LBAS and SBAS accuracy augmentation systems
LBAS and SBAS are two systems for optimizing the accuracy of geolocation by GNSS satellites. The LBAS system (Local Based Augmentation System) ensures the transmission of corrections directly to the receiver by means of communication such as the cellular network, radio, internet, …). As the SBAS (Satellite Based Augmentation System), it allows the distribution of corrections via geostationary satellites.
LBAS : the local augmentation system
The LBAS correction system is based on an observation differentiation process that removes sources of GNSS positioning errors and improves its integrity. The principle consists in evaluating, on a known point, the deviation between coordinates from the GNSS determination and its real coordinates. The effect is a set of corrections that can be transposed to nearby geographical areas. These corrections are then applied to the mobile receiver in real time or in post-processing software, known as differential GNSS or DGPS.
There are several types of local differential corrections :
- DGPS differential correction :
This is the historical method for correcting GNSS errors in real time. It is based on communication between the reference station receiver and the mobile receiver. This method uses measurements of the L1/E1 carrier code. A communication equipment transmits the corrections in real time to the mobile, to enable it to perform sub-metric positioning. - RTK :
The RTK uses the same principle of differential correction as the DGPS. The advantage of the RTK lies in the use of the phase of the carrier wave. This phase allows sub-centimetric measurements of “pseudo range corrections” and thus to position a mobile in RTK with centimetric precision. - After-treatment :
It is also possible to save the data from the base station receiver as well as the data from the mobile phone. This allows vectors to be post-processed using post-processing software. The accuracy thus achieved is equivalent to real time or even better in the case of so-called static measurements. That is to say, by continuously recording data at the same point over a long period of time.
SBAS : the spatial augmentation system
The SBAS augmentation system works differently from the LBAS. This system has a modular architecture similar to that of a GNSS constellation which includes 3 components :
- terrestrial (reference stations),
- space (geostationary satellites),
- user.
The concept : geostationary satellites deliver corrections in real time to increase accuracy. They will also transmit information to ensure the integrity of these corrections.
Example : It is the ground station network that will enable errors to be observed, e.g. EGNOS (European Geostationary Navigation Overlay Service) is the SBAS service covering Europe. To date, it has two reference stations in France.
This system takes into account the satellites of the GPS constellations and soon GALILEO. Three geostationary satellites transmit positioning corrections in Europe. Activating the use of this optimisation system on the receiver can then enable an accuracy of between 1 and 3 metres in 3D.
Several geographical areas around the world have their own service, as can be seen on the map below. As well as EGNOS, which is available in Europe, other services are implementing SBAS technology across the globe. This is the case of WAAS, MSAS, GAGAN…
Which system for which use?
It is obvious that the characteristics of each increment system induce a predisposition for certain sectors of activity. The mode of transmission and their desired level of precision will therefore make it possible to move towards one or other of the solutions.
The SBAS system was first developed for uses where telephone coverage is either limited or completely absent and where the need for accuracy is not a priority.
Such is the case of civil aviation, where at flight altitude only satellite transmission is possible. This is the case in civil aviation, where at flight altitude only satellite transmission is possible. Today this technology has spread to the agricultural sector, which does not always require high precision. However, for agricultural self-steering applications (read the article), centimetric precision is essential. The LBAS solution is preferred in this field of application. This is because it allows greater flexibility in the level of precision that can be achieved.
The exception of TERIA solutions
TERIA solutions combine the advantages of these two technologies. Indeed, the TERIA network (NRTK) provides LBAS type corrections and TERIAsat works like an SBAS system while allowing real-time centimetric positioning.
This particularity is due to the quality of the TERIA network and especially to the density of the reference stations. Indeed, if we take the case of metropolitan France, nearly 200 stations are meshed throughout the territory. This density is equivalent to having one station every 50km.
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