Method and system for providing satellite communications

A broad-band digital satellite communications system for providing broad-band data services. The system comprises a first spacecraft, generally a geo-stationary earth orbit communications device, and at least one controller having broadband communications capability with the first spacecraft. The system also includes at least one second spacecraft, generally a low earth orbit (LEO) communications device. The second spacecraft comprises ; communications capability with the at least one first spacecraft; low data rate communications capability with a land based system, generally a mobile communications service provider; and broadband communications capability with a mobile user subscriber to the mobile communications service provider.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 , there is shown a pictorial representation of a system 10 for providing data services to mobile users and incorporating features of the present invention. Advantageously this system exploits the natural asymmetry between low rate requests for data and the high rate data requested. First, through the use of the user return links that issue data requests, such as a session on the internet where large downloads are requested, and then through the application of user position information already present within the system via low rate communications links 9 between the user 16 and the LEO satellite 12 . Furthermore, the gateway operations and control center (GOCC) 13 controls low data rate, such as cellular service operations, and provides data services such as broadband data services. User position information is derived from the low data rate service to make appropriate data channel and beam assignments and to initialize and update the beam pointing algorithms that control the phased-array antennas 3 onboard the LEO satellites 12 . The Data Traffic Gateway (DTG) 14 is provided to centralize the interface to the service provider backbone and provide up-link communications with the geo-stationary earth orbit (GEO) satellite. Addition of the DTG 14 links it to the GOCC and the service provider gateways. It is appreciated that providing appropriate LEO satellites 12 data channel and beam assignments from the GOCC 13 to the DTG 14 to the GEO 11 satellite and then to the LEO satellite 12 constellation obviates the requirement for line-of-sight LEO satellite control centers. Referring to FIG. 3 there is shown a method flow chart for using the system shown in FIGS. 1 or 2 to transmit data from a ground station to a mobile user. Data is transmitted from a geo-stationary satellite (GEO) ( FIG. 1 , item 11 ) and received 47 on a low earth orbit (LEO) satellite constellation ( FIG. 1 , items 12 ), where a satellite constellation may be one or more LEO satellites. A geographical position of a user requesting the data is determined 49 and the data is transmitted 50 from the LEO ( FIG. 1 , items 12 ) constellation to the user ( FIG. 1 , item 16 ). Referring now to FIG. 1 , digital signals are transmitted from the DTG 14 to a geo-stationary (GEO) 11 satellite within multiple Fixed Satellite space-to-Earth frequency bandwidth allocations on multi-band up-link channels 5 . These signals are combined onto a signal that is transmitted from the GEO 11 satellite to a satellite constellation of low earth orbit (LEO) satellites 12 . A subset of the satellites 12 in the LEO constellation are simultaneously illuminated by the GEO satellite 11 . Each LEO satellite 12 transmits multiple, independently directed spot beams 18 , each providing a predetermined fraction of the digital signal traffic received from the GEO-LEO inter-satellite link 19 . The fraction of the digital signal may be determined by channel allocation requirements, spot beam overlap, and/or channel transmission environment. Each LEO phased-array antenna 3 independently points separate beams 18 to user terminals 16 tracked by the position information available from user voice/data low rate links 9 between the service provider gateway 15 and the LEO satellite 12 . Broadband data requests are processed through the interactive voice/data low rate links 9 that comprise a LEO constellation based cellular telephone system. Broadband data are transmitted to users from the data traffic gateway 14 through the GEO spacecraft 11 and then to the user terminal 15 through one or more of the LEO satellites 12 within view of the user terminal 16 . Thus, the system maintains a low data rate return link from each user through the conventional cellular telephone extension function and distributes high rate data as might be requested from an internet service provider through the spot beams. LEO satellite traffic and beam assignments and their associated tracking trajectories and traffic hand-offs are managed through the central Gateway Operations and Control Center (GOCC) 13 and the local service provider gateways 15 . The digital up-links to the GEO satellite are also managed from the GOCC 13 . Multiple GEO satellites and data traffic gateways are used to achieve global broadband coverage. Referring now to FIG. 4 there is shown a method flow chart for using the systems shown in FIGS. 1 or 2 to transmit data from a ground station to a mobile user. FIGS. 1 and 4 and the following numerical example illustrate features of the present invention. Select for this example frequency division multiplexed (FDM) signals with a center band spacing of 57.14 MHz, guard bands between channels of 7.14 MHz and no guard bands at the edges of the up-link and down-link bands. Identify 4.8 GHz of up-link bandwidth for the Earth-to-GEO data link that uses dual-polarization techniques to transmit 84 channels, nominally 50 MHz each, within the 27.5-to-29.9 GHz Region 2 FCC allocation for Earth-to-space communications. Data is received 31 at the data traffic gateway 14 and evaluated 32 for required up-link channel capacity. If available channel capacity is exceeded the data is parsed 34 on to multiple up-link channels before the date is substantially parallel up-linked 33 to the GEO satellite 11 . Receivers on board the GEO satellite 11 translate 35 the FDM signals into the 59-64 GHz band allocated for inter-satellite communications. All 84 channels are substantially simultaneously broadcasted 36 on a single polarization into an earth coverage beam that includes, for example, the 1414 km altitude of the GLOBALSTAR satellites. Receivers on each of the LEO satellites 12 translate the FDM channels from the inter-satellite link into the 400 MHz band of 19.7-to-20.1 GHz that is allocated for mobile satellite space-to-earth communications. The next step determines 38 if the data exceeds available down-link channel capacity. This step may be predetermined at the GOCC 13 or the GEO 11 from known channel capacity and data down-link requirements. In addition, the channel down-link capacity may be dynamically determined 38 by one or more of the LEO satellites. If the channel capacity is exceeded more channels are added 40 . For example, there are at least, 12 beams on a GLOBALSTAR LEO satellite that can be independently formed within the active phased-array antenna. With general regard to communications beam forming reference can be had to “Digital Beamforming in Wireless Communications”, by John Litva and Titus KwokYeung Lo, ISBN 0-89006-712-0, the disclosure of which is incorporated by reference in its entirety. Each beam may contain up to 7 channels from the inter-satellite link. Each beam is independently pointed to a user within the spot beam coverage. In addition, since a LEO satellite within the LEO constellation has a view of &plus;/−54 degrees to all earth terminals with at least a 10 degree elevation angle view of the satellite, many more than 12 spot beams with coverage areas of about 2 degrees may be simultaneously pointed to deliver high rate data to multiple locations without generating significant interference between beams. Thus, if the total channel capacity of an individual satellite within the LEO constellation is exceeded then channels from a satellite with overlapping beam coverage are allocated to carry a fraction of the data signal ( FIG. 2 , item 18 ). The next step 42 steers the beams associated with the allocated channels to illuminate the user. The last step spreads 37 the data onto the allocated channels to be transmitted 39 to the user. It is readily appreciated from this example that space division multiplexing (SDM) using a multi-beam phased-array antenna can provide many times frequency reuse. In this manner, all allocated up-link and down-link bandwidth with high data rate signals through the translation of the multi-band FDM traffic onto a single optical carrier signal for the GEO-to-LEO inter-satellite link are advantageously utilized. Multi-spot-beam phased-array antennas customized for each allocated down-link band may then be used to fold the many up-link channels into the many down-link beams, by utilizing a beams set for each of the various down-link bands. It is readily appreciated that the efficiency of spectral resource allocation comes from the mapping of subsets or fractions of the up-link spectrum onto spot beams; effectively using some or all of a particular bandwidth allocated for space-to-Earth communications. Referring now to FIG. 5 there is shown a pictorial diagram of another communications system incorporating features of the present invention. Referring also to FIG. 6 there is shown a method flow chart for using the system shown in FIG. 5 . Terminal 51 transmit data 62 to a satellite constellation 55 through modem 52 and ground station 53 . The data is received 63 on the satellite constellation 55 , where the satellite constellation may comprise one or more satellites. The geographical position of an internet 59 is determined 64 and the data is transmitted 65 to the internet through ground station 54 , modem bank 56 , and routers 58 . It is readily appreciated that features of the present invention allow rurally located users, or other users where internet connection is not economically feasible, to have access to internet services. In addition, it is also readily appreciated that the internet access is not limited to narrow-band data services but includes broadband services as well. Lastly, it should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.