Post on 23-Jun-2015
transcript
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