Abstract:
Due to the large up front cost in fielding a full capacity satellite constellation ( 100 ), it is desirable to have the ability to modify the constellation after deployment to add more capacity in a fashion that does not interrupt service, in order to satisfy a variable or growing market demand. In a satellite constellation including a plurality of orbital planes, each of the orbital planes precesses at a known rate. This invention employs deliberate dynamic manipulation of the orbital precession rates for different satellite planes to increase the separation distance between two adjacent planes. A new plane is then inserted between the two adjacent planes to increase the number of planes and increase the capacity. By continuing a gradual variation, a new orbital plane can be added periodically, e.g. once a year, and the variation can be stopped when no additional orbital planes are desired.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method for satellite constellation growth and more specifically to a method for adding capacity to a satellite communications network over time. 
     BACKGROUND OF THE INVENTION 
     At the present time, communications networks are being formed using space vehicles, or satellites. Several different types of networks using different types of satellites can be provided. For example, geo-synchronous satellites are used in many instances to provide fixed signals (e.g. television, telephone etc.) between fixed points. 
     An example of a more fluid system is one in which low flying satellites are provided in orbits that cooperate to form a constellation. Generally, more than one satellite is provided in the same orbit but spaced from the other satellites in the orbit to provide continuous coverage of the surface of the earth. The orbit defines a plane that may be, for example, a polar plane (or orbit) and a constellation is generally formed by a plurality of planes. 
     Due to the large up-front financial and time investment involved in fielding a satellite constellation, these systems or networks often do not reach peak capacity for a number of years after the initial start up. Once peak communications capacity is achieved, most constellation designs have no fundamental provision to add capacity, except for wholesale replacement of satellites with those having increased functionality. In many instances, it would be desirable to be able to incrementally modify the constellation after initial deployment to add more capacity in a fashion that does not interrupt service. 
     Accordingly, it is highly desirable to provide a method to grow a satellite constellation in an incremental fashion and without interruption in service. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the invention can be derived by referring to the detailed description and claims when considered in connection with the figures, wherein like reference numbers refer to similar items throughout the figures, and: 
     FIG. 1 illustrates a highly simplified diagram of a satellite-based communication system in accordance with a preferred embodiment of the present invention; 
     FIG. 2 illustrates a simplified diagram of the geometry of an ellipse and orbital parameters in accordance with a preferred embodiment of the present invention; 
     FIG. 3 illustrates a plan view of an eight plane satellite constellation in accordance with a preferred embodiment of the present invention; and 
     FIG. 4 illustrates a highly simplified diagram of a satellite beam patterns in accordance with a preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A “satellite” is defined herein to mean a man-made object or vehicle intended to orbit the earth. A “constellation” is defined herein to mean an ensemble of satellites arranged in orbits for providing specified coverage (e.g., radio communication) for portion(s) or all of the earth. A constellation typically includes multiple rings (or planes) of satellites and can have equal numbers of satellites in each plane, although this is not essential. As used herein the terms “cell” and “antenna pattern” are not intended to be limited to any particular mode of generation and include those created by either terrestrial or satellite cellular communications systems and/or combinations thereof. The present invention is applicable to systems including satellites having low-earth, medium-earth and geo-synchronous orbits. Additionally, it is applicable to orbits having any angle of inclination (e.g., polar, equatorial or other orbital patterns). “Precession” is defined by a rotation of the line of nodes of the orbit. 
     FIG. 1 illustrates a highly simplified diagram of a satellite-based communication system in accordance with a preferred embodiment of the present invention. Communication system  100  comprises communication satellites  120 , communication units (CUs)  130 , and terrestrial stations  150 . 
     Communication satellites  120  are illustrated using six polar orbits  140 , with each orbit  140  holding eleven satellites  120  for a total of sixty-six satellites  120 . However, this is not essential and more or fewer satellites, or more or fewer orbits, may be used. While the present invention is advantageously employed when a large number of satellites are being used, it is also applicable with as few as a single satellite. For clarity, FIG. 1 illustrates only a few of satellites  120 . 
     In a preferred embodiment, satellites in each orbit  140  encircle earth at an altitude of around 780 km, although higher or lower orbital altitudes may be usefully employed. Because of the relative movement between communication satellites  120  and CUs  130 , communication link  135  is a temporary one and hand-off schemes are employed to realize a continuous communication channel. 
     Communication satellites  120  communicate with other nearby communication satellites  120  through cross-links  145 . Communication satellites  120  communicate with terrestrial stations  150  using communication links  155 . Communication satellites  120  communicate with communication units  130  using communication links  135 . 
     Terrestrial stations  150  can include earth terminals (ETs). Terrestrial stations  150  can be system control centers or connected to one or more system control centers. Terrestrial stations  150  can also be gateways or connected to one or more gateways (GWs), which provide access to a public switched telephone network (PSTN) or other communications facilities. One CU  130  and one terrestrial station  150  are shown in FIG. 1 for clarity and ease of understanding. 
     CUs  130  may be located anywhere on the surface of the earth or in the atmosphere above the earth. CUs  130  are preferably communications devices capable of transmitting data to and receiving data from communication satellites  120 . By way of example, CUs  130  may be hand-held, portable cellular telephones adapted to communicate with communication satellites  120 . 
     Links  135 ,  145 , and  155  encompass a limited portion of the electromagnetic spectrum that is divided into numerous channels. Links  135 ,  145 , and  155  are preferably combinations of L-Band, S-band, and Ku-band frequency channels and can encompass Frequency Division Multiplex Access (FDMA) and/or Time Division Multiple Access (TDMA) communications (infra) and/or Code Division Multiple Access (CDMA) or combination thereof. 
     In a preferred embodiment, each satellite  120  can send messages to or receive messages from terrestrial stations  150  and to or from many CUs  130  at any given instant. The terrestrial station is responsible for maintaining data records for the satellites. Data records can include position, velocity, and precession rate. In addition, terrestrial stations  150  desirably monitor the health and status of communication system  100  and manage the operations of communication system  100 . Terrestrial stations  150  desirably include antennas and RF transceivers and preferably perform telemetry, tracking, and control functions for the constellation of communication satellites  120 . 
     Preferably, all communication satellites  120  within communication system  100  include cross-link components, earth-link components, subscriber link components, and attitude adjustment components. The attitude adjustment components are used to maintain and/or change one or more of the orbital parameters of the satellite. 
     Seam  110  is also illustrated in FIG.  1 . Seam  110  occurs between two adjacent counter-rotating planes. Two adjacent planes in which the satellites are moving in opposition directions are shown in FIG. 1, as illustrated by arrows  112  and  114 . 
     Polar orbits are illustrated but are not required for the invention. 
     As will be understood by those skilled in the art, a satellite orbiting the Earth (polar orbits are illustrated for this specific example) defines an orbital plane through the Earth. Generally, more than one satellite is provided in the same orbit but spaced from the other satellites in the orbit to provide continuous coverage of the Earth&#39;s surface. At least for purposes of this disclosure, a plurality of interconnected satellites located in a plurality of orbital planes defines a satellite constellation. 
     FIG. 2 illustrates a simplified diagram of the geometry of an ellipse and orbital parameters in accordance with a preferred embodiment of the present invention. In FIG. 2, the essential parameters of an elliptical orbit are depicted. The essential parameters depicted in FIG. 2 are defined as follows: 
     {overscore (r)} is a position vector of the satellite relative to the center of the Earth; 
     {overscore (V)} is a velocity vector of the satellite relative to the center of the Earth; 
     f is the flight-path-angle, the angle between the velocity vector and a line perpendicular to the position vector; 
     a is the semi-major axis of the ellipse; 
     b is the semi-minor axis of the ellipse; 
     c is the distance from the center of the orbit to one of the foci; 
     v is the polar angle of the ellipse; 
     r a  is the radius of apogee, the distance from the center of the Earth to the farthest point on the ellipse; and 
     r p  is the radius of perigee, the distance from the center of the Earth to the point of closest approach to the Earth. 
     The eccentricity, e, of the ellipse (not shown in FIG. 2) is equal to c/a and is a measure of the deviation of the ellipse from a circle. 
     The inclination, i, of an orbital plane, is the angle that the plane makes with the equator. As an example, each of orbital planes  140  illustrated in the simplified satellite constellation of FIG. 1 makes nearly a ninety-degree angle with the equator. It is understood, however, by those skilled in the art that a satellite network containing a plurality of orbital planes must be designed so that the planes include at least a small amount of inclination so that the satellites do not collide in the polar regions. 
     An orbital plane precesses according to the following formula: 
     
       
           W= −2.06474×10 14    a   −7/2  (cos  i ) (1 −e   2 ) −2   
       
     
     where: 
     a is the semi-major axis in km; 
     e is the eccentricity; 
     I is the inclination; and 
     w is the precession rate in degrees per day. 
     Generally, for communications networks utilizing satellite constellations, it is important that all of the orbital planes of the constellation precess at precisely the same rate or else a hole would open in the constellation. 
     Thus, in any satellite constellation a, e, and I of the above formula are substantially the same for each orbital plane, i.e. for all satellites in the constellation. 
     FIG. 3 illustrates a plan view of an eight-plane satellite constellation in accordance with a preferred embodiment of the present invention. In this specific example, the satellite constellation includes eight orbital planes, and their ground tracks are designated  321  through  328 . Each plane including a plurality of spaced apart satellites, illustrated by  320 . 
     Seam  330  is also illustrated in FIG.  3 . Satellites ground tracks to the left of seam  330  illustrate northbound satellites, and satellite ground tracks to the right of seam  330  illustrate southbound satellites. Seam  330  is not required for the invention. In alternate embodiments, the ground tracks could be used to illustrate satellites moving in either direction or in both directions. In addition, those skilled in the art will recognize that the grounds tracks can move or remain stationary with respect to the surface of the earth. 
     Desirably, each orbital plane  321  through  328  is spaced a substantially equal distance from each adjacent plane. That is, the distance between orbital planes  321  and  322  is substantially equal to the distance between orbital planes  322  and  323 , etc. This exact distance is maintained by making the planes  321  through  328  precess at substantially the same rate. 
     However, by varying the precession rate, W, slightly between orbital planes the size of the seam between two adjacent counter-rotating orbital planes can be changed, i.e. seam  330  between orbital planes  328  and  321 , of the plurality of orbital planes illustrated in FIG.  3 . The precession rate can be easily and accurately varied by varying one of: the semimajor axis, the eccentricity, and the inclination for one or more of the orbital planes  321  through  328 . 
     In a preferred embodiment, by varying at least one of the variables (a, e, or i) in the formula for w for each orbital plane  321  through  328 , orbital planes  321  through  328  are moved closer together, and seam  330  between adjacent counter-rotating orbital planes  328  and  321  is made wider. An additional plane of satellites is then inserted into this widened seam. In this fashion, the orbital plane separation distance could continually decrease at a predefined rate, allowing for systematic insertion of new planes and an increase in system capacity. For example, the number of planes could incrementally grow from 6 to 7 to 8, etc. 
     For example, in an orbital constellation at 850 km, a difference of 0.1 degrees in inclination (i) provides a relative rate change of 0.01 degrees per day. It will take approximately one year at this rate to open a seam sufficiently large to allow the insertion of an additional orbital plane. At this rate, the system capacity could grow 10 to 15 per cent in the first year. At any time that the system has grown to a desired capacity, or it is desired to stop growth for any reason, the precession rate can be easily returned to precisely the same rate. 
     In an alternate embodiment, by varying the precession rate, W, slightly between orbital planes the separation distance between two adjacent orbital planes can be changed, i.e. between orbital planes  324  and  325 , of the plurality of orbital planes illustrated in FIG.  3 . For example, by varying at least one of the variables (a, e, or i) in the formula for w for each orbital plane  321  through  328 , orbital planes  321  through  324  can be moved closer together and orbital planes  325  through  328  can be moved closer together. In this manner, the separation distance between orbital planes  324  and  325  can be made larger. An additional plane of satellites can then be inserted into this increased space between orbital planes  324  and  325 . In this fashion, the orbital plane separation distance could continually decrease at a predefined rate, allowing for systematic insertion of new planes and an increase in system capacity. 
     In alternate embodiments, the constellation growth can be achieved using other techniques. From the above formula, the precession rate is inversely proportional to the altitude (a is the semi-major axis in km). If the constellation is initially deployed with six orbital planes, and the altitude of each plane is later slightly decreased, then in a very controlled way, the distance between all orbital planes would decrease and a hole would open at the seam for insertion of another orbital plane. In this fashion, the orbital planes will continually decrease in distance at any predefined rate, allowing for systematic insertion of new planes and an increase in system capacity. If at any time it were desirable to stabilize the size of the constellation, only a small altitude change would be required. The amount of satellite fuel required to perform this altitude change is orders of magnitude less than the fuel required to perform a plane change. 
     FIG. 4 illustrates a highly simplified diagram of satellite beam patterns in accordance with a preferred embodiment of the present invention. In this view, satellite beam patterns  435  and  436  for satellites  430  in each of orbital planes  421  and  428  are illustrated schematically. Generally, beam patterns are formed so as to slightly overlap adjacent beam patterns on the Earth&#39;s surface. In an ideal situation beam pattern edges would meet exactly without overlap. Satellite beam pattern  435  illustrates the beam pattern before a new plane  429  of satellites  430 ′ has been deployed, and satellite beam pattern  436  illustrates the beam pattern after the new plane  429  of satellites  430 ′ has been deployed. 
     When a new orbital plane  429  is inserted into the constellation, the modified beam patterns  436  would ideally overlap slightly. However, in many applications, this ideal situation may not occur, and in such instances, it will be desirable to incorporate satellites with beam patterns that can be modified to provide the desired coverage. 
     When a new orbital plane  429  is inserted into the constellation, at least one set of crosslinks  410  are established between the satellites  430 ′ and satellites  430 . However, in many applications, this ideal situation may not occur, and in such instances, it will be desirable to incorporate satellites with beam patterns that can be modified to provide the desired coverage. 
     The satellite parameters and the crosslink parameters are controlled so that the crosslinks between satellites in different orbital planes are maintained when the orbital planes move with respect to one another. In addition, the satellite parameters and the crosslink parameters are controlled so that the crosslinks between satellites in the same orbital planes are also maintained when the orbital planes move with respect to one another. 
     Thus, a new and improved method of providing growth in a satellite constellation has been disclosed, the growth being in an incremental fashion without interrupting service. In addition, the method of growth can be performed using present satellites and without requiring excessive amounts of fuel to perform the changes. Further, the method can be used to grow the constellation to any desired size and can be stopped at any time. Through the use of the described method, initial costs for satellite constellations can be minimized with increases in capacity being performed at any convenient later date or period of time. This method can also aid in reducing the size of satellites because they can be designed for reduced initial capacity. 
     While we have shown and described specific embodiments of the present invention, further modifications and improvements will occur to those skilled in the art. We desire it to be understood, therefore, that this invention is not limited to the particular forms shown and we intend in the appended claims to cover all modifications that do not depart from the spirit and scope of this invention.