Global Positioning Systems

Differential Capable Handhelds

• C/A code family; Real-time (navigation) require a DGPS signal from another source, commonly a base station or service provider for a subscription fee. The US Coast Guard provides DGPS signals for free with the inland coverage expanding and primary CONUS coverage planned for the end of 2001. Double (redundant) coverage is planned for the end of '02-'03. Post-processed data (office after field work) require data acquisition from the base station, software to compute the time-tagged record corrections, and a compilation / display of the corrected file. Your operational requirements dictate real-time or post-processing. If you have to revisit a study sample site, you probably have a navigation requirement. Most likely, you do not want to search for a sample point with a 100 meter estimate of it's position.
• Y-code family; require the identical DGPS signal as the C/A code family above, but operate in Y-code without it, if it is not available or is unstable due to topography or distance from the base station. Currently, the PLGR+96 Federal Version has this capability, but not the DGPS radio equipment. Most PLGR+ receivers held by the military and some civilian agencies cannot process DGPS. Post-processing differential is NOT an option for Y-code receivers.

Military and Navigation Handheld Receivers

• C/A code family; must have DGPS to navigate at 5 meters accuracy. Without differential, this level of navigation will only yield 100 meters of accuracy or position confidence. This may be suitable for hunting, fishing, boating, or general recreation. On a good day, the fix may be tighter than 100 meters, but on a bad day, it could be 150 meters. The AVERAGE is 100 meters, 95% of the time.
• Y-code family; North American experience yields about 8 meters accuracy, open sky without WAGE. With Wide Area GPS Enhancement (WAGE) turned on, these receivers will yield about half that, at 4 meters. Addition of forest canopy, topography, urban canyons, or other obstructions will affect this accuracy just like any GPS receiver, regardless of family type or signal.

Real-time Differential Data Systems

Systems imply combinations of tools and methods, including GPS and data logging software. A real-time DGPS system not only generates a GPS position, but also tracks a base station broadcast signal, and may include details about the position (attributes). As in the handhelds, the receiver instantaneously computes the correction, and applies the differential correction to the displayed coordinate pair for the user. A viable radio link to the base station provider is required, and adds to the cost of this method, like any of the handheld options. In addition, a data logger that uses the calculated coordinate, but adds details about the data point, line, or polygon accomplishes attribution. Typically, the user carries only the data logger in their hands, with the DGPS operating in the backpack or vest-carried unit, with range poles carrying both the GPS and DGPS antennas. Some integrated antennas now allow a single antenna to serve both data needs.

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