A SPACE-BASED HIGHLY ACCURATE AND CERTIFIED TIME AND FREQUENCY DISTRIBUTION SERVICE FOR THE TELECOMMUNICATION MARKET: SYSTEM CONCEPT AND BUSINESS DRIVERS

摘要

One of the most critical requirements in telecommunication network is the synchronization and synthonization of the nodes of the network. A lack of synchronization can cause malfunctioning in audio-video services such as sound and vision out of sink, picture freezing, dropping of calls, etc. Synchronous Digital Hierarchy (SDH) networks employ a hierarchical structure of clocks for synchronization. Normally an atomic clock is used as a Primary Reference Clock (PRC) while the lower level clocks are Rubidium or Quartz oscillators which can be slaved to a GPS receiver. The generation of the time signal is in this case related to the reception of the signal from the space segment. The bias and the drift of the Rubidium clocks on board the satellites, although kept to a minimum by periodic upload of correction factors, can lead to the generation of different timescales locally at nodes of the network. The influence of the ionosphere and troposphere on the signal transmission and the error in the ephemeris determination have also an influence on the synchronization of the receiver. In the following paper the needs of the stakeholders in the telecommunication field are discussed and the main requirements for the system are gathered. A possible concept for synchronization service based on a space segment is then presented. Such concept is chosen among possible ones that have been proposed in the literature. In particular it is proposed to realize the Time and Frequency Distribution Service (TFDS) using a small Payload (less than 10Kg) on a GEO mission. The time payload which requires a quartz oscillator is kept continuously synchronized to a Precise Time Facility (PTF) on ground. The service is accessed by the user via TDMA. The user is charged a fee for using the service. The encryption of the signal allows a high level of security for the user. The signal generated by the space segment is periodically audited to guarantee and certify its accuracy. The audits are performed by certified bodies. The main business case assumptions are then presented. The business analysis shows that the profitability is linked to the ability of the system to use low cost receiver, to take advantage of synergy with future GEO missions, to provide a reliable service with a high level of accuracy, integrity, availability and continuity of service, to be certified by a competent authority and to become a well-known and recognized time service to complement the time services offered by current and future global navigation systems.