This document provides an overview of differential GPS (DGPS) concepts and techniques. It begins by explaining the primary sources of error in point positioning GPS measurements. It then describes how DGPS uses corrections from a reference station to minimize errors like atmospheric delays and orbital inaccuracies experienced by both the base and rover receivers. Real-time and post-processed DGPS methods are covered. Expected accuracy levels from point positioning, real-time DGPS, and post-processed DGPS are listed. The document concludes by relating the various GPS techniques to common accuracy requirements.
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DGPS Concepts and Accuracy Levels
1. 1
DGPS
Concepts
Learning Objectives
After completing this lecture you will be
able to:
– Explain errors in point positioning
– List accuracy expected from various
GPS measurement techniques
– Explain the process of Differential GPS
– Describe how errors are minimised by
using DGPS
Lecture Outline
• Introduction – context
• Accuracy Issues
• SA
• Dilution of Precision (DOP)
• Error sources
• Satellite Visibility
• Differential Correction
– Post-processed
– Real-time
• Conclusion/Summary
LST
2. 2
Introduction
• 3 Levels of accuracy:
– Point Positioning (Codes)
– Differential Positioning (Codes)
– GPS Surveying (also a differential
technique but uses carrier phase)
• Point positioning uses the code
observable
• Point positioning is subject to many
error sources
• Differential techniques (DGPS) can
minimise these errors
A Typical GPS Position
•Many organizations use GPS
these days
•To query a position we may get
the following:
–Latitude - 41.342565555432134
–Longitude – 114.7841234532983
•This Is Accurate……. Right?
Physical Data Collection Method
•Lat -
41.342565555432134
•Long –
114.78412345329836
When we lose knowledge
on how data was created,
we lose its accuracy
‘context’
16 decimal places ≠ high
accuracy data
LST
3. 3
Primary GPS Error Sources
•Wave path errors (ionospheric
and tropospheric)
•Satellite orbit errors
•Multipath
•Satellite Geometry (High PDOP)
•Satellite Constellation Changes
Meters
S/A (now turned off)
Atmospheric
Ephemeris
Satellite Clocks
0 20 40 60 80 100
Point Positioning Errors
• System-wide errors - DGPS
Correctable
Selective Availability (S/A)
• Turned of May 02 2000
• Government may again degrade the
accuracy
• To prevent hostile forces from using
GPS to full accuracy
• By introducing intentional errors of
timing signals and/or satellite
ephemeris
• Reduced using DGPS
LST
4. 4
-200
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
20
40
60
80
100
120
140
160
0 1 2 3 4 5 6 7 8 9 10
Time of Day (Hours UTC)
InstantaneousError(meters)
Horizontal Error (meters)
Vertical Error (meters)
2 May 2000Colorado Springs, Colorado
ANALYSIS NOTES
- Data taken from Overlook PAN Monitor Station,
equipped with Trimble SVeeSix Receiver
- Single Frequency Civil Receiver
- Four Satellite Position Solution at Surveyed Benchmark
- Data presented is raw, no smoothing or editing
SPS CEP AFTER TRANSITION: 2.8 meters
SPS SEP AFTER TRANSITION: 4.6 meters
May 2000
Satellite Orbit Errors
•Satellite isn’t really where it says
it is
•2-3 meters of error
X2 Y2 Z2
X Y Z
Ephem
eris
(X
Y
Z)
X
Constellation Changes
• Position P3 computed using ranges to satellites
1, 2, 3, 4, 5 and 6
• Position P4 computed using satellites 4, 5, 6, 7
and 8
P3
S1 S2
S3
S4
S5
S6 S7
S8
X XP1
P2
XP4
LST
5. 5
Atmospheric Errors
• Due to the ionosphere and
troposphere, measured range to
satellite is longer
True Range to SatelliteMeasured Range
Ionosphere
Troposphere
• Contributes around 4-5 meters of
range error
Atmospheric Delay
• GPS signals are delayed
as they pass through
the atmosphere
Ionosphere
Baseline not too long
Troposphere
Base
Rover
Receiver/Location Errors
DGPS won’t help:
• Multipath
– Use a ground plane on antenna
– Move away from multipath source
• Receiver Channel noise
– Use a different GPS receiver
• 2-D fix with a bad altitude
– Use Manual 3D when collecting data
• High PDOP
– Wait for the geometry to change
LST
6. 6
Multipath
• GPS computes position by measuring
ranges to satellites
• Receiving a signal reflected off anything,
will result in an incorrect range
• This results in an incorrect position
computation
Dilution of Precision (DOP)
• A measure of Satellite geometry
• Indicates the quality of position
fix
• Can be expressed in different
dimensions
– for example: PDOP, HDOP, VDOP,
TDOP
• Generally not reduced by DGPS
idealized situation
0.04 sec 0.06 sec
Dilution of Precision (DOP)
• Relative position of satellites can
affect error
LST
7. 7
Dilution of Precision (DOP)
• Real situation - fuzzy circles
Point representing position is really a box
0.04 ‘ish sec 0.06 ‘ish sec
uncertainty uncertainty
Area of uncertainty
becomes larger as satellites
get closer together
Dilution of Precision (DOP)
• Even
worse
at some
angles
Satellite Visibility and PDOP
• Use an almanac from a GPS
receiver to calculate best times of
day for GPS.
LST
8. 8
The Proximity Factor
Activity - Quiz
Which of the following errors do you
think can be reduced using DGPS?
• Multipath
• Receiver Channel noise
• High DOPs
• Satellite clock errors
• Atmospheric corrections
• Poor ephemeris data
Differential Correction
• Base Station generates
corrections for all satellites in
view
• Roving GPS receiver uses
corrections to reduce errors
• Differential correction can be
performed in either real-time or
post-processed mode
LST
9. 9
BASE
Time, t
t + 1GPS
Positions
Absolute
Reference
Position
Differential Correction
(Simplified)
• If you collect
data at one
location, there
will be errors
• Each of these
errors are
tagged with GPS
time
ROVER
Time, t
t + 1
?
Differential Correction (Cont.)
• At the same
time, the errors
occurring at one
location are
occurring
everywhere
within the same
vicinity
Satellites Seen
1 2 3 4 5 6 7 8
Satellites Used
1 2 3 4
1 3 5 6
Any Combination of Base satellites
ROVER
Time, t
t + 1
?
BASE
Time, t
t + 1
Differential Correction (Cont.)
LST
10. 10
Base
Receiver
Known
Coordinates
Shift
10m South
30m West
Block Shift Correction
Remote
Receiver
Measured
Coordinates
Apply same
Block Shift
Measured
Coordinates
Final
Coordinates
Base
Receiver
Range Correction
Range
Correction
Remote Receiver
Range
Correction
Example of DGPS CorrectionsExample of DGPS Corrections
Uncorrected
Corrected
LST
11. 11
DGPSDGPS –– BenefitsBenefits
Red = Uncorrected GPS
Green = GPS after
differential correction
Code GPS Errors and DGPS
•Wave path errors (ionospheric and
troposperic) – corrected by DGPS
•Satellite orbit errors - corrected by
DGPS
•Multipath
•Satellite Geometry (High PDOP)
•Satellite Constellation Changes –
corrected by DGPS
DDDD
DDDD
ffff
ffff
f
Activity - Quiz
• List different accuracies you might
expect from:
– Point positioning using C/A code
– Post-processed DGPS
– Real-time DGPS
• Explain two general processes used
to correct positions and minimise
errors using DGPS
LST
12. 12
Base Station Site Requirements
• Clear view to satellites
• Known coordinates
• Clear of transmitters (TV, radar)
• Line of site to rover is not
necessary
Base Station: 15° Rover: 15°
Poor Situation – Satellite
visible to Rover only.
Base Station is not
tracking it.
Possible Problem...
• setting the elevation masks
1°per 100 km distance between base and rover
Base Station: 10° Rover: 15°
Ideal Situation – Satellite
is being tracked by both
Rover and Base Station.
Solution...
• Set base elevation mask less
than rover
LST
13. 13
Sources of Base Data for
Post-Processed DGPS
Where does one get differential GPS
base station data?
• Community Base Stations (CBS)
– government, commercial or public
• Internet Access
• Set up your own
– GPS Base Station
– Virtual Reference Stations (VRS)
– Rover units used as a base
RTCM Corrections
Reference Station at
a known location
Real-Time Differential GPS
Sources of Real-Time
Corrections
Where does one get real-time
differential GPS base station data?
• AMSA or other Beacons
• Satellite Services (OmniStar)
• Set up your own real-time DGPS
– Required
Source of DGPS correction in
RTCM-SC-104 format
Data link, for example, a data radio
LST
14. 14
EGNOS
• European Geostationary Overlay System
• EGNOS V1 (Advanced Operation)
technically qualified in June 2005
• EGNOS measured performance is
excellent (e.g. 1-3m HNSE; >99% APV
availability)
• EGNOS Evolution Roadmap covering
2005-10 being defined (EGNOS V2 and
EGNOS V3 concept)
– EGNOS Service extension, non geo-
stationary broadcast, provision of new
services, GPS modernisation, introduction of
Galileo…..
IOR-W
EGNOS Geo-stationary
satellite coverage
Mean Horizontal AccuracyMean Horizontal AccuracyMean Horizontal AccuracyMean Horizontal Accuracy
(95% of time)(95% of time)(95% of time)(95% of time)
GPSGPSGPSGPS GPS & EGNOSGPS & EGNOSGPS & EGNOSGPS & EGNOS
GPS & EGNOS + GALILEOGPS & EGNOS + GALILEOGPS & EGNOS + GALILEOGPS & EGNOS + GALILEO
Note: single
frequency user,
error in meters.
LST
15. 15
EGNOSWAAS MSAS
EGNOS is an integral part
of 3 inter-regional systems
SBAS: A worldwide Initiative
Today and …
EGNOSWAAS MSAS
MEDA
EAST EUROPE
& RUSSIA
MIDAN
CHINA
SOUTH AMERICA
BRASIL AFRICA
APEC
GAGAN
… in the mid-term future
Mexico
Canada
VRS
Sydnet
Vicpos
Many Accuracies Can Be
Achieved with GPS
These depend on some variables:
• Design of receiver
• Relative positions of satellites
• Time spent on measurements
• Use of Differential techniques
LST
16. 16
< 10 m
SPS (C/A-Code)
RMS or 1 sigma
< 6 - 8 m
PPS P(Y) Code GPS
< 1- 5m
Base Station
Mapping Grade Receiver-DGPS
Post-Processed is slightly
better than real-time
LST
17. 17
< 0.5 - 1mBase Station
High Quality Mapping Receiver
SPS-DGPS
< 1 – 2 cm
RTK Base Station
Real-Time Kinematic Surveying
Not DGPS
<1/2 cm
Base Station
Survey Grade GPS
Not DGPS
Post Processed
LST
18. 18
10m
2m
1m
6m
1/2cm
1cm
Where Do You Need to Be?
Real-Time or Postprocessed?
Accuracy Requirements
• Code Phase GPS
– 40 m - Navigation (air, sea,
land)
– 10 m - Navigation to rural
property corners
– 2 m - Rural GIS data capture
– 1 m - Municipality GIS data
collection
• Carrier Phase GPS
– 1 cm - Survey stake out
– ½ cm - Control Surveys
Key Points on Accuracy
•Knowledge of how data was
collected impacts understanding
of accuracy
•Not all GPS errors are fixed
through differential correction
•If accuracy is critical, you must
use a GPS system that includes
Post-Processing
LST
19. 19
Conclusion
You can now:
– Explain errors in point positioning
– List accuracy expected from various
GPS measurement techniques
– Explain the process of Differential GPS
– Describe how errors are minimised by
using DGPS
Self Study
• Read relevant module in study books
• Do self assessment quiz
Review Questions
LST