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Tailoring GPS for the Forest Service: MTDC's Testing and Evaluation Program

Current Program Objectives

The MTDC GPS program is directed by a steering committee that meets annually to review progress and provide direction for the program. The committee includes individuals from across the country who serve as representatives of their region or resource area. The steering committee's current guidance for MTDC's GPS program is to:

  • Provide field support for GPS technology.
  • Test and evaluate new GPS receivers and technology.
  • Provide support for PPS GPS receivers used by the Forest Service.
  • Provide support for GPS technical approval and blanket purchase agreements.
  • Maintain the Forest Service national GPS Web site.
  • Support new applications of GPS.
GPS Receiver Testing and Evaluation

The focus of MTDC's GPS program has shifted as the technology has matured. In the early days, receivers were changing dramatically and the Forest Service was one of the major users of the equipment. MTDC helped test and evaluate the equipment and provided specifications to the manufacturers so their equipment met the agency's needs. As the technology has matured, MTDC's ability to steer development has decreased. However, Forest Service personnel still need to know the capabilities of GPS receivers, especially when they are used under a forest canopy. Now MTDC's GPS program focuses mainly on product evaluation (figure 8).

Most manufacturers test their receivers in open canopies and report the results as expected accuracy. This accuracy can't be achieved when the GPS receiver is used under a forest canopy. MTDC tests many new GPS receivers at Forest Service test courses and posts the tested accuracies in a spreadsheet that is available at MTDC also tests GPS receivers for their easeof- use, ruggedness, and other characteristics that are important for field users. Additionally, MTDC will evaluate new equipment such as cameras and personal digital assistants that include GPS receivers and other new technologies incorporating GPS.

Photo of two men in a forested area testing multiple GPS receivers.
Figure 8—MTDC GPS Program Leader Dick Karsky
(foreground) testing several GPS receivers.

CIO GPS Technical Approval and Blanket Purchasing Agreement

Forest Service employees must have technical approval to purchase GPS receivers and associated software (Forest Service Manual, section 6615.01c1, USDA Moratorium on Information Technology Acquisitions). MTDC works closely with the Forest Service Chief Information Office (CIO) mobile computing program to ensure that the GPS equipment and software listed in the standard technical approval meet the needs of Forest Service employees. Otherwise, individual technical approvals would have to be completed for specific hardware or software, sent to the CIO, and reviewed by qualified personnel. The process is time consuming and costly to the agency. The CIO, with MTDC's assistance, is preparing blanket purchase agreements for GPS and other mobile computing technologies.

National Forest Service GPS Web Site

MTDC maintains and hosts a national Forest Service GPS Web site. The Web site ( includes:

  • Links to CORS base stations operated by the National Geodetic Survey, Trimble Navigation Limited, and the Forest Service. The Web site includes the database for the Forest Service base stations, allowing users to postprocess GPS data for improved accuracy.

  • A section with GPS training material assembled by Forest Service GPS experts and trainers over the years.

  • A section on GPS hardware and software that includes links to the latest hardware and software available from companies, training information, software upgrades, and user FAQs (Frequently Asked Questions).

  • A section with Forest Service GPS receiver testing reports and a spreadsheet with the latest GPS receiver accuracies based on Forest Service tests.

  • A section with a table of the latest regional GPS and base station coordinators.
Aerial Spraying

For pilots to apply insecticides, herbicides, and fertilizers accurately, safely, and efficiently, they must be able to identify spray plot boundaries and position the aircraft correctly for each spray swath. Atmospheric conditions such as temperature, wind direction, and windspeed affect the drift of spray droplets. Systems that incorporate GPS and meteorological equipment in the aircraft's guidance system can optimize aerial application. Some systems integrate mechanical information such as spray pressure, flow rate, and droplet size to help account for spray drift (figures 9a and 9b).

MTDC has evaluated and tested precision aircraft spray systems incorporating GPS for more than a decade. Systems continue to become more sophisticated, incorporating new technology and modeling systems that improve the spray accuracy, even on sprayers used on all-terrain vehicles (ATVs, figure 10) and utility vehicles (UTVs).

Photo of a plane spraying a liquid substance.
Figure 9a—Aerial applicators now use GPS systems when spraying.
The GPS coordinates of the area being sprayed, windspeed, and wind
direction are used to put the spray aircraft on a path that ensures
the spray is applied where it belongs.

Photo of a man using a GPS in an aircraft.
Figure 9b—The GPS unit's screen is mounted where the pilot can use it
to position the aircraft precisely where it needs to be.

Photo of a sprayer for ATVs that uses a GPS to estimate ground speed which allows a set rate of herbicide that is distributed.
Figure 10—MTDC has developed a sprayer for ATVs (and UTVs) that
will apply a set rate of herbicide even when the ATV speeds up or slows
down. The controller uses GPS information to estimate ground speed.
That estimate is used to adjust the sprayer's pressure.