G. P. S. The Global Position System Miles Logsdon, College of Ocean and Fishery Sciences Phil Hurvitz, Urban Design and Planning College of Forest Resources

Basic Concepts zGPS yU.S. government yNAVigation System with Time And Ranging –NAVSTAR 24 satellites yRussian syste xGLONASS

Geography zLocation yindex space: coordinates xLatitude-longitude xUTM yabsolute v’s relative coordinates x121 33’ 12” x 47 24’ 15” = absolute x120km east and 40km north of Seattle

Position v’s Location zPosition : GPS ycoordinates that specify “where” zLocation: maps y“where” with respect to know objects

two Map Projections Map Projections – later??

Why use GPS(1) zAvailability: y1995, DoD NAVSTAR, civilian use foreseeable future zAccuracy: Factors ywork with “primary” data sources yHigh inherent accuracy (2.5m medium- quality properly corrected receiver) yTime Corrected to 1/1 billionth of a second

Why use GPS(2) zEase of use ystop and read a single coordinate = 20m accuracy (+/- 5m) real-time z3-D data yhorizontal (x & y) and altitude (z) yvariances in z = horizontal * 2

Satellite Transmitter Specifications(1) zRadio wave transmission (~20cm) zNot good without direct view of sky (i.e. inside, underground, under canopy, precipitation z24 solar-powered radio transmitters, 3 spares z“middle altitude”, 20,200km, below geosynchronous orbit GPS SV

Satellite Transmitter Specifications(2) zNeither polar nor equatorial zeach execute a single 12 hour orbit z4 satellites in each of 6 orbital planes zspeed of 3.87 km/sec ( 8,653 mph) zweigh ~ 1 ton with 27 feet of solar panels zOrbit tacks monitored by 4 base stations yMaster control station in Colorado Springs zEach satellite monitored twice a day

Finding distance by measuring time X A B u nAlmanac: predicted position of satellites nConstellation: set of satellites used nDOP: Dilution of Precision nPRN: Pseudo random noise code nElectromagnetic radiation (EM) 299,792.5 Km/sec 4:00 p.m. >> << 7/100 of a second after 4:00 G J K E T Y U O W V W T D H K … G J K E T Y U O W... Receiver: Satellite:

zSatellite location Given 1 satellite …We can locate our position on the surface of a sphere

zSatellite location Given 2 satellites …We can locate our position on the intersection of 2 spheres (a circle)

zSatellite location Given 3 satellites …We can locate our position on the intersection of 3 spheres (2 points)

zSatellite location Given 4 satellites …We can locate our position on the intersection of 4 spheres (1 point)

zSatellite location The point can be located on the earth’s surface

zSatellite location The precise location is determined

zMore on timing: Setting receiver clock After the correct position is determined, the receiver’s clock is adjusted Adding or subtracting time will make the location more or less precise If the receiver’s clock is ahead, the position will be over-estimated for each signal

zMore on timing If the receiver’s clock is behind, the position will be under-estimated for each signal

zMore on timing If the receiver’s clock is correct, the position will be properly estimated for each signal

zMore on timing The receiver adds and subtracts time from simultaneous equations until the only possible (correct) position is located. The receiver’s clock becomes virtually as accurate as the atomic clocks in the SVs

zSources of error: Dilution of precision (DOP) The best spread of satellites makes the best trilateration We want low DOP Satellites that are close to each other result in higher DOP: HDOP: horizontal DOP VDOP: vertical DOP PDOP: positional DOP (combination of HDOP & VDOP) TDOP: time DOP GDOP: geometric DOP (combination of PDOP & TDOP)

Major Factors of error zSatellite clock errors< 1 meter zEphemeris errors (satellite position)< 1 meter zReceiver errors< 2 meters zIonosphere errors (upper atmos.)< 2 meters zTroposphere errors (lower atmos.)< 2 meters zMultipath errors (bounced signals)??? z“Selective Availability” signal transmission0 - off (< 33m if on)

Error zAtmospheric yLight travels at 299,792,458 m/s only in a vacuum yIonospheric effects: ionizing radioation yTropospheric effects: water vapor yLight is “bent” or reflected zClock yReceiver clock errors, mostly corrected by software in receiver ySatellite clock errors ySatellite time stamp errors yTime stamp errors are not correctable ySV timing & clocks are constantly monitored and corrected zReceiver yPower interrupts yOn-board microprocessor failure yFirmware ySoftware yBlunders (user error)

zSources of error: Selective availability (S/A) Clock timing error factor introduced by the DOD Standard operation on the satellites. S/A changes the time stamp of the outgoing signals Calculated positions are erroneous SA causes locations to be in error up to 100 m Each satellite encrypts its own data separately Encryption keys shift frequently In the event of warfare, enemy forces cannot use the same accuracy as the US armed forces Military-grade have the ability to decrypt the time dithering, which lowers error to about 15 m from ~100 m uncorrected

Recording Data z180 fixes needed for maximum accuracy for a receiver and constellations z1 fix every 3 seconds zYou’ll need ~ 9 minutes

Map Projections, Coordinates, and Datums Projections Much Thanks: Denis Dean CSU

General types of map projections z Cylindrical z Conic z Azimuthal – sphere to plane zMiscellaneous

Map Projection Properties Map Projections – cont. Conformality, Shape is preserved Equaldistant Azimuths (directions) Scale Area

Geodetic Datum: The reference sphere Vertical datum: Mean Sea Level Problems …… Average Temperatures Local riparian features Seafloor Gravitational changes Earth’s rotation Etc. etc. North American Datum 1927 – NAD27 uses Clark 1866 Sphere initial point, Meades Ranch Kansas Historic data North American Datum 1983 – NAD83 uses GRS 1980 Sphere preferred for North and South America very common High Precision GPS Network - HPGN uses GRS Sphere, but is refined often created for GPS from military world wide same as NAD83 World Geodetic System 1984 – WGS84 World wide accurate system Widely used in world oceans, antartic basically same as NAD84 Much Thanks: Denis Dean CSU

YearNamea (meters)b (meters)1/fWhere Used 1830Airy6,377,5636,356, Great Britain 1830Everest6,377,2766,356, India, Pakistan, Burma 1841Bessel6,377,3976,356, Germany, Indonesia, Netherlands, Japan, Northeast China 1858Clarke6,378,2946,356, Australia 1866Clarke6,378,2066,356, USA 1880Clarke6,378,2496,356, Central, southern and western Africa, France 1907Helmert6,378,2006,356, No longer widely used 1909Hayford6,378,3886,356, USA, from 1924 used internationally in Europe and North Africa 1927NAD276,378,2066,356, North America 1948Krassovsky6,378,2456,356, Russia, Eastern countries 1960Fisher6,378,1556,356, Southern Asia 1966WGS666,378,1456,356, Worldwide 1967IUGG6,378,1606,356, Internationally, Western Europe, South America, Australia 1972WGS726,378,1356,356, Worldwide (first spheroid defined by satellite geodesy) 1980International6,378,1376,356, Worldwide 1983NAD836,378,1376,356, North America 1984WGS846,378,1376,356, Worldwide Geodetic Datum: The reference sphere Much Thanks: Denis Dean CSU

Form of a Projection PlannerCylindrical Conic Describes the shape of the surface which is being developed Much Thanks: Denis Dean CSU

Case of a projections Tangent Touches the sphere Secant Cuts the sphere Ommissive Does not touch Describe how close the surface comes to the earth spheroid Much Thanks: Denis Dean CSU

Aspect of a Projection NORMAL Transverse Oblique Describes where the surface of the earth spheroid comes in contact with the surface being developed. Much Thanks: Denis Dean CSU

Winkel “tripel” projection (1921) – National Geographic standard Average the X and Y from the Aitoff and Equirectangular projections a modified planner, secant, normal aspect projections Robinson projection (1963) – Rand McNally. the “orthophanic projection (“right appearing”, or Pseudocylindrical Projection with Pole Line a secant Tangency at 38N-38S, normal aspect projections Much Thanks: Denis Dean CSU

Mercator Projection Cylindrical, Tangent, and normal Compression (distortion of the poles) Universal Transverse Mercator (UTM) Projection and Coordinate system Cylindrical, secant (1950), and transverse Identical to Guass-Kruger projection USA uses Clarke 1866 spheroid 60 zones, North sets of coordinates all positive Coordinates in meters Much Thanks: Denis Dean CSU

Washington State Plane Coordinate System Lambert Conformal Conic projections North American Datum 1983 Zones North and South units - “often feet” for NAD 27, meters NAD83 Much Thanks: Denis Dean CSU

Coordinate Systems …. A system that allow you to use numeric values to identify any point in space … latitude/longitude, Universal Transverse Mercator, State Plane Coordinates Cartesian Coordinates – two axis crossing at right angles identifying position on a flat surface Spherical Coordinates – the angle between an axis (or axes) and a base line that runs through the origin point. Much Thanks: Denis Dean CSU

zImport into GIS Uncorrected data

zImport into GIS Differentially post-processed

zImport into GIS Real-time corrected