GPS: A Primer. presented by Jim Pugh, GISP GIS Project Manager. 2007, EMH&T, Inc.

Transcription

1 GPS: A Primer presented by Jim Pugh, GISP GIS Project Manager

2 GPS: A Primer GPS = Global Positioning System 24 Satellites in Orbit around Earth Each Broadcasts precise time and known location Receivers triangulate position

3 By the way GPS is an American system GLONASS is Russian Galileo is EU

4 GPS Orbit About 12,000 miles above Earth 6 Circular Orbital Planes Global Coverage Equipped with very precise atomic clocks

5 It s about time Distance = Velocity x Time Velocity is fixed at the Speed of Light Calculate signal travel time and distance is solved Location is somewhere within a sphere

6 It s about time A second distance can calculate an area

7 It s about time The third distance can calculate a point

8 If it were a perfect World Extremely precise clocks required Average time from Satellite <0.06 sec 1ns error = 1 ft Speed of Light Not a constant Orbits change

9 If it were a perfect World Each reading is slightly different Due to changing Satellite location Satellite DOP s & combinations Varied clock errors Points are averaged

10 GPS Error Satellite Clocks Selective Availability (SA) Orbital Errors (Ephemeris) Atmospheric Delays & Signal Strength Multipath Receiver Clock

11 GPS Error Satellite Clocks Selective Availability (SA) Orbital Errors (Ephemeris) Atmospheric Delays & Signal Strength Multipath Receiver Clock Minimal atomic clocks Disabled was upwards of 300 feet 7 feet 13 feet 3 feet 13 feet

12 Ephemeris Satellite location Monitored by DoD Updated Constantly Almanac: all SV s Adjusted Orbit Altitude Speed

13 Multipath Signal reflects off Objects Travels further and delayed Corrected with improved antenna and processors First one wins

14 Dilution of Precision (DOP) Includes: Horizontal (HDOP) Vertical (VDOP) Position (PDOP) HDOP² +VDOP² Generally below 6 Time (TDOP) Geometry (GDOP) PDOP² + TDOP² Poor DOP

15 Dilution of Precision (DOP) Includes: Horizontal (HDOP) Vertical (VDOP) Position (PDOP) HDOP² +VDOP² Generally below 6 Time (TDOP) Geometry (GDOP) PDOP² + TDOP² Good DOP

16 When to go?

17 Differential Correction Base Station Rover

18 Differential Correction Known Position All Satellites Calculate Error Unknown Error Compare to Base Apply offset Error Base Station Post Processing Rover

19 Differential Correction Base Station DGPS RTK Rover

20 Differential Correction National Public Base Station Network

21 Types of GPS Recreational Grade ± feet Mapping Grade ± 1 3 feet Survey Grade < 0.04 feet (1 cm) Vertical 2-5 times horizontal accuracy

22 Types of GPS Recreational Grade ± feet Mapping Grade ± 1 3 feet Survey Grade < 0.04 feet (1 cm) Purchased for $

23 Types of GPS Recreational Grade ± feet Mapping Grade ± 1 3 feet Survey Grade < 0.04 feet (1 cm) Purchased for $3,000-5,000+

24 Types of GPS Recreational Grade ± feet Mapping Grade ± 1 3 feet Survey Grade < 0.04 feet (1 cm) Purchased for $15,000-50,000++

25 Types of GPS Recreational Grade ± feet Mapping Grade ± 1 3 feet Survey Grade < 0.04 feet (1 cm) What do you get? Better clocks, receivers, filters, processors.

26 Types of GPS Recreational Grade ± feet Mapping Grade ± 1 3 feet Survey Grade < 0.04 feet (1 cm)

27 So what can you do with it? Tree Horticulture Inventory Ponds Streams Historical - Archeological

28 Utilities Manholes Storm Drains Sprinkler Heads Power / Light Poles Not to locate underground utilities!

29 Planimetric Building Corners Walls Trails Footpaths Bike Racks Benches Bus Stops Signage

30 Emergency Management GPS is also for Navigation Hydrants Emergency Call Boxes Valves

31 Attributes While you re collecting location, collect data! Pull-down menus Valid Values Ranges Required Fields

32 Base Map

33 Ground Control

34 Cool Tools you can Use!

35 ESRI ArcPad Mini-GIS Integrates with GPS Draw, edit Enhanced menus, attribute collection Works with many Systems Customizable

36 Laser Rangefinder Remote Positions Under Canopy or Overhang In Trees GPS Location GPS Location Offset

37 LaserCraft Rangefinder Remote Positions Under Canopy or Overhang In Trees GPS Location Pt 1 Pt 2 Offset Remote Measurements Horizontal Vertical

38 Ricoh GPS Camera GPS Ready Data Embedded in Digital Image Talks with Bluetooth enabled GPS and Rangefinders

39 GPS: A Primer References: ArcPad, ESRI. Contour XLR, LaserCraft, Inc. General Information on GPS, U.S. Coast Guard Navigation Center. Global Positioning System: The Role of Atomic Clocks, National Academy of Science. Global Positioning System Overview, Peter H. Dana, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder. Ricoh Corporation. Trimble, Inc. Product names, logos, brands, and other trademarks featured or referred to within this presentation are the property of their respective trademark holders. These trademark holders are not necessarily affiliated with EMH&T products or services nor sponsor or endorse these materials.

40 GPS: A Primer Thank you! Questions? For more information or a copy of this presentation, please [email protected]