RTN Methodologies & Best Practices

Neil D. WestonNational Geodetic Survey, NOS, NOAA

Silver Spring, MD March 20, 2014

Nor

thin

g (k

m)

Easting (km)

Single baseline approach

City limits

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Nor

thin

g (k

m)

Easting (km)

Network approach

Ref.

Ref.

Ref. Ref.

Ref.

010

00

100

100

0RTN Basics

City limits

0 100

• Reference frame – adopt latest realization of NAD 83 (NAD 83 2011 epoch 2010.00)

• CORS + RTN – include a few CORS in the RTN

• Adjustments– Constrain CORS that are included

• Base station monitoring– Process data periodically– Monitor coordinates over time

• Base station information– Coordinates, velocities, epoch

RTN Basics

RTN Basics and Formats• Receiver collection rate – minimum 1Hz• Data formats– Propriety– Open standard– Compatibility

• RTCM SC-104– Only true ‘open’ protocol– RTCM-2.1 Added RTK messages for dm levels at short distances– RTCM-2.2 Expanded for differential operations with GLONASS

RTCM Formats– RTCM-2.2, 2.3 RTK operations providing cm- level

performance at short distances– RTCM-3.x More efficient; support RTK operations

(GPS+GLONASS); network correction message

• Common message types 2.x– Type 18, 20 – RTK uncorrected carrier phase data and carrier

phase corrections– Type 19, 21 – RTK uncorrected pseudo-range measurements

and appropriate range corrections

• Common message types 3.x– 1004 – full observables– 1005 – reference station coordinates– 1007 – antenna description

RTCM FormatsRTCM 2.0 RTCM 2.1 RTCM 2.2 RTCM 2.3 RTCM 3.0

1 1,2,3 1,2,3 1,2,3 1001

2 59 59 59 1002

3 1003

59 1004

1,2,3 1,2,3 18,19 1006

59 59 20 1007

21 1008

1009

1010

1011

1012

31 23,24 1013

GPS GPS GPS+GLONASS GPS+GLONASS GPS+GLONASS

Group Name Mobile Receiver Reference Stations

Observations – GPS 1003-1004 1004

Station Coordinates 1005 + 1006 1005 or 1006

Antenna Description 1007 + 1008 1007 or 1008

Auxiliary Information 1013

Network RTK Corrections 1014 + 1015 + 1016

Network Messages

Network RTK Processing Techniques & Applications

• Virtual Reference Station – VRS• Area-Parameter Correction – FKP• Master-Auxiliary Concept – MAC

• Grading, utilities, pipelines, roads• Landscaping, cadastral surveys, mapping

Area Correction Parameters - FKP• Network described using coefficients for a surface• Parallel surface with baselines less than 100 km• Reference station at the center of the “surface”• Software in rover receiver does the interpolation

of corrections• Error information is provided for quality control

and analysis• Single and bi-directional communications

• VRS techniques is currently the most popular• Requires bi-directional communications• VRS observations constructed• Rover approx. position to NRTK center• Rover solution based on range between VRS

and rover• VRS kept to preserve ambiguities

Virtual Reference Station - VRS

Virtual Reference Station - VRSRef Sta.

Ref Sta.

Ref Sta. j

VRS Sta. i

Rover

NRTKCenter

• Rover position (NMEA) to processing center• Processing center chooses a master station –

usually closest ref. station• Auxiliary stations are chosen from a 70 km

catch circle• Rover receives MAC corrections via RTCM• Rover decides on method of interpolation of

corrections and how the position is determined – double difference, for example

Master Auxiliary Concept - MAC

Rover

Aux k3

Aux k4

Aux k2

Auxiliary k1Master j

Master Auxiliary Concept - MAC

70 km

RTK Processing

Network RTK Processes

Correction Generation Correction Interpolation

Correction Transmission

Network Ambiguities

Network Corrections

State Space Observation Space

Least Squares Co-location

Linear Combination

Grid-based Parameters

Surface Modeling

Linear Interpolation

Two way Comms.

One way Comms.

VRS

Net Adjust

MAC

FKP

Thank You

Neil D. WestonDeputy DirectorNational Geodetic Survey301-713-3222Neil.d.weston@noaa.gov