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Proceedings of Past Forest Nursery Association Meetings:

Global Positioning (GPS) Current Status and Possible Nursery Uses
Karsky, Dick
1999: Ames, IA
The GPS (Global Positioning System) is a worldwide satellite-positioning system that was funded, installed, and continues to be operated by the U.S. Department of Defense. The navigation signals are provided free and can be used by anyone who has the equipment necessary to receive them.

The Missoula Technology and Development Center (MTDC) became involved with GPS technology in August of 1983 when the Timber and Engineering Department, the programís main sponsors, recognized the potential of GPS for resource management activities. The USDA Forest Serviceís Washington Office assigned the project. Over the years, new sponsors have joined the program, which now includes the departments of Engineering, Timber Management, Fire and Aviation Management, Forest Health Protection, Recreation, Law Enforcement and Investigation, and Research, Lands, and Wildlife.

In October 1987 MTDC evaluated a GPS unit at a GPS test and training facility the MTDC had developed at the University of Montanaís Lubrecht Experimental Forest. The performance of the GPS unit was impressive, especially for digitizing a road for mapping purposes. However, that evaluation pointed out a problem with the tree canopy attenuating the signal and reducing horizontal accuracy. A report was issued covering the overall evaluation of the unit.

Other test sites were established: 1 in the eastern hardwood forests of Indiana and 1 in a typical west coast Douglas-fir forest in Oregon. The additional sites allowed the Center to test GPS receivers under a variety of field conditions. The test site on the Hoosier National Forest near Bedford, IN, was established in April 1991. This site has a 7-point polygon under a mixed oak/hickory/beech canopy typical of old-growth central hardwoods. The west coast test site was set up in 1995 on the Clackamas District of the Mount Hood National Forest. The course consists of a 13-point polygon. It is under a heavy canopy of Douglas-fir and western hemlock overstory (24 to 40 inches in d.b.h.) with a vine maple and red alder understory. In 1998 a Northeastern test site was established at Ridley Creek State Park in Pennsylvania. It consists of a 12-point polygon under a poplar, oak, maple, and beech canopy.

Position control for all of these test sites was brought in with GPS. Conventional survey equipment was used to survey in the test sites under the canopy. The test sites allow us to evaluate GPS equipment under a variety of canopy conditions typical of those encountered by Forest Service users. Users can select the GPS receiver best suited for their job based on tests conducted in conditions similar to those they will encounter while working.

The GPS navigation signal has two parts: a Standard Positioning Service (SPS) using a Coarse Acquisition (C/A) or civilian code on the L1 frequency and a Precise Positioning Service (PPS) using the P(Y) code or military signal, available on the L1 and L2 frequencies. The PPS signal is only available to the military and authorized United States Government agencies. The C/A code is intentionally degraded. Its accuracy is only guaranteed to be within 100 meters 95% of the time and within 300 meters 99% of the time. This intentional inaccuracy is called Selective Availability (SA). The PPS receivers have encryption devices that remove selective availability. These receivers provide autonomous accuracy on the order of 9 to 10 meters under the canopy.

A procedure known as differential correction was developed to improve the accuracy of the C/A signal and remove the selective availability. A second receiver is placed over a known point, or a base station is used and positions are recorded there while the roving second receiver is recording positions in the field. Because the base station receiver is on a known point, the difference between its recorded position and the known value is calculated. Position data from the roving receiver can be corrected using these values. Corrections can be made in real time if the correction signal is radioed back to the roving receiver. Otherwise, corrections can be made later by postprocessing the data.

In May 1994 the Forest Service Annex to the Memorandum of Agreement between the Department of Defense and the Department of Agriculture was signed. This agreement authorized the Forest Service and other agencies that signed annexes to use GPS receivers. Figure 1 shows some typical accuracies that can be obtained with PPS receivers and C/A code receivers.
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