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Site Positioning Accuracies Using
GPS; Y Code Receivers and Real-time
Differential Options
Karl E. Brown1
Abstract.-A
majority of the current nine DO1 Bureaus utilize the Department of
Defense created Global Positioning System in conducting their official activities.
Accuracies span a continuum from multi- to sub-meter positioning. As non-military
users, civilian bureaus have been limited to the use of the Standard Positioning Service
(SPS) with the effects of Selective Availability limiting [uncorrected] horizontal accuracy
to approximately 60-100 meters for real-time point positioning. Current differential
correction practices can correct field points to 1-3 meters accuracy, either after the hct,
or in real-time over low power radio links, or by subscription to a correction service.
Comparatively, the Precise Positioning Service (PPS) delivers 16 meter horizontal
accuracy anywhere in the world, with typical North American positioning experience at
10 meters or better. The authority to utilize the PPS was granted to DO1 by the Dept.
of Defense (DoD) on Aug. 26, 1994. Unfortunately, although commercial real-time
broadcast services can allow a user to obtain 1-3 meters positioning accuracy, a current
significant lapse in service into many wildland areas does not fully support the needs of
the DO1 Bureaus. Essentially, these services are based on urban demand and urban
subscription income, with little evidence of future planning to support users on the
remote lands administered or studied by D01. Since Natural Resources and
Environmental study sites may occur in either wildlarzd or urban etwironmentJ., both
autonomous PPS, an.d SPS receivers with real-tim,e d$?2reiztial can m.eet the positioni~lg
needs of DOI users needing to map, invmtory, or obtain geor@renct?d data on tli.~se
sites. Current studies for both the US Coast Guard and the Federal Aviation
Administration have proposed real-time GPS augmentation systems. Again, the proposed
systems are not sufficient to reach the wildland DO1 users until full augmentation by the
FAA occurs in late 1998. That full system ASSUMES approval of wide area correction
signal distribution. The DoD maintains that such a system might violate National
Security issues, and currently does not support the distribution of unencrypted d(ferentiu1
correctors over a real-time system. The DoD has recognized a positioning need in the
civilian agencies for 16 meter accuracy, and has made the PPS and the military PLGR
receiver available to Federal civilian users. Other PPS capable receivers are availahlt:
in addition to the PLGR; however, they are more expensive. In order to use the PPS,
an encryption key is utilized, antl the receivers must be keyed annually. The National
Security Agency (NSA) manages the keying materials. The keyed material is classified
as Secret [crypto], and a chain of custody with appropriate clearances is required.
Typically, NSA wants to limit the number of users of keyed material, so creating a DO1
cooperative site in 1995 would allow the DO1 to concentrate it's access for keyed
material to a secure site while still delivering the needed capabilities to the field. A full
range of accuracies are available to DO1 users, depending on varying costs, hardware antl
software installations, and traininglsecurity requirements.
'~utionwideGPS Coordinufor, USDI National Biological Service, Ilenver, CO