Research Concepts RC2000C - Manual (Page 30)

22 RC2000C Az/El Tracking Antenna Controller Chapter 4 Inclined Orbit Satellites
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Chapter 4 INCLINED ORBIT SATELLITES
This chapter describes the characteristics of inclined orbit satellites and the tracking algorithms
available on the RC2000C. The chapter begins with a tutorial on inclined orbit satellite operation, and
continues with a description of the controller's tracking algorithms. It concludes with a section that
guides the user through the hardware and software configuration procedure which must be performed
prior to initiating a track on an inclined orbit satellite.
4.1 Geostationary and Inclined Orbit Satellites
To successfully track inclined orbit satellites with the RC2000C it is necessary to become somewhat
familiar with the characteristics of an inclined orbit satellite's apparent motion as seen by the antenna.
This section briefly reviews the orbital geometry of geostationary satellites, and then discusses inclined
orbit satellites.
A geostationary satellite appears fixed in space to an observer at any point on the earth. In reality, the
earth is rotating about its axis, and the satellite appears to be stationary because it is orbiting the earth
in the earth's equatorial plane with a period identical to the earth's rate of rotation. The earth's
equatorial plane is the plane defined by the earth's equator. Please refer to figure 4.1.
Many forces act on a satellite in geostationary orbit that tend to tilt the satellite's orbital plane away from
the earth's equatorial plane and to pull the satellite out of its assigned longitudinal position. These
forces are due to the gravitational attraction of the moon (which also gives rise to ocean tides) and the
nonspherical earth. A discussion of these forces is beyond the scope of this presentation. A
geostationary satellite must expend propellant to perform station-keeping maneuvers to maintain an
orbit at the proper longitudinal position within the earth's equatorial plane.
East-West station-keeping maneuvers are performed to maintain the satellite's longitudinal position and
North-South station-keeping maneuvers are performed to keep the satellite's orbital plane aligned with
the earth's equatorial plane. Between 20 and 40 percent of the satellite's launch weight is due to
station-keeping propellant. A satellite's design life is determined by the amount of station-keeping fuel
onboard. A satellite is sometimes allowed to drift into an inclined orbit to extend its operational life. For
a geostationary satellite, approximately 90 percent of the propellant is expended for North-South
station-keeping activities. If North-South station-keeping ceases the operational life of the satellite may
be greatly extended.
The orbital plane of an inclined orbit satellite is 'inclined' with respect to the earth's equatorial plane.
Note that in figure 4.1 the inclination angle between the inclined orbit satellite's orbital plane and the
earth's equatorial plane is greatly exaggerated. Typical inclination angles are less than 10 degrees.
When satellite North-South station-keeping activities are suspended, the inclination of the satellite's
orbit increases by approximately 0.9 degrees per year. Whereas a geostationary satellite appears fixed
in space, the apparent position of a satellite in an inclined orbit varies with time.
If an inclined orbit satellite could be viewed by an observer located at the center of the earth, the
apparent motion of the inclined orbit satellite would be a figure eight centered on the earth's equatorial
plane. The motion of the satellite is periodic, which means that the figure eight pattern repeats itself.
The period of the motion is 23 hours, 56 minutes, and 4 seconds. To an observer located at the center
of the earth, the angle subtended by the figure eight pattern from North to South (i.e. height of the figure
'8') is twice the inclination angle and the angle subtended by the figure eight pattern from East to West
(i.e. width of the figure '8') is approximately the inclination angle (in degrees) squared divided by 115.
The apparent motion is slightly greater and somewhat skewed when viewed from the surface of the
earth. The exact shape of the pattern varies with the longitudinal position of the satellite and the place
on the earth from which the satellite motion is viewed. To estimate the height and width of an inclined
orbit satellite's motion as viewed from the surface of the earth, a good approximation is to multiply the
results of the equations above by 1.2.