Overview of Global Navigation Satellite Systems and Recent Developments

Prof. RAAJ Ramsankaran

Associate Professor

Department of Civil Engineering

Indian Institute of Technology Bombay, Mumbai-400076

Email: ramsankaran@civil.iitb.ac.in

13/07/2020

Webinar for Andhra Pradesh Human Resource

Development Institute (APHRDI)

Disclaimer

In this ppt, figures/text are adopted from various sources; this is only for class room explanation and training purpose and NOT for commercial activities.

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The Global Navigation Satellite SystemGeneric term for a set of high orbit satellites (~20,000 km alt.) working toprovide autonomous positioning, navigation and timing services.

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Source: NASA

Regional Navigation Satellite System (RNSS)

Four GNSS

Global Positioning System-GPS (USA)

GLObal NAvigation Satellite System-GLONASS (Russia)

BeiDou Navigation Satellite System- BDS (China)

Quasi-Zenith Satellite System (Japan)

Navigation with Indian Constellation-NavIC (India)

GALILEO Positioning System (European Union)

Two RNSS

Components of a GNSS/RNSS

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• Ground Antenna• Master Control

Stations• Monitor Stations

UPLINK DATA

AlmanacEphemeris ConstantsClock Correction Factors

DOWNLINK DATA

EphemerisAlmanacCoded Signals

Working Principle

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TRILATERATION

b

ρ: Pseudorange

b: Clock Bias; K –Satellite numbers

CARRIER PHASE IS USED MOSTLY (1-2 cm)

L1 and L2 frequency bands used as carrier

Error Sources in a GNSS

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At SPACE Level• Selective

Availability• Dilution of

precision• Ephemeris and

Clock Errors• Relativistic Errors

At PATH Level• Ionospheric • Tropospheric

Errors

At RECEIVER Level• Receiver Noise• Mutipath Errors

Source: BLACKROC

Selective Availability

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• Selective availability (SA) was an intentional degradation of GPS performance by the US government for national security reasons.

• Satellite clock corrections in the broadcast ephemeris were deliberately degraded to reduce the accuracy for civilian use of GPS to an accuracy level of 100 m for the horizontal position.

• However, on 2 May 2000, this feature was discontinued, and the USA announced that it would no longer impose this.

Dilution of Precison

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DOP or the geometric DOP (GDOP) is the general term to describe the geometry of satellites.

Figure: Illustration of GDOP

REMEDY

Most modern receivers now select those four satellites which leads to

low DOP.Manually one can look at the

SKYPLOT and start reading when geometry is strong

Ephemeral and Clock Errors

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Ephemeris is broadcasted every 30 seconds but due to electronic lags, the ephemeris used by receiver may be 2-3 hours old. This leads to incorrect satellite positions being used.

Atomic Clocks on board the satellites are prone to noise and drift errors. This leads to incorrect synchronization of satellite and receiver time leading to position errors.

REMEDY

Differential Positioning TechniquesPrecise Point Positioning

Updated Ephemeral Data provided by Internation GNSS Service (IGS)

Relativistic Errors

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Time Dilation due to difference in speeds of clocks on satellites and earth surface cause position errors of 10's of meters. This follows directly from General and Special Relativity

Sagnac Effect which causes difference in observables due to rotation of earth. The satellite observations are defined in inertial frame while observations are done in Earth centered Earth Rotating frame.

REMEDY

Modern receivers and processing softwares have built

in models to remove these errors

However, manually, they can be modelled physically and

removed

Ionospheric and Tropospheric Errors

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Ionosphere is a dispersive medium. Hence the propagation speed of signal depends on its frequency. This causes errors in position of upto 5 m.

Troposphere is a layer of highly varying refractive index due to climactic variations. This also causes changes in propagation speed and hence errors in position upto 1 m.

REMEDYUsing dual frequency receivers

Using ionospheric data from augmentation services

REMEDYUsing Geodetic Models (Require

Meteorological Data)Using Navigation Models(No

Meteorological Data)

Multipath Errors

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Multipath errors occur due to reflection of radio signals from opaque objects such as buildings, trees. Thus in addition to the direct signal from satellite, these signals also reach the receiver and cause multiple path illusion. The magnitude is highly dependent on surrounding area and can cause position errors upto 100m in worst conditions.

REMEDYSelecting satellites that are

atleast 150 above the horizonUsing speacial antennas sich as

'Choke Ring' antennas

Major Augmentation Systems

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Augmentation of a global navigation satellite system (GNSS) is a method of improving the navigation system's attributes, such as accuracy, reliability, and availability, through the integration of external information into the calculation process. (Source: Wiki)

Satellite Based (SBAS)

Ground Based (GBAS)

Additional inputs uploaded as broadcast messages to and

from satellites for global useEx:

Wide Area Augmentation Service (WAAS) - USA

GPS Aided Geo Augmented Navigation (GAGAN) - India

They are differential correction from permanently setup

monitoring stations broadcasted as RF signals.

Ex:All major airports provide

GBASIn India, IGI Delhi, RGI

Hyderabad Airports provide GBAS

SBAS Coverage

Wide Area Augmentation System (WAAS)

European Geostationary Navigation Overlay Service (EGNOS)

System for Differential Corrections and Monitoring (SDCM)

Multi-functional Satellite Augmentation System (MSAS)

GPS-aided GEO Augmented Navigation (GAGAN)

Illustration of SBAS Concept

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GPS-aided GEO Augmented Navigation GAGAN-System Overview

17 Photo: ISRO

Ground Based Augmentation System (GBAS)

➢ Also called a Local Area Augmentation Service (LAAS)

➢ GBAS aim at enhancing GNSS service levels for aviation during approach, landing and departure phases, as well as for surface operations.

➢ GBAS have a local coverage with the primary objective of meeting aviation requirements for the aforementioned operations and phases, in terms of accuracy, integrity and safety.

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Aircraft based Augmentation System (ABAS)- Special Category➢The Aircraft based augmentation can provide GNSS information as

necessary for supplemental means of navigation.

➢ It is a system that augments and/or integrates the information obtained from the other GNSS elements/navigation sensors with information available on board the aircraft.

➢The most widely used form of ABAS is Receiver Autonomous Integrity Monitoring (RAIM).

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GNSS Techniques

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STATIC

5 minutes5 minutes

5 minutes5 minutes

RAPID STATIC

30 minutes

30 minutes

CLASSIC STATIC

5 minutes

5 minutes

1 hour

REOCCUPATION

GNSS Techniques

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KINEMATIC

STOP AND GO CONTINUOUS

GNSS Techniques

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DIFFERENTIAL

X,Y,Z KNOWN

X,Y,Z OBSERVED

CORRECTIONS

RELAYED TO REFERENCE ST.

GNSS Techniques: Pros and Cons

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STATIC KINEMATIC

• 5-10 mm accuracy• Ambiguity

resolution not critical

• Minor effect of mutipath

• Undemanding on hardware/software requirements

• Time consuming• Inappropriate for

engineering applications

• 1-2 cm accuracy• High productivity• Similar procedure to

modern terrestrial surveying

• Special hardware/software requirement

• Susceptible to multipath and atmospheric errors

In actuality, these pros and cons vary according to real time or post processing usage.

Differential processing leads to sub-centimterlevel accuracy in both methods.

PR

OS

CO

NS

GNSS Receivers

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A typical reciver is a combination of antenna, front end (filtering of collected RF signals) and Analog to Digital Converter.

Important parameters used to characterize GPS receivers are:

• Central frequency (e.g.: GPS L1 = 1575.42 MHz) and bandwidth• Single or multi-frequency• Radiation pattern (directivity). GPS antennas are usually hemispherical• Impedance (typically 50 ohm)• Antenna Gain• Multipath rejection capability• Jamming mitigation capability

Basic process in a receiver is:

SKY-SEARCH SIGNAL COLLECTION PRN MATCHING TIMEAND POSITION

GNSS Receivers: Popular Brands

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GARMIN Series 60, 62, 76, 781. WAAS enabled, dual frequency2. Handheld3. Cost INR 20,000-50,000 4. 5-10 m horizontal precision

TRIMBLE Geo 7X and TRIMBLE TDC1501. WAAS, SBAS enabled2. Handheld3. Cost INR 2-3 lakh4. RTK level precision (1-5 cm)

TRIMBLE R12 INTEGRATED GNSS RECEIVER1. WAAS, SBAS enabled2. Can be used in static as well as dynamic mode3. Cost: N.A4. RTK level precision (8 mm Horizontal)

Choosing a GNSS Survey Technique #1

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WORKS GNSS TECHNIQUES

Geodetic Surveys

High Precision STATIC GNSSH: 5mm+1ppmV: 10mm+1ppm

Dual Frequency, multi-constellation and WAAS enabled Receivers

Geodetic Antennas with high multipath suppresion, broadband bandwidth

Differential Post processing optional

Atmospheric Monitoring Studies

Deformation Studies

Sea Level

Reference Frame Setups

High order land surveying

Choosing a GNSS Survey Technique #2

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WORKS GNSS TECHNIQUES

Topographical Profiling

Medium Precision STATIC GNSSH: 10mm+1ppmV: 20mm+1ppm

Dual Frequency, multi-constellation Receivers

Geodetic Antennas with high multipath suppresion,narrow to broad bandwidth

Ground Control Points for Aerial

Photography

Kinematic GNSS Reference Station

Positioning

Land Surveying

Choosing a GNSS Survey Technique #3

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WORKS GNSS TECHNIQUES

Temporary Differential GNSS Monitor Station

SetupLow Precision STATIC GNSS

H: 0.1-10mV: 2 times forH

Autonomous ReceiversHandheld Antennas with multipath

suppresion

Absolute WGS84 determination

Asset Positioning on ground

Land Surveying

Choosing a GNSS Survey Technique #4

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WORKS GNSS TECHNIQUES

Cadastral Surveys

High Precision KINEMATIC GNSSH: 10mm+2ppmV: 20mm+2ppm

Dual Frequency, multi-constellation and WAAS enabled Receivers

Geodetic Antennas with high multipath suppresion, broadband bandwidth

Gyrostablized RoversDifferential Post processing optional

Structural Monitoring

Volumteric and Area Surveys

GIS Attribute collection

Camera Points in Photogrammetry

Choosing a GNSS Survey Technique #5

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WORKS GNSS TECHNIQUES

Digital Mapping Medium/Low Precision KINEMATIC GNSSH: 0.1-4mV: 0.2-8m

Dual Frequency, phase smoothed, Code Phase Receivers

Rover based Antennas with narrow bandwidth

Gyrostablized Rovers

Yield Monitoring, Precise Farming

Facilities Mapping

GIS Attribute collection

Processing Softwares Available

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Open Source

1. MAAST: Matlab Algorithm Availability Simulation Tool, STANFORD2. RTKLIB: Real Time Kinematic Library3. GPSTk: GPS Toolkit, Univeristy of Austin, Texas4. GAMP: GNSS Analysis software for Multi-constellation and multi-frequency Precise positioning, from GFZ Postdam5. GAMIT/GLOBK/TRACK: from MIT

Freely Available

1. APPS: Automatic Precise Positioning Service, JPL NASA2. magicGNSS: by GMV3. CentrePoint RTX: Trimble

Commercial: Trimble Business Centre, Bernese, RTNet

GNSS Modernization

Space Segment:

• Inclusion of more civilian accessible frequencies besides L1/L2. Done with launch of GPS block IIF with L5(1176 MHz) and IIIF with L1C(1575.42 MHz) bands .

• Increasing the number of satellites so as to ensure that everytime there are 4Satellite Vehicles (SV) above 10°angle from horizon at any point on earth.

• Ending selective availability. The earlier GNSS constellations were built with SA features. New constellations are planned with no such feature.

Control Segment:

• Equipment upgradation in control segment in terms of improved cyber security, nuclear detonation check, jamming and spoofing of signals.

• Use of a novel message broadcast format that ensures better almanac updating in a receiver, (Since it is problematic in receivers being used after long time), updated earth orientation parameters.

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Civilian Applications of GNSS ➢ Land and Coastal Surveying

➢ Topographic Surveying

➢ Marine Studies Ex. Sono bouys deployment, Sea floor mapping, Oil rig positioning, Dredging

➢ Precision Agriculture

➢ Deformation Monitoring: At Dams, Cultural heritage sites

➢ Aerial Surveys: Drone surveying, Aerial Photography

➢ Transportation: Aerial, Marine, Road, Intelligent Transport Systems (ITS)

➢ Utility Mapping- Ex. Communication cable, pipelines

➢ Hydrology: Ex. Precipitable Water Vapour Estimation, Soil moisture, Snow depth estimations

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