2GPS concepts

By

Mahmoud El MewafiMmewafi1@ excite.com

Professor of Surveying and Geodesy, Public Works Dept., Faculty of Engineering,

Mansoura University, Egypt.Tel 002 0127440767

Current Satellite Navigation Systems

1- Global Positioning System (GPS)

– USA• First Experimental

Launch Feb, 1978• First Operational

Launch 1989 and• Operational Capability

in Feb, 1993

2- Global Navigation Satellite System (GLONASS)

– Russia

Future Satellite Navigation Systems

• Galileo

– European Union

– New services like SAR (Search and Rescue)

– Higher Accuracy for Civil Community

– Tentative Plan of Launch : 2005 – 2006

– Availability of Service : 2008?

• QZSS( Quasi-Zenith Satellite System)

– Japan

– Basically, communication satellite but also transmit

GPS like signals for navigation

• Communication

• Broadcasting

• Navigation

GPS (Global Positioning System)

Control Segment

Space Segment

User Segment

Three Segments of the GPSThree Segments of the GPS

Monitor Stations

GroundAntennas

Master Station

Kwajalein Atoll

US Space Command

Control SegmentControl Segment

Hawaii

Ascension Is.

Diego Garcia

Cape Canaveral

Ground AntennaMaster Control Station Monitor Station

Satellites (space segment)

24 NAVSTAR satellites (21 operational and 3 spares)

orbit the Earth every 12 hours

~11,000 miles altitude

positioned in 6 orbital planes

orbital period/planes designed to keep 4-6 above the horizon at any time

controlled by five ground stations around the globe

Space Segment of GPS

• GPS satellites are the space segment of the system. These are space vehicles and are responsible for sending radio signals from space.

• The Space Segment of the system consists of the GPS satellites. These space vehicles (SVs) send radio signals from space.

• The nominal GPS Operational Constellation consists of 24 satellites that orbit the earth in 12 hours. There are often more than 24 operational satellites as new ones launched to replace older satellites.

• Ground-based devices read and interpret the radio signals from several of the NAVSTAR satellites at once

• Determine their position using the time it takes signals from the satellites to reach the hand-held unit

• Calculations result in varying degrees of accuracy that depend on:

• quality of the receiver

• user operation of the receiver

• local & atmospheric conditions

• current status of system

GPS – User Segment (Receivers)

GPS – User Segment (Receivers)

• Satellites have accurate atomic clocks and all 24 satellites are transmitting the same time signal at the same time

• The satellite signals contains information that includes– Satellite number– Time of transmission

• Receivers use an almanac that includes– The position of all satellites every second– This is updated monthly from control stations

• The satellite signal is received, compared with the receiver’s internal clock, and used to calculate the distance from that satellite

• Trilateration (similar to triangulation) is used to determine location from multiple satellite signals

How It Works

Position is Based on TimePosition is Based on Time

T + 3

Distance between satellite and receiver = “3 times the speed of light”

T

Signal leaves satellite at time “T”

Signal is picked up by the receiver at time “T + 3”

GPS theory

XYZT

Signal From One SatelliteSignal From One Satellite

The receiver is somewhere on this sphere.

Signals From Two SatellitesSignals From Two Satellites

5 Sec

3 Sec

X?

Signals From Two Satellites

Three Satellites (2D Positioning)Three Satellites (2D Positioning)

5 Sec

3 Sec

X

4 Sec

2D-3 Satellites

Three Dimensional (3D) Positioning

Three Dimensional (3D) Positioning

GPS theory• That's right, if three perfect measurements

can locate a point in 3-dimensional space, then four imperfect measurements can do the same thing.

• 1. (X1 - X)² + (y1 - Y)² + (z1 - Z)² = c²(t1 - T)² • 2. (X2 - X)² + (y2 - Y)² + (z2 - Z)² = c²(t2 - T)² • 3. (X3 - X)² + (y3 - Y)² + (z3 - Z)² = c²(t3 - T)² • 4. (X3 - X)² + (y3 - Y)² + (z4 - Z)² = c²(t4 - T)²

GPS signal structure

L1 Band Frequency = 154 x 10.23 MHz = 1575.42 MHz• – C/A Code at 1.024Mhz• – P Code at 10.23 Mhz• – Navigation Data at 50Hz

L2 Band Frequency = 120 x 10.23 MHz = 1227.60 MHz• – P Code at 10.23Mhz or• – Encrypted P Code called Y Code at 10.23Mhz• – Navigation Data Bit at 50Hz• Clock Frequency : 10.23MHz

Characteristics of GPS Signal

• C/A (Coarse Acquisition) Code – Also called PRN (Pseudo Random Numbers) – PRN codes are unique for every satellite – All the Satellites have the same frequency – Modulation is done by using CDMA which is a technology based on Spread Spectrum – Spread Spectrum gives wider bandwidth – Signal is spread and hence is below the noise floor – Difficult to intercept – Requires Low power – Efficient use of Frequency Spectrum• P Code – Encrypted using Encryption Code – Encrypted P code is called Y code – Used basically for Military purpose – Higher Accuracy – Extremely difficult to intercept – A very complex code

Generation of L1 Band Signal

Error Sources

• Satellite Clock Error• Ephemeris Error – Satellite Orbit and Satellite Related• Atmospheric Propagation Delay – Ionospheric Delay – Tropospheric Delay• Receiver Noise – Receiver Clock (Oscillator Noise) – Thermal Noise – Device Induced Interference• Multipath – Surrounding Environment• Satellite Geometry – DOPs

Sources of GPS ErrorStandard Positioning Service (SPS ): Civilian Users Source Amount of Error

Satellite clocks: 1.5 to 3.6 meters Orbital errors: < 1 meter Ionosphere: 5.0 to 7.0 meters Troposphere: 0.5 to 0.7 meters Receiver noise: 0.3 to 1.5 meters Multipath: 0.6 to 1.2 meters Selective Availability (see notes) User error: Up to a kilometer or more

Errors are cumulative and increased by PDOP.

Atmospheric Delays

• As a GPS signal passes through the charged particles of the ionosphere and then through the water vapor in the troposphere it gets slowed down a bit, and this creates the same kind of error as bad clocks.

Multipath Sources of Signal Interference

Multipath Sources of Signal Interference

Earth’s Atmosphere

Solid Structures

Metal Electro-magnetic Fields

Receiver Errors are Cumulative!Receiver Errors are Cumulative!

User error = +- 1 km

System and other flaws = < 9 meters

DOP is the Geometric Orientation of Satellites with respect to the

Antenna

GPS Satellite Geometry

Satellite geometry can affect the quality of GPS signals and accuracy of receiver trilateration.

Dilution of Precision (DOP) reflects each satellite’s position relative to the other satellites being accessed by a receiver.

There are five distinct kinds of DOP. Position Dilution of Precision (PDOP) is the DOP value used

most commonly in GPS to determine the quality of a receiver’s position.

It’s usually up to the GPS receiver to pick satellites which provide the best position triangulation.

Some GPS receivers allow DOP to be manipulated by the user.

Ideal Satellite GeometryIdeal Satellite GeometryN

S

W E

Good Satellite GeometryGood Satellite Geometry

Good Satellite GeometryGood Satellite Geometry

Poor Satellite GeometryPoor Satellite GeometryN

S

W E

Poor Satellite GeometryPoor Satellite Geometry

Poor Satellite GeometryPoor Satellite Geometry

Active GOTO Waypoint

Bearing = Course Over Ground (COG) =

Cross Track Error (XTE) =Location Where GOTO Was Executed

Bearing = 650

COG = 50 XTE = 1/2 mi.

Bearing = 780

COG = 3500 XTE = 1/3 mi.

Bearing = 400

COG = 1040 XTE = 1/4 mi.

Active Leg

N

GPS Navigation: On the GroundGPS Navigation: On the Ground

Position Fix

A position is based on real-time satellite tracking. It’s defined by a set of coordinates. It has no name. A position represents only an approximation of the

receiver’s true location. A position is not static. It changes constantly as the

GPS receiver moves (or wanders due to random errors). A receiver must be in 2D or 3D mode (at least 3 or 4

satellites acquired) in order to provide a position fix. 3D mode dramatically improves position accuracy.

Waypoint A waypoint is based on coordinates entered into

a GPS receiver’s memory. It can be either a saved position fix, or user

entered coordinates. It can be created for any remote point on earth. It must have a receiver designated code or

number, or a user supplied name. Once entered and saved, a waypoint remains

unchanged in the receiver’s memory until edited or deleted.

Planning a Navigation RoutePlanning a Navigation Route

Start= Waypoint

How A Receiver Sees Your Route

How A Receiver Sees Your Route

GPS Waypoint Circle of ErrorGPS Waypoint Circle of Error

X

GPS Survey Observation

• Static Observation – Antenna is fixed at a point – Gives higher accuracy since observation is done for long time

period • Average of the observation cancels out some errors – Only Code Phase Observation – A few meters level accuracy• Kinematic Observation – Antenna is moving – Just a few or single epoch observation at a particular point – Accuracy is lower – Only Code Phase Observation – Sometimes error is too large, few hundreds of meters

Single GPS Survey Observation

• Static Observation – Antenna is fixed at a point – Gives higher accuracy since observation is done for long time

period• Average of the observation cancels out some errors – Only Code Phase Observation – A few meters level accuracy • Kinematic Observation – Antenna is moving – Just a few or single epoch observation at a particular point – Accuracy is lower – Only Code Phase Observation – Sometimes error is too large, few hundreds of meters

Differential GPS (DGPS) Survey Observation

• At least one base station and one rover is necessary• Both Base station and rover logs the data at the same

time or use the same time data for processing• Data link must be available for real time DGPS• DGPS has been widely used in many applications where

accuracy is anissue

1- Static Observation2- Kinematic Observation

GPS - Differential Correction

• GPS error when using differential correction: 1 – 3 meters

• There are two ways that differential correction can be applied:– Post-processing differential correction

• Does the error calculations after the rover has collected the points

– Real-time differential correction• Done in real time by receiving a broadcasted

correction signal (usually expensive), requiring other hardware (not just a consumer GPS receiver)

DGPS Site

x+30, y+60

x+5, y-3

True coordinates = x+0, y+0

Correction = x-5, y+3

DGPS correction = x+(30-5) and y+(60+3)

True coordinates = x+25, y+63

x-5, y+3

Real Time Differential GPSReal Time Differential GPS

DGPS ReceiverReceiver

10m10m

Base Station(w/known coordinates)

Receiver(unknown Location)

GPS ReceiverEstimated Location

Differentially Corrected Estimated Position

GPSEstimated Location

Actual (Known) Position

Differential GPS (DGPS)

• Base Station – Base Station is located at a known Point – The point is surveyed by other means of measurement – Base Station Data are broadcasted using radio channel or telephone line for real time DGPS – Base Station Data are also recorded on a PC for post processing of rover data

Rover Station

• Rover or Field Station

– Field unit observes

GPS data

– For Realtime DGPS,

a data antenna or

telephone line is used

to receive the base

station log data

– For post processing,

the field data is brought

back to the office and post processed in the computer using Base

station data

Advantage of GPS surveys

• Three Dimensional • Site Intervisibility Not Needed • Weather Independent • Day or Night Operation • Common Reference

System • Rapid Data Processing with Quality Control • High Precision • Less Labor Intensive/Cost Effective • Very Few Skilled Personnel Needed

12. What is the fastest initialization method ?

13. How much data is collected at each point in stop-and-go GPS surveying ?

15. What are the time limitations on re-occupation ?

16. Which GPS surveying method would you use for establishing control with geodetic accuracy ?

17. Which GPS surveying method would you use if you need to complete a job urgently

Wide Area Augmentation SystemWide Area Augmentation SystemGeostationary WAAS satellites

GPS Constellation

WAAS Control Station (West Coast)

Local Area System (LAAS)

WAAS Control Station (East Coast)

How good is WAAS?How good is WAAS?

+ -3 meters

+-15 meters

With Selective Availability set to zero, and under ideal conditions, a GPS receiver without WAAS can achieve fifteen meter accuracy most of the time.*

Under ideal conditions a WAAS equipped GPS receiver can achieve three meter accuracy 95% of the time.*

* Precision depends on good satellite geometry, open sky view, and no user induced errors.

QUESTIONS 1. What are the advantages of GPS surveying over conventional

surveying methods ? 2. What is the single factor that determines whether or not a GPS

survey is possible in an area and/or a project ? 3. What is a ‘baseline’ in GPS surveying ? 5. What is an ‘epoch’ in GPS terminology ? 6. Why is the ‘static’ GPS survey method so named ? 7. What is the reason for ‘minimum session length’ in static surveying ? 8. What factors determine the length of a session in static surveying ? 9. What factors determine the GPS surveying method suitable for a

given area/project ?10. What is the purpose of rover ‘initialization’ in kinematic surveying ?

• Generating mapped data for GIS databases

• “traditional” GIS analysts & data developers

• travel to field and capture location & attribute information cheaply (instead of surveying)

• Other uses (many in real time):

• 911/firefighter/police/ambulance dispatch• car navigation

• roadside assistance

• business vehicle/fleet management

• mineral/resource exploration

• wildlife tracking

• boat navigation

• Recreational

• Ski patrol/medical staff location monitoring

GPS Applications

Validation

• Accuracy• Turnaround time• Cost• Client response• Developers’

response?

• Generating mapped data for GIS databases

• “traditional” GIS analysts & data developers

• travel to field and capture location & attribute information cheaply (instead of surveying)

• Other uses (many in real time):

• 911/firefighter/police/ambulance dispatch• car navigation

• roadside assistance

• business vehicle/fleet management

• mineral/resource exploration

• wildlife tracking

• boat navigation

• Recreational

• Ski patrol/medical staff location monitoring

GPS Applications

Computers In Law Enforcement

• Officers today usually have a laptop available in their vehicles with wireless Internet to– Write tickets– Fill out accident reports– Complete routine police work– Download criminal and driving records from

databases– Check license plate registrations– Retrieve information from headquarters while on the

road

Challenges and Problems

– “We can have more data available than ever; how can we use it effectively for law enforcement?”

– “How can we combat stolen vehicles in our community?”

– “How can our officers run more license plate checks and still do their other duties?”

– “How can we increase our officers’ safety when they’re patrolling and making traffic stops?”

ALPR Mobile Vehicle Solution

– Up to two exterior cameras, mounted on roof or light bar– Fixed focal length lenses for different use cases– Manual updates to database in the field– Back Office Utility software for easy data management– Global Positioning System (GPS) and Mapping Support– Integration with Mobile Video Enforcer (MVE)

• Alert triggers MVE recording

– AirMobile and MESH / 802.11 integration for wireless updates of local database

– Certified for Motorola• MW800 / ML900 / ML850 / Panasonic Toughbook CF-29

Applications – Amber Alert Tracking

• Application– Mobile unit equipped with a single camera.

Officer can update database manually through PAGIS.

• Benefit– Increase effectiveness of response to alerts

• May save lives• Automatic reading of licenses in the background

Case StudyPennsylvania State Police Pilot

- Background– Profile

• Pennsylvania State Police are installing 1400 MW800’s Mobile Data Terminals (MDTs)

• Colonel saw technology in Europe• 22,000 stolen cars a year in PA• Customer likes trying new technology and giving

feedback

– Pilot components• 6 patrol cars out of Harrisburg• 3 shifts / 6 weeks• Broad “hot list” with CLEAN, NCIC, local wants and

warrants

Case StudyPennsylvania State Police Pilot

- Results• Immediate Results

– Vehicle recovered in the first shift• Sgt. Franks DeAndrea: “If we would end the test today,

it would already be a full success.”

– Armed and dangerous suspects apprehended in the first week

– Officer safety reinforced• Official report: “The trooper feels the ALPR system saved

his life and prevented a serious incident from occurring.”

• Click here to view the ABC News Clip with details

Tracking Evidence

• Police may enter documentation about evidence into a handheld computer– Wirelessly transmits the information to an evidence database– Prints out a bar code sticker that is placed on the physical

evidence

• DNA evidence is stored in the CODIS (Combined DNA Index System) database– Forensic and offender indexes

• Fingerprint evidence is stored in the AFIS (Integrated Automated Fingerprint Identification System) database – Fingerprint images are scanned electronically

Storing Criminal Records

• NCIC (National Crime Information Center)– FBI database of criminal justice information– Officers check subject’s previous record after arrest

• NICS (National Instant Criminal Background Check System)– FBI system to help gun dealers perform background

checks on potential buyers– Dealer calls the Bureau of Identification to run a query

on several databases

Online Criminal Database

Enforcing Traffic Laws

• Red-light systems use computer technology to catch drivers who run red lights– Cameras are positioned at intersection corners – A sensor loop triggers if a car moves over a certain

speed– The software activates the cameras to take photos– The software then creates a record with the time,

location, and the photos, and – a citation is sent to the offender

• Traffic violation fines may often be accessed and paid online

Tracking Stolen Vehicles

• LoJack is a stolen vehicle recovery system– A wireless radio-frequency transmitter is placed in the

car– The vehicle identification number of a stolen vehicle is

checked against the NCIC (National Crime Information Center)

– A signal can be sent to the device, which much like a GPS reports its exact location / a map can be created

• Key pass theft system– Uses a motion sensor and uniquely coded key pass to

detect unauthorized motion of a vehicle

Finding Missing Children

• Amber Alert system– Immediately notifies public for help in recovering

missing children– Uses EAS (Emergency Alert System) technology– EAS broadcasts alert on radio and television– Alerts can be displayed on dynamic message signs

on highways– NCMEC (National Center for Missing and Exploited

Children) provides alerts via e-mail, mobile phone, pager, or AOL Instant Messenger

Providing Wireless 911

• Enhanced 911 (E-911) displays phone number and address information on operator’s computer– The ANI (automatic number identifier) and ALI

(automatic location identifier) databases provide the information

• ANI and ALI does not work with cell phones• FCC (Federal Communications Commission)

requires that all cell phone providers equip phones with GPS (global positioning system) receivers– GPS receiver receives signals from GPS satellite– Computer links this information to the ALI database

VeriChip

• Implantable microchip that stores personal information• Useful for tracking people with illnesses or in danger • Raises privacy and legal issues

Garmin’s cheapest receivers

Garmin’s iQue 3600 PDA:

http://www.garmin.com/products/iQue3600/

Garmin’s Forerunner 201: A watch that uses GPS to determine current speed, average speed, exact distance traveled, etc. ( ) Basic features also available in the Forerunner 101 ($115).

http://www.garmin.com/products/forerunner201/

Garmin GPSmap60-C

Main Screens

Time & Date

Garmin GPSmap60-C

Satellite Page

Location

Satellite Strength

Accuracy Estimate

Skyplot

Garmin GPSmap60-C

Mark Waypoint

NameSymbol

Average

Garmin GPSmap60-C

Main ScreensSetup

By default, the GPSmap60 will record a Track (line feature) whenever the unit is turned on.

Map (GoTo) Profiles

Bluetooth GPS Receivers

Teletype’s Mini-bluetooth GPS receiver ($175)

http://www.mightygps.com/Manufacturer/minibluetooth.htm

Teletype’s USB GPS receiver for Laptops ($170)

http://www.teletype.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=1250&Category_Code=

HP’s Ipaqs and other PDAs with GPS software

Hewlett-Packard’s new iPAQ h1945 PDANow comes equipped with a hp GPS receiver and navigation system ($500)http://www.shopping.hp.com/cgi-bin/hpdirect/shopping/scripts/product_detail/product_detail_view.jsp?BV_SessionID=@@@@0280349227.1102102313@@@@&BV_EngineID=ccckadddfdjlkdgcfngcfkmdflldfgg.0&landing=null&category=handhelds&subcat1=classic_performance&product_code=PF527A%23ABA&catLevel=3

Garmin’s iQue 3600 PDA:

http://www.garmin.com/products/iQue3600/

GPS – Ellipsoid - Datum

GPS uses the WGS84 (World Geodetic System of 1984) as mathematical surface (model) of the earth

Elevations are referenced to Height Above Ellipsoid (HAE)

Navigation Review

• Position on the earth is measured in terms of latitude and longitude

• Parallels of latitude define position in the north-south direction

• Latitude is measured as angle from center of earth north or south

o

o

o

o

70

50

30

0

Navigation Review

• Longitude is defined as an angle, east or west of a reference meridian passing through Greenwich, England

• Every point on the earth has a unique address in terms of latitude and longitude

Greenwich

W E

Lat: 40 N

Lo: 20 W

60 N

40 N

20 N

20 S

40 S

60 S

Equator

Prime Meridian

20 W20 E

Lat: 40 00’.00N

Lo: 020 00’.00W

Lat: 40 00’.00S

Lo: 020 00’.00W

Every place on earth has a unique location

180 Lo approx. International Date Line

o

o

o

O

o

o

o

o

o

o

o

o

o

o o

60 04’ .20” N

149 26’.10” W

o

o

60 o

05’

149 30’Wo

20’

Navigation Review

• Horizontal datum is a chart coordinate reference system

• Nautical charts are in the process of being standardized, not all are drawn to the same datum

• The datum is shown on each chart• GPS receivers must be adjusted to

the same datum as used on the chart!

By

Mahmoud El Mewafi

Mmewafi1@ excite.comProfessor of surveting and Geodesy, Public Works Dept.,

Faculty of Engineering, Mansoura University, Egypt

6Datum Transformations

األرضية الكرة من لكل المقارنة سطوحتحويل وعملية اس بي الجي ونظام

بينهما القياسات Sat. 2

Receiver

X

Z

Y)0 ,0 ,0(

WGS-84

Local

Sat. 3

Sat. 1

Sat. 4

Datum Transformations

• A number of different procedures are available for performing coordinate transformations

• 1. Helmert Transformation

• 2. Molodenskii Transformation

• 3. Multiple Rregression Transformation

Helmert Transformations

• The most general method of transforming coordinates from one geodetic datum to another uses all 7 geometrical transformation parameters to convert Cartesian coordinates (X, Y and Z). The general formulation of this transformation is :

• Where X1, Y1, Z1 Cartesian coordinates in first datum• X2, Y2, Z2 Cartesian coordinates in second datum.

• The rotation angles θx , θ y and θ z, expressed in radians in the matrix, are assumed ’small’ angles, i.e. of the order of a few seconds-of-arc

Z

Y

X

Z

Y

X

Z

Y

X

Z

Y

X

xy

xZ

yZ

1

1

1

1

1

1

2

2

2

• the geodetic coordinates (Φ, λ, h) of a point it is first necessary to convert

these to Cartesian coordinates, before the transformation can be

applied.

• Similarly, the resulting coordinates in the new datum could also be

converted to the corresponding geodetic latitude and longitude using

the parameters of the ellipsoid associated with this new datum.