RTN Methodologies & Best Practices Neil D. Weston National Geodetic Survey, NOS, NOAA Silver Spring, MD March 20, 2014

Northing (km) Easting (km) Single baseline approach City limits * * Northing (km) Easting (km) Network approach Ref RTN Basics City limits 0 100

Reference frame – adopt latest realization of NAD 83 (NAD epoch ) 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 Formats RTCM 2.0RTCM 2.1RTCM 2.2RTCM 2.3RTCM ,2, ,2,3 18, , GPS GPS+GLONASS

Group NameMobile ReceiverReference Stations Observations – GPS Station Coordinates or 1006 Antenna Description or 1008 Auxiliary Information1013 Network RTK Corrections 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

Ref Sta. Ref Sta. j VRS Sta. i Rover NRTK Center

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 k1 Master 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. Weston Deputy Director National Geodetic Survey