Wireless communications systems are currently using space vehicles to facilitate the global exchange of information. The space vehicles typically are Low Earth Orbiting satellites which are linked via ground based stations to provide wireless access over most of the Earth's surface. A loss of information is avoided because each satellite is tracked by a ground station until about twenty seconds before its descent below the horizon. At that point, the ground station locates an ascending satellite to which the communication link can be transferred. Since the ascending and descending satellites can transmit information at any time, the ground station must be receptive to transmissions by both satellites at all times until the link transfer is complete. Such"make before break"communication ensures that no data will be lost.

Previous systems remain simultaneously linked to the ascending and descending satellites by tracking them with separate antennas. However, the separate antennas have high fabrication and maintenance costs. Moreover, such systems have a low reliability and occupy too much space for many installations.

Hence, there is a need for a communication system that uses a single compact antenna which can simultaneously track two satellites in order to increase reliability while reducing the cost of manufacturing, maintaining and operating the system.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a view of a tracking antenna within a wireless communications system; and FIG. 2 shows a receive portion of the tracking antenna.

DETAILED DESCRIPTION OF THE DRAWINGS In the figures, elements having the same reference numbers have similar functionality.

FIG. 1 shows a tracking antenna 10 within a wireless communications system 100. Antenna 10 typically is disposed in a ground based station to communicate with first and second orbiting space vehicles 12 and 13. In one embodiment, first and second space vehicles 12 and 13 comprise Low Earth Orbiting satellites. Alternatively, space vehicles 12 and 13 may comprise another type of satellite, or aircraft or other airborne vehicles.

Antenna 10 includes a receive antenna array 20 and a transmit antenna array 22 formed on a substrate 24 which is coupled to a turntable 32 by means of an axle or shaft 37. Receive antenna array 20 operates as a dual beam phased array antenna that provides coverage over fields of view 42 and 45 for simultaneously receiving microwave communication signals VC1 and VC2 propagating on signal paths 44 and 46 from space vehicles 12 and 13, respectively.

In one embodiment, communication signals VC1 and VC2 operate at about twenty gigahertz with a bandwidth of about five-hundred megahertz. Receive antenna array 20 includes N rows of antenna elements, where N is an integer and each row is organized as a linear array of antenna elements formed on a surface 48 of substrate 24 parallel to an axis 36. In one embodiment, the rows run parallel to a line 69 which is the intersection of surface 48 and a signal plane defined by signal paths 44 and 46.

Fields of view 42 and 45 determine ranges of incident angles within which signals are visible to antenna 10. For example, signal path 44 lies within field of view 42 and is therefore receivable by antenna 10. Signal path 46 lies within field of view 45 and is visible to antenna 10. Fields of view 42 and 45 are electrically rotated about axis 36 to keep fields of view 42 and 45 aligned with signal paths 44 and 46.

Transmit antenna array 22 operates as a phased array antenna that generates field of view 47 for transmitting communication signals along a signal path 40 to space vehicle 12. In one embodiment, the transmitted signals operate at about thirty gigahertz with a bandwidth of about five-hundred megahertz. The

operation of transmit antenna array 22 is similar to the operation of receive antenna array 20, except that signals are being transmitted rather than received. Since a ground station controls when signals are transmitted from antenna 10, but not when they are received, only a single field of view whose direction is electronically switched between space vehicle 12 or space vehicle 13 is necessary for transmitting to two space vehicles.

A gimbal structure 38 operates as a drive mechanism that includes first, second and third motors 26,29 and 33 coupled for tilting or rotating substrate 24.

First motor 26 rotates substrate 24 about a first axis 27 in a direction indicated by arrow 28. Second motor 29 rotates substrate 24 about a second axis 30 perpendicular to first axis 27 as shown by arrow 31. Third motor 33 rotates turntable 32 and substrate 24 about a third axis 34 perpendicular to first axis 27 and second axis 30 as shown by arrow 35. In operation, motors 26,29 and 33 rotate substrate 24 to maintain the columns substantially parallel to line 69 and perpendicular to surface 48 in order to improve the received signal strength.

As a feature of the present invention, the rotation of substrate 24 about first and third axes 27 and 34 provides acquisition alignment and tracking of fields of view 42 and 45 with space vehicles 12 and 13. Rotation about second axis 30 allows the gain of receive antenna array 20 to be optimized. For example, the strength of a captured signal is a function of the projected capture area or angle of incidence of the signal. A higher incident angle produces a larger capture area and higher signal strength. Hence, if communication signals VC1 and VC2 are of equal magnitudes, substrate 24 is rotated about second axis 30 to equalize the incident angles of VC1 and VC2. However, if VC1 has a higher magnitude than VC2, substrate 24 is rotated to reduce the incident angle and capture area of VC1 while increasing the incident angle and capture area of VC2 to equalize the gain of receive antenna array 20.

FIG. 2 shows antenna 10 in further detail, including a portion of receive antenna array 20, summing devices 73 and 74, and N phase shifters PS1 through PSN, where N is an integer. In one embodiment, N=100. Antenna elements of receive antenna array 20 are formed as openings in substrate 24 to operate as waveguides. Alternatively, the antenna elements may be formed on surface 48 as patch antenna elements or similar devices. In one embodiment, antenna elements

of receive antenna array 20 are spaced on centers which are separated by one-half a wavelength of communication signals VC1 and VC2, or about 0.8 centimeters.

As a further alternative, a skilled artisan would recognize that similar functionality of antenna 10 may be provided using a cylindrical reflector and an array of feed elements that each contain an electronically variable time delay device. One skilled in the art would recognize that a dual frequency linear array can be achieved using a dichroic Cassegrain sub-reflector.

Receive antenna array includes N linear arrays of antenna elements LA1 through LAN coupled to phase shifters PS1 through PSN as shown. The number of antenna elements within each linear array depends on such factors as the shape of receive antenna array 20, the desired gain, the shape of the fields of view and the like. In one embodiment, each linear array includes 100 antenna elements. Linear arrays LA1 through LAN include amplitude and phase distribution networks to combine the energy captured by their respective antenna elements to produce output signals VLA1 through VLAN. High frequency connectors of linear arrays LA1 through LAN are used for coupling VLA1 through VLAN on transmission lines to phase shifters PS1 through PSN. For example, output signal VLA1 is produced on a connector coupled to a transmission line 57 running from linear array LA1 to phase shifter PS1, and output signal VLA2 is produced on a connector coupled to a transmission line 58 running from linear array LA2 to phase shifter PS2.

Rays VC1A and VC1B of communication signal VC1 and rays VC2A and VC2B of communication signal VC2 are incident on antenna elements 61 and 63, respectively. Ray VC1 A is received at an incident angle 71, and ray VC2A is received at an incident angle 70. Note that the energy of captured signals increases as the incident angle increases. For example, maximum energy is captured from signals having an incident angle of ninety degrees, i. e., from directly overhead, whereas virtually no energy is captured from signals having an incident angle of zero degrees, i. e., propagating parallel to surface 48.

The operation of receive antenna array 20 proceeds as follows. Energy from ray VC1 A is captured by antenna element 61 and energy from ray VC1 B is captured by antenna element 63. Since ray VC1A travels a greater distance than ray VC1 B from space vehicle 12, ray VC1 A is captured a time delay T1 after ray VC1 B is captured. Hence, when captured, ray VC1A is phase shifted with respect to ray

VC1 B. Time delay T1 is a function of incident angle 71. In a similar fashion, rays VC2A and VC2B are captured by antenna elements 61 and 63, but ray VC2B is captured a time delay T2 after ray VC2A is captured. Time delay T2 is a function of incident angle 70. The other antenna elements operate in a similar fashion.

Recall that substrate 24 is rotated to maintain line 69 parallel to rows 51 and 52 and perpendicular to linear arrays LA1 through LAN. Hence, antenna elements of the same linear array are equidistant from a signal's originating point and consequently are captured at the same time with zero time delay, i. e., in phase. For example, signals captured by antenna elements 62 and 63 of linear array LA1 are in phase.

Note that signals VLA1 through VLAN each have components of both communication signals VC1 and VC2. For example, VLA1 includes a VC1 component captured from ray VC1 B and a VC2 component captured from ray VC2B. Similarly, VLA2 includes a VC1 component captured from ray VC1Z and a VC2 component captured from ray VC2A. Specific VC1 and VC2 components are delayed or phase shifted as described above.

Phase shifters PS1 through PSN are configured as dual, programmable, modulus 2x phase shifters which include low noise buffer amplifiers to amplify signals VLA1 through VLAN. The amplified signals are each routed through two programmable delay elements which introduce delays or phase shifts to compensate for the relative time delays among the signals captured by antenna elements. In one embodiment, the delay elements are implemented using binarily weighted transmission lines whose lengths are programmed by control signals CONTROL11 through CONTROLN1 and CONTROL12 through CONTROLN2 to vary from one-eighth to one-half of a wavelength of VC1 and VC2. Alternatively, the delay elements may include selectable passive components such as capacitors and inductors to provide the delays or phase shifts. Phase shifters PS1 through PSN typically are configured as integrated circuits.

Phase shifters PS1 through PSN have similar operation which can be understood by referring to the operation of phase shifters VPS1 and VPS2. Signal VLA1 is amplified by the low noise buffer amplifier within phase shifter PS1 to produce a first amplified signal which is routed through a first delay element of PS1 to produce component signal VPS11 having a first phase shift determined by

CONTROL11. The first amplified signal is routed through a second delay element of PS1 to produce component signal VPS12 having a second phase shift determined by CONTROL12.

Similarly, signal VLA2 is amplified by the low noise buffer amplifier within phase shifter PS2 to produce a second amplified signal which is routed through a first delay element of PS2 to produce component signal VPS21 having a third phase shift determined by CONTROL21. The second amplified signal is also routed through a second delay element of PS2 to produce component signal VPS22 having a fourth phase shift determined by CONTROL22.

Control signals CONTROL11 and CONTROL21 are selected to set the difference between the first and third phase shifts to compensate for time delay T1 so that the VC1 components of component signals VPS11 and VPS21 are substantially in phase while other components of VPS11 and VPS21 are out of phase. Similarly, control signals CONTROL12 and CONTROL22 set the difference between the second and fourth phase shifts to compensate for time delay T2 so the VC2 components of signals VPS12 and VPS22 are in phase while other components are out of phase. The other phase shifters operate similarly such that the VC1 components of VPS11 through VPSN1 are in phase while other components are out of phase. The VC2 components of VPS12 through VPSN2 are in phase while other components are out of phase.

The phase shifts produced by phase shifters PS1 through PSN determine the angles of fields of view 42 and 45. These angles can be modified by altering the phase shifts to effectively produce a rotation of fields of view 42 and 45 around line 36. In a typical embodiment in which one-hundred linear arrays each contain one-hundred antenna elements, a total N=100 phase shifters are used. Other antenna arrays with a similar number of antenna elements are configured to generate fields of view that rotate about two perpendicular axes. However, these other antenna arrays need ten thousand phase shifters (100*100) to resolve phase differences among row elements along the two axes. Since the transmission lines or passive elements of the additional phase shifters occupy a large die area of an integrated circuit and therefore have a high cost, the other antenna arrays are substantially more costly than antenna 10.

Summing devices 74 and 75 include analog adders which enhance the in-

phase components of the input signals while suppressing other components. That is, summing device 74 adds component signals VPS11 through VPSN1 to produce output signal VOUT1 at output 72. The VC1 components of VPS11 through VPSN1 are in phase, so VOUT1 primarily contains information from communication signal VC1. Similarly, summing device 75 adds component signals VPS12 through VPSN2 to produce output signal VOUT2 at output 73 which primarily contains information from communication signal VC2. Hence, summing devices 74 and 75 effectively separate the VC1 and VC2 components of signals captured by receive antenna array 20 to provide concurrent communication with space vehicles 12 and 13.

By now it should be appreciated that the present invention provides a device and method for maintaining concurrent communications with two space vehicles in different locations. An antenna has an array of antenna elements formed on a substrate to generate first and second fields of view of the antenna. A first drive mechanism is coupled to the substrate for rotating the fields of view about a first axis to acquire first and second signals. A second drive mechanism is coupled to the substrate to rotate the fields of view about a second axis to track the signals. A third drive mechanism rotates the substrate about a third axis to optimize the magnitudes of the received first and second signals. The present invention thereby provides an antenna configured as a single unit which can simultaneously track and communicate with multiple space vehicles.