Alternative Architectures for Reduced Age of Data

R. Haddad, J.R. Berg, B.B. Yoo

Abstract:	Fifteen minute Navigation Age of Data is a capability that is being considered by both GPS and Galileo. Two broad categories of GPS architectures are examined for delivering this capability: ground antenna-based and crosslinkbased. In both cases, the Control Segment clock and ephemeris processing are similar. The differences are in how the results get transmitted to the satellites. In the ground-based architecture, new navigation data is uploaded directly to every satellite in the constellation every 15 minutes. For a constellation of 30 satellites, this requires extensive ground resources for contacting the satellites and for scheduling and managing the contacts. The crosslink-based alternatives rely on the satellites being equipped with crosslinks for inter-satellite communication, which reduces the number and complexity of ground to space contacts, but requires a crosslink network in space to disseminate the uploads across the constellation. The paper identifies the components of each of the two types of architectures that are critical to maintaining the age of data below 15 minutes. A methodology is described for determining the requirements for a ground network that can deliver 15 minute age of data. The methodology shows that a ground-based architecture would need to include a network of 10 ground antennas, each capable of 8 simultaneous contacts. The ground antenna-based architecture is then examined to identify the key technological and programmatic challenges, which are then compared to challenges associated with a crosslink-based architecture. Another variant in the architectures is then examined, namely the state-vector approach to uploading navigation data, and it is compared to the upload approach currently used. Benefits from the state vector approach, which include reduced upload size, reduced on-board memory requirements, and increased responsiveness to events affecting satellite attitude are then contrasted with the challenges of maintaining synchronization between space-based predictions and predictions computed on the ground, including synchronization of reference frames and associated updates.
Published in:	Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009) September 22 - 25, 2009 Savannah International Convention Center Savannah, GA
Pages:	1519 - 1529
Cite this article:	Haddad, R., Berg, J.R., Yoo, B.B., "Alternative Architectures for Reduced Age of Data," Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009), Savannah, GA, September 2009, pp. 1519-1529.
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