Beam waveguide antenna with independently steerable antenna beams and method of compensating for planetary aberration in antenna beam tracking of spacecraft

An antenna assembly for forming and directing a transmit beam, and for controlling receive and transmit beam tracking of a spacecraft in the presence of planetary aberration. The assembly includes a main reflector, a sub-reflector centered along an optical axis of the main reflector, and a moveable transmit feed for directing electromagnetic radiation along a longitudinal axis thereof. The assembly also includes an intermediate beam waveguide assembly arranged between the moveable transmit feed and the main reflector, wherein the intermediate beam waveguide assembly includes fixed and moveable optical components for guiding electromagnetic beam energy between the moveable transmit feed and the main reflector. A beam steering mechanism is coupled with the moveable transmit feed for angularly displacing the transmit beam from the optical axis by displacing the moveable transmit feed in a direction substantially orthogonal to the longitudinal axis of the transmit feed.

FIELD OF THE INVENTION
 The present invention generally relates to a terrestrial beam waveguide
 antenna and, more particularly, to such an antenna forming a transmit
 beam, wherein the transmit beam is independently steerable with respect to
 a receive beam formed by the antenna.
 The present invention also generally relates to a method of and apparatus
 for controlling a terrestrial beam waveguide antenna and, more
 particularly, to a method of and apparatus for controlling receive and
 transmit beams of such an antenna to compensate for planetary aberration
 in the beam tracking of a spacecraft.
 BACKGROUND OF THE INVENTION
 Terrestrial stations for spacecraft communications typically include a
 large aperture antenna for communicating with a spacecraft. Such an
 antenna typically includes a beam waveguide assembly having a main
 reflector and a sub-reflector centered on an optical axis of the main
 reflector, e.g., a Cassegrain antenna. The beam waveguide assembly forms
 and directs a reciprocal pair of main antenna beams along the optical
 axis. The main antenna beams typically include a transmit beam for
 transmitting an uplink signal to and a receive beam for receiving a
 down-link signal from the spacecraft. To track the spacecraft, the main
 reflector and the sub-reflector, which are fixed relative to each other
 and rotate together, along with other optical components of the beam
 waveguide assembly, are typically driven by motors and servo-mechanisms in
 at least two rotational directions, e.g., azimuth (AZ) and elevation (EL),
 so as to align the main beams with the spacecraft. In this manner, the
 receive and transmit beams are both aligned with the same position of the
 spacecraft at a given point in time.
 A Cassegrain antenna of sufficiently high gain to track a distant
 spacecraft includes large and correspondingly heavy beam waveguide
 components, e.g., a main reflector thirty-five meters in diameter, thus
 necessitating correspondingly bulky and relatively complex motors and
 servo-mechanisms to rotate such heavy components. Antenna beam tracking
 accuracy, i.e., alignment accuracy between the main beams and a tracked
 spacecraft position, is critical when using such a high gain antenna
 because even a small alignment error, e.g., on the order of millidegrees,
 results in a significant reduction in peak antenna gain. This criticality
 is even more pronounced when the antenna is used to track an
 interplanetary spacecraft because a signal communicated between such a
 distant spacecraft and the antenna experiences substantial propagational
 attenuation, i.e., signal attenuation proportional to the square of the
 distance between the antenna and the spacecraft.
 Although the conventional antenna arrangement described above may suffice
 for communicating with a spacecraft relatively near to the earth, e.g.,
 occupying low, medium and high earth orbits, its use for communicating
 with a relatively distant, e.g., interplanetary, spacecraft is limited and
 problematic. Effective communication with the relatively distant
 spacecraft is complicated in part by a phenomenon referred to as planetary
 aberration--the phenomenon by which objects in space, as viewed from the
 earth, are not where they appear to be. Planetary aberration arises as a
 result of 1) a component of relative motion between the spacecraft and the
 antenna, specifically, a component of the spacecraft's velocity orthogonal
 to a line-of-site between the spacecraft and the antenna, and 2) the
 finite time taken for the uplink and down-link signals to travel between
 the spacecraft and the antenna due to the finite speed with which the
 signals propagate through space. The finite time taken for the uplink and
 down-link signals to travel round-trip between the spacecraft and the
 antenna is referred to as the round-trip light travel time (RTLT).
 The effect of planetary aberration can be appreciated in view of an
 astronomical coordinate system referred to as the right ascension (RA) and
 declination (DEC) coordinate system. RA/DEC coordinates define a position
 on what is referred to as a celestial sphere. The celestial sphere is a
 two dimensional projection of the sky on a sphere--the celestial
 sphere--surrounding the earth. Planetary aberration arises because the
 spacecraft moves in the RA/DEC coordinate system, and thus changes its
 position over time on the celestial sphere as observed from a point fixed
 on the earth, i.e., the antenna. The spacecraft changes its RA/DEC
 position because of its component of orthogonal velocity, without which
 the spacecraft would tend to maintain a single RA/DEC position and thus
 move directly toward or away from the antenna.
 As will become apparent from the following example, compensating for
 planetary aberration in the receive and transmit beam tracking of the
 spacecraft requires an angular separation between the receive and transmit
 beams. The conventional beam waveguide antenna system disadvantageously
 includes colinearly aligned receive and transmit beams, i.e., receive and
 transmit beams aligned in the same direction, and is without a mechanism
 for imposing such angular separation between the receive and transmit
 beams, i.e., for splitting the receive and transmit beams apart to
 compensate for planetary aberration.
 The following example serves to illustrate the detrimental effect planetary
 aberration has on communication between the spacecraft and the colinearly
 aligned receive and transmit beams of the conventional antenna. Assume a
 spacecraft initially transmits a down-link signal from a past or previous
 spacecraft position, and in the finite time taken for the down-link signal
 to travel to the antenna, i.e., half a RTLT, the spacecraft moves to a
 present spacecraft position at a present time. Assume at the present time
 the receive beam of the antenna, along with the optical axis and transmit
 beam, is aligned with the past spacecraft position to receive the
 down-link signal arriving therefrom, and, contemporaneous with the arrival
 of the down-link signal, an uplink signal is transmitted from the antenna
 via the transmit beam. Assume also in the finite time taken for the uplink
 signal to arrive at the past spacecraft position, i.e., half a RTLT, the
 spacecraft moves from the second spacecraft position to a future
 spacecraft position, i.e., in one RTLT, the spacecraft moves from the past
 spacecraft position, through the present spacecraft position, and on to
 the future spacecraft position.
 For a relatively near spacecraft, one RTLT is relatively short, e.g.,
 fractions of a second, and the displacement of the spacecraft in RA/DEC
 coordinates between the past and future positions is negligible with
 respect to the beam coverage of the receive and transmit beams.
 Consequently, effective communication can occur even though the uplink
 signal is transmitted toward the past spacecraft position, and not along a
 direction intersecting the future spacecraft position, because both
 spacecraft positions are covered by the transmit beam.
 On the other hand, for a relatively distant spacecraft, the one RTLT is
 relatively large, e.g., 160 minutes for a spacecraft near the planet
 Saturn, thus leading to an appreciable spacecraft displacement between the
 past and future spacecraft positions. In this case, the transmit beam
 coverage does not necessarily encompass the more widely separated
 positions, a situation worsened by the requirement for a highly directive,
 i.e., high gain, antenna beam. Without some form of correction or
 compensation to account for the separation of positions due to planetary
 aberration, signal loss can be significant, e.g., up to 25 dB. This is due
 to the colinear alignment of the receive and transmit beams of the antenna
 with past, present or future positions of the spacecraft. Consequently,
 ineffective communication results since the uplink signal is transmitted
 toward the incorrect spacecraft position (e.g., the past position), as a
 result of this colinear alignment of the receive and transmit beams of the
 antenna.
 For the relatively distant spacecraft, effective communication thus
 requires simultaneous alignment of the down-link and uplink signals with
 the respective past and future positions of the spacecraft at the present
 time, i.e., simultaneous alignment of the receive and transmit beams with
 respective spaced-apart spacecraft positions coinciding with times half a
 RTLT previous to and half a RTLT after the present time. Conventionally,
 achievement of such spaced alignment disadvantageously requires two
 antennas--one antenna providing receive beam tracking of the past
 position, and the other antenna providing transmit beam tracking of the
 future position--because of the colinear receive and transmit beam
 arrangement of the conventional antenna.
 Accordingly, there is a need for a high-gain beam waveguide antenna having
 a beam steering capability independent of and in addition to the
 conventional rotational mechanisms used for antenna beam steering.
 There is also a need for a high-gain beam waveguide antenna having receive
 and transmit main beams independently steerable with respect to each other
 and the optical axis of the antenna.
 There is a further need in a beam waveguide antenna system to control the
 receive and transmit beam tracking of a spacecraft moving along a space
 trajectory to compensate for appreciable planetary aberration.
 There is an even further need for using a single antenna system forming
 receive and transmit beams to beam-track a spacecraft moving along a
 spacecraft trajectory to compensate for planetary aberration.
 There is also a need to reduce the effects of propagational attenuation of
 a signal transmitted between a spacecraft and an antenna system.
 SUMMARY OF THE INVENTION
 It is therefore an object of the present invention to independently steer
 the transmit beam of a high-gain, beam waveguide antenna with respect to a
 receive beam formed by the antenna. This object also includes
 independently steering the transmit beam with respect to an optical axis
 of the antenna.
 A related object of the present invention is to control independent
 steering of a transmit beam formed by a terrestrial, high-gain, beam
 waveguide antenna with respect to an optical axis of the antenna and a
 receive beam formed by the antenna, to compensate for appreciable
 planetary aberration in the receive and transmit beam tracking of a
 spacecraft moving along a space trajectory.
 Another object of the present invention is the improvement of a
 conventional, high-gain, beam waveguide antenna having a conventional beam
 steering mechanism for steering together receive and transmit beams formed
 by the antenna, the improvement including the addition of a beam steering
 mechanism for independently steering the transmit beam with respect to the
 receive beam.
 Another object of the present invention is to reduce the effects of
 propagational attenuation of a signal transmitted between a spacecraft and
 an antenna system.
 These and other objects of the present invention are achieved through an
 improvement to a conventional, high-gain beam waveguide antenna system.
 The improved antenna system includes a beam waveguide having conventional
 components, including a large main reflector, a sub-reflector centered
 along an optical axis of the main reflector, a fixed receive feed
 associated with a receive beam formed by the antenna system, and an
 intermediate beam waveguide assembly positioned between the fixed receive
 feed and the main reflector for guiding beam energy there between. A
 conventional beam steering mechanism coupled with the main reflector and
 moveable components of the intermediate beam waveguide assembly steers
 together the optical axis of the main reflector, the receive beam and a
 transmit beam formed by the antenna system.
 The improvement in accordance with the present invention includes a
 moveable transmit feed, associated with the transmit beam. Controlled
 displacement of the moveable transmit feed, in a planar direction
 perpendicular to a beam feeding axis of the transmit feed, advantageously
 produces a corresponding angular displacement of the transmit beam from
 both the optical axis and the receive beam. The improvement also includes
 electrically driven actuators coupled with the moveable transmit feed for
 controllably displacing the transmit feed responsive to a control signal
 derived by a beam steering controller executing beam steering control
 software of the present invention. Advantageously, the electrically driven
 actuators are small, light, readily available, and easy to control because
 the transmit feed is much smaller and lighter than the large main
 reflector. As a result, high resolution transmit beam steering, on the
 order of millidegrees, is easily attained with fine displacements of the
 moveable transmit feed using the actuators coupled thereto.
 The foregoing objects of the present invention are achieved by an antenna
 assembly for forming and directing a transmit beam. The assembly includes
 a main reflector, a sub-reflector centered along an optical axis of the
 main reflector, and a moveable transmit feed for directing electromagnetic
 radiation along a longitudinal axis of the transmit feed. The assembly
 also includes an intermediate beam waveguide assembly positioned between
 the moveable transmit feed and the main reflector, wherein the
 intermediate beam waveguide assembly includes fixed and moveable optical
 components for guiding electromagnetic beam energy between the moveable
 transmit feed and the main reflector. A beam steering mechanism is coupled
 with the moveable transmit feed for angularly displacing the transmit beam
 from the optical axis by displacing the moveable transmit feed in a
 direction substantially orthogonal to the longitudinal axis of the
 transmit feed.
 The foregoing and other objects of the present invention are achieved by a
 method of controlling the improved antenna of the present invention to
 compensate for appreciable planetary aberration in receive and transmit
 beam tracking of a spacecraft moving along a space trajectory. In the
 method, the transmit and receive beams of the improved antenna
 respectively transmit an uplink signal to and receive a down-link signal
 from the spacecraft. The down-link and uplink signals travel round-trip
 between the spacecraft and the antenna in one RTLT.
 The method includes aligning the receive beam at a present time with a past
 position of the spacecraft coinciding with where the spacecraft was half a
 RTLT before the present time. The method includes contemporaneously
 aligning the transmit beam with a future position of the spacecraft
 coinciding with where the spacecraft will be half a RTLT after the present
 time. When so aligned, an angular displacement between the receive and
 transmit beams compensates for planetary aberration. The contemporaneous
 step of aligning the transmit beam includes the step of displacing the
 transmit feed of the antenna in a planar direction, thus angularly
 displacing the transmit beam from the receive beam and into alignment with
 the future position of the spacecraft.
 The foregoing and other objects of the present invention are achieved by a
 method of controlling a terrestrial antenna system to compensate for
 planetary aberration including the steps of 1) aligning a receive beam of
 the antenna system at a present time with a past position of a spacecraft,
 and 2) aligning a transmit beam of the antenna system with a future
 position of the spacecraft spaced from the past position, wherein a
 down-link signal and an uplink signal can be simultaneously received from
 the past position of the spacecraft and transmitted to the future position
 of the spacecraft by the antenna system, respectively.
 The foregoing and other objects of the present invention are achieved by a
 method of compensating for planetary aberration in an antenna system. The
 antenna system includes a beam waveguide and a transmit feed for forming
 and directing a transmit beam. The transmit beam is used to transfer a
 signal between the transmit feed and a spacecraft. The method includes
 angularly displacing the transmit beam from an optical axis of the beam
 waveguide responsive to a displacement of the transmit feed in a direction
 orthogonal to an axis of the transmit feed. Such displacement of the
 transmit feed aligns the transmit beam with a future position of the
 spacecraft, wherein the spacecraft moves from a present position to the
 future position during the approximate time taken for the transfer of the
 signal between the antenna system and the spacecraft.
 The foregoing and other objects of the present invention are achieved by an
 antenna system controller for a terrestrial antenna adapted to form and
 direct transmit and receive beams for respectively transmitting a signal
 to and receiving a signal form a spacecraft. The antenna system controller
 includes a processor, an interface coupled to the processor, and a memory
 coupled to the processor. The memory stores sequences of instructions
 which, when executed by the processor, causes the processor to 1) identify
 temporally spaced first and second apriori positions of the spacecraft
 corresponding to a round-trip travel time of the signals between the
 spacecraft and the terrestrial antenna, and 2) derive an angular
 displacement between the receive and transmit beams to contemporaneously
 align the receive and transmit beams with spacecraft positions.
 The above and still further objects, features and advantages of the present
 invention will become apparent upon consideration of the following
 detailed description of a specific embodiment thereof, especially when
 taken in conjunction with the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION
 In the following description, for purposes of explanation, numerous
 specific details are set forth in order to provide a thorough
 understanding of the present invention. It will be apparent, however, that
 the present invention may be practiced without these specific details. In
 other instances, well-known structures and devices are shown in block
 diagram form in order to avoid unnecessarily obscuring the present
 invention.
 FIG. 1 is a high-level operational diagram of an embodiment of an antenna
 system 20 operable in accordance with the principles of the present
 invention. As illustrated, antenna system 20, positioned at a
 predetermined terrestrial location 22, tracks a spacecraft 24 along its
 predetermined interplanetary trajectory 26. Trajectory 26 brings
 spacecraft 24 into the neighborhood of a distant planet 27, e.g.,
 Saturn--in one intended application of the present invention. Antenna
 system 20 forms a transmit beam 28 and a receive beam 30 for respectively
 transmitting an electromagnetic (EM) uplink signal 32 to and receiving an
 EM down-link signal 34 from spacecraft 24. Transmit beam 28 is
 approximately symmetrical about a beam axis 36 thereof substantially
 aligned with a peak gain of the transmit beam 28. Similarly, receive beam
 30 is approximately symmetrical about a beam axis 38 thereof substantially
 aligned with a peak gain of the receive beam 30.
 Antenna system 20 includes a Cassegrain high-gain antenna assembly having a
 large main reflector 40, e.g., thirty-five meters in diameter, and a
 sub-refector, not shown, aligned with an optical axis 42 of main reflector
 40. In addition to a conventional beam steering mechanism, antenna system
 20 advantageously includes a beam steering mechanism capable of angularly
 separating, i.e., angularly splitting, the receive and transmit beams
 30,28 by a predetermined angle 44. Antenna system 20 is thus capable of
 simultaneously aligning receive and transmit beams 30,28 with a first
 (i.e., past) spacecraft position p1 and a second (i.e., future) spacecraft
 position p2 having spaced-apart RA/DEC position coordinates.
 More specifically, transmit beam 28 is independently steerable in azimuth
 and elevation with respect to both receive beam 30 and optical axis 42 of
 main reflector 40, to impose angular offset or split 44 between receive
 and transmit beams 30,28 aligned respectively with the first and second
 spacecraft positions. It should be appreciated that an antenna beam is
 said to be aligned with, i.e., pointed at or in the direction of, the
 spacecraft when a peak gain of the beam is substantially aligned with the
 spacecraft; this occurs when the beam axis (e.g., beam axis 36 or 38) is
 substantially aligned with the spacecraft.
 In providing independent steering of transmit beam 28 relative to receive
 beam 30 and optical axis 42, antenna system 20 overcomes complications
 associated with planetary aberration to permit effective, contemporaneous
 reception of down-link signal 34 from and transmission of uplink signal 32
 to distant spacecraft 24 at the spaced past and future positions p1,p2, as
 the following brief operational example illustrates.
 To provide a basic understanding of the invention the following operational
 example is provided and the structure which provides this functionality is
 described in detail following the operational example. At an instant in
 time corresponding to a present time, receive beam 30 is steered into
 alignment with past position p1 where the spacecraft was half a RTLT prior
 to the present time, and contemporaneously, transmit beam 28 is steered
 into alignment with future position p2 where spacecraft 24 will be half a
 RTLT after the present time--spacecraft 26 moves from past positions p1 to
 future position p2 in one RTLT of uplink signal 32 and down-link signal 34
 between satellite 24 and antenna system 20. Down-link signal 34
 transmitted by spacecraft 24 from past position p1 is received via receive
 beam 30. Similarly, uplink signal 32 is transmitted to spacecraft 24 at
 future position p2 via transmit beam 28. Angular offset or split 44
 required between receive and transmit beams 30,28 arises due to planetary
 aberration since past and future positions p1,p2 have spaced-apart RA/DEC
 position coordinates; as described previously, the separation in positions
 arises from the relative component of spacecraft velocity orthogonal to
 the line-of-sight between the spacecraft and antenna system 20.
 As illustrated above, antenna system 20 advantageously compensates for
 planetary aberration by angularly splitting receive and transmit beams to
 respectively align the same with respective positions p1,p2. Importantly,
 aligning the peak gains of the receive and transmit beams with respective
 positions p1,p2 also reduces detrimental effects caused by propagational
 attenuation of down-link and uplink signals 34,32. Such can be appreciated
 considering that planetary aberration can require an angular offset 44 of,
 for example, up to 30 millidegrees for a spacecraft travelling near
 Saturn, while each of high-gain receive and transmit beams 30,28 has an
 exemplary 3 dB beam-width (i.e., a full beam-width 3 dB down from the peak
 gain point of the beam) of approximately 15 millidegrees.
 With reference to FIG. 2, antenna system 20 includes an antenna assembly 60
 and an antenna system controller 62. Antenna assembly 60 includes both
 conventional Cassegrain, beam-wave guiding optics, and improvements in
 accordance with the present invention, to form and direct receive and
 transmit beams 30,28. The conventional beam waveguide optics include a
 high gain, parabolic main reflector 40 rotatable in both azimuthal and
 elevational directions. Main reflector 40 is supported above ground by a
 main reflector support 63. The conventional beam waveguide optics also
 include an intermediate beam waveguide 64. Waveguide 64 guides both an
 uplink or transmit EM beam 66a and a down-link or receive EM beam 66b
 through antenna assembly 60 and feeds the EM beams to and from main
 reflector 40, respectively.
 A conventional fixed receive feed 68 receives EM beam 66b from waveguide
 64. More specifically, down-link signal 34 received via receive beam 30 is
 directed by main reflector 40 and optics associated therewith to
 intermediate beam waveguide assembly 64. Assembly 64 guides down-link
 signal 34 from main reflector 40 to an input aperture of receive feed 68.
 Conventional motors and servomechanisms, indicated generally as reference
 numeral 67, are coupled to main reflector 40, main reflector support 63,
 and moveable optical components within beam waveguide assembly 64, as will
 be described. Motors and servomechanisms 67 rotate optical axis 42 of main
 reflector 40 in both azimuthal and elevational directions responsive to a
 pair of respective azimuthal and elevational control signals 92,94, as is
 known in the art.
 An improvement to antenna assembly 60 in accordance with the present
 invention includes a conventional moveable transmit feed 70 (described
 more fully later) to independently steer transmit beam 28 with respect to
 optical axis 42 and receive beam 30. Moveable transmit feed 70 radiates
 the uplink signal, i.e., EM beam 66a, toward intermediate beam waveguide
 assembly 64. Intermediate beam waveguide assembly 64 guides beam 66a input
 thereto, along an optical path within antenna assembly 60, to an output of
 waveguide assembly 64. Beam waveguide assembly 64 directs beam 66a to main
 reflector 40, from where uplink signal 32 is transmitted into space via
 transmit beam 28.
 The improvement includes a platform assembly 72 for moveably supporting
 transmit feed 70. Specifically, a moveable upper surface or platform of
 platform assembly 72 supports transmit feed 70, whereas a lower surface of
 the platform assembly rests upon a fixed surface 76. Platform assembly 72
 displaces transmit feed 70 supported thereby responsive to a pair of
 actuator control signals 112x,112y indicative of transmit feed
 displacement, and provided from antenna system controller 62, as described
 in detail below. As will be described more filly, an independent,
 controlled displacement of transmit feed 70 in a planar direction results
 in a correspondingly controlled angular offset between transmit beam 28
 and both optical axis 42 and receive beam 30.
 Antenna system controller 62 includes both conventional beam steering
 control components and improvements in accordance with the present
 invention, which work together to control antenna assembly 60. Antenna
 system controller 62 thus controls antenna assembly 60 to track spacecraft
 24 and to compensate for planetary aberration. Conventionally, an antenna
 pointing controller (APC) 90 derives azimuthal and elevational control
 signal pair 92,94 responsive to apriori spacecraft trajectory information
 provided to APC 90 over an interface 100.
 In accordance with the present invention, a transmit feed position
 controller 110 and a beam steering controller 116 together control the
 movements or displacements of moveable transmit feed 70. Transmit feed
 position controller 110 derives actuator control signal pair 112x,112y
 responsive to transmit feed displacement commands issued thereto over an
 interface 120. High-level beam steering controller 116 controls the
 independent beam steering of transmit beam 30 to correct for planetary
 aberration, and derives the transmit feed displacement commands issued to
 controller 110 responsive to the apriori spacecraft trajectory information
 supplied thereto via an interface 118. Both APC 90 and beam steering
 controller 116 receive a signal indicative of accurate real-time, e.g.,
 Greenwich Mean Time (GMT), and are thus time-synchronized. Feed controller
 110 is also time-synchronized with controller 116 to provide controlled,
 real-time displacements of transmit feed 70.
 FIGS. 3A and 3B are schematic diagrams of an embodiment of a construction
 of the beam waveguide optics of antenna assembly 60. The conventional beam
 waveguide optics include parabolic main reflector 40 and a hyperbolic
 sub-reflector 130, both supported above an upper edifice 132. Upper
 edifice 132 is rotatively coupled to and above a fixed lower edifice 134.
 Main reflector 40 includes a central opening 136 through which beam energy
 is directed, and sub-reflector 130 is fixedly centered along optical axis
 42 of main reflector 40. Optical axis 42 extends through both a first
 focus point 138 and a second focus point 140 of the combined sub-reflector
 130 and main reflector 40.
 Moveable transmit feed 70, located within fixed lower edifice 134, provides
 the source of EM beam energy for beam 66a in the transmit direction.
 Transmit feed 70 includes a transmit horn 70a coupled to a supporting
 transmit guide or feed assembly 70b. Transmit horn 70a includes an EM
 input 142a, an EM output aperture 144, and a horn shaped body between
 input 142a and output aperture 144. Output aperture 144 is centered along
 a central, longitudinal axis 146 of transmit horn 70a. Longitudinal axis
 146 extends in a direction parallel with the Z-axis, as depicted in FIG.
 3A.
 A transmitter of antenna system 20, not shown, initially supplies uplink
 signal 32 to an input 142b of transmit guide or feed assembly 70b.
 Transmit feed assembly 70b couples uplink signal 32 to input 142a of
 transmit horn 70a. The horn shaped body of transmit horn 70a guides uplink
 signal 32 from input 142a to output aperture 144, from where the uplink
 signal is radiated, in the direction of longitudinal axis 146, toward
 intermediate beam waveguide assembly 64.
 Intermediate beam waveguide assembly 64 is conventional, and includes
 optical components within both lower edifice 134 and upper edifice 132.
 Intermediate waveguide assembly 64 guides beam 66a from an input end
 thereof proximate aperture 144, along a path through antenna assembly 60,
 to an output end of the intermediate waveguide assembly proximate opening
 136 of main reflector 40. Beam 66a exiting the output end of assembly 64
 is directed through opening 136 toward a convex outer surface of
 sub-reflector 130, to be reflected thereby back toward an inner concave
 surface of main reflector 40. This inner concave surface reflects beam
 energy incident thereto into space as a main antenna beam, e.g., transmit
 beam 30, in the direction of a main beam axis, e.g., transmit beam axis
 36.
 Beam waveguide assembly 64 includes, in series along the direction of
 guided beam 66a, 1) a hyperbolic mirror 148 and an elliptic mirror 150
 disposed within edifice 134, and 2) a plane mirror 152, an elliptic mirror
 154, an elliptic mirror 156, and a plane mirror 158 disposed within
 edifice 132. As is known, main reflector 40, sub-reflector 130 and the
 mirrors of beam waveguide assembly 64 are moveable with respect to an
 elevational axis 160 and an azimuthal axis 162 to correspondingly steer
 receiver and transmit beams 30,28 in elevational and azimuthal directions.
 An important aspect of the present invention is the layout arrangement or
 positioning of moveable transmit feed 70 and fixed receive feed 68 with
 respect to mirror 150. Such is depicted in FIG. 3C--a partial plan view of
 antenna assembly 60 of FIG. 3A--wherein transmit feed 70 is positioned at
 an origin O of an X-Y plane defined by an X axis and a Y axis, and receive
 feed 68 is fixed at an origin O'. Transmit feed origin O is concentric
 with mirror 150, and the Y-axis axis is directed radially inward from
 origin O toward mirror 150, i.e., an inward radial displacement or
 movement of transmit feed 70 form origin O toward mirror 150 coincides
 with a positive-Y displacement of the transmit feed. The X axis is
 orthogonal to the Y-axis, in a conventional right-handed Cartesian
 coordinate system with the Z-axis directed upwardly, i.e., out of the
 plane of FIG. 3C. Receive feed 68 is fixed at position O', also concentric
 with mirror 150.
 Receive and transmit beams 30,28 are aligned with optical axis 42 with
 receive and transmit feeds 68,70 positioned at respective origins O',O.
 Operationally, with longitudinal axis 146 of moveable transmit feed 70
 positioned as depicted in FIGS. 3A and 3C, i.e., aligned with origin O of
 the X-Y plane, beam 66a exiting aperture 144 impinges upon a central
 region of mirror 148, and from there traces a centralized path through
 intermediate waveguide assembly 64, as indicated in FIG. 3A by the rays
 between mirrors. It is to be appreciated that although beam 66a diverges
 and converges along its path responsive to its interaction with the
 various optical components, an axis of the beam is nevertheless
 centralized with respect to the guiding optical components. Importantly,
 since beam 66a follows the path depicted in FIG. 3A throughout assembly
 64, the beam exits the assembly in the direction of optical axis 42 and is
 centrally directed through first focus point 138. Main reflector 40 and
 sub-reflector 130 focus centralized beam 66a incident thereto into a main
 transmit beam, i.e., transmit beam 28, in the direction of optical axis
 42, as indicated by rays 164.
 FIG. 3E is a plot of antenna transmit power/gain versus angular deviation
 from optical axis 42 for antenna assembly 20 arranged as depicted in FIGS.
 3A and 3C, and operating at a transmit frequency of approximately 22 Ghz.
 The peak transmit gain PG plotted in FIG. 3E is aligned with optical axis
 42 because transmit feed 70 is positioned at origin O, as depicted in
 FIGS. 3A and 3C.
 Displacement of transmit feed 70 in the X-Y plane, i.e., in the X and/or Y
 directions, independently steers transmit beam 28 angularly away from
 optical axis 42 in either or both azimuthal and elevational directions.
 More specifically and by way of example, displacement of longitudinal axis
 146 of feed 70 from origin O by an amount .DELTA.X in the X-direction and
 an amount .DELTA.Y in the Y-direction, as depicted in FIG. 3D, imposes an
 angular offset between transmit beam 28 and optical axis 42.
 The causal effect between displacement of transmit feed 70 and angular
 displacement of transmit beam 30 is explained with reference back to FIG.
 3B. Beam 66a, originating from displaced transmit feed 70, impinges upon a
 portion of mirror 148 correspondingly displaced from the central region
 thereof, and from there traces a correspondingly displaced path, i.e.,
 displaced with respect to the centralized path of FIG. 3A, through the
 optical components of the beam waveguide assembly. Unlike FIG. 3A,
 displacement of beam 66a throughout assembly 64 causes beam 66a to exit
 assembly 64 displaced from first focus point 138 in the -Y-direction. Beam
 66a is directed through a displaced beam convergence point 166, as
 depicted in FIG. 3B. Main reflector 40 and sub-reflector 130 generally
 focus displaced or offset beam 66a incident thereto into a transmit beam
 angularly offset from optical axis 42, as indicated by rays 168. The
 magnitude and direction of the angular offset between the main beam and
 optical axis 42 is a function of the magnitude and direction of the
 displacement of longitudinal axis 146 of feed 70 in the X-Y planar
 direction. In this manner, control of transmit feed displacement
 responsively controls the angular offset of transmit beam 28 from optical
 axis 42 in azimuth and elevation.
 Another example of the above described angular offset is illustrated in
 FIG. 3F. FIG. 3F is a plot of antenna transmit power/gain versus angular
 deviation from optical axis 42 for antenna assembly 20 transmitting at
 approximately 22 Ghz, and arranged with transmit feed 70 offset
 approximately 1.66 inches from origin O in the X-direction. The 1.66 inch
 displacement between transmit feed 70 and origin O causes a 25 millidegree
 angular offset between the peak transmit gain PG' and optical axis 42, as
 depicted in FIG. 3F.
 It is to be understood that in the beam waveguide optics of antenna
 assembly 60, interaction with and control of receive and transmit EM beams
 66b,66a is reciprocal, i.e., the same, with respect to both the receive
 and transmit beam-path directions, with the exception that receive feed 68
 is fixed. The receive and transmit beams trace equivalent but reverse
 paths through the beam waveguide optics of assembly 64, and are thus
 equivalently influenced thereby. With regard to the receive beam path,
 down-link signal 34 received by receive beam 30 from a predetermined
 direction, is directed by main reflector 40 and sub-reflector 130 to
 intermediate waveguide assembly 64. Waveguide assembly 64 in turn directs
 beam 66b from main reflector 40 to receive feed 68 positioned at O'.
 Receive feed 68 directs beam energy collected thereby to a receiver of
 antenna system 20, not shown.
 In brief summary, the preferred embodiment includes moveable transmit feed
 70 and fixed receive feed 68 within edifice 134 to feed the beam waveguide
 assembly 64. Receive beam 30 is steerable through conventional beam
 steering techniques previously discussed, e.g., using APC 90 and motors
 and servomechanisms 67 controlled thereby, whereas transmit beam 28 is
 independently steerable through controlled displacement of transmit feed
 70. Transmit beam 28 is also steerable using the conventional technique.
 FIG. 4 is a perspective view of platform assembly 72 used to support and
 displace transmit feed 70. Platform assembly 72 is a commercially
 available product sold by, for example, Parker Hannifin Corporation
 located in Pennsylvania. Platform assembly 72 supports transmit feed 70
 and is adapted to displace the position of transmit feed 70 in a planar
 direction, e.g., in the X-Y plane. Platform assembly 72 is a vertically
 stacked structure including a base 200 fixed or resting on surface 76. An
 X-translation table 202 disposed above and slidingly coupled to base 200
 is displaceable in the X-direction. A Y-translation table 204 disposed
 above and slidingly coupled to X-translation table 202 is displaceable in
 the Y-direction. Transmit feed 70 is supported by an upper surface 206 of
 Y-translation table 204 and is displaced therewith.
 An upper surface 208 of base 200 includes a pair of parallel rails 210
 extending in the X direction. A set of parallel legs, not shown, depend
 vertically from a lower surface of X-translation table 202. The set of
 parallel legs slidingly engage parallel rails 210, whereby X-translation
 table 202 can be driven to slide in the X-direction. A first actuator
 assembly includes a motor 220 fixed to base 200, and a threaded rod 218
 rotatably driven by motor 220. Threaded drive rod 218 is rotatably coupled
 to X-translation table 202, whereby X-translation table 202 is driven to
 slide in the X-direction responsive to a rotative displacement of threaded
 drive rod 218 by motor 220. Specifically, X-translation table 202 is
 displaced in opposing X-directions responsive to bidirectional rotative
 displacement of threaded rod 218 by motor 220.
 Similar to the above arrangement, a pair of parallel rails 230 extending in
 the Y-direction are fixed relative to X-translation table 202.
 Y-translation table 206 is driven to slide along rails 230 by a second
 actuator including a motor 238 and an associated threaded rod 239 coupled
 to Y-translation table 204.
 Actuator control signals 112x,112y are provided to respective control
 inputs of motors 220,238 to control the rotative displacement imparted by
 these motors to respective drive shafts 218,239, to thus control the
 displacements of respective X- and Y-translation tables 202,204. Actuator
 control signals 112x,112y control the number of revolutions, the angular
 velocity, and the angular acceleration of respective drive shafts 218,239.
 In this manner, actuator control signals 112x,112y control the magnitude,
 velocity, and acceleration of the X and Y displacements of feed 70.
 FIGS. 5A and 5B are predicted performance curves for antenna assembly 20
 operating at a Ka band frequency, e.g., 34 GHz, and with a main reflector
 diameter of 35 meters. FIG. 5A is a plot of peak transmit beam gain loss
 versus transmit feed displacement along the X and Y axes. FIG. 5B is a
 plot of beam deviation, i.e., angular displacement from a reference axis,
 versus transmit feed displacement along the X and Y axes. Significantly,
 at a beam deviation or angular displacement of twenty millidegrees,
 corresponding to a feed displacement of approximately two inches from
 origin O, peak transmit beam gain loss is less than 1.5 dB. Such
 performance permits the beam tracking of a distant spacecraft in the
 presence of planetary aberration in accordance with the present invention.
 For instance, transmitter power, and thus the power of the uplink signal,
 can be increased to compensate for the relatively small decrease in
 peak-gain loss of transmit beam 28 resulting from the angular displacement
 of the transmit beam from optical axis 42 of the antenna.
 In antenna system 20, APC 90 and beam steering controller 116 control the
 beam forming/directing components of antenna assembly 60. FIG. 6A is a
 block diagram of an embodiment of controller 116. Controller 116 is a
 general purpose computer, e.g., a personal computer, as is known in the
 art. The controller includes a bus 300 for communicating information and a
 processor 302 coupled with bus 300 for processing information. A storage
 device 304, e.g., a disk, is provided and coupled to bus 300 for storing
 static information and instructions for processor 302. Controller 116
 further includes a main memory 306 coupled to bus 300 for storing
 instructions to be executed by processor 302, and for storing the apriori
 spacecraft position information downloaded via interface 118. Main memory
 306 is also used for storing temporary variables or other intermediate
 information during execution of instructions executed by processor 302.
 Controller 116 includes a two-way data communication interface 308 coupled
 to bus 300. Communication interface 308 includes interfaces 120,118.
 Controller 116 includes a display 310 for displaying information, e.g.,
 status, to antenna system operators. Operators enter information into
 controller 116 with an input device 312.
 Processor 302 executes sequences of instructions contained in main memory
 306. Such instructions are read into memory 306 from another
 computer-readable medium, such as storage device 304. Execution of the
 sequences of instructions contained in memory 306 causes processor 302 to
 perform various method and operational steps of the present invention. In
 alternative embodiments, hard-wired circuitry can be used in place of or
 in combination with software instructions to implement the invention.
 Controller 110 directly controls the movement of transmit feed 70. An
 embodiment of transmit feed controller 110 is depicted in FIG. 6B. Feed
 controller 110 includes a bus 350 coupled with the following components: a
 processor 352; a main memory 353 for storing program instructions executed
 by processor 352; a communication interface 354 for receiving beam
 steering commands from controller 116; and, a pulse generator 356 for
 generating control signals 112x,112y. Processor 352 translates transmit
 feed displacement commands received via interface 120 to pulse generator
 commands, including displacement magnitude, velocity and acceleration
 commands. Processor 352 issues the pulse generator commands to pulse
 generator 356. Pulse generator 356 derives pulsed, actuator control
 signals 112x,112y in real-time responsive to the pulse generator commands
 issued thereto.
 As mentioned above, antenna system controller 62 (FIG. 2) derives control
 signals and commands for controlling antenna assembly 60. Specifically,
 APC 90 derives antenna steering control signals 92,94 while controllers
 110 and 116 derive actuator control signals and 112x,112y to control the
 position of transmit feed 70. The following exemplary sequence of method
 steps describes the derivation and application of these control signals,
 and the control of antenna assembly 60 to thereby compensate for planetary
 aberration in the beam tracking of spacecraft 24.
 FIG. 7 is a high level flow diagram for controlling antenna assembly 60 to
 compensate for planetary aberration. At step 390, the process is started.
 At step 400, apriori spacecraft trajectory information corresponding to
 trajectory 26 is downloaded from an external source, not shown, to
 controllers 90,116 via respective interfaces 100,118.
 Next, at step 405, controller 116 uses the apriori trajectory information
 to determine an apriori past position, e.g. p1, and an apriori future
 position, e.g., p2, corresponding to an apriori present time and an
 associated apriori present position, e.g., p3, using the RTLT of down-link
 and uplink signals 34,32 between antenna assembly 60 and spacecraft
 located at apriori present position p3. This preparatory step 405 can
 occur at any time before spacecraft 24 is actually at present position p3.
 Next, at preparatory step 410, controller 116 derives an angular offset
 between receive and transmit beams 30,28, e.g., angular offset 44,
 corresponding to an alignment of receive and transmit beams 30,28 with
 respective past and future positions p1,p2
 Next, at preparatory step 415, controller 116 translates angular offset 44
 to a corresponding positional displacement of moveable transmit feed 70
 from origin O. Such displacement imposes the required angular offset 44
 between receive and transmit beams 30,28, when receive beam 30 is aligned
 with past position p1.
 The next step, step 420, is a real-time step, wherein antenna system 20
 steers receive and transmit beams 30,28 into alignment with respective
 past and future positions p1,p2 at the real-time occurrence of the present
 time, when spacecraft 24 is actually at the present position p3 along
 trajectory 26. Antenna system 20 imposes angular offset 44 between receive
 and transmit beams 30,28, and in doing so, aligns receive beam 30 with
 position p1 to receive down-link signal 34 arriving therefrom, and aligns
 transmit beam 28 so as to transmit uplink signal 32 in the direction of
 future position p2. It is to be understood that steps 400-420 are
 continuously repeated for positions p.sub.n, p.sub.n+1 so as to maintain
 alignment between receive and transmit beams 30,28 and successive
 respective past and future positions (e.g., p1,p2) as spacecraft 24
 traverses trajectory 26. In this manner, receive and transmit beams 30,28
 of antenna system 20 continuously track spacecraft 24 along trajectory 26
 and continuously compensate for planetary aberration.
 Method steps 400-420 are now explained more fully with reference to
 additional FIGS. 9, 10 and 11, wherein high-level method steps 410, 415,
 and 420 are respectively depicted in greater detail. In step 400, apriori
 spacecraft trajectory information is downloaded into the memories of APC
 controller 90 and controller 116. The apriori information is formatted to
 include a time-ordered list or series of successive spacecraft position
 entries 600 corresponding to trajectory 26 of spacecraft 24, as depicted
 in FIG. 8. Each of the entries includes the following:
 1) an apriori (e.g., predicted) spacecraft position in AZ and EL
 coordinates, e.g., p1=AZ1, EL1 etc., and
 2) an associated time index or time reference indicative of a predicted
 real-time when spacecraft 24 will arrive at the associated AZ and EL,
 e.g., at real-time t1, spacecraft 24 will be at position p1 (AZ1, EL1),
 etc.
 Such information is conventional and can be downloaded to controllers
 90,116 in advance or when needed thereby. Importantly, the time indexing
 of each of the entries permits a relatively straight forward
 identification of a future position once a past (or present) spacecraft
 position is identified. The future position is found by looking ahead in
 the position/time entries a predetermined amount of time. For example,
 once past position p1 and time index t1 associated therewith are
 identified, future position p2 is determined by adding the appropriate
 RTLT to t1, to thus establish time index t2, which is then available as an
 index by which associated future position p2 can be accessed. It is to be
 understood the positions of the spacecraft can be provided in AZ and EL
 coordinates, in RA/DEC coordinates, or in any other suitable coordinate
 system, so long as appropriate mathematical conversions there between and
 derivations therefrom ultimately permit the derivation of the transmit
 feed displacements required to align receive and transmit beams 30,28 with
 ascertained past and future positions p1,p2, in accordance with the
 present invention.
 Importantly, antenna system controller 62 also uses the time indexes for
 real-time tracking of spacecraft 24. More specifically, since APC 90 and
 controller 116 are time synchronized with each other and with real-time,
 each controller can determine in real-time the past, present and future
 positions p1-p3 of spacecraft 24 corresponding to an instant in real-time
 by comparing the real-time to the time indexes associated with the apriori
 position entries.
 As described above, at step 405, controller 116 identifies apriori past,
 future, and present positions p1(AZ1, EL1), p2(AZ2, EL2) and p3(AZ3, EL3).
 At step 410, controller 116 derives angular offset 44. A pair of angular
 coordinates or components .alpha.', .beta.' define angular offset 44, as
 illustrated in FIG. 1. Controller 116 derives angular components .alpha.',
 .beta.' at respective steps 445 and 450 (FIG. 9) in accordance with the
 following equations:
EQU .alpha.'=[(.DELTA.EL).sup.2 +(.DELTA.XEL).sup.2 ].sup.1/2
EQU .beta.'=tan.sup.-1 (.DELTA.EL, .DELTA.XEL)
EQU where .DELTA.EL=EL2-EL1, and .DELTA.XEL=(AZ2-AZ1)* cos (ELAVG), and
EQU where ELAVG=(EL1+EL2)/2
 At step 415, controller 116 translates angular offset 44(.alpha.',.beta.')
 to a corresponding positional displacement of transmit feed 70 from origin
 O, as described previously. More specifically, at step 455 (FIG. 10),
 controller 116 translates or maps angular offset 44(.alpha.',.beta.') to a
 corresponding positional displacement of feed 70 defined in terms of
 planar polar coordinates .rho., .phi., illustrated in FIG. 3D. As depicted
 in FIG. 3D, The displacement of transmit feed 70 from origin O includes a
 magnitude .rho. and a direction .phi., defined relative to the X-axis.
 This translation from angular offset 44(.alpha.',.beta.') to positional
 displacement .rho.,.phi. proceeds in accordance with the following
 equations:
EQU .rho.=[(.DELTA.X).sup.2 +(.DELTA.Y).sup.2 ].sup.1/2
 where .DELTA.X and .DELTA.Y represent displacements of transmit feed 70 in
 respective X and Y directions (see FIG. 3D), and
EQU .phi.=-.beta.'-(AZ-.phi..sub.stn)+EL+n.pi./2; n=-1
 where AZ and EL represent AZ1 and EL1, and .phi..sub.stn is a constant
 depending on the location of antenna assembly 60.
 At step 460, controller 116 translates transmit feed displacement .rho.,
 .phi. into corresponding X and Y displacements .DELTA.X, .DELTA.Y. This
 translation is necessary because in the preferred embodiment, p1atform
 assembly 72 is incrementally displaceable in X and Y directions by
 respective actuator assemblies thereof.
 After completing preparatory steps 415-460, antenna system controller 62
 has available thereto the information required to align in real-time
 receive and transmit beams 30,28 with past and future positions p1,p2, to
 thus compensate for planetary aberration. APC 90 controls real-time
 steering of optical axis 42, and both receive and transmit beams 30,28
 therewith, while controller 116, along with feed controller 110, controls
 real-time independent steering of transmit beam 28. Overall, real-time
 synchronization existing between APC 90, controller 116, and transmit feed
 controller 110 permits coordinated beam steering control of receive and
 transmit beams 30,28 by antenna assembly 62.
 Specifically, at the real-time occurrence of present time t3, i.e., at the
 time when down-link signal 34 arrives at antenna system 20 from the
 direction of past position p1, antenna system 20 performs the following
 steps:
 1) at step 463 (FIG. 11), APC 90 steers receive beam 30 into alignment with
 past position p1 to receive the down-link signal arriving therefrom. Such
 steering requires APC 90 to drive optical axis 42 of antenna assembly 62
 in azimuthal and elevational directions to bring receive beam 30 into
 alignment with past position p1; and
 2) at step 465, transmit beam 28 is steered into alignment with position
 p2. Specifically, controller 116 issues a transmit feed X,Y displacement
 command to transmit feed controller 110. The X,Y displacement command
 includes the transmit feed X and Y displacements .DELTA.X, .DELTA.Y
 required to impose angular offset 44(.alpha.', .beta.') between receive
 and transmit beams 30,28, with receive beam 30 aligned with past position
 p1 (see step 463). The X,Y displacement command also includes a time entry
 indicative of the real-time when such displacements .DELTA.X, .DELTA.Y
 must be imposed by feed controller 110. Feed controller 110 generates in
 real-time actuator control signals 112x,112y indicative of transmit feed
 displacement responsive to the X,Y displacement command. Platform assembly
 72 appropriately displaces transmit feed 70 from origin O in the X-Y plane
 responsive to supplied actuator control signals 112x,112y, as depicted in
 FIG. 3D. The planar displacement thus imposed between receive and transmit
 feeds 68,70 correspondingly imposes angular offset 44(.alpha.', .beta.')
 between receive and transmit beams 30,28, to compensate for planetary
 aberration.
 In accordance with the present invention, antenna system 20 continuously
 tracks spacecraft 24 as the spacecraft moves along its trajectory 26, to
 compensate for planetary aberration throughout the trajectory.
 Accordingly, APC 90 continuously steers receive beam 30 in real-time to
 track successive past positions of spacecraft 24. Contemporaneously,
 controller 116 and feed controller 110 steer transmit beam 28 to track
 successive future positions of spacecraft 24, associated with the
 successive past positions, by continuously updating angular offset 44
 (.alpha.', .beta.'), in response to updating of displacements .DELTA.X,
 .DELTA.Y of transmit feed 30. It can thus be appreciated that method steps
 400-465 are repeatedly traversed to provide such continuous updating to
 beam track the movement of spacecraft 24 along its trajectory 26.
 In practice, an angular alignment error 470 (see FIG. 1) typically arises
 between optical axis 42 and receive beam 28, when receive beam 28 is
 aligned with position p1. Angular alignment error 470 arises because of
 systemic errors in antenna assembly 60. At least two factors contribute to
 these systemic errors; imperfections in motors and servomechanisms 67
 leading to imperfect steering of optical axis 42 by APC 90, and
 imperfections in the optical components of the beam waveguide assembly
 leading to an angular offset error between optical axis 42 and the
 direction of receive beam 30 (and transmit beam 28).
 In the present invention, a bore-sighting calibration procedure quantifies
 angular alignment error 470, thus leading to subsequent compensation
 thereof. One such calibration procedure includes receive beam tracking of
 a distant radio source having a known location, such as a star. More
 specifically, APC 90 steers optical axis 42 into alignment with the
 positional coordinates, e.g., AZ and EL or RA/DEC, of a known star. APC 90
 systematically displaces, i.e., nutates, optical axis 42 with respect the
 position of the known star source. A receiver (not shown), coupled to an
 output of receive feed 68 and to APC 90 monitors radio signal power
 received from the star via receive beam 30, while optical axis 42 is
 nutated. A maximum received signal is detected and a corresponding angular
 offset, e.g., angular offset 470, identified. Angular offset 470 is stored
 in APC 90 memory as an angular alignment error, i.e., adjustment factor,
 for use during subsequent tracking of spacecraft 24. APC 90 applies the
 adjustment factor as necessary throughout method steps 400-465 to fine
 tune the alignment of receive and transmit beams 30,28 with respective
 positions p1,p2. For example, at step 463 APC 90 steers receive beam 30
 into calibrated alignment with position p1 by incorporation of the
 adjustment factor into AZ and EL control signal pair 92,94.
 An antenna system for and method of compensating for planetary aberration
 in the receive and transmit beam tracking of a spacecraft has been
 described. Advantageously, receive and transmit beams formed by the
 antenna system are angularly separated or split to contemporaneously align
 the receive and transmit beams with separated past and future positions of
 the satellite. By concurrently aligning the peak gains of the receive and
 transmit beams with respective down-link and uplink signals transmitted
 between the antenna system and the spacecraft, the antenna system
 advantageously reduces the effect of propagational attenuation of such
 signals.
 While there have been described and illustrated specific embodiments of the
 invention, it will be clear that variations in the details of the
 embodiments specifically illustrated and described may be made without
 departing from the true spirit and scope of the invention as defined in
 the appended claims.