Dgps concept

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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

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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

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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

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-200-180-160-140-120-100

-80-60-40-20

020406080

100120140160

0 1 2 3 4 5 6 7 8 9 10

Time of Day (Hours UTC)

Inst

anta

neou

s E

rror

(m

eter

s)

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 metersSPS 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 ZEphemeris (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 XP1P2

XP4

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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 delayedas they pass throughthe atmosphere

Ionosphere

Baseline not too long

TroposphereBase

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

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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


• Relative position of satellites can affect error LST

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• 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


• 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

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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

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BASE

Time, t

t + 1GPS Positions

Absolute ReferencePosition

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 Seen1 2 3 4 5 6 7 8

Satellites Used1 2 3 41 3 5 6Any Combination of Base satellites

ROVER

Time, t

t + 1

?BASE

Time, t

t + 1

Differential Correction (Cont.)LST

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Base Receiver Known

Coordinates

Shift 10m South30m West

Block Shift Correction

RemoteReceiverMeasured

Coordinates

Apply same Block Shift

MeasuredCoordinates

FinalCoordinates

Base Receiver

Range Correction

RangeCorrection

Remote Receiver

RangeCorrection

Example of DGPS CorrectionsExample of DGPS Corrections

Uncorrected

Corrected

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DGPS DGPS –– 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

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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

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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 CorrectionsWhere does one get real-time

differential GPS base station data?• AMSA or other Beacons• Satellite Services (OmniStar)• Set up your own real-time DGPS

– RequiredSource of DGPS correction in

RTCM-SC-104 formatData link, for example, a data radio

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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.

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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 AMERICABRASIL AFRICA

APEC

GAGAN

… in the mid-term future

Mexico

Canada

VRSSydnetVicpos

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

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< 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

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< 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

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10m2m

1m6m 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

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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

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Questions?

LST

Government & Nonprofit

Dgps concept