Research Concepts RC2500 - Manual (Page 32)

RC2500 Antenna Controller Chapter 4 Inclined Orbit Satellites 25
Research Concepts, Inc. • 5420 Martindale Road • Shawnee, Kansas • 66218-9680 • USA www.researchconcepts.com
Chapter 4 – Inclined Orbit Satellites
There are multiple versions of the RC2500 antenna controller that operate with a variety of antenna
types. These controller versions rare grouped into two categories, the RC2500A and the RC2500B. The
difference between the two groups is the ability of the ‘B’ models to track inclined orbit satellites. The ‘A’
models do not have this capability. This chapter describes the characteristics of inclined orbit satellites
and the tracking algorithms available on the RC2500B. 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 RC2500B 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.