Introduction
Continuously operating, fixed NAVSTAR Global Positioning System (GPS) reference stations are being established in increasing numbers by Federal agencies, state, and local governmental organizations, academia, and private industry. These stations are being established to support a multitude of static and dynamic positioning applications involving both real time and after-the-fact position determination. They support the use of both pseudo-range and phase data for positioning, with accuracies ranging from the centimeter level to the several tens of meters level. It is also now possible for the same fixed reference station to satisfy both navigation and positioning requirements.Permanent GPS reference stations are required for pseudo-range positioning to mitigate the effects of Selective Availability (SA) and, even in the absence of SA, to achieve higher accuracies required for some pseudo-range positioning applications. Also, with the advent of better orbits and improved processing methods, it is technically feasible and economically desirable to use a fixed reference station at an existing facility when employing phase data for GPS differential positioning.
Permanent GPS reference stations can involve differing degrees of complexity. The simplest reference station is one which serves a single user, simply recording pseudo-range and/or phase observations and saving these observations for use with field instrument observations for after-the-fact computation. The most complex reference station would be one that records both pseudo-range and phase data, broadcasts correctors and data to those performing real-time positioning and navigation, and stores both types of data for after-the-fact electronic distribution to multiple users.
Numerous Federal agencies are planning or implementing reference stations of various degrees of complexity. These fixed GPS reference stations are a valuable asset to the National Spatial Reference System (NSRS) and other Federal applications involving the National Spatial Data Infrastructure (NSDI).
Mission
The basic mission of the Work Group will be:
1. To prevent duplication of effort and facilitate the multiple use of GPS reference stations to meet all Federal agencies' needs where possible.The following activities will be undertaken by the Work Group:2. To assure that reference stations produce positions compatible with the NSRS coordinate system, i.e., North American Datum of 1983 (NAD 83).
3. To support the GPS Interagency Advisory Council (GIAC) by developing and providing required information concerning fixed reference stations.
1. Federal, state, and local governmental agencies will be monitored to determine current and planned fixed GPS station activities.Large numbers of groups are currently undertaking or planning to undertake fixed GPS reference station activities, and there is a large potential for duplication of effort. Monitoring of current activities and plans must be performed to determined if and how one group's fixed sites could be used by other groups. Thus, a continuing determination of the nature of current activities and future plans is considered essential.2. Procedures will be developed to assure all fixed sites operated by member agencies are tied to NSRS and provide NSRS positions.
3. Standards and specifications will be developed for fixed sites.
4. Common formats for data storage, distribution, and archiving will be developed and implemented through the National Geospatial Data Clearinghouse.
5. Studies will be undertaken to determine how to optimize a single Continuously Operating Reference Station (CORS) system to meet a maximum number of Federal GPS post-processing requirements.
In order for results obtained using different fixed reference stations to be combined with one another and with existing control, it is important that all Federal fixed reference stations be related to the accepted NSRS coordinate system. Standard procedures must be developed for tying fixed reference stations to the NSRS coordinate system so that the accuracy of the tie can be validated.
To assure the compatibility of fixed reference stations established by different groups and to provide guidance on establishment of fixed reference stations, standards and specifications for these stations must be developed.
If there is to be effective multiple use of GPS fixed reference station data, there must be agreement on a common format in which the data are provided and stored. This common format should allow users employing different receivers to combine results. A standard form of the Receiver Independent Exchange (RINEX), currently version 2, format must be developed and made available to GPS software vendors to satisfy this requirement.
Activities are underway to develop a single fixed reference station system which will meet the needs of all the Federal navigation and positioning users. An objective of this Work Group is to determine the requirements to be met to optimize the utility of such a network for positioning applications.
Membership
The Work Group shall be composed of a Chairperson and representatives from interested agency members of FGCS. The Chairperson, FGCS, in consultation with the member agencies, selects the Work Group Chairperson. The official agency representative shall propose Work Group members who will then be officially designated by the Chairperson, FGCS. It is preferred that Work Group members have knowledge of GPS activities underway in their agency. The Work Group Chairperson may propose additional members from FGCS and non-FGCS Federal organizations as deemed appropriate.
Operating Procedures
The Work Group shall meet as often as necessary to accomplish their objectives, but not less than twice each year. Copies of the minutes shall be forwarded to the Chairperson, FGCS.
Administration
The FGCS member agencies have supported the activities of these personnel while engaged in coordination functions; such support will be expected for the Work Group members. The FGCS Secretariat will assist in arranging meeting sites, preparing records of meetings, and finalizing reports and plans for public release.