|
NASA's Aqua satellite was launched on May 4, 2002 with six Earth-observing instruments on board. Aqua circles the Earth every 99 minutes and is in a polar orbit, passing within ten degrees of each pole on every orbit. The orbit is sun-synchronous, meaning that the satellite always passes over a particular part of the Earth at about the same local time each day. Aqua always crosses the equator from south to north at about 1:30 PM local time. One of the instruments on Aqua, MODIS, measures 36 spectral frequencies of light reflected off the Earth in a 2300-kilometer wide swath along this orbit, so that MODIS measures almost the entire surface of the Earth every day. The first animation shows the Aqua satellite orbiting for one day, August 27, 2005, showing a set of MODIS measurements taken that day that have been processed to look like a a true-color image of the Earth. Notice that MODIS only takes data during the dayside part of the orbit because it measures reflected light from the Sun, and that there is a bright band of reflected sunlight in the center of swaths over the ocean. Also visible in this animation are Hurricane Katrina, just to the west of Florida in the Gulf of Mexico, and Typhoon Talim, in the westerm Pacific between Japan and New Guinea. The second animation spans five days of Aqua orbits, from August 27, 2005 through August 31, 2005. For this animation, the orbits and data are shown over an Earth image that shows the day and night parts of the Earth at each time of the animation. The daylight part of the Earth is a cloud-free MODIS composite, while the nighttime regions show the 'city lights', the Earth's stable light sources. During the first day, August 27, the Aqua satellite is shown with a red line indicating the orbit of the satellite. Since the Earth's surface is stationary in this animation, the satellite orbit moves westward with the sun. During the second day, August 28, the most recent observation swath is shown in addition to the satellite orbit line. In this way , the drift of th orbit relative to the observations is illustrated. Starting with the third day, August 29, the orbit line disappears and the observation swaths accumulate. The observations cover the Earth during the third day except for small gaps at the equator, which are filled in during the fourth day, August 30. The animation continues to show the MODIS observations through August 31, the fifth day. The third animation shows the same composition as the second one, but the point of view has changed to that of the Sun. In this animation, the Earth rotates and the orbit is stationary. At this date, the North Pole of the Earth is tilted towards the Sun and in daylight, while the South Pole is tilted away and is in darkness.
|
