The SLE transfer services, developed by the Consultative Committee for Space Data Systems (CCSDS), provide a standard method to transport spacecraft forward and return data between various tracking stations, mission operation control centers and data-user facilities.

This standardization of the interfaces between the various facilities permits re-use of systems for successive missions and eliminates the development costs of mission-specific implementations.

Mission risk is reduced since standard SLE services facilitate the rapid substitution of ground stations in the event of a failure. Since the SLE protocols run over existing communications infrastructure and utilize TCP/IP protocols, they help integrate Space Data Systems into the global communications network.

INTEGRAL was launched in October 2002 and is transferring data between the NASA Deep Space Network (DSN) ground stations and the ESA European Space Operations Center (ESOC) by using the CCSDS Recommendations for Forward Command Link Transmission Unit (F-CLTU), Return All Frames (RAF), and Return Channel Frames (RCF) transfer services.

These services utilize the existing CCSDS Recommendations for packet telemetry and command for the space link and extend their transfer through the ground system. The F-CLTU service enables control center operators to provide CLTUs for uplink via an established forward space link physical channel to the spacecraft.

The RAF service provides the stream of telemetry frames from a single space link physical channel to spacecraft operators and other users. INTEGRAL is also utilizing the RCF service that delivers a Master Channel or specific Virtual Channels specified by the service.

RAF, RCF, and F-CLTU require implementation of SLE Service Management that controls the scheduling and provision of these transfer services. An ad hoc management scheme is being utilized for the operational missions while the CCSDS Recommendation for Service Management is being developed.

SLE services enhance interoperability by establishing a standard for services to be used in the area where most cross-support activity occurs � between the tracking stations or ground data handling systems of various organizations and the mission-specific components of a mission ground system. In addition to the INTEGRAL mission, ESOC and DSN will use the SLE transfer services for the Rosetta and Mars Express missions scheduled for 2003.

Other current and planned implementations of SLE services include:

• Deutsches Zentrum f�r Luft- und Raumfahrt e.V. (DLR) � DLR will implement SLE services in two of its ground stations in Germany to support the TerraSAR and SARLupe missions.
• National Space Development Agency of Japan (NASDA) and Institute of Space and Astronautical Science (ISAS) � Cross support will be provided by SLE services for the MUSES-C, Solar-B, and DRTS missions.
• British National Space Centre (BNSC) � A prototype SLE system has been developed by VEGA Group PLC and is installed in a QinetiQ ground station in Scotland. The Rutherford Appleton Laboratory (RAL) is planning to install a similar system to provide cross support.
• NASA Goddard Space Flight Center (GSFC) � A study team has investigated the phase-out of the NASA ground communications services based on Nascom blocks and concluded that CCSDS SLE meets the objectives of future science missions for the Space Network (SN), Ground Network (GN) and Deep Space Network (DSN). NASA now requests that Project Operations Control Centers utilize a standard SLE services interface for transferring data to and from DSN and SN sites.

The CCSDS consists of technical representatives from the major space agencies of the world who develop standard techniques for handling space data that enhance cross-support, interoperability, and science information interchange.

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