What is GPS? GPS Position Accuracy. GPS Applications. What is a GPS. How does GPS work? GPS Segments

Transcription

1 What is GPS? What is a GPS How does GPS work? GPS Segments GPS Position Accuracy GPS Applications 1

2 What is GPS? The Global Positioning System (GPS) is a precise worldwide radio-navigation system, and consists of a constellation of satellites and their ground stations, operated and maintained by the US Department of Defense (DoD). 2

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4 How Does it Work? The distance (x) from one satellite tells us we're located somewhere on the surface of an imaginary sphere centered on that satellite with a radius of x. 4

5 How Does it Work? Distance measurements from two satellites limits our location to the intersection of two spheres, which is a circle. 5

6 How Does it Work? A third measurement narrows our location to just two points. 6

7 How Does it Work? A fourth measurement determines which point is our true location 7

8 4 satellites are needed for accurate location (3 if only 2D) 4 time measurements correspond with 4 pseudo-ranges (distances) 8

9 The GPS Segments User Space Colorado Springs Control 9

10 1. Space segment: Satellite Constellation GPS is the US system Russia has GLONASS Europe plans Galileo (2014) 24 satellites at 20,000 km altitude, at 55 degree angle to equator (Glonass = 65) 10

11 2. Control segment: ground stations These 5 stations monitor the GPS satellites, check their operational health and exact position in space. The master ground station transmits corrections for the satellite's ephemeris constants and clock offsets back to the satellites 11

12 3. User segment: GPS receiver and antennas Handheld recreation grade units 5-15 m Resource grade units 1-5 m Survey grade units 1 cm 12

13 GPS Position Accuracy Many factors can affect the accuracy of GPS data. Accuracy can range from 1 centimeter to over 40 meters Significant Parameters: Number of visible satellites Satellite Geometry Occupation time Multipath Satellite Clock Errors Ephemeris Errors Atmospheric Effects Receiver Errors Operator knowledge and awareness 13

14 Environmental Factors Generally, GPS is unaffected by weather Heavy rain can weaken the signal Wet foliage impacts more than dry foliage Humidity and Temperature no effect Wind may have positive effect under forest canopy 14

15 What are the sources of error? Errors (approximate) Ionosphere Clock Ephemeris Troposphere Receiver Multipath Total 4.0 metres 2.1 m 2.1 m 0.7 m 0.5 m 1.0 m 10.4 m 15

16 Position Dilution of Precision DOP is an indicator of the quality of the geometry of the satellites PDOP < 8.0 acceptable PDOP < 4.0 : excellent High DOP (poor) Low DOP (good) 16

17 # of satellites 17

18 Multipath: GPS is line of sight 18

19 Selective Availability The random error, added to GPS signals before 2000 up to 100 metres error by scrambling last 3 decimals Turned off May 1, 2000 at midnight No intent to ever use it again 19

20 May 1, 2000 Selective Availability on May 3, 2000 Selective Availability Off 20

21 Non-Differential GPS (Autonomous or Stand-alone) d23 X 23 y 23 z 23 d 19 x 19 y 19 z 19 d 14 x 14 y 14 z 14 x21 y 21 z 21 d 21 Measured: x y z 21

22 Differential GPS X 23 y 23 z 23 x 19 y 19 z 19 x 14 y 14 z 14 x21 y 21 z 21 Measured: x y z Delta: x y z True: x y z Corrections applied after survey True: x y z Measured: x y z Delta: x y z 22

23 Real-Time Differential GPS X 23 y 23 z 23 x 19 y 19 z 19 x 14 y 14 z 14 x21 y 21 z 21 Measured: x y z Delta: x y z True: x y z Corrections applied during the survey True: x y z Measured: x y z Delta: x y z 23

24 GPS Uses Some GPS Company, breaks GPS uses into five categories: Location positioning things in space Navigation getting from point a to point b Tracking - monitoring movements Mapping creating maps based on those positions Timing precision global timing

25 Some Applications of GPS In-vehicle Navigation (car, boat, airplane) Asset Management Construction Geologic Research & Mining Military Navigation and Operations Mapping & Surveying Precision Agriculture Public Health Public Safety Wireless Communications 25

26 Other Satellite Systems Galileo Positioning System European Space Agency (ESA) alternative 30 satellites on 3 orbital planes 27 countries involved Major delays plaguing system Fully operational by 2014 Compatible with GPS (U.S.) Clocks 10 times more accurate than GPS clocks GLONASS - Russia s counterpart is the US Global Positioning System (GPS) - 21 satellites on 3 orbital paths - Magellan and Trimble sell units that can utilize GLONASS signals - Moving towards compatibility with US and EU systems 26

27 What to look for and record عن ماذا تبحث وماذا تسجل Satellites اشارة الا قمار الصناعية Elevation قياس الارتفاع Latitude دواي ر العرض Longitude خطوط الطول 27

28 صفحات العرض الري يسية Main Display Pages صفحة البوصلة صفحة الخريطة صفحة المعلومات صفحة الطريق صفحة الطريق السريع 28

29 صفحات العرض الري يسية Main Display Pages 29

30 Some GPS sites: Geocaching BC lighthouses 30

31 Thank You 31