Leroy R. Grumman Cadet Squadron

Civil Air Patrol - The official auxiliary of the United States Air Force



Balancing Forward Motion and Falling


Starting at the same altitude above the surface of a small moon, balls are thrown at different speeds. Regardless of the horizontal speed of the balls, they all fall at the same rate and hit the surface at the same time. The faster balls go further, in the same elapsed time as the slower balls, before hitting the ground. Finally, a ball travels so fast and thus so far before falling, that the ground doesn't appear flat but curves away beneath the ball. At just the correct speed, the ground curves away at the same rate that the ball drops. The ball has achieved orbit.


If the baseball player were to climb a high mountain and repeat the same exercise, the moon would appear smaller with the ground curving away more sharply. The speed that the ball needs to achieve orbit gets slower and slower as altitude increases and the curvature gets more pronounced. The combination of forward speed and falling determine the curvature of the ball's path. Matching this curvature to the curvature of the moon's surface at each altitude is at the heart of satellite orbits. The satellite drops forever towards the moon's surface but since the moon's surface curves away at the same rate, the satellite remains suspended forever at the same height above the surface. It is in orbit.


Why do satellites sometimes fall out of the sky?


Frictional forces like the drag of the atmosphere on the satellite can cause the forward speed of the satellite to decrease. As a result, the curvature of the orbit changes, decreasing so that it no longer matches the curvature of the moon's surface at this altitude. It is now like one of the "too slow" balls in the cartoon. The satellite begins to drop. As it drops it increases in speed in the vertical direction due to gravity, but never again moves fast enough in the horizontal direction to maintain its orbit. The satellite will slowly descend to the surface unless it is re-boosted to speeds sufficient to achieve orbit at its new altitude.



Satellites can operate in several types of Earth orbit. The most common orbits for environmental satellites are geostationary and polar, but some instruments also fly in inclined orbits.




Geostationary Orbits


A geostationary (GEO=geosynchronous) orbit is one in which the satellite is always in the same position with respect to the rotating Earth. The satellite orbits at an elevation of approximately 35,790 km because that produces an orbital period (time for one orbit) equal to the period of rotation of the Earth (23 hrs, 56 mins, 4.09 secs). By orbiting at the same rate, in the same direction as Earth, the satellite appears stationary (synchronous with respect to the rotation of the Earth).


Geostationary satellites provide a "big picture" view, enabling coverage of weather events. This is especially useful for monitoring severe local storms and tropical cyclones.


Because a geostationary orbit must be in the same plane as the Earth's rotation, that is the equatorial plane; it provides distorted images of the polar regions with poor spatial resolution.


Polar Orbits


Polar-orbiting satellites provide a more global view of Earth, circling at near-polar inclination (the angle between the equatorial plane and the satellite orbital plane -- a true polar orbit has an inclination of 90 degrees). Orbiting at an altitude of 700 to 800 km, these satellites cover best the parts of the world most difficult to cover in situ (on site). For example, McMurdo, Antartica, can be seen on 11-12 of the 14 daily NOAA polar-orbiter passes.


These satellites operate in a sun-synchronous orbit. The satellite passes the equator and each latitude at the same local solar time each day, meaning the satellite passes overhead at essentially the same solar time throughout all seasons of the year. This feature enables regular data collection at consistent times as well as long-term comparisons. The orbital plane of a sun-synchronous orbit must also rotate approximately one degree per day to keep pace with the Earth's surface.


Eccentric/Inclined Orbits


Eccentric/Inclined orbits fall between those above. They have an inclination between 0 degrees (equatorial orbit) and 90 degrees (polar orbit). These orbits may be determined by the region on Earth that is of most interest (i.e., an instrument to study the tropics may be best put on a low inclination satellite), or by the latitude of the launch site. The orbital altitude of these satellites is generally on the order of a few hundred km, so the orbital period is on the order of a few hours. These satellites are not sun-synchronous, however, so they will view a place on Earth at varying times.

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