Global Positioning S tGl b l P iti i System (GPS)In 1973 the U.S. Department of Defensedecided to establish, develop, test, acquire,and deploy a spaceborne Global PositioningSystem (GPS). The result of this decision is theppresent NAVSTARGPS (NAVigation Satellite ( gTiming And Ranging Global PositioningSystem).
GPS General Characteristics Developed by the US Department of Defense D t t fD f Provides Accurate Navigation g 10 - 20 m Worldwide Coverage 24 hour access Common Coordinate System Designed t replace existing D i d to l i ti navigation systems Accessible by Civil and Military
GPS TidbitsDevelopment costs estimate ~$12 billionAnnual operating cost ~$400 million $4003 Segments: Space: Satellites User: Receivers Control: Monitor & Control stationsPrime Space Segment contractor: Rockwell InternationalCoordinate Reference: WGS-84Operated by US Air Force Space Command (AFSC) Mission control center operations at Schriever (formerly Falcon) AFB, Colorado Springs
GPS SystemComponentsC t Space Segment NAVSTAR : NAVigation Satellite Time and Ranging 24 Satellites (30) 20200 Km Control Segment User Segment g 1 Master Station Receive Satellite Signal 5 Monitoring Stations
Control Segment g Monitor and ControlColoradoSprings Ascension Kwajalein Hawaii Islands Diego Master Control Station Garcia Monitor Station Ground Antenna
GPS Segments Space Segment Satellite Constellation User Segment Ground Antennas Monitor AFSCN StationsMaster Control Station FAIRBANKS ENGLAND Control Segment COLORADO SPRINGS SOUTH USNO WASH D.C. KOREA VANDENBERG, AFB CAPE CANAVERAL BAHRAINMaster Control Station (MCS) Advanced Ground Antenna HAWAII Master C Control S i l Station KWAJALEIN ASCENSIONGround Antenna (GA) Monitor Station (MS) ECUADOR DIEGO GARCIA TAHITINational Geospatial-Intelligence Agency (NGA) Tracking Station Geospatial- SOUTH ARGENTINA AFRICAAlternate Master Control Station (AMCS) NEW ZEALAND
Space Segment p g 24 Satellites • 12 Hourly orbits 4 satellites in 6 Orbital – In view for 4-5 hours 45 Planes inclined at 55 • Designed to last 7.5 years Degrees • Different Classifications 20200 Km above the Earth K b th E th – Block 1, 2, 2A, 2R & 2 F 55 Equator q
User Segment The most visible segment GPS receivers are found in many locations and applications
How It Works (In 5 Easy Steps) ( y p ) GPS is a ranging system (triangulation) The “ f Th “reference stations” are satellites moving at 4 k / t ti ” t llit i t km/s1. A GPS receiver (“the user”) detects 1-way ranging signals from several satellites Each transmission is time-tagged Each transmission contains the satellite’s position2. The time-of-arrival is compared to time-of-transmission3. The delta-T is multiplied by the speed of light to obtain the range4. Each range puts the user on a sphere about the satellite5. Intersecting several of these y g yields a user p position
Outline Principle : Range Xll Vl Range = Time Taken x Speed of Light
Outline Principle : Position The satellites are like “Orbiting Control Stations Orbiting Stations” Ranges (distances) are measured to each satellite using time dependent codes Typically GPS receivers use inexpensive clocks They clocks. are much less accurate than the clocks on board the satellites A radio wave travels at the speed of light ( (Distance = Velocity x Time) y ) Consider an error in the receiver clock 1/10 second error = 30,000 Km error 1/1,000,000 second error = 300 m error
TimingAccuracy of position is only as good as your clock To know where you are, you must know when you receive. Receiver clock must match SV clock to compute delta-TSVs carry atomic oscillators (2 rubidium, 2 cesium each) Not practical for hand-held receiverAccumulated drift of receiver clock is called clock biasTheTh erroneously measured range i called a pseudorange l d is ll d dTo eliminate the bias, a 4th SV is tracked 4 equations 4 unknowns equations, Solution now generates X,Y,Z and bIf Doppler also tracked, Velocity can be computed
Position Equations P1 = ( X − X 1 ) 2 + (Y − Y 1 ) 2 + ( Z − Z 1 ) 2 + b P2 = (X − X 2 ) 2 + (Y − Y 2 ) 2 + ( Z − Z 2 ) 2 + b P3 = ( X − X 3 ) 2 + (Y − Y 3 ) 2 + ( Z − Z 3 ) 2 + b P4 = (X − X 4 ) 2 + (Y − Y 4 ) 2 + ( Z − Z 4 ) 2 + b Where: Pi = Measured PseudoRange (Biased ranges) to the ith SV Xi , Yi , Zi = Position of the ith SV, Cartesian Coordinates , X , Y , Z = User position, Cartesian Coordinates, to be solved-for b = User clock bias (in distance units), to be solved-forThe above nonlinear equations are solved iteratively using an initialestimate of the user position, XYZ, and b- same for all satellites
Point Positioning Accuracy 10 - 100 mA receiver in autonomous mode provides navigation andpositioning accuracy of about 10 to 100 m due to theeffects of GPS errors!!? ff t f !!?
The AlmanacIn addition to its own nav data, each SV alsobroadcasts info about ALL the other SV’s In a reduced-accuracy formatKnown as the AlmanacPermits receiver to predict, from a cold start,“where to look” for SV’s when powered upGPS orbits are so predictable, an almanac may bevalid for monthsAlmanac data is large 12.5 minutes to transfer in entirety
GPS SignalsMost unsophisticated receivers track only L1M hi i d i k lIf L2 tracked, then the phase difference (L1-L2) canbeb used t filt out i d to filter t ionospheric d l h i delay. This is true even if the receiver cannot decrypt the P- code (more later) L1-only receivers use a simplified correction model
GPS Error Sources (uncertainities based on Satellite, signal propagation, and receiver based)Standard Positioning Service (SPS ): Satellite clocks: < 1 to 3.6 meters Orbital errors: < 1 meter Receiver noise: 0.3 to 1.5 meters Ionosphere: 5.0 to 7.0 meters Troposphere: 0.5 to 0.7 meters Multipath: undetermined User error: Up to a kilometer or moreErrors are cumulative
Satellite Geometry ySatellite geometry can affect the quality of signals and g y q y gaccuracy of receiver trilateration.Positional Dilution of Precision (PDOP) reflects eachsatellite’s position relative to the other satellites beingaccessed by a receiver.PDOP can b used as an i di t be d indicator of th quality of a f the lit freceiver’s triangulated position.It sIt’s usually up to the GPS receiver to pick satellites whichprovide the best position trilateration.Some receivers do allow PDOP manipulation by the user. p y
Dilution of Precision (DOP) Satellite geometry can affect the quality of signals and accuracy of receiver trilateration.• A description of purely geometrical contribution to the uncertainty in a position fix.• It is an indicator as to the geometrical strength of the satellites being tracked at the time of measurement – GDOP (Geometrical) • Includes Lat, Lon, Height & Time Good GDOP – PDOP (Positional) Poor DOP • Includes Lat, Lon & Height – HDOP (Horizontal) • Includes Lat & Lon – VDOP (Vertical) • Includes Height QUALITY DOP Very Good V G d 1-3 13 Good 4-5 Fair 6 Suspect >6
Satellite Mask AngleAtmospheric Refraction is greater for satellites atangles that are low to the receiver because thesignal must pass through more atmosphere.There is a trade off between mask angle andatmospheric refraction. Setting high angles will t h i f ti S tti hi h l illdecrease atmospheric refraction, but it will alsodecrease the possibility of tracking thenecessary four satellites.
Signal Obstruction When something blocks the GPS signal. Areas of Great Elevation Differences Canyons Mountain Obstruction Urban Environments Indoors
Selective Availability (SA) To deny high-accuracy realtime positioning to potential enemies, DoD reserves the right to deliberately degrade GPS performance Only on the C/A code By far the largest GPS error source Accomplished by: “Dithering” the clock data Results in erroneous pseudoranges Truncating the nav message data Erroneous SV positions used to compute user position Degrades SPS solution by a factor of 4 or more Long-term averaging is the only effective SA compensatorON 1 MAY 2000: SA WAS DISABLED BY DIRECTIVE
Selective Availability (SA) 100m• In theory a point position can be 30m accurate to 10 - 30 b t t 30m based on d the C/A Code The USDoD degrades the accuracy of the broadcast f th b d t information P Dither the Satellite Clocks Satellite Orbital Information +/- +/- 100m (95%) Positional accuracy P = True Position 100m (95%)
Error Budget E B d tTypical Error in Meters (per satellite) Standard GPS Differential GPSSatellite Clocks 1.5 0Orbit Errors 2.5 25 0Ionosphere 5 0.4Troposphere 0.5 0.2Receiver Noise 0.3 0.3Multipath 0.6 0.6SA 30 0Typical P i i AT i l Position AccuracyHorizontal 50 1.3Vertical 78 23-D3D 93 2.8 28 Trimble Navigation Limited
How do I Improve my Accuracy ? Use Differential GPS ( (Receiver position, satellite position, frequency- p p frequency- q yionospheric corrections, time-ambiguity of carrier phase time- measurements)
Differential Positioning g It is possible to determine the position of Rover ‘B’ in relation to Reference ‘A’ provided – The coordinates of the Reference Station (A) are known – Satellites are tracked simultaneously• Differential Positioning – eliminates errors in the sat. and receiver clocks A B – minimizes atmospheric delays – A Accuracy 0 5 cm - 5 m 0.5
Differential Positioning If using Code only accuracy is in the range of 0 5m - 5 0.5m m This is typically yp y referred to as DGPS A B
Differential PositioningIf using Ph i Phase orCode & Phaseaccuracy is in theorder of 5 - 10 mm +1ppm A B
Summary of GPS Positioning y g• Point Positioning Methods using stand alone receivers provide 10 - 100 m accuracy – Dependent on SA – 1 Epoch solution• Differential Positioning Methods using 2 receivers, simultaneously tracking a minimum of 4 satellites y g (preferably 5) will yield 0.5 cm to 5 m accuracy with respect to a Reference Station • Differential Techniques using Code will give meter accuracy • Differential Techniques using Phase will give centimeter accuracy
GPS Surveying TechniquesStaticSt ti For long baselines (>20Km), where the highest possible accuracy is required This i th t diti Thi is the traditional t h i l technique f providing for idi Geodetic Networks The only solution for large areasRapid Static For baselines up to 20Km Short Occupation times Normally used for high production
GPS Surveying Techniques y g q Stop and Go Detail Surveys. Any application Surveys where many points close together have to be surveyed Fast and economical Ideal for open areas Kinematic Used to track the trajectory of a moving object (continuous measurements) Can be used to profile roadways, stockpiles, etc.
ReferencesR f http://www.glonass- ianc.rsa.ru/pls/htmldb/f?p=202:1:15000421459964108253 http://igscb.jpl.nasa.gov/ http://igscb jpl nasa gov/ http://www.navcen.uscg.gov/gps/precise/default.htmInterface Control Documents: http://www.navcen.uscg.gov http://www.Glonass-ianc.ras.ru htt // Gl i http://www.Galileoju.com