Intro. To GIS Lecture 4 Where does spatial data come from? February 20 th, 2013

Copyright © 2011 by Maribeth H. Price 3-2 Cylindrical ConicAzimuthal 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 GPS Monitor Stations Ground Antennas Master Station

Three Segments of the GPS

Kwajalein Atoll US Space Command Control Segment Hawaii Ascension Is. Diego Garcia Cape Canaveral Ground Antenna Master Control Station Monitor Station

Orbital period ~ 12 hours Space 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 ranging step 2: measuring distance from satellite step 3: getting perfect timing step 4: knowing where a satellite is in space step 5: identifying errors GPS Key Concepts

Position 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 is and how far away from it we are… if we are lost and we know that we are 11,000 miles from satellite A… we are somewhere on a sphere whose middle is satellite A and diameter is 11,000 miles Answer…

if we also know that we are 12,000 miles from satellite B …we can narrow down where we must be… only place in universe is on circle where two spheres intersect if we also know that we are 13,000 miles from satellite C …our situation improves immensely… only place in universe is at either 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 are intersections 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 measurements 4 satellites…three (X, Y, Z) dimensions and time when clock offsets are determined, the receiver position is known 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 SourceAmount 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 errors selective 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 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 Geometry

Poor Satellite Geometry

Planning a Navigation Route Start = Waypoints

How 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 Receiver Receiver Differential GPS (DGPS) Realtime (RTK)

USCG 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 ) HW: Ch. 3 Questions: 1 and 2 Please submit last week’s assignments