S'COOL:
Frequently Asked Questions
Question: Why are we asked to
make one observation on most days, but on some days we are asked to make
two observations?
Answer: The simple answer is that on most days the CERES instrument aboard the Terra satellite will view a location on the Earth just once during daylight hours. On some days, however, CERES may see a location two or more times. And in some locations, CERES will always have more than one overpass.
In order to understand why, we need to look at how the orbit of a satellite defines where and when it will view a particular location.
Satellites can orbit the Earth in a wide variety of ways. An excellent description of the most common types of orbits can be found at the Web site for the Tech Museum of Innovation in San Jose at http://www.thetech.org/exhibits_events/online/satellite/4/4b/4b.1.html. This site also features simple animations of the orbits that illustrate the relationship between the satellite’s motion and the rotation of the Earth.
The Terra satellite orbits the Earth in a circle running north-to-south at an altitude of 702 km. This type of orbit is called a “Polar Orbit” since the satellite passes closely to both the North and South Poles. An illustration of a polar orbit is shown in Figure 1.
Figure 1: An example of a polar orbit (from http://www.thetech.org/exhibits_events/online/satellite/4/4b/4b.1.html)
Polar orbiting satellites are extremely useful for scientific studies since the orbit enables instruments aboard the satellite to see all regions of the Earth. The north-south direction of the polar orbit allows a view of all latitudes. Since the Earth is rotating as the satellite travels along the orbit circle, different longitudes are viewed with each new orbit. It takes approximately 100 minutes for Terra to complete one orbit. During this time, the Earth rotates 25°. So two successive orbits of Terra will be separated by 25° at the Equator. This is illustrated in Figure 2.
Figure 2. The paths of two successive orbits of Terra. Each dot represents the location of the satellite at one minute intervals.
Over the course of a day, Terra completes approximately 14.5 orbits. The CERES instrument on Terra therefore views regions below 14.5 orbital paths that are spread over the entire Earth. On the next day, the satellite will once again complete 14.5 orbits, but the orbit paths will be slightly changed in longitude from the previous day. In this way, the satellite can view the entire surface of the Earth over about 6 days.
This would be the whole story if the satellite instruments only looked straight down. CERES would view a S’COOL site about once every 6 days. However, both of the instruments on Terra used by the CERES project (CERES and MODIS) are designed to scan from side-to-side as the satellite moves along its orbit. This allows the instruments to see a wide swath of the Earth that is centered along the orbit. This is shown in the next three figures.
Figure 3 shows the orbit path of two successive Terra orbital paths over the U.S. on August 27, 2001. The area viewed by the MODIS instrument during two of these orbits is shown in Figure 4 by the purple boxes. Figure 5 shows the actual MODIS data that corresponds with the areas within the boxes.
From Figure 2, we can see that the separation between the orbits is greatest at the Equator. As Terra moves toward the Poles, the orbit paths become more closely spaced until they finally converge near the Poles. As can be seen in Figure 4, there is increasing overlap between the areas viewed by two successive orbits for higher latitudes. For these areas, CERES will make two observations separated by 100 minutes. For even higher latitudes, CERES may view a site three or more times. The extreme case is for the Pole, which can be seen by CERES 14 times each day.
Figure 3. Two successive orbit tracks
from August 27, 2001. The Universal Time is given for several points along
the orbits. Note that the satellite reaches a particular latitude
approximately once every 100 minutes.
Figure 4. The purple boxes show
the area viewed by the MODIS instrument during the two orbits shown in
Figure 3. Note that the intersection of the two areas increases to the
north.
Now let’s look at a specific example to see how this affects when to make S’COOL observations. We’ll consider the case of 2 schools in the United States. The first is a school in southeastern Virginia (the yellow dot in the Figures). The second is a school on the border of the U.S. states Illinois and Indiana (marked by a red dot in Figures 3-5).
The Virginia school is near the middle of the earlier orbit (the orbit on the right). This school is observed only once by CERES. The second school is located in the area of overlap between the two orbits. CERES observes this school twice on this day.
Remember that the orbit pattern changes slightly each day, so on another day the Virginia school might be seen twice, while the other school would only be seen once.
If you look closely at the images
in Figure 5, you can see that the cloud patterns have changed slightly
between the times of the first and second orbit. Therefore, if the school
can make observations at both times, we can test CERES’s ability to detect
these changes.
Figure 5. MODIS visible data over
the U.S. from August 27, 2001. The image on the right represents 5 minutes
of data observed at 16:05 GMT. The left image is 5 minutes of data from
the following orbit at 17:44 GMT.
Submit
your question
Back to help page
