CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from provisional application Ser. No. 60/281,548, filed Apr. 4, 2001. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to mobile RF terminals required to conduct bi-directional communications with a base station via a satellite link, and more particularly to a method and apparatus for quickly identifying which one of a plurality of mobile terminals is causing interference with one or more satellites orbiting in the vicinity of a target transponded satellite. 
     BACKGROUND OF THE INVENTION 
     With mobile RF terminals located on mobile platforms such as aircraft, cruise ships and other moving platforms, communicating with a ground station via a transponded satellite, there is always the remote possibility, in spite of the safeguards that may be built into the mobile terminal, that the terminal may fail in an unanticipated manner. In such event, there is the possibility that the mobile terminal may cause interference with other satellites orbiting in the geo arc adjacent the target satellite with which the mobile terminal is communicating. 
     It is also recognized that Fixed Services Satellite (FSS) operators may have difficulty in locating interference from VSAT (Very Small Aperture Terminal) systems that consist of thousands of unsophisticated terminals at remote sites. This problem is compounded when the remote terminals comprise mobile terminals that are located on fast moving platforms, such as jet aircraft. 
     Therefore, there exists a need for the ground station in communication with a plurality of mobile terminals via a transponded satellite to be able to quickly identify a malfunctioning mobile terminal which is causing interference with non-target satellites and to quickly resolve the interference incident. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a system and method for quickly determining which one of a plurality of mobile terminals communicating with a base station, such as a ground-based communication system or segment, is causing interference with one or more satellites adjacent a target transponded satellite. The system and method uses a network operations center (NOC) of the ground-based segment to command each mobile terminal of each aircraft utilizing the target transponded satellite to carry out a transmit lobing sequence, to thereby determine the off-axis effective isotropic radiated power (EIRP) of the signal transmitted by the transmit antenna of each mobile terminal. 
     The NOC uses the information obtained during the transmit lobing process to determine whether a parameter of the transmitted signals received by it during the transmit lobing process indicates that the mobile terminal is the cause of interference. If it is not, then the NOC performs the transmit lobing sequence with the mobile terminal of a second aircraft accessing the transponded target satellite, and thereafter with every mobile terminal accessing the transponded target satellite until the mobile terminal causing the interference is detected. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawing, wherein: 
     FIG. 1 is a simplified block diagram of a system for implementing the method of the present invention; 
     FIG. 2 is a simplified block diagram of a mobile terminal used on each mobile platform, for enabling the mobile platform to establish a communication link with a transponded satellite of the system; and 
     FIG. 3 is an illustration of the five points at which the transmit antenna of a mobile terminal is pointed relative to the target satellite, during the transmit lobbing sequence. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, there is shown a system  10  in accordance with a preferred embodiment of the present invention for providing data content to and from a plurality of mobile platforms  12   a - 12   f  in one or more distinct coverage regions  14   a  and  14   b . The system  10  generally comprises a ground segment  16 , a plurality of satellites  18   a - 18   f  forming a space segment  17 , and a mobile system or terminal  20  disposed on each mobile platform  12 . The mobile platforms  12  could comprise aircraft, cruise ships or any other moving vehicle. Thus, the illustration of the mobile platforms  12  as aircraft in the figures herein, and the reference to the mobile platforms as aircraft throughout the following description, should not be construed as limiting the applicability of the system  10  to only aircraft. 
     The space segment  17  may include any number of satellites  18  in each coverage region  14   a  and  14   b  needed to provide coverage for each region. Satellites  18   a ,  18   b ,  18   d  and  18   e  are preferably Ku or Ka-band satellites. Satellites  18   c  and  18   f  are Broadcast Satellite Services (BSS) satellites. Each of the satellites  18  are further located in a geostationary orbit (GSO) or a non-geostationary orbit (NGSO). Examples of possible NGSO orbits that could be used with this invention include low Earth orbit (LEO), medium Earth orbit (MEO) and highly elliptical orbit (HEO). Each of the satellites  18  includes at least one radio frequency (RF) transponder, and more preferably a plurality of RF transponders. For example, satellite  18   a  is illustrated having four transponders  18   a   1 - 18   a   4 . It will be appreciated that each other satellite  18  illustrated could have a greater or lesser plurality of RF transponders as required to handle the anticipated number of aircraft  12  operating in the coverage area. The transponders provide “bent-pipe” communications between the aircraft  12  and the ground segment  16 . The frequency bands used for these communication links could comprise any radio frequency band from approximately 10 MHz to 100 GHz. The transponders preferably comprise Ku-band transponders in the frequency band designated by the Federal Communications Commission (FCC) and the International Telecommunications Union (ITU) for fixed satellite services FSS or BSS satellites. Also, different types of transponders may be employed (i.e., each satellite  18  need not include a plurality of identical types of transponders) and each transponder may operate at a different frequency. Each of the transponders  18   a   1 - 18   a   4  further include wide geographic coverage, high effective isotropic radiated power (EIRP) and high gain/noise temperature (G/T). 
     With further reference to FIG. 1, the ground segment  16  includes a ground station  22  in bi-directional communication with a content center  24  and a network operations center (NOC)  26 . A second ground station  22   a  located in the second coverage area  14   b  may be used if more than one distinct coverage area is required for the service. In this instance, ground station  22   a  would also be in bi-directional communication with the NOC  26  via a terrestrial ground link or any other suitable means for establishing a communication link with the NOC  26 . The ground station  22   a  is also in bi-directional communication with a content center  24   a . For the purpose of discussion, the system  10  will be described with respect to the operations occurring in coverage region  14   a . However, it will be understood that identical operations relative to the satellites  18   d - 18   f  occur in coverage region  14   b . It will also be understood that the invention may be scaled to any number of coverage regions  14  in the manner just described. 
     The ground station  22  comprises an antenna and associated antenna control electronics needed for transmitting data content to the satellites  18   a  and  18   b . The antenna of the ground station  22  may also be used to receive data content transponded by the transponders  18   a   1 - 18   a   4  originating from each mobile system  20  of each aircraft  12  within the coverage region  14   a . The ground station  22  may be located anywhere within the coverage region  14   a . Similarly, ground station  22   a , if incorporated, can be located anywhere within the second coverage area  14   b.    
     The content center  24  is in communication with a variety of external data content providers and controls the transmission of video and data information received by it to the ground station  22 . Preferably, the content center  24  is in contact with an Internet service provider (ISP)  30 , a video content source  32  and a public switched telephone network (PSTN)  34 . Optionally, the content center  24  can also communicate with one or more virtual private networks (VPNs)  36 . The ISP  30  provides Internet access to each of the occupants of each aircraft  12 . The video content source  32  provides live television programming, for example, Cable News Network® (CNN) and ESPN®. The NOC  26  performs traditional network management, user authentication, accounting, customer service and billing tasks. The content center  24   a  associated with the ground station  22   a  in the second coverage region  14   b  would also preferably be in communication with an ISP  38 , a video content provider  40 , a PSTN  42 , and optionally a VPN  44 . An optional air telephone system  28  may also be included as an alternative to the satellite return link. 
     Referring now to FIG. 2, the mobile system  20  disposed on each aircraft  12  will be described in greater detail. Each mobile system  20  includes a data content management system in the form of a router/server  50  (hereinafter “server”) which is in communication with a communications subsystem  52 , a control unit and display system  54 , and a distribution system in the form of a local area network (LAN)  56 . Optionally, the server  50  can also be configured for operation in connection with a National Air Telephone System (NATS)  58 , a crew information services system  60  and/or an in-flight entertainment system (IFE)  62 . 
     The communications subsystem  52  includes a transmitter subsystem  64  and a receiver subsystem  66 . The transmitter subsystem  64  includes an encoder  68 , a modulator  70  and an Up-converter  72  for encoding, modulating and up-converting data content signals from the server  50  to a transmit antenna  74 . The receiver subsystem  66  includes a decoder  76 , a demodulator  78  and a down-converter  80  for decoding, demodulating and down-converting signals received by the receive antenna  82  into baseband video and audio signals, as well as data signals. While only one receiver subsystem  66  is shown, it will be appreciated that preferably a plurality of receiver subsystems  66  will typically be included to enable simultaneous reception of RF signals from a plurality of RF transponders. If a plurality of receiver subsystems  66  are shown, then a corresponding plurality of components  76 - 80  will also be required. 
     The signals received by the receiver subsystem  66  are then input to the server  50 . A system controller  84  is used to control all subsystems of the mobile system  20 . The system controller  84 , in particular, provides signals to an antenna controller  86  which is used to electronically steer the receive antenna  82  to maintain the receive antenna pointed at a particular one of the satellites  18 , which will hereinafter be referred to as the “target” satellite. The transmit antenna  74  is slaved to the receive antenna  82  such that it also tracks the target satellite  18 . It will be appreciated that some types of mobile antennas may transmit and receive from the same aperture. In this case the transmit antenna  74  and the receive antenna  82  are combined into a single antenna. 
     With further reference to FIG. 2, the local area network (LAN)  56  is used to interface the server  50  to a plurality of access stations  88  associated with each seat location on board the aircraft  12   a . Each access station  88  can be used to interface the server  50  directly with a user&#39;s laptop computer, personal digital assistant (PDA) or other personal computing device of the user. The access stations  88  could also each comprise a seat back mounted computer/display. The LAN  56  enables bi-directional communication of data between the user&#39;s computing device and the server  50  such that each user is able to request a desired channel of television programming, access a desired website, access his/her email, or perform a wide variety of other tasks independently of the other users on board the aircraft  12 . 
     The receive and transmit antennas  82  and  74 , respectively, may comprise any form of steerable antenna. In one preferred form, these antennas comprise electronically scanned, phased array antennas. Phased array antennas are especially well suited for aviation applications where aerodynamic drag is important considerations. One particular form of electronically scanned, phased array antenna suitable for use with the present invention is disclosed in U.S. Pat. No. 5,886,671, assigned to The Boeing Co. 
     Referring further to FIG. 1, in operation of the system  10 , the data content is preferably formatted into Internet protocol (IP) packets before being transmitted by either the ground station  22 , or from the transmit antenna  74  of each mobile system  20 . For the purpose of discussion, a transmission of data content in the form of IP packets from the ground station  22  will be referred to as a “forward link” transmission. IP packet multiplexing is also preferably employed such that data content can be provided simultaneously to each of the aircraft  12  operating within the coverage region  14   a  using unicast, multicast and broadcast transmissions. 
     The IP data content packets received by each of the transponders  18   a   1 - 18   a   4  are then transponded by the transponders to each aircraft  12  operating within the coverage region  14   a . While multiple satellites  18  are illustrated over coverage region  14   a , it will be appreciated that at the present time, a single satellite is capable of providing coverage to an area encompassing the entire continental United States. Thus, depending upon the geographic size of the coverage region and the mobile platform traffic anticipated within the region, it is possible that only a single satellite incorporating a single transponder may be needed to provide coverage for the entire region. Other distinct coverage regions besides the continental United States include Europe, South/Central America, East Asia, Middle East, North Atlantic, etc. It is anticipated that in service regions larger than the continental United States, that a plurality of satellites  18  each incorporating one or more transponders may be required to provide complete coverage of the region. 
     The receive antenna  82  and transmit antenna  74  are each preferably disposed on the top of the fuselage of their associated aircraft  12 . The receive antenna  74  of each aircraft receives the entire RF transmission of encoded RF signals representing the IP data content packets from at least one of the transponders  18   a   1 - 18   a   4 . The receive antenna  82  receives horizontally polarized (HP) and vertically polarized (VP) signals which are input to at least one of the receivers  66 . If more than one receiver  66  is incorporated, then one will be designated for use with a particular transponder  18   a   1 - 18   a   4  carried by the target satellite  18  to which it is pointed. The receiver  66  decodes, demodulates and down-converts the encoded RF signals to produce video and audio signals, as well as data signals, that are input to the server  50 . The server  50  operates to filter off and discard any data content not intended for users on the aircraft  12  and then forwards the remaining data content via the LAN  56  to the appropriate access stations  88 . 
     Referring further to FIG. 1, a transmission of data content from the aircraft  12   a  to the ground station  22  will be described. This transmission is termed a “return link” transmission. The antenna controller  86  causes the transmit antenna  74  to maintain the antenna beam thereof pointed at the target satellite  18   a . The channels used for communication from each mobile system  20  back to the ground station  22  represent point-to-point links that are individually assigned and dynamically managed by the NOC  26  of the ground segment  16 . For the system  10  to accommodate several hundred or more aircraft  12 , multiple aircraft need to be assigned to each transponder carried by a given satellite  18 . The preferred multiple access methods for the return link are code division multiple access (CDMA), frequency divisional multiple access (FDMA), time division multiple access (TDMA) or combinations thereof. Thus, multiple mobile systems  20  may be assigned to a single transponder  18   a   1 - 18   a   4 . Where a greater number of aircraft  12  incorporating a mobile system  20  are operated within the coverage region  14   a , then the number of transponders required increases accordingly. 
     The receive antenna  82  may implement a closed-loop tracking system for pointing the antenna beam and for adjusting the polarization of the antennas based on receive signal amplitude. The transmit antenna  74  is slaved to the point direction and polarization of the receive antenna  82 . An alternative implementation could use an open-loop tracking method with the pointing direction and polarization determined by knowledge of the aircraft&#39;s  12  position and attitude using an on-board inertial reference unit (IRU) and knowledge of the location of the satellites  18 . 
     Encoded RF signals are transmitted from the transmit antenna  74  of the mobile system  20  of a given aircraft  12  to an assigned one of the transponders  18   a   1 - 18   a   4 , and transponded by the designated transponder to the ground station  22 . The ground station  22  communicates with the content center  24  to determine and provide the appropriate data being requested by the user (e.g., content from the world wide web, email or information from the user&#39;s VPN). 
     An additional concern that must be taken into account with the system  10  is the potential for interference that may result from the small aperture size of the receive antenna  82 . The aperture size of the receive antenna  82  is typically smaller than conventional “very small aperture terminal” (VSAT) antennas. Accordingly, the beam from the receive antenna  82  may encompass adjacent satellites along the geosynchronous arc. This can result in interference from satellites other than the target satellite being received by a particular mobile system  20 . To overcome this potential problem, the system  10  preferably uses a lower than normal forward link data rate that overcomes the interference from adjacent satellites. For example, the system  10  operates at a preferred forward link data rate of at least about 5 Mbps per transponder, using a typical FSS Ku-band transponder (e.g., Telstar-6) and an antenna having an active aperture of about 17 inches by 24 inches (43.18 cm by 60.96 cm). For comparison purposes, a typical Ku-band transponder usually operates at a data rate of approximately 30 Mbps using conventional VSAT antennas. 
     Using a standard digital video broadcast (DVB) waveform, the forward link signal typically occupies less than 8 MHz out of a total transponder width of 27 MHz. However, concentrating the transponder power in less than the full transponder bandwidth could create a regulatory concern. FCC regulations presently regulate the maximum effective isotropic radiated power (EIRP) spectral density from a transponder to prevent interference between closely spaced satellites. Accordingly, in one preferred embodiment of the system  10 , spread spectrum modulation techniques are employed in modulator  70  to “spread” the forward link signal over the transponder bandwidth using well known signal spreading techniques. This reduces the spectral density of the transponded signal, thus eliminating the possibility of interference between two or more mobile systems  20 . 
     It is also equally important that the transmit antenna  74  meets regulatory requirements that prevent interference to satellites adjacent to the target satellite  18 . The transmit antennas used in most mobile applications also tend to be smaller than conventional VSAT antennas (typically reflector antennas that are 1 meter in diameter). Mobile transmit antennas used for aeronautical applications should have low aerodynamic drag, be lightweight, have low power consumption and be of relatively small size. For all these reasons, the antenna aperture of the transmit antenna  74  is preferably smaller than a conventional VSAT antenna. 
     VSAT antennas are sized to create an antenna beam that is narrow enough to illuminate a single FSS satellite along the geosynchronous arc. This is important because FSS satellites are spaced at 2° intervals along the geosynchronous arc. The smaller than normal antenna aperture of the transmit antenna  74 , in some instances, may create an antenna beam that is wide enough to irradiate satellites that are adjacent to the target satellite along the geosynchronous arc, which could create an interference problem. The likelihood of this potential problem occurring is significantly reduced by employing spread spectrum modulation techniques on the return link transmissions as well. The transmitted signal from the transmit antenna  74  is spread in frequency to produce an interfering signal at the adjacent satellite that is below the threshold EIRP spectral density at which the signal would interfere. It will be appreciated, however, that spread spectrum modulation techniques may not be required if the angular spacing between satellites within a given coverage region is such that interference will not be a problem. 
     In the event of an interference situation arising, the system and method of the present invention makes use of a well-known transmit lobing sequence to quickly identify which one of a plurality of mobile terminals  20  is causing an interference event with a satellite adjacent to a transponded target satellite. The present invention, however, makes use of the transmit lobing sequence in connection with the transmit antenna  74  of each mobile terminal  20  rather than the receive antenna  82 , where the lobing sequence is typically employed in detecting the beam center of the receive beam received by the receive antenna of the mobile terminal. 
     The transmit lobing sequence of the present invention involves having each mobile terminal  20 , one at a time, point its transmit antenna  74  slightly off-axis by a small angle and then transmit one or more signals to the target satellite, in this example satellite  18   a . Referring to FIG. 3, preferably two measurements are taken (points 1 and 3) with the transmit antenna  74  pointed off-axis in the azimuth plane of the transponded target satellite  18   a . Two measurements are then taken with the transmit antenna  74  pointed off-axis in elevation from the transponded satellite  18   a  (points 2 and 4), and one measurement is taken with the antenna pointed directly at the expected location of the transponded satellite. 
     The NOC  26  measures the received Eb/No (ratio of energy-per-bit to noise spectral density) of each signal it receives. During this process the NOC  26  allocates a higher off-axis EIRP density to the mobile terminal  20  to account for the off-axis pointing. This test verifies the mainlobe beam pattern, aircraft pointing accuracy and off-axis power levels. 
     Once one mobile terminal  20  completes the above-described transmit lobing process, then another mobile terminal is commanded by the NOC  26  to perform the same transmit lobing process. This testing of each mobile terminal  20 , in sequential fashion, continues until the NOC  26  detects that a signal parameter, such as the Eb/No (ratio of energy-per-bit to noise spectral density), of the received transmit lobing signals deviates significantly from the expected Eb/No. In this manner, the NOC  26  can quickly determine which one of a plurality of mobile terminals  20  is causing the interference to the adjacent, non-target satellite(s)  18 . It will be appreciated that a variety of other signal parameters, such as carrier-to-noise ratio, could also be used instead of Eb/No to provide an indication of either signal strength of signal quality of the received signals. 
     Using the above described process, a single mobile terminal can be tested and evaluated by the NOC  26  within a time span of 1-5 seconds. A transponded satellite in communication with up to  30  aircraft can be tested and evaluated in less than five minutes. 
     During the above-described process, the NOC  26  further preferably maintains aggregate power spectral density limits for all aircraft  12  having mobile terminals  20  carrying out the transmit lobing sequence. The process for monitoring the aggregate power spectral density (PSD) of the signals transmitted by the mobile terminals is described in detail in co-pending U.S. application Ser. No. 09/728,605 filed Dec. 1, 2000, and hereby incorporated by reference. The NOC  26  also maintains sufficient headroom in the transponder transponding the signals from each of the mobile platforms. The NOC  26  can also use the information gleaned from the transmit lobing process to determine the transmit power settings for the aircraft and can then command each aircraft to have its mobile terminal transmit in accordance with the power setting determined by the NOC. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification and following claims.
A method for rapidly monitoring and detecting a mobile terminal causing RF interference with one or more satellites orbiting in the vicinity of a target satellite, from a plurality of mobile terminals accessing the target satellite. Each mobile terminal performs a transmit lobing signal sequence and a ground-based component in the form of a Network Operation Center (NOC) analyzes the signals transmitted by each mobile platform. The NOC determines a ratio of energy-per-bit to noise spectral density (Eb/No) value for the signals transmitted by each mobile platform and compares these values to a predetermined Eb/No value. If the Eb/No values for a given mobile platform deviate by more than a predetermined magnitude from the predetermined Eb/No value, then the NOC identifies that given mobile platform as causing the unintended RF interference with the adjacent satellite(s).