DIFFERENTIAL GPS (DGPS)

DIFFERENTIAL GPS (DGPS)

1. DIFFERENTIAL GPS (DGPS) Mashhood arif

2. Contents Background What is Differential GPS Working of DGPS Components of DGPS Real Time DGPS Satellite Differential Services DGPS Errors Conclusion

3. What is 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 as100 meters With SA removed, a single GPS receiver from any manufacturer can achieve accuracies of approximately 10meters 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

4. Differential GPS (DGPS) DGPS is a method of improving the accuracy of your receiver by adding a local reference station to augment the information available from the satellites It also improves the integrity of the whole GPS system by identifying certain errors Differential GPS uses one unit at a known location and a rover The stationary unit compares its calculated GPS location with the actual location and computes the error The rover data is adjusted for the error

5. Differential GPS (Contd..) To obtain more accurately measurements than a single GPS unit Used to improve positional accuracy and integrity In DGPS systems, the GPS receiver broadcasts the signal it receives from a known position The GPS unit in the field simultaneously receives data from the GPS satellites and the other GPS receiver on ground through a radio signal The GPS error from the known position is compared to that of the GPS receiver in the unknown location

6. Working of Differential GPS The underlying premise of differential GPS(DGPS) is that any two receivers that are relatively close together will experience similar atmospheric error DGPS requires that a GPS receiver be set upon 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

7. Working of Differential GPS (Contd..) 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 post-processing after data capture using special processing software 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 post-processing 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

8. Components of DGPS Basic principle Errors in position are similar for all receivers in local area Corrections » Position correction » Range correction Reference station At known coordinates (in WGS 84 reference system) Must have good view of the sky Transmits corrections to users in the region Mobile station Typically single frequency C/A receivers with radio link Data links Low and high frequency transmitters “Age of correction”

9. Real Time DGPS Real-time DGPS occurs when the base station calculates and broad casts corrections for each satellite as it receives the data The correction is received by the roving receiver via a radio signal As a result, the position displayed and logged to the data file of the roving GPS receiver is a differentially corrected position

10. Satellite Differential Services Another method for obtaining real-time differential correction data in the field is by using geostationary satellites This system obtains corrections from more than one reference station, sends the information to a geostationary satellite for verification The verified information is sent to the roving GPS receiver to ensure it obtains GPS positions in real time

11. DGPS Errors With no DGPS Selective availability S/A dominates, < 100 metres (95%) Other errors amount to several metres With DGPS S/A cancels out Errors reduced in local area (increases with distance) » orbits, clocks, ionosphere, troposphere » can be reduced to a few centimeters Some errors increase (due to differencing measurements) » measurement error, multipath » therefore, this error typically remains at ~ 1 metre In addition, any error in “known” reference station position will be passed on to mobile

12. Conclusion Differential Global Positioning System ( DGPS ) is an enhancement to Global Positioning System that provides improved location accuracy, from the 15-meter nominal GPS accuracy to about 10 cm in case of the best implementations 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 baselinesusing simultaneous measurement data from two or more GPS receivers 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