GEOG 425/525 GPS Concepts and Techniques

1. GEOG 425/525GPS Concepts and Techniques John Benhart, Ph.D. Indiana University of PA Dept. of Geography & Regional Planning

2. What Are Global Positioning Systems (GPS)?

3. What is GPS Used For?

4. The Historical Context of the Global Positioning System (GPS) • The origins of surveying: • Efforts to measure distance on the earth’s surface • Efforts to specify location on the earth’s surface • Efforts to determine property/land boundaries • How?

5. How Eratosthenes Estimated the Earth’s Size

6. Early Estimates of the the Earth’s Size

7. 1753 French Survey Establishes that the Earth Bulges at the Equator

8. The Basics of Surveying • Definition: The science of measuring and mapping relative positions above, on, or below the surface of the earth; or establishing such positions from a technical plan or title description • Types: • Plane surveying: Does not take into account the curvature of the earth • Geodetic surveying: measurements covering larger distances where the curvature of the earth must be taken into account

9. Surveying: History • Evidence of surveying has been recorded as early as 5,000 years ago • Ancient Egyptian surveyors called harpedonapata (“rope stretcher” • Ropes and knots were tied at pre-determined intervals to measure distances • The 3-4-5 triangle (later formalized by Pythgoras) was discovered to derive a right angle easily by using a rope knotted at 3,4, and 5 units • Early Egyptian levels that were essentially plumb lines were derived

10. Surveying: History • Surveying inventions • Lodestone used to identify magnetic north • Thomas Diggs invents an instrument used to measure angles called the theodolite in the mid-1500s • Jean Praetorius invents the plane table in 1610 • W.J. Young invents the transit based on the theodolite, which “flips” to allow back sighting in 1831

11. Theodolite

12. How a Theodolite Works

13. Plane Table and Alidade

14. Surveyor’s Transit

15. Total Station

16. The Basics of Surveying • Starting from a position with a known location and elevation, known as a “benchmark,” the distance and angles to the unknown point are measured • Using a leveled theodolite or total station • Distance: previously chains, now lasers • Horizontal angle: from compass on theodolite • Vertical Angle: sighted in on a measuring or leveling rod at the location in question

19. The Historical Context of the Global Positioning System (GPS) • 1978 – launch of first GPS satellite • 1982 – macrometer prototype tested at MIT • 1984 – geodetic network densification in Montgomery, Co. PA • 1989 – Launch of first Block II satellite; Wide Area GPS concept tested • 1990 – GEOID90 for NAD83 datum established • 1993 – Real-time kinematic GPS implemented • 1996 – First US GPS policy expressed in presidential directive • 1999 – US GPS modernization initiative • 2000 – Selective availability (SA) deactivated

20. Overview of GPS - Logic • A continuous coverage of satellites exists within view of virtually every location on the earth’s surface (NAVSTAR) • These satellites launch signals at recorded times on specific frequencies that can be “received” by units on the earth’s surface • By calculating the amount of time it takes the signals from 4 satellites to reach a receiver on the earth surface, it is possible to determine the distance between the receiver and any satellite (pseudoranges) • By using the intersection of the radii from 4 satellites, it is possible to determine exactly where a GPS receiver is located on the earth’s surface (trilateration) ** A huge amount of science and technology has to be applied for any of these conditions above to exist…

21. Overview of GPS - Objectives • The GPS was “conceived as a ranging system from known positions in space to unknown positions on land, at sea, in air and space.” (p.11) • The original objectives of GPS were “instantaneous determination of position and velocity (i.e. navigation), and the precise coordination of time (i.e. time transfer).” • “The global NAVSTAR Global Positioning System (GPS) is an all-weather space-based navigation system under development by the DoD to satisfy the requirements of the military forces to accurately determine their position, velocity and time in a common reference system, anywhere on or near Earth on a continuous basis.”

22. Overview of GPS • GPS can be conceptually be divided into 3 segments: • Space Segment: the constellation of satellites • Control Segment: tracking and monitoring of the satellites • User Segment: varying user applications and receiver types

23. Overview of GPS • Space Segment • The NAVSTAR constellation: 24 evenly-spaced satellites in 12-hour orbits inclined 55 degrees to the equatorial plane…each is assigned a pseudorandom noise code # (PRN code) • Types: Block I, Block II, Block IIA, IIR, IIF, and Block III • Continuous signal coverage of every location on the earth’s surface • Satellite signals: launched at extremely-precisely recorded times (atomic clocks), and travel at the speed of light (through earth atmosphere) to receivers on the earth’s surface • L1: 1575.42 MHz, L2: 1227.60 MHz

26. Overview of GPS • Control Segment • Master control station: located at the Consolidated Space Operations Center at Shriver Air Force Base, Colorado Springs, CO • Collects monitoring data from global stations • Calculates satellite orbits and clock parameters for each satellite…passed to ground control stations • Responsibility for satellite control

27. Overview of GPS • Control Segment • Monitoring Stations: Five located at Colorado Springs, Hawaii, Ascension Island (South Atlantic), Diego Garcia (Indian Ocean), Kwajalein (North Pacific) • Precise atomic time standard • Continuous calculation of satellite pseudoranges • Official network for determining broadcast ephemerides • Ground Control Stations • Also located at Ascension, Diego Garcia, and Kwajalein • Communication links to upload ephemeris, and clock information to NAVSTAR satellites

28. GPS Monitoring Locations

29. Overview of GPS • User Segment • Military Users • Original envisioned users; have access to precise P-code satellite signals • Civilian Users • Range from recreational to GIS and survey grade applications – all made possible by federal infrastructure • Receiver Types • C/A code pseudorange; C/A code carrier phase; P-code carrier phase; Y-code carrier phase

30. GPS Summary • The system is predicated on: • A constantly monitored constellation of satellites • Very accurate time measurement • The ability to determine satellite location • The use of unique radio signals on specified wavelengths launched from satellites and received by receivers on the earth’s surface • The ability to translate pseudoranges into recognized coordinates through trilateration

31. Reference Systems • GPS satellites are orbiting earth and launching signals with time stamps…we are usually trying to determine locations on earth…to do this we have to define suitable coordinate and time systems • 3 Types of reference systems that are relevant in the context of GPS • Earth-fixed reference: the international terrestrial reference system • Space-fixed reference: the international conventional celestial reference system • Geodetic reference system

32. The Earth and Its Axis of Rotation • The Earth’s axis of rotation changes over time • Why? 1) Mainly caused by the gravitational forces of the moon and the sun…as well as other celestial bodies 2) changes in the mass of earth-based phenomena • Precession – a slight change in the direction of the axis of the rotating earth • Nutation - a slight irregular motion in the axis of rotation of an axially symetrical body (planet) • Polar Motion - the movement of the earth’s axis across its surface (~ 20 m westward since 1900)…due to motions in the earth’s core and mantle, and redistribution of water mass • Who Cares? These factors cause the position of the earth in its orbit (revolution) around the sun to change over time (equinoxes and solstices)

33. Precession • Nutation • Polar Motion

34. Precession: Cause The gravitational pull of the sun on the closest part of the oblate spheroid is stronger…

35. Precession: Result

36. Nutation

37. Reference Systems and Reference Frames • Reference Systems: the complete specification of a coordinate system, such as the origin, coordinate axes, coordinate units, etc. • Reference Frames: consist of a set of identifiable points along with their coordinates, which serve as a realization of the reference system

38. International (Conventional) Celestial (Space) Reference System • The celestial reference system adopted by the International Astronomical Union (IAU) for high-precision positional astronomy • Characteristics: Origin at the solar system barycenter (center of mass) and “space fixed” axis directions • Has been chosen by the IAU as the “most appropriate coordinate system for expressing reference data on the positions and motions of celestial objects.”

39. International (Conventional) Celestial (Space) Reference System • A set of specifications based on space-fixed axes defining the location of bodies in space • For example, the base plane of the system is the extension of the earth’s equatorial plane at J2000.0 (Jan. 1, 2000) • Locations (of planets and stars) are specified based on declination (north-south) and right ascension (east)

41. Earth-Centered Earth-fixed Reference • The mass center of the earth is used as a reference • Conventional Terrestrial Reference System: X axis is identical to the mean Prime (Greenwich) Meridian; Z axis is identical to the earth’s mean rotational axis (also called the Conventional International Origin (CIO)); Y axis points to the mean equatorial parallel • See www.iers.org (the International Earth Rotation and Reference Systems Service)

42. International Union of Geodesy and Geophysics (1991) Resolution on the Conventional Terrestrial Reference System • The International Union of Geodesy and Geophysics, • Considering the need to define a Conventional Terrestrial Reference System (CTRS) which would be unambiguous at the millimeter level at the Earth's surface and that this level of accuracy must take account of relativity and of Earth deformation, and • noting the resolutions on Reference Systems adopted by the XXIst General Assembly of the International Astronomical Union (IAU) at Buenos Aires, 1991, endorses the Reference System as defined by IAU at the XXIst General Assembly at Buenos Aires, 1991 and recommends the following definitions of the CTRS: • 1) CTRS to be defined from a geocentric non-rotating system by a spatial rotation leading to a quasi-Cartesian system, • 2) the geocentric non-rotating system to be identical to the Geocentric Reference System (GRS) as defined in the IAU resolutions, • 3) the coordinate-time of the CTRS as well as the GRS to be the Geocentric Coordinate Time (TCG), • 4) the origin of the system to be the geocenter of the Earth's masses including oceans and atmosphere, and, • 5) the system to have no global residual rotation with respect to horizontal motions at the earth's surface.

43. International (Conventional) Terrestrial (Earth)Reference System

45. The International Terrestrial Reference Frame • The Earth is constantly changing shape, because of plate tectonics and regional subsidence and/or used to represent the Earth when measuring its rotation in space. • To be understood in context, when the motion of the Earth's crust is observed, it must be referenced. A Terrestrial Reference frame provides a set of coordinates of some points located on the Earth's surface. It can be used to measure plate tectonics, regional subsidence or loading and/or used to represent the Earth when measuring its rotation in space.

46. The International Terrestrial Reference Frame • This rotation is measured with respect to a frame tied to the celestial reference frame. The International Earth Rotation and Reference Systems Service (IERS) was created in 1988 to establish and maintain a Celestial Reference Frame, the ICRF, a Terrestrial Reference Frame, the ITRF. • The Earth Orientation Parameters (EOPs) connect these two frames together. These frames provide a common reference to compare observations and results from different locations • Reference locations are periodically evaluated for position change to re-define the reference frame • The ITRF is regularly updated by the IERS…the latest frame is ITRF 2005

50. Earth-Centered Earth-fixed Reference • World Geodetic System of 1984 (WGS84) • The reference system utilized in GPS • Provides the basic reference frame and geometric figure for the earth, based on the USDMA (Defense Mapping Agency’s) measurements and modeling of the earth from a geometric, geodetic, and gravitational standpoint, using techniques and technology available in 1984