Introduction to gps and gnss

Introduction to GPS and GNSS

Vivek [email protected]

Early Space-Based Radio Navigation System� Launch of Sputnik – Tracking? -------------Doppler Shift.

Altitude: 985km; revolution period: 98 min

� Frank McClure, of the Applied Physics Laboratory, made

a suggestion: would it be possible to invert this problem?

– given rise to TRANSIT in late 1950’s (US- 6 sat; Altitude:

1100km; revolution period: 108 min) / TSYKLON(USSR-10

sat; 6- PARUS: Military; 4- TSIKADA-commercial/civilian;

Altitude: 1000km)

� The Navy Navigational Satellite System or TRANSIT, used

observed measurements in Doppler shift to calculate

distance and position to satellites (till 31-12-96).

� A fix requires 40 minutes for a static user-2D.

“ It is an all-weather, space based navigation system development by the U.S. DOD to satisfy the requirements for the military forces to accurately determine their position, velocity, and time in a common reference system, anywhere on or near the Earth on a continuous basis. ”

Module 2 - GPS

# In 1973 the U.S. DOD decided to establish, develop, test, acquire, and deploy a spaceborne Global Positioning System (GPS), resulting in the NAVSTARGPS (NAVigation Satellite Timing And Ranging Global Positioning System).

NAVSTAR Global Positioning System

GPS General CharacteristicsGPS General Characteristics

Module 2 - GPS

Global Positioning System

� Developed by the US DOD� Provides

� Accurate Navigation� 10 - 20 m

� Worldwide Coverage� 24 hour access� Common Coordinate System

� Designed to replace existing navigation systems� Accessible by Civil and Military

Introduction – GNSS� The theoretical definition:

� “GNSS. A worldwide position and time determination system that includes one or more satellite constellations, aircraft receivers and system integrity monitoring, augmented as necessary to support the required navigation performance for the intended operation.” [from ICAO Annex 10, Volume I]

Module 2 - GPS

• USA’s GPS (+ WAAS)• Russian GLONASS(+SBAS)• EU’s Galileo (+ EGNOS);• Japan’s QZSS (+ MSAS);• India’s IRNSS (+ GAGAN);• China’s Beidou; Compass;• Australian Augmentations

GNSS is the result of a recognition by the civilian community of the benefits that can be derived from the development of a 'true' civilian global positioning system that is: Multimodal (air, sea and land users), Capable of meeting future navigation & timing requirements, Global standard, Cost effective, Easy to use, Fundamentally based around the integration and augmentation of technologies.

GNSS elements: GPS and augmentation systems

(based on aircraft /satellite/ground inst.)

How Well Does It Work?

Navigation Accuracy Comparisons

Omega- 2 km

Inertial- 1 km

Tactical Air Navigation(TACAN)- 400m

Transit- 200m

LORAN C- 180m

GPS- 15m

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GPS Constellation

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Space Segment( Initial Operational Capability(IOC)-1993)(Full Operational Capability(FOC)-1995)

Block I

First Launch: 22 Feb 78(78-85)On-Orbit: None, Total=11

Block IIR / IIR-M(L2C civil signal & new military code M on both L1& L2)

Block II/IIA

First Launch: 14 Apr 89(89-97)Total: 28

First Launch: 2009Acquiring up to 19 SV’s

Block IIFFirst Launch: 22 Jul 1997/25Sep2005Total=21/8(R: Replenishment; M: Modernized)

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Block III

GPS System ComponentsGPS System Components

� 24 Satellites� 4 satellites in 6 Orbital

Planes inclined at 55 Degrees

� 20200 Km above the Earth

• 12 Hourly orbits – In view for 4-5 hours

• Designed to last 7.5 years• Different Classifications

– Block 1, 2, 2A, 2R & 2 F

EquatorEquator

55

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Managed by the US National Space-Based Positioning, Navigation, and Timing (PNT) Executive Committee

SpaceSpace SegmentSegment

� Master Control Station

� Responsible for collecting

tracking data from the monitoring stations and calculating satellite orbits & clock parameters

� 5 Monitoring Stations

� Responsible for measuring pseudorange data. This orbital tracking network is used to determine the broadcast ephemeris and satellite clock modeling

� Ground Control Stations

� Responsible for upload of information to SV’s

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User Segment: User Segment: The most visible segment

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� Everyone!� Merchant, Navy, Coast Guard vessels

� Forget about the sextant, Loran, etc.� Commercial Airliners, Civil Pilots� Surveyors

� Has completely revolutionized surveying� Commercial Truckers� Hikers, Mountain Climbers, Backpackers� Cars now being equipped� Communications and Imaging Satellites

� Space-to-Space Navigation� Any system requiring accurate timing

� Weather Independent

� Does not require line of sight

� Gives high Geodetic Accuracy

� Can be operated day and night

� Quicker and requires less manpower

� Economical advantages

� Common Coordinate System

� Wide Range of Applications

� Competitively Priced

Why GPS ?Why GPS ?

Module 2 - GPS

How It Works (In 5 Easy Steps)

� GPS is a ranging system (triangulation)� The “reference stations” are satellites moving at 4

km/s

1. A GPS receiver (“the user”) detects 1-way ranging signals from several satellites

� Each transmission is time-tagged� Each transmission contains the satellite’s position

1. The time-of-arrival is compared to time-of-transmission

2. The delta-T is multiplied by the speed of light to obtain the range

3. Each range puts the user on a sphere about the satellite

4. Intersecting several of these yields a user position

Module 2 - GPS

Xll

VlX

l

lll

lll

lVV

VllVlll

X

lX

Range = Time Taken x Speed of Light

Outline Principle : RangeOutline Principle : Range

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Point Positioning: Point Positioning:

Accuracy 10 - 100 mAccuracy 10 - 100 m

A receiver in autonomous mode provides navigation and

positioning accuracy of about 10 to 100 m due to the effects

of GPS errors!!?

Multi-Satellite Ranging

1 range puts user on the spherical face of the cone.

Intersecting with a 2nd range restricts user to the circular arcs.

A 3rd range constrains user to 1 of the 2 points.

Pictures courtesy http://giswww.pok.ibm.com/gps

Module 2 - GPS

� The satellites are like “Orbiting Control StationsOrbiting Control Stations”

� Ranges (distances) are measured to each satellite using

time dependent codes

� Typically GPS receivers use inexpensive clocks. They are

much less accurate than the clocks on board the satellites

� A radio wave travels at the speed of light

� (Distance = Velocity x Time)

� Consider an error in the receiver clock

� 1/10 second error = 30,000 Km error

� 1/1,000,000 second error = 300 m error

Outline Principle : PositionOutline Principle : Position

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Timing� Accuracy of position is only as good as your clock

� To know where you are, you must know when you receive.

� Receiver clock must match SV clock to compute delta-T� SVs carry atomic oscillators (2 rubidium, 2 cesium each)

� Not practical for hand-held receiver � Accumulated drift of receiver clock is called clock bias� The erroneously measured range is called a pseudorange� To eliminate the bias, a 4th SV is tracked

� 4 equations, 4 unknowns� Solution now generates X,Y,Z and b

� If Doppler also tracked, Velocity can be computed

iirsiirs Module 2 - GPS

GPS Signal StructureGPS Signal Structure� Each GPS satellite transmits a number of signals� The signal comprises two carrier waves (L1-19cm and L2-

23cm) and two codes (C/A on L1 and P or Y on both L1 and L2) as well as a satellite orbit message

� Bandwidth allocated for L1-24 MHz, L2-22 MHz, & L5-28 MHzFundamental

Frequency10.23 MHz

FundamentalFrequency10.23 MHz

x 154

x 120

L11575.42 MHz

L11575.42 MHz

L21227.60 MHz

L21227.60 MHz

C/A Code1.023 MHz

C/A Code1.023 MHz

P (Y)-Code10.23 MHz

P (Y)-Code10.23 MHz

P (Y)-Code10.23 MHz

P (Y)-Code10.23 MHz

÷ 10

50 BPS50 BPS Satellite Message (Almanac & Ephemeris)Satellite Message (Almanac & Ephemeris)


L3(1381.05MHz): L3(1381.05MHz): used only for a Nudet (Nuclear Detection) used only for a Nudet (Nuclear Detection) Detection System (NDS).Detection System (NDS).

Code Modulation

Source: Peter Dana, http://www.colorado.Edu/geography/gcraft/notes/gps/gps_f.html


Selective Availability (SA):To deny high-accuracy realtime positioning to potential enemies, DoD reserves the right to deliberately degrade GPS performance (on C/A code: deactivated on 1 may, 2000).

Coarse Acquisition (C/A) Code

� 1023-bit Gold Code

� Originally intended as simply an acquisition code for P-

code receivers

� Modulates the L1 only

� Chipping rate = 1.023 MHz (λ=290 meter)

� Sequence Length = 1023 bits, thus Period = 1 millisec

� Provides the data for Standard Positioning Service (SPS)

� The usual position generated for most civilian receivers

� Modulated by the Navigation/Timing Message code


Precise (P) Code� Generally encrypted into the Y-code (A.S.)

� Requires special chip to decode

� Modulates both L1 & L2

� Also modulated by Nav/Time data message

� Chipping rate=10.23 MHz (λ=29.30m) i.e. 10 times faster

than C/A code ensuring improved time measurement.

� Sequence Length = 2.35*1014 bits, thus Period = 266 days

� P-code rate is the fundamental frequency (provides the

basis for all others)� P-Code (10.23 MHz) /10 = 1.023 MHz (C/A code)

� P-Code (10.23 MHz) X 154 = 1575.42 MHz (L1).

� P-Code (10.23 MHz) X 120 = 1227.60 MHz (L2).


Navigation Message

� In order to solve the user position equations, one must know where the SV is:

� The navigation and time code provides this

� 50 Hz signal modulated on L1 and L2� The SV’s own position information is transmitted in a 1500-bit data

frame

� Pseudo-Keplerian orbital elements � Determined by control center via ground tracking

� Receiver implements orbit-to-position algorithm� Also includes clock data and satellite status� And ionospheric / tropospheric corrections� The International Telecommunication Union (ITU) has reserved 1559-

1610MHz band for satellite based navigation through World Radio Communication (WRC) conferences, held every three year.

� GPS bands (US Federal Communication Commission): (1215-1240MHz, 1559-1610 MHz, L5- 1164-1188MHz)


The Almanac� In addition to its own nav data, each SV also

broadcasts info about ALL the other SV’s� In a reduced-accuracy format

� Known as the Almanac� Permits receiver to predict, from a cold start,

“where to look” for SV’s when powered up� GPS orbits are so predictable, an almanac may be

valid for months� Almanac data is large

� 12.5 minutes to transfer in entirety


GPS PositioningGPS Positioning• Point Positioning Methods using stand alone receivers

provide 10 - 100 m accuracy

– Dependent on SA

– 1 Epoch solution

• Differential Positioning Methods using 2 receivers, simultaneously tracking a minimum of 4 satellites (preferably 5) will yield 0.5 cm to 5 m accuracy with respect to a Reference Station

• Differential Techniques using CodeCode will give meter meter accuracy

• Differential Techniques using PhasePhase will give centimetercentimeter accuracy


� Topo and Locations� Mapping� Monitoring� Volumes� Photo control� Construction Control

and Stakeout

� Boundaries� Seismic Stakeout� Profiles� Establishing Portable Control

Stations (sharing with Total Stations)

� Agriculture - Slope Staking� Tracking of people, vehicles� Plate movements� Sports (boating, hiking,…) � Archeology � Public Transport� Emergency services

ApplicationsApplications


User requirements in positioning accuracies User requirements in positioning accuracies Accuracy Geomatics Land Marine Airborne

Geodetic Infrastructure Earth Moving Dredging Cat II/III

Fault Monitoring Road Grading Pylon Positioning Sensor positioning< 20 cm Construction Surveys Agriculture (air- spaceborne)

Engineering Surveys MiningGeodynamics Urban cadastral survey

Resource Mapping Facility Surveys GIS Database Mapping/GIS Docking

0.2 - 1.5 m Utility Mapping Highway Surveys Buoy PositionHighway Surveys Rural cadastral survey Legal Surveys Precision farming

GIS Data Collection Automobiles Channel Navigation Sensor navigation1 - 5 m Site Specific Farming Emergency Cabling Oceanic

Navigation Public Transport ResearchTracking Harbor Entry Mapping

10 - 100 m Reconnaissance Navigation Harbor ApproachArea Navigation Oceanic

# Accuracy requirement depending on application iirs


GPS Opportunities in India• Indian GPS market can be broadly divided in following segments• National Survey Agencies(NRSC, SOI,Mil. Survey,NHO,)• Land Records (Kerala,TN,J&K, Rajasthan,Gujrat, etc.)

• Oil and Energy( OIL, ONGC, BPCL, HPCL)• Power(NHPC Assam, Himachal, J&K,Arunachal, Kaveri Basin)• Defense, Remote Sensing , GIS, Forestry AND Education• Scientific Institution(IIGM,CMMAC,WIHG, NGRI, SAC,GSI)• Mining and Geology(CMPDIL,WCL,MCL, SECL, NCL,BCCL)

• Consultancy( CRRI, CBRI, NCB, CSMRS,WAPCOS)• Engineering /Infrastructure(IRCON,RITES,Delhi Metro, NHAI)• Private Surveying Companies (ICT, Theowell, Secon, etc. )• Marine and Maritime Boards(Port Trust, MMB, NIO, etc.)


� Never get lost again� Earth deformation measurement is the main application of GNSS.� GPS is no longer primarily a military tool � Now a product with vast commercial potential

� From aviation to outdoor recreational activities � Applications will change our lives and save money

� GPS with computer mapping will help manage our natural resources

� Vehicle location and navigation lets us avoid congested freeways, find more efficient routes and save fuel, & air pollution

� Ships and aircraft are safer in all weather conditions� Businesses with large outside plant (railroads, utilities) will manage

resources more efficiently, reducing consumer costs� The GPS technology is evolving rapidly� More accurate and affordable TODAY than YESTERDAY.

Conclusioniirsiirs Module 2 - GPS

References� http://gps.gov/ � http://www.glonass-ianc.rsa.ru/pls/htmldb/f?

p=202:1:15000421459964108253

� http://igscb.jpl.nasa.gov/� http://www.navcen.uscg.gov/?pageName=GPS� Interface Control Documents:� http://www.navcen.uscg.gov� http://www.Glonass-ianc.ras.ru� http://www.Galileoju.com


Communications

Surveying & Mapping

Fishing & Boating

Off shore Drilling

Recreation

Trucking & Shipping

Personal Navigation

Aviation

Railroads

Power Grid Interfaces

Use of GPSPrecision farming
