gps technical aspects final - pennsylvania state universitypersonal.psu.edu/cdg3/gps technical...

GNSS Technical Aspects 1/9/2015

Charles "Chuck" Ghilani 1

GPS Technical Aspects

Charles Ghilani ([email protected])

Class Etiquette

• Turn off all cell phones– Or set them to vibrate

• Ask questions at any point during the class.– Simply speak up so that all can hear your question



Outline

• Observational Errors

• Localization

• Low Distortion Projections

• Processing Data

• GPS Modernization

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 5.2

GNSS Range Errors

• 1 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



Future Code Errorswith 2 more codes available

Error Source 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 10° – 15°

– 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



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



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



Precise Ephemeris

• The most accurate ephemeris with all corrections and tracking station used

• Latency: 12 – 14 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



Errors in Ephemerides

igscb.jpl.nasa.gov/components/prods.html

1 ns in time = 30 cm in range error!

Where to Get Precise Ephemerides

www.ngs.noaa.gov



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



IGS Orbit Links

Precise ephemerides

ultra-rapid/rapid ephemerides

Precise Ephemerides

Sunday

Monday

.Z means it is a zipped file



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



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 wasextended to address the special transformations of those distortedpositions to the HARN realization. So use NADCON to transformNAD83(1986) positions to the NAD83(HARN) and then use GEOCONand GEOCON11 to transform NAD83(HARN) to NAD83(2007) andNAD83(2011). For transformations between different epochs other thanthose of the HARN, 2007 and 2011, HTDP can then be used. The epochof the NAD83(2011) is 2010.0, for NAD83(2007) it is 2002.0 except inCA, WA, OR, NV and AZ where it is 2007.0. For the HARN, the situationis much more complex. The attached figure gives an approximate idea ofthe epoch for each state. In some states, there are multiple epochs, butsince the original NAD83(1986) are so distorted, you can safely take thefirst epoch to the left to be the epoch of the HARN in that state.



Process Outlined

NAD83 (1986) NADCON NAD83 (HARN)

GEOCON11 NAD83 (2011)NAD83 (2007)

GEOCON

Then use HTDP to go between different epochsand datums



With Some Badgering

With Some Badgering



To download these slides

• www.personal.psu.edu/cdg3/PSLS.pdf

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 http://www.onlyaboutall.com/articles-science/northern-light



Solar Activity• Space Weather

– 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

http://www.kowoma.de/en/gps/errors.htm

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– http://www.swpc.noaa.gov



Predicted Solar Activity

Predicted Activity



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



GPS Community Dashboard• Available at http://www.swpc.noaa.gov/communities/global-

positioning-system-gps-community-dashboard

Geomagnetic Storms• Avoid surveying when Kp index is 7 or higher (G3)



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



Space Weather

Subscription Service

Bad time for GNSS survey



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



Multipathing Conditions

Outline

• Observations

• Localization


• Processing Data




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 2005.0)

Reference Frames

• WGS 84 and ITRF 2008 agree at the cm-level.– 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!



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!

Z Axis

X Axis

Y Axis

(X,Y,Z) = P (,,h)

h

Earth’sSurface

ZeroMeridian

Mean Equatorial Plane

Reference Ellipsoid

P

GNSS-Derived Coordinates



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



What Really Happens

• Create oblique stereographic map coordinates from GNSS-derived 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: k0, which is

normally ≈ 1.0 but

1

– Central projection point (φ0, λ0)



Stereographic Projection

• Function for computing conformal latitude

χ 2 atan1 sin

1 sin

1 sin

1 sinφ90°

χ 2 atan tan4 2

1 sin

1 sinφ

/

90°

• Another common functioncos

1 sin

Direct Problem• Given: φ, λ

• Find: N and E

• Solution–

–

– cos χ sin λ λ

– cos χ sin χ sin χ cos χ cos λ λ



ExampleThe following geodetic coordinates are observedusing GNSS methods. What are the obliquestereographic map projection coordinates forStation A using a grid origin of (41°18’15”N,76°00’00”W)? (Use WGS84 ellipsoidal parameters.)

1 1299.2665

6,371,0001.0000469732

Station Latitude Longitude H (m)A 41°18'09.88223"N 75°59'58.05637"W 282.476B 41°18'21.11176"N 76°00'37.35445"W 296.571C 41°18'19.33293"N 75°59'40.39279"W 313.814D 41°18'09.67030"N 75°59'44.19645"W 304.205

average 299.2665

Solution

• WGS 84 parameters– a = 6,378,137 m

– e = 0.8181919084

• Compute zone constants

• χ 2 atan tan 45°° °

°

/

90°

41°06 48.6629753

•°

°0.7523140240



Direct Solution

• For station A: – φ = 41°18'09.88223"; λ = −75°59'58.05637"

– χ 2 atan tan/

90°

= 41°06′43.5497889″

–

= 6,369,172.5102 m

– cos χ sin χ sin χ cos χ cos λ λ 157.890m

– cos sin λ λ 45.218m

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



2D Conformal

Y

X

B

C

A

• Converts (b) to (a) and in the process transforms new points 1 – 4.

1 3

42A

B C

(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



2D Rotation• Rotation angle θ is a

clockwise angle

• x′ = xp cos θ − yp sin θ

• y′ = xp sin θ + yp cos θ

• Then scale the rotated coordinates

• sx′ = s(xp cos θ − yp sin θ)

• sy′ = s(xp sin θ + yp cos θ)

• And translate

θ θ

xpsinθxp

yp

xpcosθ

Y YN

XN

X

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 + vX

bx + ay + d = Y + vY

– This makes the equations linear!

1 0

0 1

x

y

vx y X

vYcx

b

y

a

d



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 (re and rn) about the center of the control and a translation (T0)

– 4 points recommended

Two-Step Approach• Vertical components

– Must compensate for deflection of the vertical (2 rotations) and translation between data

Re NGPS + Rn EGPS + T = HLocal − HGPS + ν

where• Re is the deflection of the vertical component in the

easting (in radians)

• Rn is the deflection of the vertical component in the northing (in radians)

• T is the translation between the data

• v is the residual error



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



Proper Configuration for Control AlignmentHorizontal control Vertical control

IIV

III II

Bridge

Don’t forget to include control on important project features

What Not to Do!Horizontal control Vertical control

IIV

III II

Bridge

DO NOT USE KNIFE-EDGE CONTROL!OR FAIL TO INCLUDE CONTROL ON IMPORTANT FEATURE!



Why???

• Error Propagation!

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

• The transformation and the computed uncertainties (in feet) are

Direction of ControlDire

ctio

n of

poin

ts to

tra

nsfo

rm



Transformation Results

Transformed Control Points (units in feet)POINT X Y VX VY

A 1.401 1.78 −0.037 −0.016B 83.296 59.154 0.063 0.012C 165.242 116.507 0.011 0.038D 247.149 173.863 −0.026 −0.026

Transformation parameters and uncertaintiesa 0.81932 ±0.0002b 0.57347 ±0.0002Tx 1.361 ±0.0366Ty 1.796 ±0.0367

Rotation 34°59'22.4"Scale 1.00008

Transformed Points

POINT X Y ±Sx ±SyO0 1.361 1.796 ±0.073 ±0.073O1 -55.987 83.728 ±0.082 ±0.082O2 -113.334 165.661 ±0.108 ±0.106O3 -170.682 247.593 ±0.139 ±0.135O4 -228.029 329.526 ±0.174 ±0.169O5 -285.376 411.458 ±0.210 ±0.204O6 -342.724 493.39 ±0.249 ±0.241O7 -400.071 575.323 ±0.286 ±0.278O8 -457.419 657.255 ±0.325 ±0.314O9 -514.766 739.188 ±0.363 ±0.353O10 -572.114 821.12 ±0.402 ±0.390

@95%



Plot of Errors in CoordinatesSx and Sy are in units of feet

Distance from X axis in feet

Unc

erta

inty

in fe

et

±0.000

±0.050

±0.100

±0.150

±0.200

±0.250

±0.300

±0.350

±0.400

±0.450

Sx

Sy

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



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 vN (m) vE (m) vH (m)

1 −0.009 0.004 0.006

2 −0.010 −0.035 0.001

3 −0.014 0.038 −0.002

4 0.034 −0.007 −0.003



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 (0.0164 ft) , 0.5 ppm, and setup errors (0.003 ft?)– (constant error in mm), scaling error (ppm), setup errors

– @95% (multiplier approximately 2); @99.7% (×3)

2 0.01640.5

10 2 0.003

– Residuals should be under a value of

– or

3



Reviewing Residuals

• Compute horizontal error as

• Compare against specifications at 99.7% level– You are looking for blunders now

Station vN (m) vE (m) vne (m) vH (m)

1 −0.009 0.004 0.010 0.006

2 −0.010 −0.035 0.036 −0.001

3 −0.014 0.038 0.040 −0.002

4 0.034 −0.007 0.035 −0.003


• 3 2

• 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

Station vN (mm) vE (mm) vne (mm) vH (mm)

1 −9 4 9.8 6

2 −10 −35 36.4 −1

3 −14 38 40.5 −2

4 34 −7 34.7 −3



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 error1 (mm) Total Error2 (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

1Centering error computed assuming 2-m rod as 2000.

2 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 1or2




• Assume 5000 m (5,000,000 mm) from base in RTK survey

– 3 2 1.2 10 5,000,000 33.9mm

• 3 out of 4 horizontal locations don’t pass!

– 3 2 3 15 5,000,000 49.1mm

• All pass

• Is rod hand held and not supported?

Station vN (mm) vE (mm) vne (mm) vH (mm)

1 −9 4 9.8 6

2 −10 −35 36.4 −1

3 −14 38 40.5 −2

4 34 −7 34.7 −3

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!



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



Outline

• Observations

• Localization


• Processing Data


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




• 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)


• 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




• 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


• 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?




• 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


• 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



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



• False Easting

• False Northing

• Scale Factor

*Single Parallel Lambert not always supported

nationalatlas.gov/articles/mapping/a_projections.html



Oblique Mercator

• Hotine AKA Rectified Skewed Orthomorphic (RSO)

• Extent other than cardinal



• Azimuth of positive skew axis at local origin

• False Easting

• False Northing

• Scale Factornationalatlas.gov/articles/mapping/a_projections.html


• 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




• 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


• 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)




• 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


• How do I use a LDP?– Define new coordinate system

– Specifics depend on software

– Several commercial packages can be used to create LDP



Example

Projection Type

Example

Longitude of origin



Example

Scale = 1 + h/Re

Example

Latitude of origin



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



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

Summary on LDPs

• Easy to create– 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



Outline

• Observations

• Localization


• Processing Data


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



NGS Antenna Calibration Data

Link to calibration data

www.ngs.noaa.gov

NGS Calibration Data@ http://www.ngs.noaa.gov/ANTCAL/



Select Your AntennaPartial listing of Topcon antennas

Sample Calibration Data



TRIMBLE NGS Calibration File;PCT converted from <ant_info.006> <MLM-04/01/23=156>

;Processor name : Joe Gabor

;Creation time : Wed Mar 31 19:46:15 2004

;Calibrated antenna : TPS GR3

;Mean phase center (mm) North East Up

L1NominalOffset = -0.4 -0.6 233.8

L2NominalOffset = 0.1 -0.5 225.2

;Elevation range (deg) Start Stop Step

ElevationRange = 0 90 5

;Azimuth step size (deg)

AzimuthStep = 0

;Azimuth/elevation corrections (mm)

AZ=0

;L1

0.0 -3.3 -4.4 -3.9 -2.6 -3.3 -0.7 1.1 2.7 3.7 4.0

3.4 1.9 -0.4 -3.3 -6.7 -10.3 -13.6 0.0 0.0

;L2

0.0 -1.7 -2.7 -3.0 -2.9 -2.5 -2.0 -1.4 -1.0 -0.6

-0.6 -0.9 -1.6 -2.9 -4.6 -6.9 -9.8 0.0 0.0

DO NOT USE!

Out of Date!

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



Analysis of GNSS Azimuths

• Accuracy of azimuths from GNSS surveys vary greatly with lengths of lines

• Static survey at 68%– 3 mm + 0.5 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"/

2 cmlength



Error in GNSS Azimuths

Length (m) SAz Length (m) SAz Length (m) SAz

100 ±41.3″ 900 ±4.6″ 1700 ±2.4″

200 ±20.6″ 1000 ±4.1″ 1800 ±2.3″

300 ±13.8″ 1100 ±3.8″ 1900 ±2.2″

400 ±10.3″ 1200 ±3.4″ 2000 ±2.1″

500 ±8.3″ 1300 ±3.2″ 2100 ±2.0″

600 ±6.9″ 1400 ±2.9″ 2200 ±1.9″

700 ±5.9″ 1500 ±2.8″ 2300 ±1.8″

800 ±5.2″ 1600 ±2.6″ 2400 ±1.7″

Static survey accuracy of ±2 cm at 99.7%

Plot

length of line (m)

Azi

mut

h er

ror

in s

econ

ds



Error in GNSS Azimuths

Length (m) SAz Length (m) SAz Length (m) SAz

100 ±123.8″ 900 ±13.8″ 1700 ±7.3″

200 ±61.9″ 1000 ±12.4″ 1800 ±6.9″

300 ±41.3″ 1100 ±11.3″ 1900 ±6.5″

400 ±30.9″ 1200 ±10.3″ 2000 ±6.2″

500 ±24.8″ 1300 ±9.5″ 2100 ±5.9″

600 ±20.6″ 1400 ±8.8″ 2200 ±5.6″

700 ±17.7″ 1500 ±8.3″ 2300 ±5.4″

800 ±15.5″ 1600 ±7.7″ 2400 ±5.2″

RTK GNSS accuracy of ±6 cm* @ 99.7%

*Assuming reported accuracy of ±10 mm @ 68%;@ 99.7% is ±3 cm

Plot

length of line (m)

Azi

mut

h er

ror

in s

econ

ds



Summary

• The vertical center of the antenna changes with the altitude to the satellite

• Always use the NGS calibration data when post-processing

• No matter what GNSS software may state, a long line is necessary to get an accurate azimuth from a GNSS survey

Outline

• Observations

• Localization


• Processing Data




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


• L5 = 115 f0– 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




• 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


• 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 extra-wide lane processing

• CL code will have P-code accuracies

• P code will be replaced by M codes



Future

• Four operational satellite positioning systems by about 2020– GPS

– GLONASS

– Galileo

– Beidou

• Expect 30 – 40 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



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.

gps technical aspects final - pennsylvania state universitypersonal.psu.edu/cdg3/gps technical...

Documents