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What Is A Differential  GPS Survey?

Differential GPS surveys or DGPS can be said to be a correction technique for GPS in order to enhance its quality of location data, which is gathered using the receivers of Global Positioning Systems (GPS). This technique can be applied in real-time directly in the field or when processing data in the office. Both methods are based on the same underlying principles, but each achieves various accuracy levels by accessing different data sources. A mineral exploration company in Canada may use the DGPS technique for automatic positioning for all their surveys, especially when the terrain is not constant.

The DGPS provides an improved location accuracy from about 15 meters nominal GPS accuracy to about 1-3cm, and DGPS uses a network of fixed ground-based reference stations for broadcasting the difference between the positions indicated by the GPS satellites system and the known fixed positions. The reference stations broadcast the difference between the measured satellite pseudo ranges and actual pseudo ranges. The receiver stations may also correct their pseudo ranges by the same amount. The correction signal is usually broadcast locally over ground-based transmitters of a shorter range. 

For instance, in a geophysical survey, geomagnetic surveys are conducted, and the measurements are compared with geomagnetic measurements obtained from a global network of satellites, which provide an absolute reference against which the geomagnetic survey measurements can be calibrated. This process enhances the accuracy of the geomagnetic survey measurements and produces high-quality geophysical maps. A good mineral exploration company in Canada will use DGPS to provide accurate and precise location data.

Difference Between GPS And DGPS

  • The GPS system’s frequency varies from 1.1 GHz to 1.5 GHz, while in DGPS, it can go as per the agency’s requirements. 
  • GPS uses the time coordinate system, while the other uses the local coordinate system. Also, the cost of GPS is lower than that of DGPS.
  • The instrument range of GPS is global, while DGPS has a local instrument range.
  • Only one receiver is used in GPS, whereas DGPS uses two receivers. 

Advantages Of DGPS

  • DGPS can improve the positional accuracy from 1.5m(with GPS) to around 40cm without any post processing.
  • An advanced form of DGPS, real-time kinematics, uses the satellite’s carrier wave to compare observations from different receivers within the system. This fine-tunes the satellite and receiver clock errors, hence improving positional accuracy.
  • The X, Y, and Z accuracy of DGPS is within millimetres.
  • As compared to the total station survey, DGPS completes the work faster. Also, the total station survey method cannot be done in the presence of thick vegetation since there is no line of sight. 
  • DGPS is used to improve the accuracy of drone surveys.

Real-Time DGPS

This occurs when the base station calculates and broadcasts corrections for each satellite as it receives the data. The roving receiver receives these corrections through a radio signal (if the source is land-based) or a satellite signal (if the source is satellite-based). The corrections are then applied to the position that is being calculated. Thus, the position displayed and logged to the data file of the roving GPS receiver is the differentially corrected position. 

Another way of obtaining real-time differential correction data is by using geostationary satellites, which receive the corrections from more than one reference station. These reference stations collect the base station GPS data and transfer it to a network control center. This center then sends the information to a geostationary satellite for verification, and this verified information is sent to the roving GPS receiver in order to ensure that it obtains GPS positions in real-time. 

Post Processing Correction

Post Processing DGPS data is done through a base GPS receiver that logs positions at a known location and a rover GPS receiver which collects positions on the field. The files from the base rover are then transferred to the office processing software, where they are computed for the correct positions for the rover’s file. A GPS base station provides the data necessary for differentially correcting GPS, and depending on the technology preferred, this data can be downloaded from the Internet. The sources of a base station for post processing are divided into four categories:

  • Public sources include government agencies which collect and store base data. Legal liability and cost recovery are some legitimate concerns of these agencies that affect the decision regarding offering this data to the public. However, the laws may differ from country to country as well as between government agencies.  
  • Commercial sources: This includes some consulting firms and universities that collect the base data, which can be purchased per hour. This is a cost-effective way of obtaining data for post processing.
  • Web-based services: This is an excellent method for large field crews.
  • Base station ownership: This is a flexible way of obtaining data for post processing but may have additional setup costs.

To Conclude

Differential correction is essential for attaining accuracy levels. The main methods of differentially correcting GPS data are widely used to ensure data accuracy.

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