Satellite Positioning and Navigation Systems

Satellite Positioning and Navigation Systems
Prof. Wu Chen Department of Land Surveying and Geoinformatics Hong Kong Polytechnic University Tel:

Where and When?

Timing Accuracy Type ms/day Time to Lose one Sec
Hydrogen ,000,000 yr Cesium & rubidium ,000 yr Quartz crystal yr Earth rotation Pendulum

Positioning Accuracy – Global
Space-based i.e. GNSS Accuracy: 1 cm (surveying) m -10 m (Nav) Ground-based i.e. Loran-C Accuracy: 200 m Celestial i.e. Stars Accuracy: a few nmi

Precise Timing is crucial for Positioning
John Harrison Chronometer (1764) James Cook’s global voyage Atomic Clock To make GPS possible Next Gen. Clock Cold Atom (2001 Noble Price) 3-4 order better than conventional Atomic Clock

Positioning Technologies – Power of nations
Development of Marine Navigation Emerged “Ocean Power Countries” Development of Modern Positioning Technologies GPS, Satellite Remote Sensing, INS, etc Military Dominance of USA Recent Wars (i.e. Iraqi wars)

Positioning Methods Land Marking Celestial Positioning
Distance and angles

Land Marking Visual Non-visual Nature land shapes Buildings Images
Lighthouse, “Aobao” Non-visual Beacons Terrain Matching Signal Matching (“Finger print”)

Celestial Positioning
Reference to Stars Direction “Star of North” Latitude Angle of star location Longitude Time when stars passing you

Direction ‘North’ True North: rotation axis of the earth
Magnetic North: ‘north’ end of magnetic needle, affected by the earth’s magnetic field only. Grid North: ‘north’ of a map, dependent on map projection Bearing: Horizontal angle referenced to the ‘north’ Measurement of North Compass Gyro Two known points

Distance Measurement  One-way ranging (GPS, GLONASS)
Measure t =  / c = one-way time of travel Calculate  = c t Two unsynchronised clocks Measure t = 2 / c = two-way time of travel Calculate  = c t / 2 One clock used to measure t Two-way ranging (PRARE) GEOMETRY OF SIGNALS Range measurements can be classified as one-way or two-way ranging methods. The time of travel in a two-way ranging system is measured e.g. with the clock of the observer. The signal travels from the observer to the satellite, and is reflected back to the observer. So, the distance between the observer and the satellite is the speed of light multiplied with half the travel time of the signal. Also, the case vice versa is applied, i.e. that the time of travel is measured with the clock of the satellite. One-way ranging demands that the clocks of the ground station and of the satellites are synchronised with each other or that the synchronisation error can be determined (GPS). The time of travel from the satellite to the observer is measured. The distance is calculated by multiplying the speed of light with the travel time of the signal.

DOPPLER EFFECT Frequency Time DOPPLER EFFECT
The Austrian physicist C. J. Doppler discovered in 1842, that waves emitted by a moving object as received by an observer will be compressed (blue-shifted) if approaching, elongated (red-shifted) if receding. It occurs both in sound as well as electromagnetic phenomena, although it takes on different forms in each. In every-day life, this phenomenon occurs, if for instance an emergency vehicle with a loud siren approaches an observer and drives past. The pitch (the received frequency) audibly changes as the vehicle passes by. When the vehicle drives towards the observer the tone is higher, when it moves from the observer the tone drops. At the moment of the shortest distance between sender and receiver the relative velocity between sender and receiver is zero and the received frequency is the same as the emitted frequency. In astronomy, the Doppler effect appears in the cosmological redshift, an effect where light emitted from a distant source appears red-shifted because of the expansion of spacetime itself. The Doppler effect can be used to determine e.g. the velocity of a car, the expansion velocity of the universe or the receiver's position with satellites (satellite system TRANSIT). The Doppler shift is also applied to determine integer ambiguities, or is used as an additional independent observable for point positioning. Frequency Time

Position Fixing Geometry
Measurements Direction distance Distance difference Distance rate Question: What does a single measurement tell us about our Position? Surface of Position (SOP, 3D) Line of Positioning (LOP, 2D)

Distance SOP (LOP) Sphere (Circle) Positioning Fix

Distance difference SOP (LOP) Hyperbolic Positioning Fix

Distance rate SOP (LOP) Positioning fix Hyperbolic
Doppler Shift N12 f - Doppler t1 N23 f t2 f + Doppler t3 +  B frequency Intersection of two hyperboloids with Earth sphere

Direction SOP (LOP) Positioning fix Plane (Line)
Intersection or resection A B C P P A B C

Mix mode Any two LOPs or three SOPs which have intersection can be used for position fix Lop Ambiguities Good approximation of initial position Extra LOP Many combinations Range-direction Two ranges Two directions Range-range difference Two range differences Two angles ………………….

Geometry of Positioning
Bad Geometry Good Geometry Pseudorange Error R2 Pseudorange Error R1 R1 R2 DOP

Modern Positioning Systems
Gyro Compass (magnetic compass) Inertial navigation systems (INS) Radar Terrestrial radio navigation systems Satellite navigation systems Charts and maps Sonar systems Doppler Systems Images Mobilephone Wireless Sensor networks ………….

Professions related to Positioning
Surveying Shape of Earth, Reference systems Maps Deformation Engineering surveying Navigation From person to spaceship Animals Guidance and Pointing Military Engineering

Navigation Questions Can I get there from here? How long will it take?
If YES, By which route How long will it take? Where are we now? Where else could we go from here? What is that thing over there, where is it? Aids of navigation, maps,.....

Basic Tasks of Navigation
Positioning Kinematic Routing Select a way to go to the destination Safe Legal Fast Cheap Convenient

Types of Navigation Space Marine Air Land: cars, trains, person…
Surface Under sea IMO, IALA Air ICAO Land: cars, trains, person… Robots bird

Information required for Navigation
My Position, course, etc. Surrounding environment Moving or static objects Radar, maps, sights Nature environments Weather, currents, tides… Routing tools

Navigation observables
Distance and angles Earth’s rotation and astronomy Earth’s properties Magnetic gravitational, electric, heat Topographic features Vehicle motion Radio signals

Bird migration Example: Artic Term migrates from Arctic and Antarctic & back (40000 km /year) Birds use Astronomy: sensing positions of sun, moon, and stars Landmarks: recognize mountains, rivers, coastlines Sensitivity to magnetic field

Kinematic positioning
To determine the kinematic state of a moving object Time Position (3D) (latitude, longitude, height) Orientation (3D) (roll, pitch heading) Velocity (6D) Acceleration (6D)

Navigating Process: Determine Pos Check Velocity Change Accel.

Homing Given a target position 1 Determine present position
2 Compute position relative to target 3 Determine present velocity 4 Calculate course and speed change required to reach target on schedule 5 apply course and speed change 6. return to 1

Tracking control Given a specified track 1. Determine position
2. compute distance off track 3. determine the course and speed 4. compute rate of change 5. alter course to come back track 6. return to 1

Required Navigation Performance (RNP)
Different user groups Air, marine, space, land,… Different stages Flight: En route, terminal, precision and non-precision approach, landing, surface operation Safety of life operation International and national standards ICAO, IMO, IALA, IHO National authorities

Performance Indicators
Accuracy Availability Capacity Continuity Coverage Dimension Integrity Reliability Update rate (Fix rate) cost

Accuracy Accuracy: The degree of conformance (95%?) between an estimated parameter of an object at a given time and the true value of that parameter Predictable accuracy (compare to the true) Repeatable accuracy (compare to itself) Relative accuracy (compare to others)

Availability: percentage of time that the system is usable.
Integrity: the ability of a navigation system to provide timely warnings to the users when the system should not be used. Reliability is the probability of performing a specified function without failure under given conditions for a specified period of time

Continuity: the ability of a system to perform a function without interruption during an intended operation. It can be described as the probability that the specified system performance will be maintained for the duration of operation, assuming that the system was available at the beginning of that phase of operation

Redundancy Redundancy allows reliability to be monitored (do you think you get the accuracy you think you get?) How to provide redundancy Repeat measurements Add more measurements First rule in navigation Never dependent upon one system

Integration Take advantages of each system to
Improve the performance of navigation Integrity Continuity Accuracy Availability Reliability Add more functions i.e. Navigation + communications

What need to be considered to integrate different systems?
Benefits Performance Functions Cost Feasibility Application driven Other factors How to integrate different systems Hardware Software Algorithms Interfaces

Developments in Surveying
Conventionally ‘Static’ Surveys (mins) Discrete Points Two receivers Post Processed Integrated Survey Systems Dynamic Surveys Multiple Observations Integrated Sensors Real-time

Position Information for other users
Scientific Research Engineering Environment monitoring Emergency Response Urban Planning GIS

Very Long Baseline Interferometry (VLBI)
Receive signal from pulse stars (quasars) (very weak signals) Two stations measure signal delay difference First technique measures distances over a thousand kilometres with accuracy of mm

Satellite Laser Ranging (SLR)
Transmit Laser pulse to Satellite Satellite Reflects signal back to earth Measure signal travel time => Distance Orbit and Station Position Determination =>Gravity and Station coordinates

Satellite Altimetry Radar to measure distance from Satellite to Earth
Ocean Circulation Direct measurement of Geoid

Synthetic Aperture Radar (SAR)
Scan distance to convert to a image Size of image: 100x100 km Not Affected by weather

Synthetic Aperture Radar (SAR)
Very Wide Applications Ocean: environmental, wave, ice, etc Land: images: Land use monitoring, flood,.. penetrate vegetation, geological feature Global DTM model soil types: moisture, ...

InSAR Technology for Ground Change Detection/Measurement
The Technology Image the ground surface with satellite radar Process data using advanced numerical techniques to detect and measure accurately ground surface change The technology is accurate, economical, efficient and highly automatic Application areas: - Land settlement measurement - Landslide monitoring - Earthquake studies - DEM generation - … … Observed land subsidence of part of reclaimed land in HK

Satellite Gradiometer GRACE MISSION (NASA)
High resolution gravity field to yield complete gravity vector to model near surface disturbing mass to help delicate geophysical interpretation

GRACE ( ) gravity map pre-GRACE (~ 40 years) gravity map

Geophysics Glacial Rebound Free Oscillation Earth Tide
Relief of Ice cover - Free Oscillation Excited by certain events Earth Tide Periodical deformation of the earth caused by the attractions from the Sun and the Moon Movement of Core

Plate Tectonics

Plate Tectonics Divergent boundaries
Continental-continental convergence

Global Tectonic Movements

Seismic Research Earthquakes Geodetic methods Movements of faults
Most of them along the boundaries of tectonic plates Geodetic methods Measure the movements of pre, in and post quakes To help understanding the earthquakes

Atmosphere Science Ionosphere Space technologies (VLBI, GPS)
Free charged particles due to solar radiation Coupled with geomagnetic field Important for communication, navigation and remote sensing Space technologies (VLBI, GPS) Determine the electron distribution in space

Atmosphere Science Troposphere refraction is the error sources in GPS positioning On the other hand, if we know the amount of refraction delay – total water vapour content along the signal pass Metrological studies, weather forecasting

Oceanography In recent years, we understand the ocean environment is an important factor to control global climate i.e. El Nino Sea surface topography Ocean circulation Wave Mean Sea Level (MSL) GPS buoys, GPS-Tide Gauge connection Altimetry, SAR……

European Tide Gauge Records
500 1000 1500 2000 Stockholm Cascais Brest Newlyn Aberdeen

Global Warming It is a fact that the earth temperature is rising
Disaster situations More flood, more draught, Ice ages, Monitor the effects: Ice sheets Sea level

Space Exploration Precise Orbits Gravity Fields of other planets
Gravity and other force models Tracking technologies Gravity Fields of other planets Remote Sensing Image correction and registration

Military Applications
Precise Gravity Precise Orbits for satellites and missiles Deflection of vertical Global Reference Frame Navigation Global DTM Cruise Missiles, airplanes Guided Weapons

Water Leakage – Need position

Position Information for ordinary people
Technology is ready GPS Mobile phone WiFi Wireless Sensor network Provide seamless positioning anywhere you are, and relevant information needed

Internet Revolution Internet
Communication + information Great Impact on human behaviour and Economy Create a “Virtue World”

Return to the real world
Mobile internet + Position Information at the location needed, And real-time Potentially, very wide and profound applications Time + Position + Information => real-world

Introduction of Global Navigation Satellite Systems (GNSS)

HISTORY OF SATELLITE NAVIGATION - Sputnik I -
Sputnik I (USSR), first artificial satellite Sputnik II (USSR), dog Laika on-board Explorer I (USA), discovery of Van-Allan Belt Venture of USA/USSR into space had begun After advent of Sputnik many activities were undertaken to determine its orbit John Hopkins Applied Physics Labs (APL) used Doppler shift measurements for orbit determination

AMERICAN TRANSIT SYSTEM (1)
1959: Development started 1964: Operational for military use 1967: Released for civil use 1997: Phase-out Orbital height: 1075 km (LEO) Orbital period: 107 min Polar orbits i  90° Two carrier frequencies: f2 = 400, f1 = 150 MHz 2-d navigation system only Time interval between position fixes: 35 to 100 min Accuracy (1 ): 200 m (400 m)

AMERICAN TRANSIT SYSTEM (2)
Since 1973: 9 OSCAR and 3 NOVA satellites have been launched Mainly designed for ship navigation (Geodetic applications) Last launch: 1988 Digital SAD Discos sensor +Z axis Rotating solar panels (2) (solar cells one side) Boom Spin SAD Momentum wheel Orbit adjust and transfer system (station seeking rocket) Command antenna (2) Fixed solar panels (2) (solar cells both sides) Microthrusters (2) (Tellon Fuel) Earth Rotate ± 180° Transmitting antenna ( MHz)

RUSSIAN TSIKADA SYSTEM
Russian equivalent to TRANSIT Exists since 1965 Eight military satellites (TSIKADA-M) Four civil satellites Orbital period: 105 min Inclination: i = 83° Dual frequency system: f1 = 150 MHz, f2 = 400 MHz Accuracy (1 ): 200 m  400 m First Soviet navigation satellite Tsyklon (Kosmos 192) launched in LEO 1967 based on Doppler principle

AMERICAN-FRENCH ARGOS SYSTEM
1970: Program start Joint program of CNES/NASA/NOAA Two meteorological NOAA satellites (h = 850 km; i = 99°) Platforms transmit an identification code at MHz Doppler receiver at satellite Doppler data down-linked to 3 telemetry stations Position determination of platform at Argos centre in Toulouse System capacity: 1000 platforms simultaneously Accuracy: several km Localisation 1 2 3 4 5

DAWN OF THE NEXT GENERATION NAVIGATION SATELLITE (1972)
Space was being used for many things Communications Listening Looking (up and down) Navigation (Transit, Tsikada) Science (Measuring) New space-based navigation systems were being proposed, but as competitors to each other Timation (US Naval Research Laboratory) 621B (US Air Force Space and Missile Organization) Transit (US Navy Special Programs) DAWN OF THE NEXT GENERATION SATELLITE (1972) During the early era of GPS development in 1972, space was already being used for many things : 1. Communications Communications satellites with their enormous capacity to span continents and handle large data rates, were already well established. 2. Listening Satellites were already being used also to eavesdrop on conversations on the earth and to measure the radio spectrum of the universe. 3. Looking (Up and Down) Optical devices were both studying the earth and examining the planets and distant stars. The stable, refraction-free environment made astronomy a particularly appealing application.. 4. Navigation The United States' Transit System and the Russian Tsikada System were being used as Doppler Navigation Systems that could give periodic updates for users whose altitude and velocity were well-known. 5. Science (Measuring) With the discovery of the Van Allen belts, a whole area of scientific measurement had been in progress for over a decade. At this time there were also new space space navigation systems being proposed within the United States. The competitors included: 1. Timation. A system under development by the U.S. Naval Research Laboratory whose principal purpose was to transfer time but which also had the capability to provide precise positioning. 2. An Air Force sponsored system called '621-B' being developed at the Los Angeles Air Force Station. This featured full 3-dimensional positioning. Ground testing of the concept had already been accomplished by the late sixties. 3. The existing Navy's Transit System which had been developed under the sponsorship of the U.S. Navy Special Program Office. While principally developed to allow the navigation of nuclear submarines, it was also starting to be used by civilians for a broad range of applications.

EXISTING AND PLANNED DOD NAVIGATION SYSTEMS IN 1972
System Sponsor Orbits Signals Characteristics TRANSIT APL/Navy Polar, Tone/ Periodic, 2D (existing) circular, Low Doppler self-jamming TIMATION NRL/Navy 8-hr, Side tone 2D, dynamic (experimental) circular Ranging easily jammed Inclined (Rb/Cs clocks) 621B SAMSO/AF Egg beater, PRN 3D, dynamic (planned, Regional Spread good A/J ground tests) Spectrum (ground control) EXISTING AND PLANNED DOD NAVIGATION SYSTEMS IN 1972 This summarizes the three existing and planned navigation systems in A Transit System was deployed in circular orbits at the relatively low aititute of 1075 km. The basic navigation technique was to listen to a steady tone and by measuring the Doppler shift be able to determine both the point of closest approach and the range at that time. It was able to provide periodic two-dimensional fixes. A handicap was a tendency for the satellites to jam each other if more than one satellite was in view. The Timation System being developed by ihe Naval Research Lab contained experimental satellites placed in eight hour circular orbits whose basic signal was a side tone or direct ranging technique. A major feature was the inclusion of Rubidium/Cesium clocks. This stable frequency output had great utility for the navigation function. The use of Timation was for two-dimensional navigation and the transfer of time. However, the side tone ranging signal was easily jammed and more than one satellite in view could vastly complicate the users' problems. The 621B System being developed by the Air Force at the Space and Missile Systems Organization (SAMSO) was to be deployed in an eggbeater configuration which allowed regional incremental deployments. The signal was a spread spectrum or pseudo-random noise (PRN) signal. That the system was to be 3-dimensional, servicing a variety of dynamic users, and because of the signal characteristics, multiple satellites could be received at the same frequency. The weak signal still had reasonably good jam resistance.

THE MAJOR TECHNICAL ISSUES
Control segment vulnerability User jamming / spoofing User equipment costs Satellite lifetime Ionospheric corrections These problems are also relevant today THE MAJOR TECHNICAL ISSUES It was determined that there were 5 major technical issues associated with any of the proposed systems. These major issues were: 1. Control svstem vulnerability. It was deemed important that the satellite system would continue to function in spite of short to medium term control segment outages. This implied that the satellites themselves had to be reasonably autonomous; thereby insuring the continuity of navigation operations. 2. User jamming and spoofing. Any radio signal received from earth satellites tends to be of relatively low power and susceptible to inadvertant or deliberate jamming or hostile spoofing of the system. Therefore, a signal structure was favored which exhibited strong resistance to jamming and spoofing. 3. User equipment costs. All major studies of the next generation of satellite navigation system had predicted millions of potential users. However, for this to be realized, user equipment costs would have to be realized, user equipment costs would have to be low. An initial target was $10,000 for the user equipment sets. 4. Satellite lifetime. If there were frequent satellite failures, they would interrupt the continuity and availability of service. But, equally important, a series of satellite failures would imply high costs for the satellite segment. The initial goals for satellite lifetime were three to five years. As will be seen, GPS more than fulfilled those goals. 5. lonospheric corrections. To obtain high precision and accuracy in the satellite ranging signals, the delaying effects of the ionosphere had to be compensated for. It was desired that this error source contribute no more than one to two meters in the ultimate navigation solution. Fortunately, the ionospheric error could be calibrated if the navigation signal was simultaneously and coherently broadcast on two different frequencies.

GPS SYSTEM Space Segment User Control Colorado Springs Hawaii
Kwajalein Ascension Island Diego Garcia Monitor Stations GPS SYSTEM The GPS configuration included monitoring stations at Hawaii, Kwajalein, Ascension Island, and Diego Garcia as well as the initial control station at Vandenberg that was later shipped to Colorado Springs. The initial 4-6 satellites were deployed to provide periodic 3-D coverage for most places around the globe. The user segments envisioned included land, sea, air and personal use as well as a wide variety of civil applications

EXPECTED GLOBAL POSITIONING SYSTEM ACCURACY
50% OF TOTAL 90% OF TIME Horizontal Vertical 4 m 4.3 m 8 m 11 m EXPECTED GLOBAL POSITIONING SYSTEM ACCURACY The advertised capability for the new Global Positioning System is shown here. This chart is for the most accurate version of the system including the dual frequency calibration of the ionosphere. In the horizontal dimension the circular error probable (CEP) was predicted to be 4 m. Vertically, half the errors were to be less than 4.3 m. Subsequent Phase One results have validated these initial predictions.

GPS SYSTEM DESIGN RESOLVED THE MAJOR TECHNICAL ISSUES
Solution Atomic clocks PRN & inertial integration Digital signal and digital receiver Redundant atomic clocks Subsystem redundancy Solid state amplifiers Dual frequency and 1575 MHz Technical Issues Control segment vulnerability User jamming /spoofing User equipment costs Satellite lifetime Ionospheric corrections GPS SYSTEM DESIGN RESOLVED THE MAJOR TECHNICAL ISSUES This chart summarizes the key technologies and design decisions which resolved the major technical issues for GPS. The key to minimizing the vulnerability of the control segment was incorporating atomic clocks whose stability contributed range errors of less than 20 ft. after a day. To reduce the vulnerability to jamming and spoofing, the spread spectrum code as well as integration with inertial components were key design features. To reduce equipment costs, a digital signal structure which could be implemented in a digital receiver was the key. For satellite lifetime the most short-lived components were designed as redundant units. This included the atomic clocks and a number of other key subsystems in addition, to avoid the infant mortality which was then associated with travelling wave tubes the satellites used solid state amplifiers at L-Band. Ionospheric Corrections were handled using the dual-frequency technique at 1227 and 1575 MHz.

GPS BLOCK I SATELLITE GPS BLOCK I SATELLITE
The GPS Block I Satellite resembles its successors in many ways. It is a earth-pointed satellite with azimuth steering and pivoting solar arrays to provide power. The earth-pointing antennas feature a shaped beam that emphasizes power that the earth limbs or the space loss is greatest. The three-axis configuration also included double and triple redundancy.

A KEY TO GPS SUCCESS - THE RESULTS OF THE RUBIDIUM FREQUENCY STANDARD
These are test results of the Rubidium Frequency Standard. This was a critical test result in the development of the GPS Phase I Spacecraft. The vertical axis is the Allan Variance, which is the stability of the atomic oscillator in parts per part. A drift of corresponds to a ranging error of approximately 10 ft. accumulated at the end of the day (one day being about 100,000 seconds). The Rubidium Clocks consistently showed test results that were an order of magnitude better than the specifications. This was a consequence of having a triple-oven temperature control system.

USE OF PSEUDOLITES AT YUMA
GPS TX (GT synch) GPS RX GT TX GPS RX GT TX GT TX GPS RX Ground Transmitter (GT) Control Interface Timing GPS Receiver SV Simulator/Transmitter Power Conditioning USE OF PSEUDOLITES AT YUMA This chart shows the system configuration of 3 pseudolites and 1 satellite for the testing of an A-6 airplane over the Yuma test range. The airplane's position was calibrated using a laser tracker not shown on this chart. The Joint Program Office performed tests at Yuma and at other selected sites around the world. Host Site Host Site Host Site Control Display Unit (CDU) Data Link Equipment

PHASE I TEST RESULTS (1) - Spherical Error Probable (Meters) -
Here are the test results for 7 vehicles using integrated GPS. The earliest tests are on the right; the later ones are on the left. The improved accuracy in part reflects system maturity. As can be seen, the A-6, which is a Navy attack airplane, showed accuracies of 6m. That is, 50% of the errors measured were within a 6m sphere. This is known as the 'Spherical Error Probable' or SEP.

SATELLITE ORBITS LEO Low Earth Orbit GEO Geostationary Orbit
IGSO Inclined Geosynchronous Orbit ICO Intermediate Circular Orbit MEO Medium Earth Orbit HEO Highly Elliptical Orbit Sun synchronous orbits HEO GEO IGSO MEO, ICO LEO Sun Sun synchronous orbit ~ SATELLITE ORBITS LEO: Up to 2000 km (navigation satellites as e.g. Transit, Tsicada, research satellites, spy satellites) Advantage for - Relatively low-cost and simple spaceborne and user equipment navigation: - Low altitude provides rapidly varying Doppler-shift, which can be used for navigation purposes - Being close to the Earth's surface, low power radio transmitters can provide adequate signal strength for simple receivers Disadvantage: - The orbital period of around 105 minutes means that the satellite is only in sight for about 15 minutes, and continuous world-wide navigation cannot be achieved without using a very large number of satellites (about 86) - Relatively large and rapid variations of orbital position that are difficult to predict, one satellite's transmissions cover only a relatively small area GEO: 36,000 km (Communication and TV satellites) Adv.: - Angular velocity like Earth - Requires smallest number of satellites - Comparatively stable orbits Disadv.: - Geometric problems prohibit fixing around Equator - Cannot provide height and cannot cover Poles - Satellite and launcher costs high IGSO: In contrast to the GEO satellites, the inclination is not zero. The ground trace is a line. Some disadvantages of the GEO can be avoided (fixing around Equator, provision of height, coverage of Poles). (e.g. GNSS satellites). ICO: Circular 5,000 to 20,000 km (GPS satellites) Adv.: - In view for reasonably long periods requiring less satellite-swapping - Stable orbits allowing more accurate long-term orbital predictions - Large area of coverage from any satellite Disadv.: - Slower Doppler-shift - More expensive in launcher cost HEO: Adv.: - Can provide good signals to users in high latitudes and to mobile users in lower latitudes in 'canyon areas' Disadv.: - Poor geometrical properties - Expensive and large number of satellites needed even for a small area - Very high Doppler-shift can cause receiver design problems - Only part of orbit usable Sun synchronous orbits: For satellites which continuously need sunlight

Digital / Analog Signals Data / Signal Input Signal Processing
SIGNAL GENERATION Antenna Oscillator Modulation Amplifier Carrier Frequency RF Signal Output Signal Information Signal Digital / Analog Signals Data / Signal Input Signal Processing SIGNAL GENERATION The carrier frequency is generated by an oscillator which consists in principle of an electrical inductivity and a capacity. The frequency is stabilised by a quartz oscillator and by control loops. The information that shall be transmitted can be in digital or analog form. A digital signal can be produced by a computer or by digital signal processors. Digital signal processors are a kind of very fast and efficient microprocessor with a specialised set of programming commands, registers and memory on chip, often used in video applications, digital encoding of speech or e.g. in modems. Analog signals are e.g. the amplified signals of a microphone. The output of this unit is the so-called baseband or information signal. The carrier frequency is modulated with the baseband signal. Dependent on the baseband signal, the carrier frequency and other factors, any adequate kind of modulation can be chosen. The emerging radio frequency signal is amplified and conducted to the antenna. Not shown in this diagram, but nevertheless important components, are filters, which select a frequency bandwidth in order to eliminate noise and perturbing frequencies. RF ... Radio Frequency

FREQUENCY AND SPECTRA OF ELECTROMAGNETIC WAVES
Frequency Bands Wavelength 100 km 10 1 100 m 10 1 100 mm 10 1 VLF LF MF HF VHF UHF SHF EHF 3 kHz 30 300 3 MHz 30 300 3 GHz 30 300 Frequency FREQUENCY AND SPECTRA OF ELECTROMAGNETIC WAVES Frequency bands are often named with letters, which are not normalised internationally and are not uniform. For example, there are different assignments for radar frequencies, US companies, rectangular waveguides, etc. The following table shows the classification of radar frequencies and the general designations of frequency ranges. Radar Frequency Bands L S C X KU K KA E W 1 2 4 8 12.5 18 26.5 40 90 110 Frequency Wave Length Symbol 3 ... 30 kHz 100 10 km VLF Very Low Frequency 300 1 LF Low Frequency 3000 1000 m MF Medium Frequency MHz HF High Frequency VHF Very High Frequency dm UHF Ultra High Frequency Ghz cm SHF Super High Frequency GHz mm EHF Extremely High Frequency Frequency Band Frequency [GHz]

SIGNAL MODULATION Technique to put information on a carrier
Basic approaches: Amplitude A(t)= AC sin(Ct) AInfo sin(Infot) ("Beating") Phase A(t)= A sin(Ct + (t)) Adaption to transmit channel Different frequencies Carrier-to-noise ratio SIGNAL MODULATION A carrier is modulated with a signal (message, information) by influencing one (or more) of its characteristics time dependently in the rhythm of the message oscillation. Basically, these characteristics are the amplitude and the angle of its phase. Often, for an application a certain frequency range is given by institutions, (e.g. commercial radio, amateur radio). The system manufacturer then has to choose the best suited characteristics which perform the modulation to get a high signal level and a low noise level at the receiver. Some criteria are: frequency, digital or analogue information, range, costs, etc. Vice versa, if it is possible to choose between frequency bands, the criteria are mainly the range of the signals and the bandwidth of the information signal. If one wants to talk with somebody in Africa over amateur radio, one would probably choose a relatively low frequency (some MHz).

AMPLITUDE MODULATION The ideal modulator is a multiplier
vC Oscillator (Carrier) vRF Output vInfo Modulation Signal (Info) AMPLITUDE MODULATION This kind of modulation is used primarily in the VLF, LF, MF, HF, and sometimes in the VHF (airborne speech transmission), because of the low noise characteristics of the modulation at lower frequencies. The carrier frequency is multiplied with the information signal (baseband signal). The resulting RF signal consists of the original carrier frequency, whose amplitude (i.e. voltage of the signal) is varying with the frequency of the information signal. Binary information with 0 and 1 signals can also be amplitude-modulated. Thereby, the RF signal consists of the carrier frequency with a certain amplitude which is so-to-say switched-on or switched-off. Carrier Frequency vC vInfo Information Signal Modulated Signal (Radio Frequency, RF) vRF

PHASE AND FREQUENCY MODULATION
FM: Information signal changes frequency of the carrier : f(t) ~ vInfo PM: Information signal changes phase of the carrier: (t) ~ vInfo Examples FM - Music PM - Modem (Binary Phase Shift Keying): PHASE AND FREQUENCY MODULATION The phase-modulated signal has a constant amplitude, but the zero crossing is shifted. The shift is proportional to the amplitude of the information signal. An important example is the Binary Phase Shift Keying (BPSK). The information signal which is 1 or -1 shifts the carrier signal by  = 180°. E.g., the output signal of a modem is BPSK modulated. BPSK is also applied to the GPS signals. Especially - but not only - in analogue applications, as e.g. commercial radio, a variant of the phase modulation is used. The characteristic of this frequency modulation is that the amplitude of the information signal changes the frequency of the carrier signal. FM ... Frequency Modulation PM ... Phase Modulation vInfo vRF vC vInfo vRF vC  

MULTIPLE ACCESS SYSTEMS
A Code Division Multiple Access (CDMA) (= Spread Spectrum Multiple Access) t PRN code (channel) S0 B t f B Frequency Division Multiple Access (FDMA) t S0 S0  f B MULTIPLE ACCESS SYSTEMS There are three multiple access systems: A Code Division Multiple Access (CDMA) (Or "spread spectrum"). CDMA is a form of multiplexing where the transmitter encodes the signal using a pseudo-random sequence which the receiver also knows and can use to decode the received signal. Each different random sequence corresponds to a different communication channel. The transmission channel is limited neither in time nor in bandwidth but in the spectral power density. Both, the the spectra and the time signals overlap in the transmission band. B Frequency Division Multiple Access (FDMA) The FDMA technique needs the separation of the frequency spectra. The total bandwidth of the system is divided in a number of narrow frequency bands, which correspond to a transmission channel. The single frequency band is available to the subscriber during the total time of the information transmission. C Time Division Multiple Access (TDMA) TDMA is a type of multiplexing where two or more channels of information are transmitted over the same link by allocating a different time interval for the transmission of each channel. I.e. the channels "take turns" to use the link. Some kind of periodic synchronising signal or distinguishing identifier is usually required so that the receiver can relate the time slots to the channels. The total bandwidth is available to every subscriber for a short time slot. f0 f0+f f t C Time Division Multiple Access (TDMA) Channel no. t S0 t t0 t0 + t B f S0 ... Power density B ... Bandwidth t ... time f ... frequency

GEOMETRY OF SIGNALS  One-way ranging (GPS, GLONASS)
Measure t =  / c = one-way time of travel Calculate  = c t Two unsynchronised clocks Measure t = 2 / c = two-way time of travel Calculate  = c t / 2 One clock used to measure t Two-way ranging (PRARE) GEOMETRY OF SIGNALS Range measurements can be classified as one-way or two-way ranging methods. The time of travel in a two-way ranging system is measured e.g. with the clock of the observer. The signal travels from the observer to the satellite, and is reflected back to the observer. So, the distance between the observer and the satellite is the speed of light multiplied with half the travel time of the signal. Also, the case vice versa is applied, i.e. that the time of travel is measured with the clock of the satellite. One-way ranging demands that the clocks of the ground station and of the satellites are synchronised with each other or that the synchronisation error can be determined (GPS). The time of travel from the satellite to the observer is measured. The distance is calculated by multiplying the speed of light with the travel time of the signal.

POLARISATION Direction of polarisation = direction of electrical field
Examples Linear polarisation E = Ex = Direction of polarisation Circular polarisation E = Ex + Ey = Direction of polarisation (e.g. GPS, GLONASS) x z Direction of propagation y E Direction of propagation S POLARISATION A wave which has an electrical field vector that lies always in one plane, is called linear polarised. This polarisation plane is fixed by the electrical field vector and the Poynting vector. The polarisation is given by the emitting antenna, but can also be changed by interaction of the wave with conductors (reflectors, ionosphere). Linear polarised waves can be divided into horizontal and vertical polarised waves. A wave which consists of two partial waves with the same amplitude but perpendicular, 90° phase shifted electrical field vectors is called circular polarised. The resulting electrical field never gets to zero. It is screwing in a spiral along the direction of propagation and has a circular cross section. E.g., GPS and GLONASS are right-hand circular polarised.

GEOMETRY OF WAVE PROPAGATION
Ground waves f < 1.6 MHz Sky waves f = MHz Line of sight waves f > 30 MHz Ionosphere GEOMETRY OF WAVE PROPAGATION Dependent on the frequency there are three forms of geometrical propagation of electromagnetic waves. Waves with frequencies up to 1.6 MHz propagate following the bending of the earth. The attenuation of these ground waves increases with higher frequencies. Sky waves are attenuated and refracted on their way through the ionosphere. The attenuation and the refraction depend on the frequency and the degree of ionisation. The ionisation again depends on the time of day. Due to the possibility of multiple refractions inside the ionosphere and between the ionosphere and the surface of the earth, sky waves can cover the earth. Waves with frequencies greater than 30 MHz are called line of sight waves. They propagate quasi-optically. This effect increases with higher frequencies. This is the reason that satellite signals - which have in general frequencies greater than 1 GHz - are only receivable if the satellites are in sight. Ionosphere Sky waves Ground wave Line of sight waves Earth

DISTURBANCES AND LOSS OF POWER AS FUNCTION OF FREQUENCY
Atmospheric losses 16 to 9 Atmospheric refraction GHz 1 10 102 103 10 2 10 1 dB/km 10 -1 10-2 Fog, Rain Rainfall 50 mm/h f Attenuation coefficient 10 0 [km] Ionosphere 80 DISTURBANCES AND LOSS OF POWER AS FUNCTION OF FREQUENCY The refractive index is modified when one introduces collisions between electrons and heavy particles, and the wave experiences absorption - which physically is due to the conversion of ordered momentum into random motion of the particles after collision. For each collision, some energy is transferred from the electromagnetic wave to the neutral molecules and appears as thermal energy. The figure on the left hand shows the attenuation coefficient versus the carrier frequency. The attenuation increases with frequency up to about 20 dB/km at 100 GHz and a rain rate of 50 mm/h. For GPS frequencies the attenuation coefficient is about 0.01 dB/km under the same conditions. The ionosphere is electrically conductible due to free charged particles. Furthermore, the magnetic field of the earth influences the refraction index n. The number of free electrons, and thus n, varies with the time of day and season. The refraction in the troposphere is mainly a function of pressure and humidity. So, the local and often unpredictable weather plays a role when estimating n. Other disturbances are galactic and cosmic noise, which come mainly from the centre of the milky way and from intensively radiating stars. They are responsible for the main noise contribution in the frequency range between 20 MHz and 12 GHz. Every absorbing medium emits radiation power. At frequencies above 1 GHz oxygen and water vapour can dominate over other noise sources. Man-made radio noise is relevant only for frequencies below 1 GHz. 50 Stratosphere Tropopause Troposphere

ATMOSPHERIC REFRACTION: IONOSPHERE
TEC measurement (Total Electron Content) Dispersive medium, km Range error as function of Frequency Sun activity (11-year sunspot cycle) Daytime / night-time ATMOSPHERIC REFRACTION: IONOSPHERE The ionosphere is referred to as the region of the atmosphere from approx. 50 to 1000 km above the surface of the earth that contains free electrons, released by an ionised fraction of the gas molecules found there. In this region, as radio waves like GPS signals encounter the free electrons, several effects occur, the most important ones being the delay of the modulation on the carrier phase and the advance of the carrier phase, the so-called group delay and phase advance. The ionospheric effects are also dispersive, meaning that radio waves of different frequencies are affected differently. The degree of the Ionisation of the ionosphere is measured as total electron content (TEC). The TEC represents the number of free electrons in a 1-square meter column along the path from ground through the ionosphere. It is measured in TEC units, where 1 TEC unit = 1016 electrons/m2. The TEC depends on the sunspot activity (approximately 11-year cycle), seasonal and diurnal variations, the line of sight which includes elevation and azimuth of the satellite, and the position of the observation site. Taking all effects into account, a GPS pseudorange may be wrong by about 0.15 m to 50 m. As the ionosphere is a dispersive medium, i.e. the velocity of the signals depends on the frequency, it is possible to calculate the TEC, and hence to correct the measurements, if measurements in two frequencies are carried out. Mathematically, the elimination of the ionosphere is possible because we have two equations (one from the L1-measurement and one from the L2-measurement) and two unknowns: the real distance and the TEC value. Thus, the TEC value can be eliminated and the real distance can be calculated. The additional range caused by the bending of the path is in general negligible. Measured range Ionosphere Geometric range Earth's surface Observation site

ATMOSPHERIC REFRACTION: TROPOSPHERE
Two components Dry = f (Temp., Pressure) Wet = f (Temp., Humidity) Local weather conditions Troposphere dry ATMOSPHERIC REFRACTION: TROPOSPHERE The neutral atmosphere is a non-dispersive medium with respect to frequencies up to 15 GHz, and thus the propagation is frequency independent. The disadvantage is that an elimination of the tropospheric refraction is not possible. The refraction index is a function of temperature, pressure and humidity, and thus of the local weather conditions. Using real data covering the whole earth, Hopfield has found empirically a representation of the dry and wet refractivity as a function of the height above the surface. The effect is a path delay of the signal that reaches m in the zenith direction and increases approximately with the cosecant ( = 1/sin) of the elevation angle, yielding about a m delay at a 5° angle. The notation tropospheric refraction is slightly incorrect because it hides the stratosphere which is another constituent of the neutral atmosphere lying roughly between 10 km and 40 km. wet hdry 40 km hwet  11 km Earth's surface Observation site

MULTIPATH - Reflections of radio signals from nearby objects -
Antenna GPS carrier phase: mm GPS pseudorange: A few meters ! Affects GPS C/A code more than P code Solution Choice of appropriate antenna Signal processing techniques MULTIPATH Multipath effects occur when the signal is not only received directly from the satellite but from surfaces near the receiving antenna which reflect the signal. Mirror effects appear at horizontal, vertical and inclined surfaces. The multipath signal superposes with the direct signal and produces phase errors, which result in faulty range measurements. These effects have periodical characteristics and can reach amplitudes up to a few centimetres with regard to carrier phase measurements and even a few metres with pseudorange measurements. To avoid this effect, appropriate antenna designs (choke rings) can be used which block multipath signals. Measurements can also be corrected by data processing, e.g. by smoothing of code pseudoranges using carrier phases.

DOPPLER EFFECT Frequency Time DOPPLER EFFECT
The Austrian physicist C. J. Doppler discovered in 1842, that waves emitted by a moving object as received by an observer will be compressed (blue-shifted) if approaching, elongated (red-shifted) if receding. It occurs both in sound as well as electromagnetic phenomena, although it takes on different forms in each. In every-day life, this phenomenon occurs, if for instance an emergency vehicle with a loud siren approaches an observer and drives past. The pitch (the received frequency) audibly changes as the vehicle passes by. When the vehicle drives towards the observer the tone is higher, when it moves from the observer the tone drops. At the moment of the shortest distance between sender and receiver the relative velocity between sender and receiver is zero and the received frequency is the same as the emitted frequency. In astronomy, the Doppler effect appears in the cosmological redshift, an effect where light emitted from a distant source appears red-shifted because of the expansion of spacetime itself. The Doppler effect can be used to determine e.g. the velocity of a car, the expansion velocity of the universe or the receiver's position with satellites (satellite system TRANSIT). The Doppler shift is also applied to determine integer ambiguities, or is used as an additional independent observable for point positioning. Frequency Time

PSEUDORANGE MEASUREMENT
Transmitted code from satellite Replica of satellite code generated in the receiver PSEUDORANGE MEASUREMENTS Comparing the transmitted code from the satellite with a replica of it generated by the receiver results in the measurement of a time shift t. Multiplying it by the velocity of light c (plus various corrections) results in a user-to-satellite distance called also pseudorange. Time shift = PSEUDORANGE MEASUREMENT (in principle, time between transmission and reception is measured) Pseudorange = t · c

CARRIER PHASE MEASUREMENT
The carrier of the signal emitted by the satellite is received (doppler-shifted) by the receiver and compared with a generated carrier. The phase difference between both is the so-called carrier phase measurement. This measurement is a subdivision of a wavelength of the signal. The integer number of additional cycles, making up the remainder of the distance, is unknown. The integer cycle count is not observed but counted by the receiver. Every loss of lock leads to a loss of the number of cycles and produces a so-called cycle slip. Thus, since the initial value of n (and the one after a cycle slip) is unknown, phase measurements are ambiguous: This ambiguity (= integer number of cycles) has to be determined in the processing.  2 n unknown 180° 360° Resolution mm

POSITION DETERMINATION (1)
Two-dimensional example with receiver clock error ( x1 - x )2 + ( y1 - y )2 = ( L + PR1)2 ( x2 - x )2 + ( y2 - y )2 = ( L + PR2 )2 ( x3 - x )2 + ( y3 - y )2 = ( L + PR3 )2 x y PR2 PR3 PR1 PR1...3: measured POSITION DETERMINATION (1) The signals from the GPS satellites have nearly the form of planes when they arrive on Earth. Nevertheless, in this explanation we assume that they propagate in spheres, and in the purely theoretical, two-dimensional case as circles. Regarding two satellites: The position is determined as the point of intersection of two signals that have gone the same distance because they have travelled the same time. The travelling time is measured falsely because of the clock error of the receiver. This error is a constant offset to the very precise caesium clock of the satellite. This unknown offset produces an additional distance (can be positive or negative) because the signal is propagating according to L= c x t. The intersection point varies, depending on L. With a third satellite there will be only one solution where all circles (with radii PR1+L, PR2+L, PR3+L) intersect. The former variable L is now fixed. solution, no receiver clock error L solution considering receiver clock error determined by third satellite L L L = toffset · c

POSITION DETERMINATION (2)
Three-dimensional case 4 satellites necessary 3 coordinates unknown 1 time parameter unknown POSITION DETERMINATION (2) The two dimensional principle can now be extended to three dimensions. The graphical illustration is of course poor, because in fact we have four dimensions (including the time), which can't be drawn. The only difference to the two-dimensional case is, that the signals propagate in the form of spheres instead of circles. Two satellites means two spheres which intersect at a circle, so a further satellite is needed. Three satellites means three spheres which intersect at a point. Still we haven't considered the receiver clock offset. In order to do that, a fourth satellite is needed. It can be seen that the position determination is independent on the absolute time of the receiver clock. The absolute time can be determined. Determined position is intersection point of all spheres (x1 - x)2 + (y1 - y)2 + (z1 - z)2 + c T = PR1 (x2 - x)2 + (y2 - y)2 + (z2 - z)2 + c T = PR2 (x3 - x)2 + (y3 - y)2 + (z3 - z)2 + c T = PR3 (x4 - x)2 + (y4 - y)2 + (z4 - z)2 + c T = PR4

Doppler Effect The received signal has different frequencies from it was transmitted The frequency difference depends on relative velocity between transmitter and receiver fr - ft =-ft(r’/c) Therefore: DOPLLER MEASURES VELOCITY Integration of Doppler shift over a period of time (t1:t2) => range difference N = -ft/c(r(t2)-r(t1))

Position Determination (3) - Hyperbolic
If the distance difference of a point to another two points is a constant, this point is on a hyperbola

DOPPLER PRINCIPLE IN SATNAV
Measurement Principle Doppler Positioning (Translocation) Doppler Shift N12 f - Doppler DOPPLER PRINCIPLE IN SATNAV The Doppler shift to one or more satellites is measured. The different slant ranges to the orbiting satellites give at the observer site a for the position of the satellite characteristical Doppler shift of the transmitted signals. This Doppler shift is evaluated comparing the received frequency to a stable frequency generated in the receiver. In addition, the satellites transmit pre-calculated orbit data to enable the determination of the satellite position. For the determination of the observer position, hyperbolic techniques are applied. t1 N23 f t2 f + Doppler t3 +  B frequency Intersection of two hyperboloids with Earth sphere

TRANSIT SYSTEM 1959: Development started
1964: Operational for military use 1967: Released for civil use 1997: Phase-out Orbital height: 1075 km (LEO) Orbital period: 107 min Polar orbits i  90° Two carrier frequencies: f2 = 400, f1 = 150 MHz 2-d navigation system only Time interval between position fixes: 35 to 100 min

Principle of TRANSIT Integrated Doppler Measurements:
N1 = f(x,y,z, Xs1, Xs2) N2 = f(x,y,z, Xs2, Xs3), N3 = f(x,y,z, Xs3, Xs4),……… Solve for position x, y, z, subject to height control (zero for sea surface) Accuracy: Real time m Multiple Passes: 1-2 m

The Global Positioning System

What is the Global Positioning System?
NAVSTAR - Navigation Satellites with Timing and Ranging Global, all weather, 24 hour, satellite based navigation, positioning and timing system Developed by the US DOD, primarily for military purposes Passive ranging to satellites enables determination of user’s position and velocity, and of time Positioning accuracy - 1mm to 20m depending on type of receiver, user dynamics, observable and processing technique Two basic positioning services PPS - Precise Positioning Service SPS - Standard Positioning Service

GPS system Three segments Space Segment Satellites in Space
Ground Segment 5 monitor station and one master control centre User Segment Any users with GPS receivers

GPS SYSTEM Monitor Stations Space Segment Hawaii User Kwajalein
Ascension Island Diego Garcia GPS SYSTEM The GPS configuration included monitoring stations at Hawaii, Kwajalein, Ascension Island, and Diego Garcia as well as the initial control station at Vandenberg that was later shipped to Colorado Springs. The initial 4-6 satellites were deployed to provide periodic 3-D coverage for most places around the globe. The user segments envisioned included land, sea, air and personal use as well as a wide variety of civil applications Colorado Springs Control Segment

Global Positioning System
24 Satellites 6 Orbital planes 55° Inclination 20200 km above the Earth 12 hour orbits

Ground Segment Monitor Station (MS) - 5 Hawaii, Colorado Springs, Ascension Island Diego Garcia, Kwajalien Receive satellite signals and relay to MCS All except Hawaii have ground antennas to communicate with satellites Master Control Station (MCS) Falcon AFB, Colorado Springs Satellite ephemerides and clock parameters estimated

Ground Segment Colorado Springs Hawaii Kwajalein Diego Garcia
Ascension

Global Positioning System Concept
User measures distance to four satellites Satellites transmit their positions in orbit User solves for position (X,Y,Z or , h) and clock error t

GPS Range Measurement One-way range (distance) between satellite and receiver Measurement of time-of-flight of coded signals Each satellite has a unique code (Gold Codes) Synchronised clocks in receiver and satellites Receiver attempts to match received code with its own code Pseudo-range Pure geometrical range corrupted by Clock offsets Atmospheric errors Other error sources

GPS Signal Structure Two L band carriers L1 1575.42 MHz L2 1227.60 MHz
Two spread spectrum modulated timing codes C/A code MHz 1ms L1 only P code MHz 38 weeks L1 and L2 Navigation message 50 bps 25 pages, 5 subframes

GPS Signal Structure

GPS Pseudo-range Observable
Main navigation observable Code correlation of timing codes Very simple, very robust Stand alone positioning Standard Positioning Service Coarse/Acquisition (C/A) code 20 metres Precise Positioning Service Precision (P/Y) code 10 metres

DGPS reference receiver
Differential GPS DGPS reference receiver DGPS correction terms Differential transmitter

Differential GPS (DGPS)
Relative Positioning using pseudo-ranges Receiver at a known location Real-time Transmit measured errors in pseudo-ranges to remote (mobile) user Post-processing Single difference of pseudo-ranges removes most ephemeris, atmospheric and satellite clock errors

GPS Carrier Phase Observable
Main surveying (high precision) observable Phase measurement on carrier signals (L1 & L2) 19cm or 24cm wavelength - resolution few mm Integer ambiguity Relative positioning Single difference / double difference More difficult to access, measure and process Static and kinematic processing

Carrier Phase Tracking
Integer Ambiguity Integer Ambiguity Carrier Phase Carrier Phase At Lock-on At a Later Epoch

Double Difference Phase Observable
T B A Observable = (TB-TA) - (SB-SA) Removes ephemeris, clock and atmospheric errors

GPS Pseudo-Range Positioning
Absolute Positioning Pseudo-ranges measured at one receiver Standard Positioning Service - 20m Precise Positioning Service - 10m Time averaging - over three days with SPS ~ 1 m Differential GPS Pseudo-range measurements at two (or more) points One receiver at know ‘reference station’ Cancels common errors Relative positioning Data link - terrestrial or satellite Real time accuracy of ~ 1 - 5m Network / Wide Area DGPS Multiple DGPS reference stations Cover large geographical area

GPS Carrier Phase Surveying
Static GPS Measure carrier phase simultaneous at two or more points Static surveying (>40mins for each line) Relative positioning of few cm and few ppm Fiducial GPS mm over thousands of kilometres Precise orbital & atmospheric modelling Fast Static Rapid/Fast initialisation techniques For short lines only a few minutes of data required Kinematic GPS surveying Carrier phase at two or more receivers One receiver stationary, one mobile from point to point Few centimetres in a few seconds Fast / On-the-fly initialisation Centimetric navigation

GPS Limitations Sky Visibility Canyons (natural and urban), quarries Tree cover Elevation mask 20º to 30º Multipath Careful positioning of sites Specialised receivers, Choke ring antennas Interference Accidental - TV, Radio, mobile phone Intentional - jamming Communications Real Time Kinematic (RTK) Differential (DGPS) Robustness of carrier phase solution Ambiguity resolution, cycle slips Range between reference stations and user

GPS Operational Capability
Initial Operational Capability (IOC) Declared operational for use by the civil community December Anti-Spoofing (AS) - January Selective Availability (SA) tests discontinued Full Operational Capability (FOC) Declared fully operational 27 April block II/IIA satellites Trials and experiments completed

Presidential Decision Directive
Presidential Decision Directive PDD NSTC-6 28 March 1996 Policy Guidelines Provide SPS continuous world-wide, free of user charges Discontinue SA within a decade (by 2006) *Suspend of SA on May 2001 NAVWAR - GPS Security Program Joint management Interagency GPS Executive Board Dept. of Defense/Transportation/State/Commerce GPS will remain responsive to National Command Authorities

GPS Modernisation More satellites in constellation - increased integrity 30-36 Auto-nav operation – up to six months More ground tracking stations – better orbit Increased radiated power - military spot beam Aft facing antennas for GEO satellite users Military use of pseudo-lites NAVWAR Enhanced Military User Equipment (UE) Separate military signal 2nd / 3rd civil signal agreements LM (Link Military) project Re-use of current spectrum, addition on new (M) code First possible launch is Option 2 of Block IIF (now 2007)

2nd / 3rd Coded Civilian Signals
March 98 agreement to implement two additional civil GPS signals January 99 ‘budget’ and frequencies announced L2 carrier will have a C/A code Third frequency, L5, MHz, in existing ARNS band L5 carrier with VS code as Safety-of-Life signal

GPS and GLNOSS Frequency Band

GPS Signal

Other Satellite Systems
GLONASS Compass (Beidou) Galileo Augmentation systems SBAS WAAS, EGNOS GBAS

Contents Introduction (week 1) GPS observables (week 2)
Reference Systems and Satellite Orbits (week3) Positioning with Pseudorange (week 4) GPS Errors and Modelling (week5) Positioning with Carrier Phase (week 6) High Precision positioning (week 7) GBAS and SBAS (week8) Terrestrial Navigation systems (week 9) Inertial Navigation Systems (week 10) System Integration (week 11) Project Presentation (week 12-14)

Assessments (50%) Report One Report Two
GPS Errors and Correction Models Ionosphere Troposphere Multipath Report Two Precise Point Positioning Methods and recent developments

Final Project (50%) Topic
Design an integrated navigation system for a specified application related to your work or experience Report Submission (week 14) Report (10-15 pages) Purposes of the application Navigation Requirements System Design Analysis on performances of the designed system Discussion and conclusion Presentation (week 12-14) Powerpoint Slides (20 min)

Satellite Positioning and Navigation Systems

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