GPS - Intro(1).pdf

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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.



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

-





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.


Receiver clock errors

Multipath


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.

Documents

GPS - Intro(1).pdf