meridian line to true north. For this exercise magnetic declination shouldn't matter. Remember, for azimuth angles 090 degrees is due east, 180 degrees in due south, 270 degrees is due west. Knowing the peak angle and the height of the satellite orbit, one may calculate the approximate distance the satellite passes away from our site location. For our example, draw a circle with a radius of 100 kilometers on the map. Use the reference scale in the bottom right-hand legend for measuring this radius. Finally plot the intersection of the satellite rise and set azimuth angle(s) and the circumference of the 100 kilometer radius circle. Once you complete this plotting; see if you can answer these questions?

Question 1: Which direction does the satellite appear to be moving with respect to the earth's surface?

Question 2: Which direction is the earth turning with respect to the sun?

Question 3: If you have access to a globe of the earth you may also ask, "What kind of orbit must the satellite be moving in order to follow this path?".

Question 4: After plotting all three, what similarities do you notice about them? How are the satellite paths similar; how are they different?

For answers, click here.

For further information on this subject, or these satellites, click on the links below:

CALIPSO, Aura, CloudSAT Outreach
Atmospheric Games (requires Flash)
Aura Satellite
Aqua Satellite
Terra Satellite
GLOBE Aerosols Protocol downloaded in Adobe.pdf format

Last Updated: 9 Sep 2004