GNSS Reference stations, how does it work?
The real-time centimetric positioning is based on data collected by GNSS reference stations, also called C.O.R.S (Continously Operating Reference Station). All PPP, RTK, NRTK, PPP-RTK positioning methods use these continuous reference stations, alone or in networks.
How are they equipped and how do they work? How do you get continuous service?
A high technology equipment
Surveyors, GIS users, engineers, scientists and the general public who collect GNSS data can now use C.O.R.S. data to improve the accuracy of their positions.
Within a few years, reference stations have been installed around the world. They now form vast interconnected networks that have enabled immense progress. This development benefits both the scientific and service fields.
A reference station C.O.R.S. combines :
- a Choke Ring* GNSS antenna and a coaxial cable connecting the GNSS antenna to the GNSS receiver,
- one multi-frequency GNSS receiver,
- one or more telecommunications equipment,
- a remote management equipment allows the remote restart of the equipments,
- a power supply.
This technology has undergone a profound evolution. The latest multi-frequency receivers support all global GNSS constellations with more than 550 frequencies. These include the GPS, GLONASS, GALILEO and BEIDOU constellations. In addition, there are also regional systems such as QZSS and SBAS.
* The Choke Ring type antenna is a robust antenna that minimizes multipath (GNSS signal deviation) or reflections against surrounding surfaces. It also allows a small variation in the position of the phase centre, for which very high precision calibrations are available. (see picture adjacent).
The choice of installation site
The place of installation must necessarily be a site free of obstacles. In other words, any mask that would prevent or attenuate the reception of GNSS signals or provide multipath. Usually, the GNSS antenna is installed on an open mountainous site or securely mounted on the roof of a perennial building. That is to say, a building that does not show any singular movement due to tectonic or climatic movements. (Therefore this excludes buildings with metal sheet structures, for example, which deform with temperature changes).
The antenna site has a clear 360° horizon, above an elevation angle of 5°. Some stations used for surveying applications have their own infrastructure. This is in order to ensure that no movement due to the environment disturbs the data acquisition. In France, this is the case of the RENAG stations and the stations of the UNAVCO network at the international.
PBO Station – Earthscope Chronicles – @UNAVCO
A permanent GNSS or C.O.R.S station then observes all the signals transmitted by the global positioning constellations continuously 24/7. This includes for the coverage of Europe: the American GPS constellation, the Russian GLONASS constellation, the European GALILEO constellation and the Chinese BEIDOU constellation.
The signals picked up by the stations are communicated by various telecommunications means to servers either in real time or in delayed time. (usually via the Internet)
As regards processing by the computer centre, the navigation message data of the main positioning constellations (ephemerides, system status, etc.) alone do not allow high accuracy to be achieved. In order to achieve centimetric precision positioning, it is necessary to take into account several variables: precise orbits and a precise reference system as well as Earth orientation parameters.
The data corrected by the computer centre are either transmitted to users in real time (RTCM format for TERIA) or stored for further processing. In this case, the data is usually in Rinex format. The RGP site of the IGN, for example, makes this type of data available : http://rgp.ign.fr
The IGS network (International GNSS Service) uses in particular the GNSS data from reference stations to define coordinate systems worldwide, through the various achievements of the ITRF. Indeed, the accuracies now obtained make it possible to go down to a few millimetres. In addition, they make it possible to measure the tectonic movements of each station very accurately and thus calculate plate displacements.
Maintenance of the reference stations is essential. The correct use of the correction calculation software is just as important. It enables users to provide a superior quality of service.
An interruption in service could have significant consequences for users in the field. All means must be put in place to prevent or resolve any service disruption.
M.O.C - Maintenance in Operational Condition
In the case of TERIA, the M.O.C. (Maintenance in Operational Conditions) service is a priority. Engineers and experts are dedicated to the maintenance and proper operation of the reference stations according to extreme quality criteria. A real “Task Force” (intervention force), this team is dedicated to providing a rapid solution in all conditions. Numerous computer interfaces provide real-time visibility of the status of the stations on site. These immediately alert the technical teams as soon as a change in status occurs.
A search protocol is then launched to quickly determine the actions to be taken:
- remote restarts
- emergency on-site intervention
The density of reference stations in the TERIA network combined with the efforts of the technical teams guarantee an availability rate of more than 99% since 2012. (ISO 9001 certification)
The M.O.C. indicators allow for continuous improvement. Moreover, it facilitates the search for possible improvements in each element. This is a necessary step to implement preventive actions on all sites. A global register lists all the actions and events that have occurred on the network of permanent stations.
Each event is qualified according to its nature:
- telephone network
- condition of the equipment,
TERIA experts constantly monitor technical developments. This includes monitoring the technological evolutions and improvements made possible by the latest versions of firmwares deployed by GNSS receiver manufacturers.
The TERIA teams are committed on a daily basis to providing a continuous quality service and personalized support to each customer.