SUR 2101 SUR 2101
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This 3 page Class Notes was uploaded by Seth Mosciski on Saturday September 19, 2015. The Class Notes belongs to SUR 2101 at University of Florida taught by Staff in Fall. Since its upload, it has received 20 views. For similar materials see /class/207063/sur-2101-university-of-florida in Surveying & Related Areas at University of Florida.
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Date Created: 09/19/15
Lec 15 The Global Positioning System Read Chap 1316 Key Problems 13120 Outline A The Global Positioning System GPS is rapidly changing surveying methods 1 We now can measure positions in real earth coordinates latitude and longitude very easily This process is called georeferencing geographic data It allows the overlay or integration of measured survey data with georeferenced survey data available from others over the intemet other maps aerial photographs space images of the earth surface Data integration creates map displays of a vast range of information for very low cost Therefore all survey data should be georeferenced GPS allows us to do this easily 2 To georeference we must shift our attention away from plane surveying into geodetic surveying In plane surveying all meridians are presumed parallel with the forward and reverse azimuth di erent by exactly 180 In reality all meridians converge to the north pole and are not parallel Geodetic surveying accounts for meridian convergence and other geometric challenges of measuring on a curved surface 3 We will have to learn about map projections Features on the curved earth surface must be geometrically and mathematically projected onto a surface such as a cylinder that can laid out at for display of earth features on a 2D map For an overview ofprojections visit website The 39 s Craft Proiect Main Page Select lecture and discussion notes then select map projections overview 4 We will also have to learn about geodetic datums Visit the same web site to review Peter Dana s notes on geodetic datums This will discuss ellipsoids as geometric figures of the earth 5 We will learn many types of coordinate systems Visit the same site and review notes on coordinate systems B NAVSTAR Constellation Further GPS material is available at Peter Dana s website The Geographer s Craft Project Main Page You can select lecture and discussion notes and find a good presentation on GPS 1 GPS was created by the US Defense Department to assist our military operations The civilian sector found that it could use the signals without decoding the military code and civilian use has grown during the 1980s and 1990s Today civilian use is beginning to outweigh military applications For example in early 2000 President Clinton deactivated what was known as selective availability SA The military purposely degraded the GPS signals making a single position uncertain by up to 200 meters when S A was turned on S A was applied at random times so civilian users could never count on more accurate locations However now SA is turned off giving all users 3 7 10 meter accuracy for a simple position measurement 2 The constellation has at least 24 satellites at an altitude of 20200 km in siX orbital planes with 4 in each Each satellite makes two orbits per day The system is designed so that four to six satellites are above the horizon visible from any location on earth at any time 3 Orbits and locations of each satellite are tracked and uploaded into the satellite for broadcast to users A GPS receiver receives the broadcast ephemeris and therefore can calculate for any location the satellite s azimuth altitude angle above horizon At any time there are usually from 4 to 8 satellites above the horizon A minimum of 3 are needed for a 2D solution of position latitude and longitude A minimum of 4 are needed for a 3D solution latitude longitude elevation Using more than minimum increases the accuracy 4 Two carrier signals transmitted Ll 157542 MHz only one used on most GPS receivers wavelength about 19 cm 7 carrier phase L2 122760 MHz used on geodetic grade receivers for atmospheric corrections 5 The military codes modulates these signals to transmit the ephemeris and other information in 300 m 7 long wavelengths 7 code phase C The GPS Solution 1 The GPS receiver measures the slant range to at least 4 satellites simultaneously for a 3D solution of lat long height The satellites are tracked and known in XYZ position in space as a function of time One measured range rl will place the observer somewhere on a sphere of radius rl measured from the known satellite position The second range r2 places the observer on a second sphere somewhere The intersection of these two spheres places the observer somewhere on a circle The third range gives a third sphere of radius r3 which intersects the circle in two places One solution is extraneous the other is the observer s location in lat long and height A fourth satellite range r4 is needed to solve for the clock error to account for the inaccuracy of the receiver s quartz clock The four measured ranges give four equations that are solved for four unknowns lat long height clock error Atypical GPS receiver requires about 2 minutes to gain lock on the minimum number of satellites measure the ranges solve the equations and display the position of the receiver 2 For a simple single GPS receiver the accuracy is about 3 7 10 meters This makes it ideal for navigation use 3 Accuracy is significantly increased by about a factor of 10 by using differential GPS Two receivers are required A control receiver is placed on a control point of known lat long and height A rover is taken to an unknown location to be positioned Both receivers read the satellites at the same time At the control station the measured position is subtracted from known position to determine AX AY and AZ corrections for all receivers in the vicinity When these corrections are applied to the rover s raw measurement the resulting position is much more accurate This will give accuracies from 03 m to 1 In
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