Questions and Answers

Geographic Information Systems Global Positioning Systems (GPS) 1. GPS A method used in surveying. It uses a constellation of satellites orbiting the earth at very high altitudes

GPS .. The GPS technology allows accurate geodetic surveys by using specially designed receivers that, when positioned at a point on the earth, measure the distance from that point to three or more orbiting satellites

Through the geometric calculations of triangulation, the coordinates of the point on the surface of the earth are determined X, Y, Z (position) and time from 4 satellites to calculate posit Pamela E. Jansma, University of Arkansas

2. NAVSTAR GPS NAVigation Satellite Timing And Ranging (NAVSTAR) Global Positioning System Developed by U.S. Department of Defense A constellation system of 28-30 satellites orbiting the earth at a high altitude twice a day Transmitting precise time and position

information For world-wide and all weather navigations NAVSTAR .. 24 satellites in 6 orbital planes Orbit the earth at approx. 20,200 km 550 inclination Satellites complete an orbit in approximately

12 hours http://www.montana.edu/places/gps/lres357/assignments.html http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html 3. Operation The basis of GPS is triangulation

Satellite GPS receiver Determination of Location Coordinate systems 3. Operation - Distance to One Satellite

Satellite - continuously broadcasts time and its location GPS receiver - receives the signal and calculates the distance between the satellite and the

receiver: Distance = Velocity x Time Span Time Span = the time a signal takes to 3. Operation - Determine Location Determination of location

- 3 or more simultaneous distance measurements are needed to determine the location of the receiver Coordinate systems - A GPS provides its position in geographical/UTM/ State Plane coordinates

and altitude 12,000 mile radius One measurement narrows down your position to the surface of a sphere http://www.montana.edu/places/gps/lres357/assignments.html 12,000 mile

radius 11,000 mile radius A second measurement narrows down your position to the intersection of two spheres, which is a circle http://www.montana.edu/places/gps/lres357/assignments.html

11,000 mile radius 12,000 mile radius 11,500 mile radius A third measurement narrows down your position to just two points, because the intersection of a circle

and a sphere is two points http://www.montana.edu/places/gps/lres357/assignments.html A four measurement narrows our position down to one point http://www.bbk.ac.uk/geog/study/courses/pgis/GPSlecture10.pdf 3. Operation- Determine location ..

Pamela E. Jansma, University of Arkansas 4. GPS Receiver GPS receivers can be handheld or installed on aircraft, ships, tanks, submarines,

cars and trucks GPS Receiver .. Radio channels Internal clock Computer

GPS Receiver .. Radio channels It needs one or more channels to receive signals of (1) the time a signal is sent, and (2) a pseudorandom code to identify the signal

Internal clock - Synchronized with the satellite in order to calculate the signal travel time precisely GPS Receiver ..

Computer - Calculates distance and location - Stores location readings - Calculates the satellite availability 5. Autonomous vs. Differential GPS Autonomous Differential

GPS: one receiver unit GPS Selective availability interference WAAS (Wide Area Augmentation System) Differential GPS .. Selective availability interference

- The Us government used to insert random errors in the signals in order to maintain optimum military effectiveness of the system. It was turned off in 2000 Differential GPS - Designed to compensate for the inserted errors,

and used after 2000 to get better position reading Differential GPS Selective availability interference Differential

GPS - Uses two receiver units, with one placed at a location with known x, y, and z coordinates, such as a surveyed control point, to detect the inserted errors and calculate the needed corrections. The needed correction information is used by another receiver to overcome its selective availability interference

Differential GPS (DGPS) .. The DGPS corrections can be applied to the GPS data in real-time (radio modems). Or, can be done later on a computer There are a series of radio beacons to

transmit the DGPS corrections for accurate navigation 5. Differential Correction http://www.bbk.ac.uk/geog/study/courses/pgis/GPSlecture10.pdf Differential Correction an example A

base GPS station where its Easting, Northing, and elevation were surveyed rigorously before and are known to be accurate Easting: 671,000m, Northing: 4,719,000m, Elevation: 304m For

the signal received 5:32PM on Sept 19,2019, the calculation for the Easting is 670,000m. An error of 1000m is inserted. Communicate to the GPS in the field to correct its Easting by 1000m for signal 5. DGPS - WAAS Satellite DGPS Wide Area Augmentation System (WAAS)

- a real-time differential correction service - is based on a network of approximately 25 ground reference stations that cover a very large service area http://www.montana.edu/places/gps/lres357/assignments.html WAAS Satellite DGPS .. - Signals from GPS satellites are received by wide area ground reference stations (WRSs)

- Each of these precisely surveyed reference stations receive GPS signals and determine if any errors exist - These WRSs are linked to form the U.S. WAAS network. This service is primarily for aviation purposes WAAS Satellite DGPS .. Improves

the accuracy, integrity and availability of the basic GPS signals Safety - critical navigation system for aviation Owned and operated by Federal Aviation Administration (FAA) Type: real-time Cost: free WAAS Stations

http://www.montana.edu/places/gps/lres357/assignments.html DGPS .. Omnistar Type: Real-time - Network of reference stations 70 reference stations

3 network control centers - Worldwide coverage Cost: http://www.montana.edu/places/gps/lres357/assignments.html 6. Static vs. Kinematic GPS Static: two or more receivers receive data for

a lengthy period of time at a fixed location Kinematic: one receiver is placed at a known location, while one or more receivers move from point to point Real Time Kinematic (RTK) 7. Advantages and Limitations

Advantages: fast, more accurate, lower cost than manual approaches Disadvantages: obstructed locations 8. Readings

Chapter 2 Omnistar How it Works http://www.montana.edu/places/gps/lres357/assignments.html http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html 5. DGPS - Omnistar Coverage North America http://www.montana.edu/places/gps/lres357/assignments.html

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