Global Navigation Satellite Systems Global Positioning System GPS

Global Navigation Satellite Systems Global Positioning System (GPS) Source: ITC, University of Twente, Enschede, the Netherlands. 1

Introduction to positioning • Determination of position is an essential component in many disciplines. The position is particularly important to; – Navigators – GIS users – Military forces – Surveyors – Explorers/hikers etc… 2

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 • Applications of GPS 3

Technological development of positioning (1) 4

Technological development of positioning (2) Theodolite Total Station Digital Theodolite 5

Advantages of Space Based System • An accurate, instantaneous, weather independent and all time positioning system was in very much demand • To fulfil the above requirement the system should be free of visual observations (optical observations), computationally accurate and fast, all weather and time independent and position independent 6

Global Navigation Satellite System (GNSS) • There are number of Satellite Systems meant for positioning NAVSTAR GPS: USA, military, fully working, will be modernized. GLONAS: Russia, military, working but not yet complete GALILEO: Europe, civil, being built up, not yet complete + other, non global ones by India and China 7

NAVSTAR Global Positioning System (GPS) • NAVigation System with Time And Ranging • It is a brain child of United States Department of Defense and Joint Program Office. • Provides accurate, continuous, worldwide, 3 D position and velocity information to the user with appropriate receiving equipment. • Precise Positioning Service (PPS) – Primarily intended for military and selected govt. agencies • Standard Positioning Service (SPS) – Available for all users worldwide 8

GPS Segments • The system can be divided in to three major segments. 1. Space Segment Space Vehicles/satellites 2. Control Segment Ground Monitor Stations 3. User Segment User community 9

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. 10

Space Segment Source: Wikipedia 11

Summary of satellites Currently in orbit and healthy Satellite launches Block Launch period In Planned preparation Success Failure I 1978– 1985 10+ 1 1 0 0 0 II 1989– 1990 9 0 0 IIA 1990– 1997 19 0 0 IIR 1997– 2004 12 +1 1 0 0 12 IIR-M 2005– 2009 8+1 1 0 0 7 IIF 2010– 2016 12 0 0 0 12 IIIA IIIF From 2017 — Total 0 0 70 0 0 3 0 0 0 12 22 34 0 0 31 Source: Wikipedia 12

Space segment of GPS system NAVSTAR GPS Satellite The space segment of GPS consists of 24 satellites on 6 orbits (approx. 20, 200 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 13

The space segment of the GPS system 14

Space segment of GLONASS system GLONASS Satellite 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 vertical 15

Space segment of Galileo system Galileo Satellite 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 centres in Europe • Accuracy of single receiver: around 1 m 16

GPS Control Segment • The GPS worldwide satellite control system consist of five monitor stations. • Remote monitor stations constantly track and gather C/A and P code from the satellites and transmit this data to the Master Control Station, which is located at Falcon Air Force Base, Colorado Springs. • Master control station computes ephemeris and satellite clock corrections and upload satellites with updated values. 17

The control segment of the GPS system • Master control station + monitor stations • Control stations receive signals from satellites, calculate precise orbital and time data for each satellite. The master control station uploads the calculated correction data to the satellites. These correction data are transmitted together the GPS signal to the receivers. 18

First GPS Control Stations Master control station 19

GPS Control Segment (Earthmap, NASA: http: //visibleearth. nasa. gov/) 21 20

GPS Users • The user community whom uses GPS as a tool for wide spectrum of applications are known as user segment. 21

The user segment of the GNSS system • Receivers and their users: – Navigation in 3 D - aircrafts, ships, ground vehicles and hand-carried instruments – Precise positioning - surveying 22

How does a GPS satellite communicate to a receiver ? • It uses Codes (binary, zeros and ones, language of computers) • In order to send codes through long distances the energy of the wave should be increased. But this distort the frequency • One remedy is to load the wave onto a wave with higher energy known as carrier wave – Phase of the carrier wave can be changed or modulated in order to carry information • These carriers are electromagnetic waves from part of the L band of the electromagnetic spectrum. 23

GPS Signals • Each GPS Satellite transmits on 2 carrier frequencies L 1 & L 2 • Their frequencies are derived from the fundamental clock frequency (f 0 = 10. 23 MHz) • L 1 = 154* f 0 (f = 1575. 42 MHz, wavelength =19 cm) • L 2 = 120* f 0 (f = 1227. 64 MHz, wavelength =24. 4 cm) 24

GPS Signal Structure of original GPS design 25

GPS Signal Structure Coarse / Acquisition code • C/A code frequency 1. 023 mega bits per second (wave length = 300 m) repeated in every millisecond. Each satellite broadcast unique C/A code. Freely available to the public. P code • P code frequency 10. 23 Mbits/S (wave length = 30 m) repeats only once a week. Therefore, the chirping rate of P code is ten times faster than C/A code. This also define the satellite uniquely. Reserved for the military applications Navigation Message • Provides detail information about each satellite’s position and the network. The GPS design has this information modulated on top of both C/A and P codes at 50 bit/s and calls it the Navigation Message 26

GPS Signal Structure (2) • Navigation Message/code : vehicle for telling the GPS receiver some of the most important things they need to know (frequency 50 Hz, modulated onto both carriers) • Sub frame 1 – Clock correction, GPS week, satellite health • Sub frame 2 & Sub frame 3 – Ephemeris – orbital information of satellite (shape/size etc. . ) • Sub frame 4 & Sub frame 5 – Almanac data – where to find all the satellites – Atmospheric correction 27

Modernized GPS signals • Military M code – Transmits Block IIR-M – Will probably replace P code • New civilian signal - L 2 C – transmits Block IIR-M and later designed satellites – broadcast on L 2 frequency • CNAV message – upgraded version of the original navigation message • L 5 Carrier– Safety of life – Block IIR-M 20 satellite – Recently started to broadcast (2009) • And many more ………. 28

Satellite clocks and GPS Time • Each satellite carries own onboard; very stable & accurate atomic clocks • While most clocks are synchronized to Coordinated Universal Time (UTC), the atomic clocks on the satellites are set to GPS time • The difference --- GPS time is not corrected to match the rotation of the Earth – it does not contain leap seconds or other corrections which are periodically added to UTC • Navigation message includes the difference between GPS time and UTC, then receivers subtract this offset from GPS time to calculate UTC and specific time zone values 29

Satellite clocks and GPS Time • The clocks in any one satellite are completely independent from those in any other. Therefore; they are allowed to drift up to one millisecond from GPS time. • Control stations monitor this drift and upload the offset into the navigation message (broadcast clock correction) • GPS receiver may relate satellite clock to GPS time with this correction 30

Range Observations A position on three dimensional space consists of three parameters (x, y, z). Determination of position is equivalent to solve for three unknowns in mathematics. Therefore, it is necessary to create three independent equations. In ranging; each equation is represented by distance (range) from a satellite to the receiver. The range = c x dt 31

Range Observations 32

How does GPS work? 0. 000214 sec Once the correlation of two codes is achieved, receiver is said “locked on” If the relation is interrupted receiver is said “lost lock”. 33

Range Observations 34

Pseudo Range Observations • Problem: But the above technique is not practically possible due to errors associated with the receiver clock. It is impossible to use precise atomic clock in the receiver due to the price and the size. • Solution: By treating the error of the receiver clock as another unknown the clock error can be computed and corrected. 35

Pseudo Range Observations 37 36

Range Observations 37

Four Satellite 38

Less satellites…… If only 3 satellites are available, only 3 unknowns can be found. If the receiver is not connected to an atomic clock, one of the coordinates should be known (well enough) - 2 D positioning. Most receivers assume, that the height did not change since the last 3 D-fix. Suitable for boats and in flat terrain, but it can cause position errors, if the height has clearly changed since the last 3 D-fix! 39

Accuracy of Pseudo Ranging Pseudo range can be resolved for about 1% of the accuracy of the code being used by using good receiver. • For C/A code (l = 300 m. ) accuracy = +/- 3 m. • For P code (l = 30 m) accuracy = +/- 30 cm. Various types of systematic errors are also associated with GPS observations. In order to reach highest accuracy these systematic errors are also to be considered. Therefore; A single receiver observing four or more satellites on its own to find a position is subjected to various errors and the errors can be large as +/-10 m. 40

What is actually measured? 41

Definition of receiver position • Position: where the pseudo-ranges of the satellites intersect 42

How to Reach Better Accuracy ? By comparing positions generated by simultaneous data, most of the systematic errors will get cancelled out and could reach much better accuracy levels when compared with positions obtained by single receiver. • This method is known as; Differential GPS (DGPS) technology. 43

Differential GPS Observations 44

Differential corrections for phase measurements By applying differential correction to phase measurements, positions can be obtained to an accuracy of cm level. Real time / post processing 45

Differential Corrections • The base station and rover should receive signals from same set of satellites in order to apply the differential correction successfully. This will reduce the errors by substantial amount. • By applying differential correction to code, positions can be obtained to an accuracy of 0. 5 m. • This is a suitable accuracy range for most applications. 46

Local & Wide Area DGPS • Correction between GPS receivers becomes weaker as the rover gets farther from the base • Local Area Differential GPS (LADGPS) – Base lines are limited to couple of hundred kilometers • Wide Area Differential GPS (WADGPS) – Uses network of base stations and distributes correction over a large area (continental/country) – e. g. “WAAS” (North America) “EGNOS” (Europe) 47

Wide Area DGPS Satellite Based Augmentation System (SBAS) • Many Base Receivers send their measurements to a data center, which computes a model for range error corrections for a large area. • Correction model uploaded to a satellite and broadcast to GPS receivers • Receiver corrects its own measurements with help of the received correction model and computes immediately more accurate positions. 48

Real Time (Differential Positioning, Code) • Base Receiver measures (pseudo) ranges to the satellites and computes the distance from coordinates. • The differences (required “range corrections”) are sent to the Rover. • Rover corrects its own measurements with the help of the received range corrections and computes immediately more accurate positions. • Real Time data comes to the receiver over a data link. 49

Communicate via …… Format RTCM (Radio Technical Commission for Maritime Services) • Radio link • GPRS/GSM • Internet as the communication link Courtesy 50

Post Processing (Differential Positioning, Code) When immediate correct positioning is not necessary; • 2 Code receivers needed, which can store the measured pseudo ranges; • differential corrections (from base) and raw observations can be downloaded to a computer • Special software uses the measurements of the base receiver to correct the measurements of the rover and compute more accurate positions. • “Post Processing” 51

Kinematic positioning • Receivers are in continuous motion • For relative positioning; – One stationary receiver – One or more moving receivers – They stop at each new point very shortly • Once the phase ambiguity is solved, receivers are kept running and their lock on to satellite is maintained • Best suitable for wide open areas • Fast and accurate 52

Real Time Kinematic (RTK) 13/1/2017 53

RTK Measurements Similar to Kinematic, but: Base station sends its observations to the rover. Rover computes position immediately. Therefore; communication (Radio / GSM / internet) is required. • Results and accuracies are known directly. • No need to measure longer (to be sure, that the result is good enough). • Solves integer ambiguity on-the-fly (very quickly) • • 54

Real Time Network Services (RTN) • Wide range of application • Setting up GPS at known station and establishing communication is expensive • RTN maintain by government or commercial bodies • Provides access for users on demand 55

The Netherlands……. • Permanent stations maintained by LNR Globalcom company • Transmit differential corrections to rover stations via GPRS/GSM, internet, Radio on demand • According to the coverage and length of baselines, you can select the base station • Can be either post processing or real time 56

WAAS/EGNOS/MSAS 57

WAAS/EGNOS/MSAS coverage 58

WAAS/EGNOS/MSAS 59

GPS receivers / Antennas • The antenna collects signals coming from the satellite. • The intercepted signals were converted to electric pulses and direct these pulses to the radio frequency section of the receiver • The most important hardware involved in GPS positioning is the receiver. • Wide array of receivers are available in the market 60

GNSS Operation modes: differential 61

GNSS Operation modes: differential 62

Accuracy • Handhelds – 10 – 25 meter • Differential GPS post processing - RTK – 2 mm 63

Sources of errors • Even if today's GPS receivers are extremely accurate, certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. • If most of the sources of error are unavoidable, it is important for the user to be aware of the ones that can influence and be prepared to take steps to reduce their impact. 64

Biases and solutions 65

Biases and solutions • Satellite clock bias; dt – Control segment gathers data for its uploads of broadcast clock corrections – Eliminates when doing DGPS • Receiver clock bias; d. T – Eliminates when doing DGPS • Orbital bias; dp – Navigation message includes the correction – broadcast ephemeris – Eliminates when doing DGPS 66

Biases and solutions • Ionospheric effect; dion – Ionosphere is the first part of the atmosphere that the GPS signal encounters – Depending on the electron density of the ionosphere, the range can be altered – Magnitude of the effect varies with; daily cycle (daylight) – The amount of time that the signal travels through the layer etc … • It affects code and carrier differently – delay and speed up • Apparent time delay contributed by ionosphere depend on frequency • To solve; – One quarter of the error can be solved from broadcast correction for single frequency receivers – Dual frequency receiver has the facility of modeling and removing significant part of this bias 67

Biases and solutions • Tropospheric effect; dtrop – Troposphere is the part of atmosphere closet to earth, it is neither ionized nor dispersive – Depending on the density of the troposphere, the range can be altered • Both L 1 and L 2 affect equally • Apparent delay contributed by troposphere independent of frequency • To solve; – Modeling the troposphere reduces 95% of the effect – Limit the distance that the signal travel through troposphere by using elevation angle – Differential GPS 68

Biases and solutions • Multipath • Limiting the effect of Multipath; – Careful site selection • Avoiding signals from low elevation angles (introducing cut off angle or mask angle) • Antenna design 69

Error model (C/A code, no SA) (SA: Selective Availability) 70 67

Error model DGPS (Code) 71

Relative static positioning • Several stationary (rovers and references) receivers simultaneously collect data • Observation session • Length of baseline • High accuracy • Slow Courtesy : Jan Van Sickle: GPS for Land Surveying 72

Quality indicators - Dilution of precision • Receiver’s position is derived from simultaneous solution of vectors between it and at least 4 satellites • The quality is then depend on these vectors; that is, The spatial distribution of the satellite configuration (satellite geometry) is directly influencing the quality of the position. • If satellites are flocked together in one area of the sky positional accuracy will be poor. The corresponding dilution of a precision will be a higher value. • Geometric dilution of precision represents the expected uncertainty of position coordinates and the clock offset. 73

Geometric Dilution of Position (GDOP) • The positional error depends on the ‘constellation’ of the satellites: the narrower the view angle between the satellites the larger the positional error 74

Quality indicators - Dilution of precision 75

More satellites Typical: • Receiver has many “Channels” (e. g. 12); • Each channel can “track” one satellite; • All satellites in view are tracked (if there are enough channels) • Position is computed by least squares adjustment using all tracked satellites • Mostly: More satellites - better GDOP 76

Handheld GPS - GARMIN 77

Future Developments • Interoperability – – GLONASS, GALILEO and GPS constellation GALILEO constellation 3 orbital planes 27 satellites + 3 spares Inclination angle – 56 degree Altitude – 23 616 km GLONASS constellation 3 orbital planes 21 satellites + 3 spares Inclination angle – 64. 8 degree Altitude – 19 100 km 78

Applications 79

ROAD TRAFFIC CONGESTION • A navigation device has a GPRS receiver for receiving real time information about or slow average speed on a stretch of motorway, indicating congestion. • The device calculates a new itinerary to avoid the congestion, based on historically record speeds on secondary roads weighed by the current average speed in the congestion area. 80

TECTONICS • GPS enables direct fault motion measurement of earthquake between earthquake GPS can be used to measure crustal motion and deformation to estimate seismic strain build up for creating seismic hazard maps 81

GPS AND TERRORISM • GPS is very important to determine the location of terrorist attacker‟s. For example, on the Gurudaspur strike, Indian intelligence agencies had determined that the GPS sets used by the terrorist were first turned on in Sargodha a home to Pakistan‟s largest airbase-on July 21, 2015, six days before the attack. • The set were then programmed with digital waypoints, which led the attackers the border to their targets in Punjab. 82

GPS AND TOURS • Location determines what content to display, for instance, information about an approaching point of interest. 83

NAVIGATION • Navigators value digitally precise velocity and orientation measurements. • With the help of GPS roads or paths available, traffic congestion and alternative routes, roads or paths that might be taken to get to the destination. • If some roads are busy (now or historically) the best route to take, The location of food, banks, hotels, fuel, airports or other places of interests, the shortest route between the two locations, the different options to drive on highway or back roads. 84

DISASTER RELIEF • Depend upon GPS for location and timing capabilities of earthquake, flood wildfires. 85

SURVEYING • Surveyors use absolute locations to make maps and determines property boundaries. • The surveying and mapping community was one of the first to take advantage of GPS because it dramatically increased productivity and resulted in more accurate and reliable data. • Today, GPS is a vital part of surveying and mapping activities around the world. 86

AUTOMATED VEHICLE • With the help of GPS location and routes for cars and trucks to function without a human driver. 87

AGRICULTURE • GPS-based applications in precision farming are being used for farm planning, field mapping, soil sampling, tractor guidance, crop scouting, variable rate applications, and yield mapping. • GPS allows farmers to work during low visibility field conditions such as rain, dust, fog, and darkness. 88

FISHING AND GPS • Synoptic maps of the main concentrations of fisherman villages, fishing ports and beach landing points, markets, processing, freezing and transhipment points, coastal landforms can be studied with the help of GPS. 89

OIL LEAK AND GPS • GPS tracking technology is helping with the study by examining how currents are influence by winds and waves and measuring wind speed to find out how oil would spread from the ocean, onto the beach. • Many instruments are being used in the study to gather as much data as possible. • After data is collected, researchers plan to use 3 D pictures of oil transports and hope to come up with more information about oil spills, how to mitigate their damage, and how to protect the environment. 90

• Fig: In 2010, GPS helped cleanup crews respond to the massive oil leak in the Gulf of Mexico. 91

GPS AND FOREST GPS • Technology Makes Tree Planting More efficient. Deforestation and disappearing wildlife habitats are a big problem in the modern world. • Manufacturing industries use state-of-the-art technologies to produce and sell more paper and wood products, but there is growing concern over the devastation wrought by their methods of obtaining materials. • One solution-orientated man is leading team, developing ways to replant forests as quickly and efficiently as possible, using GPS technology. 92

CARTOGRAPHY • Both civilian and military cartographers use GPS extensively. 93

Spatial Analysis Optimizes Malaria Prevention Measures • • Dying of Malaria Mapping from Scratch Tracking Malaria Results 94

GNSS Applications in Hydrologic Sciences • Georeferencing / ortho imaging satellite imagery/aerial photographs • DTM stake out • Cross section creation • Sample location • Measure (station) location • Ground control point collection DTM, land use, soil, etc. • (Ground)Water levels • Well location • Field/air/water navigation • Bathymetric point collection • Mobile GIS • Various engineering applications 95

Summary • • • GPS measures X, Y, Z and Time. Three segments: space, control, user Similar positioning systems: GLONASS, Galileo Signals: carrier and code Error sources Positional accuracies depends on: – Receiver type (no. of channels, code or carrier phase receiver) – Measurement method (static, observation time) – Positioning technique (absolute) • Applications of GPS 96

The end ! 97