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Satellite-based positioning

R. Knippers

Application fields

Surveying

Military operations

Engineering

Vehicle tracking

Flight navigation

Car navigation

Ship navigation

Agriculture

Mapping

Topics for discussion▪The segments of a satellite-based positioning system▪GPS, GLONASS and Galileo▪Principle of positioning▪Errors and their sources▪Positional accuracies▪Relative (differential) positioning

Three segments▪Space segment:

the satellites that orbit the Earth, and the radio signals that they emit. ▪Control segment: the ground stations that monitor and maintain the space segment components. ▪User segment: the users with their hard- and software to conduct positioning.

Space segment of GPS system

The space segment of GPS

consists of 24 satellites on 6 orbits (approx. 22,000 km from the centre of the Earth): ▪

Each satellite carries a

clock.

Each satellite completes 2

orbits/day.

24 hour complete GPS

coverage anywhere on the

Earth.

Accuracy: 21 meters 95% of

time

NAVSTAR GPS

Satellite

L2 Carrier

L1 Carrier

P-Code

P-Code

C/A Code

Navigation

Message

Navigation

Message1227.60 MHz1575.42 MHz

GPS Signal Structure

Control Segment of GPSSpace Segment24+ Satellites

Current ephemeris is

transmitted to users

Monitor Stations• Diego Garcia• Ascension Island• Kwajalein • Hawaii• Colorado Springs

GPS Control Colorado Springs

End User

Control Segment of GPS

Master Control StationMonitor StationGround Antenna

Colorado

SpringsHawaii

Ascension

Islands

Diego

Garcia

Kwajalein

User segment of GPS

Receivers and their users:▪

(Military)

Navigation in 3D-aircrafts,

ships, ground vehicles and hand-carried instruments

Precise positioning -

Surveying

(Time dissemination - astronomy) (Research projects on atmospheric distortions) EI Q

GPS-Receivers

Selection of a GPS receiver▪Application (boating, flying, driving, mapping, surveying)▪Accuracy requirements▪Power consumption requirements▪Operational environment▪Signal processing requirements▪Cost▪Data exchange standards

Space segment of GLONASS system

Russian system

(Globalnaya Navigatsionnaya

Spunikova Sistema - GLONASS)▪

24 satellites (21 operational and

3 spare).

Three orbital planes at 65º

inclination.

Two codes as GPS, but all

satellites broadcast identical codes but using slightly different carrier frequencies for each satellite.

The positioning principal is the

same as GPS

Accuracy: 20 m horizontal and

~30 m verticalGLONASS Satellite http://www.glonass-ianc.rsa.ru

Space segment of Galileo system

Galileo is in the implementation

phase, first satellite to be launched in 2006, planned operation start 2008.▪

Designed for civil purposes

30 satellites

3 orbits (23,222 km high)

Network of ground stations, 2

control centresin Europe

Accuracy of single receiver:

around 1 m

Galileo Satellite

Principle of positioning

GPS-receiverGPS-satellite

Distance

(velocity of light) x (travel time)

The GPS-receiver computes the

distances (ranges) to the satellitesThe GPS-receiver computes the distances (ranges) to the satellites

How does the GPS-receiver

computes the travel time?How does the GPS-receiver computes the travel time?

L2= 1227.60 MHzL

1= 1575.42 MHz

GPS code on Carrier wave

(C/A or P code)

It receives GPS-codes and

Carrier waves from the satelliteIt receives GPS-codes and

Carrier waves from the satellite

Code from SatelliteCode from Receiver

Time difference

between

Receiver and Satellite signal

Δt

Code comparison

Principle of positioning

GPS-receiverGPS-satellite

Pseudo-range

(velocity of light) x (travel time) receiver clock error) + (other errors)

The GPS-receiver measures in fact

pseudo distances (pseudo-ranges) to the satellitesThe GPS-receiver measures in fact pseudo distances (pseudo-ranges) to the satellites

Principle of positioning

(X,Y,Z)1 2 3 distance 1 distance 2 distance 3

To determine a position in a

3 dimensional space it takes in theory

3 distance measurements from

3 satellitesTo determine a position in a

3 dimensional space it takes in theory

3 distance measurements from

3 satellites

Pseudorange positioning

Three-satellite fix position

(trilateration)Two-satellite fix position

One-satellite fix position

(X,Y,Z,

ΔΔΔΔt)

12 34
pseudorange 1 pseudo- range 2 pseudo- range 3pseudorange 4

Pseudorange

velocity of light * travel time receiver clock error + other errors

Accurate positioning requires

an extra distance measurement from a fourthsatellite to eliminate the receiver clock errorAccurate positioning requires an extra distance measurement from a fourthsatellite to eliminate the receiver clock error

Principle of positioning

Synchronization bias of the receiver clock

Error sources in absolute positioning▪Selective availability▪Satellite clock and orbit errors▪Ionospheric and tropospheric delays▪Receiver"s environment (multi-path)▪Satellite constellation

Receiver"s environment errors

Magnitude of the error sources*

* Absolute, single-point positioning based on code measurements*

Good satellite constellation

Low PDOP (1.5)Good satellite constellation

Low PDOP (1.5)

Bad satellite constellation

High PDOP(5.7)Bad satellite constellation

High PDOP(5.7)

Satellite constellation

positional error

Positional accuracy in absolute positioning

Absolute, single-point positioning

based on code measurements:

Typical error:

5-10 m

(horizontal accuracy)

Typical error:

2-5 m (horizontal accuracy) when using a dual- frequency receiver or the encrypted military signals (P-code)

Location errors: noise, bias and blunder

Noise (random) errors:

noise in code and noise in receiver, multi-path.

Bias (systematic) errors:

clock, satellite position, ionosphere, troposphere,

GDOP effects.

Blunder:incorrect

geodetic datum, software failures, hardware problems etc.

Systematic errors (bias) removal is essential

to improve the positional accuracy! (X,Y,Z)

Reference point

Reference (or base) receiverTarget (or field) receiver

Differential (or relative)

positioning

Relative positioning

Positional accuracy in relative positioning

Relative, single-point positioning

based on code measurements

Typical error:

0.5 - 5m

(horizontal accuracy)

Positional accuracy in relative positioning

Relative, single-point positioning

based on carrier phase measurements

Typical error:

2mm - 2cm

(horizontal accuracy) Carrier phase measurementsCarrier phase measurement is a technique to measure the range (distance) of a satellite by determine the number of cycles of the (sine-shaped) radio signal between sender and receiver. The number of cycles is determined in a long observation session from the change in carrier phase (Phase Shift Keying). This change happens because the satellite is orbiting itself.

L1/L2 Carrier

Relative (differential) survey techniques using carrier phase measurements▪Static▪Stop and go kinematic▪Pseudo-kinematic▪Kinematic▪Rapid static▪On-the-fly (OTF)/real-time kinematic (RTK)

Real-time kinematic positioning

Network positioning

Network positioning Relative positioning using a network of reference stations

NLR Globalcom

http://www.lnrglobalcom.nl

GlobalNET 2005

Network positioning GlobalNET 2005: Reference Station at ITC

Satellite-Based Augmentation

Systems (SBAS)

(X,Y,Z)

Reference point

Ground stationField receiver

(X,Y,Z)

Reference point

Ground stationGeostationary satellite

Satellite

-based Augmentation

Systems

(X,Y,Z)

Reference point

Ground station

(X,Y,Z)

Reference point

Ground stationGeostationary satellite

Satellite

-based Augmentation

Systems

Operational systems▪WAAS (Wide-Area Augmentation System) for North America▪EGNOS (European GeostationaryNavigation Overlay Service) for Europe▪MSAS (Multi-functional SatelliteAugmentation System) for eastern Asia

WAAS, EGNOS, MSAS

Wide Area Augmentation System (WAAS)

Local Area Augmentation System (LAAS)

INTERNATIONAL INSTITUTE FOR GEO-INFORMATION SCIENCE AND EARTH OBSERVATION

The end !

INTERNATIONAL INSTITUTE FOR GEO-INFORMATION SCIENCE AND EARTH OBSERVATION

Mobile GIS applications

Data collection with a mobile computer

INTERNATIONAL INSTITUTE FOR GEO-INFORMATION SCIENCE AND EARTH OBSERVATION

Location-Based Services (LBS)

Location-Based Services on a Mobile

computer or mobile phone. INTERNATIONAL INSTITUTE FOR GEO-INFORMATION SCIENCE AND EARTH OBSERVATION

Location-Based Services - Applications1.

Location based information services (e.g.

search for the nearest restaurant or the nearest banking cash machine) 2.

Location based emergency service (e.g.

pinpoint your location on dialing 9-1-1) 3.

Location based billing service (e.g. preferential

billing for calling by establishing personal zones such as a home zone or work zone). 4.

Fleet applications (tracking a vehicle and/or

operator). INTERNATIONAL INSTITUTE FOR GEO-INFORMATION SCIENCE AND EARTH OBSERVATION

LBS application - Mobile phone tracking

http://geotracing.com/tlandquotesdbs_dbs19.pdfusesText_25