DETAILED DESCRIPTION OF THE INVENTIONReferring toFigure 1, nomenclature used throughout this disclosure and the Figures and Equations is presented. In addition, in order to effectively illustrate and explain the invention, the diagrams and equations presented are for a 2 dimensional view of the system, with diagrams being illustrated inFigures 2 through 13, and with equations (1 through 19) being presented herein and in a listing at the end of this section. One feature of the invention is that the LEO communications satellites in the constellation be in polar orbits. Because the plane containing geosynchronous satellites for which the frequencies are to be re-used are in an orbit which is orthogonal to any plane of the polar orbiting LEO communications satellite constellation disclosed, the 2D configuration disclosed in the diagrams and equations is the simple projection from any LEO orbital plane of a 3D configuration. Therefore, 3D diagrams and equations are projected extensions of the 2D depictions which are well understood and simple to produce by those practiced in the art of orbital mechanics and analysis.
Furthermore, more complex equations than those presented herein, which accommodate the slight ellipsoid shape of the earth and other higher order factors, are well known to those practiced in the art of orbital mechanics. The assumption of the earth as being perfectly spherical is used throughout this disclosure to illustrate the principles involved and the mechanics of the invention, but is not meant to form a limitation on any matter disclosed herein. The principles disclosed herein and the invention may be extended to accommodate a non-spherical earth and higher order orbital elements without departure from the scope of the disclosure.
Figure 2 illustrates the situation and provides foundation for the subsequent Figures. In
Figure 2, a single plane of multiple LEO satellite planes of a LEO satellite constellation in a polar orbit is represented, with two of many satellites which would be in the orbital plane OP shown, indicated at LEO1 and LEO2. As would be typical for a LEO communications constellation, and as indicated in the figure as LEO1 BEAM and LEO2 BEAM, LEO satellites typically create overlapping beams of coverage, for both transmission to, and reception of, signals from ground stations. Within each beam of coverage of a single LEO satellite, there may be multiple sub-beams, enabling frequency and polarization re-use within a beam, in the communications functions with the earth stations. Furthermore, as is well understood by those practiced in the art, the beams and/or sub-beams may be directed in real-time in order to accommodate various orbital elements and earth station practicalities. By fully populating an orbital plane, and by positioning multiple orbital planes at regular angular longitudinal spacing, the entire earth can be covered at all times with beams of at least one satellite in the LEO communications constellation. The Iridium communications satellite constellation is an example of such a constellation, owned and operated by Iridium Satellite LLC. However, the Iridium system, as well as other systems, employ spectrum which is not the same as that employed by GEO satellite communications systems, and no more of such spectrum is available. Examples of satellites and their operations are disclosed in
US Patents 5,410,728 and
5,604,920.
Still referring toFigure 2, points on the surface of the earth in the northern hemisphere are indicated asP1NE at 70 degrees north latitude, throughP8NE, which lies on the equator. For each pointP, a vector is drawn indicating the bore sight of a directional antenna which would point at a geostationary satellite if located at that point. In addition, the LEO satellite orbit track OP shown orbits in a clockwise fashion, however this is simply for convention in the Figures and nomenclature in this disclosure and does not limit the generality of this disclosure. Under the convention used herein, for any pointP in Quadrant 1 or 2, the LEO satellites in polar orbit ascend from the North and descend toward the South (in the direction of travel indicated).
Still referring toFigure 2, as can be seen, anytime a LEO satellite using the same frequencies as a GEO satellite for communications with an earth station passes over a pointP, there is a spot in the orbit track where the LEO satellite is directly in line between the GEO satellite and the earth station. Therefore, at that point, if the LEO satellite were transmitting on the same frequency that the earth station is set to receive from the GEO satellite, then interference would result, and the GEO satellite's signal could be interfered with by the LEO satellite's signal, as both signals on the same frequency would be simultaneously received by the earth station's antenna and RF front end, even with a highly directional earth station antenna pointed specifically at the GEO satellite.
Referring now toFigure 3, the actual elevation pointing angle between a directional antenna at any pointP on the surface of the earth, pointing to any geostationary satellite, is indicated. The azimuth pointing angle is not shown and not relevant to illustrating the operative principles since any azimuth angle in 3D would have the same projection into the orthogonal polar plane shown inFigure 3. The governing Equations 1 and 2 provide the solutions to computation ofγ for any latitude Φ. Equations 1 and 2 are set forth in the Equations Table and below (the equation number appearing next to the equation, in parentheses):
By way of example, and without limitation, the table inFigure 3 computes the approximate elevation angle for a directional antenna at increments of 10 degrees of latitude, starting 80 degrees of latitude, which is approximately the highest latitude under which GEO-to-earth station line-of-sight communication links could be sustained, to 0 degrees of latitude, which is the equator.
Next, referring toFigure 4, the geometry of two satellites in a LEO orbital plane is presented. In thisFigure 4 and including Equations 3 and 4, the relationship between the spacing,s, of the LEO satellites in-plane and the subtended angleθ as measured from a vertex at the earth's geometric center,C, is indicated. In thisFigure 4,P is indicated at the equator. Equations 3 and 4 are set forth below (the equation number appearing next to the equation, in parentheses). Expressed in terms ofΘ, Equation 5 provides a solution for determining the angleΘ.
Next, referring toFigure 5, the two sets of geometries are overlaid, with the heavy blue lines indicating direction vectors to LEO satellite locations as the LEO satellite constellation orbits, as seen fromP1NE andP8NE. In theFigure 5 overlay it can be seen that as a LEO satellite approaches any pointP on the earth from the north, a directional antenna atP pointing toward the GEO satellite is pointing south.
However, as any specific LEO satellite passes over and then goes beyond any pointP, if it continues transmitting backwards toward the earth station atP, at some point it would transmit down the boresight of any antenna pointing toward a GEO satellite.
In order for a LEO satellite constellation to provide continuous coverage to anywhere on earth, at least one satellite must be in view at all times from any pointP on the earth, and the pointing direction from that satellite to the pointP must not be the along the same vector as the pointing direction between pointP and a GEO satellite. Therefore, during the period when a first satellite in LEO orbit must cease transmitting to pointP in order to avoid interfering with GEO signals arriving at the same time and place on the same frequency, another second satellite in LEO orbit must be available in view of the pointP in order to carry on whatever communications may be taking place between the LEO communications satellite constellation and the earth station at pointP.Figures 6 and7, including the 'zoom-in' views of those diagrams, will be used to demonstrate two end-case situations, first inFigures 6A,6B at the near maximum latitude where any earth station may be practically anticipated to communicate with a GEO satellite (approximately 70 degrees latitude) and secondly inFigures 7A,7B at the equator.
Referring now toFigures 6A,6B, the computation is shown to compute the maximum spacing of two satellites in a polar LEO orbit which are, or could be, in communication with pointP1NE at 70 degrees latitude, such that both (a) the earth station is never without line-of-sight to an orbiting LEO satellite suitably far enough above the local horizon to be available for reliable communications, and (b) during the period when any orbiting LEO satellite is within a guard band around the vector between the earth station and a GEO satellite, another LEO satellite is within view (and far enough above the local horizon), to take over any communications function with the earth station from the first LEO satellite (since the first LEO cannot transmit to the earth station when it is within the guard band, so that it does not interfere with the GEO satellite communications to the earth station).
InFigures 6A,6B, s must be found (see Equations 3 and 4), which is used to computeθ (see Equation 5) and thus the number of satellites in the LEO orbital plane required, subject to the constraints that the satellite orbiting at altitudeh must be at least an angleα above the local horizon and maintain a guard band angle ofβ around the vector between the earth station and a GEO at angleγ. The cosine formula is employed, first with respect to triangleC-P1NE-D to compute d, then with respect to triangleC-P1NE-A to computea, then finally with respect to triangleA-P1NE-D to compute s, given the previously computedd, previously computeda and known angleω1. (Although Equation 8 may provide two solutions, the meaningful solution is utilized for the distance d.) The relevant equations are indicated as Equations 5 through 8, 9 through 12, and 11 through 13, which are set forth below (the equation number appearing next to the equation, in parentheses).
As can be seen by comparingFigures 6A,6B andFigures 7A,7B, as a satellite in the LEO constellation approaches the equator and covers a pointP with its communications beam, the distance between when the satellite ascends above the horizon and when it must stop transmitting to the pointP, asP also approaches the equator, is reduced. Unlike the more northerly positions of the satellite and pointsP, however, pointP at the equator can also be communicated with by a LEO satellite which is departing the equator, or descending in the sky to the South.
Figure 8 shows one portion of one plane of the invention, which comprises a LEO-based constellation for communications with earth stations anywhere in the world, and which can operate simultaneously in spectrum allocated for GEO-to-earth station use, including to an earth station at the same time and place, which operates as will be further described below.Figure 8 shows three satellites (represented by the circles designated 1, 2 and 3) in one orbital plane at two different conceptual times, called T=1 and T=2, operating near the equator to service a ground station at the equator. The satellites 1, 2 and 3 at time T=1 are represented by solid line circles and at time T=2 are represented by broken line circles. Operations near the equator are the limiting case for the invention, and so are shown in detail and the focus of much of the disclosure. InFigure 8, one of the satellites labeled "2" approaches the equator at T=1, and then crosses the equator, with pointP8NE below it. In this Figure, the pointP8SE is introduced, being the nearly identical point toP8NE, except just south of the equator whilstP8NE is just north of the equator. The northern horizon is indicated asNH and the southern horizon asSH.
In the disclosed invention, as satellite 3 rises above the northern horizonNH by a chosen angleα with respect to an earth station atP8NE at the equator, satellite 3 is able to create a communications link withP8NE. At the same time T=1, excepting a necessary time for hand-off, satellite 2, which was previously serving communications withP8NE, ceases its communications with P8NE as it entersP8NE's GEO satellite guard band. As satellite 3 continues ascending acrossP8NE's northern sky, it continues serving any communications needs ofP8NE, which can be on the same frequency as that used with any GEO satellite, without interfering with any communications which may be ongoing with said GEO satellite, until it arrives at the position indicated of satellite 2 at T=1. At that time a satellite 4 (not shown) will begin rising above the northern horizon with respect toP8NE, so that satellite 3 can turn off its communications link withP8NE while it transitions acrossP8NE's GEO guard band.
Meanwhile, as satellite 2 comes out of the guard band ofP8NE at T=2, it can begin servingP8SE, which is assumed at the same place on the equator asP8NE, except south of the equator. Prior to satellite 2 beginning service ofP8SE, P8SE was served by satellite 3, which is setting to the south, with respect toP8SE. In the same way, every point around the globe is covered by a satellite in the constellation.
Referring now toFigure 9, the communications beams associated with two satellites in the disclosed invention are described as they transition across the sky over the equator from North to South. As stated previously, the beams described are the antenna patterns created by real-time adjustable beam antennas on the LEO satellites, such as can be created with phased array antennas, which are well known and understood to those practiced in the art. As also previously stated, the beam envelopes may have within each of them various sub-beams for specific frequency-reuse, polarization re-use or accommodation of other orbital elements or earth station elements which are nevertheless within the scope of the invention.
Referring still toFigure 9, the forward beam angle with respect to the satellite is indicated as angleψ and the backward angle of the beam is indicated as angleλ.
As indicated inFigure 9, around the descending semi-circle of the polar orbit of the LEO communications satellite, for that portion during which a satellite is in quadrant 1, the LEO projects its communications beam forward in the direction it is travelling, continuously, at an angle ofψ, which can be as large an angle as reasonable or feasible for communications with earth stations until the satellite's latitude,σ, reaches a so-called latitude limit, as it approaches the equator. With respect to the forward portion of the beam, as it approaches the equator, the control means of the LEO satellite's directional antenna begins to reduce the forward angle of its forward beam as indicated as the satellite labeled SAT2 progresses from T=1 to T=6 toward the equator.
Also inFigure 9, now noting the satellite labeled SAT1, as it progresses from T=1 to T=6, its beam is extinguished over the equator and no communications occur with that satellite from any ground station while it transits across the GEO guard band at the equator. After crossing the equatorial guard band into quadrant 2, the SAT1 then expands what is now the backward pointing portion of its communications beam as indicated, such that when the satellite has reached the latitude limit angle away from the equator, the backward beam cover a maximum region behind it, as a mirror image to the forward beam communications coverage area produced in quadrant 1.
Each satellite also controls the angle,λ, of a so-called backward beam as shown inFigures 10A,10B.Figures 10A,10B also indicate the latitude of the satellite at any particular point in its orbit,σ. The parameters and labelsα,β,γ,A, P, C, a, and d are as previously discussed with respect toFigures 6 and7, and Equations 5 through 15 are operative as previously described with respect toFigures 6 and7 to compute the relevant geometric angles and lengths of the triangles. Once the lengthsa andd are found, the Equations 17, 18 and 19 are employed to calculateψ at the latitude limit andλ as a function ofσ, for a givenβ andγ. Equations 14 through 19 are set forth below (the equation number appearing next to the equation, in parentheses).
Tabulations of the various parameters appear in Table 1 (Figure 14) for the input parameters ofα=5 degrees,β=5 degrees andh=1,800km. For those parameters, the computations outlined in red define the primary elements of the disclosed invention, and the implementation of the satellite antenna control mechanism which regulates the beam and/or sub beam projection, showing: that 11 satellites are required in each polar orbital plane, that the maximum forward beam angle required is 50.96 degrees, that the forward beam should begin limiting at a satellite latitude of 34.04 degrees (maintaining pointing to just past the equator as it approaches the equator), and that the angle of the back beamλ should track the values indicated in the columns titled withλ andσ, whereσ to the left of the long black line is treated as a dependent variable based on the LEO sat communicating with an earth stationP as indicated in the first column. Notice that as the equator is approached, the back beam angle becomes negative, indicating that the back beam must begin pointing somewhat ahead of the satellite, rather than behind it, as the satellite nears the equator, in order to avoid transmitting down the bore sight of an antenna pointed at a GEO satellite. In the limiting case whereβ=0, the back angle never goes negative and the back beam angleλ at the equator is 0 (this situation is shown in Table 2,Figure 15). The columns underσ andψ to the right of the long black line in Table 1 (Figure 14) compute the forward beam angle as a function of the latitude of the satellite, where now the latitude of the satellite is treated as an independent variable.
To show how the disclosed invention is applicable to other parameters, Table 3 (Figure 16) shows the computations for input parameters ofα= 10 degrees,β=10 degrees andh=800km. For those parameters the computations show: that 21 satellites are required in each polar orbital plane to implement the method, that the maximum forward beam angle required is 61.04 degrees, and that the forward beam should begin limiting at a satellite latitude of 18.96 degrees (maintaining pointing to just past the equator as it approaches the equator). For example, still referring to Table 3 (Figure 16), when one of the satellites in the constellation approaches the equator in quadrant 1, when its latitude references to the center of the earth is at 8.06 degrees North, its back beam must be -0.05 degrees or less, therefore actually then the back beam is pointing forward.
As can be seen inFigure 9 and10, as one of the satellites in the constellation approaches the equator, its overall beam width declines by the governing equations to zero. However, as the beam width approaches zero, there is some practical limit for realizable satellite antennas. This practical limit can change based on implementation methods for the antenna and its associated control function, and at that limit the beam can simply be turned off (no longer transmitting). Any additional margin associated with such minimum beam width can be accommodated by adjusting the guard band,β.
The satellites may be configured with a satellite control mechanism that controls the satellite operations. For example, the control mechanism may determine the position of the satellite, including its latitude, and may use the latitude position to regulate the beam projected form the satellite. According to some embodiments, the satellite control mechanism preferably includes computing components that are carried by the satellite. The computing components preferably include a computer that is provided with software that includes instructions for monitoring the positions of the satellite along its orbit and regulating beams projected by antennas of the satellite. Any suitable mechanism for directing the beam of the antenna may be employed, including mechanical or electronic controls that limit, expand, direct, or combine these methods to regulate the beam angle. The beam also may be formed from sub-beams. The satellite may be provided with one or more real-time adjustable beam antennas, such as, for example, a phased array antenna, or other antennas that are known in the art. The satellite antennas may generate beam envelopes that may comprise various sub-beams for specific frequency-reuse and/or polarization re-use. According to some embodiment, the beam envelope sub-beams may be configured to accommodate other orbital elements and/or earth station elements. According to preferred embodiments, the satellite is configured to produce one or more beams, and preferably, a beam may be provided with one or more forward portions, and one or more rearward portions (for example, where a beam forward portion may comprise a first beam, and a beam rearward portion may comprise a second beam). The beam portion may be regulated (e.g., by turning it on or off) to limit the beam field or projection. According to some embodiments, the satellite control mechanism may be powered using the power source of the satellite. According to some embodiments, the system components may be powered with solar panels that may be deployed on the satellite for this and other purposes. The satellite beam control mechanism preferably includes computing components configured to process the satellite location information, and determine the beam angle to be provided by an antenna, such as, for example, a transmitting antenna of the satellite. The control mechanism preferably manipulates the beam angle in accordance with determinations from the satellite location information and the application of the positioning as set forth herein, and, in particular, according to the embodiments provided represented by the equations herein (seee.g., Equations 5 through 13). Satellites may be provided with suitable antennas for communications with earth stations. For example, phased array antennas, helical antennas, or other suitable antennas may be provided. In addition, the satellites may be configured to communicate with other satellites. Suitable antennas, such as lenses for satellite cross-link communications may be provided. For example, adjacent satellites may communicate with each other. The satellites also may be provided with devices for routing signals, such as, communications and data. For example, the satellite may be configured with one or more switching units that process information as to the communication destination, and route the communication through an appropriate satellite. According to some embodiments, the satellites are configured to route communications to an earth station within the beam range of the satellite, and the earth station may be connected to a network that routes the communication to a designated destination. Similarly, transmissions from an earth station may be received by a satellite, and the satellite may route that communication to a destination, such as, a device. For example, according to some embodiments, the satellites and the satellite system preferably may transport of datagrams between any satellite and a ground terrestrial network. Earth stations, which may be configured as or in association with a gateway station may receive and transmit signals, such as datagrams, between a satellite. This may be carried via immediate re-transception of the datagram to a gateway station in view of the same satellite (i.e., bent pipe), where the data is transmitted to the satellite from an earth station or gateway, and the satellite sends it right back down again. In some embodiments, the signal or data may be sent without modification, other than processing to retransmit the signal back (which may involve one or more of signal amplification, shifting the uplink/downlink frequency for the re-transmission. According to other embodiments, the satellite may be configured with equipment that may be used to carry out on-board processing of the signal, such as for example, to demodulate, decode, re-encode and/or modulate the signal (e.g., through a regenerative transponder). According to some preferred embodiments, transport of datagrams between any satellite and a ground terrestrial network may be carried out via a crosslink to one or more other satellites in the satellite constellation, and then from these other satellites to a gateway. For example, the intended gateway may not be in view of the first satellite at any particular time, but may be in view of one of the other satellites of the satellite constellation. A satellite in view of the gateway may receive a datagram routed from another satellite (e.g., the first satellite). The satellites of the constellation preferably may be configured to crosslink, and route transmissions through their respective cross-links.
The equations and tables may be re-arranged with straightforward mathematical manipulations well known to those practiced in mathematical arts, to enable any particular parameter shown to be a free variable, allowing the rest of the satellite constellation orbital elements and satellite antenna pointing arrangements to be computed thereafter, without going outside of the scope of the present invention.
It is readily understood from the symmetry of the disclosed invention that the satellites operate in mirror image of each other in each quadrant. That is, the geometry, antenna patterns and operation of the satellites in a plane in quadrant 1 are mirrored around the equator to generate quadrant 2, which is then mirrored around the North-South axis of the earth to generate quadrant 3, and then mirrored up around the equator to generate quadrant 4. The details of a satellite crossing the equator have been presented in detail as this is the place of highest potential for interference, and this when crossing the equator, the most effective interference avoidance technique is to simply have the equator-overpassing satellite cease transmitting to the earth stations when within the guard band around the GEO pointing vector. This also permits the satellite sufficient time to re-orient the antenna system to the subsequent quadrant. When a satellite is over the poles, it must also re-orient its antenna pointing system however the mechanics of that can be performed in an unconstrained way in any manner suitably designed by those practiced in the art, because there are no geostationary communications possible at the poles, since no geostationary satellite can be in view from the poles.
It should be noted that the drawings inFigures 8,9 and10 imply that the LEO communications beam intersects the ground at exactly the equator. In practical implementation, the beam would extend forward beyond the equator as the satellite approaches the equator, by an extent necessary to accommodate various uncertainties in orbit and antenna patterns, as well to accommodate the time necessary to handoff from the other LEO satellite as it enters the guard zone over the equator. This practical matter is easily accommodated without going outside of the scope of the invention.
An option that complicates the design of the satellite antenna and antenna control system is as follows, but remains within the scope of this invention. The beam envelope which has been disclosed above is typically composed of many sub-beams. Certain sub-beams may be turned off or re-directed as the satellite passes over or near the equator, enabling additional communications support for areas above and below the equator, without causing the satellite to cease all transmission to earth stations. This option however requires careful control of side lobes of the satellite antenna pattern, which can add expense and may not be technically possible for certain antenna implementation methods.
Because the orbit of the disclosed satellite constellation is polar, the azimuth plane of the LEO satellite constellation can be operated independently of the elevation plane which has been thoroughly described herein. As such, the number of planes for complete global coverage can be designed as an independent variable with respect to the operation of the satellites and their antenna patterns in a plane. For example, the LEO-based communications system can be designed to cover 30 degrees of longitude to the right or left of the plane of orbit, while simultaneously operating as provided for in this disclosure, and as above with respect to Table 1 (Figure 14) for example, within the plane of orbit.
Figure 11 shows a complete satellite constellation (not to scale) which complies with the specifications of Table 1 (Figure 14), with 11 satellites per plane and 6 planes (only three being depicted in the Figure), at an orbit height of 1800km, in the required polar orbit. As seen inFigure 11, each plane may be populated such that the satellite within the plane have the equator crossing time of each satellite slightly offset from the neighboring plane, which, depending on plane spacing selected, may provide further assistance to cover earth stations near the equator which are closer to a neighboring orbital plane.
The method of onward communications between the class of LEO communications constellations disclosed herein and a further earth terminal or earth gateway is flexible and may be either via an earth gateway within view of each satellite in a so-called bent pipe architecture, or may be via a cross-linked architecture, such as that employed by the Iridium satellite constellation. Both implementations are possible with the disclosed invention, and either may be employed to complete a communications link between an earth station, a satellite operated within a satellite constellation as disclosed herein, and another earth station, or other terrestrial data or communications network.
In addition to the isolation provided between the GEO communications satellites, GEO-involved earth stations and the LEO-based communications system disclosed herein that is provided by the geometry of the operations and the antenna system, there are additional features of the disclosed system related to the earth stations which communicate with the LEO satellites which can now be described. When an earth station transmits up to a LEO satellite in the disclosed system, the transmission must only overcome the distance to the LEO satellite, which requires considerably less signal power than that required to overcome the distance to a GEO satellite at the same frequency. This situation is shown in Table 4 ofFigure 17, which shows the free space path loss (FSPL) in-plane calculated for various angles from a LEO satellite in a constellation orbiting at an 800km altitude to an earth station, compared to the FSPL to a GEO satellite, at the same communications frequency of 12 GHz (Ku Band). The calculations show a minimum difference in path loss of 33dB. The difference in path loss provides a significant link margin to further reduce the possibility that a signal transmitted by an earth station with an omnidirectional antenna which is intending to transmit only to a LEO satellite is nevertheless recognized by a GEO satellite listening on the same frequency, thereby causing an interference with the GEO satellite communications system.
Referring still to Table 4 shown inFigure 17, the same path loss data provides the basis for the ability of an earth station associated with the LEO satellite communications constellation which has an omnidirectional antenna to avoid interference to it from a GEO-based communications signal. Because the LEO satellite is isolated from GEO-based receiving stations by geometry, the LEO satellite may transmit at powers such that the signal power received on-ground by the earth stations associated with the LEO system may be much higher than the same power received by an omnidirectional antenna at the same frequency from a GEO system, thus providing the ability for the LEO-associated earth station to reject the much weaker signal from the GEO satellite, by means commonly known to those practiced in the art of receiver design.
Notwithstanding the previous paragraph, additional link margin may be desired to accommodate wider operating envelopes in a given system design. Therefore, the LEO system disclosed may be paired with earth stations which are designed to communicate solely with the LEO satellites, even though they are communicating on the same frequency as an earth station communicating with a GEO satellite right next to it. One additional optional element of the LEO satellite based communications system disclosed is to add a directional antenna to the earth station. While a fully azimuth and elevation directional antenna with a small beam width is an option, such an antenna is often prohibitive in cost, size, weight or power for certain applications. However, with the LEO satellite operating as described above provides for a communications direction that is always pointing towards the North in the Northern hemisphere, and pointing South in the Southern hemisphere. This fact enables a dramatically simpler directional antenna to be employed by the earth station. The in-plane antenna pattern of a simple loop antenna, which is oriented perpendicular to the ground, is shown inFigure 12. Even this simple antenna provides as much as 12 dB of additional link margin at a point P on the equator, and even more for higher latitudes. The only requirement on the earth station is that the direction of the antenna's highest gain be pointed generally South, if the earth station is in the Southern hemisphere, or generally North, if the earth station is in the Northern hemisphere.
Referring now toFigure 13, it is shown that by pointing the maximum antenna gain away from the direction of the GEO satellite, and towards the direction of the LEO satellite, additional margin is obtained to assist in minimizing the possibility that the LEO earth station's transmissions is received with sufficient power by a GEO satellite system to be recognized. The requirement to be generally pointing North or South, depending only on which hemisphere the earth station is in, is a much simpler requirement on an earth station directional antenna than the requirement to be fully azimuth and elevation directional capable, thus making the LEO-based system disclosed more economical for mass deployment. Other similar patterns from other types of antennas can be created which are well known to those practiced in the art of directional antennas which can provide for the same or greater additional margins with economical implementations, without departing from the scope of this disclosure.
References are made to an earth station that receives and transmits communications between it and LEO satellites. The earth station may include antennas that are located on earth to receive transmissions from and/or send transmissions to LEO satellites. The antennas of the earth station may be any suitable antennas for receiving and/or transmitting suitable frequencies, and in particular RF frequencies to and from LEO satellites. Each LEO satellite may be configured with one and preferably a plurality of antennas. For example, an LEO satellite may have a first antenna that transmits a forward beam in a forward direction and a second antenna that transmits a rearward beam in a rearward direction (e.g., relative to the satellite orbit direction), where the antennas may be independently controlled, and may restrict or expand their respective beams or extinguish them. Satellite antennas may comprise one or more phased array antennas. For example, the phased array antenna may be configured with a number of individual radiating elements that are controllable to control the beam coverage, and in particular the beam configuration and angle. A computer on the satellite, which, according to some embodiments, may comprise a dedicated computer programmed with instructions for manipulating the beam angle (which, for example, may include software stored on a chip or other circuitry component that contains the instructions), may be used to control the antenna array to generate a beam projection that may be increased or decreased in accordance with the satellite orbit, and which may be carried out to maximize the coverage for an antenna. The computer preferably is configured with software containing instructions to regulate the operation of the antenna to eliminate transmissions that may otherwise interfere with GEO satellite communications (including where the LEO satellite and GEO satellite transmissions use the same spectrum). This may be carried out by controlling the beam angles of the projections from the antennas as well as turning off the antennas as needed (e.g., when within the guard band range of a GEO earth station antenna). According to some preferred embodiments, the computer may be configured to manipulate the beam projections in accordance with the determinations set forth herein. The satellite beams preferably are manipulated mechanically, electronically, or by both ways, to generate a desired beam of coverage and avoid transmissions within the guard band of a GEO satellite antenna (e.g., of a GEO earth station).
These and other advantages may be realized with the present invention. While the invention has been described with reference to specific embodiments, the description is illustrative and is not to be construed as limiting the scope of the invention. Various modifications and changes may occur to those skilled in the art without departing from the scope of the invention described herein and as defined by the appended claims.
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