What is GPS?What is GPS?

What is a GPS

How does GPS work?How does GPS work?

GPS SegmentsGPS Segments

GPS Position Accuracy GPS Position Accuracy

GPS Applications GPS Applications

What is GPS?What is GPS?

The Global Positioning System (GPS) is a preciseworldwide radio-navigation system, and consists of aconstellation of satellites and their ground stations,operated and maintained by the US Department ofDefense (DoD).

What is GPS ?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.

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)

How does GPS work?How does GPS work?

GPS GPS TriTrilatlaterationeration

Distance = Time x Speed

(Speed = 300,000 km/sec)

Atomic clocks measure time in seconds to 10 decimal placesCode is transmitted many thousand times a second and includes

Time: Which satellite it is XYZ coordinates (ephemeris)

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

How Does it Work?How Does it Work?

Distance measurements from two satellites Distance measurements from two satellites limits our location to the intersection of two limits our location to the intersection of two spheres, which is a circle.spheres, which is a circle.

How Does it Work?How Does it Work?

A third A third measurement measurement narrows our narrows our location to location to just two just two points.points.

How Does it Work?How Does it Work?

A fourth A fourth measurement measurement determines determines which point is which point is our true our true locationlocation

How Does it Work?How Does it Work?

4 satellites are needed for accurate location (3 if only 2D)

4 time measurements correspond with 4 ‘pseudo-ranges’ (distances)

The GPS SegmentsThe GPS Segments

SpaceSpace

Colorado Springs

UserUser

ControlControl

11. Space segment: . Space segment: Satellite ConstellationSatellite 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)

Space SegmentSpace Segment24 24 satellites in satellites in 6 6 inclined orbitsinclined orbits

4 4 satellites per orbit satellites per orbit -- 12 12 hour revolutionshour revolutions1212,,600 600 miles (miles (2020,,000 000 km) altitudekm) altitude

22. Control segment: ground stations. 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

•• Managed by the U.S. Air ForceManaged by the U.S. Air Force•• 4 4 monitoring and monitoring and 3 3 upload stationsupload stations•• Daily ephemeris (location) updatesDaily ephemeris (location) updates•• Transmits clock and orbit correctionsTransmits clock and orbit corrections•• Disable use of satellitesDisable use of satellites•• Degrade accuracy of the signalsDegrade accuracy of the signals

Control SegmentControl Segment

U SU SAirAir

ForceForce

33. User segment: GPS receiver and antennas. User segment: GPS receiver and antennas

Handheld recreation grade units 5-15 m

Resource grade units 1-5 m

Survey grade units 1 cm

U.S. MilitaryU.S. MilitaryCivilian shippingCivilian shippingScientific high Scientific high accuracy accuracy applicationsapplicationsResource Resource managers (GIS managers (GIS data capture)data capture)Survey and Survey and mapping controlmapping control

User SegmentUser Segment

Selective AvailabilitySelective 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

May May 11, , 2000 2000 –– Selective Availability onSelective Availability on May 3, 2000 – Selective Availability Off

GPS Position Accuracy GPS Position Accuracy Many factors can affect the accuracy of GPS Many factors can affect the accuracy of GPS data. Accuracy can range from data. Accuracy can range from 1 1 centimeter to centimeter to

over over 40 40 metersmeters

•Satellite Clock Errors•Ephemeris Errors•Atmospheric Effects•Receiver Errors•Operator knowledge and awareness

•Number of visible satellites•Satellite Geometry•Occupation time•Multipath

Significant Parameters:

Environmental FactorsEnvironmental Factors•• Generally, GPS is unaffected by weather Generally, GPS is unaffected by weather

•• Heavy rain can weaken the signalHeavy rain can weaken the signal

•• Wet foliage impacts more than dry foliageWet foliage impacts more than dry foliage

••Humidity and Temperature Humidity and Temperature –– no effectno effect

•• Wind may have positive effect under forest canopyWind may have positive effect under forest canopy

Types of GPS units and errorsTypes of GPS units and errors

Recreation Grade: hiking, boating, hunting $100-500-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

What are the sources of error?What are the sources of error?

Errors (approximate)

Ionosphere 4.0 metresClock 2.1 mEphemeris 2.1 mTroposphere 0.7 mReceiver 0.5 mMultipath 1.0 mTotal 10.4 m

Dilution of PrecisionDilution of Precision

•• DOP is an indicator of the quality of the geometry of the satellitesDOP is an indicator of the quality of the geometry of the satellites

High DOP (poor) Low DOP (good)

PDOP < 8.0 acceptable PDOP < 4.0 : excellent

Dilution of Precision Dilution of Precision –– 15 15 Degree Mask AngleDegree Mask Angle

# of satellites# of satellites

Multipath: GPS is line of sightMultipath: GPS is line of sight

Differential CorrectionDifferential Correction

Uncorrected GPS ~10mCorrected (DGPS) ~1m

Applications of GPSApplications of GPS InIn--vehicle Navigation (car, boat, vehicle Navigation (car, boat,

airplane)airplane) Asset ManagementAsset Management ConstructionConstruction Geologic Research & MiningGeologic Research & Mining Military Navigation and OperationsMilitary Navigation and Operations Mapping & SurveyingMapping & Surveying Precision AgriculturePrecision Agriculture Public Health Public Health Public SafetyPublic Safety Wireless CommunicationsWireless Communications

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

Other Satellite SystemsOther Satellite Systems

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

Integrating GIS and GPSIntegrating GIS and GPS Collect a series of latitude and Collect a series of latitude and

longitude points using GPS receiverslongitude points using GPS receivers•• add them into an existing digital map add them into an existing digital map

Skills in Skills in •• data collection/creationdata collection/creation•• GPS readingGPS reading•• observationobservation

Digital photosDigital photos

Some GPS sites:Some GPS sites:

www.garmin.com

Geocaching

BC lighthouses

www.gpsdrawing.com