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error sources affecting high accuracy GNSS positioning

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1 error sources affecting high accuracy GNSS positioning
overview of GPS error 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: Satellites Receivers Software

3 GPS Satellites                                                                                                                                  The 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 Frequency Effective Output 500W Line of Sight Pass through clouds, glass, plastic Blocked by buildings, mountains, etc. Weaker signals under trees Satillite down to earth

5 Time Difference The 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 = distance If delay = s then distance = 20,446 km One sat and earth picture, one sphere graphic, work out math to give distance of 20,000 km Delay = 68.2 ms

7 Time Delay = Distance Therefore, we know we are located on a sphere 20,446 km from satellite One sat and earth picture, one sphere graphic, work out math to give distance of 20,000 km

8 3-D Trilateration 1 Satellite 2 Satellites 3 Satellites

9 Time Correction Error of 1/1000 second = 300 km
Atomic Clocks used in Satellites Quartz Clock in GPS receiver Needs to be corrected Corrected by seeing fourth satellite Atomic clocks are accurate to nano-seconds fact check. Add picture of timing corrections

10 More Satellites are Better
Receiver selects best signal Geometry affects accuracy Watch satellite page If accruing signal from additional satellites good to wait Able to watch accuracy improve Screen capture of grays vs black satellites showing still in process of accruing signal from additional Satellites

11 GPS Signals Each 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 Components Almanac (telemetry) updated location of all satellites Unique Satellite identification code Pseudorandom noise code – similar to a song Work 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 by BPSK (binary phase shift keying) method QPSK (quadrature phase shift keying) method

14 The GPS Codes – C/A Code The Coarse Acquisition (C/A) Code
Modulated on the L1 carrier only For each satellite, a unique pseudorandom code of 1023 bits The chip rate is Mbps* In distance domain, one chip is 293 m Code 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 carriers Encrypting the P code w/ secret W code results in Y code Referred to as anti-spoofing (A-S) Also called P(Y) code P code chip rate is Mbps - 10x faster than C/A In distance domain, one chip is 29.3 m P code sequence is very long - A unique pseudorandom noise (PRN) code of 2.35 x 1014 binary digits 266 day period; divided into 38 segments of 7 days each Each 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 km Consists of 25 frames of 1,500 bits each, or 37,500 bits total Complete transmission takes 750 seconds, or 12.5 minutes Navigation message includes: Coordinates of the GPS satellites in time Satellite health, clock correction, almanac & atmospheric data Info about other satellites

17 Execution of Surveys; Sources of Error
Errors may be characterized as random, systematic, or blunders Random error represents the effect of unpredictable variations in the instruments, the environment, and the observing procedures employed Systematic error represents the effect of consistent inaccuracies in the instruments or in the observing procedures Blunders 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 errors Receiver errors Signal propagation errors Various schemes can be employed to reduce or even eliminate these errors

19 GPS errors include: Satellite errors
GPS Errors & Biases GPS errors include: Satellite errors orbit error center-of-mass to L1 antenna phase center L1-L2 antenna phase center offset atomic clock error selective availability (SA) Satellite geometry, DOP Transmission multipath Satellite inter-channel bias

20 Ephemeris Error Satellite orbits are modeled (predicted)
Modeling not perfect – Ephemeris error on the order of 2 m to 5 m Between-receiver differencing does not entirely remove ephemeris error, but better over short baselines (<10 km). Two receivers located closely see about the same orbit error In relative positioning, the ratio of ephemeris error to SV range is proportional to the baseline error to baseline length ratio Expected baseline error is 2.5 mm for a 5 m ephemeris error for a 10 km baseline Precise ephemeris data can be downloaded and applied during post-processing.

21 Satellite Clock Errors
Block II/IIA SVs carry 4 clocks 2 Rubidium and 2 Cesium atomic clocks Block IIR carry Rubidium clocks only Ru & Cs clocks are not perfect – some drift Drift error corresponds to a range error ~2.6 to 5.2 m Or ~8.6 to 17.3 ns per day SV time monitored by control segment Amount of drift is calculated and transmitted in the navigation message Still 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 1990 Motivated by C/A code receiver accuracies which approached P-code receiver accuracies Even though P-code designed to be 10x more accurate SA dithered the SV clock time (delta error) & slowly varied the orbital error (epsilon error) SA discontinued in 2001 Too many schemes developed to get around SA For short baselines, DGPS effectively cancelled SA Eliminating SA improves autonomous accuracies 22 m horizontal, 33 m vertical, 95% of the time DGPS 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 geometry DOP – Dilution of Precision (no dimensions) computed from approximate receiver & satellite coordinates PDOP – Position Dilution of Precision HDOP – Horizontal Dilution of Precision VDOP – Vertical Dilution of Precision TDOP – Time dilution of Precision

25 Satellite Geometry (2) The lower the DOP the better
Recommended PDOP < 5 Recommended HDOP < 2.5 DOPs are easy to predict using receiver approximate position & ephemeris predictions (SV almanacs).

26 GPS errors include: Receiver errors
GPS Errors & Biases GPS errors include: Receiver errors receiver noise receiver clock Antenna phase center L1-L2 phase center offset L1-L2 phase center variation Circular polarization Receiver inter-channel bias Stability Antenna height Mark stability Earth tides Ocean tide loading Atmospheric loading Crustal motion

27 Receiver Clock Errors Receivers use inexpensive crystal clocks, so experience much larger clock drift. Can be treated as an unknown in the solution estimation process Better yet, can be removed with differencing techniques.

28 ANTENNA HEIGHT ARP MARK The height is measured vertically (NOT slant height) from the mark to the ARP in meters. ARP = Antenna Reference Point, shown at I 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 TYPE The antenna phase centers are located somewhere around here. phase ctr. The antenna offsets are the distance between the phase centers and the ARP ARP You do not need to know these offsets. They are passed to the processing software through the antenna type This 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 ARP NGS 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 orbit Elevation Angle (deg.)

31 Antenna Phase Center Variation
SV 20 SV 20 SV 14 . SV 14 Different Phase Patterns Note that SV elevation and varying phase patterns affect signal interpretation differently Antenna Type B Antenna Type A But 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 site For short baseline surveying, use the same antenna on each receiver Orient each antenna in the same direction Differencing techniques will cancel phase center variations

34 GPS errors include: Signal Propagation errors
GPS Errors & Biases GPS errors include: Signal Propagation errors Ionospheric effects Dry troposphere delay Wet troposphere delay multipath

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 Refraction Sometimes 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
Ionosphere Greatest at 1400 (local time) Typical 5 to 15 m at zenith Extreme 0.15 to 50 m at zenith Higher frequencies have less effect Error correction by dual frequency “Wet” Troposphere 10% of total effect Model accuracy only 10 to 50% Need humidity along path About 20 cm at zenith Hydrostatic (“Dry”) Troposphere 90% of total effect Model accuracy only 2 to 5% Need surface atmospheric pressure and temperatures Accurate pressure is critical About 2.2 m at zenith

38 GPS Signal Delays Caused by the Atmosphere

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 altitude Ionosphere is a dispersive medium – it bends GPS signals and changes propagation speed of signal. Bending (signal refraction) causes negligible range errors Propagation speed changes cause significant range errors Speeds up carrier phase beyond speed of light, so ranges appear short Slows down PRN code, so ranges appear long Ionosphere 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 horizon Differencing 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 solutions But increases measurement noise, so not always reliable Loose integers of cycle ambiguities Single 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 pressure Low temperature 90% of total delay relatively constant and easy to correct for The more water vapor in the atmosphere the slower the signal (wet delay) High humidity 10% of total delay Highly variable and hard to correct for Ionosphere troposphere

43 Tropospheric Delay The troposphere extends from earth’s surface up to about 50 km altitude It is considered electrically neutral Non-dispersive medium for RF below 15 GHz Varies with temperature, pressure, and humidity So 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 component Dry component can be predicted ok. Wet component cannot be predicted that well Tropospheric delay cannot be removed with differencing techniques Use 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 cycle Sunspots increase the radiation hitting the earth's upper atmosphere and produce an active and unstable ionosphere

45 Signal Interference Sometimes 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 receiver caused by reflected signals within the receiver environment chain link fences vehicles road signs To reduce the errors associated with multipathing use a ground plain and avoid sites near reflective surfaces B>

47 Multipath Errors (1) A major error source for both pseudorange and code-phase measurements Occurs 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 antenna Function of objects around antenna Varies with SV geometry

48 Multipath Errors (2) Multipath affects both carrier-phase and pseudorange measurements Carrier-phase multipath max value is ¼ cycle, or about 4.8 cm on L1 Pseudorange multipath can theoretically reach tens of meters for C/A code measurements. Pseudorange multipath mitigated w/ receiver technology Since multipath is a function of SV geometry, it’s characteristics repeat every sidereal day Assuming a static receiver configuration This 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 surfaces Use an antenna w/ a groundplane, or a choke ring antenna Choke ring features ¼ wavelength slots in a concentric pattern to attenuate reflected signals In static observations, conduct data acquisition over multiple days during different time blocks

50 Multipath Satellite signal arriving at receiver via multiple paths due to reflection (Leick 1995) Quasi-periodic signal; 5 to 50 minutes Maximum multipath is a fraction of wavelength (L1 = 19 cm; L2 = 24 cm) typically 2 to 5 cm Geometric relationship between satellite, antenna, and surroundings Same pattern in same satellite geometry on consecutive days

51 h ø ø Figure 1 Multipath Description
Multipath Delay : (meters) T = 2hSinø c Multipath Freq. : (Cycles/hr) d(T )ƒ~ hCosø dt  h d ø ~ 2 rad. dt hr. ø ø Figure 1 Multipath Description August Ionospheric refraction and Multipath Effects in GPS Carrier Phase Observations Yola Georgiadou and Alfred Kleusberg IUGG 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 -

53 Differential GPS Positioning

54 DGPS Radio Beacon Systems

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

56 WAAS-Wide Area Augmentation System

57 WAAS-Wide Area Augmentation System
Satellite based augmentation system Developed by FAA & DOT for precision flight approaches Approximately 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.

59 Postprocessing / Real-time
Before After

60 Carrier Phase Positioning
Resolving the integer ambiguity allows phase measurements to be related to distances Nl Distance Nl = Integer Ambiguity Dl = First Partial Wavelength Dl Distance = Nl + Dl

61 Carrier Phase Differencing
( ) 2 1 N - + D = l p Difference Phase Distance Phase Difference

62 Datasheet Basics what's on a datasheet
The NGS Data Sheet See file dsdata.txt for more information about the datasheet. DATABASE = Sybase ,PROGRAM = datasheet, VERSION = 7.56 National Geodetic Survey, Retrieval Date = NOVEMBER 20, 2007 AE8289 *********************************************************************** AE8289 CBN This is a Cooperative Base Network Control Station. AE8289 TIDAL BM This is a Tidal Bench Mark. AE8289 DESIGNATION AE8289 PID AE8289 AE8289 STATE/COUNTY- MN/ST LOUIS AE8289 USGS QUAD - DULUTH (1993) AE8289 AE *CURRENT SURVEY CONTROL AE8289 ___________________________________________________________________ AE8289* NAD 83(2007) (N) (W) ADJUSTED AE8289* NAVD (meters) (feet) ADJUSTED AE8289 EPOCH DATE AE8289 X , (meters) COMP AE8289 Y ,373, (meters) COMP AE8289 Z ,624, (meters) COMP AE8289 LAPLACE CORR (seconds) DEFLEC99 AE8289 ELLIP HEIGHT (meters) (02/10/07) ADJUSTED AE8289 GEOID HEIGHT (meters) GEOID03 AE8289 DYNAMIC HT (meters) (feet) COMP AE Accuracy Estimates (at 95% Confidence Level in cm) AE8289 Type PID Designation North East Ellip AE AE8289 NETWORK AE AE8289 MODELED GRAV , (mgal) NAVD 88 AE8289 VERT ORDER - FIRST CLASS II AE8289.The horizontal coordinates were established by GPS observations AE8289.and adjusted by the National Geodetic Survey in February 2007. what's on a datasheet Position Accuracy How to locate and identify help files (dsdata.txt, glossary, …)

63 W GEODETIC CONTROL BASICS: 1) Position? 2) Accuracy?
95% confidence interval W NAD 83(2007) (N) (W)

64 A horizontal plane parallel with the ellipsoid
horizontal accuracy radius north a a b W W east W NGS 2007 textbook FGDC 1998 A horizontal plane parallel with the ellipsoid

65 vertical accuracy W 95% confidence interval
--- NAVD (meters)

66 tie to adjacent stations?
old vs new accuracies old order (1st order class II) new order (1 cm) distance dependent? YES angles, distances, leveling have proportional errors. NO “rigid” GNSS network. tie to adjacent stations? ties are required. local accuracy is computed. accuracy metric? TECHNIQUE equipment resolution, design, technique, closure. STATISTICS observation quality, adjustment results.

67 Datasheet W North = 0.86 cm East = 0.69 cm Ellip = ± 1.45 cm
FZ1046 *********************************************************************** FZ *CURRENT SURVEY CONTROL FZ1046 ___________________________________________________________________ FZ1046* NAD 83(2007) (N) (W) ADJUSTED FZ1046* NAVD (meters) (feet) LEVELING FZ1046 FZ Accuracy Estimates (at 95% Confidence Level in cm) FZ1046 Type PID Designation North East Ellip FZ FZ1046 NETWORK FZ1046 E FZ1046 VERT ORDER - THIRD ? North = 0.86 cm East = 0.69 cm (N) (W) (meters) W Ellip = ± 1.45 cm

68 using local & network accuracies
Network = CORS %w %a wz xy wx yz “%z” “aw” “bx” ad bc cd ab “%d” z w a d y x b c “%y” “%x” “%b” “%c”

69 using local & network accuracy horizontal example
0.0 cm CORS CORS network 5.5 cm North East North East Ellip network 0.86 cm local 2 cm “local” 5.91 cm “network” 5.85 cm

70 Readjustment: What changed?

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