Systems for communication via satellites are well known in the art and have been used in many applications for several decades. Many of these systems use geostationary satellites, i.e. satellites seen at fixed positions in the sky relative to the earth due to being located at the Clark Belt (i.e. approximately 36,000 kilometers above the earth's equator, where the angular velocity of the satellite matches the angular velocity of the earth rotation around its axis). As these satellites seem stationary, ground stations in some of these communication systems (for example VSAT systems, television broadcasting systems, etc.) may employ relatively simple and inexpensive antennas (e.g. ones not including moving parts and/or motors). This economical aspect allows wide usage of such communication systems for many applications.
However, geostationary satellites are actually not stationary at all but rather very much in motion. While most of this motion is synchronized with the earth rotation, some of it causes the satellite to deviate from its nominal position above the earth's equator. In order to keep a geostationary satellite in place, i.e. within a small distance from its nominal position (a distance which does not affect the performance of communication systems and/or which allows use of antennas lacking any tracking mechanisms (non-tracking antennas)), the satellite course must be periodically corrected. Thus, geostationary satellites have engines for performing course corrections.
Unfortunately, course-correction engines of geostationary satellites run on fuel (gas). Each geostationary satellite is launched with a certain amount of fuel, designed to suffice for making all the required course corrections over the predetermined operation duration of the satellite (usually around 15 years). However, since building and launching new geostationary satellites is very expensive (usually in the hundreds of millions of USD per satellite), operators of geostationary satellites try to prolong the use of already deployed satellites before replacing them with new ones. Thus, as a geostationary satellite runs out of fuel (leaving little fuel for taking the satellite out of orbit when it is no longer usable), the satellite operator reduces the frequency of course corrections and ultimately stops making them altogether.
As a result of making less or no course corrections, geostationary satellites develop a cyclic inclination motion around the nominal position (relative to the earth). Thus, a satellite in such condition is often referred to as an inclined orbit satellite. The path the satellite makes in the sky, as a result from the cyclic inclination motion and as seen from the earth, resembles the figure “8”. An inclined orbit satellite completes a full inclination cycle every 23 hours, 56 minutes and 4 seconds, during which it passes the center of the inclination path (i.e. the satellite's nominal position) twice. The magnitude of the satellite's inclination is measured in degrees (i.e. an angle, as seen from a location on earth looking at the satellite). With its course being uncorrected, the inclination of an inclined orbit satellite increases by approximately one (1) degree per year.
FIG. 4 shows an exemplary satellite communication system, comprising a ground station 410 and an inclined orbit satellite 420. Due to the inclination, satellite 420 may be seen from ground station 410 as moving around orbital arch 430 in accordance with inclination path 440, crossing orbital arch 430 at point 460 twice in each inclination cycle. As a result of the inclination, satellite 420 may be seen from ground station 410 as moving across inclination angle 450.
As previously described operators of geostationary satellites keep operating inclined orbit satellites as long as they can sell the capacity over these satellites (i.e. for use by communication systems) or until they manage to replace them by new ones. Using inclined orbit satellites requires using tracking antennas at the ground stations of communication systems. These antennas are usually more expansive than regular antennas that may be used with properly positioned geostationary satellites. However, some communication systems operators may opt to using the more expensive equipment since the satellite capacity is usually sold at lower prices (to balance the higher cost of equipment) and due to capacity shortage over properly positioned satellites.
It is well known in the art that tracking antennas may include one or more motors, for adjusting the antenna alignment along one or more axis, and at least one controller, which may control the motor(s). The controller may be configured to use methods for determining the satellite position and for keeping track of its position as it moves along the inclination path in order to insure continuous communication via the satellite between the ground station and any other ground station(s) comprising the communication system.
However, the existing methods used for tracking inclined orbit satellites have deficiencies. Most of these deficiencies are related to a process called peaking The peaking process may be used for determining the correct alignment of the antenna with the satellite (i.e. for pointing the center of the antenna beam at the satellite's position). In order to do so, the antenna must be moved off the center of the beam and/or around the expected position (i.e. the center position) while reception quality of a signal arriving from the satellite is measured at each of the attempted positions. Once reception level measurements covering the entire peaking scanning range are acquired, the measurements and information regarding antenna positions corresponding to those measurements may be processed to produce a calculated center position.
However, in order to achieve accurate alignment (i.e. pointing the exact center of the antenna beam towards the satellite with minimal alignment error), the antenna must be moved quite significantly away from the center position, causing perhaps significant deterioration in reception quality of received signals. On one hand, if the peaking process is done with a purpose to yield a highly accurate result and/or a minimal alignment error, the antenna must be moved over a relatively large range, hence the process may take considerable time to complete and during which service at the ground station is significantly affected (i.e. as moving further away from the center position further decreases at least the reception level). Moreover, during that time the satellite keeps moving, i.e. the longer it takes to complete the process the further the satellite may move and the larger the resulting alignment error may get after all. On the other hand, if the peaking process is shortened and the search is limited to a smaller range, the resulting antenna alignment is likely to be less than optimal and include a significant alignment error. This alignment error reduces overall performance of the ground station (as at least the reception quality is less than optimal) and increases the probability for needing to repeat the peaking process thereafter (hence further causing performance degradation as well as expediting wear processes of motors and moving parts, which results in higher maintenance costs).
Thus, a more efficient method for tracking inclined orbit satellites is needed.