LEO SATELLITE DATA TRANSMISSION ARRANGEMENT

Abstract: This invention relates to low earth orbit (LEO) satellite communication system, and discloses a method of remotely activating a deactivated tracking mechanism associated with the directional tracking antenna of a terrestrial user terminal of such a system when the user terminal is not in use during quiescent periods. The activated tracking mechanism enables the directional antenna to target a passing satellite of the system so that data can be transmitted to the user terminal during these periods. The method is accomplished by providing an omnidirectional auxiliary antenna and an associated receiver at the user terminal to receive a radio control signal which is selectively transmitted through the satellite to enable a control device that controls the tracking mechanism.

Description: LEO SATELLITE DATA TRANSMISSION ARRANGEMENT Technical Field This invention relates to low earth orbit (LEO) satellite communication systems, and in particular to a method and an arrangement for remotely activating the deactivated tracking mechanism of a directional tracking antenna associated with a terrestrial user terminal of such a system. Background Art LEO satellite communication systems are known and typically comprise a constellation of satellites orbiting at an altitude of about 1500km, a plurality of terrestrial user terminals and a plurality of terrestrial gateway stations, the user terminals and the gateway stations being located in the footprint of a passing satellite's antenna beam. The satellites provide a seamless radio communication path, typically in the Ku band, between the user terminals and the gateway stations, via their associated directional antennas. This communication path provides a connection between a user terminal and a telecommunications network interfaced with a gateway station. Users may be, for example, businesses, schools, hospitals, or private individuals in houses or apartments. There is a growing demand by users of telecommunication services for new services such as, for example, internet, video conference, distance learning, tele- medicine and tele-working. These new services combine text, still and moving images, sound and data files, and are interactive. Therefore transmission rates for transmitting such data are relatively high and require a transmission path of higher capacity than narrow band transmission paths normally provided for plain old telephone services (POTS). Although trunk networks have high capacity optical transmission paths for the transmission of broad band data, the local access transmission path to the user, sometimes called "the last mile", is normally a narrow- band twisted copper pair which impairs high transmission rate data. It is known to use a LEO communication system to provide a wide band local access transmission path or "the last mile". An example of such a system used for this purpose is the SkyBridge LEO communications system. Because the transmission path being provided by the passing satellite is a broad band transmission path the antennas associated with the users terminal must be high gain directional antennas such as parabolic reflector antennas or Luneberg lens antennas, having a tracking mechanism to track the passing satellite. In order for a local access LEO communication system to be commercially attractive, the tracking directional antennas associated with a user terminal must be low cost. This is achieved by using relatively simple low cost tracking mechanisms as compared with more expensive complex precision tracking mechanisms. However, the working life of simple tracking mechanisms is less than the working life of the precision tracking. To extend the working life of the low cost simple tracking mechanism it is known to deactivate the tracking mechanism when the user terminal is not in use during quiescent periods. Deactivation of the tracking mechanism, however, introduces a problem in that the communication path between the user terminal and the passing satellite is severed because the satellite is no longer targeted by the user terminal's antenna. Satellite carriers wanting to take advantage of low traffic periods that occur during the quiescent periods to download data to the user terminal, such as daily newspaper text, stock market information, or software upgrades, cannot do so because the tracking mechanism is turned off. Also terminating telephone calls and emails cannot be delivered to the user. Disclosure of the invention It is an object of the present invention to provide an arrangement to activate a deactivated antenna tracking mechanism of a user terminal of a LEO communications system, so that data transmitted to the user terminal via a passing satellite during a quiescent period can be received and stored in the user terminal. According to the invention, in a low earth orbit satellite communications system comprising a constellation of satellites, a plurality of terrestrial user terminals and a plurality of terrestrial gateway stations geographically remote from said user terminals, said user terminals and said gateway stations being geographically located in the footprint of a passing satellite's antenna beam, and each user terminal including at least one directional antenna means and an associated controllable antenna tracking mechanism means which is deactivated when said user terminal is not in use, there is provided a method of remotely activating a deactivated controllable antenna tracking mechanism, comprising the steps of: Provide an auxiliary antenna means and an operatively associated radio receiver means whose output is coupled to control means of said controllable antenna tracking mechanism means: Cause a radio control signal of a predetermined frequency to be transmitted from a remote transmitter means to be received by said auxiliary antenna means and passed to said radio receiver means to produce an enabling signal at its output means; Enable said control means with said enabling signal to activate said controllable antenna tracking mechanism to cause said directional antenna means to target and track a passing satellite for receiving data transmitted through said gateway station to said user terminal via the satellite. According to a further aspect of the invention, in a low earth orbit satellite communications system comprising a constellation of satellites, a plurality of terrestrial user terminals and a plurality of terrestrial gateway stations geographically remote from said user terminals, said user terminals and said gateway stations being geographically located in the footprint of a passing satellite's antenna beam, and each user terminal including at least one directional antenna means and an associated controllable antenna tracking mechanism means which is deactivated when said user terminal is not in use, there is provided an arrangement for remotely activating a deactivated controllable antenna tracking mechanism, said arrangement comprising an auxiliary antenna means, a radio receiver means operatively associated with said auxiliary antenna means and whose output is coupled to control means of said controllable antenna tracking mechanism means, means to cause a radio control signal of a predetermined frequency to be transmitted from a remote transmitter means to be received by said auxiliary antenna means and passed to said radio receiver means to produce an enabling signal at its output means to enable said control means to activate said controllable antenna tracking mechanism to cause said directional antenna means to target and track a passing satellite for receiving data transmitted through said gateway station to said user terminal via the satellite. Brief Description of the Drawings In order that the invention may be readily carried into effect, embodiments thereof will now be described in relation to the accompanying drawings, in which: Figure 1 shows a first embodiment of the present invention. Figure 2 shows a second embodiment of the present invention. Figure 3 shows a third embodiment of the present invention. Figure 4 shows a fourth embodiment of the present invention. Best mode of carrying out the invention Referring to Figure 1 , there is shown part of a LEO communications system including a terrestrial gateway station 1 having a directional tracking antenna means 2 operatively associated therewith, an orbiting satellite 3, and a geographically remote user terminal which comprises a directional tracking antenna means 4, a tracking mechanism (not shown) operatively associated with an antenna tracking control means 4a, a transceiver 5, a processor 6, the users PC and telephone equipment 7, and an auxiliary omnidirectional antenna means 8 operatively associated with processor 6 via a receiver 9. In use, if high transmission rate data is to be selectively sent to the user terminal during a quiescent period, a radio control signal is transmitted through the gateway station 1 and relayed by satellite 3. At the user terminal, auxiliary omnidirectional antenna 8 receives and passes this control signal to receiver 9 whose output is processed in processor 6 to produce an enabling signal to enable the antenna tracking control means 4a to activate the tracking mechanism to enable the directional tracking antenna means 4 to target and track satellite 3 to receive and store the data. Preferably, after the transmission is completed the directional tracking antenna is deactivated. Referring to Figure 2, in this embodiment, during quiescent periods when the tracking mechanism is deactivated, the directional antenna means 4 is utilised as the auxiliary antenna and is fixedly aimed at a geostationary satellite 10 which relays a radio control signal selectively transmitted through the geostationary satellite's communication system and received by the fixed directional antenna 4 and passed to the associated transceiver to enable the control means 4a to activate the user terminal's tracking mechanism and enable the directional antenna 4 to target and track satellite 3 to receive and store transmitted data relayed by satellite 3. Referring to Figure 3, this embodiment is similar to that described in relation to Figure 1 in that a radio control signal is passed from the omnidirectional antenna 8 to receiver 9 to produce an enabling signal to enable the antenna tracking control means 4a to activate the tracking mechanism, however in this embodiment the radio control signal is transmitted from a terrestrial paging system 1 1 . Figure 4 shows a further implementation of the invention. In this alternative embodiment to that shown in Figure 1 , a precursor satellite 12 may be used to initiate the terminal by sending a wake-up signal to be received by the user terminal's auxiliary omnidirectional antenna 8. The user terminal tracking antenna 4 and its associated tracking mechanism and tracking control means are initiated by the wake-up signal and process of locking on to the orbiting satellite 3 is commenced. Preferably the precursor satellite has the ability to broadcast the wake-up signal over a paging channel to which the auxiliary receiver 9 is tuned. The wake-up call may be a coded signal directed to a single user terminal, or it may be arranged to initiate a group of user terminals. The precursor satellite's paging channel may be a narrow band channel, e.g., 9.6 kHz, as the wake-up call can be a brief message. The precursor satellite 12 may be part of the communication system constellation, part of a separate constellation, or a geostationary satellite.

Claims: The claims defining the invention are as follows: 1 . In a low earth orbit satellite communications system comprising a constellation of satellites, a plurality of terrestrial user terminals and a plurality of terrestrial gateway stations, said user terminals and said gateway stations being geographically located in the footprint of a passing satellite's antenna beam, and each user terminal including at least one directional antenna means and an associated controllable antenna tracking mechanism means which is deactivated when said user terminal is not in use, a method of remotely activating a deactivated antenna tracking means, comprising the steps of: - providing an auxiliary antenna means and an operatively associated auxiliary receiver means whose output is coupled to control means of said controllable antenna tracking mechanism means: causing a radio control signal to be transmitted from a remote transmitter means to be received by said auxiliary antenna means and passed to said auxiliary receiver means to produce an enabling signal at its output means; enabling said control means with said enabling signal to activate said controllable antenna tracking mechanism to cause said directional antenna means to target and track a passing satellite for receiving data transmitted through said gateway station to said user terminal via the satellite. 2. A method as claimed in claim 1 , wherein said auxiliary antenna means is an omnidirectional antenna. 3. A method as claimed in claim 2, wherein said radio control signal is transmitted through said gateway station and relayed by said satellite. 4. A method as claimed in claim 2, wherein said radio control signal is transmitted through a terrestrial paging system. 5. A method as claimed in claim 1 , wherein the auxiliary antenna means is the user terminal's directional antenna means fixedly aimed at a geostationary satellite, said radio control signal being relayed by said geostationary satellite to be received by said user terminal's fixed directional antenna means. 6. A method substantially as herein described with reference to Figures 1 - 4 of the accompanying drawings. 7. In α low earth orbit satellite communications system comprising a constellation of satellites, a plurality of terrestrial user terminals and a plurality of terrestrial gateway stations geographically remote from said user terminals, said user terminals and said gateway stations being geographically located in the footprint of a passing satellite's antenna beam, and each user terminal including at least one directional antenna means and an associated controllable antenna tracking mechanism means which is deactivated when said user terminal is not in use, an arrangement for remotely activating a deactivated antenna tracking means, said arrangement comprising an auxiliary antenna means, a radio receiver means operatively associated with said auxiliary antenna means and whose output is coupled to control means of said controllable antenna tracking mechanism means, means to cause a radio control signal of a predetermined frequency to be transmitted from a remote transmitter means to be received by said auxiliary antenna means and passed to said radio receiver means to produce an enabling signal at its output means to enable said control means to activate said controllable antenna tracking mechanism to cause said directional antenna means to target and track a passing satellite for receiving data transmitted through said gateway station to said user terminal via the satellite. 8. An arrangement as claimed in claim 7, wherein said auxiliary antenna means is an omnidirectional antenna. 9. An arrangement as claimed in claim 8, wherein said radio control signal is transmitted through said gateway station and relayed by the passing satellite. 10. An arrangement as claimed in claim 8, wherein said radio control signal is transmitted through a terrestrial paging system. 1 1 . An arrangement as claimed in claim 7, wherein the antenna means is the user terminal's directional antenna means fixedly aimed at a geostationary satellite, said radio control signal being relayed by said geostationary satellite to be received by said user terminal's fixed directional antenna means. 12. An arrangement substantially as herein described with reference to Figures 1 - 4 of the accompanying drawings. 13. A user terminal associated with a low earth orbit satellite communication system, including an arrangement as claimed in any one of claims 7 - 12. DATED THIS THIRD DAY OF AUGUST 1999 ALCATEL

Inventors: RUDOLPH COLIN, RUDOLPH, COLIN
Applicants: CIT ALCATEL, RUDOLPH COLIN, ALCATEL, RUDOLPH, COLIN
Application year: 2000
Main classification: H04B7
IPC codes: H01Q1, H04B7