Global Positioning Systems

The DOI user community utilizes the full range of GPS services available, from autonomous 5-20 meter handheld positions, to 3-8 meter real-time navigation, to sub-meter survey applications. Project Center staffs have a long history of GPS use, and they continue to press for access to emerging technologies and methods to make the collection of geospatial data more efficient, timely, and cost-effective. Our DOI activities include access to both the original, traditional civilian side of GPS called the Standard Positioning Service (SPS), and the Precise Positioning Service (PPS), with encrypted Y-code signal access.

DOD turned off the Selective Availability feature that limited the accuracy of the SPS signal in May of 2000, making higher accuracy positioning available to all civil users. The status of Selective Availability affects the needs review for Departmental users of PPS. Many users may move to the Standard Positioning Service and collect data in the 3-10 meter horizontal error range. The Department continues to support science conducted on, near, or around military reservations in both the US and abroad. Jamming will confound these scientists near these facilities, and unannounced (by design) jam testing will continue. Continued use of Y-code will support their mission uninterrupted and allow them more robust positioning options, especially in areas of steep terrain and heavy forest cover because of the signal differences, penetration, and anti-jamming functionality.

Our use of PPS began in 1994 with the Pentagon's approval of our DOI Memorandum of Understanding signed August 26, 1994. We currently support over 1300 PPS receivers, and have to re-encrypt them every 54 weeks to access the PPS. Users of PPS routinely obtain 5-10 meter real-time positions with equipment costing $1,500 to $4,000. The majority of the DOI PPS users are equipped with the Rockwell Precise Lightweight GPS Receiver (PLGR+96); however, a few Project Centers also have the Trimble P(Y) Centurion Receiver. The remaining SPS users own an extensive variety of equipment and mix of manufacturers, from $200 handhelds to $40,000 survey grade instruments. The DOI has a complete user community in the real-time and post-processed Differential Correction mode of data reduction, yielding about 1-2 meters accuracy. The DOI owns and operates several Base Stations for collection of correction files for differential use, and these Project Centers access and serve their local base station data via a variety of electronic services or over the Internet.

WHAT'S NEW
• S/A ramps to zero in May, 2000 (Selective Availability)
• DOI Coordinators will meet in Denver, Colorado in September 2005. This group met last September at ION and compared post-S/A experiences.
• PLGR2 added to procurement information (12 channel, L1/L2 handheld receiver).

What can I get for $$...? Year-end questions on what to buy are best resolved by deciding what resolution you need, followed by when you need it. Are you a real-time user, or can you live with an after-the-fact answer? Are you detail mapping (1-3 meters) with attributes, or are you navigating (5-10 meters) to survey plots or sampling points on a river? Do you need absolute position confidence, or relative on-the-fly positioning?



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