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Introduction

Introduction. Summary of Topics. - GPS - WAAS - Coordinate Systems & Map Projections - Datum. The Global Positioning System . Space Segment. User Segment. Control Segment. How to Calculate a Position. Measure the Distance to the Satellites.

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Introduction

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  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

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