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Using RTK GNSS Wisely

1Todd Horton, PE, PLS

Michigan Society of Professional Surveyors

Using RTK

GNSS Wisely

Todd Horton, PE, PLSFebruary 2017

Autonomous Positioning

Differential PositioningConcept: Detect and cancel identical errors with

simultaneous observation.

F + E = G + E




Static & RTK Computations

Static survey: post process data in office

RTK survey: process data immediately at rover

Key Element = Simultaneous Observation

Ionospheric Effects

Ionospheric effects

cancel when receivers

are within 10 km (6.1 mi).

Real-Time Kinematic Positioning

RTK message

Base

receives and

rebroadcasts

GPS data as

RTK

message.

Rover

receives

GPS data

and RTK

message to

compute

vector.




Single Base RTK Corrections

Known

Position

CorrectionCorrection

Corrected

Position

message

Single base unit broadcasts uniform

RTK corrections in all directions.

Single Base RTK Corrections

RTK Positional Error

RTK positional error increases with distance from the base.




Trimble R8 Accuracy Specs

http://trl.trimble.com/docushare/dsweb/Get/Document-140079/022543-079J_TrimbleR8GNSS_DS_1109_LR.pdf

Constant error

Scalar error

RTK Vector Errors

• Vector is the line from base to rover.

– 10 mm + 1 ppm horizontal error for RTK

vectors is typical.

– 10 mm = 0.033 ft = constant error

– 1 ppm = scalar error (distance dependent)

• 1 part error to 1,000,000 parts measurement

• 1 mm error / 1 km RTK vector

• 0.005 ft error / 1 mile RTK vector

RTK Vector Errors

• 1 ppm = scalar error (distance dependent)• 1 part error to 1,000,000 parts measurement

• 1 mm error / 1 km RTK vector

• 0.005 ft error / 1 mile RTK vector

• This error approximates the misfit between

a uniform RTK correction and reality.

– RTN eliminates most of this.




Differential Position Errors

)( 22

scalarconstdist EEE

RTK vector(miles)

E const(feet)

E scalar(feet)

E dist(feet)

1 0.033 0.005 0.033

2 0.033 0.011 0.034

4 0.033 0.021 0.039

8 0.033 0.042 0.053

16 0.033 0.084 0.091

32 0.033 0.169 0.172

RTK relative accuracy

Base can occupy a known point or an unknown (assumed) point.

Absolute & relative accuracy

• Absolute accuracy

– Accuracy relative to

map surface

• Relative accuracy

– Accuracy relative to

adjacent positions

• Position = coordinate




Absolute

accuracy = ±0.07’

A: known B: unknown

Absolute

accuracy = ±0.05’

Absolute

accuracy = ±5’

Absolute & relative accuracy

A: known B: unknown

Relative accuracy = ±0.05’

Absolute

accuracy = ±5’

Relative accuracy = ±0.05’

Poor absolute accuracy

Good relative accuracy

Good absolute accuracy

Good relative accuracy

C Continuously

O Operating

R Reference

S Stations

• CORS is a permanently mounted survey-grade GPS

receiver which broadcasts RTK corrections.

• Each CORS collects GPS data 24 hours per day, 7 days

per week, 365 days per year.

• Stored data from each CORS is available for download

to enable survey post-processing.

Continuously Operating Reference Stations

• The CORS network is

the anchor of our

National Spatial

Reference System.

– Much higher accuracy

than first order

monuments.

– Actively monitored.

– Tracks crustal motion.

– Data is used for research.

http://www.ngs.noaa.gov/CORS/




Regional

CORS

Network

Single Base RTK Solutions

Uniform RTK correction

within each zone

Uniform RTK correction

within each zone

Potential for mismatched corrections

Reference

Station

3

Reference

Station

2

Reference

Station

1

Single Base RTK Solutions

Co

rre

ctio

n 1

Co

rre

ctio

n 2

Co

rre

ctio

n 3

Reference

Station

3Reference Station 2

Reference

Station

1

Uniform correction

within each zone

http://www.ngs.noaa.gov/CORS/




Network RTK (RTN) Solutions

Interpolated RTK

correction within

polygon

Reference

Station

3

Reference

Station

2

Reference

Station

1

Network RTK (RTN) Solutions

Co

rre

ctio

n 1

Co

rre

ctio

n 2

Co

rre

ctio

n 3

Reference

Station

3Reference Station 2

Reference

Station

1

RTK correction

interpolated for rover location

RTN Flavors

• All RTN solutions provide a single vector

solution from a physical reference station.

• Common RTN solution methods:

– Non-physical Reference Station (VRS)

– Master Auxiliary Corrections (MAC)




RTN Solution

1. Data from the reference stations streams to the processing and control center.

2. The RTK rover sends its approximate position.

3. The processing center selects reference stations forming a polygon around the rover.

4. The processing center transmits correction data to the rover.

Master Auxiliary Corrections (MAC)

Rover

position

Correction

data

Streaming GPS data

Auxiliary

Master Auxiliary

Master Auxiliary Corrections (MAC)

Streaming GPS data

Rover computes vector from nearest reference

station.

Master

Auxiliary Auxiliary




MAC & VRS

• MAC networks make

the rover compute

RTN corrections.

• VRS networks

compute RTN

corrections at the

control center.

What is a VRS?

• A virtual reference station is the center of an

area for which an RTN correction applies.

– First VRS is your first initialization point.

– Control center creates new VRS points as you move.

Virtual Reference Station (VRS)

Rover

position

VRS

correction

Streaming GPS data

Virtual Reference

Station




Virtual Reference Station (VRS)

Streaming GPS data

Rover computes vector from nearest reference

station.

Virtual Reference

Station1

Virtual Reference

Station 2

RTN Positional Error

RTK constant

error still applies.

PPM error

(distance dependent)

approaches zero.

Trust

but

Verify




Truth versus Address

• Physical monument

– Truth

– Relatively stable (in most of CONUS)

• Point coordinate

– Merely an address

– Contains error

– Subject to change

New Accuracy Standards

for NGS Datasheets

(http://fgdc.er.usgs.gov/standards/status/swgstat.html)

• Local Accuracy: adjacent points

• Network Accuracy: relative to CORS

• Numeric quantities, units in cm (or mm)

• Both are relative accuracy measures

• Will not use distance dependent expression

• Order/Class codes will no longer be used

OLD: Order & Class Codes

LC1766 ***********************************************************************

LC1766 CBN - This is a Cooperative Base Network Control Station.

LC1766 DESIGNATION - ALEXANDER 2

LC1766 PID - LC1766

LC1766 STATE/COUNTY- IL/PIATT

LC1766 USGS QUAD - SEYMOUR (1970)

LC1766

LC1766 *CURRENT SURVEY CONTROL

LC1766 ___________________________________________________________________

LC1766* NAD 83(1997)- 40 06 20.45303(N) 088 29 42.90776(W) ADJUSTED

LC1766* NAVD 88 - 231.3 (meters) 759. (feet) GPS OBS

LC1766 ___________________________________________________________________

LC1766 X - 128,287.167 (meters) COMP

LC1766 Y - -4,883,624.381 (meters) COMP

LC1766 Z - 4,087,096.112 (meters) COMP

LC1766 LAPLACE CORR- -0.58 (seconds) DEFLEC99

LC1766 ELLIP HEIGHT- 199.36 (meters) GPS OBS

LC1766 GEOID HEIGHT- -31.94 (meters) GEOID99

LC1766

LC1766 HORZ ORDER - B

LC1766 ELLP ORDER - FOURTH CLASS I

http://fgdc.er.usgs.gov/standards/status/swgstat.html




NEW: Relative AccuracyLC1766 ***********************************************************************

LC1766 CBN - This is a Cooperative Base Network Control Station.

LC1766 DESIGNATION - ALEXANDER 2

LC1766 PID - LC1766

LC1766 STATE/COUNTY- IL/PIATT

LC1766 COUNTRY - US

LC1766 USGS QUAD - SEYMOUR (1970)

LC1766

LC1766 *CURRENT SURVEY CONTROL

LC1766 ______________________________________________________________________

LC1766* NAD 83(2011) POSITION- 40 06 20.45302(N) 088 29 42.90701(W) ADJUSTED

LC1766* NAD 83(2011) ELLIP HT- 199.346 (meters) (06/27/12) ADJUSTED

LC1766* NAD 83(2011) EPOCH - 2010.00

LC1766* NAVD 88 ORTHO HEIGHT - 231.3 (meters) 759. (feet) GPS OBS

LC1766 ______________________________________________________________________

LC1766

LC1766 FGDC Geospatial Positioning Accuracy Standards (95% confidence, cm)

LC1766 Type Horiz Ellip Dist(km)

LC1766 -------------------------------------------------------------------

LC1766 NETWORK 0.77 2.21

LC1766 -------------------------------------------------------------------

LC1766 MEDIAN LOCAL ACCURACY AND DIST (039 points) 1.00 2.67 69.56

LC1766 -------------------------------------------------------------------

Observed positions and errors

Positions lose their credibility without error estimates.

• 100 distance measurements

• Mean = 2000.00 feet

• Standard deviation = ±0.10 feet

Mean = 2000.00

1999.80

1999.90

2000.20

2000.10

http://www.ngs.noaa.gov/datums/vertical/VerticalDatums.shtml#NAVD88




• 100 distance measurements

• Mean = 2000.00 feet

• Standard deviation = ±0.04 feet

Mean = 2000.00

1999.92

1999.96

2000.08

2000.04

Standard Deviation

68%

95%

99%




2D Position Quality

)( 2

2

2

1 EEDRMS

ObservedPosition

(address)

ObservedPosition

(address)

39% confidence

65% confidence

Commonly Known As

)( 2

2

2

1 EEDRMS

ObservedPosition

(address)

• HRMS

– Trimble

– Topcon

• 2DCQ

– Leica




Positional Errors

1000.00 ft calculated

999.94 ft

1000.06 ft

HRMS = 0.03 ftHRMS = 0.03 ft

Effects of Positional Errors


Any of these lines

are possible.


Weakness of GNSS


99.94 ft

100.06 ft





Match the tool to the task

• Can you stake this straight boundary line

accurately with GNSS?

80.00ft 80.00ft 80.00ft 80.00ft 80.00ft 80.00ft

Match the tool to the task

• GPS methods give greater accuracy over long distances.

• Total station methods give greater accuracy over shorter distances.

Confidence Levels

Observed Position

GroundTruth

65%95%99%

• 65% confidence = 65%

probability that the TRUTH

falls within 1 HRMS of the

ADDRESS (mean).

• 95% confidence = 95%

probability that the TRUTH

falls within 2 HRMS of the

ADDRESS (mean).




Reported Precision

Displayed at

data collector

Specified in contracts

Reported in NGS

datasheets

Reported in OPUS

solutions

Comparing Results

Previous

Observation

Current

Observation

Difference

Northing 1255110.131 1255110.182 +0.051 ft

Easting 1028009.036 1028008.987 -0.049 ft

HRMS 0.037 ft 0.040 ft

65% confidence overlap

Published

Coordinate

Observed

Coordinate

Radius =

1 X HRMS




95% confidence overlap

Published

Coordinate

Observed

Coordinate

Radius =

2 X HRMS

Strengthening

Field

Practices

www.trimble.com/GNSSPlanningOnline

DOP is always changing

PDOP < 3 gives best results




Adding Redundancy

• Repeat the observation

with different satellite

geometry.

– Re-measure control

points before, during, and

after survey session.

Satellite geometry changes.

9:00 AM 11:00 AM




MOLA to RV22 10.8 Km

Day 264dh

(m)Day 265

dh

(m)

Mean dh

(m)

14:00-14:30 -10.281 17:00-17:30 -10.279 -10.280

14:30-15:00 -10.278 17:30-18:00 -10.270 -10.274

15:00-15:30 -10.281 18:00-18:30 -10.278 -10.280

15:30-16:00 -10.291 18:30-19:00 -10.274 -10.283

16:00-16:30 -10.274 19:00-19:30 -10.274 -10.274

16:30-17:00 -10.287 19:30-20:00 -10.276 -10.282

17:00-17:30 -10.279 20:00-20:30 -10.261 -10.270

17:30-18:00 -10.270 20:30-21:00 -10.251 -10.261

18:00-18:30 -10.277 15:00-15:30 -10.270 -10.274

18:30-19:00 -10.271 15:30-16:00 -10.276 -10.274

19:00-19:30 -10.277 16:00-16:30 -10.278 -10.278

19:30-20:00 -10.271 16:30-17:00 -10.286 -10.279

20:00-20:30 -10.259 14:00-14:30 -10.278 -10.269

20:30-21:00 -10.254 14:30-15:00 -10.295 -10.275

Mean

14:00-21:00 -10.275 14:00-21:00 -10.276 -10.276

-10.254

-10.251> -10.253

Spread = 0.003 m

Mean = -10.276

Two Days / Same Time

Difference = 0.023 m

Difference

= 0.001 m

Two Days/

Different Times

-10.254

-10.295> -10.275

Spread = 0.041 m

Mean = -10.276

Importance of

Redundancy

Repeat with a new base location

Measuring points in the network that have already

been located creates redundancy.

www.dot.state.wy.usSurvey Manual Appendix M

Avoiding Multi-Path Error

http://www.dot.state.wy.us/




Avoiding Multi-Path Error

• Set the base at a wide open site.

• Expect multi-path near buildings.

• Chain link fences reflect GPS

signals.

– Mesh openings are too small to

allow signals to pass.

– Set offset points with GPS from

which to measure by other means.

– Raise antenna.

Wrap Up

• What was the most

valuable thing you

learned today?

Contact Information

Todd Horton, PE, PLS

[email protected]

(217) 493-3371 mobile

[email protected]

(217) 373-3785

mailto:[email protected]

mailto:[email protected]

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