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Fundamentals ofGlobal Positioning

System Receivers

Lecture Notes by

He-Sheng Wang

September 19, 2008


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Preface The purpose of this course is to present detailed fundamental

information on a global positioning system (GPS) receiver.

Although GPS receivers are popularly used in every-day life,their operation principles cannot be easily found in one book.

In a GPS receiver, the signal is processed to obtain therequired information, which in turn is used to calculate theuser position. Most other types of receivers process the input signals to obtain the

necessary information easily, such as in amplitude modulation (AM)and frequency modulation (FM) radios.

At least two areas of discipline, receiver technology andnavigation scheme, are employed in a GPS receiver. Thiscourse covers both areas.

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Preface In the case of GPS signals, there are two sets of

information: the civilian code, referred to as the

coarse/acquisition (C/A) code, and the classifiedmilitary code, referred to as the P(Y) code. Thiscourse concentrates only on the C/A code.

The material in this course is presented from thesoftware receiver point of view. It is likely that narrow band receivers, such as the GPS

receiver, will be implemented in software in the future.A software receiver approach may explain the operation

better.

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Preface Aim: To introduce the principles of the

operation of the GPS system and itsapplications There is flexibility in the exact content of the

course depending on student interests Generic topics include standalone, millimeteraccuracy positioning and kinematic GPS

Emphasis is on fundamental principles andlimitations

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Topics to be Covered Coordinate and time systems:

When working at the millimeter level globally, how do youdefine a coordinate system

What does latitude, longitude, and height really mean atthis accuracy

Light propagates 30 cm in 1 nano-second, how is timedefined

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Topics Satellite motions

How are satellite orbits described and how do the satellitesmove

What forces effect the motions of satellites

What do GPS satellite motions look like and what are themain perturbations to the orbits

Where do you obtain GPS satellite orbits

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Topics GPS observables Satellite motions

GPS signal structure and its uniquenessCode Phase measurements

Carrier phase measurements

Initial phase ambiguitiesEffects of GPS security: Selective availability (SA) and

antispoofing (AS)

Data formats (RINEX)

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Topics Estimation Procedure

Simple weighted-least-squares estimation

Stochastic descriptions of random variables andparameters

Kalman filtering

Statistics in estimation procedures Propagation of variance-covariance information

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Topics Propagation medium

Neutral atmosphere delayHydrostatic and water vapor contributions

Ionospheric delay (dispersive)

Multipath

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Topics Mathematic models in GPS positioning

Basic theory of contributions that need be to included formillimeter level global positioning

Use of differenced data

Combinations of observables for different purpose

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Topics Methods of processing GPS data

Available software

Available data (International GPS service, IGS; Universityconsortium

Cycle slip detection and repair

Relationship between satellite based and conventionalgeodetic systems (revisit since this is an important topic)

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Topics Applications and examples from GPS

Tectonic motions and continuous time seriesEarth rotation variations; measurement and origin

Kinematic GPS; aircraft and moving vehicles

Atmospheric delay studies

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Text Books and References Text

Pratap Misra and Per Enge, Global Positioning System:

Signals, Measurements, and Performance, Ganga-JamunaPress.

James Bao-Yen Tsui, Fundamentals of Global PositioningSystem ReceiversA Software Approach, Wiley-Interscience.

References Kayton & Fried,Avionics Navigation Systems, Second Edition,

Wiley Interscience.

E. D. Kaplan, Understanding GPS: Principles andApplications, Artech House.

Global Positioning System: Theory and Applications, 2Volumes, edited by B. Parkinson, J. Spilker, P. Axelrad, and P.

Enge, AIAA, http://www.aiaa.org, 19962008/9/19 13


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Contents1. Introduction2. GPS: An Overview3. GPS Coordinate Frames, Time References, and

Orbits4. GPS Measurements and Error Sources5. PVT Estimation6. Precise Positioning with Carrier Phase

7. GPS Signals8. Signal-to-Noise Ratio and Ranging Precision9. GPS Receivers

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Introduction toRadionavigation SystemsPredecessors to GPS

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Global Positioning System Satellite Navigation

System Multilateration based

on one-way rangingsignals from 24+satellites in orbit.

Operated by the UnitedStates Air Force Nominal Accuracy

10 m (Stand Alone) 1-5 m (Code

Differential) 0.01 m (Carrier

Differential)

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Navigation Terminology Navigation

Answer to the question Where am I?

Implies the use of some agreed upon coordinate system. Coordinates systems will be the subject of future lectures.

Related Terminology

Guidance: Deciding what to do with your navigation information Control: Orienting yourself/vehicle/weapon to follow out the guidance

decision

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Latitude, Longitude and Attitude One of many coordinate systems used to

described a location on the surface of the

earth Lattitude

Range: 90 North latitude are + South latitude are -

Longitude Range: 180 East longitude is + West longitude is -

Altitude Normally Upward is + In a North East Down (NED) coordinate

system up is -

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Definition of Latitude and

Longitude

Latitude (Paralles) are formed by theintersection of the surface of the

earth with a plane parallel to theequatorial plane

Longitude or Meridians are formedby the intersection of the surface of

the earth with a plane containing theearths axis.

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Latitude Determination Using

Polaris

Actual location of Polaris is 89o05

2008/9/19 20The Sky Above Stanford on Jan 6, 2002


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Instruments of Navigation

A SextantAn Astrolabe2008/9/19 21


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View Through a Sextant

Easier to alignSuns (or othercelestial bodys)limb with thehorizon.

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Latitude Determination Using the

Sun

= 900

Suns Altitude Suns Declination

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The Longitude Problem

Celestial map changes because of Earths 15o/hr (approximately)rotation rate.2008/9/19 24


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Longitude Determination Longitude Determination Methods

Methods based on time Compare the time between a clocks at the

current location and some other referencepoint.

Requires Stable Clocks

Celestial Methods Eclipses of Jupiters Moons

Lunar Distance Method

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Stability of Clocks

A $20 wrist watchhas an oscillator

stable enough tomeet theaccuracyrequirements of

the longitudeprize. The size and cost

of the super-

stable clocksmakes themunsuitable for usein mass produced

device.2008/9/19 26


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Fundamentals Radionavigation

Radio Frequency (RF) signals emanating from a source or sources.

The generators of the RF signal are at known locations

RF signals are used to determine range or bearing to the known

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Classification of Radio

Frequencies

Propagation characteristic of RF signals is a function of their frequency2008/9/19 28

Li f Si ht T i i


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Line of Sight Transmission

VHF (VOR, ILS Localizer) and UHF (ILS Glide Slope, TACAN/DME) are line ofsight systems.

Limited Coverage area

LORAN and OMEGA are over the horizon systems Large coverage area

In the case of Omega, coverage was global

Frequency band in which GPS operates makes it a line of sight system.

However, because of the location of the satellites, it is able to cover a largegeographic area.

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INS and Radionavigation

Systems

* INS is not a radionavigation system but is normally used inconjunction with such systems

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Phases of Flight

The required navigation accuracy and reliability (i.e., integrity,continuity and availability) depend on the phase of flight

Currently, as well as in the past, this meant that an aircraft had to be

equipped with various navigation systems.2008/9/19 31


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VHF Omni-directional Range

(VOR) Provides Bearing ()

Information Operates 112 118

MHz Accuracy 1o to 2o.

Principles of Operation Transmits 2 Signals

1st signal has azimuthdependent phase

2

nd

signal is a reference Phases differencebetween 1st signal and2nd signal is

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Distance Measuring Equipment

(DME) Measures Slant Range () Operates between 962 and 1213 MHz

Based on Radar Principle Airborne unit sends a pair of pulses Ground Station receives pulses After short delay (50 s) ground station resends the pulses back Airborne unit receives the signal and calculates range by using the following equation:

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Instrument Landing System (ILS)

Used extensively during approach and landing to provides vertical and lateral guidance Principle of Operation

Lateral guidance provided by a signal called the Localizer (108-112 MHz) Vertical guidance provided by another signal called the Glide Slope (329-335 MHz)

Distance along the approach path provided by marker beacons (75 MHz)2008/9/19 34

G G S R


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Generic GPS Receiver

Functional Block Diagram

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A F d t l S ft GPS


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A Fundamental Software GPS

Receiver

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Software Approach This course uses the concept of software radio to present the subject. The software radio idea is to use an analog-to-digital converter (ADC)

to change the input signal into digital data at the earliest possible stagein the receiver. The input signal is digitized as close to the antenna as possible.

Once the signal is digitized, digital signal processing will be used to

obtain the necessary information. The primary goal of the software radio is minimum hardware use in aradio.

Conceptually, one can tune the radio through software or even changethe function of the radio such as from amplitude modulation (AM) to

frequency modulation (FM) by changing the software.

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Software Approach The main purpose of using the software radio concept to

present this subject is to illustrate the idea of signal

acquisition and tracking. A software approach should provide a better understanding of

the receiver function because some of the calculations can beillustrated with programs.

Once the software concept is well understood, the readersshould be able to introduce new solutions to problem such asvarious acquisition and tracking methods to improve efficiency

and performance.

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P t ti l Ad t f th


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Potential Advantages of the

Software Approach An important aspect of using the software approach

to build a GPS receiver is that the approach candrastically deviate from the conventional hardwareapproach.

The software approach is very flexible. New algorithms can easily be developed without

changing the design of the hardware.

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OUTLINE1. Introduction

2. GPS: An Overview3. GPS Coordinate Frames, Time References, and Orbits

4. GPS Measurements and Error Sources

5. PVT Estimation6. Precise Positioning with Carrier Phase

7. GPS Signals

8. Signal-to-Noise Ratio and Ranging Precision9. GPS Receivers

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GPS: An Overview


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GPS: An Overview Objectives, Status, and Policies System Architecture

Signals Receivers and Measurements

Augmentation System and Differential GPS (DGPS)

Civil Applications Modernization Plans

Summary

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Objectives, Status, and Policies The principle objective of GPS was to offer the U.S. military accurate estimates of position,velocity, and time (PVT). Position error: 10 m Velocity error: 0.1 m/s

Time error: 10 ns The U.S. DoD decreed that the civil users of GPS would be provided with a reasonable

accuracy consistent with the national security considerations. Standard Position Service (SPS) for peaceful civil use Precise Positioning Service (PPS) for the DoD-authorized users

Access to the full capability of the system (i.e., PPS) is restricted by cryptographic techniques Anti-Spoofing (AS) SPS signals were degraded throughout the 1990s by introducing controlled errors to reduce

their precision Selective Availability Deactivated by a Presidential Order on 2 May 2000

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Predecessors Applied Physics Laboratorys TRANSIT: Navy Navigation Satellite System Doppler Shift Broadcast Satellite Ephemeris (Satellite prediction algorithm) Limitation: Velocity Sensitivity, Mutual Interference

Naval Research Laboratorys Timation Satellites Provide very precise time and time transfer between various points on the Earth Navigation Information: Side-tone ranging

U.S. Air Force Project 621B Satellite-ranging signal based on pseudorandom noise (PRN)

All satellites could broadcast on the same nominal frequency Anti-jamming capability Slow communication link (50bps)

Joint Program Office NAVSTAR (Navigation System with Time and Ranging) GPS

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GPS Design Choices & Enabling


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GPS Design Choices & Enabling

Technology Design Choices

Active or passive

Passive system need only receive transmission Positioning method: Doppler, Hyperbolic, or Trilatertion Trilateration time synchronized signals from satellites

Pulsed or continuous wave (CW) CW signal in the form of code division multiple access spread spectrum

Carrier frequency L-band offering line-of-sight with minimal atmospheric attenuation

Satellite constellation and orbits MEO constellation of 24 satellites

Enabling Technology Stable space platforms in predictable orbits

Ultra-stable clocks Spread spectrum modulation/signaling Integrated circuits

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Global Navigation Satellite


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Global Navigation Satellite

Systems (GNSS) GPS is not the only modern satellite-based

navigation system. GLONASS is a Russian parallel to GPS24 satellite FDMA navigation system

Galileo is expected to be EU offering for satellitenavigation in 2005

Beidou () experimental satellite navigation

system is Chinas developing testbed.

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GPS GLONASS


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GPS vs. GLONASS

GPS GLONASS

24+ 24-

6 3

4 8

55 64.8

26,560km 25,510km

1158 1115

CDMA FDMA

L1:1575.42MHz

L2:1227.60MHz

L1:1602.5625~1615.5MHz

L2:1246.4375~1256.5MHz

UTC(USNO) UTC(SU)

WGS84 SGS85

(SA) ()

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System Architecture Space Segment

Control Segment

User Segment

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Space Segment

Constellation

Number of Satellites 24

Number of Orbital Planes 6

Number of Satellites PerOrbit

4

Orbital Inclination 550

Orbital Radius 26560km

Period 11h57m57.26s

Ground Track Repeat Sidereal Day

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GPS Nominal Orbit Planes

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Control Segment

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GPS Control Monitor

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User Segment

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GPS Positioning Services Precise Positioning Service (PPS)Authorized users with cryptographic equipment and keys

and specially equipped receivers use the PrecisePositioning System.

Standard Positioning Service (SPS)Civil users worldwide use the SPS without charge or

restrictions. Most receivers are capable of receiving andusing the SPS signal. The SPS accuracy is intentionallydegraded by the DOD by the use of Selective Availability.(SA Turn off on May 1, 2000)

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Positioning and Timing Accuracy


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Positioning and Timing Accuracy

Standard (SPS)

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Signals Signal Structure Anti-Spoofing (AS) and Selective Availability (SA)

Signal Power

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Signals Currently, each GPS satellite transmits continuously usingtwo frequencies in the L-band referred to as Link 1 (L1) andLink 2 (L2) L-band covers frequencies between 1GHz and 2 GHz

Subset of the ultra-high frequency (UHF)

L1:fL1 = 1575.42 MHz L2:fL2 = 1227.60 MHz

Two signals are transmitted on L1, one for civil users, and theother for DoD-authorized users.

The lone signal on L2 is intended for the DoD-authorized

users only.

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Signal Structure Carrier: RF sinusoidal signal with frequencyfL1 orfL2. Ranging Code: a unique sequence of 0s and 1s assigned to

each satellite which allows the receiver to determine thesignal transit time instantaneously. PRN (Pseudo-random noise) codes allow all satellites to transmit at

the same frequency without interfering with each other

Each satellite transmit two different codes Coarse/Acquisition (C/A) code

Precision (Encrypted) [P(Y)] code

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Signal Structure Navigation Data: a binary-coded message consistingof data on the satellite health status, ephemeris

(satellite position and velocity), clock biasparameters, and an almanac giving reduced-precision ephemeris data on all satellite in the

constellation data rate: 50 bits per second (bps)

bit duration: 20 ms

12.5 minutes for the entire message to be received

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Signal Structure The three components of a signal are derivedcoherently from one of the atomic standard aboard

the satellite. 10.23 MHz

fL1 = 1575.42 MHz = 27710.23 MHz fL2 = 1575.42 MHz = 26010.23 MHz

The specific form of modulation used is called binary

phase shift keying (BPSK)

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Signal Structure

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Signal Structure

)2cos()()(2

)2cos()()(2)2sin()()(2)(

22)()(

2,

11

)()(

1,11

)()()(

LLkk

LY

LL

kk

LYLL

kk

C

k

tftDtyP

tftDtyPtftDtxPts

++

+++=

where PC is the signal power of C/A-code, PY,L1, and PY,L2 are the signal powers of

P(Y)-code on L1 and L2, respectively;x(k)

(t) =

1 andy(k)

(t) =

1 represent the C/A-code and P(Y)-code sequences, respectively, assigned to satellite number k;D(k)(t) =1 denotes the navigation data bit stream;fL1 andfL2 are the carrier frequenciescorresponding to L1 and L2, respectively; L1 and L2 are the initial phase offsets.

(1)

Note: In order to express the BPSK signals as (1), we haveswitched the binary values of the codes and navigation data to 1.From our old notation, a bit 0 maps into 1; and a bit 1 map into -1.

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L2 1227 6 MHz

L1 1575.42 MHz


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P(Y)-Code

Encrypted

U.S.military use

P(Y)-Code

Encrypted

U.S.military use

C/A-Code

Degraded

Civil use

L2 1227.6 MHz

GPS signals. Currently, each GPS satellite transmits three signals,two on L1 and one on L2 frequency. The BPSK-modulated signals areshown. The signal carrying C/A-code on L1 was degraded purposelythroughout the 1990s, but this practice has now ended. Access to

P(Y)-code is limited to the DoD-authorized users via encryption.2008/9/19 63


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Spread Spectrum The modulation of a carrier by a binary code spreads thesignal energy, initially concentrated at a single frequency,over a wide frequency band: over 2 MHz for the C/A-code and

about 20 MHz for the P(Y)-code, centered at the carrierfrequency.

While the signal power is unchanged, this step reduces thepower spectral density below that for the background RFradiation

Such signals, referred to as spread spectrum signals, havemany properties which make them attractive for use incommunication and navigation.

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Spread Spectrum

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Power Spectra

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Pseudo-Random Noise (PRN) PRN sequences are nearly orthogonal to each other. For satellites kand l,

which are assigned unique PRN sequences called C/A-codesx(k) andx(l),

).()1023(where,,allfor,0)()( )()(1022

0

)()(mxmxlknnixix

i

lk

=

=++

.1allfor,0)()(1022

0

)()( +=

nnixix

i

kk

The left hand side of (2) defines the cross-correlation function of thetwo sequences for shift n.

(2)

A PRN sequence is nearly uncorrelated with itself, except for zeroshift. For a C/A-code

(3)

The left hand side of (3) defines the auto-correlation functionof a sequence for shift n. The auto-correlation function of aPRN is nearly zero except for zero shift where it has a sharp

peak.2008/9/19 67

Anti-Spoofing (AS) and Selective


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Availability (SA) Anti-Spoofing: The main mechanism for limiting access to the fullcapabilities of GPS has been encryption of the P-code broadcast on bothL1 and L2.

Encrypted P-code is referred to as Y-code Access to the Y-code is under cryptographic key

SPS limits civil users to the C/A-coded signal on L1 but dual-frequencymeasurements are essential for precise positioning. Receiver manufacturers have devised proprietary techniques to gain access to

measurements on both L1 and L2. The same P(Y)-code is being transmitted by a satellite on both frequencies. The L2 measurements are more fragile and noisier than they would be if the

codes were known.

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Anti-Spoofing (AS) and Selective


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Availability (SA) Throughout the 1990s, the signal available for unrestricteduse were purposefully degraded under the policy of Selective

Availability (SA) by adding controlled errors in themeasurements.A five-fold increase in positioning error

Dithering the satellite clock

Can be eliminated via differential corrections SA was deactivated on 2 May 2000 in accordance with a Presidential

Decision Perhaps the European plans to develop Galileo accelerated the U.S. move

to drop SA

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Signal Power The GPS signals received on the earth are extremely weak.

RF power at the antenna input port of a satellite is about 50 watts Half is allocated to the C/A-code

In order to deal simply with a wide range of power levels, electrical engineers expresspower ratios on a logarithmic scale in units of decibel (dB), defined as

comparedbetolevelspowerare,where,log10 010

110

0

1 PPP

P

P

P

dB

=

dB3,dB30,dBW103

2

1

21 =+==

P

P

P

PP

(4)

Absolute values of power can be expressed similarly in relation to 1 watt or 1milliwatt in units of dBW or dBm, respectively. Consider a signal with power (P1) of0.1 watt. This power level can also be represented as -10dBW or 20dBm. A secondsignal, with a power (P2) of 100 watt is 30dB more powerful than the first signal. Athird signal, with 200-watt power (P3), is 3dB stronger than the second signal. Wecan capture these relationships as follows.

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Signal Power


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g

P C/A

L1 -133 dBm -130 dBmL2 -136 dBm -136 dBm*

*Presently not in L2 frequency

The actual signal powers in recent years have been 3-5 dB higher than thespecifications. Even so, the powers are still only around 10-16 watt. Interestingly, 10-16watt is enough power to navigate with if we were among friends and people of good will.

The GPS signals are well below the background RF noise level sensed by an antenna. Itis the knowledge of the signal structure that allows a receiver to extract the signalburied in the background noise and make precise measurements. The signal boost sorealized is called processing gain.

The low signal power is the Achilles heel of GPS, especially in military use.

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Power of Received Signal


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g

Zenith 5o Elevation

SV Transmit Power 27 W 27 W

SV Antenna Gain 10.5 dB 16.2 dBEffective Power Radiated Towards

Earth294 W 467 W

Path or Spreading Loss 1.95x10-16m-2 1.20x10-16m-2

Received Power Density5.51x10-14W/m2

5.26x10-14W/m2

Effective Area of Receive Antenna 2.87x10-3

m2

2.87x10-3

m2

Atmospheric Losses 2 dB 0.63 0.63

Effective Received Power 1.00 x 10-16 W 0.95 x 10-16 W

In dBm = 10log10

(Power in mW) -130 dBm -130 dBm2008/9/19 72


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Comparable Power Tracking -130dBm is roughly equivalent to listening to a

500 mW baby monitor a thousand miles away.

1,000 miles

16,000 miles

0.5 W

27 W

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Receivers and Measurements


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Signal Acquisition and Tracking

Estimation of Position, Velocity, and Time (PVT)

Evolution of Receiver Technology

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Signal Acquisition and Tracking The basic functions of a GPS receiver are: to capture the RF signals transmitted by the satellites

spread out in the sky, to separate the signals from satellites in view,

to perform measurements of signal transit time and

Doppler shift, to decode the navigation message to determine the

satellite position, velocity, and clock parameters,

to estimate the user position, velocity, and time

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GPS Collected Data Time


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Domain Plot

This is the digital data that results from the GPS analog front end ASIC.

Important parameters: sampling frequency=5.0425MHz, IF=1.25MHz2008/9/19 76


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Q. So How Does GPS Work?


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Q Answer: By integrating the signal until SNR >> 0 dB

This is the key to everything from here on.

As we will see, the GPS signal has an element that repeatsevery 1 millisecond, and we can accumulate manyidentical signals until the SNR is high enough.

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Estimation of Position, Velocity,


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and Time (PVT) The quality of the PVT estimates obtained by a user from GPS

depends basically upon two factors:

number of the satellites in view and their spatial arrangement in the sky The spatial distribution of the satellites relative to the user is referred to as

satellite geometry

quality of the range and range rate measurements There are several sources of biases and random errors. Errors in the navigation message parameters which specify satellite position

and signal transmission time introduce errors in the pseudorangemeasurements.

Propagation delays in the ionosphere and troposphere, signal distortion dueto multipath, and receiver noise also introduce measurement errors.

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Basic GPS Positioning Concept -- Trilateration


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How GPS Works


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Basic Equations for Finding User


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Position( ) ( ) ( )

( ) ( ) ( )

( ) ( ) ( )232

3

2

33

2

2

2

2

2

22

2

1

2

1

2

11

uuu

uuu

uuu

zzyyxx

zzyyxx

zzyyxx

++=

++=

++=

Nonlinear Equations: Difficult to Solve

Relatively Easily Solved with Linearization and Iterative Approach

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Measurement of Pseudorange


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g

utuu

isisi

btt

btt

+=

+='

'

Every satellite sends a signal at a certain time tsi. The receiver will receivethe signal at a later time tu.

iT= c(tutsi) ---- true value of pseudorange or geometric range

From a practical point of view it is difficult to obtain the correct timefrom the satellite or the user. The actual satellite clock time and actualuser clock time are related to the true time as

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ContdBesides the clock error, there are other factors affecting thepseudorange measurement. The measured pseudorange ican be written as

Di: satellite position error, Ti: tropospheric delay error, Ii:

ionospheric delay error, i: receiver measurement noise error,i: relativistic time correction

)()( iiiiutiiiTi ITcbbcD +++++=

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Contd

( ) ( ) ( )( ) ( ) ( )

( ) ( ) ( )

( ) ( ) ( ) uuuu

uuuu

uuuu

uuuu

bzzyyxx

bzzyyxx

bzzyyxx

bzzyyxx

+++=

+++=

+++=+++=

2

4

2

4

2

44

2

3

2

3

2

33

2

2

2

2

2

22

2

1

2

1

2

11

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GPS/SPS Performance Specificationsfor Global Positioning and Time


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Dissemination

Error (95%) PPS SPS

SA Active SA OFF*

PositionHorizontal

Vertical

22 m

28 m

100 m

156 m

10 m

15 mTime 200 ns 340 ns 50 ns

*Estimates2008/9/19 87

Evolution of Receiver Technology


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Several generation of GPS receivers came to marketbetween 1980 and 2000.

The receivers available today bear the sameresemblance to the early receivers as the laptop andpalm computers do to the minicomputers of the early1980s.

The advent of very large scale integration (VLSI) hasled to powerful microprocessors and memory chips,which have changes the look and feel of all

electronic equipment, including the GPS receivers.

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Complete Receivers


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Handheld receivers for hikers, backpackers and sailors.Small in size with lat-lon displays or simple maps

$100 - $300

In-car navigation systems. Detailed street maps andturn-by-turn directions

$400 - $2000

Marine navigation. Fixed mount large screens withelectronic charts

$400 - $3000

Aviation. FAA certified, panel mounted, with maps $3000 - $15,000

Survey and mapping. Oftentripod mounted, exclusivelyDifferential GPS, one meter tocentimeter accuracy

$3500 - $30,000

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Modules


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Plug-in modules. Integrated receiverand antenna, used for tracking and

monitoring

$100 - $300

OEM boards. Receiver circuitry for

customer integration

$60 - $100

Chip sets $10 - $30

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Augmentation Systems and

Diff ti l GPS (DGPS)


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Differential GPS (DGPS) The accuracy of the different navigation and positioning applications vary widely. Horizontal positioning accuracy of tens of meters is generally more than adequate

for navigation in wide-open spaces: maritime navigation on the open seas;

aircraft navigation in en route, terminal, and non-precision approach phases of flight; recreational use by hikers and backpackers.

Many important applications require greater accuracy: under poor visibility conditions, harbor entry by ships, taxiway guidance on airport

surface, Category I precision approaches by aircraft typically require meter-level

accuracy Automobile navigation over roads and highways has a similar accuracy requirement Category III precision approaches require decimeter-level accuracy vertically

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Reducing Measurement Errors and/or

I i S t llit G t


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Improving Satellite Geometry Satellite geometry can be improved by adding satellites to theconstellation to provide additional ranging signal.

A user can improve the geometry by deploying pseudo-satellites, called pseudolites, which transmit GPS-like signals.

The pseudolites can be deployed on the ground, in the air, oron a ship.

A GPS receiver has to be modified to receive and processthese signals.

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Differential GPS


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Hierarchy of GPS Capability


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Civil Applications


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High-precision (millimeter-to-centimeter level) positioning

Specialized applications such as aviation and space

navigation Land transportation and maritime uses

Consumer products

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Modernization Plans


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P(Y)-Code

Encrypted

M-code (starting 2003)

P(Y)-Code

Encrypted

M-code (starting 2003)

C/A-Code

Degraded(2 May 2000)

L2 1227.6 MHz

L1 1575.42 MHz

C/A-Code

(starting 2003)

L5 1176.45 MHz

Civil signal

(starting 2005)

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Summary


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Basic Description Space-based radionavigation system broadcasting synchronized

timing signals to provide estimates of position, velocity, and time based

on passive, one-way ranging to satellites. Milestones

1973: Architecture approved

1978: First satellite launched 1995: System declared operational

2000: Purposeful degradation of the civil signal stopped

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SummaryS C


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Satellite Constellation

Twenty-four satellites in six orbital planes inclined at 55o;near-circular orbits with radius 26,560 km; orbital period:11h 58m; ground track repeats each sidereal day

Reference Standards

Coordinate frame: WGS 84

Time: UTC (USNO)

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SummarySi l


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Signals Carrier Frequency (Wavelength)

L1: 1575.42 MHz (0.19029 m)

L2: 1227.60 MHz (0.24421 m) Multiple Access Scheme

Code division multiple access (CDMA)

PRN Codes

C/A-code on L1 P(Y)-code on L1 and L2

Code Frequency (Mcps) C/A-code: 1.023 P(Y)-code: 10.23

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Summary


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Performance Achievable

Real time: Typically, absolute positioning error of several

meters with a single receiver, decimeters in differentialmode

Batch processing: millimeter-level relative positioning

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lecture01 updated

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