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ST133 Sensor Network Approach to GPS RTK Nicholas Zinas Supervised by: Prof Paul Cross Dr Marek Ziebart University College London


Overview • Fundamentals of GPS Positioning

• The concept of Network RTK • Research – The Sensor Network Approach • Data campaign and outcomes

• Conclusions


GPS Fundamentals (1)

from : Canadian space agency


GPS Fundamentals (2) • L1 Carrier Wave

1575.42MHz C/A Code 1.023 MHz Navigation Message 50Hz

Precise Code 10.23 MHz Observables L1 : carrier phase C/A,P1: pseudorange


GPS Fundamentals (3) • L2 Carrier Wave

1227.6 MHz Navigation Message 50 Hz Precise Code 10.23 MHz

Observables L2 : carrier phase P2: pseudorange


GPS Positioning • Sample Text

GPS Positioning

Absolute Positioning

PseudoRanges

Carrier Phases &Pseudoranges

Relative/Differential Positioning

One Reference Station

Static Fast Static Stand Alone

OTF Kinematic Stop & Go

Multiple Reference Stations

PseudoRanges

Carrier Phases & Pseudoranges

Precise Point Positioning CORS LADGPS

WADGPS Network RTK


Evolution of GPS Networks 1980

2000 Centimetre Level Single Baseline RTK GPS Positioning

GPS used for geodesy First Active Networks of GPS

Network RTK GPS

Metre Level RTK Differential GPS


Advantages & Benefits Single Base RTK

Network RTK

• Fewer Reference Stations needed • Kinematic Techniques ensure high accuracy for distances of tens of kilometres • Improved error modelling


The Concept (1)

Network RTK architecture


Selection of the Ref Station site •

Well founded and stable antenna construction

Free of radio frequency interference

Adequate sky visibility

Not subject to sources of signal multipath

Position has to be determined in a global reference frame with high accuracy, days from GPS observations

Transformation to a local reference frame has to be accurate enough to support surveying activities


Radio Technical Commission for Maritime Services •

Special Committee 104 responsible for producing standards for differential GNSS, maritime & terrestrial

Currently in version 3.0

In version 3.1 a Network RTK technique will be implemented (MAC)


The Concept (2)

Network RTK theoretical development


How it works – Double difference • Network RTK Main Observable 1ABi  (1A  iA )  (1B  iB )

• Phase (Φ) 1ABi  p1ABi   N 1ABi  errors1ABi • Pseudorange (PR) 1i AB

PR

p

1i AB

 errors

1i AB


How it works - ambiguity • Introducing the ambiguity

• Double Difference Ambiguity N 1ABi  ( N 1A  N Ai )  ( N B1  N Bi )

N Ai


Network RTK Process (1)

a) Resolve Ambiguities between Reference Stations b) Compute Corrections c) Apply Corrections


How it works – ambiguity resolution between Ref Stations

1i N BC

N 1ABi i N 1AC

N B1 iD i N 1AD

NC1 iD


Network RTK Process (2)

a) Resolve Ambiguities between Reference Stations

b) Compute Corrections c) Apply Corrections


How it works – generating the corrections geometric distance

1i AB

errors

 

1i AB



1i AB

 N

observed phase

1i AB

integer ambiguity


How it works – generating the corrections geometric distance 11i i Corrections AAB B

errors

 

1i AB



1i AB

 N

observed phase

1i AB

integer ambiguity


Network RTK Process (3)

a) Resolve Ambiguities between Reference Stations b) Compute Corrections

c) Apply Corrections


How it works – applying the corrections (1) • Virtual Reference Station Central Processing Facility


How it works – applying the corrections (1) • Virtual Reference Station Central Processing Facility


How it works – applying the corrections (2) ••Flachen Flachen Korrektur Korrektur Parameter Parameter Central Processing Facility


••Flachen Flachen Korrektur Korrektur Parameter Parameter

Single Baseline Positioning

How it works – applying the corrections (2)

Central Processing Facility


How it works – applying the corrections (3) •Master Auxiliary Concept

Central Processing Facility


How it works – applying the corrections (3) •Master Auxiliary Concept

AUXILIARY Central Processing Facility

AUXILIARY MASTER

AUXILIARY


How it works – applying the corrections (3) •Master Auxiliary Concept

AUXILIARY Central Processing Facility

AUXILIARY MASTER

AUXILIARY


Having the Network

Having the Network


Having the Network (2) GPS Networks

public organizations

private businesses

return on investment?


Service Providers • Subscription Fees Coverage

Full Service* / annual fee

Provider A

173,000 square miles

$2160

Provider B

82,000 square miles

$5940

Provider C

18,000 square miles

$6600

* L1/L2 centimetre level GPS/GLONASS GPS RTK corrections for 1 receiver


Main disadvantage

No control over the quality of the position computed by the user


Research

How can we overcome this?


Research (2)

RTCM type 18/19 messages

Central Processing Facility

b c

a e f

d


Research (2)

Central Processing Facility

b c

a e f

RTCM type 18/19 messages d


Research (2)

Central Processing Facility

b

e f

Positions

c

a

d


Research (3) How we can solve the double differenced ambiguities between the rovers

N 1ABi b

N B1 iE

a

N

1i FA

f

1i N BC

N

1i AE

N E1 iF

c

N

1i CE

e

N

1i DE

NC1 iD d

since their position is not accurately known?


Research (4) Closed Loop Principle N 1ABi

b

1i N BC

c

a

N

NC1 iD

1i DA

d

N

1i AB

N

1i BC

N

1i CD

N

1i DA

0


Research (5)

Central Processing Facility

b c

a Central Processing Facility

e

d

f

a) Solve the ambiguities between the rover and the closest Reference Station (FKP or MAC)


Research (6)

Central Processing Facility

b c

a Central Processing Facility

e

d

f

b) Ambiguity = Phase - Pseudorange


Research (6)

Central Processing Facility

b c

a Central Processing Facility

e

d

f

c) Bundle Adjustment


Data Campaign - Athens, Greece GPS Antennas

No

Simulating

R8 Int

9

Rover

5800 Int

3

Rover

Zephyr

1

Rover

Zephyr Geodetic

1

Reference Station

Existing CORS: JGC,DION,GEOT


Data Campaign (2)

3hrs, 1sec data rate


Software (UCL) Models

Algorithms

Troposphere ESA, Sastamoinen

Single Epoch Carrier Phase Positioning

Ionosphere IONEX, Klobuchar

Multiple Epoch Carrier Phase Positioning

Antenna Phase Centre Offsets and Variations

Point Positioning

Geoid (EGM 96)

LAMBDA Reference Station Ambiguity Resolution


Outcomes and Continuing Research Approach a • The standard Network RTK techniques can easily be applied without any modifications

Approach b • Phase – Pseudorange not always successful

Approach c • Stochastic properties of the regional ionosphere and troposphere have to be investigated


Advantages (1)


Advantages (2) • No need for sophisticated algorithms at the user site • Consistency of the models used for the computations • Statistical interpretation of different solutions

• All information for each individual system available • Better Atmospheric modelling • Rover Positions transmitted to the rover in a frame of preference • Integrity and quality of the computed position can be determined


Conclusions • Infrastructure maintenance is a running cost for the service provider that at least has to ‘break even’. • A Fully Centralized GPS Network approach introduces a new concept in handling the network data that can prove to be more business efficient. • Based on that new business models can be developed • This hi tech infrastructure has to have it’s potential exploited!


Acknowledgments

• Mr Christopher Zinas, Surveyor Engineer • Mr Nikolas Simpas, CEO of Topomet SA • Geotech Ltd


Thank You


Presentation - Trimble2007