The Difference Between Handheld GPS Receivers & Surveying Grade GPS Receiversby Richard Asmus
GPS receivers collect signals from navigation satellites in orbit. The signals are available at all locations on Earth, and anyone can use them free of charge. GPS receivers come in a wide variety of models for various professional and recreational uses. Whether a GPS is worn on the wrist, is handheld, is installed in a vehicle or is mounted on a tripod, its accuracy determines its grade. The most accurate GPS is called "survey grade."
The 24 GPS satellites orbiting the Earth each transmit two navigation signals identified as L1 and L2. The pattern sends signals from at least four satellites to all points on Earth. All GPS receivers must pick up signals from at least three to calculate a position. GPS receivers with lesser accuracy use only the L1 frequency. More elaborate units use both the L1 and L2 frequencies to calculate the position more accurately. Handheld units may be designed in either version, but survey grade GPS devices never come in a handheld version.
Most handheld GPS receivers on the market have recreational grade accuracy. You can use them to find your location when enjoying various recreational and sporting events such as running, cycling, skiing, golfing, motorcycling, boating, hiking or camping. Some models show only longitude, latitude and compass bearings, and they must be compared to a map to identify the location. Others have build-in mapping software and superimpose your location onto a selection of maps. Specialized maps include details for areas such as ski resorts and golf courses. Accuracy of recreational GPS receivers usually falls within 50 feet.
Other Handheld GPS Grades
The next level of accuracy for GPS is mapping grade, and that grade is divided into two sections by the USGS: commercial grade and differential grade, with handheld models available in both. Commercial grade uses only the L1 frequency and has an accuracy of about 10 feet. Because their price does not go up radically, people use them for recreational purposes. Differential grade GPS devices use both the L1 and L2 frequencies for an accuracy of about 3 feet. A differential grade GPS device may cost more than 10 times the price of a commercial grade.
Survey Grade GPS
The most accurate GPS device, the survey grade, does not come in handheld models. It requires two units to operate: a base station and a rover, each of which must receive signals from at least four separate satellites for a minimum total of eight. The distance between the base station and rover varies, depending on the particular location. A surveyor is responsible to know that the accuracy specifications of a GPS device is within the requirements of the surveying project. Typically, a survey grade GPS has an accuracy of within 1 centimeter horizontally and 2 centimeters vertically.