To go further technically

Precision GNSS receivers capable of accepting corrections will integrate components as well as additional software modules dedicated to precision calculations.

Principle of correction :

By placing a fixed GPS Reference Station (or base station) of which we know the exact and precise position in longitude, latitude and altitude, we can calculate the positioning error that a GPS returns to us at any time.
It then becomes easy to calculate the correction that gives us its exact position to the nearest centimeter (and sometimes better).
In use, we have found that it is possible to apply this same correction to other GPS units that are nearby (a few km – for information, the error is estimated at around 1 mm/km of distance). The fixedly positioned GPS is called a reference or pivot station.To distribute this correction to the GPS (in standard RTCM format) that are nearby, it is necessary to be able to communicate with them, either by radio or by internet via mobile telephony (modem).
It is therefore important to note that the mobile (or rover) sends its position to the network calculation center (NMEA-GGA frame) then will calculate from the correction information received in return, its positioning solution by double differences, in order to eliminate uncorrelated errors in space, such as satellite clock error or their electronics bias.

For more than 15 years, Géodata Diffusion has installed a network of 215 reference stations to cover the whole of France. All of the observations from each of these stations are collected on our servers, allowing us to then provide them to our customers via a subscription to our corrections for satellite positioning.

The single Station mode :

By connecting a mobile GPS to a single reference station (or pivot) by radio or mobile telephony, it is possible to calculate and therefore take advantage of the corrections of the centimetric precision brought by the reference station to the mobile GPS.

The distance that separates the mobile GPS from the Reference Station becomes the baseline and the accuracy of the correction then becomes dependent on its length. The further away from the base station, the more the accuracy deteriorates. It is observed that within a radius of 30km the centimetric precision is preserved.

This mode allows a very fast start-up (reduced initialization time) and a moderate investment but remains fragile in the event of a stoppage of the base station and the need to secure this equipment.

The network mode :

The network mode is an advanced mode for calculating corrections for positioning by satellites which implements several reference stations which surround a mobile receiver, thus creating a cell. By combining the individual corrections of each of the reference Stations of the cell, it is then possible to obtain a centimetric correction that is more homogeneous and more stable than with the single station.

This mode, which therefore does not depend on a single observation station, is of great interest because it then offers a correction which no longer depends on the length of the baseline. The quality of the corrections is homogeneous everywhere within the cell.

The network also offers an undeniable advantage because it does not depend on a single base station to generate quality corrections but on a cell in the event of a stoppage of one of the stations, the network continues to operate correctly and to deliver centimetric quality and precision corrections because it compensates for the absence of one of the bases.

The network allows a very fast start-up and a very short initialization time. Of course, the network mode represents a major investment because it requires a major infrastructure and great expertise that cannot be improvised.

Repeatability :

In some applications, it is essential to be able to return to exactly reposition oneself in the same place and sometimes after several years. This repeatability is obtained thanks to the centimetric precision of the corrections delivered on the one hand, but also by the precise and fixed positioning of the reference stations. Stability and control of their positioning is one of the keys.

Technically :

The operation of GNSS is based on measuring the propagation time of the signal transmitted by a satellite until it is measured by a receiver. The measurement of the propagation time of the signal coming from several satellites makes it possible by intersection to determine the position of the receiver.

The satellites therefore emit electromagnetic waves in the direction of the Earth which propagate at the speed of light.

The receiver on Earth measures the time taken by the wave to reach it. The receiver can then estimate the distance separating it from the satellite from the propagation time of the wave (approximately 70 ms). The precise measurement of this propagation time is essential since an error of 10 µs generates an error of 300 m: an accuracy of 1 ns is therefore needed to achieve a resolution of the order of one meter.

Accurate measurement of this propagation time requires perfect synchronization of the satellites with each other and with the receiver [Hofmann-Wellenhof et al., 2008 HOFMANN-WELLENHOF, B., LICHTENEGGER, H. and WASLE, E. (2008).Ó ]. This desynchronization occurring in the same way in all the measurements made in the direction of the satellites at a given time, it is therefore sufficient to estimate a desynchronization parameter (called clock error) to remove this uncertainty.

For standard positioning by GNSS, 4 unknowns must therefore be determined:

• • • • • Three position unknowns, in a geocentric frame, linked to a reference system specific to GNSS;
        • A time unknown, linked to the desynchronization of the receiver with GNSS time.

Positioning methods

different positioning methods :

• • • • • Standard: Absolute positioning on real-time or delayed code; the precision is of the order of 5 m
        • DGNSS: Differential GNSS, relative positioning on real-time or delayed code measurement; the precision is of the order of 50 cm.
        • RTK: Real Time Kinematic, relative positioning on real-time phase measurement; the precision is of the order of 4 cm.
        • Static: Relative positioning on real-time or delayed phase; the accuracy varies between a few millimeters and a few centimeters depending on the occupation time.

Code measurement

The receiver continuously receives the code from the satellite with a delay coming from the time taken by the wave to travel the distance between the satellite and the receiver. The receiver therefore measures this delay or shift, equal to the difference between the instant of reception (receiver clock) and the instant of transmission (transmitter clock). Since the receiver and transmitter (satellite) clocks are not synchronized, the time difference is marred by an error that must be estimated.

Mesure de Code

4 unknowns must therefore be determined: for instantaneous positioning, the measurement of at least 4 pseudo-distances is therefore necessary.

The receivers currently used make it possible to carry out the code measurement with an accuracy of the order of a hundredth of a cycle (length of a bit).

Phase measurement

Another technique than code measurement that can be used for positioning by GNSS is based on measuring the phase difference between the signals received and generated by the receiver. This measurement can be made on the different carriers used by the GNSS. Of course, the receiver and transmitter (satellite) clocks not being synchronized, the measured phase shift is tainted by a synchronization error which it is always necessary to estimate.

Mesure de Phase

The signal transmitted by the satellite is received by the receiver. an integer number of cycles, the period of the signal, the fractional part of the measured cycle.

However, only the fractional part of the propagation time of the signal between the satellite and the receiver counted in number of cycles can be measured by the receivers: the whole number of cycles elapsed since the start of the measurement is unknown: we speak of ambiguity of the phase measurement. The integer ambiguity is then defined as being the integer number of cycles elapsed at the start of the measurement.

During an observation session, care is therefore therefore taken not to interrupt the signal observed in the direction of a satellite. In the event of an interruption, it is called a cycle jump: the value of the entire ambiguity changes.

Note: For GPS the wavelengths associated with carriers L1 and L2 are respectively A1 = 19.0 cm and A2 = 24.4 cm. The precision on the phase measurement is therefore sub-millimeter.

Navigation Messages

In addition to being phase modulated using the code, GNSS signals are also modulated using the navigation message. The navigation message contains information that is used by the receiver, including:

• • • • • The position of the satellites (ephemeris) and information about it (satellite status).
        • The elements making it possible to obtain the date of the emission of the signal, in the time scale of the satellite.
        • The satellite clock correction to be applied to overcome its drift with respect to GNSS time.
        • More general information: global parametric model of the ionosphere (for correcting its effect), almanac of all satellites (health, approximate position)..

RTK Definition (Wikipedia)

Real Time Kinematics (RTK) is a satellite positioning technique based on the use of measurements of the phase of the carrier waves of the signals emitted by the GPS, GLONASS or Galileo.nition RTK systems.

A reference station provides real-time corrections to achieve centimeter-level accuracy. In the particular case of GPS, the system is then called Carrier-Phase Enhancement or CPGPS.

• • • • • Differential phase processing of GNSS signals
        • Centimetric precision inversely proportional to the “baseline” due to the spatial decorrelation of error sources and in particular atmospheric errors.
        • Receive the approximate position of the user (NME-GGA) connected by TCP/IP (GPRS, EDGE, UMTS (3G) see xDSL or Internet by satellite)
        • Model in real time the errors affecting the user’s site and send him “corrections” specially generated for his work area (MAC, VRS, FKP network concept)
        • Disseminate corrections reliably and transparently by relying on existing Telecom networks

NTRIP Client

The NTRIP (Network Transport of RTCM data over IP) protocol is used to send RTK correction data from the base to the rover via the Internet.

This is especially useful in areas where traditional radios do not work well due to trees or hills.

The NTRIP client is part of the software integrated into the mobile. It obtains the correction data from the NTRIP Orpheon server and sends it internally in the GPS to the positioning calculation module. This program is a more useful alternative to GNSS internet radio.

Access to the Internet is generally via conventional means:

• • • • • A Wi-Fi connection
        • A Bluetooth connection
        • A mobile phone connection

As long as the GPS is not connected and does not receive RTCM correction messages from the Orpheon server, the GPS works like an ordinary GPS with an accuracy of several meters (natural mode).

To connect to the Orphéon NTRIP server, the mobile’s NTRIP client must be configured to identify itself to the server and obtain the corrections in the format adapted to its calculation mode.

• • • • • Server IP address (or DNS)
        • The communications port
        • User ID
        • The user’s password
        • The mounting point corresponding to the calculation mode

At the time of connection, the NTRIP Client will identify itself then it will send its current position to the server (frame $GxGGA) so that the server can define the reference stations that will be used in the correction solution (network mode) or the nearest reference station (single station mode).

The server will then send the correction message every second in the RTCM format selected by the mount point.

The GPS will integrate these correction parameters into its calculations to improve its precision which in a few cycles will reach the desired precision. The time taken to obtain this precision is the convergence time.

The GPS will also regularly send its position to the server in order to:

• • • • • to receive the best correction parameters (especially if the GPS is moving in a vehicle).
        • to allow the server to determine that the mobile is still in the work area covered by the service.
        • to allow the server to confirm that the connection with the mobile is still valid.

The different steps

Without going into the details of these programs, it is interesting to understand what is happening at each of these stages.

Correction message formats

The format and content of GPS correction messages are dictated by the standard of the RTCM organization. Each RTCM message consists of a variable number of 30-bit words, the first two of which serve as the header. In the context of real-time localization, messages 18 and 19 are of major interest, and the minimum quantity of information to be disseminated for an RTK correction is given by the formula:
(1) [bytes/s] = [30-bit words] x 5 = f x2 xFREQ x(3 + 2x N) x51
Where f denotes the rate of measurements; FREQ, equal to 1 or 2, describes the single or dual frequency nature of the receiver; N represents the number of satellites.

The official version of the standard at the moment is called:
RTCM 10403.3, Differential GNSS (Global Navigation Satellite Systems) Services – Version 3 + Amendment 1 (April 28, 2020).

See the complete list of RTCM messages and their usage: https://www.use-snip.com/kb/knowledge-base/rtcm-3-message-list/

version 3 groups messages together with associated data instead of sending separate messages to accomplish the same task. For example, in version 2, sending a complete RTK message required a message type of 18 for corrections and 19 for pseudo-range measurements, whereas in version 3 this information is grouped into a single type 1003. Multiple message types are defined for the same types of information to further improve efficiency; Type 1001 has GPS data only on the L1 frequency, while 1002 adds various additional information, while 1003 and 1004 do the same with L1 and L2 data for stations that can take advantage of the second carrier.

Version 3.x uses a variable-length message format and a single 24-bit cyclic redundancy check (CRC) over the entire message, as opposed to 6-bit parity for each 30-bit word. Like previous versions, the message format begins with an 8-bit extended preamble, followed by a 6-bit reserved area, and then a 10-bit message length that allows up to 1024 bytes of data. The message, each with its privately defined header and data, follows the header and is then limited by the CRC. The saving of data, especially in the case of RTK, is significant, an RTK version 3 correction set is generally half as long as version 2.

Example: RTCM 1004: Extended L1 and L2 GPS RTK Observables
This type of GPS message is the most common type of observation message, with L1/L2/SNR content. This is the most frequently found legacy message.

The mounting point

The mount point allows you to choose the type of data stream and the list of RTCM messages that will be exchanged between your hardware and our server, this corresponds both to the language that the hardware will use but also to the method of calculating the positioning that will be used.

i-MAX, MAX, MAC, FKP, VRS corrections ?

In the configuration of your equipment which will guide your choice of mounting point, the choice of the type of corrections will depend on your equipment, the measurements you are going to perform and the availability of services.

MAC

On the initiative of Leica Geosystelms and GEO++ in 2001, first NRTK standard for the correction of network messages which would make it possible to overcome the problems of the various existing approaches. Concept based on the notion of master and mobile (Master Auxiliary Concept) allowing the Mobile to receive raw observation data from the nearest Master station in order to perform its correction calculations.

MAX

This Leica Geosystems format is based on the principle of using a cell of several reference stations around the rover with a reference station (the closest). MAX corrections contain all of the cell’s correction information and provide the maximum level of accuracy and reliability for the rover.

These corrections optimize the bandwidth required to transmit the corrections.

FKP

The oldest FKP method, (Flächen-Korrektur-Parameter) developed by Geo++, consists of the principle of transmissions of a model of errors calculated for the place where the mobile is specifically located. Subsequently, the rover calculates the corrections itself in the same way as in VRS. In addition the correction of a physical reference station is used in combination with the FKPs to calculate the individualized corrections for the position of the mobile.

FKP corrections are transmitted in RTCM 2.3 format. This method, which provides the mobile with more information, is bandwidth-intensive and requires stable communication quality without being full GNSS compatible.

i-MAX

Introduced by Leica Geosystems RTK network corrections are based on the position of the rover are calculated from information relating to a virtual master station created by calculations in close proximity to the rover in order to reduce the length of the baseline and present a level of lower error. The position of the master station can change to follow the movements of the mobile.

Full GNSS compatible corrections transmitted in RTCM-3 MSM format

VRS

With the Virtual Reference Station, a concept introduced by Trimble, the rover interprets and uses network-correction data from a virtual base as if it were operating with a single physical base station on a very short baseline, this which dramatically increases RTK performance. However, it does not create a virtual vector.

The different formats for transmitting corrections

Format RTCM-2.x (DGPS) 

This message is the most common way to send DGPS style code corrections for GPS or FKP

Format RTCM-3.1 ( RTK GPS Glonass)

RTCM versions 3.0 and 3.1 focus on optimizing bandwidth usage, higher integrity and management of RTK networks

Format RTCM-3.x MSM (RTK Full GNSS)

The RTCM-3 MSM standard thus ensures data interoperability with all compatible GNSS receivers by transmitting standardized observations of GPS + Glonass + Galileo + Beidou signals for all L1/L2 and L5 frequencies.

Message formats were previously limited to the L1 and L2 bands and to a single signal per band, which is why it has become necessary to use a new universal data format in real time: the MSM (for Multiple Signal Message) which is the new RTCM-3 key concept allowing all GNSS observation data to be presented in generic form.

List of RTCM messages used

Here is the list of RTCM messages used for each mount point for port 8500 (all applications except agriculture. For agriculture see list of RTCM messages for port 7500).