# Presentation: 258093

## Network Virtual Reference Station Systems / Recent Minnesota DOT Activities

- Don Seitz
- Minnesota DOT
- Herbert Landau
- Trimble

## Concept of VRS

- Uses a Network of Reference Stations (ABCD) to create Virtual Reference Stations (VRS)
- A VRS is created for each user location.

## VRS  -  How does it work?

- Uses observations from multiple reference stations
- Continuously monitors integrity of reference station data
- Models systematic errors including:
- ionosphere
- troposphere
- satellite orbit errors
- multipath
- Creates a unique virtual reference station for each user’s location
- Delivers the data in RTCM or CMR+ format to the rover

**Notes:**
Productivity lost through having to establish base station – locate co-ordinates, set up comms and power, provide security (additional person).
Radio frequency and range issues.
Battery charging.

## Why use VRSTM ?

- Extended operating range with improved initialisation and accuracy
- Increased productivity
- Eliminates need to establish reference station
- Set-up, power, physical security become non-issues
- Provides integrity monitoring
- All users in common, established coordinate frame
- Eliminates dependency on single reference station
- Uses established communications

**Notes:**
Productivity lost through having to establish base station – locate co-ordinates, set up comms and power, provide security (additional person).
Radio frequency and range issues.
Battery charging.

## Data Flow in Network 

- Router

## What‘s New with VRS

- Improved modelling of ionosphere
- Increase network correction availability by new models
- Optimal smoothing for pseudorange corrections for DGPS applications
- Raw Data instead of Differential analysis
- RTCM 1,9,20,21 messages for DGPS services
- RTCM 59 message for additional VRS information to the rover
- CMR+ generation

## New Ionospheric Models

- Ionosphere is the major limiting factor for RTK on long baselines
- Focus on ionosphere
- Improved models in GPSNet for modelling local ionospheric effects
- Sample Comparison of effects on 32 km baseline
- with raw data
- with VRS corrected data

## Double Difference Iono Error on 32 km Baseline

- Double Difference Iono Error on 32 km Baseline

- Local Time

## Double Difference Iono Error on 32 km with VRS corrections

- Double Difference Iono Error on 32 km with VRS corrections

- Local Time

## Test Hardware

- RTK Receiver 1

- RTK Receiver 2

- GPSNet

- RTCM/VRS

- RTCM/Raw

- Modem 1

- Modem 2

- Antenna Splitter

- Reference Network

## Set-Up

- Rover Höhenkirchen

- Reference Munich

## Fixing Performance

## Fixing Performance

## Fixing Performance

## Correction Availability 

- Correction Availability is one of the major problems in VRS
- VRS needs to provide the same satellites like single station RTK
- Major improvements for availability in latest versions
- Satellites are tracked from 5° elevation
- Ambiguity fixing in most cases at 10° or below
- GPSNet provides correction terms for all satellites of interest for RTK

## Availability of VRS Corrections

- Number of Satellites

- Local Time

## Special VRS RTCM 59 message

- RTCM 59 provides information on
- The nearest reference station coordinates
- Linear correction terms for each corrected satellite with respect to
- Geometry
- Ionosphere
- Message provides information on actual ionospheric conditions to the rover
- RTCM 59 format was distributed to all major manufacturers

## Implementation of VRS at MNDOT

- System Diagram
- Reference Stations
- Central Server
- Mobile Units

## System Diagram

## Metro CORS VRS Stations

## Reference Station Hardware

- Compact L1/L2 antennas w/ Groundplane
- Snow cone radomes
- type of monuments??
- 4000SSI receivers
- Precidia Ether232 terminal server
- LAN connection to terminal server

## Central Server

- Central server @ St. Paul station
- One RTCM generator for every CDPD rover (unique port number)

## Mobile/Roving Units

- Trimble 4700 & 5700 receivers
- Novatel Wireless Sage CDPD modems

## MN DoT Testing  Results

- Point Minnesota: .6 km from nearest base
- Point Weltzen: 20.5 km from nearest base

## MN DoT Testing  Results

- Station Weltzen

## MN DoT Testing  Results

- Station Weltzen

## MN DoT Testing  Results

- Station Minnesota

## MN DoT Testing  Results

- Station Minnesota