451-102 ERT 247 GEOMATICS ENGINEERING Global Positioning System (GPS) Siti Kamariah Md Saat School of Bioprocess Engineering GPS Surveying 1

Why GPS and RS in GIS? GPS and remote sensing imagery are primary GIS data sources, and are very important GIS data sources. GPS data creates points (positions), polylines, or polygons Remote sensing imagery and airphotos are used as major basis map in GIS Information digitized or classified from imagery are GIS layers

Overview Basics of GPS Applications and uses GPS Segments 451-102 GPS Surveying 1

Globe Positioning System (GPS) GPS is a Satellite Navigation System GPS is funded and controlled by the U. S. Department of Defense (DOD). While there are many thousands of civil users of GPS world-wide, the system was designed for and is operated by the U. S. military. GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. At least 4 satellites are used to estimate 4 quantities: position in 3-D (X, Y, Z) and GPSing time (T) 20,000 km http://maic.jmu.edu/sic/glossary.htm#Projection

What does GPS provide? 3D position Navigation information Time 451-102 What does GPS provide? 3D position Navigation information position heading velocity Time GPS Surveying 1

Parts of GPS The three parts of GPS are: Satellites Receivers Software

Satellites There are quite a number of satellites out there in space. They are used for a wide range of purposes: satellite TV, cellular phones, military purposes and etc. Satellites can also be used by GPS receivers.

GPS Satellites The GPS Operational Constellation consists of 24 satellites that orbit the Earth in very precise orbits twice a day. GPS satellites emit continuous navigation signals.

Receivers and Satellites GPS units are made to communicate with GPS satellites (which have a much better view of the Earth) to find out exactly where they are on the global scale of things.

451-102 GPS Signals Each GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant. Physically the signal is just a complicated digital code, or in other words, a complicated sequence of “on” and “off” pulses. Signal chosen because: The complex pattern ensures that the receiver does not accidentally synchronize up to some other signal or so the receiver won’t accidentally pick up another satellite’s signal GPS Surveying 1 10

451-102 What is GPS used for? ... personal, vehicle, marine and aircraft Navigation Surveying, deformation monitoring,geodesy, geodynamics Precision farming, soil mapping, ionospheric modeling, asset management Time transfer GPS Surveying 1

451-102 In-car navigation GPS Surveying 1

Building/engineering set out 451-102 Building/engineering set out GPS Surveying 1

451-102 Tectonic measurement GPS Surveying 1

451-102 Machine guidance GPS Surveying 1

451-102 Air navigation GPS Surveying 1

451-102 Mapping GPS Surveying 1

Deformation monitoring 451-102 Deformation monitoring GPS Surveying 1

451-102 Missile guidance GPS Surveying 1

451-102 Three GPS segments GPS Surveying 1

Space segment Constellation of 27 satellites Six orbital planes 451-102 Space segment Constellation of 27 satellites Six orbital planes Altitude of approx. 20,200km Period of 11h 58m Atomic clocks accurate to 1x10-13 s/day GPS Surveying 1

451-102 Space segment … GPS Surveying 1

451-102 Space segment … GPS Surveying 1

Signal structure

451-102 Control segment GPS Surveying 1

Control segment ... Monitor and control the constellation 451-102 Control segment ... Monitor and control the constellation Establish and maintain GPS time Provide satellite location (ephemeris) GPS Surveying 1

Control segment ... Master Control Station Ground Control Station 451-102 Control segment ... UPLOAD NAVIGATION MESSAGE TRACK Master Control Station Ground Control Station Monitor Station Ground Control Station Monitor Station Ground Control Station Monitor Station GPS Surveying 1

User segment… Some examples already discussed… Military applications include : Navigation (land, sea, air) Missile guidance Search and rescue Civilian applications include : Recreational uses (hiking, 4WD…) Professional uses (from metres to millimetres) Position, dynamics, time

Position from satellites

Differential positioning

451-102 Capabilities of GPS GPS Surveying 1

Factors Influencing Position Accuracy 451-102 Factors Influencing Position Accuracy The number of satellites (channels) the receiver can track. The number of channels a receiver has is part of it’s design. The higher the number of channels---the greater the potential accuracy. The higher the number of channels---the greater the cost. GPS Surveying 1

Factors Influencing Position Accuracy 451-102 Factors Influencing Position Accuracy The number of satellites that are available at the time. Because of the way the satellites orbit, the same number are not available at all times. When planning precise GPS measurements it is important to check for satellite availability for the location and time of measurement. If a larger number of channels are required (6-10), and at the time of measurement the number available was less than that, the data will be less accurate. GPS Surveying 1

Factors Influencing Position Accuracy 451-102 Factors Influencing Position Accuracy The system errors that are occurring during the time the receiver is operating. The GPS system has several errors that have the potential to reduce the accuracy. To achieve high levels of precision, differential GPS must be used. GPS Surveying 1

Factors Influencing Position Accuracy 451-102 Factors Influencing Position Accuracy Differential GPS uses one unit at a known location and a rover. The stationary unit compares its calculated GPS location with the actual location and computes the error. The rover data is adjusted for the error. Real Time Kinematic (RTK) Post processing GPS Surveying 1

451-102 Location Once the GPS receiver has located its position it is usually displayed in one of two common formats: Latitude and longitude Universal transverse mercator (UTM). GPS Surveying 1

Latitude and Longitude 451-102 Latitude and Longitude Latitudes and longitudes are angles. Both use the center of the earth as the vertex, and both utilize the equator, but they use a different zero reference. GPS Surveying 1

451-102 UTM Zones The world is divided into 60 zones of latitude, each 6o wide at the equator, that extend from 84o N to 80o s. These zones begin at 180o longitude and are numbered consecutively eastward. GPS Surveying 1

451-102 UTM Zones--cont. The conterminous United States is covered by 10 UTM grid zones. In the Northern Hemisphere each zone's northing coordinate begins at the equator as 0,000,000 and is numbered north in meters. The easting coordinates are measured from an artificial reference line drawn perpendicular to the equator and centered in the zone at the equator. GPS Surveying 1

UTM--cont. The UTM system uses a different grid for the polar regions. 451-102 UTM--cont. The UTM system uses a different grid for the polar regions. These areas are covered by a different conformal projection called the Polar Stereographic. GPS Surveying 1

451-102 UTM--cont. Since compass directions have little meaning at the poles, one direction on the grid is arbitrarily designated "north-south" and the other "east-west" regardless of the actual compass direction. The UTM coordinates are called "false northing" and "false easting.” GPS Surveying 1

Using Location Information 451-102 Using Location Information Each system has its advantages and disadvantages. Latitude and longitude UTM Advantages With the proper instruments, a person can determine their position at the site without using GPS. Used by most maps Advantages Best method for determining distances between two points. Disadvantages Difficult to determine distances between two or more points. Disadvantages Not very useful for finding a location. GPS Surveying 1

Time Difference The GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is.

Velocity x Time = Distance Calculating Distance Velocity x Time = Distance Radio waves travel at the speed of light, roughly 186,000 miles per second (mps) If it took 0.06 seconds to receive a signal transmitted by a satellite floating directly overhead, use this formula to find your distance from the satellite. 186,000 mps x 0.06 seconds = 11,160 miles

Triangulation Geometric Principle: You can find one location if you know its distance from other, already-known locations.

Triangulation

Triangulation

3-D Trilateration 1 Satellite 2 Satellites 3 Satellites

451-102 Atomic Clocks GPS satellites use Atomic Clocks for accuracy, but because of the expense, most GPS receivers do not. the dashed lines show the actual intersection point, and the gray bands indicate the area of uncertainty. the solid lines indicate where the GPS receiver "thinks" the spheres are located. Because of errors in the receiver's internal clock, these spheres do not intersect at one point. The GPS receiver must change the size of the spheres until the intersection point is determined. The relative size of each sphere has already been calculated, so if the size of one sphere is changed, the other spheres must be adjusted by exactly the same amount. GPS Surveying 1 49

Line of Sight Transmissions Line of sight is the ability to draw a straight line between two objects without any other objects getting in the way. GPS transmission are line-of-sight transmissions. Obstructions such as trees, buildings, or natural formations may prevent clear line of sight.

Light Refraction Sometimes the GPS signal from the satellite doesn’t follow a straight line. Refraction is the bending of light as it travels through one media to another.

Signal Refraction Signals from satellites can be like light. When they hit some interference (air patterns in the atmosphere, uneven geography, etc.) they sometimes bend a little.

Signal Interference Sometimes the signals bounce off things before they hit the receivers.

PDOP PDOP = Positional Dilution of Precision All of this combines to make the signal less accurate, and gives it what we call a high “PDOP.” 11,000 miles 11,000 miles 11,000 miles 11,000 miles A PDOP of <4 is excellent A PDOP of 4-8 is good A PDOP of >8 is poor

Satellite Distribution When the satellites are all in the same part of the sky, readings will be less accurate.

Differential Correction Differential correction is a technique that greatly increases the accuracy of the collected GPS data. It involves using a receiver at a known location - the "base station“- and comparing that data with GPS positions collected from unknown locations with "roving receivers." ISU Base Station - http://134.50.65.125/

Postprocessing / Real-time Before After

Waypoints Waypoints are locations or landmarks that can be stored in your GPS. Waypoints may be defined and stored in the unit manually by inputting latitude and longitude from a map or other reference. Or more usually, waypoints may be entered directly by taking a reading with the unit at the location itself, giving it a name, and then saving the point.

Data Dictionary GPS units collect data in: These are called features. Points Lines Areas These are called features. A data dictionary is a means by which we collect specific information about a data feature.

451-102 GPS Errors Noise Biases Blunder Clock GPS Surveying 1

451-102 Noise Error Noise errors are the combined effect of code noise (around 1 meter) and noise within the receiver noise (around 1 meter). GPS Surveying 1

Bias Error Selective Availability (SA) Ephemeris data errors: 1 meter 451-102 Bias Error Selective Availability (SA) SA is the intentional degradation of the SPS signals by a time varying bias. SA is controlled by the DOD to limit accuracy for non-U. S. military and government users. Selective availability is turned off. Ephemeris data errors: 1 meter Satellite orbits are constantly changing. Any error in satellite position will result in an error for the receiver position. GPS Surveying 1

451-102 Bias Error SV clock errors uncorrected by Control Segment can result in one meter errors. GPS Surveying 1

Bias Error Tropospheric delays: 1 meter. 451-102 Bias Error Tropospheric delays: 1 meter. The troposphere is the lower part (ground level to from 8 to 13 km) of the atmosphere that experiences the changes in temperature, pressure, and humidity associated with weather changes. Complex models of tropospheric delay require estimates or measurements of these parameters. GPS Surveying 1

Bias Error Unmodeled ionosphere delays: 10 meters. 451-102 Bias Error Unmodeled ionosphere delays: 10 meters. The ionosphere is the layer of the atmosphere from 50 to 500 km that consists of ionized air. The transmitted model can only remove about half of the possible 70 ns of delay leaving a ten meter un-modeled residual. GPS Surveying 1

Bias Error Multipath: 0.5 meters. 451-102 Bias Error Multipath: 0.5 meters. Multipath is caused by reflected signals from surfaces near the receiver that can either interfere with or be mistaken for the signal that follows the straight line path from the satellite. GPS Surveying 1

Blunder Blunders can result in errors of hundred of kilometers. 451-102 Blunder Blunders can result in errors of hundred of kilometers. Control segment mistakes due to computer or human error can cause errors from one meter to hundreds of kilometers. User mistakes, including incorrect geodetic datum selection, can cause errors from 1 to hundreds of meters. GPS Surveying 1

451-102 Blunder Receiver errors from software or hardware failures can cause blunder errors of any size. GPS Surveying 1

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