Global Positioning System A GADGET WHICH CHANGED THE WAY THE WORLD OPERATESSeminar by:B V AparnaECECMR College of Engg. And Tech
Why do we need GPS? Trying to figure out where you are is probable man’s oldest pastime. Finally US Dept of Defense decided to form a worldwide positioning system. Also known as NAVSTAR ( Navigation Satellite Timing and Ranging Global positioning system) provides instantaneous position, velocity and time information.
Four Basic Functions of GPS Position and coordinates. The distance and direction between any two waypoints, or a position and a waypoint. Travel progress reports. Accurate time measurement.
How does the GPS work? Requirements Triangulation from satellite Distance measurement through travel time of radio signals Very accurate timing required To measure distance the location of the satellite should also be known Finally delays have to be corrected
GPS Satellite Signal: L1 freq. (1575.42 Mhz) carries the SPS code and the navigation message. L2 freq. (1227.60 Mhz) used to measure ionosphere delays by PPS receivers 3 binary code shift L1 and/or L2 carrier phase The C/A code The P code The Navigation message which is a 50 Hz signal consisting of GPs satellite orbits . Clock correction and other system parameters
Signal generation in a GPS SatelliteL1 90 Q1575Mhz I 1.023C/A code Mbps SUM C/A + NAV 50 bps MultipliersNavigationmessage L1+P+C/A +NAV 10.23 L1 outputP code Mbps P+NAV L2 outputL2 I L2+P+NAV1227 Mhz
Triangulation Position is calculated from distance measurement Mathematically we need four satellites but three are sufficient by rejecting the ridiculous answer
Measuring Distance Distance to a satellite is determined by measuring how long a radio signal takes to reach us from the satellite Assuming the satellite and receiver clocks are sync. The delay of the code in the receiver multiplied by the speed of light gives us the distance
Getting Perfect timing If the clocks are perfect sync the satellite range will intersect at a single point. But if imperfect the four T satellite will not intersect at the same point. The receiver looks for aT+3 common correction that will make all the satellite intersect at the same point
Space Segment: 24 GPS space vehicles(SVs). Satellites orbit the earth in 12 hrs. 6 orbital planes inclined at 55 degrees with the equator. This constellation provides 5 to 8 SVs from any point on the earth.
Control Segment: The control segment comprises of 5 stations. They measure the distances of the overhead satellites every 1.5 seconds and send the corrected data to Master control. Here the satellite orbit, clock performance and health of the satellite are determined and determines whether repositioning is required. This information is sent to the three uplink stations
User Segment: It consists of receivers that decode the signals from the satellites. The receiver performs following tasks: Selecting one or more satellites Acquiring GPS signals Measuring and tracking Recovering navigation data
User Segment:There are two services SPS and PPSThe Standard Positioning Service SPS- is position accuracy based on GPS measurements on single L1 frequency C/A codeThe Precise Position Service PPS is the highest level of dynamic positioning based on the dual freq P-code Only authorized users, this consists of SPS signal plus the P code on L1 and L2 and carrier phase measurement on L2
User Segment Military. Search and rescue. Disaster relief. Surveying. Marine, aeronautical and terrestrial navigation. Remote controlled vehicle and robot guidance. Satellite positioning and tracking. Shipping. Geographic Information Systems (GIS). Recreation.
Three Segments of the GPS GPS Space Segment Space Segment Control Segment User Segment User Segment Control Segment Ground AntennasMaster Station Monitor Stations
Errors in GPS The GPS is designed as accurate as possible.However, there are still errors and the most significant of these are discussed below: Atmospheric conditions Ephemeris errors Clock drift/measurement noise Selective availability multipath
Sources of GPS ErrorStandard Positioning Service (SPS ): Civilian Users Source Amount of Error Satellite clocks: 1.5 to 3.6 meters Orbital errors: < 1 meter Ionosphere: 5.0 to 7.0 meters Troposphere: 0.5 to 0.7 meters Receiver noise: 0.3 to 1.5 meters Multipath: 0.6 to 1.2 meters User error: Up to a kilometer or more Errors are cumulative and increased by PDOP.
Errors due to geometry Poor GDOP When angles from the receiver to the SVs used are similar Good GDOP When the angles are different
Sources of Signal Interference causing errors Earth’s Atmosphere Solid Structures Metal Electro-magnetic Fields
DGPS Errors in one position are similar to a local area High performance GPS receiver at a known location. Computes errors in the satellite info
DGPS Data Links Land Links MF,LF,UHF/VHF freq used Radiolocations,local FM, cellular telephones and marine radio beacons Satellite links DGPS corrections on the L band of geostaionary satellites Corrections are determined from a network of reference Base stations which are monitored by control centers like OmniSTAR and skyFix
Applications of GPS system Tracking is useful because it enables a central point to monitor the position of several vehicles or people, in real time, without them needing to relay that information explicitly. This can include children, criminals, police and emergency vehicles or military applications.o GPS vehicle tracking is also used to locate stolen cars, or stolen mobiles. Once we know our location, we can, of course, find out where we are on a map, and GPS mapping and navigation is perhaps the most well-known of all the applications of GPS.
Applications of GPS system A tracking applications are not that much popular as the navigation applications. But, so many people take uses of them. It enables users to find a location of any object that is tagged with a system. Navigation applications are the most famous GPS applications. The latest releases of those applications allow users to have much advanced features and facilities. Other common applications:Car navigation,Hand held ,Tracking,GIS,Survey ,Manufacturing,Military Related 1%