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AUG:23 GPS INTRO28 – 30 GPS Use Orientation (pls bring 2 AA batteries /grp)
GPS FIELDWORK
SEP: 4 – 6 GPS LEC - GPS FIELDWORK
11 – 13 GPS LEC - EXAM #3
18 – 20 GIS LEC - GIS LEC
25 – 27 GIS LEC - GIS LEC
OCT:2 – EXAM #44 – GRP WORK - GRP PRESENTATION
Identify your group members;Submit on a piece of paper
UVLE PASSWORD: GE1gonzalez_(1)2011
DAILY “BOMBARDMENT”
OF DATADATA FROM SPACE
The Earth is now surrounded by:(as of 23 Aug 2012)
38,751 orbiting satellites(21,988 are inactive)http://www.celestrak.com/satcat/boxscore.asp
To protect the
GLOBAL ENVIRONMENTGLOBAL ENVIRONMENT
USEFUL INFORMATIONUSEFUL INFORMATION
Turn into
BIG RESPONSIBILITY FOR S&T PROFESSIONALSBIG RESPONSIBILITY FOR S&T PROFESSIONALS
The GPS Satellites
For geo-referencing spatial events to map their distribution at any time.
GPS Receivers
Introduction
to
NAVSTAR GPS
Introduction
to
NAVSTAR GPS
What it is
GPS: Global Positioning System is a worldwideradio-navigation system formed from aconstellation of 24 satellites and their groundstations.
Uses the principle of
triangulation and time-
of-arrival of signals to
determine the location
of a GPS receiver.
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Feasibility studies began in 1960’s.
Pentagon appropriates funding in 1973. First satellite launched in 1978.
System declared fully operational in April, 1995.
Cost of maintenance = $750M per year (US DoD)
1982 - Russians developed GLONASS (Global'naya NavigatsionnayaSputnikovaya Sistema) – fully operational in 2010
2005 - Europe developed GALILEO – fully operational in 2012
2006 - India is developing IRNSS (Indian Regional Navigation SatelliteSystem) – fully operational in 2013
History of GPS GPS FACTS
Name : NAVSTAR
Altitude: 12,660 miles (~20,250 km)
Weight: 1,900 lbs (in orbit)
Size: 17ft. (with solar panels)
Orbital period: 12 hours
GPS Facts
Consists of 24 Earth-orbiting satellites
Solar cells power each satelliteAntennas transmit timing information
Operated and maintained by U.S.
Department of Defense
Position and coordinates.
The distance and direction between any two
waypoints, or a position and a waypoint.
Travel progress reports.
Accurate time.
Four Basic Information from GPS
Control Segment
Space Segment
User Segment
Three Segments of the GPSThree Segments of the GPS
Monitor Stations
Ground
Antennas
Master Station
Kwajalein Atoll
US Space Command
Control SegmentControl Segment
Hawaii
AscensionIs.
Diego Garcia
Cape Canaveral
Ground AntennaMaster Control Station Monitor Station
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Basic Functions of Control/Monitor
Segment
These stations are the eyes and ears of GPS,monitoring satellites as they pass overhead bymeasuring distances to them every 1.5 seconds
These data are then smoothed using ionosphericand meteorological information and sent to theMaster Control Station at Colorado Springs, USA.
The ionospheric and meteorological data areneeded to get more accurate delay measurements,which in turn improve location estimation.
c o r r e c t e d
Functions of Control/Monitor
Stations (Cont’d)
Master control station estimates parametersdescribing satellites' orbit and clockperformance. It also assesses health status of thesatellites and determines if any re-positioningmay be required.
The information are then returned to threeuplink stations (collocated at the AscensionIsland, Diego Garcia and Kwajalein monitorstations) which eventually transmit theinformation to the satellites.
Space Segment A system of24 satellites
Space Segment
The space segment is the satellite constellation
24 satellites with a minimum of 21 operating98% of the time
6 Orbital planes
Circular orbits
20-200 km above the Earth's surface
11 hours 58 minute orbital period
Visible for approximately 5 hours above thehorizon
Military.
Search and rescue.
Disaster relief.
Surveying.
Marine, aeronautical and terrestrial navigation.
Remote controlled vehicle and robot guidance.
Satellite positioning and tracking.
Shipping.
Geographic Information Systems (GIS).
Recreation.
User Segment How the GPS Works
by triangulation
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How the GPS Works
Imagine you are somewhere in Quezon City. You are TOTALLY lost. You ask someone, “Where am I?”He/she says, “You are 5 km from the Quezon City Hall.”
It doesn’t really help you. You could be anywhere on acircle 5 km around the Quezon City Hall.
Imagine you are somewhere in Quezon City. You are TOTALLY lost. You ask someone, “Where am I?”He/she says, “You are 5 km from the Quezon City Hall.”
It doesn’t really help you. You could be anywhere on acircle 5 km around the Quezon City Hall.
Quezon City Hall
5km
How the GPS Works You ask someone else where you are.He/she says, “You are 7 km from SM-Marikina.”
If you combine this information with the QC Hall information, youhave two circles that cross.
You ask someone else where you are.He/she says, “You are 7 km from SM-Marikina.”
If you combine this information with the QC Hall information, youhave two circles that cross.
Quezon City Hall
SM-Marikina -
-7km
5km
Now you know that you must be at one of these two pointsif you are 5 km from QC Hall and 7 km from SM-Marikina.
How the GPS Works
Quezon City Hall
SM-Marikina
-
A third person tells you that you are 4 km from Celebrity Sports.
The 3rd circle will only cross the other circles at one point.
A third person tells you that you are 4 km from Celebrity Sports.
The 3rd circle will only cross the other circles at one point.
Melchor Hall
7km
5km
Celebrity Sports
4km
Now you know exactly where you are.
The only place allthese circles crossis at Melchor Hall!
Imagine that QC Hall,SM-Marikina, andCelebrity Sports areSatellites that giveclues about theirrespective distance fromthe GPS receiver youare carrying.
How the GPS Works
by triangulation
Position is Based on TimePosition is Based on Time
T + t
Distance between satellite andreceiver = t x speed of light
TSignal leaves satelliteat time “T”
Signal is picked up by thereceiver at time “T + t”
Distance = velocity x timeDistance = velocity x timeSpeed of light c
Pseudo Random Noise (PRN) CodePseudo Random Noise (PRN) Code
Receiver PRN
Satellite PRN
TimeDifferenceEach satellite
generates its ownunique PRN
Each receiverhas a database of all Satellites’ PRN
Complex sequenceof 1s and 0s pulses
Signal matching at the same time that PRN is sent (time T from the Almanac)
(Not really random but is a precise unique code that identifies a particular satellite)
(at time T)
(at time T)
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What Time is It?What Time is It?
Military Time = UTC(military time is simply local time on a 24 hour clock)
e.g., 1800 means 6pm local time wherever appliedbut 1800 UTC is 6pm only in Greenwich
Universal Coordinated Time (UTC) = official time of the world
Greenwich Mean Time
Local Time: AM and PM (adjusted for local time zone)
GPS Time = UTC + 13sec*
* GPS Time is ahead of UTC by approximately 13 seconds (leap seconds adjustment)
Or “Zulu time”(military term)
or (the basis of all time)
(divides the world into 24 time z ones of 1 hour each; 0 000 is midnight in Greenwich)
(all GPS use UTC)
* UTC - Temps Universel Coordonné
Triangulating Correct PositionTriangulating Correct Position
-
4th Satellitefor ACCURACY
12
3
NEEDS 4 SATELLITES FOR ACCURACY
Signal From One SatelliteSignal From One Satellite
The receiver issomewhere onthis sphere.
Signals From Two SatellitesSignals From Two Satellites
Three Satellites (2D Positioning)Three Satellites (2D Positioning) Triangulating Correct PositionTriangulating Correct Position
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Three Dimensional (3D) PositioningThree Dimensional (3D) Positioning Triangulation Requirements
To triangulate, a GPS receiver measures distance
using the travel time of radio signals.
To measure travel time, GPS receiver needs veryaccurate timing.
Along with distance, receiver needs accurate dataon where the satellites are (geometry) in space.
The system will also need to correct for any delaysthe signal experiences as it travelsthrough the atmosphere.
Selective Availability (S/A)
The Defense Department dithered the satellite timemessage, reducing position accuracy to some GPS users.
S/A was designed to prevent US enemies from using GPSagainst US and its all ies.
In May 2000 the Pentagon reduced S/A to zero meterserror.
But S/A could be reactivated at any time by the Pentagon.
Sources of GPS Error
Standard Positioning Service (SPS ): Civilian Users
Source Amount of Error
Satellite clocks: 1.5 to 3.6 meters
Orbital errors: < 1 meter (due to gravitational pull)
Ionosphere (upper): 5.0 to 7.0 meters
Troposphere (lower): 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. Position DilutionOf Precision
Mountains
Sources of Signal InterferenceSources of Signal Interference
Earth’s Atmosphere
Solid Structures
Metal
Electro-magnetic Fields
= ERRORS
Forest
GPS Satellite Geometry
Satellite geometry can affect the quality of GPS signals and accuracy ofreceiver trilateration (triangulation).
Geometric Dilution of Precision (GDOP) reflects each satellite’s positionrelative to the other satellites being accessed by a receiver; therefore errors.
There are five distinct kinds of DOP : Horizontal-HDOP, Vertical-VDOP,Position-PDOP, Time-TDOP, General-GDOP.
Position Dilution of Precision (PDOP) is the DOP value most commonlyused in GPS to determine the quality of a receiver’s position.
It is usually up to the GPS receiver to pick satellites which provide the bestposition t riangulation.
Some GPS receivers allow DOP to be manipulated by the user.
* GDOP Geometric Dilution of Precision
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Ideal Satellite GeometryIdeal Satellite GeometryN
S
W E
FAR APART FOR BETTER GEOMETRY
Good Satellite GeometryGood Satellite Geometry
Good Satellite GeometryGood Satellite Geometry
FAR APART FOR BETTER GEOMETRY
Poor Satellite GeometryPoor Satellite GeometryN
S
W E
TOO CLOSE TOGETHER
Poor Satellite GeometryPoor Satellite Geometry Poor Satellite GeometryPoor Satellite Geometry
TOO CLOSE TOGETHER
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Receiver Errors are Cumulative!Receiver Errors are Cumulative!
User error = +- 1 km
System and other flaws = < 9 meters
Using GPS Receivers for
Positioning and Navigation
Planning a Navigation RoutePlanning a Navigation Route
Start= Waypoint
How A Receiver Sees Your RouteHow A Receiver Sees Your Route
- satellite
- waypoint
N (0000)
(00)N
Desired Track
(DTK) (xº)
PresentLocation
Tracking (TRK) (xº)
Active GOTOWaypoint
GPS Navigation TerminologiesGPS Navigation Terminologies
Course Made on Ground (CMG)
(CMG) (xº)
ActiveFromWaypoint
Cross Track Error (XTE)
Active GOTOWaypoint
Bearing =Course Over Ground (COG) =
Cross Track Error (XTE) =Location Where GOTOWas Executed
Bearing = 650
COG = 50
XTE = 1 km
Bearing = 780
COG = 3500
XTE = .75 km
Bearing = 400
COG = 1040
XTE = .5 km
N
GPS Navigation: On the GroundGPS Navigation: On the Ground
(constantly being computed & averaged)(constantly being computed & averaged)
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On Position Fix
A position is based on real-time satellite tracking.
It is defined by a set of coordinates.
It has no name.
A position represents only an approximation ofthe receiver’s true location.
A position is not static. It changes constantly asthe GPS receiver moves (or due to random errors).
A receiver must be in 2D or 3D mode (at least 3 or4 satellites acquired) to provide a position fix.
A 3D mode dramatically improves positionaccuracy.
About Waypoints
A waypoint is defined by 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 ornumber, or a user supplied name.
Once entered and saved, a waypoint remainsunchanged in the receiver’s memory until editedor deleted.
About GPS Clocks
Atomic clocks cost ~ $50,000 - $100,000 very expensive foreveryday consumer use.
The GPS solution to this problem:Every satellite contains an expensive atomic clock, but thereceiver itself uses an ordinary quartz clock, which itconstantly resets.
So, the receiver looks at incoming signals from four or moresatellites and gauges its own inaccuracy.
The receiver calculates the necessary adjustment that willcause the four spheres to intersect at one point(proportionally incorrect).
How the GPS Works
Quezon City Hall
SM-Marikina
-
A third person tells you that you are 4 km from Celebrity Sports.
The 3rd circle will only cross the other circles at one point.
A third person tells you that you are 4 km from Celebrity Sports.
The 3rd circle will only cross the other circles at one point.
Melchor Hall
7km
5km
Celebrity Sports
4km
Now you know exactly where you are.
The only place allthese circles crossis at Melchor Hall!
Imagine that QC Hall,SM-Marikina, andCelebrity Sports areSatellites that giveclues about theirrespective distance fromthe GPS receiver youare carrying.
Real-time kinematic Differential GPS (applyingcorrection during field measurements)
Static Differential GPS - Post-processing (applyingcorrection after field measurements)
Differential GPS
DGPS Base Stn.
54, 98current reading
29, 35
True coordinates = 24, 38
DGPS correction = 54-5 and98 +3
True coordinates of Roaming = 49, 101
DGPS Receiver
RoamingReceiver
-5, +3
Real Time Kinematic (RTK) GPSReal Time Kinematic (RTK) GPS
5units more than the true coordinates
3units less than thetrue coordinates
Correction = -5, +3
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DGPS Base Stn
x +30, y +60
x +5, y -3
True coordinates = x +0, y +0
Correction = x -5, y +3
DGPS correction = x +(30-5) andy +(60+3)
True coordinates = x +25, y +63
RoamingReceiver
Static (Post-processing) Differential GPSStatic (Post-processing) Differential GPS
Corrections areapplied after fieldwork
NDGPS Ground StationsNDGPS Ground StationsNational Differential Global Positioning System
Yellow areas show o verlap between NDGPS stations. Green are as are little to no cove rage.
Topography may also limit some areas of coverage depicted here.
NDGPS Ground StationsNDGPS Ground StationsNational Differential Global Positioning System
Yellow areas show ov erlap between NDGPS stations. Green area s are little to no covera ge.
Topography may also limit some areas of coverage shown here.
Using GPS Data
A GPS receiver essentially determines the receiver's
position on the Earth.
Once the receiver makes this calculation, it can tell
you the latitude, longitude and altitude of its current
position.
To make the navigation more
user-friendly, most receivers
plug this raw data into map
files stored in memory.
Using GPS Data (Cont’d)
You can:
use maps stored in the receiver's memory,
connect the receiver to a computer that can hold
more detailed maps in its memory, or
simply buy a detailed map of your area and find your
way using the receiver's latitude and longitude
readouts.
Some receivers let you download detailed maps into memory or
supply detailed maps with plug-in map cartridges.
Using GPS Data (Cont’d)
A standard GPS receiver will not only place youon a map at any particular location, but will alsotrace your path across a map as you move.
If you leave your receiver ON, it can stay inconstant communication with GPS satellites to seehow your location is changing.
This is what happens if car isequipped with a GPS.
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Using GPS Data
With this information and its built-in clock, the receiver
can give you several pieces of valuable information:
How far you've traveled (odometer)
How long you've been traveling (timer)
Your current speed (speedometer)
Your average speed
A "bread crumb" trail showing you exactly
where you have traveled on the map
The estimated time of arrival at your
destination if you maintain your current speed
SatellitesSatellites
Geostationary – directly above equator,
orbits at the samespeed as the earth
- communication satellites (Comsat)
Sun Synchronous – passes over a point
on the earth at the samelocal solar time
- earth observation satellites (EOS)
Wide Area Augmentation SystemWide Area Augmentation System
Geostationary
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 Availabilityset to zero, and under idealconditions, a GPS receiverwithout WAAS can achieve15 meter accuracy most ofthe time.*
Under ideal conditions a
WAAS equipped GPSreceiver can achieve3 meter accuracy 95% ofthe time.*
* For airplanes, precision depend s on good satellite geometry, open sky view, and
no user induced errors.
GPS POSITIONINGEach satellitecontinuously transmitsorbital data for the entireconstellation in additionto timing data and otherinformation.
A GPS receiver's job is tolocate four or more ofthese satellites, figureout the distance to each,and use this informationto deduce its ownlocation.
Position Error
The position reported by the receiver is not absolute and containssome amount of uncertainty or error.
Troposphere
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Sources of Signal Errors
Ionosphere and tropospheredelays -
Orbital (ephemeris) errors.
Sources of Signal Errors
Receiver clock errors
Multipath
Sources of Signal Errors
Number of satellites visible.
Satellite geometry/shading
Measure of Accuracy
Dilution of precision (DOP) (low is good)
Position DOP (PDOP) is commonly used and a value of 6
or less is generally acceptable
Error Correction
Averaging multiple GPS fixes
Differential correction or differential GPS (DGPS)
BASE Stn
Configuring a GPS receiver
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Configuring a GPS receiver
Coordinate system
Datum
Protocol
Units of measure
Elevation Mask
2D or 3D Mode
Using GPS in the field
The following are some general tips while out in the fieldscollecting data:
1. Perform mission planning and testing2. Get good satellite fix3. Batteries are fully charge and bring a spare4. Keep antenna pointed skyward5. Use position offset technique6. Capture GPS fixes in 3D mode with PDOP of 6 or less,7. Always verify your receiver is receiving enough signals,
8. Do not rely solely on GPS!Bring along a paper map of the area & a compass.