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Unit 1 Introductory slides
- 1. This work is supported by the National Science Foundation’s Directorate for Education and Human Resources (TUES-1245025, IUSE- 1612248, IUSE-1725347). Questions, contact education-AT-unavco.org INTRODUCTORY INFORMATION ABOUT GPS Module: Measuring the Earth with GPS Unit 1: Collecting GPS Data By Karen M. Kortz and Jessica J. Smay Version: 10 July 2019
- 3. WHERE IS GPS? • Have you used GPS? When? ?
- 4. WHERE IS GPS? • GPS receivers are in phones, cars, etc.— basically anything that tells you where you are.
- 5. HOW DOES GPS WORK? • A receiver obtains signals from GPS satellites Artist’s conception of a GPS Block II-F satellite in Earth orbit
- 6. HOW DOES GPS WORK? • Network of 24–32 satellites • Precisely known orbits and time • A receiver needs 4 satellites to accurately determine position http://spaceplace.nasa.gov/gps-pizza/en/
- 7. HOW DOES GPS WORK? http://spaceplace.nasa.gov/gps-pizza/en/ • Receiver position is determined by calculating the distances to at least 4 satellites • Solve for the variables: • North–South position • East–West position • Elevation • Time
- 8. 8 GPS antenna inside of dome Monument solidly attached into the ground with braces. If the ground moves, the station moves. Solar panel for power Equipment enclosure • GPS receiver • Power/batteries • Communications/radio/modem • Data storage/memory HIGH-PRECISION PERMANENT GPS STATION
- 9. HIGH-PRECISION PERMANENT GPS STATION What is the function of what the arrow is pointing to? a. Power source b. Receives satellite signal c. Records data d. Communication e. Anchors station to ground ?
- 10. HIGH-PRECISION PERMANENT GPS STATION What considerations need to be made when installing a GPS station? ?
- 11. HOW CAN STUDYING GPS MOTION BE USEFUL TO SOCIETY? (Brainstorm) What GPS can measure: • Movement of ground… • …due to plate motion • …near earthquake faults • …during earthquakes • …due to movement of magma underground • …due to changing size of glaciers • …due to changing snow depth • …as it compacts (subsides) • …due to changing amount of groundwater • …due to changing size of lakes and reservoirs • ….from a landslide • Amount of water in the atmosphere • Amount of soil moisture • Vegetation growth • Sea level • Amount of ash in the atmosphere ?
- 12. HOW CAN STUDYING GPS MOTION BE USEFUL TO SOCIETY? (Alternate) ? Which of these things that GPS measures has the largest impact on society? a. Small motions of the ground that build up to lead to earthquakes b. How much glaciers push down the ground, which changes as the size of glaciers change c. The height of the ground, which is influenced by how much water is in or on it d. How much the ground is sinking because of removal of oil or water
- 13. GPS Jigsaw
- 14. MEASURING EARTH WITH GPS UNIT 1: INTRODUCTION TO GPS JIGSAW • Part 1: Becoming an “expert” in your team • Team 1: Reference frames • Team 2: Direction of motion • Team 3: Speed of motion • Part 2: Applying your “expert” knowledge in new groups https://www.unavco.org/software/ visualization/GPS-Velocity- Viewer/GPS-Velocity-Viewer.html
- 16. Think–Pair–Share List as many observations as you can from the data in these graphs from a GPS station near the Wasatch Front in Salt Lake City. ?
- 17. Think–Pair–Share List as many observations as you can from the data in this graph from a GPS station near the tip of the Olympic Peninsula, west of Seattle. GPS station P403 ?
Editor's Notes
- Whether they know it or not, students use GPS receivers daily in their cars, phones, cameras, and handheld recreational GPS. Most have heard the term “GPS,” but few know much about how the system actually works or high-precision GPS for science and industrial use. Note: Although the term GPS (Global Positioning System) is more commonly used in everyday language, it officially refers only to the USA's constellation of satellites. GNSS (Global Navigation Satellite System) is a universal term that refers to all satellite navigation systems including those from the USA (GPS), Russia (GLONASS), European Union (Galileo), China (BeiDou), and others. In this module, we use the term GPS even though, technically, some of the data may be coming from satellites in other systems. Images from UNAVCO
- From the users point of view, GPS is a passive system: you need only a low-powered receiver to obtain the signals, which contain all the information you need to determine your position. This requires that the satellites carry atomic clocks to keep accurate time and transmit to the user the position of the satellite in the sky. The orbits are computed by the Department of Defense (DoD) by tracking the satellites from ground control stations. GPS control segment consists of a global network of ground facilities that track the GPS satellites, monitor their transmissions, perform analyses, and send commands and data to the constellation. The current (as of 2015) operational control segment includes a master control station, an alternate master control station, 12 command and control antennas, and 16 monitoring sites. The ground control stations have known positions that are uploaded to the satellites, which then transmit the information continuously along with the time. With handheld receivers, it is possible to find your position to within a few meters, limited primarily by the accuracy of the broadcast orbits. As we shall see, the civilian scientific community uses more than 200 stations to determine the orbits with an accuracy of a few centimeters, allowing millimeter positioning after several hours of tracking. Image: Artist’s conception of a GPS Block II-F satellite in Earth orbit. (public domain from NASA)
- From the user’s point of view, GPS is a passive system: you need only a low-powered receiver to obtain the signals, which contain all the information you need to determine your position. This requires that the satellites carry atomic clocks to keep accurate time and transmit to the user the position of the satellite in the sky. The orbits are computed by the Department of Defense (DoD) by tracking the satellites from ground control stations. GPS control segment consists of a global network of ground facilities that track the GPS satellites, monitor their transmissions, perform analyses, and send commands and data to the constellation. The current (as of 2015) operational control segment includes a master control station, an alternate master control station, 12 command and control antennas, and 16 monitoring sites. The ground control stations have known positions that are uploaded to the satellites, which then transmit the information continuously along with the time. With handheld receivers, it is possible to find your position to within a few meters, limited primarily by the accuracy of the broadcast orbits. As we shall see, the civilian scientific community uses more than 200 stations to determine the orbits with an accuracy of a few centimeters, allowing millimeter positioning after several hours of tracking. Top image: https://en.wikipedia.org/wiki/Global_Positioning_System Bottom image: NASA (public domain) http://spaceplace.nasa.gov/gps-pizza/en/
- The time element is needed because of the difference between the highly accurate atomic clock on the satellites (US$100,000 each) and the much cheaper clock in the GPS receivers. Although 4 is a technical minimum number of satellites, for geoscience research work it is highly recommended to have at least 6 satellites. Although people will often refer to this process and “triangulation,” in fact it is “trilateration” because it depends on distances rather than angles. For a nice explanation of trilateration versus triangulation visit https://gisgeography.com/trilateration-triangulation-gps/. Images from NASA (public domain) http://spaceplace.nasa.gov/gps-pizza/en/
- Example of a typical remote GPS station. In urban areas, some are also fixed to the roofs of large reinforced buildings and hooked into grid power supply with battery backup. Many stations are now being equipped with “real-time” communication to allow immediate transfer of data to researchers. In other places logistics or country-specific regulations require that data not be transmitted but is instead downloaded manually on a given schedule (monthly, quarterly, or annually, depending on the site).
- Answer: b Image from https://www.unavco.org/instrumentation/networks/status/pbo/photos/P056
- Examples: Things that can make the signal noisy: plants/trees, bird poop. The site should be accessible for maintenance. The location should be useful to answer scientific questions. The ground should be stable relative to the surrounding area (unless you’re trying to measure a landslide). Left image from https://www.unavco.org/instrumentation/networks/status/pbo/photos/ana1 Right image from https://www.unavco.org/instrumentation/networks/status/pbo/photos/p316
- There is no correct answer! The point of this slide is to get students to think about and discuss what GPS can measure and how those measurements are relevant to society. After students respond individually, have them find someone who answered differently than they did and convince that person that their response is the best answer.
- Map screenshot from https://www.unavco.org/software/visualization/GPS-Velocity-Viewer/GPS-Velocity-Viewer.html
- Data from https://www.unavco.org/instrumentation/networks/status/pbo/overview/coon Description from eq_gps_infinitesimal_strain_analy.v5.docx Example 2: Wasatch Fault – extensional strain This triplet (PBO stations P116, P088, COON) is located along the Wasatch Front in the Salt Lake City metropolitan area (Fig. WF.1). The stations form a triangle that spans the Wasatch fault zone (Black et al., 2004)—one of the most active normal faults in North America—as well as several other active faults. The long-term horizontal velocities for the stations are quite robust, but there are some non-tectonic variations with decadal and annual periods that students might wonder about (Fig. IN.2). Elósegui et al. (2003) found that the decadal variation is consistent with elastic deformation due to changes in the water level and thus the mass of the Great Salt Lake, while the annual signal most likely reflects more local site-specific effects. This triplet could be used as a launching point for a discussion of the Wasatch Fault, intra-continental normal faulting, and seismic hazards in the region (e.g. Chang and Smith, 2002). Figure IN.2. Time-series plot of location for station COON. Annual (seasonal) signal is most clearly visible in East component. Decadal signal is most clearly visible in the vertical (PBO data).
- Data from: https://www.unavco.org/instrumentation/networks/status/pbo/overview/p403 Description from eq_gps_infinitesimal_strain_analy.v5.docx Example 1: Olympic Peninsula—compressional strain This triplet (PBO stations NEAH, P401, P403) is located at the tip of the Olympic Peninsula, west of Seattle (Fig. OP.1). The stations are located near the down dip limit of the locked portion (Chapman and Melbourne, 2009) of the Cascadia subduction zone (Fig. OP.2) and experience compression as they are forced northeastward by the down-going Juan de Fuca plate. One complication that might confuse/interest students is that the time-series shows effects of slow slip events (also called episodic tremor and slip) (Fig. IN.1). The majority of the slow slip occurs in the transition zone immediately to the west, but these coastal stations also move in response to the nearby slip. The data still show a fairly constant long-term velocity, however, and thus work well as a compressional example for this exercise. Another interesting thing to consider is that the triangle of stations also spans the Calawah fault (Fig. OP.3), inferred to be a left-lateral strike slip fault active in the last 15,000 years (Lidke, 2004). Figure IN.1. Time series plot of east component of location for station P403. Arrows point to likely slow slip events (also called episodic tremor and slip). (PBO data) This triplet could be used as a launching point for a discussion of the Cascadia subduction zone, the Seattle Fault, and seismic hazards in the region (e.g. Johnson et al., 1994 and more recent work).