What is a GPS How d oes es GP G S work? G S Segmen en s GPS sition A c A cura r cy GPS PS p A plications

Transcription

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

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).

3 What is GPS? The Global Positioning System a satellite-based navigation system consisting of a network of 24 orbiting satellites that transmit radio signals to GPS receivers.

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5 How does GPS work? Process: 1. Satellite sends out signal/code e.g. at midnight (with date stamp) 2. GPS units receives code at midnight plus travel time (decimal seconds) 3. The delay or lag when the GPS receives it is the signal s travel time. 4. GPS unit multiplies the time by the speed of light to determine how far the signal travelled = how far you are from that point in space 5. Software combines the >4 readings to generate a ground location (with some degree of error)

6 GPS Trilateration Distance = Time x Speed (Speed = 300,000 km/sec) Code is transmitted many thousand times a second and includes Time: Which satellite it is XYZ coordinates (ephemeris) Atomic clocks measure time in seconds to 10 decimal places

7 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.

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

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

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

11 4 satellites are needed for accurate location (3 if only 2D) 4 time measurements correspond with 4 pseudo-ranges (distances)

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14 The GPS Segments User Space Colorado Springs Control

15 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)

16 Space Segment 24 satellites in 6 inclined orbits 4 satellites per orbit - 12 hour revolutions 12,600 miles (20, km) altitude

17 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

18 Control Segment Managed by the U.S. Air Force 4 monitoring and 3 upload stations Daily ephemeris (location) updates Transmits clock and orbit corrections Disable use of satellites Degrade accuracy of the signals U S Air Force

19 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

20 User Segment U.S. Military Civilian shipping Scientific high accuracy applications Resource managers (GIS data capture) Survey and mapping control

21 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

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

23 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

24 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

25 Types of GPS units and errors Recreation Grade: hiking, boating, hunting $ No differential correction -No Setting of mask angle -No setting of PDOP collection values -Tracks only one frequency or signal -No ability to reject multi-path measurements Resource/ Mapping Grade: sub-metre accuracy $5,000 10,000 -Differential correction post processing or real time -Setting mask angles -PDOP value collection -Ability to reject multi-path measurements Survey Grade: cms accuracy $20,000-50,000 -All of above plus: -Tracking two frequencies

26 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

27 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)

28 Dilution of Precision 15 Degree Mask Angle

29 # of satellites

30 Multipath: GPS is line of sight

31 Differential Correction Uncorrected GPS ~10m Corrected (DGPS) ~1m

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34 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

35 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

36 Integrating GIS and GPS Collect a series of latitude and longitude points using GPS receivers add them into an existing digital map Skills in data collection/creation GPS reading observation Digital photos

37 Some GPS sites: Geocaching BC lighthouses