36. Global Positioning System

Transcription

1 36. Introduction to the Global Positioning System (GPS) Why do we need GPS? Position: a basic need safe sea travel, crowed skies, resource management, legal questions Positioning: a challenging job local navigation is relatively easy, but at sea there are no landmarks Navigation and Positioning Systems Landmarks subject to change, only works in local areas Dead reckoning complicated, errors accumulate quickly Celestial complicated, only works on clear nights, limited precision LORAN limited coverage, limited and variable accuracy OMEGA radio direction beacons, limited accuracy and subject to radio interference SatNav doppler radar-based, few satellites, infrequent updates Global Positioning System this is the one! What is GPS? Where did it come from? A very accurate system designed and operated by the U.D Dept of Defense originally designed for positioning nuclear submarines A large investment ($12 billion) has been made in its development over the past 25 years Congress approved because of other potential applications Only possible with today s computer, clock and satellite technology NAVSTAR GPS System Space Segment Control Segment User Segment Ground Antennas Master Station Monitor Stations FR 3262 /

2 Space Segment: Constellation of GPS Satellites 28 satellites in 6 orbital planes 20,200 km altitude NAVSTAR Satellites Altitude: 10,900 miles very high orbit for accuracy, coverage, and survivability Size: 17 feet Weight: 1900 pounds Orbital period: 12 hours Orbital plane: 55 degrees to equatorial plane 28 satellites 24 active, 4 spares Control Segment: User Segment: GPS Receiver X, Y, Z, T Latitude, longitude and elevation to tens of feet in real time Components Antenna Satellite Receiver Electronics to receive satellite signals Microprocessor to process the data that determines the antenna position Controls to enable user input to the receiver Display screen How Does GPS Work? 3. Depends on accurate timing -- good clocks 2. Measures distance using travel time of radio signals 4. Must know exact locations of satellites 5 Steps 1. Basis of system is triangulation from satellites 5. Correct for atmospheric and ionspheric delays FR 3262 /

3 Triangulation from Satellites By measuring the distance to several satellites we can determine our location A second measurement narrows our location to the intersection of two spheres 11,000 miles One measurement puts us somewhere on the surface of a sphere 11,000 miles Intersection of two spheres is a circle 12,000 miles Satellite A third measurement narrows the possible locations to two points The fourth measurement only intersects one of the two points Intersection of three spheres is two points Measuring the Distance to the Satellites Measure how long it takes the GPS signal to reach the receiver Radio waves travel at the speed of light Time (seconds) X 186,000 miles/sec = miles If satellite is overhead the time is 0.06 seconds Estimated ranges to each satellite intersect within a small area when corrected for the receiver clock bias With good clocks, all we need to know is exactly when the signal left the satellite FR 3262 /

4 How do we know when the signal left the satellite? Measure time difference between same part of the coded signal Use the same code at satellite and receiver Synchronize satellites and receivers so they generate the same code at the same time Then look at the incoming code from the satellite and see how long ago our receiver generated the same code Satellite Receiver Time Difference 3 System depends on very accurate clocks to measure travel time ΔT ΔT 2 Satellites have atomic clocks accurate to 1 billionth of a second 1 ΔT 3 ΔT 4 also very expensive However, ground receivers only need to have consistent clocks Receiver Synchronous Satellite Outputs Differing Times Signals Received How do we know the location of the satellites? High orbits are very stable, symmetric and there is no atmospheric drag Corrections are transmitted by Defense Dept to the satellites Corrections are transmitted by satellites to ground receivers Solving the Distance Equation R FR 3262 /

5 Atmospheric and Ionospheric Corrections The ionosphere and atmosphere slow down the signals Models are used to correct for these effects Selective Availability In the past the government sometimes introduced artificial clock and ephemeris errors to prevent hostile forces from using it unfortunately the errors were also in the data of friendly forces us and SA was the greatest source of error On May 2, 2001 selective availability was turned off and accuracy of most receivers should be at least 20 meters Differential GPS Differential GPS Uses two GPS receivers together, one stationary, one moving Stationary receiver is at a known location Stationary receiver compares calculated position with known location to determine amount of error Then transmits the error to moving receivers Garmin Corp. How Accurate is GPS? GPS Errors Depends on Receiver design Time spent on measurements Relative positions of satellites Survey systems can provide sub-centimeter accuracy Other good systems can deliver meter accuracy, 1 to 3 meters with differential GPS FR 3262 /

6 Error Budget Applications of GPS Typical Error in Meters (per satellite) Satellite Clocks Orbit Errors Ionosphere Troposphere Receiver Noise Multipath Total Standard GPS Differential GPS Aircraft, ship and vehicle navigation Aerial photography acquisition Location of features and boundaries for input to GIS and digital image classification and accuracy assessment Mapping topography, soils, forests, geology, wetlands, utilities,. Survey and legal land description Civil engineering, construction of highways, bridges, dams,. Applications of GPS, cont. Resource inventory (plot location) Precision farming Vehicle tracking (trucks, buses, taxis, ) Emergency (police, fire, ambulance) and rescue Timing (precise to a billionth of a second) Some GPS sites on the Internet GPS Overview Commercial homepages with background information as well as product descriptions has a very nice tutorial Eventually everyone will have a GPS address Combining GIS and GPS Capability with Satellite Imagery for In-Field Forestry Applications Rick Kerns Tom Burk Marvin Bauer Outline Hardware Software Imagery Field Applications FR 3262 /

7 Hardware Compaq ipaq with Magellan GPS315 for early field prototyping due to software support, cost and easy availability Hardware Fujitsu Stylistic 3500R with Magellan GPS Receiver Increased functionality and power with Windows 2000 Software ESRI ArcPad Runs on WinCE or Windows Familiar ArcView like interface Supports MrSID compressed images StarPal HGIS+ GPS Mapping Software Runs on WinCE or Windows Simple user interface Customizable wizards Supports various sensor input besides GPS Software Imagery Imagery depends on user s application We have focused on Digitized aerial photographs IKONOS Geo product (1 meter resolution) Imagery must be compressed to be stored and viewed on small platform computers Possible with LizardTech and ER Mapper products Specific Field Applications Regeneration Surveys Imagery allows evaluations of spatial homogeneity Corner Location and Line Running Effort, as measured by amount of time, reduced FR 3262 /

8 Regeneration Surveys Regeneration Surveys Survey traverse and sample plot locations are mapped in GIS prior to entry into the field GPS is used to navigate through the traverse GIS is used in the field to collect data at the sample plots The digital satellite data is used to create a greenness map (NDVI) Suspected holes in the vegetation are mapped on the desktop GIS While traversing the survey the holes are verified, mapped and attributed. Corner Location and Line Running ArcPad s Custom Data Entry Forms Approximate corner locations are mapped using a PLS layer at the 40 level (¼ of ¼ PLS section) A roads layer is used as a visual reference during navigation GPS is used for navigating to the corners and running the boundary lines StarPal s Custom Grid Back-up data in the field Data collected in the field can be downloaded via a standard serial connection to a laptop computer FR 3262 /

9 Updated data in ArcView Urban Forest Health Field Data Collection ArcPad displaying parcels and DOQ data The field computer with ArcPad and GPS allowed us to view DOQs and parcel data from the county surveyor departments while running the real time tracking option in the field. Additionally, we had the capability to call up information from the parcel file data table. The aerial view of the study sites from the DOQs along with the information from the parcel data table, combined with the guidance of the real time tracking option, allowed us to know exactly where we were and better acquire our field data. Summary: How important is it to know where you are? GPS, like GIS, is very complementary to remote sensing, providing a cost effective way to acquire precise location information, including elevation. It is an integral part of aerial and satellite data acquisition, as well as field data collection and mapping. FR 3262 /