APN-041 Rev 1A July 2013

Application Note on Network RTK for the

OEMV Receivers

Introduction

This application note includes background information on Network RTK and general guidance on how to use

it. The majority of the content in this paper is from a distribution report done for NovAtel Inc. titled Network

RTK Status Western Europe by Hans-Jürgen Euler from inPosition gmbh.

About Network RTK

Network RTK development started over ten years ago with the introduction of real-time GPS baseline

solutions. Around the world, there were activities surrounding Network RTK but in Germany, the idea to

install permanent base stations for high-precision RTK applications was fostered. Network RTK uses

permanent base station installations, allowing kinematic GNSS users to achieve centimeter accuracies

without the need for setting up a GNSS base station on a known site.

Using permanent base stations is advantageous in areas with considerable GNSS surveying activity

because many users can share the infrastructure and the associated costs. Some of the installations are

operated by companies that provide service to the surveying community. Installations can be a single base

station, a number of single base stations, or networked base stations. A single base station within 20-30 km

from the rover is typically required if a user is operating in a standard base to rover mode. Otherwise the

performance, accuracy, and the reliability of user's RTK may be degraded.

For the base operator, networking reduces the number of base stations that are needed to provide a given

level of accuracy to the rover so integrating several base stations into a network improves the accuracy and

increases the overall user system performance. Networking also reduces the number of base stations

required to cover a large area. These permanent base station networks require real-time communication to a

networking computation center and real-time estimation of biases between bases.

An understanding of applied corrections or the processing procedure is also very important because the

installations rely on proprietary computation algorithms and formats that could restrict the field equipment. It

is in the interest of service providers to supply the service for more than a single type of RTK field

equipment.

Many Network RTK methods exist, but currently, VRS (Virtual Reference Station), iMAX, FKP (Flächen

Korrektur Parameter - the German phrase for spatial correction parameter) and MAC (Master-Auxiliary

Concept) are the most common. See pages 2 and 3 for more information on individual methods.

The calculations required to produce the observation information follow these principal sequences for data

handling:

1. Base station observation information transfer to a central location

2. Integer ambiguity resolution for observations between base stations

3. Fit model parameters to deviations

4. Determination of corrections for rover position and correction of observations

5. Calculation of the rover’s position based on its observation information and network information

These principal steps are used by different Network RTK software and remote firmware. Some of these

calculation steps might be combined into one step, but this does not change the overall principle. See Figure

Hans-Jürgen Euler (December 2005), Reference Station Network Information Distribution. International

Association of Geodesy Commision 4: Positioning & Applications. Retrieved October, 2007 from, IAG WG 4.5.1:

Network RTK.

APN-041 Rev 1A July 2013

Independent RTCM Format

Traditionally, the communication interface between different manufacturers’ GNSS equipment is the

manufacturer independent RTCM format. Networking services based on either FKP or VRS approaches

provide the observations via the RTCM standard, but are currently operating in a mode not defined in the

standard document and/or disseminating additional information in a proprietary message. RTCM delivers a

container (message type 59) for proprietary information, but the content is not specified in the standard.

Figure 1: Data flow between reference stations and rover system

The figure above shows sequence of operations and calculations required until a rover's position is

calculated. Current network approaches are to distribute the principle calculations between the software of

the network and the rover. As long as calculation steps are performed within the same software, these steps

can be combined into one step. For more information on these steps, refer to

Reference Station Network

Information Distribution by Hans-Jürgen Euler.

Network RTK Methods Supported by OEMV Receivers

VRS

The virtual reference station (VRS), or virtual base station (VBS), method, introduced by Trimble, calculates

a set of corrections from a virtual base station in the vicinity of a rover receiver. All baseline-length-

dependent errors, such as abnormal troposphere variation, ionospheric disturbances and orbital errors are

removed from the VRS because the virtual base is so close to the rover. The rover sends the position to the

network in a standard NMEA (National Marine Electronics Association) GGA message and the network

software calculates the position of the VRS. The VRS corrections are sent back to the rover and the rover

calculates its position. Most rovers receive VRS data for a calculated base station that is only a few meters

away.

VRS data is supplied as RTCM messages RTCM1819 or RT2021and RTCM V3 messages RTCM1003 and

RTCM1004

The VRS approach requires bi-directional communication for supplying the rover’s position to the networking

software.

Refer to the OEMV Family Firmware Reference Manual for details on these RTCM and RTCM V3 messages.

APN-041 Rev 1A July 2013

iMAX

The iMAX method, introduced by Leica Geosystems, calculates a set of corrections based on the rover

position, similar to VRS. However, instead of calculating the base station observations for a location close to

the provided rover position, original observation information is corrected at the base station position and sent

to the rover in a compact form. In VRS, use of a virtual base station, close to the rover, may disguise the

residual ionospheric errors in the base observations. However, iMAX provides actual base station position

information therefore, the real distance to the base can be computed from the base station coordinates, and

the rover may be able to open its settings to estimate the remaining residual ionospheric delay. The iMAX

method may trigger the rover to open its settings further than required since the networking software

removes at least part of the ionospheric disturbances. However, compared to VRS above, this approach is

safer since it notifies the rover when there might be baseline-length-dependent errors in the observation

information.

iMAX uses

RTCM 3.0 Network RTK messages and requires bi-directional communication to the networking

software for supplying the base station observation information.

FKP Reference Station Information

The FKP method is another approach that delivers the information from a base station network to the rover.

No precise knowledge of the rover’s position is required for providing the correct information. The corrections

are deployed as gradients to be used for interpolating to the rover’s actual position.

FKP is the preferred method of Geo++ for disseminating network RTK information. In principle, any format

capable of transporting base station raw observations can be used together with a message to transport the

FKP coefficients. Geo++ combines the FKP information with RTCM version 2.3, RTCM2021 messages

which has been adopted as the standard for SAPOS. For the FKP information, no standard has been

adopted yet but some users and providers argue that there is a standard because the information is being

transmitted in an RTCM59 message. The layout of the message is described in RTCM Message Type 59-

FKP for transmission of FKP document from the Geo++ webpage found at

http://www.geopp.de/download/geopp-rtcm-fkp59-1.1.pdf

. This document describes the layout and the

interpretation of the bit stream.

In contrast to VRS, FKP can be used for broadcast installations. Bi-directional communication between the

rover and the network is not mandatory, but for large networks different access numbers or points are

required. When bi-directional communication is available, FKP installations require NMEA coordinates.

These positions are used to determine the most suitable base station FKP constellation for the user. The

advantage of this is that only one access point is required to cover the whole network. This is important for

users covering large regions with different base stations close to the working area.

Other Proprietary Network RTK methods

Mac (Master-Auxiliary Concept)

The basic principle of the master-auxiliary concept is to provide, in compact form, as much of the information

from the network and the errors it is observing to the rover as possible. With more information on the state

and distribution of the dispersive and non-dispersive errors across the network, the rover is able to use more

intelligent algorithms in the determination of its position solution. Each supplier of reference station software

will have their own proprietary algorithms for modeling or estimating these error sources. The rover system

can decide to use or to neglect the network RTK information depending on its own firmware algorithm

performance.

MAC uses defined RTCM messages so, in theory, each installation should be identical and have no

proprietary messages. The formats of these messages are still under discussion by the RTCM standards

committee but once they are defined and the initial algorithm implementation is complete, it will be easier for

manufacturers of rover equipment to support MAC.

APN-041 Rev 1A July 2013

Commands

RTKNETWORK

This command sets the RTK network mode for a specific network.

Abbreviated ASCII Syntax: Message ID: 951

RTKNETWORK mode [network#]

Factory Default:

rtknetwork auto

Input Example:

rtknetwork imax

Field Field Type

ASCII

Value

Binary

Value

Description

Binary

Format

Binary

Bytes

Binary

Offset

RTKNETWORK

header

This field contains the command name or

the message header depending on

whether the command is abbreviated

ASCII, ASCII or binary, respectively.

mode

RTK network mode see Table 1: Network

RTK Mode.

The factory default is auto where the

receiver switches to the first available

network RTK source.

Enum

network#

Specify a number for the network

default = 0

Ulong

H+4

APN-041 Rev 1A July 2013

Table 1: Network RTK Mode

Binary ASCII Description

Disable

Single reference station RTK mode. All received network RTK corrections are ignored.

1-4

Reserved

VRS

The virtual reference station (VRS), or virtual base station (VBS), idea, introduced by Trimble, is

that a base station is artificially created in the vicinity of a rover receiver. All baseline-length-

dependent errors, such as abnormal troposphere variation, ionospheric disturbances and orbital

errors, are reduced for this VRS. The rover receiving VRS information has a lower level of these

errors than a distant base station. The VRS is calculated for a position, supplied by the rover

during communication start-up, with networking software. The VRS position can change if the

rover is far away from the initial point. The format for sending the rover’s position is standard

NMEA format. Most rovers receive VRS data for a calculated base station that is within a couple

of metres away. The VRS approach requires bi-directional communication for supplying the

rover’s position to the networking software.

IMAX

The iMAX idea, introduced by Leica Geosystems, is that networking software corrections, based

on the rover’s position, are calculated as with VRS. However, instead of calculating the base

station observations for the provided position, or another position closer to the base station,

original observation information is corrected with the calculated corrections and broadcast. VRS

works so that although the rover is unaware of errors the VRS is taking care of, there still might be

ionospheric remains in the base station observations. iMAX provides actual base station position

information. The rover may assume the base station is at a distance and open its settings for

estimation of the remaining ionospheric residuals. The iMAX method may trigger the rover to open

its settings further than required since the networking software removes at least part of the

ionospheric disturbances. However, compared to VRS above, this approach is safer since it

notifies the rover when there might be baseline-length-dependent errors in the observation

information. iMAX requires bi-directional communication to the networking software for supplying

the base station observation information.

FKP

The FKP method delivers the information from a base station network to the rover. No precise

knowledge of the rover’s position is required for providing the correct information. The corrections

are deployed as gradients to be used for interpolating to the rover’s actual position.

MAX

The basic principle of the master-auxiliary concept is to provide, in compact form, as much of the

information from the network and the errors it is observing to the rover as possible. With more

information on the state and distribution of the dispersive and non-dispersive errors across the

network, the rover is able to use more intelligent algorithms in the determination of its position

solution. Each supplier of reference station software will have their own proprietary algorithms for

modeling or estimating these error sources. The rover system can decide to use or to neglect the

network RTK information depending on its own firmware algorithm performance.

Reserved

AUTO

Default value, assume single base. If network RTK corrections are detected then the receiver will

switch to the appropriate mode. iMAX and

VRS can only be detected using RTCMV3 however it is

not possible to distinguish between iMAX or VRS. If iMAX or VRS is detected then iMAX will be

assumed

Final Points

If you require further information, regarding the topics covered by this document, please contact:

NovAtel Customer Service

1120 – 68 Ave. N.E.

Calgary, Alberta, Canada, T2E 8S5

Phone: +1-800-NOVATEL (in Canada or the U.S.) or +1-403-295-4500

Fax: +1-403-295-4901

E-mail: support@novatel.com

Website: www.novatel.com