GPS Technical Aspects

Transcription

1 GPS Technical Aspects Charles Ghilani Turn off all cell phones Or set them to vibrate Class Etiquette Ask questions at any point during the class. Simply speak up so that all can hear your question Charles "Chuck" Ghilani 1

2 Observational Errors Localization Low Distortion Projections Processing Data GPS Modernization Outline GNSS Range Errors Error Source Maximum Value (m) Ionospheric refraction 4.0 Ephemeral errors 2.1 Clock 2.1 Troposphere 0.7 Receiver 0.5 Multipath 1.0 Uncorrelated Error Total ns of clock error results in a 30 cm range error Clock and hardware errors are eliminated by differencing Ephemeral errors can be eliminated by using a precise ephemeris in post-processing Avoid multipathing conditions Charles "Chuck" Ghilani 2

3 Error Source Future Code Errors with 2 more codes available Error Size (m) Satellite Clock and Ephemeris Errors ±2.3 Ionospheric refraction ±0.1 Tropospheric refraction ±0.2 Receiver Noise ±0.6 Other (multipathing, etc.) ±1.5 Error in Sum ±2.9 Error Sources Refraction Can be largest error in range observations (±1 4 m) Can be reduced by elevated "mask" angle 10 to 20 used to avoid lower satellites Suggestion: Collect at 10 and process at Much of the error can be removed from equations if two frequencies are available On code-based positions L2C and L5 will allow code-based units to do this Charles "Chuck" Ghilani 3

4 Ephemerides Ephemeris provides positions of satellites with respect to time This is your control Four types of ephemerides Broadcast ephemeris Ultra-rapid ephemeris Rapid ephemeris Precise ephemeris Broadcast Ephemeris Near-future prediction of the satellite s orbital position Part of the navigation message Uploaded twice daily Contains Keplerian parameters at a single epoch in time and rates of change of parameters Computed accuracy about ±0.1 m in position ± 5 nanoseconds in time ±0.15 m in range error Charles "Chuck" Ghilani 4

5 Ultra-Rapid Ephemeris Published 4 times daily; contains 48 hrs of data Two components First half computed from observations Second half predicted Accuracy orbit: ±5 cm (predicted) and ±3 cm (observed) time: ±3 ns in time (predicted) and ±150 ps (observed) ±0.90 m in range error and ±0.045 m As tracking stations make their data available accuracies are improved Rapid Ephemeris Satellite positions given in ECEF coordinates in 15 min intervals Latency: 1 2 days Accuracy ±2.5 cm in position ±75 ps in time ± 22.5 mm in range error Charles "Chuck" Ghilani 5

6 Precise Ephemeris The most accurate ephemeris with all corrections and tracking station used Latency: days Accuracy 2.5 cm in position ±75 ps in time ±22.5 mm in range error Comments on Ephemerides All three precise ephemerides provide sufficient accuracy for typical surveying applications. Should use one of the precise ephemerides for post-processing data when available Accuracy of baseline is approximately Broadcast errors vary by satellite Charles "Chuck" Ghilani 6

7 Errors in Ephemerides igscb.jpl.nasa.gov/components/prods.html 1 ns in time = 30 cm in range error! Where to Get Precise Ephemerides Charles "Chuck" Ghilani 7

8 Types of Data The NGS in June 30, 2012 changed to IGS08 (epoch 2010) The DoD changed the broadcast ephemeris in GPS week 1674 (2/1/2011) to ITRF08 (epoch 2005) as realized by the DoD tracking station! No GNSS ephemeris is the original NAD83 nor the current NAD 83 (2011) GPS File Naming Convention igrnnnnx.aaa, where igr = International GPS Service rapid ephemeris igu = ultra-rapid; igs = precise nnnn = GPS week number, e.g.1775, 1776,... x = day of the week where Sunday = 0, Monday = 1,..., Saturday = 6 aaa = file type where sp3 is the precise ephemeris erp is the Earth rotation parameters Files are zipped Example Wednesday, January 20, 2014 is igr17763.sp3 Charles "Chuck" Ghilani 8

9 IGS Orbit Links ultra-rapid/rapid ephemerides Precise ephemerides Precise Ephemerides Sunday Monday.Z means it is a zipped file Charles "Chuck" Ghilani 9

10 What Does This Mean? The NGS or IGS precise ephemerides are computed using IGS08 And before that ITRF 2005! Thus your coordinates are not NAD83 GPS coordinates from earlier surveys are not in agreement with today s coordinates! Broadcast ephemeris (DoD) uses ITRF2008 You must localize GNSS coordinates to get NAD83 coordinates or GNSS coordinates from earlier surveys One Method Take your existing coordinates and transform them into the current realization NAD83 (2011) Be aware that the NGS HTDP can not always be used. Contact with NGS provided the following information Charles "Chuck" Ghilani 10

11 NGS HTDP Does not transfer previous NAD83 realizations Implies that NAD83 (CORS96) = NAD83 (2007) = NAD83 (2011) Converting NAD83 (2007) to (2011) The NAD83(1986) positions are highly distorted. Program NADCON was extended to address the special transformations of those distorted positions to the HARN realization. So use NADCON to transform NAD83(1986) positions to the NAD83(HARN) and then use GEOCON and GEOCON11 to transform NAD83(HARN) to NAD83(2007) and NAD83(2011). For transformations between different epochs other than those of the HARN, 2007 and 2011, HTDP can then be used. The epoch of the NAD83(2011) is , for NAD83(2007) it is except in CA, WA, OR, NV and AZ where it is For the HARN, the situation is much more complex. The attached figure gives an approximate idea of the epoch for each state. In some states, there are multiple epochs, but since the original NAD83(1986) are so distorted, you can safely take the first epoch to the left to be the epoch of the HARN in that state. Charles "Chuck" Ghilani 11

12 Process Outlined NAD83 (1986) NADCON NAD83 (HARN) GEOCON NAD83 (2007) GEOCON11 NAD83 (2011) Then use HTDP to go between different epochs and datums Charles "Chuck" Ghilani 12

13 With Some Badgering With Some Badgering Charles "Chuck" Ghilani 13

14 To download these slides What is Space Weather? Solar activity results in ejection of material from the sun Results in solar winds Most commonly known event from this is the northern lights Results from excited oxygen atoms Image from Charles "Chuck" Ghilani 14

15 Space Weather Solar Activity Ionospheric refraction Signal refracts (bends) when traveling through the ionosphere resulting in an elongated signal Dual (or multiple) frequencies are used to estimate this error Error can be in the range of 1 to 4 meters Expect higher values in this year Solar Activity Space Weather/Ionospheric Activity can result in greater refraction or even severe degradation of the signal Sun Spots Activity is cyclical and occurs in 11-year cycles Currently in peak period (2012 to 2014) NOAA Space Weather Prediction Center Charles "Chuck" Ghilani 15

16 Predicted Solar Activity Predicted Activity Charles "Chuck" Ghilani 16

17 Space Weather Now Avoid satellite surveying when any are listed as strong or worse (extreme) Geomagnetic storms may cause satellite orientation problems and communication problems (G3 G5) Solar radiation storms may create problems with satellite operations, orientation, and communications (S4 S5) Radio blackouts may cause intermittent loss of satellite and radio communications (R3 R5) Possible radio problems at R2 level 2002 Experiment Had 2 students do an independent study to determine the point precision of GPS without selective availability Required one 2-hr session with a PDOP spike Errors in position did not match PDOP! PDOP Errors in position Charles "Chuck" Ghilani 17

18 GPS Community Dashboard Available at Geomagnetic Storms Avoid surveying when Kp index is 7 or higher (G3) Charles "Chuck" Ghilani 18

19 Solar Radiation Storms Avoid surveying when flux level 10,000 (S4) Radio Blackouts Avoid surveying when X-ray peak is R3 or hieher R2 periods may cause radio problems Charles "Chuck" Ghilani 19

20 Space Weather Subscription Service Bad time for GNSS survey Charles "Chuck" Ghilani 20

21 Project Planning Site Recon Is the site suitable for GPS? Google Earth or Site Visit Avoid sites with canopy RTK & Kinematic: Avoid sites with signal obstructions Bridges, overpasses, etc. Multipathing Reception of an elongated signal Signal bounces off a surface before being received by antenna Results in a longer signal path Avoid sites with multipath issues Urban Canyons Buildings, fences, reflective surfaces, parked vehicles, etc. Antennas with large ground planes or choke ring can help reduce multipath Charles "Chuck" Ghilani 21

22 Multipathing Conditions Observations Localization Low Distortion Projections Processing Data GPS Modernization Outline Charles "Chuck" Ghilani 22

23 Reference Frames The NGS is currently using NAD83 (2011) for GPS Based on IGS08 (epoch 2010) IGS08, which is the IGS realization of ITRF2008 The DoD is currently using WGS84 (G1674) Based on ITRF2008 (epoch ) Reference Frames WGS 84 and ITRF 2008 agree at the cmlevel. DoD states transformation parameters are 0 NAD 83 and WGS 84 disagree by about 1.5 m (~5 ft) in their origin JUST TELLING YOUR RECEIVER/SOFTWARE TO PROVIDE NAD83 COORDINATES WILL NOT WORK! Charles "Chuck" Ghilani 23

24 Localization of Survey Brings WGS 84 (G1674) coordinates into NAD 83 (2011) datum or any other datum of choice Options used by manufacturers Three-dimensional transformations Helmert Molodensky Both require 3D local coordinates» such as SPCS + Elevation 2-D + 1-D transformations Most common since it can use any coordinate system» Including arbitrary systems! GNSS-Derived Coordinates Z Axis h P (X,Y,Z) = P (,,h) Earth s Surface Zero Meridian Reference Ellipsoid Y Axis X Axis Mean Equatorial Plane Charles "Chuck" Ghilani 24

25 What Really Happens! You occupy control points in your local/arbitrary coordinate system with receiver The receiver determines the geodetic position of the occupied points in the WGS 84 Software then converts the latitude and longitude to oblique stereographic map projection coordinates (N,E) What Really Happens Software applies the geoid model to the GNSS-derived geodetic height as where h is the GNSS-derived geodetic height N is the geoidal height at the point (modeled) H is the GNSS/geoid model-derived orthometric height Important to use geoid model (GEOID12A) since this represents a systematic error Failure to do so will result in part of the model appearing as residuals Charles "Chuck" Ghilani 25

26 What Really Happens Create oblique stereographic map coordinates from GNSSderived geodetic coordinates Convert the stereographic map projection coordinates using a 2D conformal coordinate transformation into local/arbitrary coordinates 2 points required 4 or more recommended Stereographic Projection Defining parameters are semimajor axis, a, and eccentricity, e, for ellipsoid Scale factor at origin: k 0, which is normally 1.0 but 1 Central projection point (φ 0, λ 0 ) Charles "Chuck" Ghilani 26

27 Stereographic Projection Function for computing conformal latitude χ 2atan 1 sin 1 sin 1 sin 1 sinφ 90 χ 2atan tan 4 1 sin 2 1 sinφ Another common function cos 1 sin / 90 Given: φ, λ Find: N and E Solution Direct Problem cosχsin λ λ cosχ sin χ sin χ cos χ cos λ λ Charles "Chuck" Ghilani 27

28 Example The following geodetic coordinates are observed using GNSS methods. What are the oblique stereographic map projection coordinates for Station A using a grid origin of ( N, W)? (Use WGS84 ellipsoidal parameters.) Station Latitude Longitude H (m) A 41 18' "N 75 59' "W B 41 18' "N 76 00' "W C 41 18' "N 75 59' "W D 41 18' "N 75 59' "W average ,371,000 WGS 84 parameters a = 6,378,137 m e = Compute zone constants χ 2atan tan Solution / Charles "Chuck" Ghilani 28

29 Direct Solution For station A: φ = 41 18' "; λ= 75 59' " χ 2 atan tan / 90 = = 6,369, m cos χ sin χ sin χ cos χ cos λ λ cos sinλ λ m m 2D Conformal Unknown parameters 1 scale factor 1 rotation Translations in x and y Also called four parameter similarity transformation Used in localization of GNSS surveys to local coordinate systems Charles "Chuck" Ghilani 29

30 2D Conformal Converts (b) to (a) and in the process transforms new points 1 4. Y B C B C 1 3 A X A 2 4 (a) (b) 2D Conformal Must have at least two common control points known in both coordinate systems Four or more are preferable 1. Control should be on the edges of the points to transform 2. The control should lie in each quadrant of the points to be transformed Items 1 & 2 avoid extrapolation and larger transformation errors in transformed points Control points Points to Transform Charles "Chuck" Ghilani 30

31 Rotation angle θ is a clockwise angle x = x p cos θ y p sin θ y = x p sin θ + y p cos θ 2D Rotation Y YN Then scale the rotated coordinates sx = s(x p cos θ y p sin θ) sy = s(x p sin θ + y p cos θ) And translate θ x p θ x p cosθ y p X x p sinθ XN 2D Conformal So the transformation is X=(S cos θ)x (S sin θ)y + Tx Y = (S sin θ)x + (S cos θ)y + Ty Letting a = S cos θ, b = S sin θ, c = Tx, and d = Ty then ax by + c = X + v X bx + ay + d = Y + v Y This makes the equations linear! a 1 0 x y b X v x 0 1 y x c Y v y d Charles "Chuck" Ghilani 31

32 What Really Happens A two-dimensional conformal coordinate transformation is used to convert the GPS-derived oblique stereographic map projection coordinates into your local coordinates 2 points required Vertical control is transformed using two rotations (r e and r n ) about the center of the control and a translation (T 0 ) 4 points recommended Two-Step Approach Vertical components Must compensate for deflection of the vertical (2 rotations) and translation between data R e N GPS + R n E GPS + T = H Local H GPS + ν where R e is the deflection of the vertical component in the easting (in radians) R n is the deflection of the vertical component in the northing (in radians) T is the translation between the data v is the residual error Charles "Chuck" Ghilani 32

33 Two-Step Method To do this procedure properly The control must surround the intended area of the survey. (Some can be in the interior, but the edges are critical to avoid extrapolation) GNSS-derived heights (ellipsoid) must be converted using the latest geoid model to elevations/orthometric heights (H). Recommendations for Control Control must be in proper locations 4 horizontal control points 4 vertical control points 1 point in each quadrant of the project Include any crucial design points such as control for bridges Have additional control points that are NOT included in the localization Use these as checks on the localization and field work Charles "Chuck" Ghilani 33

34 Proper Configuration for Control Alignment Horizontal control Bridge Vertical control Don t forget to include control on important project features IV I III II What Not to Do! Horizontal control Bridge Vertical control DO NOT USE KNIFE-EDGE CONTROL! OR FAIL TO INCLUDE CONTROL ON IMPORTANT FEATURE! IV I III II Charles "Chuck" Ghilani 34

35 Error Propagation! Why??? Example Points (0,0), (0,100),..., (0,1000) are transformed using control (0,0), (100,0), (200,0), and (300,0) are used to define the control for the 2D transformation Direction of points to transform Direction of Control The transformation and the computed uncertainties (in feet) are Charles "Chuck" Ghilani 35

36 Transformation Results Transformed Control Points (units in feet) POINT X Y VX VY A B C D Transformation parameters and uncertainties a ± b ± Tx ± Ty ± Rotation 34 59'22.4" Scale Transformed Points POINT X Y ±Sy O ±0.073 ±0.073 O ±0.082 ±0.082 O ±0.108 ±0.106 O ±0.139 ±0.135 O ±0.174 ±0.169 O ±0.210 ±0.204 O ±0.249 ±0.241 O ±0.286 ±0.278 O ±0.325 ±0.314 O ±0.363 ±0.353 O ±0.402 ±0.390 Charles "Chuck" Ghilani 36

37 Plot of Errors in Coordinates Sx and Sy are in units of feet Uncertainty in feet ±0.450 ±0.400 ±0.350 ±0.300 ±0.250 ±0.200 ±0.150 ±0.100 ±0.050 ±0.000 Sx Sy Distance from X axis in feet Remember Extrapolation of data is bad! Interpolation of data is GOOD! Perform localization only once per project Failure to do so will result in you creating different realizations of the same coordinates Caused by errors in GPS observations Charles "Chuck" Ghilani 37

38 Proper Configuration for Control You need the control to surround the project area Horizontal control (4 or more recommended) to properly define scale and rotation With at least one in each quadrant of project Vertical control (4 or more recommended) to properly orient level surface. Need to define deflection of vertical components and provide a stable surface To isolate blunders in control Be sure to include any important project features! Field Localization of Survey Occupy local control with receiver Review residuals and correct if residuals are outside of range estimated from GNSS What you will see when 3 or more control are occupied Station v N (m) v E (m) v H (m) Charles "Chuck" Ghilani 38

39 Are the Residuals Acceptable? What are acceptable residuals? That depends!!! How good is your local control? How good are the GNSS-derived coordinates? How good are your setups? Are the Residuals Acceptable? Static survey (constant + ppm) and setup errors 5 mm ( ft), 0.5 ppm, and setup errors (0.003 ft?) (constant error in mm), scaling error (ppm), setup (multiplier approximately ( 3) Residuals should be under a value of or 3 Charles "Chuck" Ghilani 39

40 Reviewing Residuals Compute horizontal error as Compare against specifications at 99.7% level You are looking for blunders now Station v N (m) v E (m) v ne (m) v H (m) Field Localization of Survey Station v N (mm) v E (mm) v ne (mm) v H (mm) where 3 is multiplier for 99.7% (95% has a multiplier of 2) Setup error is the estimated error in centering over the point Typically 1 3 mm Constant and ppm depends on type of survey and manufacturer s specifications d is distance from base Charles "Chuck" Ghilani 40

41 Setup Error Analysis Typical mark is about 2 mm So centering rod horizontally should be within ±1 mm Also affected by centering of circular bubble Misleveling (minutes) Centering error 1 (mm) Total Error 2 (mm) ±1 ±0.6 ±1.2 ±2 ±1.2 ±1.5 ±3 ±1.8 ±2.0 ±4 ±2.3 ±2.5 ±5 ±2.9 ±3.1 1 Centering error computed assuming 2-m rod as Total error computed as 1 Setup Error Analysis Typical depth of mark is 2 mm This is a constant error in vertical Height of rod to ARP typically within ±1 2 mm Can be measured with some error Vertical setup error estimated at ±3 4 mm or more! Computed as 2 1 or 2 Charles "Chuck" Ghilani 41

42 Field Localization of Survey Station v N (mm) v E (mm) v ne (mm) v H (mm) Assume 5000 m (5,000,000 mm) from base in RTK survey ,000, mm 3 out of 4 horizontal locations don t pass! ,000, mm All pass Is rod hand held and not supported? Summary on Localization You need the control to surround the project area Horizontal control (4 or more recommended) to properly define scale and rotation At least one in each quadrant of project Include important design points! Charles "Chuck" Ghilani 42

43 Summary on Localization You need the control to surround the project area Vertical control (4 or more recommended) to properly orient level surface. At lease one in each quadrant of the project Should be at/near edges of project Include a GEOID model to remove systematic errors Be sure to include any important design points! Summary on Localization Never perform a localization more than once for a project Doing it more than once will Create different realizations of the transformation You will be creating multiple coordinate systems Isolate and remove/correct any blunders Charles "Chuck" Ghilani 43

44 Observations Localization Low Distortion Projections Processing Data GPS Modernization Outline Low Distortion Projections The problem with Map Projections All map projections introduce some distortion Curved surface to plane Conformal projections distort distances Map projection origin is typically nowhere near the project Increases distortion Map projections are typically at/near sea level Elevation increases distortion A problem of Grid vs Ground Charles "Chuck" Ghilani 44

45 Low Distortion Projections Doesn t localization/site calibration solve this problem? Typically only used for small areas Should only use one localization per project File/Parameters may not be compatible with other software (present or future) Not always appropriate for large projects Corridor projects Phased projects Projects with multiple consultants and/or crews May not be linked to the National Spatial Reference System (NSRS) Low Distortion Projections What is a Low Distortion Projection (LDP)? Conformal map projection Maintains correct depiction of distances and azimuths Correct depiction of shapes is important in surveying Projection origin is at or near project center Projection is at or near project elevation This removes the ground vs grid problem Charles "Chuck" Ghilani 45

46 Low Distortion Projections Advantages of Low Distortion Projections Distance distortion is minimized Projection can cover a large area Large Project City County Uses standard map projections Easily linked to NSRS or other reference frames Can transform to/from other coordinate systems Low Distortion Projections Disadvantages of Low Distortion Projections Easy to create can lead to numerous LDPs for the same area No central registry for LDPs No requirement to document or provide metadata Short-term solution? Charles "Chuck" Ghilani 46

47 Low Distortion Projections Examples: WISCRS Wisconsin Coordinate Reference System Overcomes issues with WCCS Wisconsin County Coordinate System Enlarged Ellipsoids Projection designed for all 72 counties Some counties share the same projection When combined with Height Modernization there is no need to localize! Also used in several other states/counties/cities Low Distortion Projections How do I create a LDP? Define the project area Choose a projection based on extent Use a conformal projection Many exist but recommend Transverse Mercator Lambert Conformal Conic Single Parallel Oblique Mercator Charles "Chuck" Ghilani 47

48 Transverse Mercator North-South extent Latitude of local origin Longitude of local origin False Easting False Northing Scale Factor nationalatlas.gov/articles/mapping/a_projections.html Lambert Conformal Conic Single Parallel Good for surveys that are long in East-West extents Latitude of local origin Longitude of local origin False Easting False Northing Scale Factor *Single Parallel Lambert not always supported nationalatlas.gov/articles/mapping/a_projections.html Charles "Chuck" Ghilani 48

49 Oblique Mercator Hotine AKA Rectified Skewed Orthomorphic (RSO) Extent other than cardinal Latitude of local origin Longitude of local origin Azimuth of positive skew axis at local origin False Easting False Northing Scale Factor nationalatlas.gov/articles/mapping/a_projections.html Low Distortion Projections How do I create a LDP? Determine an average elevation and geoid height for the project area Choose a central meridian/parallel near the center of the project area Estimate scale factor for central meridian/parallel 1 h is average ellipsoid height for project area R is the radius of the earth Charles "Chuck" Ghilani 49

50 Low Distortion Projections How do I create a LDP? Compute distortion at project boundaries and preferably throughout the project area Is the distortion acceptable? If not, choose a new scale factor, project origin, or maybe projection Low Distortion Projections Recommendations Define Latitude and Longitude to nearest minute or 10 seconds Define Scale Factor to 6 or 8 decimal places Define False Easting and False Northing using a rounded number (100,000 not 102,842.58) Define False Easting and False Northing to avoid conflicts with other coordinate systems (State Plane) Charles "Chuck" Ghilani 50

51 Low Distortion Projections How do I create a LDP? Document it! Linear Units Meters US Survey Foot International Foot Ellipsoid Datum Projection type and parameters Project/Projection name or identifier Low Distortion Projections How do I use a LDP? Define new coordinate system Specifics depend on software Several commercial packages can be used to create LDP Charles "Chuck" Ghilani 51

52 Example Projection Type Example Longitude of origin Charles "Chuck" Ghilani 52

53 Example Scale = 1 + h/re Example Latitude of origin Charles "Chuck" Ghilani 53

54 Example False Easting and Northing Example Best-practices Create LDP for your area Enter values appropriate for your work Document work for future reference Remember you are creating your own coordinate system Charles "Chuck" Ghilani 54

55 Summary on LDPs Low distortion projections place the mapping surface at the level of the ground Avoids the Grid vs Ground problem GNSS will provide distances that match EDM distances Easy to create Summary on LDPs Software can create these projections for you Disadvantage is that the metadata for the LDP must be saved for future use Lose the metadata and you lose the projection Charles "Chuck" Ghilani 55

56 Observations Localization Low Distortion Projections Processing Data GPS Modernization Outline Processing Data Centering of antenna over point Must let software "know" the antenna Electrical center of satellite does not coincide with physical center Antennas calibrated to provide offsets from electrical center to physical center Electrical center varies with altitude of satellite Use NGS calibration data when post-processing Charles "Chuck" Ghilani 56

57 NGS Antenna Calibration Data Link to calibration data NGS Calibration Charles "Chuck" Ghilani 57

58 Select Your Antenna Partial listing of Topcon antennas Sample Calibration Data Charles "Chuck" Ghilani 58

59 TRIMBLE NGS Calibration File ;PCT converted from <ant_info.006> <MLM-04/01/23=156> ;Processor name : Joe Gabor ;Creation time : Wed Mar 31 19:46: ;Calibrated antenna : TPS GR3 ;Mean phase center (mm) North East Up L1NominalOffset = L2NominalOffset = ;Elevation range (deg) Start Stop Step ElevationRange = ;Azimuth step size (deg) AzimuthStep = 0 ;Azimuth/elevation corrections (mm) AZ=0 DO NOT USE! Out of Date! ;L ;L Processing Data Make connections with CORS or HARN stations when possible CORS are almost always possible with long sessions Process baseline data Always use the most precise ephemeris available Process baselines by session to avoid trivial baselines Charles "Chuck" Ghilani 59

60 Analysis of GNSS Azimuths Accuracy of azimuths from GNSS surveys vary greatly with lengths of lines Static survey at 68% 3 mm ppm RTK survey 10 mm + 1 ppm Multiply computed values by 3 for 99.7% Analysis of GNSS Azimuths For static surveys assume ±1 cm error in position Error in azimuth. 206,264.8"/ length 2 cm Charles "Chuck" Ghilani 60

61 Error in GNSS Azimuths Static survey accuracy of ±2 cm at 99.7% Length (m) S Az Length (m) S Az Length (m) S Az 100 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±1.7 Plot Azimuth error in seconds length of line (m) Charles "Chuck" Ghilani 61

62 Error in GNSS Azimuths RTK GNSS accuracy of ±6 cm 99.7% Length (m) S Az Length (m) S Az Length (m) S Az 100 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±5.2 * Assuming reported accuracy of ± % is ±3 cm Plot Azimuth error in seconds length of line (m) Charles "Chuck" Ghilani 62

63 Summary The vertical center of the antenna changes with the altitude to the satellite Always use the NGS calibration data when postprocessing No matter what GNSS software may state, a long line is necessary to get an accurate azimuth from a GNSS survey Observations Localization Low Distortion Projections Processing Data GPS Modernization Outline Charles "Chuck" Ghilani 63

64 Modernization of GPS By 2022 the modernized GPS will be operational (est. by DoD) Includes 1 new frequency, L5 2 new codes (CM and CL) Increased power in signals due to new processing techniques Should be able to receive signals in canopy conditions Modernization of GPS L5 = 115 f 0 λ = 25 cm C codes being added (CM and CL) Power of signal will increase 251 times! Most of this is due to better processing capabilities Charles "Chuck" Ghilani 64

65 Modernization of GPS Future Civilian receivers will be able to make Real-time atmospheric corrections Better ambiguity resolution with extra wide-lane processing Receive signal in canopy New navigation message (CNAV) Now loaded but not available except for testing Modernization of GPS Will result in better code solutions and faster/better carrier phase-shift solutions Code-based solutions Some estimate solutions within ±10 cm Today this is 5 10 m Faster ambiguity resolution due to extrawide lane processing CL code will have P-code accuracies P code will be replaced by M codes Charles "Chuck" Ghilani 65

66 Future Four operational satellite positioning systems by about 2020 GPS GLONASS Galileo Beidou Expect satellites in view at all times Future? Signals expected to be accessible in canopy conditions All boundary surveys can be done with satellite positioning Future boundary surveys can be defined by ITRFxx/NGS coordinates Fixed in future Charles "Chuck" Ghilani 66

67 Future Technology will change the way we do things! Questions? T F 1. Localization/site calibration is the process of putting GNSS coordinates into a local/arbitrary coordinate system T F 2. We are in a peak period of solar activity. T F 3. GNSS surveys should not be performed during an S1 solar storm. T F 4. A low distortion projection minimizes the differences between the grid and ground distances. T F 5. Azimuths determined by GNSS surveys are always accurate. Charles "Chuck" Ghilani 67