1 / 43

Understanding GPS, WAAS, and Coordinate Systems: A Comprehensive Overview

This guide delves into the foundational elements of the Global Positioning System (GPS) and the Wide Area Augmentation System (WAAS), exploring how they operate and enhance positioning accuracy. It covers fundamental concepts such as satellite trilateration, the significance of satellite geometry, and common sources of GPS error. Moreover, it explains coordinate systems, map projections, and the importance of datums in georeferencing spatial data. Learn how to effectively measure distances and positions, along with best practices for data collection in various environments.

hovan
Télécharger la présentation

Understanding GPS, WAAS, and Coordinate Systems: A Comprehensive Overview

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Introduction

  2. Summary of Topics - GPS - WAAS - Coordinate Systems & Map Projections - Datum

  3. The Global Positioning System Space Segment User Segment Control Segment

  4. How to Calculate a Position Measure the Distance to the Satellites

  5. Minimum # of Satellites Required-Trilateration 1 satellite – somewhere on a sphere

  6. Minimum # of Satellites Required-Trilateration 2 satellites – somewhere on a circle

  7. Minimum # of Satellites Required-Trilateration 3 satellites – one of two points

  8. Minimum # of Satellites Required-Trilateration 4 satellites – one point 3D GPS Location Note: with 3 satellites, one point is on the earth’s surface and one is nowhere near. However, we still need the 4th satellite because receiver clocks are inaccurate.

  9. Ranging • When working perfectly, receivers and satellites generate an identical week-long code in perfect synchrony with one another. • Measure the time delay between identical portions of receiver and satellite code, and multiply by the speed of light (~1 meter/3 ns). • A receiver synchronizes its code with that of satellites by adjusting the timing of its code generation until the intersection of all satellite ranging spheres converges on a single point.

  10. When There are Only 3 Satellites(2d GPS Location) – Horizontal Error is Greater Elevation - last known 2-5 X Error Rule

  11. Sources Of GPS Error • Errors that can be differentially corrected • Satellite clock errors • Satellite orbit errors • Atmospheric delay errors • Errors that can’t be differentially corrected • Receiver noise and electromagnetic fields • Multi-path • Sky obstructions • User mistakes

  12. Ideal Satellite Geometry N E W S

  13. Good Satellite Geometry

  14. Poor Satellite Geometry N Note: if poor satellite geometry, then on the receiver the Accuracy value will be higher W E S

  15. Poor Satellite Geometry Note: if poor satellite geometry, then on the receiver the Accuracy value will be higher N W E S

  16. Poor Satellite Geometry N Note: if poor satellite geometry, then on the receiver the Accuracy value will be higher W E S

  17. Poor Satellite Geometry

  18. Signal Strength • Satellite signal strength affects position quality. • Signal strength bar height indicates signal strength. • Good satellite arrangement cannot compensate for low signal strength.

  19. GPS Positions Aren’t Absolute Example: • 5 co-located receivers report positions scattered across 30 meters during a 3-hour test period. • Repeating the test would produce a new pattern. • Positions fluctuate constantly due to satellite arrangement, environment, and receiver sophistication.

  20. WAAS Is Differential Correction • GPS signals reaching reference stations • Reference stations compute corrections based on their own known position. • The WAAS control station receives correction messages from reference stations, and transmits them to WAAS satellites. • WAAS satellites transmit corrections to GPS receivers. • GPS receivers apply corrections to incoming GPS signals in real time.

  21. Using WAAS • Under ideal conditions, Garmins receiving 100% WAAS corrections may achieve 1 – 3 meter accuracy

  22. Good Data Collection Techniques Hold Upright & High External Antenna Sleeve Mounts Vehicle Brackets

  23. Coordinate Systems, Map Projections & Datum

  24. Coordinate Systems, Map Projections & Datum • Spatial data are referenced to earth’s surface, or georeferenced, using either a geographiccoordinate system (GCS) or a projected coordinate system. • Map Projections use a mathematical formula to transform the earth’s 3-dimensional surface to a flat 2D surface. e.g., Universal Transverse Mercator (UTM) • Datum provides a reference system that describes the size and shape of the earth

  25. Latitude & Longitude PrimeMeridian (Longitude) 30º N 10º N 0º 0º 10º S Equator (Latitude) PointofOrigin

  26. Longitude(X) Latitude (Y) Geographic Coordinate System • Geographic coordinates use longitude and latitude to define locations on the earth’s spherical surface • Not a projection! • Always has an associated datum • Units: Decimal Degrees, Degrees Decimal Minutes, Degrees Minutes Seconds

  27. ParallelsofLatitude 20º N 10º N 10º 690 miles 0º N 10º 690miles 10º S 10º 690miles

  28. Meridians of Longitude 10º ToNorthPole 240 mi 10º 460 miles Equator 10º 690 miles ToSouthPole 110º W 120º W

  29. Three Ways To Express Latitude / Longitudeon a Garmin

  30. Three Ways To Express Latitude / Longitude(for the Same Location) hddd.ddddd° Degrees (Decimal Degrees) N 43.68216°, W 116.28725° hddd° mm.mmm’ Degrees-Minutes (Decimal Minutes)N 43° 40.930’, W 116° 17.235’ hddd° mm’ ss.s” Degrees-Minutes-Seconds (Decimal Seconds)N 43°40’ 55.8”, W 116°17’ 14.1”

  31. Example: Error in Latitude 35° 24´ 45˝ N 35° 24.450’ N 1/3 of a mile

  32. Projected Coordinate System Projected coordinate systems use a mathematical conversion to transform latitude and longitude coordinates on earth's three-dimensional surface to a two-dimensional surface. • Why project data? • Make accurate measurements, distance calculations from map • Preserve area, shape, distance, or direction in your map • Small-scale (large area) maps won’t have curved lines associated with Lat/Lon

  33. Projecting a Sphere Onto a Plane Three-dimensional sphere to two-dimensional flat map.

  34. Examples of Several Projections Depending on the projection, a certain amount of distortion occurs when portraying the earth on paper.

  35. Universal Transverse Mercator (UTM) Projection • measured in meters • located in zones (1 - 60) • include northing and easting • are positive Zone Easting Northing Latitude Band Coordinates

  36. UTM Grid Overlay 21 84º N X W V U T T S R 21 T Q P Latitude Bands N M L K J H G F E D C 80º S 60 Zones, and 20 Latitude Bands Zones 1 60 Equator

  37. UTM Zones in the Lower 48 19 10 11 12 18 13 17 16 14 15 UTM Zones

  38. UTM Location Format on a Garmin

  39. Ellipsoid (GPS) Polar Axis Semi-Minor Axis Topographic Surface Equatorial Axis (Semi-Major Axis Earth Mean Sea Level (Geoid) Map Datum The Geodetic datum defines a reference system that describes the size and shape of the earth, e.g., North American Datum of 1983 (NAD83)

  40. Datums We Use – 3 Datums WGS 84 NAD 83 NAD 27 10 – 120+ meters 1 meter Garmin - >100 Map Datums

  41. Example: Datum Shift in Arizona NAD83 N34.555o, W111.195o 210 meters NAD27 N34.555o, W111.195o

  42. Always Ask When Transferring! Coordinate System/Projection? Datum?

  43. Summary of Topics - GPS - WAAS - Coordinate Systems & Map Projections - Datum

More Related