Global Navigation Satellite Systems (GNSS)

Dr. Mahesh K. Jat

Malaviya National Institute of Technology Jaipur

Introduction

• GNSS GPS (USA), GLONASS (Russia), Galileo(Europe), Augmentation

Systems (SBAS, GBAS), IRNS (India), QuasiZenth (Japan)

• Fuelling growth during the next decade will be next generation GNSS

that are currently being developed.

Global Navigation Satellite Systems (GNSS) involve satellites, ground stations

and user equipment to determine positions around the world and are now

used across many areas of society

Global Navigation Satellite Systems (GNSS)

• NAVSTAR– USA

• GLONASS– Russians

• Galileo– Europeans

GLONASS from Russia

• GLONASS-M (L1 and L2 bands ) satellites with an improved 7-year design

lifetime.

• 2007 to 2008 planned to launch GLONASS-K satellites with improved

performance, also transmit a third civil signal (L3).

• Stated intention is to achieve a full 24-satellite constellation transmitting

two civil signals by 2010.

• Full constellation is planned to be broadcasting three sets of civil signals

by 2012.

• Indian Government announced at the end of 2004 that it would be

contributing funds to assist Russia to revitalize GLONASS.

Galileo from the European Union

• Constellation of 30 satellites, increased altitude (approximately 3000km higher

than GPS) which will enable better signal availability at high latitudes.

• Exact signal structure is still liable to change,

• Galileo satellites broadcast signals compatible with the L1(E5a E5b) and L5 GPS

signals. Galileo will also broadcast in a third frequency band at E6; which is not

at the same frequency as L2/L2C of GPS.

• Current plan is to offer 5 levels of service:

o Open Service uses the basic signals, free-to-air to the public with performance similar to

GPS and GLONASS.

o Safety of Life Service allows similar accuracy as the Open Service but with increased

guarantees of the service, including improved integrity monitoring to warn users of any

problems.

o Public Regulated Service is aimed at public authorities providing civil protection and

security (eg police), with encrypted access for users requiring a high level of performance

and protection against interference or jamming.

o Search and Rescue Service is designed to enhance current space-based services (such as

COSPAS/SARSAT) by improving the time taken to respond to alert messages from distress

beacons.

o Commercial Service allows for tailored solutions for specific applications based on

supplying better accuracy, improved service guarantees and higher data rates.

06/09/07 Veena G Dikshit, Sc 'E' , ADE, Bangalore

GNSS Signal Spectrum

Galileo E5/A

GPS L2

1151

MH

z

1300

MH

z

Galileo E5/B

1164

MH

z

1188

MH

z

1215

MH

z

1239

MH

zGlonass

G2

1237

MH

z

1261

MH

z12

60M

Hz

Galileo E6

1559

MH

z

1610

MH

z

GPS L1

1563

MH

z

1587

MH

z

Glonass

G2

5010

MH

z

5030

MH

z

1591

MH

z

1254

MH

z12

58M

Hz

1593

MH

z

ARNS

RNSS* RNSS*RNSS

960M

Hz

RNSS

ARNS ARNS

5250

MH

z

Galileo C1

RNSSG

alile

o E3

GPS L5

RNSS* shared with other services

1214

MH

z

Galileo E5/A or E5/B frequency band optionsG

alile

o E1

Gal

ileo

E2

Gal

ileo

E4

Upper L-BandLower L-Band C-Band

Galileo E5/A

GPS L2

1151

MH

z

1300

MH

z

Galileo E5/B

1164

MH

z

1188

MH

z

1215

MH

z

1239

MH

zGlonass

G2

1237

MH

z

1261

MH

z12

60M

Hz

Galileo E6

1559

MH

z

1610

MH

z

GPS L1

1563

MH

z

1587

MH

z

Glonass

G2

5010

MH

z

5030

MH

z

1591

MH

z

1254

MH

z12

58M

Hz

1593

MH

z

ARNS

RNSS* RNSS*RNSS

960M

Hz

RNSS

ARNS ARNS

5250

MH

z

Galileo C1

RNSSG

alile

o E3

GPS L5

RNSS* shared with other services

1214

MH

z

Galileo E5/A or E5/B frequency band optionsG

alile

o E1

Gal

ileo

E2

Gal

ileo

E4

Upper L-BandLower L-Band C-Band

06/09/07 Veena G Dikshit, Sc 'E' , ADE, Bangalore

BENEFITS OF GNSS

• Availability of Signals

• Extra satellites improve continuity

• Extra satellites and signals can improve accuracy

• Extra satellites and signals can improve efficiency

• Extra satellites and signals can improve availability (of satellites at a particular location)

• Extra satellites and signals can improve reliability

What is the GPS?

• Orbiting navigational satellites – Transmit position and time data

• Handheld receivers calculate– latitude

– longitude

– altitude

– velocity

• Developed by Department of Defense

The Global Positioning System

• Baseline 24 satellite constellation in medium earth orbit

• Global coverage, 24 hours a day, all weather conditions

• Satellites broadcast precise time and orbit information on L-band radio frequencies

• Two types of signals:– Standard (free of direct user fees)

– Precise (U.S. and Allied military)

• Three segments: – Space

– Ground control

– User equipment

History of the GPS

• 1969—Defense Navigation Satellite System (DNSS) formed

• 1973—NAVSTAR Global Positioning System developed

• 1978—first 4 satellites launched

Delta rocket launch

History of the GPS

• 1993—24th satellite launched; initial operational capability

• 1995—full operational capability

• May 2000—Military accuracy available to all users

Control Segment

Space Segment

User Segment

Three Segments of the GPS

Monitor Stations

GroundAntennas

Master Station

Components of the SystemSpace segment• 24 satellite vehicles• Six orbital planes

– Inclined 55o with respect to equator

– Orbits separated by 60o

• 20,200 km elevation above Earth

• Orbital period of 11 hr 55 min

• Five to eight satellites visible from any point on Earth

Block I Satellite Vehicle

GPS Satellite Vehicle• Four atomic clocks

• Three nickel-cadmium batteries

• Two solar panels– Battery charging

– Power generation

– 1136 watts

• S band antenna—satellite control

• 12 element L band antenna—user communication

Block IIF satellite vehicle (fourth generation)

GPS Satellite Vehicle

• Weight– 2370 pounds

• Height– 16.25 feet

• Width– 38.025 feet including

wing span

• Design life—10 years

Block IIR satellite vehicle assembly at Lockheed Martin, Valley Forge, PA

GPS Space Segment

• The space segments nominally consists of 24 satellites, currently:– 28 (24+4 spares) active GPS satellites (26 Block II, 2 Block IIR)

– Constellation design: at least 4 satellites in view from any location at any time to allow navigation (solution for 3 position + 1 station clock unknowns)

– “Right Time, Right Place, Any Time, Any Place”

• GPS Orbit characteristics:

– Semi-Major Axis (Radius): 26,600 km

– Orbital Period : 11 h 58 min

– Orbit Inclination: 55 degrees

– Number of Orbit Planes: 6 (60 degree spacing)

– Number of Satellites: 24 (4 spares)

– Approximate Mass: 815 kg, 7.5 year lifespan

– Data Rate (message): 50 bit/sec

– PRN (Pseudo-Random Noise) Codes: Satellite-dependent Codes

– Transmit, Frequencies L-Band L1: 1575.42 MHtzL2: 1227.60 MHtz

GPS Space Segment

Currently: 26 Block II, 2 Block IIR, no Block I satellites are active.Picture of a Block II Satellite

Components of the System

User segment• GPS antennas & receiver/processors

• Position

• Velocity

• Precise timing

• Used by– Aircraft

– Ground vehicles

– Ships

– Individuals

Components of the System

Ground control segment

• Master control station

– Schreiver AFB, Colorado

• Five monitor stations

• Three ground antennas

• Backup control system

GPS Control Segment

MCS Colorado Springs

Hawaii

Buenos Aires

US NIMA Tracking Sites

Diego Garcia

Ascension

Bahrain

Kwajalein

Smithfield

US Airforce Tracking Sites

US Airforce Upload Sites

Hermitage

Ouito

US Air Force and NIMA Control and Tracking Stations

See also map at <http://164.214.2.59/GandG/sathtml>MCS – Master Control Station

GPS Communication and Control

GPS Ground Control Stations

• Trilateration– Intersection of spheres

• SV Ranging– Determining distance from SV

• Timing– Why consistent, accurate clocks are required

• Positioning– Knowing where SV is in space

• Correction of errors– Correcting for ionospheric and tropospheric delays

GPS involves 5 Basic Steps

Accurate Timing is the Key

• SVs have highly accurate atomic clocks• Receivers have less accurate clocks• Measurements made using “nanoseconds”

– 1 nanosecond = 1 billionth of a second

• 1/100th of a second error could introduce error of 1,860 miles

• Discrepancy between satellite and receiver clocks must be resolved

• Fourth satellite is required to solve the 4 unknowns (X, Y, Z and receiver clock error)

Satellite Positioning

• Also required in the equation to solve the 4 unknowns is the actual location of the satellite.

• SV are in relatively stable orbits and constantly monitored on the ground

• SV position is broadcast in the “ephemeris” data streamed down to receiver

How does GPS work?

• Satellite ranging– Satellite locations

– Satellite to user distance

– Need four satellites to determine position

• Distance measurement– Radio signal traveling at speed of light

– Measure time from satellite to user

• Low-tech simulation

How does GPS work?

Pseudo-Random Code

• Complex signal

• Unique to each satellite

• All satellites use same frequency

• “Amplified” by information theory

• Economical

Signal Structure

• Each satellite transmits its own unique code

• Two frequencies used– L1 Carrier 1575.42 MHz

– L2 Carrier 1227.60 MHz

• Codes– CA Code use L1 (civilian code)

– P (Y) Code use L1 & L2 (military code)

How GPS works?

• Range from each satellite calculatedrange = time delay X speed of light

• Technique called trilateration is used to determine you position or “fix” – Intersection of spheres

• At least 3 satellites required for 2D fix • However, 4 satellites should always be used

– The 4th satellite used to compensate for inaccurate clock in GPS receivers

– Yields much better accuracy and provides 3D fix

How does GPS work?

• Distance to a satellite is determined by measuring how long a radio signal takes to reach us from that satellite.

• To make the measurement we assume that both the satellite and our receiver are generating the same pseudo-random codes at exactly the same time.

• By comparing how late the satellite's pseudo-random code appears compared to our receiver's code, we determine how long it took to reach us.

• Multiply that travel time by the speed of light and you've got distance.

• High-tech simulation

How does GPS work?

• Accurate timing is the key to measuring distance to satellites.

• Satellites are accurate because they have four atomic clocks ($100,000 each) on board.

• Receiver clocks don't have to be too accurate because an extra satellite range measurement can remove errors.

How does GPS work?

• To use the satellites as references for range measurements we need to know exactly where they are.

• GPS satellites are so high up their orbits are very predictable.

• All GPS receivers have an almanac programmed into their computers that tells them where in the sky each satellite is, moment by moment.

• Minor variations in their orbits are measured by the Department of Defense.

• The error information is sent to the satellites, to be transmitted along with the timing signals.

Position is Based on Time

T + 3

Distance between satellite and receiver = “3 times the speed of light”

T

Signal leaves satellite at time “T”

Signal is picked up by the receiver at time “T + 3”

Pseudo Random Noise Code

Receiver PRN

Satellite PRN

Time Difference

What Time is It?

Zulu Time

Military Time(local time on a 24 hour clock)

Universal Coordinated Time

Greenwich Mean Time

Local Time: AM and PM (adjusted for local time zone)

GPS Time + 13*

* GPS Time is ahead of UTC by approximately 13 seconds

Signal From One Satellite

The receiver is somewhere on this sphere.

Signals From Two Satellites

Three Satellites (2D Positioning)

Triangulating Correct Position

Three Dimensional (3D) Positioning

GPS Position Determination

System Performance

• Standard Positioning System– 100 meters horizontal accuracy

– 156 meters vertical accuracy

– Designed for civilian use

– No user fee or restrictions

• Precise Positioning System– 22 meters horizontal accuracy

– 27.7 meters vertical accuracy

– Designed for military use

System Performance

Selective availability

• Intentional degradation of signal

• Controls availability of system’s full capabilities

• Set to zero May 2000

• Reasons

– Enhanced 911 service

– Car navigation

– Adoption of GPS time standard

– Recreation

System Performance

• The earth's ionosphere and atmosphere cause delays in the GPS signal that translate into position errors.

• Some errors can be factored out using mathematics and modeling.

• The configuration of the satellites in the sky can magnify other errors.

• Differential GPS can reduce errors.

Differential Correction

• Technique used to correct some of these errors

• Referred to as “differential GPS” or DGPS

• In DGPS, two GPS receivers are used

• One receiver is located at an accurately surveyed point referred to as the “base station”

• A correction is calculated by comparing the known location to the location determined by the GPS satellites

• The correction is then applied to the other receiver’s (known as the “rover”) calculated position

DGPS Methods

• Post-processing – Corrections performed after the data is collected

– Special software required

• Real-time – Corrections are performed while the data is being

collected

– Need special equipment to receive the DGPS signal

Wide Area Augmentation System - WAAS

• New “real-time” DGPS

• Satellite based

• FAA initiative….now fully operational

• Series of ~25 ground reference stations relay info to master control station

• Master control station sends correction info to WAAS satellite– http://gps.faa.gov/programs/waas/howitworks.htm

GPS Accuracy Comparison

GPS Device Autonomous WAAS

DGPSReal-time

DGPSPost-process

DGPS

Garmin GPSMap 76s ~ 10 - 15 ~3 3 1 - 3

Rockwell – PLGR

Federal Users Only ~ 8 - 15 NA 3 NA

Trimble - GeoXT ~ 10 ~3 1-3 Sub-meter

Some common GPS devices used by FWS:

Accuracy given in meters

GPS Accuracy Issues

• Ways to improve the accuracy of your GPS collected data– Standardize data collection methods– Establish protocols for your applications– Employ averaging techniques– Perform mission planning– Utilize DGPS – Understand how the selection of datums and coordinate systems

affect accuracy• GPS data collected in wrong datum can introduce ~200 meters of error into your

GIS!

Application of GPS Technology

• Location - determining a basic position

• Navigation - getting from one location to another

• Tracking - monitoring the movement of people and things

• Mapping - creating maps of the world

• Timing - bringing precise timing to the world

Application of GPS Technology

• Private and recreation– Traveling by car

– Hiking, climbing, biking

– Vehicle control

• Mapping, survey, geology

• English Channel Tunnel

• Agriculture

• Aviation– General and commercial

– Spacecraft

• Maritime

GPS Navigation

Handheld GPS Receivers

• Garmin eTrex

– ~$100

• Garmin-12

– ~$150

• Casio GPS wristwatch

– ~$300

• The GPS Store

Some issues to consider when purchasing GPS devices

• What is the accuracy level required for your application?

(10 meters or sub-meter)

• How is unit going to be used in field?– External antenna required, in heavy canopy, ease of use, durability, data

dictionary capability, waterproof…

• Cost…… from $100 to $12K

• Staff expertise..training..support network

• How well does unit interface with GIS?

Mobile mapping software for WindowsCE devices

TerraSync (Trimble)

ArcPad (ESRI)

Multi-path rejection technology

Trimble GeoXT

Bluetooth

Allows for cable free operation

Latest Technology

Bring GIS data into the field!

Custom forms for data collection

ArcPad Software

Integrate GPS with GIS

Websites

• GPS Links from IGS: http://igscb.jpl.nasa.gov/overview/links.html

• U.S. Coast Guard Navigation Information Center: http://www.navcen.uscg.mil

• U.S. Department of Transportation: http://www.dot.gov

• NIMA Satellite Geodesy: http://164.214.2.59/GandG/sathtml/

• UNAVCO: http://www.unavco.ucar.edu/

• GPS Environmental & Earth Science Information System: http://genesis/html/index.shtml

• GPS Joint Program Office http://gps.laafb.af.mil/

Books:

• Institute of Navigation, Global Positioning System, Vol. I, Papers published in NAVIGATION,

ISBN: 0-936406-00-3, 1980 (Spilker, Van Dierendonck, etc.

• American Institute of Aeronautics and Astronautics (AIAA), Global Positioning System: Theory and

Applications, Volume I & II, Progress in Astronautics and Aeronautics ISBN or Order Number: 1-

56347-107-8 , 1996

• Kleusberg, A. P. Teunissen,ed. GPS for Geodesy, Lecture Notes in Earth Sciences, Springer-

Verlag, ISBN 3-540-60785-4, 1996

• Springer, T.A., “Modeling and Validating Orbits and Clocks Using the Global Positioning System”,

Doctoral Thesis, University of Bern, Switzerland, November 1999

References

67

U.S. Augmentations

Nationwide Differential GPS Wide Area Augmentation System

Local Area Augmentation SystemContinuously Operating Reference Stations

Galileo EU/ESA GPSUSA

GLONASSRussia

Global Navigation Satellite Systems

GNSSAccuracy 10m or better

CompassChina

PlannedIndia, Japan, Korea

Space Based Augmentation SystemsImproves GNSS accuracy to 3 metres

Part copied from http://www.kowoma.de/en/gps/errors.htm

GNSS Errors

Geometry up to 100m Orbits up to 5m

Canyon Effect – 1 metreIonospheric & Tropospheric

diffraction 10 + 1 metres

Timimg errors 4m –Rounding errors 1m