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New Concepts In Long RangePositioning

Gordon T. JohnstonRacal Survey Group Limited

Civil GPS Service Interface Committee1st December 2000

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IntroductionIntroduction

l Background to developments

l Requirements & Objectives

l Technical challenges

l Description of techniques and methods

l Solutions and results

l Summary and conclusions

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BackgroundBackground

l GPS hardware and software developments

l Increase in positioning accuracy requirements

– MBES, construction and deep water projects

l Cost and availability of positioning services

l Dual frequency data

l WAAS, EGNOS and Competition.

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DGPS Hardware DevelopmentsDGPS Hardware Developments

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5m5m 5cm5cm1m1m

GPS SystemsGPS Systems Performance Performance

100m

10m

1-5m

1-5cm

Standalone GPS (with SA)

Standalone GPS (no SA)

Code-based DGPS

Carrier-based DGPS

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GPS GPS ObservablesObservables

l Two independent range observables

– Code (pseudo-range)

– Carrier phase

l GPS receiver only measures fractional partof observables

l Both measurements ambiguous by aninteger number of wavelengths

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GPS Measurement WavelengthsGPS Measurement Wavelengths

CodeCodeL1: C/A code - civilian

(code wavelength ≈ 293m)P (Y) code - military

L2: P (Y) code - military

Carrier frequenciesCarrier frequenciesL1: 1575.42 MHz (wavelength ≈ 19cm)L2: 1227.60 MHz (wavelength ≈ 24cm)

CYCLE SLIPS⇓

New Integer Ambiguity (N)

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GPS Error SourcesGPS Error Sources

Satellite ClocksSelective Availability

Ephemeris

Atmospheric Delays

Multipath

Receiver Clocks, etc

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Carrier-Phase DGPSCarrier-Phase DGPS

Advantagesl Lower levels of multipath errorl Lower levels of measurement

noise

Disadvantagesl Unknown number of wavelengthsl Positioning affected by similar

biases to Pseudo-Ranges:- Ionosphere and troposphere- Satellite & receiver clocks

-5

0

5

10

15:00 15:30 16:00 16:30 17:00

Cod

e ob

serv

atio

n (m

)

-5

0

5

10

15:00 15:30 16:00 16:30 17:00

Pha

se o

bser

vatio

n (m

)

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Genesis - The RACAL LRTK SystemGenesis - The RACAL LRTK System

OVERVIEW

l Accurate to better than 20 cm

l Satellite-based LRTK system

l Operational over long baselines (>500km)

l Provides a robust LRTK System

(User Equipment, Quality Parameters,

smooth positioning mode, Network Solution)

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RACAL LRTK ChallengesRACAL LRTK Challenges

l Technique– Error Modeling (Atmosphere, Multipath)– QC Statistics (reliable QC statistics)– Robust Transition between solution types

l Data Transmission– Data Compression– Transmission mode vs Baseline lengths

(i.e. performance at long ranges required tosupport Satellite-based Transmission mode)

RACAL Genesis General ArchitectureRACAL Genesis General Architecture

GPS SatellitesGPS SatellitesData Link SatelliteData Link Satellite

Reference StationNetwork

Reference StationNetwork

Compressed DataCompressed Data

UserUser

Satellite Uplink HubSatellite Uplink Hub

GPS SignalsGPS Signals

Compressed Data

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RACAL Genesis System ComponentsRACAL Genesis System Components

Repair and FilterObservations

• Combine• Synchronise• Transform

DataRX

+

Mobile StationMobile Station

Position Estimatesand

Quality Control

Phase\Code Observables +

-

-Multipath Map

Almanac\Time\Ephemeris

+

Almanac\Time\Ephemeris

Phase\Code Observables

Repair and FilterObservations

DataTX

Compress

PositionCalculation

Weights

Base StationBase Station

GPS Receiver GPS Receiver

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RACAL Genesis Results - 50kmRACAL Genesis Results - 50kmBaselineBaseline

50km baseline

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RACAL Genesis Results - 50km BaselineRACAL Genesis Results - 50km Baseline

Typical Static real time performanceover 2 hours including initialization:

Mean SD(2σ)Delta East -0.002m 0.17mDelta North 0.070m 0.11mDelta Height -0.026m 0.21m

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RACAL Genesis Results - 200kmRACAL Genesis Results - 200kmBaselineBaseline

200km baseline

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RACAL Genesis Results - 200kmRACAL Genesis Results - 200kmBaselineBaseline Typical Static real time performance

over 2 hours including initialization:Mean SD(2σ)

Delta East 0.110m 0.14mDelta North -0.004m 0.12mDelta Height -0.013m 0.20m

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RACAL Genesis Results - 900kmRACAL Genesis Results - 900kmBaselineBaseline

900km baseline

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RACAL Genesis Results - 900kmRACAL Genesis Results - 900kmBaselineBaseline Typical Static real time performance

over 2 hours including initialization:Mean SD(2σ)

Delta East 0.490m 0.09mDelta North -0.185m 0.35mDelta Height 0.316m 0.38m

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RACAL Genesis NetworkRACAL Genesis Network

l 4 Station Network LRTK Solution v Single UserSV Tracking (24 Hours)

l 2 periods in particular would not have enoughDD’s for Single Baseline

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RACAL Genesis Results - NetworkRACAL Genesis Results - Network

LRTK 3-STN Network

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RACAL Genesis Results - NetworkRACAL Genesis Results - Network

Typical Static real time performanceover 2 hours including initialization:

Mean SD(2σ)Delta East 0.100m 0.08mDelta North 0.002m 0.09mDelta Height 0.062m 0.10m

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Delta Position – Wrong Integer DeterminationDelta Position – Wrong Integer Determination

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

12 12.05 12.1 12.15 12.2 12.25 12.3

Time (Hours)

Del

ta P

osi

tio

n

Delta North

Delta East

Delta height

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Statistical QC Control of LRTK Position Calculation

0

5

10

15

20

25

30

35

12 12.05 12.1 12.15 12.2 12.25 12.3

Time (Hours)

Sta

tist

ical

Val

ue

Statistic

Threshold

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Delta Position – Right Integer DeterminationDelta Position – Right Integer Determination

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

19.4 19.6 19.8 20 20.2 20.4 20.6

Time (Hours)

Del

ta P

osi

tio

n

Delta NorthDelta EastDelta Height

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Statistical QC Control of LRTK Position Calculation

0

5

10

15

20

25

30

35

40

45

50

19.4 19.6 19.8 20 20.2 20.4 20.6Time

Sta

tistic

al T

hres

hold

Statistic

Threshold

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Summary & ConclusionsSummary & Conclusions

l Sophisticated GPS Processing using a networkof stations demonstrates LRTK potential.

l Genesis available in North and Norwegian Seasin 2001.

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Genesis North West Europe CoverageGenesis North West Europe Coverage

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Applications for High AccuracyApplications for High AccuracyPositioningPositioning

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Positioning & Vertical ControlPositioning & Vertical Control

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High Accuracy NavigationHigh Accuracy Navigation

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High Accuracy 3D ControlHigh Accuracy 3D Control