Abstract:
A dual phased array payload ( 100 ) for use onboard a communications satellite is disclosed. The payload includes one or more phased array receive antennas ( 102-108 ) including numerous individual receiving elements distributed in a predetermined configuration. Each of the individual radiating elements is selectively adjustable in amplitude and phase to achieve scanning beams for receiving information transmitted from the ground in an uplink beam. The payload includes a packet switch ( 114 ) connected to the phased array receive antennas ( 102-108 ). The packet switch ( 114 ) includes a set of inputs and a set of outputs. The set of inputs are selectively connectable to the set of outputs. The payload ( 100 ) includes one or more phased array transmit antennas ( 120-126 ) connected to the packet switch ( 114 ). The phased array transmit antennas ( 120-126 ) include numerous individual radiating elements distributed in a predetermined configuration.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a communications satellite transmission and reception architecture that includes phased array antennas. In particular, the present invention relates to a satellite payload that coordinates a transmit phased array antenna, packet switch, and a receive phased array antenna. 
     Satellites are a common feature of modern communications networks and have long provided communications services on a global scale. A communications satellite often flies in a geostationary (GSO) orbit (at approximately 35,784 km altitude with an inclination and eccentricity of zero) so that the satellite always appears in the same spot in the sky. Satellites, however, may also be placed in other orbits, including Non-geostationary orbits (NGSO). 
     A NGSO satellite typically orbits between 250 and 12000 km above the Earth. NGSO satellites orbit the Earth independently of the Earth&#39;s own rotation and therefore do not maintain a constant location in the sky. Because the orbit of a NGSO satellite periodically takes the NGSO satellite over various locations on the Earth, the NGSO satellite may be used to provide periodic communications services to those locations. A constellation of many NGSO satellites may be used to provide nearly continuous coverage to virtually all areas of the Earth. 
     As an example, Teledesic LLC, located in Kirkland Wash., United States, has proposed a NGSO constellation referred to as the Teledesic Network which flies 288 NGSO satellites. The Teledesic Network incorporates 12 orbital planes with 24 NGSO satellites per plane. Each orbital plane is approximately perpendicular to the equator and separated from adjacent orbital planes by approximately 15 degrees. The altitudes of the satellites in each orbital plane are staggered so that the satellites pass below and above one another at the North and South poles, where each orbital plane converges. 
     Two sets of intersatellite links (ISL) connect the satellites in the Teledesic Network. North-South links provide continuous connections between the satellites in individual orbital planes. Any first satellite in an orbital plane has a connection to a second satellite ahead of its current position and a third satellite behind its current position. Similarly, East-West links provide a connection between the satellites in a first orbital plane and the satellites in a second orbital plane and a third orbital plane on either side of the first orbital plane (the adjacent orbital planes). 
     In the past, satellite antenna technology has provided GSO and NGSO satellites with Multi-Beam Antennas (MBAs) which provide transmission and reception of fixed size and direction beams. Because the MBAs are fixed, the satellite using an MBA antenna cannot adjust its antenna pattern to accommodate the variations in user demand in a region of interest (ROI) as the satellite moves overhead. For example, an MBA pattern setup to cover the United States would not efficiently cover South America. 
     As a result, satellites in the past have required payloads using complex configurations of MBAs. Each MBA is selectively activated or deactivated to provide coverage for a ROI as the satellite moves from one ROI to another ROI. The complex configurations of MBAs require the satellite to include a complex switching network to activate and deactivate the MBAs. Furthermore, the complex switching network and additional MBAs increase the power demand on the satellite&#39;s limited power supply as well as drive up the size and weight of the satellite (making the satellite more expensive to build, transport, and launch). 
     In the past, satellites have attempted to cope with the complexity of MBA design by purposefully creating payloads with sub-optimal MBA configurations using fewer MBA components. Because an MBA design providing optimal coverage for one ROI generally results in very poor coverage for the other ROIs, a compromised, sub-optimal MBA configuration is typically used. The sub-optimal configuration reduces the complexity of the MBA design and relieves the very poor coverage for the other ROIs by providing sub-optimal coverage for all ROIs. The sub-optimal configuration also reduces the amount of communications capacity available to the ROIs and thereby limits the amount of information that may be transmitted and received as the satellite moves overhead. Revenues are correspondingly reduced. 
     A need has long existed in the industry for a satellite payload design that eliminates the added complexity, power requirements, and cost of complex MBA configurations. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a communications satellite with an increased optimality in antenna and payload architecture. 
     It is another object of the present invention to lower the cost, size, and weight of a communications satellite. 
     Yet another object of the present invention is to provide a communications satellite with the ability to flexibly adjust its uplink and downlink beam coverage using phased array antennas. 
     Another object of the present invention is to allow a communications satellite to route information between a receive phased array antenna and a transmit phased array antenna using a packet switch. 
     The present invention provides a dual (i.e., including both transmit and receive antennas) phased array payload for use onboard a communications satellite. The payload includes a phased array receive antenna including numerous individual receiving elements distributed in a predetermined configuration. Each of the individual radiating elements has a low noise amplifier (LNA) and is selectively adjustable in phase and amplitude to achieve scanning beams for receiving information transmitted from the ground in an uplink beam. 
     The payload also includes a packet switch connected to the phased array receive antenna. The packet switch includes a set of inputs and a set of outputs. Each input is selectively connectable to one or more outputs. 
     In addition, the payload includes a phased array transmit antenna connected to the packet switch. The phased array transmit antenna includes numerous individual radiating elements distributed in a predetermined configuration. Each of the individual radiating elements has a power amplifier, as well as a controllable amplitude and phase excitation used to electronically steer a downlink beam produced by the radiating elements in combination. 
     A payload computer is connected to the packet switch. The payload computer generates outputs that control the connection of the packet switch inputs to the packet switch outputs. The payload may further include a downconverter connected to the phased array receive antenna, an analog to digital converter connected to the downconverter, and a demodulator/decoder connected to the analog to digital converter. An encoder/modulator may additionally be connected to the packet switch, and an upconverter may also be connected to the encoder/modulator and the phased array transmit antenna. 
     The communications satellite may additionally communicate with the ground, or with other satellites, using a beacon transmitter and receiver. The beacon transmitter and receiver operate under control of the payload computer to transmit and receive command, control, and status information. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates one embodiment of the dual phased array payload of the present invention, including a phased array receive antenna, packet switch, and phased array transmit antenna. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to FIG. 1, that figure shows one embodiment of the dual phased array payload  100  according to the present invention. The payload  100  includes the phased array receive antennas  102 - 108  (“receive antennas”) connected to a low noise amplifier and next to downconverter  110 . The downconverter  110  is in turn connected to channel recovery circuitry  112  including an Analog to Digital Converter (ADC), a channelizer, and demodulators/decoders. A packet switch  114  is included and connected to the channel recovery circuitry  112  as well as an encoder/modulator  116 . An upconverter  118  is connected between the encoder/modulator  116  and the phased array transmit antennas  120 - 126  (“transmit antennas”). 
     A beacon  128  and associated beacon control  130  are also included. A payload computer  132  controls the operation of the payload  100 , and in particular, is connected to the packet switch  114  and the beacon control  128 . A frequency generator  134  is provided to produce the frequencies required by the downconverter  110  and the upconverter  118 . The payload may also include an ISL North transmitter and receiver pair  136  and an ISL South transmitter and receiver pair  138  as well as an ISL East transmitter and receiver pair  140  and an ISL West transmitter and receiver pair  142 . 
     The phased array receive antennas  102 - 108  replace the complex MBA antenna discussed above and receive RF energy forming uplink beams. The uplink beams may, for example, contain RF energy at approximately 28.6-29.1 GHz and contain numerous individual channels. Similarly, the transmit antennas may be used to form downlink beams containing one or more channels, for example, in the 18.8-19.3 GHz band. 
     Each phased array antenna is composed of a group of individual radiators which are distributed in a predetermined configuration, for example a linear or two-dimensional spatial configuration. The radiators may be implemented, for example, as dipoles, slots, open-ended waveguides, spirals, microstrip or patch elements. 
     In accordance with phased array antenna operation principles, the amplitude and phase excitation of each radiator is individually controlled to form a radiated beam of any desired shape. The direction of the beam in space is controlled electronically by adjusting the phase of the excitation signals at the individual radiators. As a result, the phased array antenna may steer a transmitted beam while the phased array antenna remains fixed in space. In other words, no mechanical motion is required to accomplish beam steering. During reception, each radiator may be monitored or scanned in turn to determine the RF energy incident upon the radiator to recover a transmitted RF signal. Phased array antennas suitable for use with the present invention are available from TRW, Inc., Redondo Beach Calif. 90278. Additional information on phased array antennas may be found in the  Antenna Engineering Handbook  (Richard C. Johnson, ed.) 20-1-20-67 (1993). Pages 20-1-20-67 of the Antenna Engineering Handbook are incorporated herein by reference in their entirety. 
     No particular number of receive antennas or transmit antennas is required; one or more of each may be used. In certain preferred embodiments, however, power dissipation requirements may suggest a certain structure for the transmit antennas. For example, current technology allows approximately 30% of DC power applied to the transmit antennas to result in useful RF power. Thus, 70% of the DC power generally turns into heat. As a result, several smaller transmit antennas  120 - 126  that generally point NE., NW., SE., and SW. (and that dissipate heat more readily) may be used instead of one large transmit antenna. 
     The receive antennas  102 - 108  are connected to the low noise amplifier and downconverter  110 . The downconverter  110  uses a mixer to shift the frequency of the received RF energy to an intermediate frequency that may be operated on by the channel recovery circuitry  112 . The frequency generator  134  is responsible for generating the frequencies used by the downconverter  110  and may use Voltage Controlled Oscillators (VCOs), Numerically Controlled Oscillators (NCOs) and the like. 
     The channel recovery circuitry  112  includes an analog-to-digital converter (ADC) to convert the received uplink beam to digital samples. A channelization function follows which separates out the individual channels in the uplink beam and routes the channels to demodulators. The demodulators/decoders remove modulation and encoding applied to the uplink beam, for example Quadrature Phase Shift Keying (QPSK) or 16 level Quadrature Amplitude Modulation (16-QAM). The resulting demodulated digital bits from each channel are input to the packet switch  114 . 
     In most instances, information sent in an uplink beam will be broken into discrete packets. The demodulated and decoded digital bits recovered by the channel recovery circuitry  112  therefore represent those packets. The packets, in turn, typically include header information and data information. The header information may include, for example, synchronization information, error correcting information, and routing information that describes the source and destination for the data information. The payload computer  132  may use the routing information in part to control the packet switch  114 . Packets may also be routed to or from the payload computer  132  to facilitate communications resource control. 
     The packet switch  114  generally includes N inputs and M outputs and is able to connect the N inputs to any of the M outputs. In operation, the N inputs may be assigned to each uplink or intersatellite link (ISL) channel and the M outputs may be assigned to each downlink or intersatellite link (ISL) channel. As one example, the routing information may indicate that a particular packet recovered from the first uplink channel is destined for a particular region of interest (ROI). The payload computer  132  may then determine which downlink beam covers that ROI and adjust the packet switch  114  to connect the input used for the packet to the output used to generate the downlink beam covering the ROI. The packet switch also supports functions such as multicast (in which a single packet is forwarded to more than one downlink beam or ISL) and broadcast (in which a single packet is forwarded to all downlink beams and ISLs). A packet switch suitable for use in the present invention is available from TRW, Inc., Redondo Beach Calif., 90278. 
     As noted above, the outputs of the packet switch  114  are used to generate downlink or intersatellite links (ISL) beams. The first step in generating downlink beams is performed by the encoder/modulator  116  which takes digital bits from the packet switch  114  and generates symbols that may simultaneously carry multiple bits. Modulation added by the modulator  116  may include, for example, QPSK (two bits per symbol) or 8-PSK (three bits per symbol). The upconverter  118  shifts the symbols produced by the modulator  116  up in frequency for transmission, typically using a mixer. As an example, the upconverter  118  may use an approximately 19 GHz frequency (generated by the frequency generator  134 ) to generate downlink beams in the 18.8-19.3 GHz range. The downlink beams are applied to the transmit antennas  120 - 126 , whose individual radiating elements are controlled by the payload computer  132  for electronic steering. 
     The payload  100  may optionally include optical or radio frequency (RF) intersatellite links  136 - 142 . For example, an ISL North transmitter and receiver  136  and an ISL South transmitter and receiver  138  may be provided to establish communications between satellites in the same orbital plane. Similarly, an ISL East transmitter and receiver  140  and an ISL West transmitter and receiver  142  may be provided to establish communications between satellites in adjacent orbital planes. The North, South, East, and West links may be implemented with laser optics or RF packages and may be used to interconnect the communications satellites in a network. Worldwide routing of packets may then flow through the payload  100 , either to additional communications satellites over the intersatellite links  136 - 142 , or to and from the ground using the receiver antenna  102 - 108  and the transmitter antennas  120 - 126 . 
     Additionally, the payload  100  may include a beacon  128  and associated beacon control  130 . The beacon  128  includes a transmitter and receiver that operate on RF frequencies to transmit and receive information to and from the ground. The beacon control  130  receives information to transmit from the payload computer  132  and forwards to the payload computer  132  information received by the beacon  128 . The beacon control  130  may also include hardware for modulation, demodulation, upconversion, downconversion, and analog to digital conversion. 
     The beacon provides a dedicated control channel between the payload  100  and the ground (although the beacon could conceivably be pointed at another satellite as well). The control channel may be used to carry command, control, and status information regarding the frequencies in use by the transmit antennas  120 - 126  and the receive antennas  102 - 108 , the present amount of bandwidth available at the payload  100 , communication or call setup and teardown information, synchronization information and the like. The payload computer  132  may also, for example, use the control channel to indicate to the ground any errors or malfunctions the payload  100  is experiencing, or may use the control channel to transmit a signal that a ground station searches for to determine whether a satellite is passing overhead and is available for service. 
     While particular elements, embodiments and applications of the present invention have been shown and described, it is understood that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teaching. It is therefore contemplated by the appended claims to cover such modifications and incorporate those features which come within the spirit and scope of the invention.