LOW-EARTH ORBIT SATELLITE ACQUISITION AND SYNCHRONIZATION SYSTEM USING A BEACON SIGNAL

Abstract: A system for acquiring the beacon of a satellite and synchronizing data transmission between the satellite and a ground terminal is disclosed. The ground terminal conducts a search for a satellite to be acquired. The search may be based on previously developed information from which the location of the satellite can be predicted and, thus, limited to a small area of the sky, or cover a large area of the sky in accordance with a search routine. After the beacon of a satellite is acquired, the geographic area served by the satellite is determined. If the satellite does not serve the cell within which the ground terminal is located, a further satellite search is conducted, which may be based in part on information contained in the beacon of the acquired satellite. After the satellite serving the cell containing the ground terminal is acquired, a test is made to determine how long the satellite will continue to cover the cell. If the time period is short, communication waits until the next satellite to cover the cell is acquired. If the time period is long, communication is allowed to begin. The beacon is also used by the ground terminal to: (i) accurately time uplink data transmissions; (ii) estimate uplink Doppler, Doppler rate, and Doppler rate derivative and use this information to pre-compensate the carrier frequency of the uplink data transmissions for Doppler variations; and (iii) estimate the carrier frequency of downlink data transmission(s) by continuously tracking the beacon carrier frequency and scaling the result by a suitable scaling factor.

Description: LOW-EARTH ORBIT SATELLITE ACQUISITION AND SYNCHRONIZATION SYSTEM USING A BEACON SIGNAL Field of the Invention This invention relates to data communication systems and, more particularly, to satellite data communication systems. Background of the Invention In recent years the need for global data networking capability has rapidly expanded. In order to meet this need, broadband satellite communication systems have been proposed as an alternative to land-based communication systems. One type of satellite data communication system is described in a variety of U.S. patents assigned to the assignee of this patent application, including U.S. Patent Nos. 5,386,953; 5,408,237; 5,527,001; 5,548,294; 5,641,135; 5,642,122; and 5,650,788. These patents and other pending applications assigned to the assignee of this patent application describe a satellite communication system that includes a constellation of low-Earth orbit (LEO) satellites that implement an Earth-fixed cellular beam approach to transmitting data from one location on the Earth's surface to another location. More specifically, each LEO satellite has a communication "footprint" that covers a portion of the Earth's surface as a satellite passes over the Earth. The communication footprint defines the area on the Earth within which ground terminals can communicate with the satellite. Located within each footprint are a large number of cells. During the period of time a cell remains within the borders of a satellite footprint, ground terminals located in the cell transmit data to and receive data from the serving satellite. When a satellite reaches the end of its serving arc, another orbiting satellite is positioned to serve the Earth-fixed cell previously covered by the satellite reaching the end of its serving arc. During serving, -2- the antennas of ground terminals located within the cells continuously point toward the serving satellite as it moves in orbit and antennas on the satellite point toward the cell during the time period within which the cell is allowed to transmit data. Data to be sent from one location on the Earth to another location is transmitted from a ground terminal located within the cell to the satellite serving the cell via an uplink data channel. The data is routed through the constellation of LEO satellites to the satellite serving the cell within which the ground terminal of the designated receiver is located. The latter satellite transmits the data to the receiver ground terminal via a downlink data channel. Thus, the constellation of LEO satellites and the ground terminals form a satellite data communication network wherein each ground terminal and satellite forms a node of the network. In order for a LEO satellite data communication system to be competitive, it must have a wide bandwidth. In the United States, the frequency spectrum is crowded at lower frequencies due to pre-allocated terrestrial and satellite users. Such a wide bandwidth is therefore generally only available in the gigahertz (GHz) range. In order to be competitive, it is also advantageous that the satellite data communication system be designed such that each ground terminal is able to rapidly acquire and synchronize its operation to the satellite serving the ground terminal. The orbit of the satellite in a LEO constellation causes each satellite to be visible to a ground terminal for only a limited period of time. In order to maximize the amount of time (and hence data) that the ground terminal can communicate with each satelUte, it is therefore important to minimize the acquisition and synchronization period. The present invention is directed to a satellite acquisition and synchronization system that accomplishes these objectives. Summary of the Invention In accordance with this invention a system for acquiring the beacon of a satellite serving a ground terminal and synchronizing data transmissions between the satellite and the ground terminal is provided. The ground terminal conducts a search for a satellite to be acquired. The search may be based on information resulting from prior data transmissions from which the location of the satelUte can be predicted and, thus, Umited to a small area of the sky, or cover a large area of the sky when no prediction information exists or the prediction information is unreUable. After the beacon of a satellite is acquired, the footprint of the sateUite is determined. If the footprint of the satellite does not cover the ceU in which the ground terminal is located, a further search is conducted, which may be based in part on information -3- contained in the beacon of the acquired satelUte. After the satelUte serving the ceU in which the ground terminal is located is acquired, the ground terminal is synchronized to the satellite. In accordance with further aspects of this invention, after the beacon of the satelUte serving the ground terminal cell is acquired, a test is made to determine how long the satellite will continue to serve the ceU. If the time period is short, communication waits until the next satelUte to serve the ground terminal cell is positioned for acquisition. If the time period is long, communication is aUowed to begin. In accordance with other aspects of this invention, the ground terminal uses the beacon to accurately point the antenna of the ground terminal toward the satellite serving the cell within which the ground terminal is located. In accordance with further aspects of this invention, the ground terminal uses the beacon to accurately time upUnk data transmissions. In accordance with still other aspects of this invention, the ground terminal also uses the beacon to estimate uplink Doppler, Doppler rate, and Doppler rate derivative and uses the estimate to pre-compensate the carrier frequency of the uplink signal for Doppler and Doppler rate. In accordance with yet other aspects of this invention, the ground terminal uses the beacon to continuously track downlink carrier frequency by continuously tracking the beacon carrier frequency and scaling the result by a suitable scaling factor. As will be readily appreciated from the foregoing summary, the ground terminals of the LEO satelUte data communication system use the beacon of the satelUte serving a ground terminal in various ways to accomphsh various results. Specifically, the beacon is used to enable accurate antenna pointing, uplink carrier frequency synchronization, data frame synchronization, and downlink carrier frequency synchronization. Such usage of the beacon enables the ground terminals to rapidly achieve network (e.g., satelUte) acquisition and synchronization, and accurately time uplink traffic transmissions in order to maximize that data that may be transmitted to and from the serving LEO satelUte. Brief Description of the Drawings The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: FIGURE 1 is a pictorial diagram showing the orbital paths of the satellites of a consteUation of low-Earth orbit (LEO) satellites positioned to cover the entire surface of the Earth; FIGURE 2 is a plan view of a portion of the consteUation of LEO satelUtes depicted in FIGURE 1; FIGURE 3 is a pictorial view showing the various signal paths to and from a constellation of LEO satellites of the type depicted in FIGURES 1 and 2; FIGURE 4 is a pictorial view that shows a single LEO sateUite communicating with a single earth ceU in three sequential positions; FIGURE 5 is a functional block diagram of a satellite acquisition and synchronization system formed in accordance with the invention; FIGURE 6 is a pictorial diagram illustrating the relationship between the communications footprint of a satellite, the coverage area of a sateUite beacon, and the relationship between a LEO satelUte, a ground station, and global positioning system (GPS) satellites; FIGURE 7 is a pictorial diagram iUustrating the sateUite acquisition aspect of the invention using a beacon signal; FIGURE 8 is a table Ulustrating various satelUte spatial search modes based on the amount of prior sateUite position knowledge available to a ground terminal; FIGURES 9A and 9B are top views of a satelUte footprint and terminal space search based on the assumption that the satellite moves from point A to point B at height h; FIGURE 10 is a pictorial diagram illustrating approximate relationships between search grid dimensions and antenna beam width; FIGURES 11A and 11B are diagrams illustrating the Doppler and Doppler rate error that occurs when an antenna is pointing at the center of a grid location and the satellite is located elsewhere in the grid; FIGURE 12 is a flow diagram iUustrating the steps used to determine if a cell lies within a satellite coverage area; FIGURE 13 is a pictorial diagram iUustrating sateUite footprints at different latitudes; FIGURE 14 is a pictorial diagram comparing beacon constituent options; -5- FIGURE 15 is a table illustrating the bits required for the different beacon constituent options illustrated in FIGURE 13, based on certain presumptions; FIGURE 16 is a flow diagram Ulustrating a first beacon constituent option; FIGURE 17 is a flow diagram illustrating a second beacon constituent option; FIGURE 18 is a flow diagram Ulustrating a third beacon constituent option; FIGURE 19 is a data diagram illustrating the preferred beacon frame structure and its relationship to terminal-to-satellite link (TSL) data frames; FIGURE 20 is a block diagram illustrating a beacon modulator for creating the beacon frame structure Ulustrated in FIGURE 19; FIGURE 21 illustrates the preferred location of the beacon frequency with respect to the data spectrum; FIGURE 22 is a flow diagram iUustrating data frame synchronization; FIGURE 23 is a timing diagram iUustrating data frame synchronization; FIGURE 24 is an example of the error in uplink burst arrival time given a downlink slant range error; FIGURE 25 is a block diagram of an uplink Doppler compensation system; FIGURE 26 is a block diagram of a downlink Doppler compensation system; and FIGURES 27A-27C are graphs depicting a frequency spectrum of signals received by the downlink Doppler compensation system of FIGURE 26. Detailed Description of the Preferred Embodiment The present invention is directed to an acquisition and synchronization system using a beacon signal that is ideally suited for use in a low-Earth orbit (LEO) satellite communication network. A LEO sateUite communication network includes a constellation of satelUtes orbiting the Earth such that a majority of the Earth is within the view of at least one sateUite at any point in time. One proposed LEO sateUite communication network employs 288 satellites, plus spares, located in 12 polar orbit planes. Each plane includes 24 satelUtes at an altitude of approximately 1,350 kilometers. The path of travel of the satellites of such a network is generaUy depicted in FIGURE 1. More specifically, FIGURE 1 depicts the Earth 11 surrounded by a plurality of rings that depict the orbital planes of the plurality of sateUites 13. FIGURE 2 Ulustrates a number of the sateUites 13a, 13b, 13c, . . . that make up the consteUation of satelUtes included in a LEO satelUte communication network of the type illustrated in FIGURE 1. The sateUites are shown close to one another for Ulustrative purposes only. As shown in FIGURE 2, a data signal 15a consisting of one or more data packets is transmitted from a ground terminal 16 to a first satellite 13f that forms part of the constellation of satellites via an uplink data channel. Depending on network traffic, the data packets are routed by various paths through the satelUte constellation. For example, the receiving or uplink sateUite 13f may forward one or more data packets to a second sateUite 131, which forwards the data packets to a third satellite 13 m, which forwards the data packets to a fourth satellite 13n. The routing continues until the data packets reach the sateUite 13o serving the ground terminal 18 that is to receive the data signal. The satellite serving the ground terminal, caUed the sending or downlink sateUite, transmits the data packets as a data signal 15b to the ground terminal 18 via a downlink data channel. After receipt at the ground terminal, the data is forwarded to an end user. It will be appreciated that each of the data packets may be routed through different paths in the satellite constellation and may arrive at the end user in a different order than the order in which they were sent. Upon receipt at the receiving ground terminal, the data packets are reordered in the correct order. FIGURE 3 further Ulustrates the LEO satellite communication network. End users 17a, 17b, 17c, . . . are connected either through networks 19a and 19b, or directly to ground terminals 21a, 21b, 21c . . . The networks 19a and 19b may, for example, be conventional switched public telephone system networks, corporate networks, or other proprietary networks. Network operations and control systems 25 a and 25b are shown as communicating with the satelUtes via separate ground terminals 23a and 23b. All of the ground terminals are designed to transmit signals to and receive signals from the constellation of sateUites via uplink and downlink data channels. Preferably, the LEO satellite communication network employs Earth-fixed ceUular beam technology. More specifically, as a LEO satelUte passes over the Earth, the satellite's antenna beams define a region of the Earth, called a footprint. Since LEO sateUites, in contrast to geosynchronous satelUtes, are moving with respect to the Earth, the region of the Earth covered by a sateUite's footprint is constantly changing. Located within each footprint are a plurality of cells. In an Earth-fixed cellular beam system, a satellite's antennas are controlled to remain pointed at the ceUs located within the footprint as the sateUite moves in orbit. As the servicing satellite moves, particular cells are handed off from the presently servicing sateUite to the next servicing sateUite. For a predetermined period of time each cell is serviced by the same sateUite. An Earth-fixed cellular satellite communication system is believed to substantially reduce communication problems when compared to other sateUite communication systems. An Earth-fixed cellular beam system is iUustrated in FIGURE 4. As a satellite moves in orbit with respect to the Earth, the ceUs located within a footprint defined by the satellite's antennas are continuously serviced by the sateUite. As shown, at the beginning of the arc, at a time 1, the boundary 31 of the beam of one of the antennas of the satellite covers a ceU 33. At time 2, midway in the arc, the beam from the same or a different antenna is electronically or mechanically steered so that the cell is still covered by the same satellite. At the end of the arc, time 3, the beam from the same satelUte still covers the cell 33. Since the sateUite antenna footprint covers the cell 33 from time 1 to time 3, the cell with respect to the satellite is "fixed." Prior to time 1, the antenna of another (upstream) satellite covered the ceU 33. At time 3, the antenna of a further (downstream) satellite wUl begin covering the ceU 33. Rather than serving a single ceU, a satellite antenna beam can be electronicaUy controUed to service a group of cells by hopping from cell to ceU located within the satellite footprint. Preferably, the uplink and downlink data channels use a time division multiple access (TDMA) air interface to coordinate spectrum sharing between terminals located within a cell. As cells leave sateUite footprints, the appropriate antenna of the succeeding sateUite picks up the prior ceUs, and the antennas of that satellite point at the cells from time 1 to time 3 as the sateUite moves through its serving arc. The size of the satellite footprints is such that aU regions of the Earth are covered at all points in time. As is well known in the cellular telephone communication and other arts, data to be transmitted is transformed into digital "packets." Each packet includes a header and payload. The header includes packet address bits and the payload contains the data being transmitted. While various transmission schemes can be utilized, preferably the ceUs include one or more terminals that assemble the data packets from a variety of sources into a stream of data packets for transmission to the sateUite serving the ceU. This aUows the bandwidth to be allocated to the end users in a variety of ways. For example, by paying a higher fee, an end user can obtain a priority portion of the avaUable bandwidth. Other less priority-sensitive messages can be communicated at a lower cost. On the uplink, each satellite includes a pluraUty of antennas designed to receive data from a plurality of cells in a sequential manner. In contrast to the uplink transmission of data, preferably downlink data is transmitted as a burst of data packets. Each receiving terminal determines which packets are intended for it based on information contained in the header. In order for a sateUite communication network of the type illustrated in FIGURES 1-4 to send data in a cost-effective manner, when compared to entirely land-based commercial networks, the bandwidth of the satelUte-to-ground terminal links must be relatively large, e.g., 500 MHz. One avaUable frequency range offering such a bandwidth is in the Gigahertz range, specificaUy the 18.8 GHz to 19.3 GHz portion of the Ka band. Even with such a broad bandwidth, however, maximizing the amount of data that may be transmitted requires, among other things, that the ground terminal quickly acquire and remain synchronized with a new servicing sateUite. More specifically, ground terminals must be able to: (1) rapidly acquire the satellite serving the ground terminal when data communication is to begin; (2) accurately time uplink data communication; (3) pre-compensate upUnk carrier frequency to account for Doppler shift; and (4) continuously track downlink carrier frequency to receive bursts of downlink traffic. The present invention allows the ground terminal to accomplish all of these functions based on a single beacon signal received from a satelUte. That is, each satellite 13 a, 13b, 13 c, . . . contains a beacon transmitter for transmitting a beacon signal that is received by the ground terminals 21a, 21b, 21c, . . . As will be discussed in additional detaU below, the beacon signal contains sufficient information for a terminal to determine if it has acquired the sateUite serving the cell within which the ground terminal lies, the ephemeris (position and velocity) of the sateUite, a clock that can be used to establish frame timing, and the identity of the contention channel, i.e., the channel used to request service. As shown in FIGURE 5, the ground terminals 21a, 21b, 21c, . . . and 23a, 23b, . . . each include a satellite acquisition system 35 and a synchronization system 37 formed of software and hardware components. The sateUite acquisition system 35 and synchronization system 37 utilize the received beacon signal to accomphsh the foregoing functions, preferably in the manner described below. 1. Satellite Acquisition Satellite acquisition involves spatiaUy locating a satellite in the region above a ground terminal through reception of the sateUite' s beacon signal, determining whether the satelUte serves a terminal's geographic area, and determining the communication channel used to request service (called the contention channel). SateUite synchronization involves synchronizing the sateUite and the terminal in both time and frequency. Satellite acquisition and synchronization are required each time a terminal begins a new communication sequence. The present system for acquisition and synchronization with a satellite requires the ground terminals 21a, 21b, 21c, 21d, . . . and 23a, 23b, and the satelUtes, 13a, 13b, 13c, . . . to have accurate knowledge of their respective positions. In accordance with the invention, signals produced by Global Positioning System (GPS) sateUites are used by the LEO satellites to accurately determine their positions in space. More specifically, as shown in FIGURE 6, the LEO satelUtes 13a, 13b, 13c, . . . receive GPS signals from the GPS satellites 41a, 41b, 41c, . . . and use the GPS signals to determine their position. The GPS signals are also used to synchronize the time and frequency of all of the satellites 13a, 13b, 13c, . . . . The location of the ground stations 21a, 21b, 21c, . . . 23a, 23b may simUarly be determined using GPS signals. Preferably, however, each ground station is programmed with its precise location when the ground station is instaUed at a fixed position on the Earth. As depicted in FIGURE 6, each satellite 13a, 13b, 13c, . . . transmits a beacon signal in a beam towards the Earth. Obviously, the beacon transmitted by each sateUite must cover all of the ground terminals located within a satelUte's footprint. This means that the beacon must have a predetermined beam width β, i.e., the beacon must cover a cone having an apex angle equal to β. In the case of a sateUite system of the type described above, i.e., one including 288 satellites, plus spares, located in 12 polar orbit planes, each plane including 24 satellites at an altitude of approximately 1,350 kUometers, β is equal to 80°. An 80° beam width covers an area on the Earth that is sUghtly larger than the area of the sateUite footprint centered at any longitudinal along the equator. The size of the satellite footprint is based on the number of satellites in orbit, each of which covers a predetermined angle, γ, with respect to the center of the Earth. In the case of 24 equaUy spaced satellites, γ is equal to 15°. Obviously, sateUite footprints at latitudes other than the equator overlap, whereby ground terminals can be "Uluminated" by multiple beacons. As wiU be better understood from the following description, the confusion that could occur as a result of such multiple illumination is avoided by including satellite information in the beacon signals, such as the position of the satelUte transmitting the beacon. The ground terminals use the satelUte information to determine what sateUite is the one and only one serving the cell within which a ground terminal Ues. As also shown in FIGURE 6, each terminal has a high gain antenna having a beam width, α, that is used to acquire and track the beacon of the serving sateUite. -10- The antennas of two terminals 21a and 21b are illustrated in FIGURE 6. The antenna of one terminal, 21a, is shown in a search mode and the antenna of the other terminal, 21b, is shown pointed toward a sateUite 13 a, which is iUustrated as lying directly overhead. While various antenna beam width angles α can be used, a suitable angle is 3.5°. FIGURE 7 iUustrates the LEO satelUte acquisition process. In general, the process begins when a terminal conducts a spatial and frequency search for a sateUite beacon. After a satellite beacon is detected, the terminal demodulates and decodes the beacon, and uses the information contained in the beacon to determine the geographic area covered by the satelUte and if the cell within which the terminal lies falls in the geographic area. If the acquired satellite does not serve the ceU, the terminal searches for another satellite. Once the sateUite serving the cell within which the ground terminal is located has been acquired, the ground terminal estimates the time to handover to another satelUte. If the acquired satelUte is about to stop serving the cell within which the ground terminal Ues, i.e., the sateUite is about to go out of view, the terminal points toward the rising sateUite, or searches for it. After acquiring a serving satelUte with a relatively long time to handover, the terminal refines its antenna pointing direction, estabUshes timing and data frame synchronization, and begins data transmission on the contention channel. Refining antenna point is necessary to minimize pointing losses. EstabUshing timing and frame synchronization is required to accurately time the service request data transmission on the contention channel. Turning now to a more detailed description of the LEO sateUite acquisition process shown in FIGURE 7, at the start of the acquisition process 51, the antenna of the ground station is pointed to a predicted satelUte position. As wttl be better understood from the following description, the prediction is based on when a sateUite was last communicating with the acquiring ground terminal. It may be the first position in a search grid, or may be based on previously acquired satellite information. In any case, after pointing in the predicted satelUte direction, a fast fourier transform (FFT)-based search for the satelUte beacon is conducted 55. If the beacon is not detected, the antenna is pointed to another predicted sateUite position 53. This sequence is repeated until a satelUte beacon is detected. After detection, the beacon is demodulated and decoded 57. SatelUte information contained in the beacon is used to determine the sateUite coverage area 59. As will be better understood from the following description, depending upon the nature of the satelUte information -11- contained in the beacon, if the satellite does not cover the geographic area in which the cell containing the terminal is located, it may contain information about the coverage area of other satelUtes. If the sateUite does not cover the ceU within which the terminal is located, depending upon the sateUite information contained in the beacon, the antenna of the ground terminal is either pointed toward the appropriate satelUte position and another FFT search conducted for the satelUte beacon, or pointed toward another predicted satellite position 61 and the foregoing steps repeated. If the acquired satellite covers the cell within which the terminal is located, an estimate of the time to handover the ceU within which the terminal is located to another satellite is made 63. If the time is short, the antenna of the ground terminal is either pointed toward the handover sateUite position and another FFT search is conducted or pointed toward another predicted sateUite position 61 and the foregoing steps repeated. If the time to handover is long, the antenna pointing direction is refined for accuracy and the slant range to the sateUite determined. More specifically, the GPS- based satellite position information and the terminal position information is used to determine the slant range between the satelUte and the ground terminal. In this regard, as wUl be better understood from the foUowing description, part of the satellite information contained in the beacon is the position of the satelUte. The slant range information is used to establish data frame timing. Examining the initial spatial search identified in FIGURE 7 by reference numbers 51-55 in more detaU, it is desirable to minimize the amount of time it takes for the ground terminal to acquire a satellite. The speed with which the satellite is acquired depends on the information known to the ground terminal searching for a satelUte. For purposes of discussion, three possible search modes are identified depending on the state of the ground terminal. These search modes wiU be referred to as "frozen," "cold," and "warm." As shown in FIGURE 8, the frozen search mode is defined as an initial ground terminal instaUation or where no communication has occurred between the ground terminal and the satellite network for a predetermined period of time—one week or more. Further, the ground terminal has no ephemeris information regarding any satellite in the constellation. The frozen mode causes the ground terminal to search a large region of space. The region is the maximum field of view of the ceU within which the ground station Ues, which is the same as the satellite footprint at the -12- equator, i.e., γ or 15° latitude and longitude for the satelUte consteUation example described above. The cold search mode presumes some sateUite orbital plane knowledge, but no communication with the sateUite network for a period of time less the predetermined period, i.e., less than one week. SpecificaUy, the ground terminal knows the position of at least two satelUtes per orbital plane; sateUite position and velocity were made avaUable to the ground terminal within the last week; and no sateUite orbital maneuvers have taken place since the last position update. The cold search mode requires the same degree of latitude, i.e., γ or 15°, but substantially less longitude ( γ)--3° for example. The warm search mode presumes that: (i) there has been no communication with the satellite network for a relatively short period of time—one hour or less— but that there was communication prior to this short period; (U) the ground terminal knows the position and velocity of at least two satellites in the plane serving the ground terminal prior to communication termination and in the plane to the east of the serving plane; and (in) satellite position and velocity information has been provided to the ground station within the last hour. In this case, the search routine can cover a substantially smaUer region of space, i.e., a region of space that is «γ in both latitude and longitude~0.6° latitude by 0.6° longitude, for example. In summary, the frozen search mode assumes the terminal either has never been used before or has had no communication with the satelUte network for at least a predetermined period of time-one week. As a result, the ground terminal has no ephemeris updates for at least the past week, and does not know the latitude or longitude of the satellite serving its area. The cold search mode assumes that the terminal has not been used during the last week, but before that has been used extensively and has built up an ephemeris database that consists of ephemeris data for at least two satellites per orbital plane. Based on this knowledge, the ground terminal can predict the longitude of the sateUites serving its geographical area to within a predetermined range, i.e., three degrees, but cannot predict the satellite's latitude. The warm search mode assumes that the ground terminal has not communicated with the satellite network for less than an hour, and before that has been used long enough to know the position of at least two satelUtes in the plane that has been serving the ground terminal and the plane to the east of the serving plane. Knowledge of the plane to the east is needed in case of a cross-plane handover. -13- FIGURES 9A, 9B, and 10 depict a spatial space search using the cold search mode. As noted above, the cold search mode covers a spatial region 3° longitude by 15° latitude for the satellite network example described herein, i.e., a sateUite network comprising 288 satelUtes in 12 orbital planes at an altitude of approximately 1,350 kilometers. The searched space is divided into a search grid, one segment of which is shown as a lightly shaded box in FIGURE 9A. The antenna of the ground terminal is pointed to the center of each grid segment in sequence. As shown in FIGURES 9 A and 10, the grid segment size is, preferably, equal to the size of the region of space searched during warm acquisition, i.e., 0.6° latitude by 0.6° longitude. As previously discussed with respect to FIGURE 7, when the antenna terminal is pointed to the center of a grid segment a fast fourier transform (FFT) is performed to determine if a beacon signal is being emitted by a satellite located within the grid segment. When a satellite is located in a grid segment, but not at the center, there is a pointing loss and frequency prediction error, i.e., a Doppler and Doppler rate prediction error. For each grid segment there wUl therefore be a frequency range surrounding the transmitted beacon frequency that must be searched in order to detect the Doppler shifted beacon frequency at the ground terminal. The maximum prediction error occurs when the sateUite is at one of the corners of the grid segment. The magnitude of the error is a function of grid segment size, with larger grid segments resulting in higher error due to the corners being further away from the center. FIGURES 11A and 11B show the Doppler and Doppler rate error for 0.6° latitude by 0.6° longitude grid segments belonging to a search strip at the same longitude as the terminal. The horizontal axis of each graph shows the search grid segment latitude relative to the latitude of the terminal. Zero on the horizontal axis indicates that the search grid segment center is at the same latitude and longitude as the terminal. A positive horizontal axis value indicates that the search grid segment center is north of the terminal along the same longitude. A negative horizontal axis value indicates that the search grid segment center is south of the terminal along the same longitude. The top curve in FIGURE 11A shows the Doppler error when the antenna is pointing to the center of the grid segment and the sateUite is at either the SW or SE corner of the grid segment. The bottom curve shows the error when the antenna is pointed to the center and the sateUite is at the NE or NW corners. FIGURE 1 IB shows the Doppler rate error. In order to detect signals over the entire frequency range, the FFT sampUng rate must be at least twice the highest Doppler shift. Preferably, the FFT is preceded -14- by a downconversion process that includes a predict driven numericaUy controUed oscillator (NCO) that is tuned to remove the Doppler and Doppler rate error with respect to the center of the grid segment. The FFT bin size should be large enough to avoid smearing, preferably a bin size greater than the maximum Doppler rate times the FFT duration. The signal level in each FFT bin is compared against a preset threshold. The beacon is detected when one or more FFT bins have a signal level greater than the threshold. For a fixed FFT size, the detection probability can be increased by increasing the sample signal to noise ratio (SNR) and for a fixed sample SNR the detection probability can be increased by increasing the FFT size. Preferably, these parameters are traded off in a manner that allows the system to operate at the lowest sample SNR to enable the system to minimize the received power to noise ratio (P/N 0 ). For a given P/NQ and sample SNR, there is a maximum Doppler shift that can be supported which, in turn, means that there is a maximum grid segment size that can be used. The finer the grid segment size, the longer it takes to cover the space to be searched and find a sateUite. If a satellite is detected in the grid segment using the above method, sateUite information contained in the satellite's beacon is evaluated to determine if the satellite is serving the cell in which the terminal is located. FIGURES 12 and 13 lustrate how a ground terminal determines if an acquired sateUite is the one serving the ceU within which the ground terminal is located. As noted above and described below with respect to FIGURE 15, the position of the sateUite transmitting the beacon is encoded in the beacon signal. Based on the sateUite's position, the ground terminal determines the sateUite's subsateUite point. See block 121. The subsateUite point is the point lying on the Earth's surface along a line running between the satelUte and the center of the Earth. Based on the subsateUite point, at block 123, the sateUite coverage area is determined. FIGURE 13 graphically depicts the method of determining a satelUte coverage area. As shown in FIGURE 13, the geographic area covered by a satelUte is relatively square at the equator and changes into a trapezoidal shape away from the equator. The satellite positional information contained in the beacon plus information about the location of the ground terminal aUows the ground terminal to determine the exact size of the satellite footprint and if the center of the ceU within which the ground station is located lies within the footprint. Although the shape of the satelUte footprint changes with the latitude of the subsatelUte point, each satelUte's footprint has boundaries that are a constant distance (in terms of Earth-fixed latitude and longitude) away from the -15- subsatellite point. Satellite footprint boundaries may therefore be readUy computed by calculating the subsateUite point of each satellite based on the position of the sateUite and adding constant amounts to the subsateUite point to determine the boundary locations. Returning to FIGURE 12, once the sateUite coverage area is determined a test is made to determine if the ceU containing the ground terminal lies within the satelUte coverage area. See block 125. If the answer is yes, the sequence of steps depicted in FIGURES 16, 17, or 18 and described below are followed. If the answer is no, the antenna is pointed toward another grid segment and the search continued for another sateUite as shown in FIGURE 7 and described above. When another satellite is acquired, the sequence shown in FIGURE 12 is repeated. As noted above, when a ground terminal first acquires a satellite, it is possible to acquire a satellite not serving the cell within which the terminal Ues before acquiring the satellite that serves the ceU. In this regard, attention is directed to FIGURE 14, which shows four Earth-fixed cells 31a, 31b, 31c, and 3 Id aU positioned to communicate with a satelUte 13m. That is, the communicating sateUite 13m Ues within the field of view of all four ceUs 31a, 31b, 31c, and 3 Id. The communicating satelUte 31m is surrounded by eight other sateUites 13g, 13h, 13i, 131, 13n, 13q, 13r, and 13 s. For purposes of lustration, the direction of satellite travel in FIGURE 13 is denoted from top to bottom and the Earth's rotation is from left to right. In addition to the communicating satellite 13m, three additional satelUtes Ue within the field of view of each of the Earth-fixed ceUs. At a minimum, the beacon produced by each sateUite must contain enough satellite information for a terminal to determine if it has acquired the satellite serving the cell within which the ground terminal Ues, the ephemeris (position and velocity) of the sateUite, a clock that can be used to estabUsh frame timing, and the identity of the contention channel, i.e., the channel used to request service. Beyond this minimum information, the beacon may include information about adjacent satellites, which can be used to repoint the antenna of the terminal toward the satellite serving the ceU within which the terminal lies, or enable repointing of the terminal's antenna when handover is required. Increasing the amount of satellite information contained in the beacon reduces the amount of time it takes a ground terminal to acquire an adjacent sateUite serving the ground station. For example, as shown in FIGURE 14, prior to acquiring the serving satellite 13m a ground terminal located within one of the Earth- fixed cells 31a could have acquired any of three other satellites within the ceU's field -16- of view, 13g, 13h, or 131. If the beacon only contains information about the acquired sateUite, the ground terminal would have to re-execute a complete search routine and, potentially, could locate the other two incorrect satellites prior to acquiring the satellite 13m serving the geographic area within which the Earth-fixed ceU 31a lies. In contrast, if the beacon contains information about other nearby sateUites, acquiring the beacon of the wrong satellite results in the ground terminal receiving information that can be used to immediately point the ground terminal's antenna toward the correct satellite. There are three meaningful options that need to be considered when determining the type of satellite information to be included in the beacon. Option 1 covers a beacon containing information only about the acquired satelUte. This is the minimum information necessary for a ground terminal to acquire a LEO satelUte and establish data frame timing. Option 2 covers a beacon containing information about the acquired sateUite plus three adjacent satelUtes that are potential handover candidates. This beacon information aUows a ground terminal to acquire a LEO satellite and estabUsh data frame timing, if the acquired satelUte is the one serving the cell within which the ground terminal Ues. Option 2 also aUows a terminal to track the position of sateUites that could be assigned at the time of handover. Option 3 covers a beacon containing information about the acquired satellite plus the eight satellites surrounding the acquired satelUte, all of which are potential handover candidates, or potential "false" acquisition candidates. FIGURE 15 is a table that, based on certain assumptions, illustrates the relative magnitude of the satellite information that must be contained in the beacon for each of the three options. The beacon information constituents are: sateUite identifier, satelUte ephemeris (e.g., position and velocity), time word, contention channel identifier, and unique word. The function and purpose of the unique word is described below. Each constituent is updated at some predetermined rate. Preferably, the update rate for all of the constituents except the unique word is the same—5 seconds, for example. The unique word is updated at a faster rate— 0.11506 seconds, for example. The update rate affects both sateUite acquisition time and uplink and downlink margin. While a slower update rate increases uplink and downlink margin, it also increases sateUite acquisition time. The sateUite identifier relates satellite position and velocity at any given time to the corresponding sateUite. By way of example only, the table Usts: 10 databits for satelUte identity; 192 databits per satelUte for satellite ephemeris, i.e., sateUite position and velocity; -17- 64 databits (per satellite) for time word; and 5 databits for contention channel identification, i.e., to identify the channel to be used by the ground terminals to request service. In addition, 21 databits are used to identify a unique word whose purpose is described below. With respect to satelUte ephemeris, the number of bits used to represent each component of a three-dimensional position and velocity coordinate system is 32. While the databit choices set forth in FIGURE 15 are to be taken as exemplary, not limiting, it is clear from FIGURE 15 that Option 2 requires a greater bandwidth (more than three times) than Option 1, and Option 3 requires a greater bandwidth than Option 2 (more than twice). FIGURE 16 is a functional flow diagram Ulustrating the steps performed by a system implementing Option 1. InitiaUy, as previously described in connection with FIGURE 7, the antenna of the ground terminal is pointed toward a first predicted satellite position, which may be the first segment of a search grid. See block 71. After the antenna is pointed, an FFT search is conducted for a sateUite beacon. See block 72. At block 73, a test is performed to determine if a satellite beacon has been detected. If a satellite beacon has not been acquired, the predicted satelUte position is updated (block 74) and the antenna pointed toward the next predicted sateUite position (block 71), i.e., the next segment in the search grid. When a satellite beacon is acquired, at block 75, a test is made to determine if the correct satelUte has been acquired. If the correct satellite has not been acquired, the predicted sateUite position is again updated (block 74) and the antenna repointed toward the new position (block 71 )— the next segment in the search grid. When the correct sateUite is acquired, at block 77, a test is made to determine if the time to handover is short. If the time to handover is short, the predicted satelUte position is updated again and the foregoing steps repeated. If the time to handover is not short, the process terminates. FIGURE 17 is a functional flow diagram Ulustrating the steps performed by a system implementing Option 2. As before, initiaUy, the antenna of the ground terminal is pointed to a predicted satellite position. See block 81. Next, an FFT search is conducted for a satellite beacon. See block 82. Then, at block 83, a test is made to determine if a satelUte beacon has been detected. If no sateUite beacon has been detected, the predicted satelUte position is updated (block 84) and the antenna of the ground terminal pointed toward the new position. When a satellite beacon is detected, a test is made to determine if it is the correct satellite. See block 85. If the acquired satellite is not the correct satellite, i.e., the satellite serving the cell within which the ground terminal is located, the -18- predicted satellite position is updated (block 84) and the antenna of the ground terminal pointed toward the new position. If the correct satelUte has been acquired, a test is made to determine if the time to handover is short. See block 87. If the time to handover is not short, the process ends. If the time to handover is short, at block 89, the information contained in the beacon of the acquired satellite is analyzed to determine the location of potential handover sateUites. Then, at block 91, the antenna of the ground station is pointed toward the location of one of the potential handover satellites and the beacon of that satellite is acquired. Then a test is made to determine if it is the correct satellite. See block 93. If the potential handover satellite is not the correct sateUite, the antenna is pointed toward the location of another potential handover satellite (block 91) and the test (block 93) is repeated. If the correct satelUte has been acquired, a test is made (block 87) to determine if the time to handover is short. If the time to handover is not short, the sequence ends. If the time to handover is short, the sequence of steps is repeated. FIGURE 18 is a functional flow diagram iUustrating the steps performed by a system implementing Option 3. As before, initially, the antenna of the ground terminal is pointed toward a predicted satellite position. See block 101. Then, at block 102, an FFT search is conducted for a satellite beacon. Then a test (block 103) is made to determine if a sateUite beacon has been detected. If no satellite beacon is detected, the predicted satelUte position is updated (block 104) and the sequence is repeated. When a sateUite beacon is detected, at block 105, a test is made to determine if the sateUite is the correct satellite, i.e., the satelUte serving the ceU within which the ground terminal is located. If the satelUte is not the correct satellite, information contained in the beacon of the acquired satellite is analyzed to determine the location of the correct sateUite, the antenna of the ground terminal is pointed toward the location of the correct satelUte, and the beacon of that sateUite is acquired. See block 107. Next, at block 109, a test is made to determine if the time to handover is short. If the time to handover is not short, the sequence ends. If the time to handover is short, at block 111, the location of potential handover sateUites is determined based on the information contained in the beacon of the acquired sateUite. Then, the antenna of the ground terminal is pointed toward the location of a potential handover satellite and the beacon of that satellite is acquired. See block 113. Then, at block 115, a test is made to determine if the newly acquired satellite is the correct satellite, i.e., the satellite serving the cell within which the terminal is located. If the -19- sateUite is not the correct satellite, the antenna is pointed toward the location of another potential handover satellite (block 113) and the test (block 115) is repeated. When the correct handover satellite is acquired, a test (block 109) is made to determine if the time to handover is short and, then, the sequence ends. As w l be readily appreciated from the foregoing description and reviewing FIGURES 16, 17, and 18, all options have various advantages and disadvantages. Clearly, Option 1 (FIGURE 15) has the advantage of lower cost and faster beacon repeatabiUty because the beacon contains the smallest amount of information. The disadvantage of Option 1 is that a terminal performing sateUite acquisition without any prior knowledge about satellite positions can acquire three incorrect satellites (worst case) before acquiring the satellite serving its cell. Thus, acquisition time is likely slow in most instances Another disadvantage is that a traffic channel has to be used to convey information about the position and velocity of potential handover satellites, if this information is to be conveyed to ground terminals, since the beacon does not contain this information. Option 2 has the same initial sateUite acquisition problem as Option 1. However, Option 2 has the advantage of not having to rely on a traffic channel for information about the position and velocity of potential handover candidates. In the case of Option 3, a terminal with no prior knowledge can acquire at most one wrong satellite before acquiring the one serving its geographic area. This is because when the ground terminal acquires one satellite it knows the position of eight adjacent satellites, one of which is guaranteed to be serving the cell within which the ground terminal lies. Hence, the ground terminal can immediately repoint its antenna toward the correct sateUite. Also with this option, the position of all potential handover candidates is known. Returning to FIGURE 7, assuming a five-second update period, a ground terminal has to wait a maximum of just under ten seconds after detecting a sateUite beacon and synchronizing the ground terminal to the satelUte before receiving a complete set of satellite position and velocity data. A test is then made to determine if the correct sateUite is acquired as determined by the satellite coverage area 59, and the search continued toward another predicted sateUite position 61 if the correct sateUite is not acquired. Once the correct satellite is acquired, the ground terminal estimates the time to handover 63, which can be easUy calculated because the acquired satellite's position and velocity are known to the ground terminal. If it is a "long" time to handover, the ground terminal synchronizes to the satellite frame clock -20- and begins transmitting on the contention channel. Otherwise, the ground terminal waits to point to a rising sateUite before completing network acquisition. Obviously the length of the "short" and "long" times are relative system parameters that may be adjusted depending on a variety of factors, including the amount of data to be transmitted or received by the ground terminal. As illustrated in FIGURE 15 and described above, satellite position and velocity at a given time are related to the corresponding satellite by a satellite identifier. The exemplary baseline illustrated in FIGURE 15 aUocates 64 databits to denote time, 10 databits to identify a satellite, and 5 databits to identify the contention channel. The number of databits used to represent each component of the three- dimensional position and velocity vector is 32. Given that the maximum radio distance between the center of the earth and the highest orbital plane satellite is less than 10,000 km for the exemplary satelUte network described above, it can be shown that a 32 databit representation of position results in a quantization error of about 4.65E-6km. If this level of quantization is not necessary, the 32 databits can be reduced. In addition to the foregoing, the satellite beacon includes a 21 databit unique word. The unique word serves two purposes: to denote a new set of ephemeris data, and to establish frame timing. Preferably, the unique word is inserted once every predetermined interval, such as 115.06 μsec, and detected by correlating the unique word against a locally (terminal) generated unique word. The correlation of the two unique words produces a beacon frame clock that is related to the rate of the data frame clock by some factor, such as 1/9. As depicted in FIGURE 19, the unique word is preferably used to denote the start of ephemeris updates by reversing its polarity at predetermined mtervals, such as once very five seconds, and correlating it with the same terminal generated unique word, i.e., the terminal generated unique word used to detect beacon frame boundaries. The result is a negative correlation peak once every reversal period, e.g., once every five seconds, and a positive peak every period, e.g., every 115.06 ms. It should be noted that increasing the time between unique words increases link frame synchronization error because errors in beacon frame synchronization are scaled by the ratio of beam frame duration to data frame duration. Decreasing the time between unique words decreases the error, but increases the information rate because unique words have to be inserted more often. The unique word is also used to estabUsh frame timing, as described more fully below. -21- FIGURE 20 illustrates the presently preferred baseUne beacon modulation system. Information databits 131, i.e., databits identifying the sateUite, the sateUite's position and velocity, the satellite's time, and the contention channel, are forward error correction (FEC) encoded by a convolutional encoder 133. The FEC coded data is multiplexed with the unique word 135 by a multiplexer 137. The output of the multiplexer is precoded by a differential precoder 139. The output of the differential precoder is mixed with a signal produced by a pseudonoise (PN) signal generator 141 in a mixer 143. The output is modulated by a suitable modulator 145, such as a gaussian minimum shift keying (GMSK) modulator. As a result, the beacon is, in effect, a direct sequence spread spectrum GMSK signal. While the beacon spectrum can be located in any portion of the bandwidth of the downlink communication spectrum, preferably, it is positioned at the top edge of the downlink spectrum. Positioning the beacon at the upper edge of the downlink spectrum minimizes the interference with the downlink data signals. In this regard, attention is directed to FIGURE 21, which illustrates an exemplary downlink spectrum of 500 MHz lying between 18,800 MHz (18.8 GHz) and 19,300 MHz (19.3 GHz). The beacon has a bandwidth of 1 MHz located at the upper end of the band, i.e., between 19,299 and 19,300 MHz. Since the beacon demodulator is coherent, the beacon carrier frequency acquisition and tracking can be accomplished using closed loop techniques. Preferably, the beacon frequency is estimated from the FFT information developed in the manner previously described. The estimate is used to set the carrier frequency of a numerically controlled oscillator (NCO). The frequency estimation error is equal to 1 FFT bin, which is equal to 1/FFT observation time, which, in turn, is equal to 1/FFT dwell time. Preferably, the bandwidth of the NCO loop is set to be about 2-4 times greater than the maximum estimation error to ensure a high probability of pull in. After beacon demodulation, the start of beacon updates are identified by detecting the negative going unique word, which is inserted periodically at the start of each ephemeris update, as illustrated in FIGURE 19 and previously described. While beacon detection can occur in the presence of a substantial, e.g., 3dB, antenna pointing loss, it is not desirable to communicate on the contention channel with such a large pointing loss. Antenna pointing losses can be reduced by using the most recent satelUte coordinates to update the antenna pointing coordinates. The use of the global positioning system satellites aUows the LEO sateUite position to be -22- known to a spherical accuracy of 0.1 km or better, allowing a pointing accuracy of 0.1 km or better. 2. Uplink Data Frame Synchronization The use of a TDMA air interface to coordinate spectrum sharing between terminals requires that the terminals time their upUnk data transmissions so that the transmissions arrive at the serving satellite within predetermined timeslot boundaries. Once a satelUte is acquired it is therefore necessary to synchronize the uplink data framing, that is, synchronize the frames at the satellite and the ground terminal to a common clock. In a preferred embodiment of the system, the ground terminal must know the satellite's frame timing to within a predetermined amount, e.g., ±3.67 μsec, in order to insure that the uplink data transmissions are accurately timed. FIGURES 22 and 23 depict how to synchronize the upUnk data frame based on the received beacon. As noted above, the beacon includes a unique word that is inserted once every predetermined number (e.g., 9) of uplink data frames. The upUnk data frame rate is therefore obtained by multiplying the received beacon frame by the predetermined number, e.g., 9. At this point, the Earth-based data frame clock and the satellite-based data frame clock have the same rate, but are not phase synchronous. Aligning the phases requires delaying the Earth-based clock by T - Tphat, where T is the data frame duration and T^t is an estimate of the downUnk propagation time, T p , i.e., the time it takes for the beacon to propagate from the satellite to the ground station. As shown in FIGURE 22, an estimate of the downUnk propagation time 151 is determined by computing the slant range 153 between the sateUite and the ground terminal and dividing the slant range by the speed of light 155. As noted above, the GPS signals aUow the sateUite to determine its position to an accuracy of 0.1 km. The sateUite position information is included in the position and velocity information contained in the beacon. Those skiUed in the art w l recognize that the slant range may therefore be easUy computed by the ground terminal using the received satellite position information and the known position of the ground terminal. In the preferred embodiment of the system, the slant range may be computed to within a maximum error of ±1.1 km. After computing an estimated propagation time Tp h at , a test is made at decision block 159b to determine whether the propagation time is less than the data frame duration T. FIGURE 23 shows the case when T^ . = T p and T p < T. The first line of FIGURE 23 shows the sateUite data frame clock. The second line shows the -23- satellite beacon frame clock, which is synchronized to the satelUte data frame clock, but at a lower frequency— 1/9 in the example shown in FIGURE 23 (frame factor of N = 9). The third line shows the timing of the beacon frame clock received at the ground terminal. As shown, the latter clock is delayed by the propagation time T p . The fourth Une shows the received beacon frame clock multipUed by the frame factor, 9. This clock frequency is the same as the frequency of the sateUite data frame clock, but is out of phase with the satelUte data frame clock. As shown in the fifth line, the clocks are synchronized by delaying the ground terminal clock by a time T - T phat . The delay step is indicated at a block 157b in FIGURE 22. Delaying the ground terminal clock by T - T^t is only effective if the estimated delay time T P hat is less than the data frame duration T. Depending on the relative locations of the ground terminal and the transmitting satelUte, the transmission delay time may exceed the data frame duration T. At a decision block 159a, a test is therefore made to determine if the estimated delay time is greater than the data frame duration, i.e., if T p hat > T. If T P hat is greater than T, at a block 157a the ground terminal clock is delayed by a time 2T - T,**. Delaying the ground terminal clock in this amount ensures that the ground terminal and transmitting satelUte clocks are phase synchronous to within an estimated propagation time error. The error in estimating the propagation time is determined by the accuracy in measuring the slant range between the satelUte and the ground terminal. In the preferred embodiment of the system, the maximum error in estimating the propagation time is ±3.67 usec based on a sateUite positional error of ±1.1 km. In order to ensure that uplink bursts arrive at the sateUite at TDMA slot boundaries, the terminal needs to estimate the upUnk propagation time and, then, begin transmitting its uplink bursts that much eariier. As discussed above, the ground terminal is synchronized to the data frame boundaries to within a predetermined amount (±3.67 μsec). Since the TDMA slots are fixed relative to the frame boundary, the ground terminal knows the TDMA slot positions to within this amount, i.e., ±3.67 μsec. FIGURE 24 shows the error and uplink arrival time for three different cases when the downlink propagation time has been overestimated by 3.67 μsec and (l) the uplink has no error; (2) the uplink propagation time is underestimated by 3.67 μsec; and (3) the uplink propagation time is overestimated by 3.67 μsec. When the uplink propagation time is predicted correctly, the uplink burst arrives 3.67 μsec early due to the error in estimating the downlink time. When the uplink time is underestimated, the uplink burst starts 3.67 μsec later than it should and arrives at a -24- time slot on time due to the cancellation of downUnk and uplink errors. When the uplink and downlink are both overestimated, the error is doubled and the uplink burst arrives up to 2* (3.67) μsec early. Siπύlarly, underestimating the downlink propagation time by 3.67 μsec can lead to an uplink arrival time that is up to 2*(3.67) μsec late relative to the satelUte-based time slot. Appropriate guard times are therefore incorporated at the satelUte to aUow such timing errors without causing interference between signals transmitted from ground terminals assigned to adjacent TDMA timeslots. 3. Uplink Carrier Frequency Compensation The relative motion between the sateUite and the ground terminal wUl cause a Doppler shift in the frequency of the signal transmitted from the terminal to the satellite. It is advantageous to compensate for this Doppler shift prior to transmission because pre-compensation reduces the complexity of the sateUite-based demodulators and minimizes the guard bands between uplink TDMA channels operating at different frequencies. Without upUnk Doppler compensation, data transmissions received at a satelUte can have a relatively large Doppler shift— ±250 kHz— when operating in the exemplary Ka frequency band at the sateUite altitude described above. FIGURE 25 is a block diagram iUustrating a system for pre-compensating the frequency of the signal transmitted from the ground terminal to account for the expected Doppler shift. As shown in FIGURE 25, a terminal-based orbit determination program 161 is updated by ephemeris updates received from the satellite every predetermined period, e.g., every five seconds. The ground terminal uses this information to compute the sateUite slant range and its first, second, and third derivatives. Dividing the slant range derivatives by the speed of Ught and scaling them to the desired frequency, f c , yields the appropriate estimates for the Doppler, Doppler rate, and change in Doppler rate. The signal transmitted from the ground terminal will arrive at the satellite at the desired frequency, ., if the signal is transmitted at a frequency . - O^ + dD jj a dt + d^im f /dt 2 ) where D^ is the estimated Doppler shift, dD^/dt is the estimated Doppler rate, and d^^/dt 2 is the estimated change in Doppler rate. The estimated uplink Doppler, Doppler rate, and Doppler rate derivative are therefore subtracted 163 from the desired uplink transmit frequency, f c , to remove the Doppler effect. The results of the subtraction controls a numerically controUed oscillator 165, which pre-adjusts the frequency of the signal transmitted to the satellite. The end -25- result is that the transmitted signal arrives at the satellite at the desired frequency, f c , plus any errors, Δ, that are introduced by the estimation process. 4. Downlink Carrier Frequency Acquisition and Tracking Preferably downlink data transmissions to ground terminals involve hopping (switching) antenna beams that periodicaUy Uluminate each Earth-fixed ceU. Consequently, downUnk data transmissions are received by the ground terminal in bursts and at irregular intervals. As on the uplink, the Doppler shift of the downlink carrier can be very high. The Doppler shift and the fact that downlink data transmission bursts can be relatively short (e.g., four packets) present a chaUenging frequency acquisition problem for the ground terminal. FIGURE 26 is a block diagram of a preferred system used to estimate, acquire, and track the downlink carrier frequency ft using the beacon signal. At the sateUite level, a reference osciUator 171 is used to generate a narrowband beacon signal having a frequency ft by scaling the reference oscUlator signal by a constant kj,- The reference oscUlator 171 is also used to generate a downlink carrier signal having a frequency ft,d„ by scaling the reference oscillator signal by a constant ka. Both signals are transmitted from the satellite to the ground terminal, the beacon signal continuously and the downlink carrier signal in bursts, as noted above. During transmission, each signal is Doppler shifted in accordance with the relative motion of the satellite with respect to the ground terminal. At the ground terminal, the beacon signal having a carrier frequency f j , is continuously tracked as the Doppler shift varies using techniques known to those skUled in the art. FIGURE 27A-27C graphically depict the frequency spectrum of the signals received at the ground terminal. As shown in FIGURE 27A, prior to a burst of data arriving at the ground terminal, only the beacon signal is detected at a frequency ft. As shown in FIGURE 26, the beacon carrier frequency is scaled by the known constant ka/k (see block 173) and the result used to determine the downlink carrier frequency, which is used to control a downconverter 175. Since the beacon signal and the downlink carrier signal are approximately equaUy affected by the satelUte-motion induced Doppler shift, the Doppler shift is removed and the downlink carrier frequency ft,d n is easUy estimated. As shown in FIGURE 27B, when a data burst from the sateUite arrives at the ground terminal, the pre-estimated downlink carrier frequency ft,d„ allows the data burst to be quickly acquired, even though both the downlink carrier frequency and the beacon carrier frequency have been Doppler shifted upward in frequency. Following receipt of the bursted data, the ground -26- terminal continues to monitor the beacon signal, as depicted in FIGURE 27C. Continuously tracking the beacon signal allows the downUnk carrier frequency ft,d„ to be quickly and easUy acquired and tracked. In accordance with this invention, the foregoing and other chaUenges associated with a LEO satellite communication network of the type illustrated in FIGURE 1 are overcome by each sateUite producing a beacon signal that, preferably, faUs within the communication spectrum. The beacon is utilized by the ground terminals to estabUsh and maintain a communications link. More specificaUy, the beacon is used by the ground stations to accomphsh: accurate antenna pointing; downlink carrier frequency synchronization; data frame synchronization; and uplink carrier frequency synchronization. While the preferred embodiment of the invention has been Ulustrated and described, it wUl be appreciated that various changes can be made therein without departing from the spirit and scope of the invention. For example, whUe the invention has been described in connection with a LEO Earth-fixed cell sateUite communication network, the invention can be used with other types of satellite networks including LEO satelUte communication networks using satellite-fixed beams and geosynchronous sateUite communication networks. Hence, within the scope of the appended claims it is to be understood that the invention can be practiced otherwise than as specifically described herein.

Claims: -27- The embodiments of the invention in which an exclusive property or privUege is claimed are defined as follows: 1. A ground terminal beacon acquisition and synchronization system for a sateUite communication network comprising a pluraUty of orbiting satelUtes and a plurality of ground terminals that communicate with one another via uplink and downlink data channels to and from the orbiting satelUtes, each of said orbiting sateUites producing a beacon, said ground terminal beacon acquisition and synchronization system comprising: (a) an antenna; (b) a satellite acquisition system for: (i) pointing said antenna toward a predicted satelUte position and determining if a satelUte is located at a predicted sateUite position based on whether a beacon is emanating from said predicted satelUte position; (u) if no sateUite is located at the predicted satelUte position, causing the antenna to point toward another predicted sateUite position and determining if a satelUte is located at said another predicted satellite position based on whether a beacon is emanating from said another predicted sateUite position; (iii) if a satelUte is located at the predicted sateUite position, determining if the satellite is serving the geographic area in which the ground terminal lies based on information contained in the beacon; and (iv) if the satellite is not serving the geographic area in which the ground terminal lies, causing the antenna to point toward still another predicted sateUite position and determining if a sateUite is located at said still another predicted satelUte position based on whether a beacon is emanating from said still another sateUite position; and (v) repeating (U) through (iv) untU the sateUite serving the geographic area in which the ground terminal Ues is located; and (c) a synchronization system for synchronizing the ground terminal to the satelUte serving the geographic area in which the ground terminal Ues based on information contained in the beacon. 2. A ground terminal beacon acquisition and synchronization system as claimed in Claim 1 wherein the satelUte acquisition system also determines how long the satellite will continue to serve the geographic area in which the ground terminal -28- lies prior to the synchronizing system synchronizing the ground terminal to the satellite. 3. A ground terminal beacon acquisition and synchronization system as claimed in Claim 2 wherein the synchronizing system does not synchronize the ground terminal to the sateUite serving the geographic area in which the ground terminal Ues if the period of time is short and, instead, the satelUte acquisition system points the antenna toward yet stUl another predicted satellite position and determines if a sateUite is located at said yet stUl another predicted sateUite position based on whether a beacon is emanating from said yet stUl another satelUte position. 4. A ground terminal beacon acquisition and synchronization system as claimed in Claim 3 wherein said synchronizing system synchronizes said ground terminal to the satellite serving the geographic area in which the ground terminal Ues if the period of time is long. 5. A ground terminal beacon acquisition and synchronization system as claimed in Claim 4 wherein the sateUite acquisition system uses the beacon to accurately point the antenna of the ground terminal toward the sateUite serving the geographic area in which the ground terminal Ues. 6. A ground terminal beacon acquisition and synchronization system as claimed in Claim 5 wherein the synchronization system uses information contained in the beacon to accurately time uplink data transmissions. 7. A ground terminal beacon acquisition and synchronization system as claimed in Claim 6 wherein the synchronization system uses information contained in the beacon to estimate upUnk Doppler, Doppler rate, Doppler rate derivative, and uses the estimate to pre-compensate the carrier frequency of the uplink data channel for Doppler and Doppler rate. 8. A ground terminal beacon acquisition and synchronization system as claimed in Claim 7 wherein the synchronization system uses information contained in the beacon to continuously track downUnk carrier frequency by continuously tracking beacon carrier frequency, scaling the result by a suitable scaling factor, and using the scaled result to locate the downlink carrier frequency. -29- 9. A ground terminal beacon acquisition and synchronization system as claimed in Claim 8 wherein the scaUng factor is equal to a scaling factor applied to the beacon by the sateUite serving the geographic area in which the ground terminal Ues divided by a sealing scale factor applied to the downlink carrier frequency by the sateUite serving the geographic area in which the ground terminal Ues. 10. A ground terminal beacon acquisition and synchronization system as claimed in Claim 8 wherein the beacon includes information regarding the identity of the satellite with which the beacon is associated and wherein the satellite acquisition system uses the identity information to assist in determining if a sateUite is serving the geographic area in which the ground terminal Ues. 11. A ground terminal beacon acquisition and synchronization system as claimed in Claim 10 wherein said beacon also includes information about the satellite position and velocity and wherein the sateUite synchronization system uses the position and velocity information to determine the slant range between the ground terminal and the satellite serving the geographic area in which the ground terminal Ues. 12. A ground terminal beacon acquisition and synchronization system as claimed in Claim 11 wherein the sateUite synchronization system uses said sateUite position and velocity information to estimate uplink Doppler, Doppler rate, and Doppler rate derivative. 13. A ground terminal beacon acquisition and synchronization system as claimed in Claim 12 wherein the beacon includes an identifier for identifying the contention channel to be used by a ground station. 14. A ground terminal beacon acquisition and synchronization system as claimed in Claim 13 wherein the beacon includes information about the identity of sateUites other than the sateUite associated with the beacon. 15. A ground terminal beacon acquisition and synchronization system as claimed in Claim 14 wherein said sateUite identity information includes information about the identity of handover satellites. -SO- lό. A ground terminal beacon acquisition and synchronization system as claimed in Claim 15 wherein in addition to the identity of handover satelUtes, the beacon also includes information about the position and velocity of the handover satelUtes and wherein the sateUite acquisition system utilizes the position and velocity information to point the antenna toward said still another predicted satellite position. 17. A ground terminal beacon acquisition and synchronization system as claimed in Claim 16 wherein said beacon includes a unique word and wherein said unique word is used by said satellite synchronization system to identify a beacon frame. 18. A ground terminal beacon acquisition and synchronization system as claimed in Claim 17 wherein said synchronization system also utilizes said unique word to determine when satellite position and velocity is updated. 19. A ground terminal beacon acquisition and synchronization system as claimed in Claim 1 wherein the sateUite acquisition system uses the beacon to accurately point the antenna of the ground terminal toward the satellite serving the geographic area in which the ground terminal Ues. 20. A ground terminal beacon acquisition and synchronization system as claimed in Claim 1 wherein the synchronization system uses information contained in the beacon to accurately time upUnk data transmissions. 21. A ground terminal beacon acquisition and synchronization system as claimed in Claim 1 wherein the synchronization system uses information contained in the beacon to estimate uplink Doppler, Doppler rate, Doppler rate derivative, and uses the estimate to pre-compensate the carrier frequency of the uplink data channel for Doppler and Doppler rate. 22. A ground terminal beacon acquisition and synchronization system as claimed in Claim 1 wherein the synchronization system uses information contained in the beacon to continuously track downlink carrier frequency by continuously tracking beacon carrier frequency, scaling the result by a suitable scaling factor, and using the scaled result to locate the downlink carrier frequency. -31- 23. A ground terminal beacon acquisition and synchronization system as claimed in Claim 22 wherein the scaling factor is equal to a scaUng factor appUed to the beacon by the satellite serving the geographic area in which the ground terminal lies divided by a sealing scale factor appUed to the downlink carrier frequency by the satellite serving the geographic area in which the ground terminal lies. 24. A ground terminal beacon acquisition and synchronization system as claimed in Claim 1 wherein said beacon includes information regarding the identity of the sateUite with which the beacon is associated and wherein the sateUite acquisition system uses the identity information to assist in determining if a sateUite is serving the geographic area in which the ground terminal lies. 25. A ground terminal beacon acquisition and synchronization system as claimed in Claim 24 wherein said beacon also includes information about the satellite position and velocity and wherein the satelUte synchronization system uses the position and velocity information to determine the slant range between the ground terminal and the satelUte serving the geographic area in which the ground terminal Ues. 26. A ground terminal beacon acquisition and synchronization system as claimed in Claim 1 wherein the beacon includes an identifier for identifying the contention channel to be used by a ground station. 27. A ground terminal beacon acquisition and synchronization system as claimed in Claim 1 wherein the beacon includes information about the identity of sateUites other than the sateUite from which a beacon emanates. 28. A ground terminal beacon acquisition and synchronization system as claimed in Claim 27 wherein said satellite identity information includes information about the identity of handover satellites. 29. A ground terminal beacon acquisition and synchronization system as claimed in Claim 28 wherein in addition to the identity of handover satelUtes, the beacon also includes information about the position and velocity of the handover sateUites and wherein the satellite acquisition system utilizes the position and velocity information to point the antenna toward said stUl another predicted sateUite position. -32- 30. A ground terminal beacon acquisition and synchronization system as claimed in Claim 1 wherein said beacon includes a unique word and wherein said unique word is used by said satelUte synchronization system to identify a beacon frame. 31. A ground terminal beacon acquisition and synchronization system as claimed in Claim 30 wherein said synchronization system also utilizes said unique word to determine when satellite position and velocity is updated.

Inventors: GHAZVINIAN FARZAD, HINEDI SAMI M, SARRAFAN SHIVA S, SHAH BIREN N, STURZA MARK A, GHAZVINIAN, FARZAD, HINEDI, SAMI, M., SARRAFAN, SHIVA, S., SHAH, BIREN, N., STURZA, MARK, A.
Applicants: GHAZVINIAN FARZAD, HINEDI SAMI M, SARRAFAN SHIVA S, SHAH BIREN N, STURZA MARK A, TELEDESIC LLC, GHAZVINIAN, FARZAD, HINEDI, SAMI, M., SARRAFAN, SHIVA, S., SHAH, BIREN, N., STURZA, MARK, A.
Application year: 1999
Main classification: H04B7
IPC codes: H04B7