Lecturer: Michael O'Grady

Course: MSc Ubiquitous & Multimedia Systems

Unit: Context Sensitive Service Delivery

Lecture: GPS

Navigation & Positioning Systems

Objectives

Introduce some principles of electronic navigation

Describe state-of-the-art in satellite navigation

Introduce techniques based on cellular telephony

Outlined current developments in Satellite Based Augmentation Systems

Why the sudden interest?

Location-aware services also called proximity services

Examples include:• personal locator services• asset tracking

Emergency services provision E-911 E112

Exercise: Find some research reports and see what they say regarding market projections and

types of services required

Classification

Terrestrial RadioSatelliteCellular NetworkHybrid

Terrestrial Radio - Techniques

Radio Direction Finding directional antenna

• Tune into radio station with known coordinates• Determine bearing• Repeat with a second station• Construct two lines using appropriate bearing• Intersection indicates position

Hyperbolic Technique• Tune into two stations at known positions• Calculate time difference between signals• Repeat with a second station• Construct hyperbola• Intersection indicates position

Terrestrial Radio - Examples

DECCA• 1937 in USA

Problems• short range signal

– large number of transmitters– expensive to implement and maintain

LORAN-C• Successor to DECCA• 1950 in USA• Deployed worldwide• Civilian use since early 1990

– Now operated by North West Europe LORAN-C System (NELS)

Satellite - History

1950s• Russian space program

Doppler ShiftMeasurements of the Doppler Shift in the signals broadcast by a satellite following a known and well defined orbit could be used to estimate position

Example: Transit• Navy Navigational Satellite System• 5 satellites following low altitude (1100km) polar

obits• Decommissioned in 1996 ( due to GPS!)

Satellite - GPS

Global Positioning System (GPS)• Navstar GPS• US Department of Defense (DoD)• Military ownership and control

History• Initial tests in 1973 using ground transmitters• First satellites launched in 1978• 10 satellites launched by 1989• 24 satellites in operation• 1995 - Full operational capacity

Services• Navigation (accuracy variable but 20m is

realistic)• Time

GPS - Architecture

GPS comprises of three segments Space Segment Control Segment User Segment

GPS - Space Segment (I)

GPS Satellites• Space Vehicles (SVs)• 24 SVs in constellation• 3 spares• Orbit every 12 hours• Altitude of 20,200 km

Space Vehicle Characteristics• 2 solar panels• I small rocket• 4 types (Block I, II etc)• 4 atomic clocks (2 caesium & 2 rubidium)

GPS - Space Segment (II)

Constellation structure• 6 equally spaced (60 degrees) orbital planes• 4 SVs per plane• Each plane inclined at 55 degrees with respect

to the equatorial plane

Objective• Ensure that between 5 and 8 SVs are visible

from any point on earth at any given time

GPS - Space Segment (III)

GPS - Control Segment (I)

Network of 5 monitoring stations Function

• Track SVs• relay satellite time and ranging information to

master control station in Colorado

At the master control station• orbit and timing parameters calculated for each

SV• information then uploaded to the appropriate

SVs

Note: All SVs tracked 92% of time due to the geographic spread of the monitoring stations

GPS - Control Segment (I)

GPS - User Segment

Two groups of users Military

• standard military applications

CivilianCommercial

• Surveying• Vehicle monitoring• Precision agriculture

Recreational• Geocaching

GPS - Theory (I)

Trilaterationa basic geometric principle that allows the position of an object be determined if its distance from three objects is knownIn brief….if distance to one object is known

I am on a sphere with that object is centre.if distance to a second object is known

I am on a circle that denotes the intersection of the two spheres.if distance to a third object is known

I am on either of two points where the three spheres intersect.

GPS - Theory (II)

But clock on receiver NOT synchronised with those on the satellites!

So fourth satellite required to eliminate clock bias

• 4 equation- 4 unknowns• 3D fix (longitude, latitude, altitude), time bias eliminated• Extra satellites can be used to improve calculation• World geodetic System 1984 (WGS84)

If altitude assumed fixed - three satellites adequate for a reading (2D fix)

Note: distance to satellite referred to technically as a pseudorange

GPS - Signals

Each satellite broadcasts at 2 frequencies L1

• 1575.42 MHz• Civilian users

L2• 1227.6 MHz• Military users• Encrypted

GPS - Sources of Error

clock and ephemeris errors ionospheric & tropospheric delays multipath & shading satellite geometry or Geometric

Dilution of Precision (GDOP) receiver noise & delay

Note: Selective availability has been removed since May 2000Note: One nanosecond of inaccuracy results in 30

centimeters error approximately

GPS - DOP

Dilution of Precision (DOP) Measurement of the geometry of the visible

satellite constellation•satellites spread across sky gives good (small) DOP

values•clustered satellites give poor DOP values (

– Geometric Dilution of Precision (GDOP) -position, time

– Horizontal Dilution of Precision (HDOP)- Latitude, longitude

– Vertical Dilution of Precision (VDOP) - altitude– Positional Dilution of Precision (PDOP) - position– Time Dilution of Precision (TDOP) - time

Examples:– 1 indicates perfection– if HDOP greater than 5, reading is rejected!– PDOP of 8 is acceptable provided HDOP less than 5

Differential GPS (DGPS)

1. if the position of a GPS receiver is known, the error in the pseudoranges can be calculated

2. These “corrections” may be broadcast to nearby users thus improving their position readings accordingly

DGPS demands• network of reference stations• Allocated FM frequency• specialist receiver

Readings of up to 5 meters may be obtained easily sub-meter accuracy may also be obtained subject to

cost and quality of the equipment

DGPS

DGPS Signal

GPS Receiver Station DGPS TransmitterShipboard GPS &DGPS Receivers

Satellite - GLONASS

Soviet equivalent of GPSInitial satellite launched in 1992Last satellite deployed in 1996Working constellation of 24

satellitesArchitecturally similar to GPSControl segment within borders of

former Soviet Union

Satellite - Galileo

Conceived in 1999Initial satellites scheduled for launch in

2004Initial services scheduled for 2006Full operational capacity by 2008Objectives

state-of-the-art positioning and timing services guarantees regarding accuracy & availability designed for civilian requirements

Galileo - Architecture

30 satellites in circular orbitsOrbit planes inclined at 55-60

degrees to equatorial planeincrease coverage in Northern Europe

Ground segment consists of 14 ground stations spread around the globe

Interoperable with GPS and GLONASS

Cellular Techniques

Recall E-911 directive

Solution 67% of Calls 95% of Calls

Handset Based 50 meters 150 meters

Network Based 100 meters 300 meters

E-911 focused attention on how a cellular network’s features and topology might be used to ascertain a subscriber’s position

Cell-ID

Also called Cell of Origin (COO)Networks knows position of subscriber to

cell level• Map geographic coordinates to Base Station

Advantages include:• speed & cost• Minimum modifications to network infrastructure

Disadvantages include• variable accuracy (as cell sizes vary)• Propagation effects may mean serving cell is not

actually the nearest cell

Timing Advance (TA)

GSM Timing Advance parameter• used for synchronising time slots• may be defined as the time delay (distance!)

between handset and Base Station• 6 bits in length • precision of about 550 meters

Requires augmentation• other positioning mechanism• directional antenna

Angle of Arrival (AOA)

Triangulation Measure angles of signal at Base Station

using sophisticated antenna Repeat for second Base Station Intersection of lines projecting outwards

will indicate subscriber’s position

Disadvantages• Susceptible to interference and fading• Line-of-Sight (LOS) conditions essential• Will not work well in city

Advantage• may work very well in rural area

Time of Arrival (TOA)

Similar to GPSMeasure distance between handset and

Base StationRepeat for two more Base Stations

• if time is synchronized this is adequate• if not, a further reading is required

Advantages• easy to implement

Disadvantages• Accurate clocks required at each station• Susceptible to errors in urban environments• May not access enough base stations in rural

areas

Time Difference of Arrival (TDOA)

Uses Time Difference as distinct from absolute time Hyperbolic curves must be

constructed for at least two Base Stations

Intersection indicates position

Enhanced Observed Time Difference (E-OTD)

Also called Observed Time Difference (OTD) Observed Time Difference of Arrival (O-

TDOA)

Identical to TDOA except Calculations are performed on the handset

Disadvantages Significant modifications required on the

handset

Advanced Forward Link Trilateration (A-FLT)

In principle, identical to TDOA or E-OTD

Can be implemented on handset or network

Implemented only on CDMA networks Recall: CDMA is time synchronized so

time differences are easier to measure

Location Fingerprinting

Combines signal features such that a unique signature (fingerprint) for a given location is created

Database is assembled by driving a vehicle through the area which transmits signals to the Base Station

Signature is generated by analyzing the incoming signal

By comparing the subscriber’s signal with the signatures in the database, a location estimate can be obtained

Assisted GPS (A-GPS)

Utilizes GPS and topology of cellular network

GPS receivers are placed throughout the PLMN These “advise” the handset on which

satellites to observe Information is then sent back to the server for

position calculation Advantages

•quick & accurate•easy to incorporate DGPS•can track weak signals

Disadvantages•expensive

Assisted GPS - Schematic

MSC

Fixed GPS Receiver

AGPS Server

Assistance Information

Pseudo-ranges

Pseudo-ranges

Position

Base Station

Hybrid Positioning Systems

No technique capable of delivering accurate positions under all circumstances

But Pragmatic combination may lead to better

results sometimesExample

A-GPS TDOA

Each system has complementary strengths and weakness

Standardization - Satellite Navigation

Recall: GPS and GLONASS remain under the control of their respective militaries

International Civil Aviation Organization (ICAO)

active but limited in its effectiveness

Galileo SAGA (Standardization Activities for Galileo)

work is currently ongoing

Standardization - 2G Cellular Networks

Access Technology

Analogue Mode – TIA (TR45.1) A-GPS

CDMA – TIA (TR45.5)A-GPS

A-FLT

GSM – 3GPP (3G TS 23.271)

A-GPS

TOA

E-OTD

TDMA – TIA (TR 45.3) A-GPS

Standardization - 3G Cellular Networks

Radio Access Network Positioning Mechanism

GPRS/EDGE

TA

E-OTD

GPS

UMTS

Cell-ID

OTDOA

A-GPS

Product: Garmin NavTalk

Product: Benefon Esc!

References E-911

• http://www.fcc.gov/911/enhanced/ GPS

• http://tycho.usno.navy.mil/gpsinfo.html• http://www.colorado.edu/geography/gcraft/notes/

gps/gps_f.html GLONASS

• http://www.glonass-center.ru/ Galileo

• http://www.esa.int/export/esaSA/GGGMX650NDC_navigation_0.html

EGNOS• http://www.esa.int/export/esaSA/GGG63950NDC_navi

gation_0.htmSISNet

• http://esamultimedia.esa.int/docs/egnos/estb/sisnet/sisnet.html

The End