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Space




Wideband Gapfiller System

Communications Payload

The space segment will support communication services in two military frequency bands: X-band and Ka-band. The Wideband Gapfiller Satellite payload shall be capable of supporting at least 1.2 Gbps aggregate simplex throughput. The Gapfiller satellites will operate in X band and in WGS Broadcast Ka band, similar to the Phase II GBS in service today, in order to interoperate with existing and new X band and GBS terminals. The Gapfiller satellites will also provide a new two-way military Ka-Band capability to support the expected military mobile/tactical two-way Ka terminal population with greatly increased system capacity. Each Wideband Gapfiller Satellite orbital configuration will provide services from 65 North latitude to 65 South latitude and for all longitudes accommodated within the field of view of the satellite. As an objective, the satellite will provide services to 70 North latitude. X-band services will augment services provided by DSCS III satellites. The Ka-band services will augment broadcast service provided by GBS payloads on UHF Follow-On satellites; the Ka-band services will also support two-way network services besides broadcast. The Gapfiller satellites also will support services that require crossbanded connectivity: X-band uplinks to Ka-band downlinks and Ka-band uplinks to X-band downlinks.

Capacity: WGS will offer 4.875 GHz of instantaneous switchable bandwidth. The system will provide capacity ranging from 1.2 Gbps to more than 3.6 Gbps to tactical users, depending on the mix of ground terminals, data rates and modulation schemes employed. Thus, each WGS can supply more than 10 times the capacity of a DSCS III Service Life Enhancement Program (SLEP) satellite.

Working with Boeing, Harris is developing the Ka-band antennas for the next generation Wideband Gapfiller Satellite (WGS). The 10 steerable, solid graphite offset antennas on each satellite provide the critical communications link to user terminals, enabling WGS satellites to securely transmit and receive digital-quality voice, data and imagery. The WGS Ka-band antennas will be ready to integrate onto the Boeing 702 satellite bus, providing a relatively simple plug-and-play interface.

Coverage: The WGS design includes 19 independent coverage areas that can be used throughout the field of view of each satellite to serve warfighters between 65 degrees North and South latitude. This includes eight steerable/shapeable X-band beams formed by separate transmit and receive phased arrays; 10 steerable Ka-band beams served by independently steerable, diplexed gimbaled dish antennas, including three with selectable polarization; and one X-band Earth coverage beam.

A Wideband Gapfiller Satellite will be capable of providing coverage for terminals in several distinct regions located anywhere in the Earth field of regard. The distinct geographic regions are contained within the field of regard. The density of terminals and networks in these regions can vary according to the distance from the focal area of a conflict or operation. The total number of coverage areas defined here will contain focal areas in a theater as well as regions outside the theater that support operations therein. The Gapfiller satellites will support Ka-band terminals located in several Narrow Coverage Areas and in at least one Expanded Narrow Coverage Area. The Gapfiller satellites will provide two-way and broadcast services within Narrow Coverage Areas to deployed tactical forces in theater as well as to fixed gateways, broadcast injection sites, satellite control sites, and out-of-theater tactical users such as air bases and naval battle groups. The Expanded Narrow Coverage Area is several times larger than the Narrow Coverage Area.

Connectivity: The enhanced connectivity capabilities of WGS enable any user talk to any other user with very efficient use of satellite bandwidth. A digital channelizer divides the uplink bandwidth into nearly 1,900 independently routable 2.6 MHz subchannels providing any-coverage-to-any-coverage connectivity (including X-to-Ka and Ka-to X crossbanding) for maximum operational flexibility. In addition, the channelizer supports multicast and broadcast services and provides an extremely effective and flexible uplink spectrum monitoring capability for network control.

The Gapfiller satellites will support a variety of network topologies that include broadcast, hub-spoke, netted, and point-to-point connectivities. Limited protection against jamming or interference will, in general, only be possible for those communications networks that employ modems with modulation schemes capable of providing protection against jamming. In certain situations, gain discrimination that may be inherent in the design and emplacement of the Gapfiller satellite antenna patterns may also provide some measure of protection against jamming and interference sources located at various distances from friendly forces.

Protection of Gapfiller satellite communication services against electronic attack (e.g., jamming) and electronic support (e.g., signal interception) will be provided by the Universal Modem (UM) operating at select user terminals. Similarly, protection of Gapfiller satellite in-band command links will be provided via the frequency and geographical diversity of the command channels and SC2C locations as well as by Government furnished cryptographic equipment operating within the Gapfiller configuration and control elements at the X-band and Ka-band control terminals. Additional protection will be provided by the format and protocols implemented within the command channels to ensure that only "valid" commands are sent and received. A command is "valid" if it can be successfully decrypted and decoded by the satellite.

AFOTEC completed an early operational assessment (EOA) of the Wideband Gapfiller Satellite (WGS) system September 2000. The EOA noted that the complexity of the crossbanding between X-band and Ka-band on board the satellite, along with the concurrent development of the Gapfiller Satellite Configuration Control Element (GSCCE) with the automation upgrades of the Satellite Operations Center (SOC) and Defense Satellite Communications System (DSCS) Operations Center (DSCSOC) networks posed risks to successful WGS development and implementation.



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