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Differential GPS Explained
By Morag Chivers, Trimble

Differential correction techniques are used to enhance the quality of location data gathered using global positioning system (GPS) receivers. Differential correction can be applied in real-time directly in the field or when postprocessing data in the office. Although both methods are based on the same underlying principles, each accesses different data sources and achieves different levels of accuracy. Combining both methods provides flexibility during data collection and improves data integrity.

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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. The base station receiver calculates its position based on satellite signals and compares this location to the known location. The difference is applied to the GPS data recorded by the roving GPS receiver.

What Is GPS?

GPS is a satellite-based positioning system operated by the United States Department of Defense (DoD). GPS encompasses three segments—space, control, and user. The space segment includes the 24 operational NAVSTAR satellites that orbit the earth every 12 hours at an altitude of approximately 20,200 kilometers. Each satellite contains several high-precision atomic clocks and constantly transmits radio signals using a unique identifying code.

One Master Control Station, five Monitor Stations, and Ground Antennas comprise the control segment. The Monitor Stations passively track each satellite continuously and provide this data to the Master Control Station. The Master Control Station calculates any changes in each satellite's position and timing. These changes are forwarded to the Ground Antennas and transmitted to each satellite daily. This ensures that each satellite is transmitting accurate information about its orbital path.

The user segment, comprised of both civilian and military users worldwide, acquires signals sent from the NAVSTAR satellites with GPS receivers. The GPS receiver uses these signals to determine where the satellites are located. With this data and information stored internally, the receiver can calculate its own position on earth. This positional information can be used in many applications such as mapping, surveying, navigation, and mobile GIS.

What GPS Can Do for GIS

GPS is an excellent data collection tool for creating and maintaining a GIS. It provides accurate positions for point, line, and polygon features. By verifying the location of previously recorded sites, GPS can be used for inspecting, maintaining, and updating GIS data. GPS provides an excellent tool for validating features, updating attributes, and collecting new features.

Mobile GIS accesses enterprise GIS in the field. Because GPS provides accurate location information in the field, it is an essential component for mobile GIS. Field inspectors, maintenance teams, utility crews, and emergency workers all require timely access to enterprise GIS data so they can make informed decisions. To facilitate the flow of information to and from the field, mobile GIS solutions leverage advances in wireless technology and the Internet. With mobile GIS, data is directly accessible to field-based personnel whenever and wherever it is needed.

How GPS Works

A GPS receiver must acquire signals from at least four satellites to reliably calculate a three-dimensional position. Ideally, these satellites should be distributed across the sky. The receiver performs mathematical calculations to establish the distance from a satellite, which in turn is used to determine its position. The GPS receiver knows where each satellite is the instant its distance is measured. This position is displayed on the datalogger and saved along with any other descriptive information entered in the field software.

Some Limitations

GPS can provide worldwide, three-dimensional positions, 24 hours a day, in any type of weather. However, the system does have some limitations. There must be a relatively clear "line of sight" between the GPS antenna and four or more satellites. Objects, such as buildings, overpasses, and other obstructions, that shield the antenna from a satellite can potentially weaken a satellite's signal such that it becomes too difficult to ensure reliable positioning. These difficulties are particularly prevalent in urban areas. The GPS signal may bounce off nearby objects causing another problem called multipath interference.

What's the Differential?

Until 2000, civilian users had to contend with Selective Availability (SA). The DoD intentionally introduced random timing errors in satellite signals to limit the effectiveness of GPS and its potential misuse by adversaries of the United States. These timing errors could affect the accuracy of readings by as much as 100 meters.

With SA removed, a single GPS receiver from any manufacturer can achieve accuracies of approximately 10 meters. To achieve the accuracies needed for quality GIS records—from one to two meters up to a few centimeters—requires differential correction of the data. The majority of data collected using GPS for GIS is differentially corrected to improve accuracy.

The underlying premise of differential GPS (DGPS) is that any two receivers that are relatively close together will experience similar atmospheric errors. DGPS requires that a GPS receiver be set up on a precisely known location. This GPS receiver is the base or reference station. The base station receiver calculates its position based on satellite signals and compares this location to the known location. The difference is applied to the GPS data recorded by the second GPS receiver, which is known as the roving receiver. The corrected information can be applied to data from the roving receiver in real time in the field using radio signals or through postprocessing after data capture using special processing software.

Real-Time DGPS

Real-time DGPS occurs when the base station calculates and broadcasts corrections for each satellite as it receives the data. The correction is received by the roving receiver via a radio signal if the source is land based or via a satellite signal if it is satellite based and applied to the position it is calculating. As a result, the position displayed and logged to the data file of the roving GPS receiver is a differentially corrected position.

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