Intro. To GISLecture 4

Where does spatial data come from?

February 20th, 2013

Copyright © 2011 by Maribeth H. Price 3-2

Cylindrical Conic Azimuthal

Review: Projections

Line of contact:Equator

Point of contact:North Pole

Lines of contact:Equator

Review: Map Basics

Source/Date

Review: Isopleth/Contour Map

Review: Symbols and Scale

• Features may become more generalized

• Point vs. Polygon

Credits

• Some slides from UT Austin, Geology program and US National Park Service

Data Collection Techniques

• Digitizing (tracing features)– Scanned maps– Raster data

• Surveying data points using GPS, surveying equipment

• Remote sensing • Drawing files (CAD)

Surveying

• The technique, profession, and science of accurately determining the terrestrial or three-dimensional position of spatial features

• In general, the outputs is in the form of a map

• Tools– Total stations, theodolite, etc.– GPS

Global Navigation Satellite Systems (GNSS)

• Global positioning system (GPS) is the first deployed set of GNSS for positioning. It was developed by DoD.

• Russia has been developing GLONASS• Galileo is planned by a consortium of

European governments and industries• The fourth system is under development is the

Chinese Compass Satellite Navigation System• The concepts of GNSS are described using

GPS as a reference

Global Positioning System

A brief history of the Global Positioning System

Segments of the GPS A primer on how the GPS works Problems with the GPS Advancements in the GPS

There was a lack of consistent positioning Department of Defense finally said: “we need something better: all-day and all-night; all terrain”

• system (constellation) of 24 satellites in high altitude orbits(cost ~ $12 billion)

• coded satellite signals that can be processed in a GPS receiver to compute position, velocity, and time• parts of system include: space (GPS satellite vehicles) control (tracking stations) users

first one launched in 1978 ….June 26, 1993 Air Force launched 24th satellite

Global Positioning System

Control Segment

Space Segment

User Segment

Three Segments of the GPSThree Segments of the GPS

Monitor Stations

GroundAntennas

Master Station

Three Segments of the GPS

Kwajalein Atoll

US Space Command

Control SegmentControl Segment

Hawaii

Ascension Is.Diego Garcia

Cape Canaveral

Ground AntennaMaster Control Station Monitor Station

Orbital period ~ 12 hours

Space SegmentSpace Segment

• The constellation is designed such that at any point on the earth at least four satellites are “seen”

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

step 1: using satellite rangingstep 2: measuring distance from satellitestep 3: getting perfect timingstep 4: knowing where a satellite is in spacestep 5: identifying errors

GPS Key Concepts

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”

Measuring Time

• Satellites have atomic clocks– Very expensive: $100K

• Receivers have “ordinary” clocks– Inexpensive and not as accurate as satellite’s clocks

Therefore, our measurements are subject to errors due to inaccurate time measurements made by receivers

Hold on! There is a way to get around this problem

Out of 24, how many satellites are needed for positioning?

Question?

GPS is based on satellite ranging, i.e. distance from satellites …satellites are precise reference points

…we determine our distance from them

we will assume for now that we know exactly where satellite isand how far away from it we are…

if we are lost and we knowthat we are 11,000 miles

from satellite A…we are somewhere on a sphere

whose middle is satellite Aand diameter is 11,000 miles

Answer…

if we also know that we are12,000 miles from satellite B

…we can narrow down wherewe must be…

only place in universe is oncircle where two spheres intersect

if we also know that we are13,000 miles from satellite C

…our situation improvesimmensely…

only place in universe is ateither of two points where

three spheres intersect

Answer…

three can be enough to determine position… one of the two points generally is not possible (far off in space)

two can be enough if you know your elevation …why?

one of the spheres can be replaced with Earth… …center of Earth is “satellite position”

generally four are best and necessary….why?Because of the clock errors associated with receivers…

this is basic principle behind GPS……using satellites for triangulation

Answer…

how do we know that it is wrong?…measurement from third satellite

X

3rd satellite at 3 seconds

all 3 intersect at X…if time is correct

if time is not correct…

Answer… With a Perfect Receiver

add our one second error to the third receiver…

XX

…circle from 3rd satellite cannot intersect where other two do

purple dots areintersections of

2 satellites

define area of solutions …receivers calculate best solution(add or subtract time from each satellite)

Answer… With Typical Receivers

position determined from multiple pseudo-range measurements4 satellites…three (X, Y, Z) dimensions and time

when clock offsets are determined, the receiver position is known

Answer…

Answer…

• That’s why we need at least four satellites

OK…

• We already know that satellites constantly transmits signals at known times

• As a user, we need to see at least four satellites above our horizon to determine the position

• Well, how well do we know satellites’ positions?

Where is the Satellite?

• Satellites operate in known orbits• orbits known in advance and programmed into receivers• satellites constantly monitored by DoD …identify errors (ephemeris errors*) in orbits …usually minor• corrections relayed back to satellite “data message” about their “health”

* Ephemeris are data describing the altitude, position and speed of the satellite

Sources of Errors When Positioning with GPS

• Standard Positioning Service (SPS ): Civilian Users• Source Amount of Error

Satellite clocks: 0.5 to 1 meter Orbital errors (ephemeris): < 1 meter Ionosphere: 5.0 to 10.0 meters Troposphere: 0.5 to 1 meter Receiver noise: 0.3 to 1.5 meters Multipath: 0.6 to 1.0 meters Selective Availability (SA) Does not exist any more User error: Up to a kilometer or more

• Errors are cumulative and increased by DOP.• Note that the numbers are not current (absolute). However,

you can get a feel for which errors are more significant than the other (relative).

• tropospheric water vapor: affects all frequencies; difficult to correct

• multipath: reflected signals from surfaces near receiver

• noise: receiver noise

• Satellite clock errors; ephemeris errorsselective availability: SA; error introduced by DoD;

turned off May, 2000

• blunders: human error in control segment user mistakes (e.g. incorrect geodetic datum)

…more on this in a minute receiver errors

• Dilution of precision (DOP): Satellite geometry

Sources of Errors When Positioning with GPS

ionosphere: electrically charged particles (50-500km above earth);affects speed of electromagnetic energy…amount of affect depends on frequency (need “dual-frequency” receivers to correct)

Sources of Errors When Positioning with GPS : Ionosphere

Sources of Signal InterferenceSources of Signal Interference

Earth’s Atmosphere

Solid Structures

Metal Electro-magnetic Fields

Note that the boxes indicates causes of multipath effects

N

S

W E

Satellite Geometry

• Affects the positional accuracy • Satellite geometry is estimated by th Dilution of

Precision (DOP)… can also be expressed as GDOP, PDOP, HDOP, or VDOP

• A number from 1 to 100 (unitless)… The lower the better. Typically around 4

Ideal geometry for four satellites

N

S

W E

Poor Satellite Geometry

Good Satellite GeometryGood Satellite Geometry

Poor Satellite GeometryPoor Satellite Geometry

Planning a Navigation RoutePlanning a Navigation Route

Start= Waypoints

How A Receiver “Sees” Your RouteHow A Receiver “Sees” Your Route

Yellow stars: where you want to go.

Green stars: where the GPS receiver may take you.

Blue circles: the potential circle of GPS error at each waypoint.

Differential GPS (DGPS)

• One of the ways to significantly improve accuracy• corrects errors at one location using measured errors at a known

position (base station)• requires software in reference receiver that can track• The base station and the rover (user/remote) should not be too far

from each other ( <50km). The accuracy diminishes with increase in the distance between the two units.

• There are two modes of measurements:– Realtime (Real Time Kinematic GPS: RTK)– Post-process

Differential GPS (DGPS)

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

DGPS ReceiverReceiver

Differential GPS (DGPS)

• Realtime (RTK)

USCG NDGPS Ground StationsUSCG NDGPS Ground Stations

Yellow areas show overlap between NDGPS stations. Green areas are little to no coverage. Topography may also limit some areas of coverage depicted here.

Differential GPS (DGPS)

• Known GPS brands– Leica– Trimble– Garmin

Summary

• GNSS is a satellite-base positioning system. GPS is the first deployed by DoD

• GNSS is based on range (distance) measurements • Three satellites must be seen by the receiver for

positioning. However, the position may not be accurate due to…?

• GNSS measurements contain errors coming from ionosphere, troposphere, receiver noise, multipath, poor geometry, etc.

• The measurement accuracy can be improved using DGPS techniques

• Low DOP can also improve accuracy

• Think about: What would be ideal GPS measurements for different map scales?

Suggestion

• GPS is still a black box to you!• Take the summer course to get hands-

on experience with GPS measurements techniques

Homework & Lab

• Read Ch.3 (p. 55-67)• HW: Ch. 3 Questions: 1 and 2

• Please submit last week’s assignments