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GNSS

an introduction

“ Imagine, accurate positioning dropping right out of the sky at

the touch of a button!”

Jeff Hurn for Trimble Navigation, 1993

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The use of out-of-Earth references was always a natural

approach to the problem of positioning. The use of artificial

satellites – the GPS is the most used of such systems – is in

this evolution path: it allows to easily get the coordinates of a

point with low-cost devices.

Using it for topographic or geodetical purposes requires a

distinct methodology and equipment from the simple

navigation tasks most users perform.

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Spatial positioning

Uraniborg Obs.

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Astronomical positioning

Radar and distancemeter – WWII

TRANSIT

Navigation system (low precision)

Doppler process allowing submetric precision

1970

VLBI Very Long Baseline Interferometry

SLR Sattelite Laser Ranging

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VERY LONG BASELINE INTERFEROMETRY

Observations of a sender (object) that are made simultaneously by a set of receivers (telescopes) to be combined: emulates a large receiver

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Goddard Geophysical and Astronomical Observatory. LAGEOS

SATELLITE LASER RANGING

LUNAR LASER RANGING

Laser Ranging Station

at McDonald Observatory

Satellites with

retroreflectors and

measurements of

millimeter level

precision, useful for

geodynamic studies

2

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1978 – 1st satellite block I

1989 – 1st satellite block II

1985 – 10 satellite block I

1994 – Operational

2000 – S/A discontinued (selective availability) allowing users to receive a non-degraded signal

1973 Navigation System with Timing and Ranging Global Positioning System (DoD)

1981 – 1º receiver code/phase

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GPS

Block II satellite

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WHAT IS GNSS FOR? Get the position

Get the velocity

Get [precise] time

BASIC PRINCIPLE Measure the receiver-to-satellite distances

GNSS APPLICATIONS

Navigation (sea, earth, air)

Geodesy and geodynamics

Topography and cartography

GIS

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SPACE SEGMENT

CONTROL SEGMENT

USER SEGMENT

GPS SYSTEM COMPONENTS

GPS is the most used GNSS

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Constellation of 24 satellites:

8 per orbital plane

Altitude: 20200 km

Period: 11h58m

Constellation period: 23h56m

Inclination: 50o (tilt relative to equator)

SPACE SEGMENT

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CONTROL SEGMENT

1 Control Station (Schriever Air Force Base - Colorado Springs) adjust the ephemeris of each

satellite's internal orbital model, and internal

clock

4 Monitoring Stations

(Hawaii and Kwajalein, Pacific; Diego Garcia, Indian and Ascension Island,

Atlantic) for information retrieval on each

satellite’s behavior, and communication with

Colorado Springs

3

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USER SEGMENT

“CIVIL” : SPS (10m)

“MILITARY – USA”: PPS (1m)

SA - SELECTIVE AVAILABILITY

(currently disabled)

AS – “Anti Spoofing”

(encrypted, preventing access to code P)

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New satellites (Block IIR-M, IIF)

Change of signal: L1 and L2 (increased signal power)

New signal (L5)

New codes have been created

- L2C (Civilian L2)

- M (Military)

Next generation - GPS III: NASA has requested that Block III

satellites carry laser retro-reflectors: that makes tracking the orbits of the

satellites independent of the radio signals, which allows satellite clock

errors to be disentangled from errors of the ephemeris.

System evolution

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Other GNSS

Galileo satellite (ESA)

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GLONASS – Global Orbiting

Navigation Satellite System

Conceived by:

Russian Federation Dept. of Defense

Space segment: 21 satellites in 3 orbital planes, + 3 spare; orbit

19100 km, period 11h15m, system period 8 days

Completed constellation in 1995: completion, decay, modernization

Send both standard precision (SP) and an obfuscated high precision

(HP) signals

GPS and GLONASS use different coordinate systems

As of 2008, GLONASS availability: Russia 66.2% whole Earth 56.0%

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EGNOS- European Geostationary Navigation Overlay

Service ( perspective: increase the positional precision from 20m to 2m)

•SPATIAL SEGMENT

•Three geostationary satellites (A geostationary orbit (GEO) is a

geosynchronous orbit directly above the Earth's equator)

•Inmarsat III Atlantic Ocean region – East (15.5ºW)

•ESA ARTEMIS (21.5ºE)

•Inmarsat III F5 (25ºE)

And:

Precise information on each GPS

satellite’s position;

Information from on board clocks;

Ionosphere parameters

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CONTROL SEGMENT

34 additional stations Main control stations

USER SEGMENT

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WAAS – Wide Area Augmentation System

Federal Aviation Administration (FAA) and Department of Transportation

(DOT)

OPERATION:

25 stations in USA; 2 control stations

Receive GPS information, calculate and

broadcast corrections to apply to GPS data

(orbital drifts, clock errors, ionosphere and

troposphere delays)

USE IN: North America and West Europe

Operational Sept- 2002 : position precision: horizontal 1-2 m

vertical 2-3 m

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Coverage areas for WAAS, EGNOS and

MSAS

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SPACE SEGMENT

30 satellites (orbit 24 000 km, ~ 4 000km more

than GPS) in 3 orbital planes, inclination 56º.

9+1 satellites per orbital plane.

CONTROL SEGMENT

2 main centers in Central Europe

GALILEO – European Satellite Navigation System

European Commission and ESA

In 2007 the 27 EU transportation

ministers anounced operationality by

2013

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Four future different navigation services:

•Open Service (OS), free access. Accuracy of <4 m

(horiz.); <8 m (vert.) in simultaneous use both OS bands;

Single band <15 m (h.) / <35 m (v.)

•Commercial Service (CS), encrypted, available for a

fee. Accuracy < 1 m. If complemented by ground stations:

accuracy <10 cm.

•Public Regulated Service (PRS) and Safety of Life

Service (SoL), both encrypted, similar accuracy to OS.

Robustness against jamming and reliable detection of

problems within 10 seconds. Exclusive use by police,

military and safety-critical transport applications (air-traffic

control, automated aircraft landing, etc.), respectively.

GALILEO – European Satellite Navigation System

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OPERATION

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6 seconds

4 seconds

AB

In a perfect world

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6 seconds4 seconds

5 seconds(wrong time)

7 seconds(wrong time)

AB

hypothetical 1s delay in receiver’s clock A

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6 seconds4 seconds

8 seconds

AB

C

again the perfect world

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5 seconds(wrong time)

7 seconds(wrong time)

9 seconds(wrong time)

BA

C

hypothetical 1s delay in receiver’s clock

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OPERATION

•The basis of GPS is triangulation from satellites: to

triangulate, a GPS receiver measures the distance

using the travel time of radio signals

•To measure travel time, GPS needs very accurate

timing which it achieves with some tricks

•Along with distance, the satellites’ position must be

known. High orbits and careful monitoring are critical

•The correction of any delays the signal experiences as

it travels through the atmosphere, must be accounted

for

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Fo=10,23 MHz

L1= 154xFo = 1575.42 MHz (19,0cm wavelength)*

L2= 120xFo = 1227.60 MHz (24.4cm)

C/A (Coarse/Acquisition) 1.023 MHz

P (Precise) 10.23 MHz

GPS SIGNAL

(oscillation of the carrier wave)

(Carrier Phase)

Code

* this makes the carrier signal act as a

much more accurate reference than

the pseudo random code by itself

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32 BLOCK OF SIGNAL BY DENSE VEGETATION AND BUILDINGS

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r r

r

r

1

2 3

4 t o

t 1 r 4 = t 1 t o v ( - )

PSEUDO-DISTANCE (Pseudo-Range)

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.)()()(

,)()()(

,)()()(

,)()()(

2424244

2323233

2222222

2121211

kkkkk

kkkkk

kkkkk

kkkkk

cdtZZYYXX

cdtZZYYXX

cdtZZYYXX

cdtZZYYXX

(Xi,Yi,Zi) – Satellitei position (Xk,Yk,Zk) – Receiver position dtk – Receiver clock status

CALCULATION OF COORDINATES BASED ON PSEUDO-DISTANCES

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D t

USE OF CODE

Receiver-generated signal

Received satellite signal

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Complete and integer number of cycles from the

emission instant to the reception instant

USE OF CARRIER WAVE SIGNAL

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Epoch (0)

Ambiguity

Epoch (i)

Ambiguity

Counted cycles

Measured phase

It is necessary to measure the ambiguity of the

carrier wave phase, in order to use its information

as a distance measuring procedure along with time

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METHODS FOR SOLVING THE

AMBIGUITIES

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OPERATION A

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0,50 m 3-5 min 1 sec.

< 40 km

ABSOLUTE POSITIONING

DGPS

2,00 m 3-5 min 1 sec.

< 300 km

WADGPS 4 m

SPS 5 to 15 m

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Single frequency Double frequency

C/A + L1 C/A + L1,L2

Static (Conventional)

Static (Rapid)

Kinematic

0,02 m 30 min to 10 km

+ 3 min/km 5 to 10 s < 20 km

0,01 m 20 min to 10 Km

+ 2 min/km 5 to 15 s < 20 km

0,005 m 20 min to 10 km

+ 2 min/km 5 to 30 s < 20 Km

0,01 m 10 min to 10 km

+ 1 min/km 5 to 10 s < 20 km

0,01 m (RTK) < 20 km -

RELATIVE POSITIONING

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Navigation mode

SPS (C/A) : 10m

PPS (P) : 1m

post-processing

real time

ABSOLUTE POSITIONING

WADGPS: regional differential correction

DGPS: local differential correction

0,5m-2m

4m

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Reference station at Instituto Superior Técnico (http://websig.civil.ist.utl.pt/gps)

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Rinex format

Receiver Independent Exchange Format Version

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Differential correction broadcast

Coast stations (Beacon Stations)

Geostationary (Racal/OmniStar)

GSM antennas

Other (via radio)

RTCM – Radio Technical Commission for Maritime Services

(Standard for differential correction transmissions)

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SPS receivers

Most future mass market receivers, such as these, will process

both the GPS C/A and the Galileo OS signals, for maximum

coverage.

automotive navigation systems recreational navigation systems

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navigation system

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DGPS receivers

(real time or post-processing)

Border demarcation in

Timor-Leste / Indonesia

Trimble Geo-Explorer 2005

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DGPS receivers

(real time or post-processing)

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AFTER DGPS CORRECTION

BEFORE DGPS CORRECTION

EXAMPLE

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Static

RELATIVE POSITIONING

Conventional

Rapid Static

Kinematic

Real Time Kinematic

(RTK)

Post-processing

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B A

RELATIVE

POSITIONING

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RENEP

REDE NACIONAL DE ESTAÇÕES PERMANENTES

(INSTITUTO GEOGRÁFICO PORTUGUÊS)

Cascais

Ponta Delgada

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Accuracy (m)

1.00

0.10

0.01

Conventional static 0 120

Rapid static

0 2 5

Time (min)

Unsolved ambiguities

Solved ambiguities

When the ambiguities (number of integer cycles) are

solved, accuracy will not increase significantly

COLLECTING TIME

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For large bases (>20 km), where the greatest possible precision is required Useful for implementing geodetical networks Useful for large areas

For bases up to 20 km Short occupying times Great productivity in most applications

Static mode

Static-rapid

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For moving objects (simulation of continuous measurements of the position) Useful for road mapping and related applications

KINEMATIC POSITIONING

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Post-processing is not needed Availability of results right on occasion Several applications in navigation

RTK – Real-Time Kinematic

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Mono frequency receivers

Base measurement with accuracy up to 2 cm + 1 ppm

(rms) Use carrier wave L1

Topographical applications with base length up to 15 km Inexpensive solution, but difficult to achieve the above

mentioned precision

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Base measurement with accuracy up to 3 mm + 0.5 ppm (rms) Use in geodesy and topography :

Geodetic networks, Geodynamics, Network densification, Photogrammetric control, Topographic detail

Double frequency receivers

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MEASUREMENTS

With Selective Availability

on

2001

2006

http://www.mar-it.de/NavGen/navgen_r.htm

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Multi-path

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Ionospheric delay

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Re

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ive

r n

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Mu

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pa

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Sa

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locks

Tro

po

sp

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Ep

he

me

ris

Ion

osp

he

re 0

10

20

30

40

m

Relative importance of error sources A

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Error propagation

PDOP – Position Dilution of Precision

HDOP – Horizontal Dilution of Precision

VDOP – Vertical Dilution of Precision