5.6 Orbit of Satellite

The orbit of a satellite is referred to by several names with respect to orbit figure, inclination, period and recurrence as shown in Figure 5.6.1.

The circular orbit is the most basic orbit and is explained as follows. The orbit can be expressed as the polar coordinates (r,).

r = r e + h s = 0 t

where re : radius of the earth 6,378,160 m
hs : altitude of satellite
t : time
0 : angular velocity

The angular velocity and the period are expressed as follows.


where : gravity constant ; 3.986005

a. Geosynchronous orbit
The orbit with the same earth rotation rate (h24 = the sidereal day ; 86164.1 sec) is called an earth synchronous orbit or geosynchronous orbit. The geosynchronous orbit with an inclination of i = 0 is called a geostationary orbit because the satellite looks stationary over the equator from a ground surface view. As such, a geostationary satellite is useful for covering wide areas. Many meteorological satellites and communication satellites are geosynchronous types.

b. Sun synchronous orbit
Most earth observation satellites, such as Landsat, with lower altitudes have sun synchronous and semi-recurrent orbits. The sun synchronous orbit can be defined as the orbit in which the orbital plane rotates in a year in unison with the one revolution / year apparent motion of the sun. The model precession rate , is a function of inclination i, orbit altitude hs and orbital period T as shown in Figure 5.6.2. As seen in the figure, the sun synchronous orbit has W=1 (revolution / year). For example, in case of i = 100 degree, the altitude of the sun synchronous orbit is about 1,200 km with about 108 minutes of orbit period. The advantage of the sun synchronous orbit is that the observation conditions can be kept with a constant solar incident angle.

c. Semi-recurrent orbit
While the recurrent orbit can be defined as the orbit which returns to the same nadir point in a day, the semi-recurrent orbit returns to the same nadir point in N days repetition (N>1), which is much better for covering all of the earth than the recurrent orbit.


Copyright © 1996 Japan Association of Remote Sensing All rights reserved