GST 101 Introduction to Geospatial Technology��Unit 5 – Creating and Editing Geospatial Data�Module 5.2 – Global Navigation Satellite Systems (GNSS) and Global Positioning Systems (GPS)����� � �

Ann Johnson

Associate Director

ann@baremt.com

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Based upon work supported by the National Science Foundation under Grants DUE 1304591, DUE 164409, DUE 1700496, DUE 1937177, Due 1938717 DUE 1937237, 2030206 and 2015927. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

www.gps.gov

A Definition and Difference Between �GNSS and GPS

• Global Navigation Satellite System or GNSS is the umbrella term for any system that provides location information to receivers by using the timing and positioning data encoded in the signals from satellites
• Global Positioning System (GPS) is the term used specifically for the USA’s NAVSTAR GNSS
• This Module will refer to the system in the general way using the term GPS

A Few GNSS and GPS Examples

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• NAVSTAR: USA’s Global Positioning Systems (GPS)
• Started testing in 1978 and became fully operational in 1995
• Selectively availability turned off in 2000
• Now under the direction of the USA Space Force
• Updated satellites have been launched and as of November 13, 2020 there are 30 operational satellites

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• GLONASS: Russian: Global Navigation Satellite System
• Starting in 1982
• 28 satellites with 23 operational
• Galileo: European Union and European Space Agency
• First launched on December 28, 2005
• The full completion (2020) includes 30-satellites (24 operational and 6 spares)

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Creating Accurate Location Information

• GPS can be used to provide location information for features
• Location collected as Points, Lines and Polygons (areas)
• Accuracy of locations can depend on:
• Device being used to receive the location,
• Multi-paths error from buildings or other objects,
• Error introduced by atmosphere and satellite orientation,
• Satellite and Ground Based Augmentation systems to improve accuracy
• Many sensors use GPS data for locations including in car navigation systems to track location of vehicles in real time
• Real-time GPS data can be used in many applications that need to track or guide vehicles

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Global Positioning Systems – Three Segments

(3) User Segment

(2) Satellite Segment

(1) Ground Control Segment

GPS Satellites�

• The GPS Operational Constellation consists of 24 satellites that orbit the Earth in very precise orbits twice a day
• The orbits are monitored and maintained by the Ground Control segment of the system
• Up to 12 satellites can be visible from any position on earth
• The dashed green lines in the graphic to the right indicates which satellites are visible to a receiver
• The number of GPS satellites and their distribution visible to a receiver when data is collected can affect the accuracy of the calculated location

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https://www.youtube.com/watch?v=qHqly38BgTQ

Time Difference – Travel Time

GPS units include an ephemeris which provides orbital position information, timing and health of the GPS satellite and the satellite constellation

The GPS receiver compares the time a signal was transmitted by a satellite with the time it was received by the receiver

Distance = rate x time

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Calculating Distance to a Satellite Using Time of Travel

Distance = rate x Time

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• Rate is the velocity of the satellite signals which travel as radio wave at the speed of light or 186,000 miles per second (mps) or 299,792 km/second

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• Time = the elapsed time between when the satellite sent the signal and the receiver received it�

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If the elapsed time was 0.0003 second, then distance:

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Distance = 186,000 mps x 0.0003 seconds = 558 miles

We can find distance for one satellite, but what about determining a location on Earth’s Surface?

558 miles

�Multiple Satellites Are Needed to Determine a Location

Trilateration

• You can know the distance from one satellite fairly easily
• With one data point you know where you are relative to that satellite but not where you are on the Earth
• Therefore, you need more data

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Trilateration

With two satellites, you are narrowing down your location

There are only two points that are possible

Still, you don’t know your location�

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Trilateration

With three satellites, there is only one point where are three circles converge

That’s where you’re located!

Why Is Elevation the Least reliable value from a GPS?

Some Applications that Use GPS

• Location of Features
• Survey, Mapping & GIS
• Resource and Asset Management - Environmental & Forestry, mining, oil and gas
• Agriculture
• Construction
• Utilities
• Transportation
• Military, although civilian uses now exceeds military
• Tracking (including animal and people)
• Emergency Services – Fire, Police, Search and Rescue
• Natural and man caused disasters – tornadoes, hurricanes, landslides, earthquakes, volcanoes
• Timing & Synchronization

Banking, Stock Market, Telecommunications

• LBS - Location Based Services (sensor IoT location)
• Agriculture, autonomous vehicles, Railroads, Cell phone location

Location-Based Services (LBS)

A location-based service (LBS) is a service, accessible on mobile devices (such as Smart Phones) through a combination of GPS and Assisted GPS using cell tower network to more quickly determine the geographical position of the mobile device

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• Can identify a location of a person or object and other nearby features (such as the

nearest ATM)

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• Can also be used for parcel tracking and vehicle tracking services and even location-

based games.

http://healthmap.org/iphone/

GPS Uses – Recreation and Agriculture

Bolstad, Fig. 5-27 GPS System Mounted on a Tractor

Integrated GPS and Mapping System for recreation use

See GeoTech Center website (https://geotechcenter.org) �for additional Model Courses and other curriculum resources. ���������Note: some content is a derivative of other authors���

Ann Johnson

Associate Director

ann@baremt.com

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2-15-2021 V5