Combined navigation and mobile communication satellite architecture

A navigation and mobile communication satellite system including a Geo-Mobile (GEM) satellite (14) and a plurality of Global Position System (GPS) satellites (50). The GEM satellite (14) is in operative communication with at least one ground segment (12) and at least one user segment (16), and includes a GEM processor for providing communication services to the user segments (16). The GPS network is also in operative communication with at least one of the user segments (16) and at least one of the ground segments (12). The GPS network provides navigation data to the user segment (16). Each of the ground segments (12) include a gateway station (28) for receiving the navigation data and uplinking the navigation data to the GEM satellite (14) such that the navigation data is rebroadcast to the user segments (16) at a higher power than from the GPS network alone. In this manner, the GEM system provides a navigation signal at a much higher power than the present GPS system. This signal is of sufficient strength to overcome the effects of a lower power signal jammer, which may be effective against the weaker GPS signal.

TECHNICAL FIELD
 The present invention relates to satellite communication systems, and more
 particularly, to an improved Global Position System (GPS) architecture
 having robust navigation capability in combination with a mobile
 communication network.
 BACKGROUND OF THE INVENTION
 The present GPS navigation network is vulnerable to signal jamming and low
 signal acquisition probability depending upon the environment with which
 the user is in. This is due primarily to the low power signals associated
 with present GPS navigation service. For military applications in
 particular, there is a need for uninterrupted GPS navigation service with
 robust anti-jam capability to small, hand held mobile terminals. There is
 also a need for integrated navigation and mobile communication services
 into a single such terminal.
 DISCLOSURE OF THE INVENTION
 Accordingly, it is an object of the present invention to provide an
 improved GPS network architecture having robust anti-jam capability.
 Another object is to provide integrated communication and navigation
 services to mobile user terminals.
 According to the present invention, the foregoing and other objects and
 advantages are attained by a navigation and mobile communication satellite
 system comprising a Geo-Mobile (GEM) satellite (14) in operative
 communication with at least one ground segment (12) and at least one user
 segment (16). The GEM satellite (14) includes a GEM processor for
 providing communication and navigation services to the user segments (16).
 A plurality of Global Position System (GPS) satellites (50) are also in
 operative communication with at least one of the user segments (16) and at
 least one of the ground segments (12). The GPS satellites (50) provide
 navigation data to the user segment (16). Each of the ground segments (12)
 include a gateway station (28) for receiving the navigation data and
 processing and uplinking the navigation data to the GEM satellite (14)
 such that the processed navigation data is rebroadcast to the user segment
 (16) at a higher power than from the GPS satellites (50).
 Other objects and advantages of the invention will become apparent upon
 reading the following detailed description and appended claims, and upon
 reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION
 The invention combines the commercial Geo-Mobile (GEM) architecture with
 Global Position System (GPS) transmit ability integrated into the existing
 GEM payload operating on the L-band. This architecture, referred to
 NAVCOMM (Navigational & Communication) 10 is shown in FIG. 1. NAVCOMM 10
 supports GPS civil and military missions by providing global reach,
 preventing disruption with robust anti-jam capability, preventing
 unauthorized use while maintaining worldwide civilian access, and aiding
 disadvantaged users in signal acquisition for all environments thereby
 allowing assured navigation or in-flight retargeting. In addition, the
 system offers satellite mobile communications capabilities including:
 voice, facsimile services, supplementary services, netted broadcast voice
 communications, and point-to-point data transmissions.
 The NAVCOMM system uses the high power GEM payload to overcome hostile
 jammers in the theater of operations and provide for users in low
 reception areas. The GPS signal is uplinked from a ground segment 12 to
 the NAVCOMM satellite processor on-board the GEM satellite 14, and
 rebroadcast down to the user segment 16 in the theater of operations. The
 rebroadcast signal aids the user terminals in acquiring the current
 constellation of GPS satellites.
 Users access the system at L-band via terminals that are similar to those
 used with ground-based cellular systems such as handsets 18 or vehicular
 terminals 20. Users can also access the system from fixed terminals 22. In
 this case, however, at least some of the user segments 16 such as the
 handset 18 also include GPS receivers such as are known in the art to
 provide navigation information to the user.
 The ground segment 12 comprises a primary site 24 for satellite control, an
 optional backup site 26, and a plurality of gateway stations 28. The
 primary site for satellite control includes the satellite operation center
 30 as well as a gateway station 36. The satellite operation center 30
 provides telemetry, tracking and control (TT&C) signals to the to the
 satellite 14.
 The optional backup site 26 includes a backup satellite operation center
 38. The backup satellite operation center 38 operates in the exact same
 manner as the satellite operation center 30 of the primary site 24, and
 become operational in the event that the primary site 24 is disabled or
 otherwise not functioning properly.
 Each gateway station 28, including the gateway station 36 of the primary
 site 24, is preferably in operative communication with a public switched
 telephone network (PSTN) 44 and public land mobile network (PLMN) 46. In
 this manner, each gateway station 28 provides communication services to
 the user segments 12 via the GEM satellite 14 as is known in the art. A
 summary of the communication aspects of the GEM satellite system follows.
 The GEM components of the NAVCOMM system 10 will be described with
 reference to FIG. 1. The GEM system is a mobile satellite service that
 includes a geosynchronous satellite 14 integrated with a ground segment 12
 and user segment 16 to provide mobile communication services similar to
 those provided by terrestrial cellular systems, e.g., voice, data,
 facsimile, and other supplementary services. The system ground segment 12
 includes gateway stations 28 that are interfaced with the PSTN and PLMN so
 that mobile subscribers can access users of the PSTN and PLMN as well as
 other GEM mobile subscribers from anywhere within the satellite service
 region. Users access the system via low power, omni-directional, dual or
 single mode, terminals 18, 20 that are similar in design to those used
 with ground-based cellular systems. Dual mode terminals operate with both
 the GEM system and the local ground-based cellular systems. In the present
 example, however, at least some of the terminals also include a GPS
 receiver for receiving navigation data from the GPS network 50.
 The space segment consists of a geosynchronous satellite 14 and integrated
 single large L-band antenna and on-board digital signal processing
 payload.
 The ground segment 12 consists of a primary gateway site 24 and one or more
 gateway stations 28. Satellite operation center 30 provides overall
 control of the network and the satellite as well as communication
 equipment to provide PSTN/PLMN connectivity. Each gateway station 28
 includes an antenna 32 and a transmitter 34 to enable communication with
 the GEM satellite 14 and GPS network.
 The communications payload of satellite 14 includes a single L-band antenna
 aperture 42 for both transmit and receive, and a digital signal processor
 100. The large antenna reflector, along with the multi-feed network, and
 digital beam forming functions, which are performed by the on-board
 digital processor 100, provides the cellular coverage of the mobile
 satellite service area with more than 200 beams that are approximately 0.7
 degrees in beam width. The L-band coverage area is approximately 12
 degrees in diameter as viewed from geosynchronous altitude and could be
 tailored as allowed by the digital beam forming capability of the system.
 The system creates a regional cellular coverage pattern that can be
 deployed anywhere in the world. Mobile users in any given cell are
 assigned a carrier frequency that is unique within that cell that may be
 reused, on a non-interfering basis, within another (nonadjacent) system
 cell. Ku-band coverage is provided to gateways 28 via an area coverage
 antenna. L-band to L-band links are connected by the satellite processor
 100 to support mobile-to-mobile calls at a single hop through to satellite
 14. Satellite 14 also provides Ku-band to Ku-band links for an inter
 network communication subsystem for transmission of control information
 between the primary gateway site 24 and the gateway stations 28. Beacon
 tracking stations uplink special L-band signals that the satellite 14
 tracks to maintain precise pointing of its mobile link beams.
 More than 2000 carriers, each with nominally eight
 time-division-multiple-access (TDMA) signals, are available to distribute
 traffic to mobile users in various beams. The network control and resource
 allocation operations within the primary site 24 and gateway stations 28
 dynamically distribute these TDMA signals among the beams in accordance
 with the instantaneous traffic demand. This traffic demand can be spread
 non-uniformly across all of the beams covering the GEM system service
 region.
 The system utilizes low rate encoded voice transmission. The terminals 18,
 20, 22 may be either single or dual mode. Dual mode terminals allow
 communications either via the satellite 14 or the local terrestrial
 cellular system. Mobile switches at the gateway stations 28 support
 mobility as the users move from beam-to-beam within the coverage area.
 The NAVCOMM system 10 augments the existing GPS satellite network to
 provide more robust GPS communications by combining the communication
 functions and transmit capabilities of the GEM satellite architecture with
 the GPS network.
 The existing GPS satellite network provides highly accurate, real-time
 positioning and timing data. The GPS includes a constellation of
 radio-navigation satellites 50 which continuously transmit precise timing
 and location information to substantially the entire surface of the earth.
 Position detectors located within user terminals 18 acquire several
 transmissions from a corresponding plurality of GPS satellites 50 to
 determine the location of the user terminal 18. In this case, the
 positioning data can be used for navigation, mobile communications,
 in-flight weaponry targeting, as well as digital battlefield
 synchronization, for example.
 In order to make the system compatible with the existing processor design
 used in the GEM satellite system, it is necessary to preprocess the
 navigation signal before it is broadcast up to the GEM satellite. This
 prevents distortion of the wide band navigation signal after it is
 processed by the GEM on-board digital processor.
 FIG. 2 shows a schematic block diagram of the preprocessing hardware and
 software required at the gateway stations 28 to precondition the
 navigation signal before it is broadcast up to the GEM satellite 14.
 Referring to FIG. 2, the GEM processor 100 on-board the GEM satellite,
 expects to receive data in a specific format, i.e., approximately 200
 narrow-band channels arranged in approximately 25 MHz of bandwidth. The
 GEM processor then performs certain linear transformations on the signals.
 These transformations, when applied to the frequency spectrum of the
 wide-band, high data-rate system, (the GPS system) creates undesired
 warping of the frequency spectrum. To accommodate this, a NAVCOMM
 preprocessor (NPP) 102 is positioned between the output of the wide-band,
 high data-rate system 104 and the input to the GEM processor 100. The NPP
 102 is included as part of the gateway stations 28. The advantage of the
 NPP 102 is to reduce the need to build a separate space-based processor
 system for the wide-band, high data-rate system 50 or GEM satellite 14.
 FIG. 3 shows a schematic block diagram of the NPP 102 of FIG. 2. As
 mentioned above, the function of the NPP 102 is to preprocess the
 wide-band, high data-rate signal (the GPS data) in such a way that the
 warping imposed on it by the GEM satellite system processor is nullified
 or canceled out. One embodiment of the NPP 102 is shown in FIG. 3. In this
 example, the NPP 102 includes an analog receive section 102, a digital
 processor 112, and an analog transmit portion 114. The output
 characteristics of the wide-band, high data-rate system consist of a 20
 MHz code-division-multiple-access (CDMA) channel and not more than 5 MHz
 of spectrum divided into several narrow band channels. In contrast, the
 GEM processor 100 expects to receive 128, 200 kHz channels in each forward
 gateway sub-band. Thus, it is necessary to subdivide at least the 20 MHz
 CDMA channel into smaller spectral sub-bands prior to transmit into the
 GEM satellite system.
 Accordingly, the analog receive section 110 is responsible for performing
 the frequency conversions from the wide-band, high data-rate transmit band
 to a lower frequency range suitable for implementing based-band
 processing. Thus, the analog receive section 110 includes a down
 conversion stage, a filtering stage, and an amplification stage, although
 its exact architecture would depend on knowledge of the desired transmit
 frequency plan of the wide-band, high data-rate system.
 The NPP digital processor 112 performs the function of digitizing the
 based-band version of the wide-band, high data-rate signal such that
 discreet time frequency manipulations can take place. This would require
 additional amplification and filtering as well as analog-to-digital
 conversion. The NPP digital processor 112 divides the wide-band, high
 data-rate spectrum into 200 kHz slices compatible with the GEM input
 spectrum channelization. This is accomplished through the use of a Fast
 Fourier Transform (FFT) algorithm, the implementation of digital filters,
 and electronic bookkeeping to keep the spectral portions intact such that
 they can be reassembled at a later time. The NPP digital processor 112
 also includes an analog back end consisting of a digital-to-analog stage,
 an interpolation filter and an amplifier. These components return the
 newly assembled GEM-like spectrum to the analog transmit section 114 of
 the NPP 102.
 The analog transmit section 114 of the NPP functions to up-convert the NPP
 baseband output to the expected GEM input spectrum. Analog transmit
 section 114 accordingly includes an up converter, a filtering stage, and
 an amplification stage.
 Many of the sub-blocks in the NPP processor 102 have counterparts in
 existing systems designed for the GEM satellite family and as such readily
 present themselves to these of skill in the art.
 The advantage of the above-described architecture is that the GEM system
 provides a navigation and timing signal at a much higher power (up to +50
 dB greater) than the current GPS signal. The present system provides a
 signal of sufficient strength to overcome the effects of a lower power
 jammer--one that would be effective against the much weaker GPS signal,
 but not against the GEM supplied signal. Thus, the system permits the GPS
 user equipment such as terminal 18 to acquire a GPS timing and aids that
 equipment in the acquisition of weaker GPS signals. At the same time,
 however, the system presents users with the full range of communications
 capabilities. Moreover, since the system incorporates the existing GEM
 architecture, the system includes the ability to dynamically allocate
 payload resources to support both navigation and communication demands.
 From the foregoing, it will be seen that there has been brought to the art
 a new and improved GPS communication architecture which overcomes the
 problem associated with the present GPS architecture. While the invention
 has been described in connection with one or more embodiments, it will be
 understood that the invention is not limited to those embodiments. On the
 contrary, the invention covers all alternative, modifications, and
 equivalents as may be included within the spirit and scope of the appended
 claims.