Background of the Invention In a communications system where earth-based subscriber units communicate with an orbiting space vehicle, the space vehicle must generate energy beams in order to service areas occupied by the earth-based subscriber units in order to convey information. In a communication system of this type, the energy beams typically generated by the space vehicle move relative to the earth- based subscriber units at a velocity which is commensurate with the velocity of the space vehicle. Thus, an individual earth-based subscriber unit may be illuminated by a particular energy beam for a limited period of time before the energy beam moves beyond the area occupied by the subscriber. When this occurs, the space vehicle must assign the subscriber unit to an adjacent beam so that communications with the space vehicle can continue. This process of subscriber beam-to-beam hand over may be repeated throughout the entire field of view of the space vehicle as the space vehicle moves overhead.

A more favorable approach to facilitating communications between earth- based subscriber units and an orbiting space vehicle is to make use of energy beams which are fixed relative to an area occupied by the earth-based subscriber unit. This allows the energy beam generated from the space vehicle to move relative to the space vehicle while remaining stationary in relation to a constant area fixed to the surface of the earth. This technique is preferred since it reduces the need to hand over a particular subscriber unit from one communications beam to another as the space vehicle moves in its orbit. This reduction in the need for beam-to-beam hand over reduces the frequency management and administration tasks which the space vehicle must execute in order to maintain contact with the earth-based subscriber.

Thus, it is highly desirable for the multi-beam antenna of a space vehicle to generate communications beams over the field of view of the space vehicle, and to keep these beams fixed to areas on the earth's surface as the space vehicle moves relative to the earth's surface. It is further desirable for the technique to be implemented in a simple and inexpensive manner. This enables the space vehicle

to provide services to earth-based subscribers while reducing the need for beam-to- beam hand over as the space vehicle moves relative to the subscriber. This, in turn, can reduce the cost to subscribers for these types of services.

Brief Description of the Drawings The invention is pointed out with particularity in the appended claims.

However, a more complete understanding of the present invention may 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 shows a space vehicle providing communication services to an earth- based subscriber unit in accordance with a preferred embodiment of the invention ; FIG. 2 is a block diagram of an apparatus for generating multiple earth-fixed beams by a space vehicle in accordance with a preferred embodiment of the invention ; FIG. 3 shows relative beam pattern areas at nadir and at edge of coverage generated by an orbiting space vehicle in accordance with a preferred embodiment of the invention ; FIG. 4 shows an exemplary coverage pattern of beams in real space generated from the apparatus of FIG. 2 in accordance with a preferred embodiment of the present invention ; FIG. 5 shows a subset of the beam coverage pattern of FIG. 4 when the subset is used to illuminate a surface of the earth in accordance with a preferred embodiment of the invention ; FIG. 6 shows the effect on beam overlap which results from over sampling in a multiple beam former in accordance with a preferred embodiment of the present invention ; FIG. 7 is a diagram showing the geometrical quantities used in the derivation of angular beam rate for a phased array antenna which provides multiple earth-fixed beams in accordance with a preferred embodiment of the invention ; FIG. 8 is a diagram which shows the geometrical quantities used in the derivation of beam grid velocity relative to the earth for a phased array antenna which provides multiple earth-fixed beams in accordance with a preferred embodiment of the invention ; FIG. 9 is a graph showing the relationship of beam velocity (as a fraction of space vehicle velocity) as a function beam pointing angle using a space vehicle mounted phased array antenna which provides multiple earth-fixed beams in

accordance with a preferred embodiment of the invention ; FIG. 10 is a graph showing the relationship of beam position error as a function the of beam pointing angle using a phased array antenna which provides multiple earth-fixed beams in accordance with a preferred embodiment of the invention ; and FIG. 11 is a flow chart of a method for multiple earth-fixed beam generation from a space vehicle in accordance with a preferred embodiment of the invention.

Description of the Preferred Embodiments A method and system for generating multiple earth-fixed beams from a space vehicle enables a multi-beam antenna of a space vehicle to project communications beams over the field of view of the space vehicle and to keep these beams fixed to areas on the earth's surface as the space vehicle moves overhead. This reduces beam-to-beam hand over as the space vehicle moves relative to an earth-based subscriber. The method and system are relatively simple to implement and can be used with a variety of beam forming networks without modifying the basic multiple beam forming network architecture. Additionally, beam velocity and beam position error are negligible throughout most of the satellite multi-beam antenna field of view.

FIG. 1 shows a space vehicle providing communication services to an earth- based subscriber unit in accordance with a preferred embodiment of the invention.

In FIG. 1, satellite 10 is representative of any type of space vehicle which moves relative to surface of the earth 60. Thus, satellite 10 can represent a space vehicle or airborne vehicle which provides navigation information, broadcast or multicast audio or video, or any other type of information. Satellite 10 can also represent a space vehicle which receives information from various locations on surface of the earth 60. In a preferred embodiment, satellite 10 radiates multiple energy beams, such as communications beam 20, which provides communication services to earth-based subscriber unit 30.

Satellite 10 preferably illuminates surface of the earth 60 with numerous other communications beams, such as communications beam 25 of FIG. 1. In a preferred embodiment, satellite 10 is a low earth orbit satellite which moves relative to surface of the earth 60 and earth-based subscriber unit 30. As satellite 10 moves in its orbit, a set of multiple communications beams, such as communications beams 20 and 25, is steered in the opposite direction so that areas illuminated by each communications beam remain constant and fixed to a corresponding area on the surface of the earth.

FIG. 2 is a block diagram of an apparatus for generating multiple earth-fixed

beams by a space vehicle in accordance with a preferred embodiment of the invention. In FIG. 2, phased array antenna 120 provides the physical means by which energy beams, such as communications beams 20 and 25 of FIG. 1, are generated. The radiating elements which comprise phased array antenna 120 can be any type of elements which are capable of launching an electromagnetic wave in response to an electrical current present on a surface of the radiating element.

Suitable examples include dipoles, helices, and patch antennas. Phased array antenna 120 can also make use of radiating elements such as waveguide slots and horns which launch an electromagnetic wave as a function of a potential generated across an aperture. It is contemplated that the number and arrangement of the radiating elements which comprise phased array antenna 120 are designed in accordance with the particular, power, gain, and directivity requirements of the communication system represented by satellite 10 of FIG. 1.

In a preferred embodiment, each of phased shifters 140 is coupled to a corresponding one of the"N"radiating elements which comprise of phased array antenna 120. Phase shifters 140 can be of any conventional type, such as those that employ ferroelectric devices, field effect transistors, PIN diodes, or micro- electromechanical switches. However, it is preferable that phase shifters 140 be capable of modifying phase by at least a substantial portion of a 2-rr radian range under the control of controller 130. Desirably, this broad range of control over the phase of phase shifters 140 allows beam scanning over the entire field of view of phased array antenna 120.

Phase shifters 140 are coupled to outputs of controller 130 and to multiple beam forming network 110. Preferably, multiple beam forming network 110 incorporates a number of outputs equal to the number of elements of phased array antenna 120. It is contemplated that multiple beam forming network 110 will accept a number of inputs commensurate with the number of beams which are generated by multiple beam forming network 110, typically within the range of 50 to 500 beams, although a greater or lesser number of beams (and inputs) can be used according to the requirements of the particular application.

Multiple beam forming network 110 can be any type of conventional multiple beam forming network. Thus, multiple beam forming network 110 can make use of a Butler matrix. Other suitable multiple beam forming network architectures include Nolan and Blass matrices. Although it is desirable that multiple beam forming network 110 is an orthogonal network, since an orthogonal network can exhibit a lesser amount of loss through the beam forming network in comparison with other beam forming techniques, orthogonality is not mandatory in order to practice the invention. Any multiple beam forming network capable of providing either receive or

transmit beams can be exploite in multiple beam forming network 110.

Switch matrix/power combiner 100, coupled to multiple beam forming network 110, generates"M"number of beam outputs for use by multiple beam forming network 110. In a preferred embodiment, switch matrix/power combiner 100 incorporates beam switches 105, and power combiners 107. Beam switches 105 and power combiners 107 are representative of any conventional design, provided that each is capable of functioning to combine and separate the beams formed by multiple beam forming network 110. Although power combiners 107 are illustrated as three-way combiners, this is not intended to be limiting in any way, as the use of different types of combiners, such as two-way, four-way, and five-way power combiners can be used as well, according to the number of beams generated by multiple beam forming network 110 and the desired complexity of switch matrix/power combiner 100.

The capability for beam combination and beam separation is useful in order to keep constant the area serviced by a particular communications beam as the beam is scanned from an edge of coverage in a forward direction, to the space vehicle nadir, and back to an edge of coverage behind the space vehicle. The desirability of the beam combination and separation functions is discussed in greater detail in relation to FIG. 3, herein.

Controller 130 supplies steering commands to each of phase shifters 140 in the form of a uniform phase shift amount. These steering commands function to steer the beams created by phased array antenna 120. As each beam is steered across the field of view of phased array antenna 120, controller 130 can inactivate any beams that are scanned beyond an edge of coverage. This ensures that space vehicle transmit beams illuminate only locations on surface of the earth 60, and do not transmit into free space. Controller 130 also maps subscriber units to each of the"M"beams generated by multiple beam forming network 110 and manages the beam combination and separation functions.

FIG. 3 shows relative beam pattern areas at nadir and at edge of coverage generated by an orbiting space vehicle in accordance with a preferred embodiment of the invention. In FIG. 3, satellite 10 moves relative to surface of the earth 60.

Satellite 10 generates numerous energy beams, however, only three such beams are shown (communications beams 210, 220, and 230, for example). Preferably, each of the aforementioned communications beams occupies a substantially constant angular area, denoted as angle,. As can be seen from FIG. 3, communications beams 210 and 230 encompass a larger area on surface of the earth 60 than communications beam 220, even though each of communications beams 210, 220, and 230 subtends a substantially constant angular area.

Consequently, the number of earth-based subscribers which can be serviced by each of communications beams 210 and 230 is greater than the number of subscribers which can be serviced by communications 220. Thus, as beam 210 is scanned from the edge of coverage in the forward direction, the area encompassed by the beam becomes smaller, reaching a minimum area at the nadir of satellite 10.

As the beam is scanned from nadir, to an area behind the space vehicle, the beam pattern area begins to increase in size until reaching the area encompassed by communications beam 230.

Thus, in a communications system which makes use of a space vehicle, such as satellite 10, which generates multiple communications beams, it is useful to combine beams as the beam moves from a forward edge of coverage, toward the nadir of the space vehicle. Similarly, it is advantageous to separate beams as the beams move from the nadir to an edge of coverage behind the space vehicle.

These function to keep the number of subscribers within a beam pattern relatively constant as the space vehicle moves in relation to the surface of the earth, thus reducing the need for beam-to-beam hand over. This, in turn, reduces the management and administration tasks which must be performed by satellite 10 in order to maintain the communications services provided to earth-based subscribers, and lowers the cost of service provided to the subscribers.

FIG. 4 shows an exemplary coverage pattern of beams in real space generated from the apparatus of FIG. 2 in accordance with a preferred embodiment of the present invention. The beam coverage pattern of FIG. 4 illustrates a smaller number of the 256 beams available from a 16x16 element phased array antenna, such as phased array antenna 120 of FIG. 2. Both the horizontal and vertical axes of FIG. 4 represent beam pointing angles (in radians) and are linear in sine space.

Sine space has been chosen in FIG. 4 since it allows the-4 dB beam contours within the field of view to be illustrated without significant distortion. The inner circular area indicates an exemplary 40 degree grazing angle contour for a space vehicle located at an altitude of 1400 Km above the surface of the earth.

FIG. 5 shows a subset of the beam coverage pattern of FIG. 4 when the subset is used to illuminate a surface of the earth in accordance with a preferred embodiment of the invention. The horizontal and vertical axes of FIG. 5 represent range from a nadir point for a space vehicle located at an altitude of 1400 Km above the surface of the earth. In FIG. 5, it can be seen that those beams at nadir (near the center of the figure) embrace a smaller area than those at edge of coverage (near the edges of the figure) embrace a larger area. Additionally, the beams nearest the corners of FIG. 4 include the largest area of any of the 96 beams subset

of FIG. 4, FIG. 6 shows the effect on beam overlap which results from over sampling in a multiple beam former (FIG. 3, 110) in accordance with a preferred embodiment of the present invention. The horizontal and vertical axes of FIG. 6 represent range from a nadir point for a space vehicle located at an altitude of 1400 Km above the surface of the earth. In FIG. 6, coverage between beams generated by a phased array antenna is improved by way of over sampling an exemplary Butler matrix in the vertical axis. For the example of FIG. 6, a rank 18 Butler matrix has been used with 2 inputs being loaded per column. This reduces the spacing between beams, and increases beam overlap to a power level of-3dB. Although such over sampling tends to increase the loss in multiple beam forming network 110, the additional loss can be traded against the advantages of increased beam overlap.

To achieve this result, the rank of the column (vertical) combiners is increased, with additional inputs terminated. Column rank can be found by solving the following transcendental equation for Nr : Where Na is the number of rows of elements (oriented perpendicular to the axis of motion of the space vehicle) in the array and N, is the column rank of multiple beam forming network 110. Note that when Na= Nr, and Na is large, the voltage ratio is 2/7r, or-3. 9 dB.

FIG. 7 is a diagram showing the geometrical quantities used in the derivation of angular beam rate for a phased array antenna which provides multiple earth-fixed beams in accordance with a preferred embodiment of the invention. In FIG. 7, satellite 10 moves with a velocity vector denoted by"V'and orbits above surface of the earth 60 at an altitude"h". Provided below is the derivation of the rate of change in the phase of each of phase shifters 140 of FIG. 2 in order to maintain the beams at constant areas fixed to the surface of the earth.

In a simple analogy for determining the ideal beam rate, imagine that surface of the earth 60 defines an outer boundary (i. e. the"teeth") of a fixed sprocket of radius"/", as shown in FIG. 7. In this analogy, satellite 10 is at the center of a second rotating sprocket of radius"h", the altitude of satellite 10 above surface of the earth 60. The teeth of the two sprockets mesh at the nadir of satellite 10, along the line joining"r"and"h"of FIG. 7. The velocity of a point"p", where the teeth of the two sprockets mesh, is equal to 0 (neglecting the earth's rotation) since the sprocket of radius"r"does not rotate. Thus, <BR> <BR> <BR> <BR> <BR> <BR> d6 d# d6 d# r d#<BR> VP = v - h = 0. but since v = (r+h), = + ,<BR> <BR> <BR> <BR> dt dt dt dt h dt where d6/dt is the angular velocity of the sprocket of radius"h" (the satellite sprocket), and-=-is the angular velocity of satellite 10. dz r+h The earth's rotation can be easily considered by defining"V'as the velocity of satellite 10 relative to the earth. Since satellite 10 makes one revolution per orbit in order to keep its nadir directed toward surface of the earth 60, the angular beam rate of an antenna of satellite 10 should be d6 dQ dQ r vr -= relative to the satellite.<BR> <BR> <BR> dt dt dt h 9r+h)l1 In accordance with conventional techniques, the phase"YJn"at each antenna element"r/"of phased array antenna 120 is incremented so that "=-nkd (sin0o) where k=21T/A, 0ois the direction of the main beam, and d is the spacing between the elements of the phased array antenna.

Taking a time derivative yields : d#@/dt=-nkd(cos#0) d#0/dt.

Thus, d0cJdt = -(d##/dt)/(nkd(cos #0)). d#n/dt is chosen chosen produce produce proper beam rate, ##0/dt, at the nadir of the space vehicle.

Now, let c= -(d#n/dt)/(nkd), so d#0/dt=c/cos #0. Then c= vr/(h (r+h)) in order to synchronize the beam rate at nadir. The beam rate at edge of coverage is a factor of 1/ (cos (49. 7°), or approximately 1. 5 times the rate at nadir, for an altitude "h"equal to 1200 kilometers, and edge of coverage defined by elevation equal 25 degrees (or oxo=50 degrees).

Note that dSo/dt is the angular rate of the beam relative to the antenna (which is fixed to the space vehicle), while d9/dt is the total angular rate of the beam relative to the earth. The two quantities differ by the angular velocity of the space vehicle dD/dt. Therefore, a controller, similar to controller 130 of FIG. 2, would compute the phase shifter settings according to (Pn=-nkdvrtl (h (r+h)), where t= time.

The phase shifter settings (which can be used by phase shifters 140 of FIG.

2 to generate earth-fixed beams) cause an increase in beam angular velocity towards the edges of coverage in front of and behind the space vehicle. This increase is useful in synchronizing the beam to the ground. However, the effect overcompensates for the curvature of the earth, as will be shown in description of FIG. 8, herein. In order to determine the velocity of a beam on the surface of the earth at any point in the coverage area, it is desirable to consider the curvature of the earth.

It is possible to generalize the phase shifter algorithm to two dimensions, in order to consider the areas to the left and to the right of the space vehicle. In practice, however, the two-dimensional beam arrangement grid on the surface of the earth (hence, the array itself) is aligned in the apparent direction of motion of the space vehicle as seen from the surface of the earth. Thus, the beam grid need only be scanned in one dimension (along the axis of motion for the space vehicle).

FIG. 8 is a diagram which shows the geometrical quantities used in the derivation of beam grid velocity relative to the earth for a phased array antenna which provides multiple earth-fixed beams in accordance with a preferred embodiment of the invention. In FIG. 8, the subsatellite point, p, moves at the rate v"= (d ? ldt) r, in a direction tangent to surface of the earth 60. Beams at other points in the coverage area move at the rate Vb= (v, + v). t^ relative to nadir, where # is tangent to the earth and v, has magnitude s (d9sldt) where"s"is the slant range from the space vehicle to the coverage point. From this geometry, the tangential velocity of the beam in the positive direction (counterclockwise around satellite 10) is given by : By making the following substitution, this expression can be written in terms of 0 only : where 9 is used instead of 00.

Since di/dt is the sum of the angular beam velocity plus the angular velocity of the space vehicle, when the expression is also substituted, the following results are obtained in the equation, below : where b iS the velocity of the beam on the ground as fraction of the space v vehicle velocity. In the ideal case, this quantity would equal zero. Note that the"*" in the above equation denotes multiplication of the two"f"quantities. When i=0 is inserted, b =0, since the beam rate has been chosen to synchronize with the v beam at nadir. When the values r--6378. 2 Km (earth radius) and h=1200 are substituted, the beam velocity as a function of the beam pointing angle (0) can be plotted, as is done in FIG. 9, herein.

FIG. 9 is a graph showing the relationship of beam velocity (as a fraction of space vehicle velocity) as a function beam pointing angle using a space vehicle mounted phased array antenna which provides multiple earth-fixed beams in accordance with a preferred embodiment of the invention. From FIG. 9, it can be seen that the beam velocity as a fraction of the space vehicle velocity (b) is zero v at the nadir of the space vehicle, as predicted by the equation for Additionally, v for beam pointing values which approach. 9 radians, b remains a relatively small v quantity, indicating little motion of the beam across the surface of the earth. Only for values which approach 1. 00 radian does becomes significant, indicating v substantial motion of the beam relative to the surface of the earth.

FIG. 10 is a graph showing the relationship of position error, P (Km), as a function the of beam pointing angle using a phased array antenna which provides multiple earth-fixed beams in accordance with a preferred embodiment of the

invention. FIG. 10 results from the integration of the expression for b derived in v relation to FIG. 8. This yields the position of the beam relative to a point on the earth that the beam is intended to cover, as a function of 0. where the substitution i9l=clt has been made. Additionally, cl is the total angular velocity of the beam, which has been set to : to synchronize the beam at nadir. For example, if tl--O, the beam and coverage point are synchronized at nadir, then the velocity can be integrated numerically for various points in the coverage up to the edge of coverage. This results in the graph of FIG. 10.

From FIG. 10, it can be seen that the beam position error, p (t,, t,), is zero at the nadir of the space vehicle, as predicted by the equation for p (t,, t2).

Additionally, for beam pointing values which approach. 5 radians, p (t,, t2) remains a relatively small quantity, indicating less than 15 Km error in beam positioning. Only for values which approach. 75 radian does p (t"t2) becomes significant, indicating substantial error of the beam relative to the surface of the earth.

FIG. 11 is a flow chart of a method for multiple earth-fixed beam generation from a space vehicle in accordance with a preferred embodiment of the invention.

The apparatus of FIG. 2 is suitable for performing the method of FIG. 11. The method begins with step 310, where weighting coefficients for elements of a phased array antenna are determined. These weighting coefficients include a relative amplitude and phase and are desirably applied to each element of the phased array antenna in order generate multiple earth-fixed beams. Also in step 310, a determination is made as to which of the multiple beams will be used to transmit communications information from the space vehicle to one or more earth-based subscriber units, and which beams will be used to receive communications information from earth-based subscriber units.

In step 320, the phase component of the weighting coefficients are shifted by a proportionality constant which is approximately in the range of -nkdvrtl ( (r+h) h) wherein n represents a particular element of said phased array, k represents a wave number, d represents a spacing between elements of the phased array antenna, v represents a velocity of the space vehicle, r represents a radius of the earth, h represents an altitude of the orbiting space vehicle, and t

represents time, thereby illuminating a substantially constant area fixed to a surface of the earth as the space vehicle moves relative to the earth.

At step 330, a determination is made as to whether a particular beam is pointing beyond an edge of coverage. If the decision of step 330 indicates that a beam is pointing beyond an edge of coverage, step 340 is executed in which the beam is inactivated. This beam inactivation ensures that any transmitted energy is incident on a surface of the earth, where earth-based subscribers can be expected to be located, and that energy not broadcast to an unoccupied region of free space.

If the decision of step 330 indicates that the beam is not being pointed beyond an edge of coverage, step 360 is executed in which the space vehicle activates the particular receive or transmit beam. Following step 360, step 370 is executed in which the beam is combined with a second beam as the beam is being pointed from an edge of coverage to a nadir. Also in step 370, the space vehicle performs any subscriber beam-to-beam hand over which is required in order to continue communications between the space vehicle and the earth-based subscribers.

In step 380, the space vehicle performs any required separation of beams as the combined beam is scanned from nadir to an edge of coverage. Step 380 also includes any assigning of earth-based subscriber units to appropriate communications beams as the beams are separated. In step 390, the space vehicle determines if a beam is pointing beyond and edge of coverage. If the decision of step 390 indicates that a communications beam is not pointing beyond and edge of coverage, step 400 is executed in which the beam continues its scan.

If, however, the decision of step 390 indicates that the beam is pointing beyond and edge of coverage, step 410 is executed in which the particular beam is inactivated.

The method then returns to step 330 where another beam generated by the multiple beam forming network is evaluated to determine if the beam is scanning beyond an edge of coverage. Steps 330 through 410 are repeated for each beam generated by the multiple beam forming network. In a preferred embodiment, several beams generated by the multiple beam forming network are being processed by various steps within the sequence of steps 330 to 410. Thus, at any given instant, several beams may be undergoing processing by step 370, in which individual beams are being combined with others as the beams are scanned from an edge of coverage to nadir. Meanwhile, other beams may be undergoing processing by step 380, in which beams are being separated as they are scanned from nadir to an edge of coverage.

A method and system for multiple earth-fixed beam generation from an orbiting space vehicle provides the capability for a space vehicle to generate

multiple earth-fixed beams and reduce beam-to-beam hand over as the space vehicle moves relative to an earth-based subscriber. The method and system can be operated in conjunction with a variety of multiple beam forming networks, such as those that employ Butler, Nolan, and Blass matrices. Additionally, proper control over the phase shifters results in beam position and velocity errors which are negligible over large ranges of beam pointing angles.

Accordingly, it is intended by the appended claims to cover all of the modifications that fall within the true spirit and scope of the invention.