There must be a relatively clear "line of sight" between the GPS antenna and four or more satellites.
However, the system does have some limitations. GPS can provide worldwide, three-dimensional positions, 24 hours a day, in any type of weather. This position is displayed on the datalogger and saved along with any other descriptive information entered in the field software. The GPS receiver knows where each satellite is the instant its distance is measured. The receiver performs mathematical calculations to establish the distance from a satellite, which in turn is used to determine its position. Ideally, these satellites should be distributed across the sky. How GPS WorksĪ GPS receiver must acquire signals from at least four satellites to reliably calculate a three-dimensional position. With mobile GIS, data is directly accessible to field-based personnel whenever and wherever it is needed. To facilitate the flow of information to and from the field, mobile GIS solutions leverage advances in wireless technology and the Internet. Field inspectors, maintenance teams, utility crews, and emergency workers all require timely access to enterprise GIS data so they can make informed decisions. Because GPS provides accurate location information in the field, it is an essential component for mobile GIS. Mobile GIS accesses enterprise GIS in the field. GPS provides an excellent tool for validating features, updating attributes, and collecting new features. By verifying the location of previously recorded sites, GPS can be used for inspecting, maintaining, and updating GIS data. It provides accurate positions for point, line, and polygon features.
GPS is an excellent data collection tool for creating and maintaining a GIS. This positional information can be used in many applications such as mapping, surveying, navigation, and mobile GIS. With this data and information stored internally, the receiver can calculate its own position on earth. The GPS receiver uses these signals to determine where the satellites are located. The user segment, comprised of both civilian and military users worldwide, acquires signals sent from the NAVSTAR satellites with GPS receivers. This ensures that each satellite is transmitting accurate information about its orbital path. These changes are forwarded to the Ground Antennas and transmitted to each satellite daily. The Master Control Station calculates any changes in each satellite's position and timing. The Monitor Stations passively track each satellite continuously and provide this data to the Master Control Station. One Master Control Station, five Monitor Stations, and Ground Antennas comprise the control segment. Each satellite contains several high-precision atomic clocks and constantly transmits radio signals using a unique identifying code. The space segment includes the 24 operational NAVSTAR satellites that orbit the earth every 12 hours at an altitude of approximately 20,200 kilometers. GPS encompasses three segments—space, control, and user. GPS is a satellite-based positioning system operated by the United States Department of Defense (DoD).
The difference is applied to the GPS data recorded by the roving GPS receiver. The base station receiver calculates its position based on satellite signals and compares this location to the known location. The underlying premise of differential GPS (DGPS) requires that a GPS receiver, known as the base station, be set up on a precisely known location. Combining both methods provides flexibility during data collection and improves data integrity. Although both methods are based on the same underlying principles, each accesses different data sources and achieves different levels of accuracy. Differential correction can be applied in real-time directly in the field or when postprocessing data in the office. Differential correction techniques are used to enhance the quality of location data gathered using global positioning system (GPS) receivers.