A Differential Global Positioning System (DGPS) is an enhancement to the Global Positioning System (GPS) which provides improved location accuracy, in the range of operations of each system, from the 15-meter nominal GPS accuracy to about 1-3 cm in case of the best implementations.
Each DGPS uses a network of fixed ground-based reference stations to broadcast the difference between the positions indicated by the GPS satellite system and known fixed positions. These stations broadcast the difference between the measured satellite pseudo ranges and actual (internally computed) pseudo ranges, and receiver stations may correct their pseudo ranges by the same amount. The digital correction signal is typically broadcast locally over ground-based transmitters of shorter range.
Ground control points (GCP’s) are locations on the surface of our planet with a known X,Y (e.g. latitude and longitude) and Z (e.g. height above mean sea level in meters). In order to use GCPs in the ortho-rectification process, or for any Survey application, they need to be extremely accurate. One cannot collect them with a low-priced handheld global positioning system (GPS) unit or with a smart phone. What is required is a survey-grade GPS unit which is often referred to as a differential GPS unit. These units work by locking on to multiple GPS satellites in space that then triangulate your position on the ground. The longer you wait, the more accurate your position will be calculated as more satellites lock onto the unit. The position of each GPS satellite is then verified and corrected by a series of ground stations that monitor their orbit, velocity and direction of movement, usually called as post processing of the DGPS observations using a suitable software.
Post-processing is used in Differential GPS to obtain precise positions of unknown points by relating them to known points such as survey markers. The GPS measurements are usually stored in computer memory in the GPS receivers, and are subsequently transferred to a computer running the GPS post-processing software. The software computes baselines using simultaneous measurement data from two or more GPS receivers.
The baselines represent a three-dimensional line drawn between the two points occupied by each pair of GPS antennas. The post-processed measurements allow more precise positioning, because most GPS errors affect each receiver nearly equally, and therefore can be cancelled out in the calculations. Differential GPS measurements can also be computed in real time by some GPS receivers if they receive a correction signal using a separate radio receiver, for example in Real Time Kinematic (RTK) surveying or navigation.
Real-Time Kinematic (RTK) positioning is a satellite navigation technique used to enhance the precision of position data derived from satellite-based positioning systems (global navigation satellite systems, GNSS) such as GPS, GLONASS, Galileo, NavIC and BeiDou. It uses measurements of the phase of the signal's carrier wave in addition to the information content of the signal and relies on a single reference station or interpolated virtual station to provide real-time corrections, providing up to centimeter-level accuracy. With reference to GPS in particular, the system is commonly referred to as carrier-phase enhancement, or CPGPS.
“Prashant Surveys” owns 15 numbers of Survey grade, dual frequency Trimble & Leica RTK DGPS (Differential Global Positioning system) receivers viz. Leica GS14, Trimble R8, 5800 & 4800 receivers for establishing the precise Ground Control Points (GCP’s) upto the accuracy of + / 5 mm on the ground. All the GCP’s are observed in static mode for sufficient duration as per industry standards, by triangulation method, in order to obtain the best results. The control points can also be obtained by RTK method for projects requiring ortho rectification of satellite images, requiring relatively lesser accuracy about 2 to 5 cm.
The post processing of collected DGPS static observation data is done by using required Datum / projection and suitable post processing software’s like Leica Geo Office, Leica Infinity, Trimble Business Centre or sometimes AUSPOS which is a free online GPS data processing facility provided by Geoscience Australia. The processed points data (Pt. ID., Latitude, Longitude, Elevation, X, Y, Z) is submitted in various formats like MS Excel, AutoCAD dwg files, pdf files, Google KML files as per the requirements of the client.