Intro to GIS Spring 2012

 GPS = Global Positioning System  GNSS = Global Navigation Satellite Systems  Satellite based technologies that give location on Earth’s surface

 Navigation: knowing where you are and where to go  Defense: precise locations for targets  Now: many uses – commercial, field mapping and surveying, automobile travel, recreation

 Navigating the oceans  Compass: points to magnetic north so know direction traveling  Sextant: instrument that can determine angles of stars, moon and sun over horizon. Indicates your latitude  Chronometer: shipboard timepiece that indicates your longitude

 Early 20 th century: radio-based navigation systems used during WWII  Limitations of ground radio systems  Very accurate but doesn’t cover wide area  Or, one that covers wide area but is not very accurate

 NAVSTAR: operated by the US Department of Defense, 1 st satellite launched in 1978, last satellite launched in 1994  GLONASS: Russian, little used internationally  Galileo: being developed by a consortium of European governments and industries  Chinese Compass Satellite Navigation System: in development

 Satellites  Receivers(users)  Control stations

 24 satellites orbiting the Earth  About 20,000 km above surface  Complete orbit in 12 hours

 Each satellite is carefully placed and monitored in orbit  Each contains a very accurate clock – to 3 billionths of a second, or

 Each broadcasts a signal that includes:  Pseudorandom code: unique to identify which satellite  Ephemeris data: identifies satellite position in space at any given moment  Almanac data: exact time signal was sent  Electromagnetic radiation  Low power radio waves that pass through clouds, glass and plastic, but not Earth or buildings  Speed of light

 Distance = velocity x time  Know velocity: signals traveling at speed of light 3 x 10 8 m/s  Time: determine time between when signal was sent by satellite and when received by the receiver (GPS unit)

 Need precise clocks  Each satellite emits a pseudorandom code  Signal so complicated that it looks random  Receiver compares the signal it receives with the signal should be exactly when it is received Satellite directly overhead: takes about 0.06 second to reach receiver Source Bolstad, p. 180

 Detect, decode and process signals from satellites within range  Contain accurate clock, although not as accurate as the ones on satellites  Measure the distance between the time the signal was sent and the time it was picked up by the receiver.  Used to determine the distance to satellite  Need signals from at least 4 satellites to determine location and elevation  More is better

 Know location of satellites (at least 4)  Know distance of each satellite from receiver Source Bolstad, p. 181

 5 ground stations around the world  Hawaii, Ascension Island (South Atlantic Ocean), Diego Garcia (Indian Ocean), Kwajalein (Marshall Islands), and Colorado Springs  Master station in Colorado, USA  Responsible for:  Tracking  Communications  Data gathering  Integration  Analysis

 Cannot pass through buildings, underground, sides of mountains, and dense foliage  Buildings and terrain can reduce visible sky and block signal reception  Signals can reflect off buildings and outcrops, thus increasing the length of time the unit receives the signal  Signal slows through the atmosphere. GPS uses a correctional factor  Locations of satellites should be at wide angles to each other

 Use 2 receivers: 1 stationary and the other roving (e.g., hand-held unit)  Location of stationary known. Can be used to apply correction to signals due to atmospheric interference  Works in reverse: known location so will know the length of time a satellite signal should take to reach it  If the roving unit is within a few hundred kilometers, can use the error correction Source Bolstad, p. 186

 Improves accuracy  Some GPS receivers can receive correction while collecting data  Others require post processing: corrections are applied later in a lab Source Bolstad, p. 188