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The Global Positioning System (GPS) is a space based navigation system maintained and operated by the Department of Defense. It consists of a nominal constellation of 24 satellites in high altitude orbits. Its primary mission is to provide passive, real-time, 3-D positioning,navigation and velocity data for land, air and sea based applications (both military and civilian). GPS satellites orbit the earth every 12 hours at an altitude of 14,000 miles from the earth's center.Each satellite is equipped with four atomic clocks, which keep the time to an extremely high degree of accuracy. They each broadcast their precisely timed radio signals through the atmosphere and onto the earth's surface at the speed of light. The signals from each satellite arrive at any particular point on or above the earth's surface at slightly different times. A ground based receiver contains a sensitive antenna and a timetable for the satellites. It measures the time difference between the arrivals of the signals and compares it with the timetable. With the application of trigonometry, the longitude, latitude and altitude of the receiver can be calculated. Three satellites are needed to calculate the longitude and latitude, and a fourth satellite is needed to calculate the altitude. Additional satellites simply increase the accuracy. GPS Receiver Categories GPS Tracking is an essential element of Mobile GIS data gathering. Selection of the proper GPS receiver for a particular project is critical to its success. Receiver selection is to be based on the following criteria: Application for which the receiver is to be used Accuracy requirement for data being gathered Operation environment Cost GPS receivers range from high cost, high accuracy geodetic quality to low cost, low accuracy “resource grade” or recreational models. District owned and operated GPS receivers can be separated into roughly 3 main categories: Resource Grade (Level 1), Professional Grade (Level 2)and Survey Grade (level 3) Resource Grade GPS Receivers (Recreation/Navigation) Currently several types of inexpensive resource grade, non-differentially corrected receivers are in use at the District. The accuracy of this type of hand held receiver is around 7-10 meters.Circular Error Probable (CEP) without differential correction. While accuracy for these receivers has increased since Selective Availability has been turned off by the Department of Defense, they are still not accurate enough to gather coordinates for input into the District’s numerous corporate databases that require high accuracy. These units, however, can be used for trip planning, emergency operations and navigation. Professional Grade GPS Receivers Professional Grade Online GPS Tracking systems are capable of achieving positional accuracy of less than one meter. The receivers also incorporate differential GPS corrections from government operated differential navigation reference stations or the WAAS for highly accurate coordinate data gathering. Survey Grade GPS Receivers Survey Grade GPS receivers are professional GPS receivers used by Surveyors for high precision position determination. Survey grade GPS receivers are extremely accurate and expensive systems that require long acquisition time for high accuracy data gathering. These systems are not covered in this document.
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