Today s Agenda. Global Positioning System (GPS) Brief History of GPS. Global Positioning System. Brief History of GPS (contd.)

Transcription

1 Global Positioning System (GPS) Cons 340 Today s Agenda What is GPS History of GPS How GPS works Components Signals Trianglation GPS Errors GPS and GIS Global Positioning System Brief History of GPS The Global Positioning System (GPS) is a collection of U.S. government satellites designed to provide accrate positioning and navigation information anywhere on the srface of the earth D.O.D. directed that navigation systems be normalied First satellite lanch of the NAVSTAR Global Positioning System. The first GPS Satellite, lanched in 1978, was the Block I. Brief History of GPS (contd.) 1983 Following the downing of Korean Flight 007, GPS goes pblic Second generation of NAVSTAR satellites were lanched (completed in 1995). The second generation of GPS Satellites, were the Block II. They weigh 1900 ponds, and are abot the sie of a Volkswagen Brief History of GPS (contd.) 1990 s Rapid growth of GPS se dring the 90 s, started t with the Persian Glf War. May 2, 2000 Selective Availability (SA) deactivated. 5.5 hors of degraded GPS signal Selective Availability degraded GPS signals in an effort to prevent their se for targeting. 1

2 How GPS Works Components of the System Signals Determining Position Time = Distance Trianglation Three Components of GPS Space Segment Satellites Control Segment Grond stations User Segment Receivers Space Segment Control Segment System ses 24 satellites (21 active and 3 in reserve). Each satellite makes a complete orbit every 12 hors. This constellation provides 5 8 satellites above horion from any point on earth. Five Grond Based Control Stations User Segment Consists of GPS receivers and the ser commnity. Receivers se satellite signals to compte position, velocity, and time estimates. These estimates are sed primarily for navigation. 2

3 How the system works How It Works (In 5 Easy Steps) Space Segment 24+ Satellites Monitor Stations Diego Garcia Ascension Island Kwajalein Hawaii Colorado Springs GPS Control Colorado Springs The Crrent Ephemeris is Transmitted to Users along with a coded signal End User GPS is a ranging system (trianglation) The reference stations are satellites moving at 4 km/s 1. A GPS receiver ( the ser ) detects 1-way ranging signals from several satellites Each transmission is time-tagged Each transmission contains the satellite s position 2. The time-of-arrival is compared to time-oftransmission 3. The delta-t is mltiplied by the speed of light to obtain the range 4. Each range pts the ser on a sphere abot the satellite 5. Intersecting several of these yields a ser position Trianglation for Location Distance Measring 11,000 Miles We re somewhere on this sphere. T t llit t Three Satellites pts s at one of two points. Generally only one of these is reasonable. Two satellites pts s somewhere on this circle. More satellites sed to trianglate position, eqals higher accracy. The whole system revolves arond time!!! Distance = Rate x Time Rate = 186,000 miles per second (Speed of Light) Time = time it takes signal to travel from the SV to GPS receiver Measring Travel Time SV Clocks 2 Cesim & 2 Rbidim in each SV $100,000-$500,000 $, each Receiver Clocks Clocks similar to qart watch Always an error between satellite and receiver clocks ( t) Reqire 4 satellites to solve for x, y,, and t GPS Signals GPS signals are broadcast on 2 L-band carriers L1: MH Modlated by C/A-code & P-code L2: MH Modlated by P-code only (L3: 3 rd carrier, Global alert system) Most nsophisticated receivers only track L1 If L2 tracked, then the phase difference (L1-L2) can be sed to filter ot ionospheric delay. This is tre even if the receiver cannot decrypt the P-code L1-only receivers se a simplified correction model 3

4 NAVSTAR Carrier Signals (L1 & L2) Combination of Carrier and Code L1 signal carries navigation message and SPS code signals L2 signal is sed to measre ionospheric delay by PPS eqipped receivers SPS is the Standard Positioning Service. PPS is the Precise Positioning Service. NAVSTAR Signals (C/A) C/A (coarse acqisition) code is a repeating 1 MH psedo random noise (PRN) code. This PRN code is a binary nmber that repeats every 1 millisecond. Carrier Phase GPS is based on L1 & L2 signals. Code Phase GPS is based on C/A Code The Almanac In addition to its own nav data, each SV also broadcasts info abot ALL the other SV s In a redced-accracy format Known as the Almanac Permits receiver to predict, from a cold start, where to look for SV s when powered p GPS orbits are so predictable, an almanac may be valid for months Almanac data is large 12.5 mintes to transfer in entirety Time is Distance For example, the PRN code is sent from a satellite, and generated by the GPS receiver. Code from satellite reaches receiver. Difference between code received and code generated by receiver = time = distance. Time Difference If Greater yo wanted difference to make in it difficlt time, eqals for someone greater to se distance this system, between what cold satellite yo and do receiver. to degrade its accracy?? Sorces of GPS Positional Error y y GPS Error Noise Bias Things to Know! Becase the satellite is sing radio waves there are some things to keep in mind. Radio waves cannot pass throgh some objects. Ex: bildings, some trees, montains. Will not receive a signal. Radio waves can bonce or be reflected off of certain objects. Ex: water, large metal objects, bildings. Signal will not be accrate. These can reslt in an inaccrate measrements de to MULTI PATH ERRORS. y Blnders Soltions Obstrcted Reflection 4

5 To get the best measrement: Remain in the open. Avoid bildings and tall trees. Take several measrements. Wait, have patience. Remember the satellites are orbiting, and a satellite that was not visible earlier may come into view of the receiver and give yo a more accrate measrement. Diltion Of Precision (DOP) A Measre of The Geometry Of The Visible GPS Constellation Good DOP Poor DOP Diltion Of Precision PDOP = Position Diltion Of Precision (Most Commonly Used) VDOP = Vertical Diltion Of Precision GDOP = Geometric Diltion Of Precision HDOP = Horiontal Diltion Of Precision TDOP = Time Diltion Of Precision QUALITY DOP Very Good 1-3 Good 4-5 Fair 6 Sspect >6 A bit of Atmospheric Science Cost of Errors (in meters) Device & PRN Noise 1-2 Clock Errors 1 Ephemeris Errors 1 Tropospheric Delays 1 Ionospheric Delays 10 Mltipath Errors 0.5 Selective Availability Total SA Disabled May 2, hors with SA enabled 8 hors with SA disabled 5

6 Cost of Errors disabled SA Device & PRN Noise 1-2 Clock Errors 1 Ephemeris Errors 1 Tropospheric Delays 1 Ionospheric Delays 10 Mltipath Errors 0.5 Selective Availability Total Soltions differential correction Differential correction depends on a base station, at a known position, to compte corrections for remote receivers. Corrections can be made in real time, or by post-processing. Cost of Errors After Correction Types of Differential Correction Device & PRN Noise 1-2 Clock Errors 0 Ephemeris Errors 0 Tropospheric Delays 0 Ionospheric Delays 0 Mltipath Errors 0.5 Selective Availability 0 Total 1-3 Coverage: Local Area Wide Area Methods: Real-Time Post Processing 34 Real-Time DGPS Positioning Wide Area Agmentation System Error correction message Wide Area Differential GPS Reference receivers GPS signals Field receiver Real-time Corrections to Remove S/A etc. 6

7 Error - conclsions 100 meters: Selective Availability (SA) 15 meters:typical GPS position accracy withot SA. 3-5 meters:typical differential GPS (DGPS) position accracy. < 3 meters:typical WAAS position accracy. CDGPS CDGPS is a Canadian differential GPS service. It is a wide-area, or satellite- based service that broadcasts CDGPS corrections optimied for Canada, reslting in sperior meterlevel accracy with most GPS receivers and sb-meter with dal freqency technology GPS to GIS Use a combination of GPS hardware and software to create point, line, and area objects. Use software to attach attribte data to these objects. Where to Start Check for high PDOP vales & # of SVs Create a Data dictionary Set critical vales Featre Logging Rate PDOP Mask (Points, Line/Area, Min PDOP Switch (2D / 3D) # of Posn) Antenna Height Not in Featre Rate Log DOPs Position Mode (2D / 3D) Coordinate system Elevation Mask Datm SNR Mask Units 7