Around The World For 26 Years - A Brief History Of The Nga Monitor Station Network

The Defense Mapping Agency, a predecessor of the National Geospatial-Intelligence Agency (NGA), founded the Global Positioning System (GPS) Monitor Station Network (MSN) in December 1985. Since then, the NGA MSN has collected GPS observables and related data in support of the publicly available NGA precise ephemeris and the WGS 84 reference frame products. More recently, the data collected by the MSN has played an integral role in improving both the monitoring and the quality of the GPS broadcast ephemeris. In this latter role, the MSN not only supports the positioning and navigation needs of the US DoD, but also of all GPS users throughout the world. The MSN is one of the oldest global GPS tracking networks, celebrating its 25th anniversary in December 2010. We will provide an overview of the evolution of NGA's MSN, which is an important part of the enabling infrastructure of GPS.We view the evolution of the network as primarily a response to the the evolution of the MSN mission. The changing mission of the network has driven the evolution of facets of the system and is still doing so. While the evolving mission and network may not be apparent to the general users, our discussion of the history will focus on highlighting those topics that have an impact on the broader GPS user community. Specifically, we feel it is important to address the following questions in our discussion.How did the MSN come to contribute to the production of the GPS broadcast ephemeris and the monitoring of GPS performance? What has the impact been?Why is the distribution of the MSN tracking network important, and how has it evolved?How and why did the MSN evolve from manned stations that collected and forwarded data on a daily basis to an automated near-real-time network operating under central control?Which observables are collected by the MSN, and how has the quality of the observables changed?We will show how the evolution of the MSN was influenced by, and responded to, the greater backdrop of the evolution of the GPS system as a whole.