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

10. 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”

11. 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

12. 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

13. 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

14. 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