US Department of Agriculture, USDA Forest Service, Technology and Development Program Banner with Logos.
Images from various aspects of the T&D Program.
HomeAbout T&DT&D PubsT&D NewsProgram AreasHelpContact Us
  T&D > T&D Pubs > Tailoring GPS for the Forest Service: MTDC's Testing and Evaluation Program T&D Publications Header

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

History of MTDC's GPS Program

MTDC has been working with GPS technology since 1983 when the Washington Office engineering and timber staffs asked the center to investigate potential uses of GPS in resource management. Since then, MTDC has worked closely with GPS equipment manufacturers to evaluate and test receivers for use by Forest Service employees (figure 2). The GPS program at MTDC was officially chartered in 1989.

Photo of a man in a forested area using the old, bulky, heavy GPS.
Figure 2—In 1988, GPS receivers were bulky, heavy, and expensive.
It sometimes took hours to determine a position. Now hand-held GPS
receivers that cost just a few hundred dollars can do an even better job.

History of GPS

The U.S. Department of Defense (DOD) launched the first GPS satellite in 1978. Seventeen years after the launch of the first test satellite, the satellite constellation was fully operational. It included 24 NAVSTAR satellites: 21 operational satellites, and 3 spares.

The satellites are arranged in six orbital planes inclined 55 degrees relative to the earth's equator and separated by 60 degrees from one another. Four satellites are in each plane. The satellites orbit at an altitude of 12,550 miles (20,200 kilometers), completing each orbit in about 12 hours. This arrangement ensures that at least four satellites will always be in view above the horizon at any time from any point on earth.

The broadcast signal identifies each satellite, its status, its location, and the time. This precise time is used by the ground-based GPS receiver to calculate the distance to the satellite. The receiver determines the time of broadcast and the position of the satellite at the time of broadcast using the satellite's identification and an almanac stored on the GPS receiver. The difference between the broadcast time and reception time is used to determine the distance to a given satellite. If distances can be calculated to a minimum of four satellites, the receiver's location can be determined in three dimensions. If more satellite signals are available, accuracy can be increased.

The signal is broadcast from each satellite on two different codes: the precise P(Y) code for military users and the course/acquisition (C/A) code for civilian users. Until a few years ago, the C/A code was purposely degraded to reduce its accuracy. This degradation, called selective availability (SA), could be turned on and off by the military.

When SA is turned off, civilian GPS receivers are typically 10 times more accurate than when SA is turned on. SA was permanently turned off in 2000. In September 2007, President Bush mandated that future GPS satellites would not have the SA capability.

In 1995, the DOD began allowing some civilian government agencies, such as the Forest Service, to use P code receivers. When the P code is encrypted, as it is under DOD policy, it is referred to as the P(Y) code. The P(Y) code, which is inherently more accurate than the C/A code used by civilians, can only be decrypted by GPS receivers with a valid decryption key.

Differential GPS

Several systems have been developed to improve the real-time accuracy of civilian GPS receivers. These systems are called differential GPS or DGPS. DGPS uses the fixed location of a reference station to determine just how far off the location based on the satellite signal is from the station's surveyed location at any moment. The difference, or inaccuracy, can be transmitted to DGPS receivers, which use the inaccuracy to correct the location determined by the satellite signal. The DGPS systems in the United States include:

NDGPS—This land-based nationwide DGPS system has 86 operational sites, each with a precisely surveyed GPS antenna. When a site is brought online, its GPS receiver calculates the pseudorange (estimated distance) from the antenna to each of the satellites in view. The pseudorange is compared to the actual distance from the receiver's antenna based on the survey. The difference between these values is the correction factor or differential correction. This differential correction is broadcast for users in the coverage area of the NDGPS facility. To use NDGPS, your GPS receiver must be able to receive broadcasts from NDGPS sites. Such receivers are called beacon receivers. The NDGPS system can provide 1- to 3-meter accuracy. The closer you are to the NDGPS site, the better the accuracy.

WAAS—WAAS (Wide Area Augmentation Service) consists of about 25 ground reference stations across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for drifts in the satellites' orbits and their clocks as well as signal delays caused by the atmosphere and ionosphere. The corrected differential message is broadcast through one of two geostationary satellites (satellites with a fixed position over the equator). The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal. WAAS can improve GPS accuracy to within 3 meters.

CORS—Some GPS manufacturers have developed software packages that allow users to improve the accuracy of information that has already been collected. For instance, a recreation specialist can collect GPS points on a trail and process the information later to improve the accuracy. The National Geodetic Survey, an agency of the National Oceanic and Atmospheric Administration, coordinates the Continuously Operating Reference Station (CORS) network. These reference stations (sometimes called base stations) collect correction information from each satellite and store the data in files. Software packages use the information from the CORS files to correct the user's data.

The Forest Service operates several base stations around the country to provide correction information for users in those areas. MTDC maintains and archives the correction information on the Forest Service's GPS Web site (figure 3, http://www.fs.fed.us/database/gps).

Screen shot of the Forest Service GPS web site.
Figure 3—MTDC maintains the national GPS Web site for the
Forest Service.

GPS in the Forest Service

The Forest Service began to use GPS receivers for surveying in the mid-1980s. At the time, each receiver cost $75,000. A company provided the receivers, processing software, and support for the Forest Service.

These survey-grade GPS receivers were large, heavy, and power hungry. A 12-volt automotive battery would operate the unit for just 3 to 4 hours. Preparing to use the GPS units required quite a bit of planning because the satellite constellation included only four to five satellites in the early days, the minimum required. The satellites were available simultaneously only for 3½ to 4½ hours a day. It took 45 to 90 minutes to get an accurate position at each station.

As the technology matured, the Forest Service separated GPS activities into two functions: surveying and resource management. The two functions had different requirements. Survey-grade receivers have to track both the C/A code and the carrier phase code (an additional code broadcast from GPS satellites that enables higher accuracy). Doing so requires a clear view of the sky 10 to 15 degrees above the horizon to prevent losing the lock on the satellite signal. This type of receiver is not suited for resource management activities where much of the work is under the forest canopy. Resource management work requires a receiver that tracks just the C/A code and that can tolerate signal interruptions.

Land surveyors in each Forest Service region were responsible for implementing survey-grade GPS technology. MTDC was given the responsibility for implementing GPS technology in resource management activities throughout the Forest Service.

In 1986, a workshop was held for GPS manufacturers, Forest Service users, and representatives from the DOD's GPS Joint Program Office. Nine manufacturers participated in the workshop, as did 25 Forest Service employees representing most resource functions. This workshop gave the GPS industry a better understanding of the requirements and needs of Forest Service field employees.

GPS Test Courses

MTDC established a GPS test course at the Lubrecht Experimental Forest northeast of Missoula, MT, by surveying geodetic control points under a typical forest canopy. These control points were used to evaluate the accuracy of GPS systems being tested. This course, established in 1986, was the first of its kind for the Forest Service. The course also was used to help instructors teach students GPS procedures and methods. This course has been used extensively by MTDC over the years for testing the accuracy of new receivers.

Other test courses have been located and surveyed across the United States (figure 4). In 1991, an eastern test course was established under a heavy canopy of mixed oak, hickory, and beech at the Hoosier National Forest near Bedford, IN. A West Coast test course was set up in 1995 at the Clackamas District of the Mount Hood National Forest. This course is in a stand of dense, second-growth Douglas-fir and western hemlock. In 1998, a northeastern hardwood test course was established at Ridley Creek State Park, PA. In 2000, a test course was established at Powell, ID, under a canopy of old growth cedar on the Montana-Idaho border (figure 5). Another test course was established at the El Yunque National Forest in Puerto Rico to test receivers in a tropical forest ecosystem (figure 6). These courses allow MTDC to evaluate GPS equipment under canopy conditions representative of those that Forest Service users could expect to encounter.

Graphic of the United States. In the graphic there are six stars that indicate where courses are set up to test GPS recivers.
Figure 4—Six courses are set up across the United States to test GPS
receivers under different canopy types. Tests conducted at these
courses help Forest Service employees know how much accuracy to
expect when using GPS receivers under different types of
forest canopies.

Photo of a GPS and an antenna that are set up in a dense forest.
Figure 5—The old growth cedars at the Powell, ID, test course form
an especially dense forest that is a challenge for many GPS receivers.

Photo of a man carrying an antenna and a GPS in a tropical area.
Figure 6—A test course in the El Yunque National Forest in Puerto
Rico allows Forest Service employees to test GPS receivers under
a tropical canopy.

PPS Receiver Support

In 1994, the Forest Service and several other civilian agencies were authorized to acquire and operate military precise positioning service (PPS) receivers through a memorandum of understanding with the DOD. These receivers are accurate within 9 to 10 meters under a dense forest canopy. The security modules in these receivers must have a decoding key installed each year. MTDC established a communication security (COMSEC) account with the National Security Agency. This account allows the center to acquire the classified keying material. In 2007, hundreds of PPS receivers were in use throughout the Forest Service.

Each PPS receiver contains a code module that must receive a "key" to be activated. This key is changed on a date that is kept secret by DOD. Without this key, the receivers cannot access the P(Y) code. All Forest Service PPS receivers (figure 7) must be inventoried and rekeyed at least once a year at the Missoula COMSEC facility. In addition, some receivers must be keyed more than once a year because their battery fails or the key is canceled inadvertently.

Photo of a precision lightweight GPS receiver.
Figure 7—Some Forest Service employees use the
Rockwell PLGR, a precision lightweight GPS receiver.
This receiver needs to be keyed annually by MTDC
so it can receive the military P(Y) code that allows
increased accuracy in dense forests and in deep canyons.