# Geographic Information Systems

## Presentation on theme: "Geographic Information Systems"— Presentation transcript:

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 position
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 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

3. Operation Principles The basis of GPS is triangulation Satellite
GPS receiver Determination of Location Coordinate systems

3. Operation - Distance to One 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 where time = the time a signal takes to travel from the satellite to the receiver, velocity = ?

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

One measurement narrows down our position to the surface of a sphere
12,000 miles is the radius of a sphere centered on the satellite. Our position could be anywhere on the surface of that sphere. One measurement narrows down our position to the surface of a sphere

The intersection of two spheres is a circle
The intersection of two spheres is a circle. Now we know that our position is somewhere on that circle. A second measurement narrows down our position to the intersection of two spheres, which is a circle

The three spheres intersect at only two points
The three spheres intersect at only two points. Usually we can discard one of the two points because one point might be nowhere near the earth or it might be moving at a ridiculous speed. The computers in GPS receivers have various techniques for distinguishing the correct point from the incorrect one. But there is a reason we need a fourth measurement... A third measurement narrows down our position to just two points, because the intersection of a circle and a sphere is two points

A four measurement narrows our position down to one point

3. Operation- Determine location ..
Pamela E. Jansma, University of Arkansas

4. GPS Receiver GPS receivers can be hand carried out or installed on aircraft, ships, tanks, submarines, cars and trucks

GPS Receiver .. Radio channels Internal clock Computer

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 GPS: one receiver unit Selective availability interference Differential GPS 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

Differential GPS Selective availability interference Differential GPS
- Uses two receiver units, with one placed at a known location, such as a surveyed control point, to detect the inserted errors and calculate the needed corrections. The 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 PC There are a series of radio beacons to transmit the DGPS corrections for accurate navigation

5. Differential Correction

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 WAAS, a real-time differential correction service operated by the FAA, provides flight and approach navigation for civil aviation. Its coverage area includes the continental US and outlying parts of Canada & Mexico. The WAAS system has 25 ground reference stations throughout the U.S. The reference stations compute corrections and transmit the data to WAAS satellites, which then re-transmit the correction data for WAAS-enabled GPS receivers. The GeoXTs are capable of receiving WAAS signals, and most recreational grade GPS receivers now include WAAS receivers.

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, a real-time differential correction service operated by the FAA, provides flight and approach navigation for civil aviation. Its coverage area includes the continental US and outlying parts of Canada & Mexico. The WAAS system has 25 ground reference stations throughout the U.S. The reference stations compute corrections and transmit the data to WAAS satellites, which then re-transmit the correction data for WAAS-enabled GPS receivers. The GeoXTs are capable of receiving WAAS signals, and most recreational grade GPS receivers now include WAAS receivers.

WAAS Stations

DGPS .. Omnistar Type: Real-time - Network of reference stations
3 network control centers - Worldwide coverage Cost: ~ \$800/year Omnistar is the leader in worldwide DGPS satellite services. A previously operating DGPS satellite service, Racal-LandStar, was recently acquired by the group of companies that owns Omnistar. Satellite differential providers offer correction signals sent by communication satellites. The satellites allow widespread coverage throughout the world. Use of satellite differential correction requires a subscription (approx. \$800/yr).

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 OmniSTAR's unique "Virtual Base Station" technology generates corrections optimized for the user's location. OmniSTAR receivers at monitor sites output both high quality RTCM-SC104 Version 2 corrections and differentially corrected Lat/Long in NMEA format. Multiple OMNISTARtm GPS monitor sites send GPS corrections via a lease line to the Houston Network Control Center, where data corrections are checked and repackaged for uplink to L- Band Geostationary Satellites. Correction data are received by user equipment and applied in real-time.

5. DGPS - Omnistar Coverage North America
Omnistar provides coverage throughout the world. This slide shows Omnistar’s coverage in North America.