Inclined orbit and type of controller

Orbital inclination is the angle between the plane of an orbit and the equator. An orbital inclination of 0° is directly above the equator, 90° crosses right above the pole, and 180° orbits above the equator in the opposite direction of Earth’s spin.

Orbital inclination measures the tilt of an object’s orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object.

For a satellite orbiting the Earth directly above the equator, the plane of the satellite’s orbit is the same as the Earth’s equatorial plane, and the satellite’s orbital inclination is 0°. The general case for a circular orbit is that it is tilted, spending half an orbit over the northern hemisphere and half over the southern. If the orbit swung between 20° north latitude and 20° south latitude, then its orbital inclination would be 20°.

The diagram of Inclined orbit

Various satellites we are using for broadband communications are in GEO orbits which are located 22,300 miles above the equator, and the goal is for them to stay fixed in that one spot over the same piece of the planet, right over the equator.

However, Geostationary orbits are not very stable. The satellite is affected by many gravitational forces such as the sun and moon. These forces may cause the satellite to drift north and south on a daily basis and the effect would worsen with time. Therefore the satellite has a fuel or propellant generally hydrazine which through small jets or thrusters, keep the satellite in position. The most efficient use of the fuel is to have the satellite trace a figure 8 in space, keeping it within a small ‘box.’ The fuel is used as efficiently as possible to extend its use. This process is called station keeping.

Usually the propellant will take 10 to 15 years to run out, the satellite must be discarded and replaced ; often moved to a “graveyard orbit” where it is out of the way by then. To delay this inevitable ending, satellite owner may elect to omit the inclination maneuvers that control north/south movement and instead, only control eccentricity or east/west movement. This results in an elongated figure 8 in the sky, where the satellite appears to drift north and south, while otherwise remaining in the same place over the earth. Also some satellite operators decide to put the satellite in an inclined orbit. The greater the inclination of the orbit is, the larger the surface area of the earth that the satellite will pass over at some time in its flight.

Satellites are usually put into inclined orbit at the end of their life when station-keeping fuel is very low, by suspending North-South station-keeping maneuvers (while continuing East-West station-keeping), a significant savings of propellant can be realized. Typically, for a geostationary satellite, 90 percent of the total propellant usage is due to North-South station-keeping maneuvers.

A satellite in an inclined orbit has certain known characteristics (to a close approximation). The inclination of the satellite’s orbital plane relative to the earth’s equatorial plane increases at a rate of between 0.6 and 0.9 degrees per year. The rate varies from year to year. The apparent motion of the satellite is periodic with time, the period is approximately 23 hours, 56 minutes, 4 seconds. The apparent motion of the satellite about a nominal position as viewed from the center of the earth is a figure eight pattern.

While an inclined-orbit satellite poses a problem for the end user that the earth station antenna must track the satellite. For this purpose, the antenna must be equipped with a dual-axis steerable mount and a tracking controller.

A dual-axis steerable mount is a motorized mount which can be moved independently about two axes: east-west and up-down. Any of the following mount types can be used.

EL/AZ mount. This mount is dual-axis by definition: it adjusts the antenna in the azimuth (east-west) and elevation (up-down) axes.
Polar mount. A polar mount can be used for tracking if it is equipped to allow adjustment about the declination axis throughout the full range of the satellite’s figure-8 pattern.
Hybrid mount.I t is a dual-axis mount which is neither EL/AZ nor polar. The up-down axis is elevation; the east-west axis falls between azimuth and hour angle.

The tracking controller moves the antenna automatically to track the satellite. Two types of controllers are available commercially:

AGC-controlled.This type of controller monitors the level of the received signal (typically by monitoring receiver AGC voltage), and moves the antenna periodically to peak the signal.

Most AGC-controlled controllers are equipped to maintain a “history,” or ” map,” of the figure-8 pattern. The history is established during the first day’s operation; thereafter, the history is used in two ways: it tells the controller which way to move the antenna when peaking the signal, and it allows the controller to continue tracking if the satellite signal fails.

The controller updates the history during the course of normal tracking. Thus, it responds automatically to changes in the shape of the tracking-8 pattern as the satellite continues to drift along its north-south axis.

AGC-controlled controllers move the antenna in a series of short steps. For this reason, they are sometimes called “step-track” controllers. The original data used to generate the Figure-8 pattern illustrated above was produced by an AGC-controlled controller. Note that the individual steps are clearly evident in this illustration.

Program-controlled.This type of controller mathematically calculates the pointing angles to the satellite and moves the antenna accordingly. Calculations are based on program data entered into the controller.

This type of controller is capable of moving the antenna continuously, rather than in a series of steps. This technique is advantageous in low-signal situations where any change in AGC voltage would result in degraded signal quality.

Because this type of controller calculates pointing angles from program data, it does not respond automatically to changes in the tracking pattern as the satellite drifts. For this reason, the program data must be updated periodically.