Presentation on theme: "error sources affecting high accuracy GNSS positioning"— Presentation transcript:
1 error sources affecting high accuracy GNSS positioning overview of GPSerror sources affecting high accuracy GNSS positioning
2 What is GPS?GPS, or Global Positioning System, is able to show you your position on the Earth anytime, in any weather, anywhere.The three parts of GPS are:SatellitesReceiversSoftware
3 GPS SatellitesThe GPS Operational Constellation consists of 24 satellites that orbit the Earth in very precise orbits twice a day. GPS satellites emit continuous navigation signals.
4 The Signal from the Satellite Microwave Radio FrequencyEffective Output 500WLine of SightPass through clouds, glass, plasticBlocked by buildings, mountains, etc.Weaker signals under treesSatillite down to earth
5 Time DifferenceThe GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is.
6 Time Delay = Distance Signal travels at speed of light (c) Time delay x c = distanceIf delay = s then distance = 20,446 kmOne sat and earth picture, one sphere graphic, work out math to give distance of 20,000 kmDelay = 68.2 ms
7 Time Delay = DistanceTherefore, we know we are located on a sphere 20,446 km from satelliteOne sat and earth picture, one sphere graphic, work out math to give distance of 20,000 km
9 Time Correction Error of 1/1000 second = 300 km Atomic Clocks used in SatellitesQuartz Clock in GPS receiverNeeds to be correctedCorrected by seeing fourth satelliteAtomic clocks are accurate to nano-seconds fact check. Add picture of timing corrections
10 More Satellites are Better Receiver selects best signalGeometry affects accuracyWatch satellite pageIf accruing signal from additional satellites good to waitAble to watch accuracy improveScreen capture of grays vs black satellites showing still in process of accruing signal from additional Satellites
11 GPS SignalsEach GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant.Physically the signal is just a complicated digital code, or in other words, a complicated sequence of “on” and “off” pulses.Signal chosen because:The complex pattern ensures that the receiver does not accidentally synchronize up to some other signal or so the receiver won’t accidentally pick up another satellite’s signal
12 Signal ComponentsAlmanac (telemetry) updated location of all satellitesUnique Satellite identification codePseudorandom noise code – similar to a songWork on second mouse click that will lock in the PRN signal once it has locked in.Offset = 68.2 milliseconds
13 The GPS Signals GPS satellites transmit on two L-band frequencies: L1 = MHz (19 cm wavelength)L2 = MHz (24.4 cm wavelength)These two carrier frequencies (sine waves) are modulated by two digital codes and a navigation message.Code modulations are achieved byBPSK (binary phase shift keying) methodQPSK (quadrature phase shift keying) method
14 The GPS Codes – C/A Code The Coarse Acquisition (C/A) Code Modulated on the L1 carrier onlyFor each satellite, a unique pseudorandom code of 1023 bitsThe chip rate is Mbps*In distance domain, one chip is 293 mCode looks like noise, but is generated mathematically, hence “PRN Code” (pseudorandom noise)Available to all users* A “chip” is the length of time to transmit 0 or 1 in a binary pulse code. The chip rate is the number of chips per second.
15 The GPS Codes – P Code Precision (P) Code Modulated on both L1 and L2 carriersEncrypting the P code w/ secret W code results in Y codeReferred to as anti-spoofing (A-S)Also called P(Y) codeP code chip rate is Mbps - 10x faster than C/AIn distance domain, one chip is 29.3 mP code sequence is very long - A unique pseudorandom noise (PRN) code of 2.35 x 1014 binary digits266 day period; divided into 38 segments of 7 days eachEach satellite transmits a unique 1-week segment of P-code, initialized every 0000
16 The GPS Navigation Message Navigation (telemetry) data messages are modulated on both L1 and L2 carriers at a rate of 50 kbps.In distance domain, one chip is 5950 kmConsists of 25 frames of 1,500 bits each, or 37,500 bits totalComplete transmission takes 750 seconds, or 12.5 minutesNavigation message includes:Coordinates of the GPS satellites in timeSatellite health, clock correction, almanac & atmospheric dataInfo about other satellites
17 Execution of Surveys; Sources of Error Errors may be characterized as random, systematic, or blundersRandom error represents the effect of unpredictable variations in the instruments, the environment, and the observing procedures employedSystematic error represents the effect of consistent inaccuracies in the instruments or in the observing proceduresBlunders or mistakes are typically caused by carelessness and are detected by systematic checking of all work through observational procedures and methodology designed to allow their detection and elimination
18 GPS Errors & Biases (1) GPS errors include: Satellite errorsReceiver errorsSignal propagation errorsVarious schemes can be employed to reduce or even eliminate these errors
20 Ephemeris Error Satellite orbits are modeled (predicted) Modeling not perfect – Ephemeris error on the order of 2 m to 5 mBetween-receiver differencing does not entirely remove ephemeris error, but better over short baselines (<10 km).Two receivers located closely see about the same orbit errorIn relative positioning, the ratio of ephemeris error to SV range is proportional to the baseline error to baseline length ratioExpected baseline error is 2.5 mm for a 5 m ephemeris error for a 10 km baselinePrecise ephemeris data can be downloaded and applied during post-processing.
21 Satellite Clock Errors Block II/IIA SVs carry 4 clocks2 Rubidium and 2 Cesium atomic clocksBlock IIR carry Rubidium clocks onlyRu & Cs clocks are not perfect – some driftDrift error corresponds to a range error ~2.6 to 5.2 mOr ~8.6 to 17.3 ns per daySV time monitored by control segmentAmount of drift is calculated and transmitted in the navigation messageStill leaves an error of several nanoseconds (where one nanosecond equates to a range error of ~ 30 cm)Best removed by differencing techniques
22 Selective Availability SA added by DoD with the Block II SVs in 1990Motivated by C/A code receiver accuracies which approached P-code receiver accuraciesEven though P-code designed to be 10x more accurateSA dithered the SV clock time (delta error) & slowly varied the orbital error (epsilon error)SA discontinued in 2001Too many schemes developed to get around SAFor short baselines, DGPS effectively cancelled SAEliminating SA improves autonomous accuracies22 m horizontal, 33 m vertical, 95% of the timeDGPS accuracy not improved, but can lower transmission rate of differential correctors
23 Satellite Distribution When the satellites are all in the same part of the sky, readings will be less accurate.
24 Satellite Geometry (1) Good geometry when satellites are spread out Measures of geometryDOP – Dilution of Precision (no dimensions) computed from approximate receiver & satellite coordinatesPDOP – Position Dilution of PrecisionHDOP – Horizontal Dilution of PrecisionVDOP – Vertical Dilution of PrecisionTDOP – Time dilution of Precision
25 Satellite Geometry (2) The lower the DOP the better Recommended PDOP < 5Recommended HDOP < 2.5DOPs are easy to predict using receiver approximate position & ephemeris predictions (SV almanacs).
27 Receiver Clock ErrorsReceivers use inexpensive crystal clocks, so experience much larger clock drift.Can be treated as an unknown in the solution estimation processBetter yet, can be removed with differencing techniques.
28 ANTENNA HEIGHTARPMARKThe height is measured vertically (NOT slant height) from the mark to the ARP in meters.ARP = Antenna Reference Point, shown atI promised we would talk more about antenna heights.The height of the antenna is measure vertically from the from the mark, to the Antenna Reference Point which is almost always the center of the bottom-most, permanently attached piece o f the antenna.Some antennas have a short pipe or extension to them.If in doubt you can find photo’s and diagrams of each antenna (that we have calibrated) at the URL shown.So if you leave the antenna height as “0” this will be the height OPUS returns.
29 The antenna phase centers are located somewhere around here. ANTENNA TYPEThe antenna phase centers are located somewhere around here.phase ctr.The antenna offsets are the distance between the phase centers and the ARPARPYou do not need to know these offsets. They are passed to the processing software through the antenna typeThis is a close up of the last slide. You can see where the ARP is.When you enter the correct Antenna type OPUS knows the antenna offsets and passes them to the software to determine the ARPNGS uses the ARP rather than the phase center. The ARP is a known point on the antenna whereas the phase center varies depending on the elevation angle from the antenna to the satellite.The Antenna Reference Point (ARP) is almost always located in the center of the bottom surface of the antenna.Incorrect or missing antenna type big vertical errors
30 Antenna Phase Center Variation Phase Center Variation (mm)This shows the difference in vertical position of the L1 Phase Center as the satellite gains altitude.You can see the phase pattern changes by 15 mm as the satellite climbs in its orbitElevation Angle (deg.)
31 Antenna Phase Center Variation SV 20SV 20SV 14.SV 14DifferentPhase PatternsNote that SV elevation and varying phase patterns affect signal interpretation differentlyAntennaType BAntennaType ABut where the problem comes in is that different types of antennas have different phase pattern.This is the point in space where the antenna thinks it is receiving the signal. And it is different for the L1 & the L2.So the satellites elevation and the varying phase patterns affect signal interpretation.
32 Antenna Phase Center Variation The antenna phase center does not coincide with the physical center of the antenna’s active element.In fact, the phase center can vary with SV elevation and azimuth.Antenna phase center errors typically on order of a few centimeters.
33 Antenna Phase Center Variation Use correct antenna type/calibration parameters for acquisition and processing.NGS Antenna Calibration web siteFor short baseline surveying, use the same antenna on each receiverOrient each antenna in the same directionDifferencing techniques will cancel phase center variations
35 Line of Sight Transmissions Line of sight is the ability to draw a straight line between two objects without any other objects getting in the way. GPS transmission are line-of-sight transmissions.Obstructions such as trees, buildings, or natural formations may prevent clear line of sight.
36 Light RefractionSometimes the GPS signal from the satellite doesn’t follow a straight line.Refraction is the bending of light as it travels through one media to another.
37 Atmospheric Error Sources IonosphereGreatest at 1400 (local time)Typical 5 to 15 m at zenithExtreme 0.15 to 50 m at zenithHigher frequencies have less effectError correction by dual frequency“Wet” Troposphere10% of total effectModel accuracy only 10 to 50%Need humidity along pathAbout 20 cm at zenithHydrostatic (“Dry”) Troposphere90% of total effectModel accuracy only 2 to 5%Need surface atmospheric pressure and temperaturesAccurate pressure is criticalAbout 2.2 m at zenith
38 GPS Signal Delays Caused by the Atmosphere TECIPWV
39 Atmosphere based Ionospheric Delay (Advance) Ionosphere> 10 km< 10 km
40 Ionospheric Delay (1) The ionosphere is A region of earth’s atmosphere where uv and x-ray radiation from sun cause gas ionization.Extends from 50 km to ~1,000 km altitudeIonosphere is a dispersive medium – it bends GPS signals and changes propagation speed of signal.Bending (signal refraction) causes negligible range errorsPropagation speed changes cause significant range errorsSpeeds up carrier phase beyond speed of light, so ranges appear shortSlows down PRN code, so ranges appear longIonosphere is not homogenous – described in layers within which electron densities vary.Total Electron Count (TEC) varies with time of day, time of year, 11-year solar cycle, geographic location.
41 Ionospheric Delay (2) Ionospheric delay is frequency dependant L2 ( MHz, 24.4 cm) is greater than L1 delay ( MHz, 19 cm)Range errors on the order of 5 to 15 meters, but can be as high as 150 m during extreme solar events at midday and near the horizonDifferencing techniques over short baseline distances can effectively remove much of the ionospheric delay.Dual-frequency receivers combine L1 & L2 carrier phase measurement for iono-free solutionsBut increases measurement noise, so not always reliableLoose integers of cycle ambiguitiesSingle frequency receivers must use empirical models in post-processing or from real-time sources.
42 Troposphere Delay Ionosphere troposphere The more air molecules the slower the signal (dry delay)High pressureLow temperature90% of total delayrelatively constant and easy to correct forThe more water vapor in the atmosphere the slower the signal (wet delay)High humidity10% of total delayHighly variable and hard to correct forIonospheretroposphere
43 Tropospheric DelayThe troposphere extends from earth’s surface up to about 50 km altitudeIt is considered electrically neutralNon-dispersive medium for RF below 15 GHzVaries with temperature, pressure, and humiditySo minimized at receiver’s zenith (~2.3 m error), maximum at low elevations (~20-28 m error at 5° elevation)Separated into dry (90%) and wet componentDry component can be predicted ok.Wet component cannot be predicted that wellTropospheric delay cannot be removed with differencing techniquesUse standard meteorological data as default.1010 mb, 20° C, 50% rel humidity
44 Sunspot cycle Sunspots follow a regular 11 year cycle We are just past the peak of the current cycleSunspots increase the radiation hitting the earth's upper atmosphere and produce an active and unstable ionosphere
45 Signal InterferenceSometimes the signals bounce off things before they hit the receivers.
46 Multipath chain link fences vehicles road signs Multiple signal paths between the satellite and receivercaused by reflected signals within the receiver environmentchain link fencesvehiclesroad signsTo reduce the errors associated with multipathing use a ground plain and avoid sites near reflective surfacesB>
47 Multipath Errors (1)A major error source for both pseudorange and code-phase measurementsOccurs when the GPS signal arrives at the antenna from a reflected path.The reflected path is longer, so the receiver-to-satellite range appears greater.Reflected signal interferes with the direct signal at the receiver antennaFunction of objects around antennaVaries with SV geometry
48 Multipath Errors (2)Multipath affects both carrier-phase and pseudorange measurementsCarrier-phase multipath max value is ¼ cycle, or about 4.8 cm on L1Pseudorange multipath can theoretically reach tens of meters for C/A code measurements.Pseudorange multipath mitigated w/ receiver technologySince multipath is a function of SV geometry, it’s characteristics repeat every sidereal dayAssuming a static receiver configurationThis means it can be correlated from one day to the next in the position solution residual estimate
49 Multipath Errors (3) To reduce effects of multipath Select a receiver antenna location free of obstructions or reflecting surfacesUse an antenna w/ a groundplane, or a choke ring antennaChoke ring features ¼ wavelength slots in a concentric pattern to attenuate reflected signalsIn static observations, conduct data acquisition over multiple days during different time blocks
50 MultipathSatellite signal arriving at receiver via multiple paths due to reflection (Leick 1995)Quasi-periodic signal; 5 to 50 minutesMaximum multipath is a fraction of wavelength (L1 = 19 cm; L2 = 24 cm) typically 2 to 5 cmGeometric relationship between satellite, antenna, and surroundingsSame pattern in same satellite geometry on consecutive days
51 h ø ø Figure 1 Multipath Description Multipath Delay :(meters)T = 2hSinøcMultipath Freq. :(Cycles/hr)d(T )ƒ~ hCosødt hd ø ~ 2 rad.dt hr.øøFigure 1Multipath DescriptionAugust Ionospheric refraction and Multipath Effects in GPS Carrier Phase ObservationsYola Georgiadou and Alfred KleusbergIUGG XIX General Assembly Meeting, Vancouver, Canada
52 Differential Correction Differential correction is a technique that greatly increases the accuracy of the collected GPS data. It involves using a receiver at a known location - the "base station“- and comparing that data with GPS positions collected from unknown locations with "roving receivers."ISU Base Station -
55 DGPS Radio Beacon Systems Accuracy of DGPS method between <1 m and <3 m, depending on base-rover distance, transmission rate (“age of correctors”), and quality of C/A code receiver.Higher accuracy w/ short baseline distances, higher transmission rates, and carrier-smoothed C/A code ranges.Range is limited by VHF signal
57 WAAS-Wide Area Augmentation System Satellite based augmentation systemDeveloped by FAA & DOT for precision flight approachesApproximately 25 ground reference stations across the US that monitor GPS satellite data.Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message.
58 WAAS-Wide Area Augmentation System This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere.The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator.The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.Better than 3 m error 95% of the time.
62 Datasheet Basics what's on a datasheet The NGS Data SheetSee file dsdata.txt for more information about the datasheet.DATABASE = Sybase ,PROGRAM = datasheet, VERSION = 7.56National Geodetic Survey, Retrieval Date = NOVEMBER 20, 2007AE8289 ***********************************************************************AE8289 CBN This is a Cooperative Base Network Control Station.AE8289 TIDAL BM This is a Tidal Bench Mark.AE8289 DESIGNATIONAE8289 PID AE8289AE8289 STATE/COUNTY- MN/ST LOUISAE8289 USGS QUAD - DULUTH (1993)AE8289AE *CURRENT SURVEY CONTROLAE8289 ___________________________________________________________________AE8289* NAD 83(2007) (N) (W) ADJUSTEDAE8289* NAVD (meters) (feet) ADJUSTEDAE8289 EPOCH DATEAE8289 X , (meters) COMPAE8289 Y ,373, (meters) COMPAE8289 Z ,624, (meters) COMPAE8289 LAPLACE CORR (seconds) DEFLEC99AE8289 ELLIP HEIGHT (meters) (02/10/07) ADJUSTEDAE8289 GEOID HEIGHT (meters) GEOID03AE8289 DYNAMIC HT (meters) (feet) COMPAE Accuracy Estimates (at 95% Confidence Level in cm)AE8289 Type PID Designation North East EllipAEAE8289 NETWORK AEAE8289 MODELED GRAV , (mgal) NAVD 88AE8289 VERT ORDER - FIRST CLASS IIAE8289.The horizontal coordinates were established by GPS observationsAE8289.and adjusted by the National Geodetic Survey in February 2007.what's on a datasheetPositionAccuracyHow to locate and identifyhelp files (dsdata.txt, glossary, …)
63 W GEODETIC CONTROL BASICS: 1) Position? 2) Accuracy? 95% confidence intervalWNAD 83(2007) (N) (W)
64 A horizontal plane parallel with the ellipsoid horizontal accuracyradiusnorthaabWWeastWNGS 2007textbookFGDC 1998A horizontal plane parallel with the ellipsoid
65 vertical accuracy W 95% confidence interval --- NAVD (meters)
66 tie to adjacent stations? old vs new accuraciesold order(1st order class II)new order(1 cm)distance dependent?YESangles, distances, leveling have proportional errors.NO“rigid” GNSS network.tie to adjacent stations?ties are required.local accuracy is computed.accuracy metric?TECHNIQUEequipment resolution, design, technique, closure.STATISTICSobservation quality, adjustment results.
67 Datasheet W North = 0.86 cm East = 0.69 cm Ellip = ± 1.45 cm FZ1046 ***********************************************************************FZ *CURRENT SURVEY CONTROLFZ1046 ___________________________________________________________________FZ1046* NAD 83(2007) (N) (W) ADJUSTEDFZ1046* NAVD (meters) (feet) LEVELINGFZ1046FZ Accuracy Estimates (at 95% Confidence Level in cm)FZ1046 Type PID Designation North East EllipFZFZ1046 NETWORK FZ1046 EFZ1046 VERT ORDER - THIRD ?North = 0.86 cmEast = 0.69 cm(N) (W) (meters)WEllip = ± 1.45 cm
68 using local & network accuracies Network = CORS%w%awzxywxyz“%z”“aw”“bx”adbccdab“%d”zwadyxbc“%y”“%x”“%b”“%c”
69 using local & network accuracy horizontal example 0.0 cmCORSCORSnetwork 5.5 cmNorth EastNorth East Ellipnetwork 0.86 cmlocal2 cm“local”5.91 cm“network” 5.85 cm