RU2584957C2 - System for locating trains with real-time check on position assessment integrity - Google Patents

Abstract

FIELD: transportation.

SUBSTANCE: group of inventions relates to field of automation and remote control of railway sector. Satellite terminal is designed to be installed on board a train is configured for receiving navigation signals from satellites belonging to one or more satellite navigation systems. Said satellite terminal is characterised in that it is moreover configured for storing georeferencing data of a railway route of train and for determining, based on georeferencing data stored and navigation signals received, a position of train along railway route and an integrity level associated to said calculated position. System comprises said satellite terminal and is designed to be installed on board a train, acquiring from an odometer installed on board a current estimate of position, current assessment of situation and receiving exact position of train from alarm system installed along railway route, as well as possibility of correcting current situation assessment.

EFFECT: increase of reliability in train positioning and traffic safety.

7 cl, 5 tbl, 7 dwg

Description

FIELD OF THE INVENTION

The present invention, in General, relates to the determination of the location of trains and, in particular, to a system designed to assess the position of the train and to verify in real time the reliability of the position estimate.

State of the art

As you know, in the railway industry, there is a noticeable need for the development of positioning systems that would have increased reliability for controlling trains during movement and for guaranteeing the safety of railway communications. In the field of aeronautics, such a need was met through the use of Satellite Based Differential Correction Systems (SBAS), which provide an opportunity to improve the accuracy of position estimation, and therefore can be used to support air navigation. In addition, SBASs are also designed to provide a “safety for life” signal and therefore can be used to support air traffic control systems.

KNOWN SBAS SYSTEMS REPRESENT:

The European Geostationary Auxiliary Navigation System (EGNOS), designed to provide services to improve the accuracy of assessing the situation on the European continent and in North Africa (in particular in northern Morocco, Tunisia, Algeria and Libya);

The Wide Area Differential Correction System (WAAS), developed in the United States of America and designed to provide services to improve position accuracy in a vast area of the North American continent; and

Japan's Multi-Functional Satellite Differential Correction System (MSA), designed to provide services to improve position accuracy in a vast area of the Asian continent.

When all of the SBASs mentioned above are fully operational, an airplane that, for example, takes off in New York, heading for London and then to New Delhi, always remains within the reach of such SBAS.

In particular, SBAS guarantee accuracy of the order of two meters when assessing the position. In addition, SBAS also guarantees the reliability of data received from Global Navigation Systems (GPS) and provides a much more accurate altitude calculation, which can also be used for air navigation in the future.

In more detail, SBAS was used to provide information that provides improved position estimates based on signals received from GPS and to provide “safety for life” signals:

many geostationary satellites (that is, in fixed positions relative to the surface of the Earth, compared with GPS satellites that move in orbit);

a plurality of ground stations that are appropriately geographically linked, they provide the appropriate means of reference time (high-precision clocks), and which are configured to determine the delays of signals transmitted by GPS satellites due to ionization of the troposphere; and

many base stations for data processing.

In more detail, SBAS, for determining position estimation errors based on signals received from GPS, operate as described below. Ground stations detect the error of data transmitted by GPS satellites (which can be, for the most part, related to ionization in the lowest layers of the atmosphere). To this end, ground stations compare their own position, calculated on the basis of signals received from GPS satellites, with GPS satellite orbits and the corresponding certified locations. As you know, GPS take basic calculations of their own position by the delay with which they receive a signal from GPS satellites. Since each ground station knows the corresponding exact position and positions of the GPS satellites from which it received GPS signals (the mentioned positions are determined not based on the received signals, but rather based on the data of the orbits of the satellites themselves), each ground station is therefore easily configured Identify the error associated with the propagation of GPS signals through the atmosphere. Each terrestrial radio station, therefore, can generate, based on the calculated errors, the corresponding array of surrounding points and detect the allowable error for each of these points, thus expanding the area in which the calculated GPS errors are valid. Therefore, in this way, each ground station determines a corresponding error model that is valid for the corresponding area of competence. The data generated by the ground stations is then transmitted to at least one data processing base station, which generates a very dense grid of correction factors. If this corresponds, in practice a large number of points of a known position are processed, for each of which correction data for a signal received from each GPS satellite is processed. These data are updated in real time, taking into account the fact that the propagation conditions of the GPS signal through the atmosphere, obviously, change in accordance with the conditions of the atmosphere itself. These correction factors are then transmitted to SBAS satellites so that they can ultimately be transmitted to the earth using the same frequency as GPS signals (i.e., L1 frequency) and then received by the respective user terminals. The terminal, which receives SBAS signals, selects the data valid for the grid points closer to it, applies it to the satellites that it receives at this point in time, and uses them to calculate its own position.

In the railway industry, using SBAS is not so simple. In fact, the “safety for life” signaling service was considered mainly as procedures in aeronautics, which are fundamentally different from procedures on the railway. In fact, the procedures on the railway begin with the idea that each train can be controlled at any time along its route, determining in real time, on the basis of the condition of the train and the level of knowledge about the position of the train, both speed and possible stops in the event that security procedures are required. A more complex development of these train control processes is presented in the European Rail Traffic Management System (ERTMS) and in the European Train Traffic Management System (ETC).

In particular, the integrated system, ERTMS-ETC, is an advanced system for the administration, management, protection and transmission of railway signals, designed to replace many mutually incompatible systems related to the movement and safety of various European railways, to guarantee the interoperability of trains in various European railway networks and the maximum level of performance of European railway networks, as for high orostnyh, and for having the Greatest commercial interest.

ERTMS-ETC consists of different equipment, which is designed to carry out the functions mentioned above, and is characterized by three different functional levels, in particular, the first functional level, the second functional level and the third functional level. The definition of each functional level depends on how the railway line is equipped, and on how information is exchanged between the train and monitoring stations.

At the first level, ERTMS-ETC authorization for movement and the corresponding route information is transferred to the train and continuously displayed in the driver’s cabin using balises (point track sensors) called “eurobalises”, which are distributed along the railway lines, which provide their own determination of the position of the train and transmit conditions on the route, and all of them can be integrated by the following sequence of transmission points, which continuously transmit information and relevant traffic control data to the train and position.

In particular, modern trains are equipped with on-board odometers that are capable of measuring the speed of the trains on which they are installed and for assessing the position of the mentioned trains by integrating the measured speed. At the first level, ERTMS-ET Sevrobalyses are used to calibrate airborne odometers, that is, to adjust position estimates transmitted by airborne odometers based on certified positions transmitted by Eurobalyses.

First level ERTMS-ETCs provide on-board signals that can be added to traditional signaling systems currently installed on railway lines, leaving the latter operational for the circulation of conventional trains.

The fixed transmitting balises (eurobalises) transmit, through appropriate coding, the information provided by the fixed linear signals and provide the necessary authorization for movement to the on-board devices of the train. The computer on board the trains processes the maximum speed and braking curves based on data received from the Eurobalys. In order to ensure the possibility of obtaining the necessary information from ground-based balis, the necessary authorization, in particular, the necessary authorization for subsequent movement, it is necessary that the train accesses the mentioned bali, passing by them. Information related to the reliability of the train and for the corresponding determination of the position is detected through the rail track. By installing additional Eurobalances (Eurocircuits) between the signal of the beginning of the length and the signal of the end of the length of the path, it is possible to obtain a fairly continuous transmission of information. Information can be transmitted during the passage of a locomotive through an inductive means or using a radio device.

In this regard, in FIG. 1 shows an example scenario in which the first level of ERTMS-ETC operates.

In particular, in FIG. 1 is schematically illustrated:

a section of the railway line (designated 11 as a whole), which contains two Eurobalances (indicated by numbers 111 and 112, respectively), which are connected to a module on the line (designated 113), which, in turn, is remotely connected to the control center ( marked 12); and

a train (generally indicated as 13) that runs along a section of railway line 11 and has an on-board computer installed (indicated as 131), which is connected to a receiver (indicated as 132) and to a control panel (designated 133) made with the possibility of supplying information to the driver (designated 134) of the train 13.

In more detail, the control center 12 transmits to the module 113 on the line information related to the section of the railway line 11, such as, for example, authorization for the movement of trains, deceleration of their speed and the maximum permitted speed. The module 113 on the line feeds into the Eurobalysis 111 and 112 information received from the control center 12 together with other information transmitted by the fixed signal transmission systems (not shown in Fig. 1 for simplicity) installed along the section of the railway line 11. Each of the two Eurobalyses 111 and 112 is geographically attached, that is, it knows the corresponding exact position and transmits the corresponding position, along with information received from module 113 on the line, when the trains pass through inductive means or by radio. When the train 13 passes through the Eurobalisms 111 and 112, the receiver 132 receives the information transmitted by the said Eurobalises 111 and 112 and feeds it to the on-board computer 131. The on-board computer 131 displays on the control panel 133 information received through the receiver 132, together with other information ( for example, the current braking profile of the train 13) obtained by processing the received information and other information related to the train 13 (for example, speed, weight and length of the train 13).

In addition, the on-board computer 131 is connected to the on-board odometer (not shown in FIG. 1 for simplicity) of the train 13 to receive the latest estimates of the position of the train 13. The on-board computer 131 corrects the estimates based on the positions adopted from the Eurobalances 111 and 112. The on-board computer 131 displays on the control panel 133 the position estimates transmitted by the airborne odometer when the exact positions transmitted by the eurobalises 111 and 112 are available to it, while when it takes the exact positions transmitted by the eurobalisms 111 and 112, it displays the exact positions indicated on the control panel 133.

In this regard, instead, a second-level ERTMS-ETC provides distance management between trains using radio link between trains and a base station called the Radio Block Center (RBC), which, knowing the line status and the status of other trains, continuously transmits train information related to the line (such as, for example, authorizing the movement of trains, reducing their speed, and the maximum permitted speeds), using connections based on the international standard of mobile telephony, to transfer data on the railway the road of the Global System for Mobile Communications - Railway (GSM-R). Trains, therefore, can also determine their own speed profile based on their own characteristics of weight and braking. The system intervenes in a timely manner in case of possible security risks.

In particular, the ERTMS-ETC of the second level is a system for transmitting signals and protecting the train based on digital data transmission. In the train control cabin, on specially provided control panels, information related to the route and authorization of train movement received directly from RBC are displayed. The positions of trains, the direction of movement, together with all other necessary information, are automatically transmitted to trains in RBC at predetermined intervals. Train traffic is thus constantly monitored by the RBC.

In the ERTMS-ETC of the second level, the Eurobalances perform only the function of reference points for controlling and correcting the position of the train along the line. The on-board computer constantly processes the transmitted data and the allowed maximum speed.

In this regard, in FIG. 2 shows an example scenario in which a second level ERTMS-ETC operates.

In particular, in FIG. 2 is schematically illustrated:

a section of the railway line (generally designated as 21), which contains two Eurobalances (designated, respectively, as 211 and 212);

RBC (designated as 22); and

a train (designated 23 as a whole), which passes through a section of railway line 21, and on board which an on-board computer is installed (designated 231), which is connected to a receiver (designated 232), to a GSM-R terminal 233, which exchanges information with RBC 22, and with a control panel (indicated as 234) configured to provide information to the driver (designated as 235) of the train 23.

In detail, RBC 22 transmits to the GSM-R terminal 233 information related to the section of railway line 21, such as, for example, authorization of train movement, their deceleration and maximum permitted speeds. The GSM-R terminal 233 supplies information received from the RBC 22 to the on-board computer 231. The on-board computer 231 displays information received from the RBC 22 on the control panel 234 via the GSM-R terminal 233, together with other information (for example, the current braking profile train 23) obtained by processing said information received from RBC 22 and other information related to train 23 (for example, speed, weight and length of train 23).

In addition, each of the two Eurobalisms 211 and 212 is geographically linked, that is, it knows the corresponding exact location, and when passing trains through inductive means or by radio, the corresponding location. When the train 23 passes through the Eurobalises 211 and 212, the receiver 232 receives the positions transmitted by the mentioned Eurobalises 211 and 212, and feeds them to the on-board computer 231.

In addition, the on-board computer 231 is connected to the on-board odometer (not shown in FIG. 2 for simplicity) of the train 23, for receiving from the latter estimates the position of the train 23. The on-board computer 231 corrects the said estimates based on the positions adopted from the Eurobalances 211 and 212. The on-board computer 231 displays, on the control panel 234, position estimates transmitted by the airborne odometer when it does not have the exact positions available by the eurobalises 211 and 212, whereas when it takes the exact positions transmitted by the eurobalisms 211 and 212, it displays said exact positions on the control panel 234.

Ultimately, the position of the train 23, the direction of the train 23, together with all other necessary information, is automatically transmitted via the on-board computer 231 to the RBC 22 via the GSM-R terminal 233. Thus, RBC 22 tracks the movement of train 23.

At the same time, ERTMS-ETC study of the third level is still being carried out, since some aspects related to train safety still require more in-depth study. In a broad sense, the third-level ERTMS-ETC involves the elimination of a large number of ground-based devices and the transfer of location and reliability control of trains to specially designed airborne transmitting devices that constantly exchange data with the center to process and manage data regarding the movement of trains in a particular section. In addition, the ERTMS-ETC of the third level surpasses the concept of sections from fixed blocks by introducing sections from dynamic blocks that are not modeled in a predetermined physical space, but are formed in accordance with the circulation requirements and with the capabilities provided by the radio transmission system.

SUMMARY OF THE INVENTION

The present invention was developed to meet the need for reliable positioning systems for controlling trains in motion and, accordingly, a comprehensive study was conducted aimed at developing a new train positioning system that would satisfy the aforementioned need in the railway industry and guarantee the safety of the railway movement.

The aim of the present invention, therefore, is to provide a system for determining the location of trains, which allows for reliable location and guarantee the safety of railway traffic.

The above goal is achieved by the present invention, since it relates to a satellite terminal and a train positioning system in accordance with what is defined in the attached claims.

In particular, the satellite terminal, in accordance with the present invention, is designed to be installed on board a train, configured to receive navigation signals from satellites belonging to one or more satellite navigation systems, and characterized in that, in addition, it is configured to :

storing geo-referenced data of the train railway route; and

determining, based on the stored geolocation data and received navigation signals, the position of the train along the railway route and the level of confidence associated with the estimated position.

In detail, the confidence level indicates the maximum error associated with the calculated position.

In particular, the satellite terminal is configured to:

extracting from the received navigation signals location data corresponding to the satellites that transmitted said navigation signals;

determining, based on the stored geolocation data and location data corresponding to at least two satellites, the position of the train along the railway route; and

determining, based on the stored geo-referenced data and location data corresponding to at least three satellites, the confidence level associated with said estimated position.

Preferably, the satellite terminal is configured to:

calculation, for each set of three satellites from which the navigation signals are received, of the corresponding position associated with the railway route, based on the stored geo-referenced data and location data corresponding to the three satellites, and the corresponding level of protection based on said corresponding position associated with the railway route, and location data corresponding to the three satellites mentioned, the corresponding level of protection indicates a maxim nuyu error associated with said corresponding position bound to the railway route;

selecting a set of three satellites in accordance with selection criteria based on at least calculated protection levels; and

determining the position of the train along the railway route and the level of confidence associated with the said position based on, respectively, the position associated with the railway route and the level of protection calculated for the selected set of three satellites.

In particular, each position associated with the railway route, calculated for a set of three satellites, contains:

the corresponding first coordinate, denoting the average height of the railway route, calculated on the basis of geographical data;

the corresponding second coordinate equal to zero; and

the corresponding third coordinate corresponding to the curved abscissa associated with the railway route and calculated on the basis of the stored georeferencing data and location data corresponding to the three satellites.

In more detail, for each position associated with a railway route calculated for a set of three satellites,

the corresponding first coordinate corresponds to the first reference axis, essentially vertical relative to the surface of the Earth; and

the corresponding second coordinate and the corresponding third coordinate correspond, respectively, to the second reference axis and the third reference axis, respectively, which are mutually perpendicular and are in a plane tangent to the Earth's surface.

Preferably, the satellite terminal is configured to calculate for each set of three satellites from which it receives navigation signals:

the corresponding position associated with the railway route and the corresponding time offset associated with the navigation signals received from the three satellites, based on the stored geolocation data and location data corresponding to the three satellites;

the corresponding average error associated with the second coordinate of the corresponding position associated with the railway route based on the first and third coordinates of the corresponding corresponding position associated with the railway route, the corresponding time offset, and location data corresponding to the three satellites; and

an appropriate level of protection based on the corresponding average error, so that the maximum error associated with said corresponding position associated with the railway route is less than said corresponding level of protection.

In particular, the satellite terminal is configured to calculate for each set of three satellites from which it receives navigation signals:

the corresponding variance associated with the corresponding average error based on a predetermined probability distribution; and

appropriate level of protection based on a multiple of the corresponding variance.

Thus, the satellite terminal is configured to select a set of three satellites for which the minimum level of protection has been calculated.

Alternatively, the satellite terminal is configured to:

calculating, for each set of three satellites from which it receives navigation signals, the corresponding accuracy reduction index based on the corresponding position associated with the railway route and the location data corresponding to the three satellites and the corresponding reliability index based on the corresponding accuracy reduction index and appropriate level of protection; and

selection of a set of three satellites based on calculated reliability indices.

In addition, the train positioning system in accordance with the present invention, designed for installation on board a train, comprises the satellite terminal mentioned above and is configured to:

receiving from the odometer installed on board the train, a current position estimate transmitted by said odometer;

receiving the exact position of the train from the alarm system installed along the railway route;

if it receives the exact position of the train from the alarm system, supplying, as the current position of the train, the said exact position and correcting the current position estimate supplied by the odometer based on said exact position;

if it does not accept the exact location of the train from the alarm system, and the satellite terminal determines the current position of the train along the railway route, related to the level of confidence that meets the specified safety conditions on the railway, the current position determined by the satellite terminal as the current position of the train is determined and correcting the current position estimate supplied by the odometer based on said current position determined by the satellite terminal;

if it does not accept the exact position of the train from the alarm system, and the satellite terminal determines the current position of the train along the railway route, associated with a confidence level that does not satisfy the specified safety conditions on the railway, the current position estimate transmitted by the odometer as the current position of the train; and

if it does not accept the exact position of the train from the alarm system, and the satellite terminal does not determine the current position of the train along the railway route, the current position estimate supplied by the odometer as the current position of the train.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, some preferred embodiments, which are provided solely for explanation and by way of non-limiting example, will be presented with reference to the attached drawings (which are not drawn to scale), in which:

in FIG. 1 is a schematic illustration of an example scenario in which a first level ERTMS-ETC operates;

in FIG. 2 is a schematic illustration of an example scenario in which a second level ERTMS-ETC operates;

in FIG. 3 is a schematic illustration of a train positioning system in accordance with a preferred embodiment of the present invention;

in FIG. 4 is a schematic illustration of an architecture of an ERTMS-ETCS system, within which an architectural layer is integrated to determine a satellite’s location, in accordance with a preferred embodiment of the present invention;

in FIG. 5 shows a typical odometer error and an odometer error corrected using a satellite location in accordance with a preferred embodiment of the present invention;

in FIG. 6 shows a Cartesian support system, presented as an example used in calculating the position of a train in accordance with a preferred embodiment of the present invention; and

in FIG. 7 is a graph that represents errors and levels of protection that can be obtained in locating a train using the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description is presented to provide a person with skills in the art, the possibility of implementation and use of the invention. Various modifications of the presented embodiments will be immediately apparent to those skilled in the art, and the general principles disclosed herein can be applied to other embodiments and applications, thus without departing from the scope of the present invention.

Therefore, the present invention should not be understood as limited only to the described and presented embodiments, but it should be afforded the greatest degree of protection in accordance with the principles and characteristics set forth herein and defined in the attached claims.

The present invention is based on the idea of the present applicant to use one or more global satellite navigation systems (GNSS), such as, for example, GPS, the European Galileo satellite navigation system, the Russian GLONASS satellite navigation system, etc., to determine the position of a train. In fact, this applicant, on the basis of intuition, understands that the use of GNSS to control the movement of trains would significantly simplify the travel infrastructure, significantly reducing the number of balis and, consequently, the costs associated with maintaining the infrastructure, which are currently especially high. In addition, the present applicant also intuitively understands that through the use of satellite navigation information, it would be possible to switch to the concept of continuous balise, since satellite data could potentially be used anywhere in the railway network.

Ultimately, the present applicant also understands that at the present time, when GNSS is operated to determine the location of trains, it is necessary, in order to guarantee the safety of railway communication, to have also available certification of satellite navigation data, that is, information about the reliability of the position estimate.

Therefore, a first aspect of the present invention relates to a satellite terminal that has been designed to be installed on board a train and configured to:

receiving navigation signals from satellites belonging to one or more satellite navigation systems, for example, belonging to GPS and / or the Galileo system and / or the GLONASS system;

storing geo-referenced data for the train’s railway route, and

determining, based on the stored geo-referenced data and received navigation signals, the position of the train along the railway route and the level of confidence associated with the estimated position.

In particular, the confidence level indicates the maximum error associated with the estimated position.

Since the aforementioned satellite terminal determines the position of the train with the provision of real-time certification, that is, the level of its reliability, it becomes possible to guarantee the safety of railway communications. In particular, said satellite terminal, in order to certify a train position calculated on the basis of navigation signals received from a plurality of GNSS satellites, verifies in real time the correct functioning of said GNSS satellites.

Next, operation of the satellite terminal in accordance with the present invention will be described in detail.

As you can see, a train can usually move along a predetermined route. This feature allows you to operate a reduced number of GNSS satellites to calculate the position of the train. In particular, it is possible to calculate the position of the train using navigation signals received from only two GNSS satellites. If more than two GNSS satellites are available, it also becomes possible to obtain information about the reliability of the satellite data itself.

Furthermore, in addition to the reliability information, it is also possible to improve accuracy based on a data accuracy index that can be obtained by represented Geometric Attenuation Accuracy (GDOP), which is composed of the contribution associated with potential PDOP uncertainty (position DOP) and the contribution associated with with uncertainty of TDOP time (DOP time); this index depends on the angular distance that separates each of the GNSS satellites that are in the field of view of the train whose location you want to determine.

Within the framework of position uncertainty, it is also possible to identify the vertical uncertainty of VDOP (vertical DOP) associated with the vertical coordinate and the uncertainty of direction in the HDOP motion plane (horizontal DOP). These concepts, which are well known in the field of aeronautical engineering, have undergone strong reinterpretation in the context of the analysis of train movement, in fact, in the railway industry, it becomes possible to introduce the concept of sDOP, where s denotes a curved abscissa that identifies the track imposed on the train by rail.

In particular, the sDOP uncertainty corresponding to the curvilinear abscissa s can be estimated by projecting the components of the position error known from the classical approach onto the direction of the route that the train follows, which is the reliability of the data provided by a possible inertial navigator on board the train, for example, an odometer. In itself, the calculation of DOP will in any case be modified in relation to what happens in accordance with the classical approach in the field of air navigation. In fact, the presence of a geometric constraint imposed by railroads reduces the number of degrees of freedom, and therefore the number of GNSS satellites, needed to evaluate the position. In particular, if zero is known, that is, the initial position on the curvilinear abscissa s, that is, if the starting point of the train is known, the number of GNSS satellites needed to estimate the curvilinear abscissa s and correct the time offset drops to two. This means that in the presence of multiple GNSS satellites it is always possible to identify the best pair, or set of three GNSS satellites, in order to minimize sDOP, therefore, improving accuracy in addition to validation.

Therefore, based on what has been described above, the satellite terminal, in accordance with the present invention, is configured to:

extracting from the received navigation signals navigation data corresponding to GNSS satellites that transmitted said navigation signals;

determining, based on the stored geolocation data and location data corresponding to at least two GNSS satellites, the position of the train along the railway route; and

determining, based on the stored geo-referenced data and location data corresponding to at least three GNSS satellites, the confidence level associated with said estimated position.

To determine the position of the train, the aforementioned satellite terminal usually uses a Cartesian support system installed in such a way that the z axis coincides with the local vertical of the Earth’s surface, the y and x axes, which are perpendicular to each other, are located in a plane tangent to the Earth’s surface, and the y axis oriented in a direction consistent with curvilinear abscissa s.

Using the curvilinear coordinate s and the Cartesian reference system mentioned above and taking into account that the coordinate of the train with respect to the x axis is 0 and that the coordinate of the train with respect to the z axis is equal to the average local radius of the Earth increased by the average local elevation (the mentioned values are known for the satellite terminal, thanks to the stored geo-referenced data regarding the length of the railway route that the train should go through), it becomes possible to reduce the number of unknown x systems of pseudorange equations; that is, the residual unknowns are the values of the curvilinear coordinate s and the time offset δt.

In particular, the mentioned time offset δt arises from

mainly time offsets between the clock of the satellite terminal and the clock of the GNSS satellites, of which the said satellite terminal receives navigation signals; and

secondly, phase shifts introduced into the navigation signals due to various factors, for example, due to the phenomenon of multipath propagation, propagation through the atmosphere, in particular, the ionosphere, etc.

Therefore, two GNSS satellites are enough to solve a system of two pseudorange equations of two unknowns; namely, it is possible to calculate the value of the curvilinear coordinate s and the time offset δt based on location data corresponding to only two GNSS satellites, whereas if location data corresponding to three or more GNSS satellites were available, it would also be possible to introduce a criterion for estimates of the error obtained in determining the curvilinear coordinate s.

For example, based on the hypothesis that the satellite terminal receives navigation signals from five GNSS satellites, the satellite terminal can usually perform the following operations to calculate the position of the train and estimate the error:

протяженности железнодорожного маршрута, по которому должен двигаться поезд, упомянутую среднюю высоту

рассчитывают на основе данных географической привязки на протяженности железнодорожного маршрута, сохраненных спутниковым терминалом); и,for each possible combination of three GNSS satellites, the satellite terminal determines, based on the location data corresponding to the three GNSS satellites, the corresponding time offset Δt and the corresponding y coordinate value (i.e., the curved coordinate s), imposing, in the corresponding system of three equations pseudorange = 0 and z equal to the average local radius of the Earth, increased by the average local elevation (that is, by setting the level z equal to the average height

the length of the railway route on which the train should move, the aforementioned average height

calculated based on geo-referenced data along the length of the railway route stored by the satellite terminal); and,

for each possible combination of three GNSS satellites, the satellite terminal enters into the corresponding system of three pseudorange equations the corresponding values calculated for y and δt, excluding for x the restrictions of its equality to zero, and thus determines the error with which each pseudorange equation is introduced along the x coordinate, based on the pair of corresponding solutions found for y and δt.

Thus, the satellite terminal receives, for each possible set of three GNSS satellites, a corresponding value for y, a corresponding value for δt, and three corresponding errors for x. With five GNSS satellites available, a satellite terminal can view N sets of three GNSS satellites, i.e., N simple combinations of three GNSS satellites, where

As a result of the error analysis, the satellite terminal can thus exclude two GNSS satellites for which the largest error is obtained and, therefore, only considers the combination or combinations generated by GNSS satellites that produce the smallest error. Thus, GNSS satellites with significantly erroneous data can be excluded from the calculation of the position of the train.

In particular, assuming that usually one or more sets of three GNSS satellites can be identified, it becomes possible to calculate, for each set of three satellites in question, the corresponding average error corresponding to the x coordinate; in particular, it becomes possible to calculate the average of the three corresponding errors calculated in accordance with the x coordinate. In addition, if we assume that the errors have an isotropic characteristic, the average error corresponding to the x coordinate also denotes the average error corresponding to the y coordinate, that is, corresponding to the curvilinear coordinate s.

For each set of the three GNSS satellites in question, it is therefore possible to calculate:

based on the corresponding average error corresponding to x, the corresponding variance σ (which, based on the isotropic error hypothesis, denotes the corresponding variance of the error corresponding to y), if, for example, we take the Gaussian error distribution curve; and,

based on the corresponding change in variance σ, the corresponding level of protection L _P , which indicates the maximum error potentially introduced into the estimation of the position of the train, and therefore inversely proportional to the accuracy of the estimation of the position of the train; for example, the protection level L _P can be conveniently calculated as a multiple of the variance σ, that is _, L _P = A · σ, where A≥2.

At this time, the satellite terminal discards, based on the protection level L _P calculated for different sets of three GNSS satellites, those GNSS satellites that, when taken into account to calculate the position of the train, determine the highest level of protection L _P , choosing the set to determine the position of the train or sets of three GNSS satellites, which are formed only by GNSS satellites, allowing to obtain the lowest level of protection L _P.

), которое связано с минимальным уровнем защиты L_P, следовательно, уровень достоверности, связанный с упомянутым положением поезда, определяют на основе упомянутого минимального уровня защиты L_P.In particular, the satellite terminal can determine the position of the train based on the estimated position (0, y,

), which is associated with a minimum level of protection L _P , therefore, the confidence level associated with said position of the train is determined based on said minimum level of protection L _P.

Alternatively, a satellite terminal may:

), рассчитанных на основе данных местоположения, соответствующих упомянутым трем спутникам GNSS, иcalculate for each set of three GNSS satellites from which it receives navigation signals, the corresponding DOP index based on location data corresponding to the three GNSS satellites and the corresponding position (0, y,

) calculated based on location data corresponding to the three GNSS satellites mentioned, and

an appropriate reliability index based on said corresponding DOP index and the corresponding level of protection L _P ;

choose a set of three GNSS satellites based on calculated reliability indicators; for example, a satellite terminal may select a set of three GNSS satellites that corresponds to a reliability metric that minimizes the appropriate combination of the DOP index and protection level L _P ; and

) и уровня защиты L_P, рассчитанных для набора из трех выбранных спутников.determine the position of the train and the level of confidence associated with the said position, based on, respectively, the position (0, y,

) and protection level L _P calculated for a set of three selected satellites.

Thus, a satellite terminal that receives navigation signals from five GNSS satellites can identify up to two “erroneous” GNSS satellites; namely, it cannot be used to calculate the position of the train. In the case where there are three GNSS satellites that are causing an error, the satellite terminal still allows you to choose the best configuration, but the error cannot be completely eliminated, and the value of the protection level increases.

In the case where there are more than three GNSS satellites that cause errors, the satellite terminal can no longer determine the reliability, but provides a higher level of protection.

In this regard, five examples of satellite data reliability analysis are presented below when a satellite terminal receives navigation signals from five GNSS satellites, each example being summarized in the corresponding table.

In particular, the following are:

Table 1, which summarizes the data from the first scenario of the example in which the satellite terminal receives navigation signals from five GNSS satellites, none of which lead to an error (satellites that do not lead to errors, are connected in the five tables below with the symbol “• ") that is, all of these five GNSS satellites can be used by a satellite terminal to determine the position of a train with a minimum level of protection, that is, receiving a minimum error (the mentioned minimum level of protection is indicated in the following tables as 1);

Table 2, which summarizes the data of the second scenario of the example in which the satellite terminal receives navigation signals from five GNSS satellites, only one of which leads to an error (the satellites that lead to errors, are indicated by the symbol “X” in the following tables), i.e. in which four GNSS satellites can be used by a satellite terminal to determine the position of a train with protection level 1;

Table 3 summarizes the data of the third scenario of the example in which the satellite terminal receives navigation signals from five GNSS satellites, two of which lead to errors, that is, in which only three GNSS satellites can be used by the satellite terminal to determine the position of the train with the level protection 1;

Table 4, which summarizes the fourth scenario of the example in which the satellite terminal receives navigation signals from five GNSS satellites, three of which lead to errors, that is, in which the satellite terminal allows you to determine the position of the train with an average level of protection, that is, with an average an error (the mentioned average level of protection is indicated in the following tables 2); and

Table 5, which summarizes the fifth scenario of the example in which the satellite terminal receives navigation signals from five GNSS satellites, four of which lead to errors, that is, in which the satellite terminal allows you to determine the position of the train only with a high level of protection, that is, with very high error (the mentioned high level of protection is indicated in the following tables 3).

Table 1 5 SATELLITES, ALL OF WHICH CAN BE USED Three satellite combination from ₁ c ₂ c ₃ c ₄ c ₅ c ₆ c ₇ c ₈ c ₉ c ₁₀ Satellites S ₁ • • • • • • S ₂ • • • • • • S ₃ • • • • • • S ₄ • • • • • • S ₅ • • • • • • Protection level one one one one one one one one one one

table 2 5 SATELLITES, 4 OF WHICH ARE SUITABLE FOR USE Three satellite combination c ₁ c ₂ c ₃ c ₄ c ₅ c ₆ c ₇ c ₈ c ₉ c ₁₀ Satellites S ₁ • • • • • • S ₂ X X X X X X S ₃ • • • • • • S ₄ • • • • • • S ₅ • • • • • • Protection level 2 2 2 one one one 2 2 2 one

Table 3 5 SATELLITES, 3 OF WHICH ARE SUITABLE FOR USE Three satellite combination c ₁ c ₂ c ₃ c ₄ c ₅ c ₆ c ₇ c ₈ c ₉ c ₁₀ Satellites S ₁ • • • • • • S ₂ X X X X X X S ₃ • • • • • • S ₄ X X X X X X S ₅ • • • • • • Protection level 2 3 2 2 2 one 3 2 3 2

Table 4 5 SATELLITES, 2 OF WHICH ARE SUITABLE FOR USE Three satellite combination c ₁ c ₂ c ₃ c ₄ c ₅ c ₆ c ₇ c ₈ c ₉ c ₁₀ Satellites S ₁ • • • • • • S ₂ X X X X X X S ₃ X X X X X X S ₄ X X X X X X S ₅ • • • • • • Protection level 3 3 2 3 2 2 3 3 3 3

Table 5 5 SATELLITES ONLY ONE OF WHICH IS SUITABLE FOR USE Three satellite combination c ₁ c ₂ c ₃ c ₄ c ₅ c ₆ c ₇ c ₈ c ₉ c ₁₀ Satellites S ₁ X X X X X X S ₂ X X X X X X S ₃ X X X X X X S ₄ X X X X X X S ₅ • • • • • • Protection level 3 3 3 3 3 3 3 3 3 3

All of the above examples relate to the case where a satellite terminal receives navigation signals from five GNSS satellites, which is the most common case for a GNSS receiver. In any case, the methodology for calculating the position of the train described above can obviously also be applied to the case when the satellite terminal receives navigation signals from four GNSS satellites. In this case, four sets of three GNSS satellites can be considered, and therefore it is possible to identify only one satellite with an error. In addition, the methodology for calculating the position of a train described above can be applied, obviously, also in cases when a satellite terminal receives navigation signals from more than five GNSS satellites. In these cases, the number of satellites with an error that can be identified increases.

In accordance with a second aspect of the present invention, the satellite terminal described above can preferably be used with a first level and a second level of ERTMS-ETC.

In particular, the position of the train transmitted by the satellite terminal, together with the reliability information regarding the said position, can preferably be used to correct the position estimate provided by the on-board odometer of the train. Thus, it becomes possible to eliminate the need for a dense distribution of Eurobalysis (one per kilometer or less) and limit their use to just a few points. In fact, reliable location data transmitted by the satellite terminal replaces and extends the concept of balise. In fact, the use of satellite position data eliminates the odometer integration error, which, based on angular velocity data, and therefore satellite position data, if associated with a significant point, constitutes a virtual balis. In addition, satellite location data are much more representative: associated with a fully geographically linked railway line, they can be used at any time along the route, therefore, they revolutionize the very idea of fixed significant points.

The technical advantage associated with the use of the satellite terminal described above is represented by the fact that the latter allows the use of on-board odometers, which have less accuracy and, therefore, are less expensive (both in production and in terms of service life).

In FIG. 3 schematically illustrates a system for locating trains in accordance with a preferred embodiment of the present invention.

In particular, in FIG. 3 illustrates, by way of non-limiting example, the integration of said position determination system in the second level of ERTMS-ETC.

In detail, in FIG. 3 are shown:

a section of the railway line (generally indicated by number 31) that contains Eurobalysis (indicated as 311);

RBC (designated as 32); and

a train (generally labeled 33) that travels along a portion of the railway line 31 and an on-board computer (labeled 331) is installed on board, which is connected to a receiver (labeled 332), to the GSM-R terminal 333, which provides exchange of RBC 32 information with a GNSS terminal 334, which contains a geographically linked route followed by train 33 and calculates the position of train 33 on the route described above, and a control panel (indicated as 335) configured to provide information to the driver (indicated , as 336) trains 33.

In detail, RBC 32 transmits to the GSM-R terminal 333 information related to the section of the railway line 31, such as, for example, authorizing the movement of trains, their deceleration and the maximum permitted speed. The GSM-R terminal 333 transmits information received from the RBC 32 to the on-board computer 331. The on-board computer 331 displays on the control panel 335 the information received from the RBC 32 via the GSM-R terminal 333, together with other information (for example, the current braking profile train 33) obtained based on processing of said information received from RBC 32 and other information related to train 33 (for example, speed, weight and length of train 33).

In addition, Eurobalis 311 is geographically linked, that is, it knows its own exact location, and it transmits, when passing the train, through inductive means or by radio, the exact location mentioned. When train 33 passes through Eurobalis 311, receiver 332 assumes the position transmitted by said Eurobalis 311 and transfers it to the on-board computer 331.

In addition, the on-board computer 331 is connected to the on-board odometer (not shown in FIG. 3 for reasons of simplicity) of the train 33 for receiving from the last assessment of the position of the train 33. The on-board computer 331 is configured to:

if he receives from the receiver 332 the exact location supplied from the Eurobalism 311, determining how the position of the exact position train 33 transmitted by the Eurobalism 311, and correcting the position estimate transmitted by the airborne odometer based on said exact position;

if he does not receive from the receiver 332 the exact position transmitted by Eurobalysis 311, and the GNSS 334 terminal provides a position related to a confidence level that satisfies the specific safety conditions on the railway, determining as the position of the train 33, the position transmitted by the GNSS terminal 334, and determining an estimate of the position transmitted by the airborne odometer based on said position supplied from the GNSS terminal 334;

if he does not receive from the receiver 332 the exact position provided by Eurobalysis 311, and the GNSS terminal 334 submits a position related to a confidence level that does not satisfy the specific safety conditions on the railway, determining the position estimate transmitted by the airborne odometer as the position of the train 33;

if he does not receive the exact position transmitted by Eurobalysis 311 from the receiver 332, and the GNSS terminal 334 does not provide any position (for example, since the train 33 is located in an area not covered by any GNSS), the position estimate of the position 33 of the train 33 is determined transmitted by an onboard odometer.

The specific safety conditions on the railway can preferably be stored by the on-board computer 331 and / or can be determined dynamically by the on-board computer 331, and / or can be fed dynamically to the on-board computer 331 RBC 32 via the GSM-R terminal 333. For example, the on-board computer 331 may determine specific railway safety conditions based on information relating to a portion of a railway line 31 received from RBC 32 and data regarding a train 33, such as, for example, speed, weight and length of a train 33. B in particular, the on-board computer 331 can preferably evaluate whether the current level of confidence associated with the position provided by the GNSS terminal 334 satisfies the railway safety conditions for the railway section 31 to guarantee railway safety on the mentioned section of the railway line 31.

Finally, the position of the train 33, the direction of movement of the train 33, together with all other necessary information, is automatically transmitted via the on-board computer 331 to the RBC 32 via the GSM-R terminal 333. Thus, RBC 32 monitors the movement of train 33.

Based on what has been described above, it becomes clear that the satellite terminal in accordance with the present invention, in particular, is useful in the future ERTMS-ETC, in so far as:

it guarantees real-time location data certification procedures; that is, he is able to submit in real time the level of reliability of the calculated position;

it allows you to carry out its integration at the first level and at the second level ERTMS-ETC with minimal modifications to the current configuration of the mentioned system; in particular, it does not require any significant modifications of the Radio Block Center and requires only minor modifications of the system on board trains;

it allows you to function as a virtual balis, thus providing the possibility of evolution of the use of balis from the concept of discrete use to a specific innovative concept of use without any gap, which allows you to correct errors on the odometer at any point along the length of the railway route and, therefore, provides the opportunity entering the third level ERTMS-ETC, that is, the mobile unit.

In general, a satellite location in accordance with the present invention can preferably be integrated into the ERTMS-ETC architecture as an overlay level, as schematically shown in FIG. four.

In particular, in FIG. 4 is a block diagram illustrating the architecture of an ERTMS-ETCS system that integrates a satellite location in accordance with the present invention.

In more detail, the architecture shown in FIG. 4 contains:

architectural level ERTMS-ETC type 41; and

GNSS location architecture level 42, in accordance with the present invention, which is partially superimposed on the ERTMS-ETC type 41 architecture level.

As described above, the GNSS system for locating trains in accordance with the present invention operates as follows:

if balsa is present, the position of the train is the position given by balsa, and the onboard odometer error is reduced to zero using the position supplied by balsa;

if there is no balsa, and the confidence level supplied to the GNSS location satisfies the specific safety conditions on the railway, the train position is the position obtained through the GNSS location, and the on-board odometer error is corrected using the position obtained through the GNSS location;

if there is no balsa and the level of confidence supplied to the GNSS location does not satisfy the specific safety conditions on the railway, the train position is the position supplied by the airborne odometer; and

if there is no balsa and the GNSS location does not allow to obtain any position, the position of the train is the position given by the airborne odometer.

Assuming that balises can be installed with extreme accuracy (of the order of one meter), due to geographical reference (for example, using GPS receivers) having statistical characteristics that are quite long in time, errors in the case of ERTMS-ETC mainly depend on the accuracy of the airborne odometer, the type of route that the train traveled (slipping on rails, braking, etc.), and the distance between two consecutive balises.

In FIG. 5 is a graph representing an odometer error and an odometer error corrected based on a position obtained through GNSS location determination as a function of a train position (assuming a train speed of 300 km / h and a linear error due to slippage).

As shown in FIG. 5, the maximum error due to the odometer after 10 km is 300 m, while the odometer error corrected based on the position obtained through GNSS location determination is always on the order of several meters.

The GNSS location error, in principle, depends on the position measurement, and is of the order of several meters, regardless of the state of the train speed, since the position is obtained directly on the basis of satellite triangulation, and not on the basis of speed integration (as in the case of an odometer). In addition, it is possible to reduce the ionospheric error by using GNSS signals at two frequencies.

An important advantage of the present invention is derived from the possibility of obtaining error information from position calculation data using a constraint for a train that follows geographically linked tracks. In fact, in this way, as described above, two unknowns remain for the train: the curvilinear coordinate and the time offset.

In this regard, in FIG. Figure 6 shows the Cartesian support system z _s x _s y _s shown as an example used in calculating the position of a train in accordance with the present invention.

, где

представляет собой среднюю высоту упомянутого маршрута 61, рассчитанную на основе данных географической привязки упомянутого маршрута 61. Предполагая, что x_s и y_s являются изотропными, и в отношении распределения ошибок (учитывая, что обе координаты x_s и y_s лежат в плоскости, касательной к поверхности Земли), как описано выше, становится возможным рассчитать значение y_s (и смещение по времени), путем решения системы уравнений псевдодальности и с последующим перерасчетом ошибки по x_s в отношении положения номинального "0".In particular, as shown in FIG. 6, on the axis y _s is represented curvilinear abscissa s, along which a train is moving, x _s-axis represents the direction normal curvilinear abscissa s, and z _s is the local earth's vertical surface. In FIG. 6, in addition, the number 61 denotes the route along which the train follows, which, as described above, is located so that each point along the mentioned route 61 x _s = 0 and z _s =

where

represents the average height of said route 61, calculated on the basis of the geo-referenced data of said route 61. Assuming that x _s and y _s are isotropic, and with respect to the distribution of errors (given that both x _s and y _s are in the plane tangent to the Earth’s surface), as described above, it becomes possible to calculate the value of y _s (and the time offset) by solving the system of pseudorange equations and then recalculating the error in x _s with respect to the position of the nominal “0”.

An estimate of these errors for each satellite makes it possible to calculate the protection level L _P in such a way that the mentioned protection level L _{P is} always greater than the error in y _s . The developed algorithm, in addition, allows you to provide important information on the various error components that must be obtained, of which the contribution of the ionosphere is of no small importance.

In FIG. Figure 7 shows a typical graph of the error and protection level L _P over a route of approximately 60 km.

As shown in FIG. 7, the error is always within the protection level L _P , which is calculated in real time for the best set of the three available GNSS satellites. The GNSS system for locating trains allows you to identify malfunctions of GNSS satellites and eliminate GNSS satellites that represent a malfunction from the calculation of the train position. The train route can be either rectilinear or curved, and can be approximated with a high degree of accuracy. In FIG. 7, it can be noted that, compared with a 300 m error of more than 10 km obtained using the on-board odometer (error shown in FIG. 5), the GNSS location introduces errors of less than 30 m on a route of almost any length. This implies that in order to obtain an error of less than 30 m, it is preferable to provide balises, not every 2-3 km, but one every 50-60 km, without changing the accuracy of measurement and the safety of railway transport. From this point of view, it should be noted that baliz can be installed in places that are easily accessible for maintenance and easily manageable, also from the point of view of system security.

Another important observation relates to the continuous availability of location data, which allows, at the same cost, to enter the ERTMS-ETC third-level mobile unit.

It is clear that if the satellite data is not available, or the error indicated by the confidence level is too high, for example, more than 50 m (a situation that can last for several seconds), the system allows you to transmit a signal (lack of protection or error outside the border), and only the odometer during this period will be the only source of information that it can use (data merging procedure, based on an exclusivity mechanism) to exclude many sources of information and.

From the above description, the advantages of the present invention can be easily understood.

In particular, it should be emphasized once again that the present invention can preferably be integrated into existing systems and future systems (that is, those that are already under development) for the administration, management, protection and transmission of signals about railway traffic; in particular, they can preferably be used with all three levels of ERTMS-ETC. In fact, the present invention:

provides location data that guarantees an efficient train location service;

guarantees the accuracy of location data, which allows to improve the monitoring and control procedures for train traffic;

provides real-time validity of location data, thus guaranteeing the safety (in the sense of "safety for life") of railway transport in real time; and

submits location data in such a way that their interaction with Eurobalances is possible.

Logically, the GNSS location data associated with a geographically anchored point along the railroad tracks (significant point) constitute a virtual balis. This leads to the fact that the number of physical balances can be reduced with the benefit of simpler and more economical management and administration of the system.

The real advantage of satellite data, however, is not that it does not need to be tied to a rigid, albeit virtual, system for determining the position by reference points, providing what can be called a continuous balise system. The concept of continuous balise is a fully finished with the third level ERTMS-ETC, which is not tied to a fixed section of the railway route. A key element for the adoption of satellite data in the ERTMS-ETC architecture, therefore, is the reliability of the data themselves in real time.

In addition, the present invention preferably corresponds to the development scenario of Italian and European railways, in which it is hypothetically proposed in the future to use the European Galileo navigation satellite system, which, as you know, will transmit information about the certification of satellite performance and the error introduced in the provision. But, since the present invention can preferably be used with any GNSS, it becomes possible to extend the use of satellite data in locating trains. An important reason for using a system based not only on the Galileo system is the “control” factor. In fact, it will, unlike Russia, China or India, use a non-private system (i.e., Galileo) for strategic and critical infrastructure, such as the railway industry. In this perspective, in the case of railways, it may be more appropriate to adopt a strategy of using multiple constellations of satellites (both as technologies for backing up information and as comparison technologies), instead of the usual control of only one system (in Europe, Galileo, in Russia GLONASS etc.). Therefore, since the present invention can be used with one or more GNSSs, it could allow for the development of a system for locating trains, which would allow inter-rail connectivity in different countries.

In the end, it should be emphasized once again that the present invention allows to determine at any time not only the position of the train, but also the maximum error that was made during the measurements, and to verify the correct operation of the satellites.

In the end, it becomes clear that various modifications can be made in the present invention, all of which are within the scope of the invention as defined in the appended claims.

Claims (7)

1. Satellite terminal (334), designed with the possibility of installation on board the train (33) and made with the possibility of:
storing geo-referenced data of the train's railway route (33);
receiving navigation signals from satellites belonging to one or more satellite navigation systems;
extracting from the received navigation signals location data corresponding to the satellites that transmitted said navigation signals; and
determining, based on the stored geolocation data and received navigation signals, the position of the train (33) along the railway route and the level of confidence associated with the specified position;
characterized in that it is additionally configured to:
if the said satellite terminal (334) receives navigation signals from only two satellites, determining the position of the train (33) along the railway route by calculating the position of the train attached to the railway route based on the stored geographic data and location data corresponding to the two satellites; and
if said satellite terminal (334) receives navigation signals from three or more satellites,
calculating for each set of three satellites from which the navigation signals are received, the corresponding position of the train tied to the railway route, based on the stored georeferencing data and location data corresponding to the three satellites, and the corresponding level of protection based on the said corresponding position of the train tied to the railway route, and location data corresponding to the three satellites, the aforementioned corresponding level of protection is the maximum error associated with said corresponding position of the train, bound for the railway route; selecting a set of three satellites in accordance with a selection criterion based on at least calculated protection levels; and
determining the position of the train (33) along the railway route and the level of confidence associated with the said position, based on, respectively, the position of the train associated with the railway route and the level of protection calculated for the selected set of three satellites. 2. The satellite terminal according to claim 1, configured to, if said satellite terminal (334) receives navigation signals from only two satellites, determine the position of the train (33) along the railway route by:
calculating the first coordinate of the position of the train tied to the railway route, based on the stored data of the geographic reference, said first coordinate indicating the average height of the railway route;
imposing a restriction that the second coordinate of said position of a train tied to a railway route is zero; and calculating a third coordinate of said position of a train tied to a railway route based on said first and second coordinates of said position of a train tied to a railway route and location data corresponding to said two satellites, said third coordinate corresponding to a curved abscissa associated with a railway route ;
wherein the satellite terminal (334) is configured to, if it receives navigation signals from three or more satellites, calculate for each set of three satellites from which the navigation signals are received:
the corresponding position of the train tied to the railway route by calculating the first coordinate of said corresponding position of the train tied to the railway route based on the stored geographic data, the first coordinate indicating the average height of the railway route;
imposing a restriction that the second coordinate of said corresponding position of the train tied to the railway route is zero; and calculating a third coordinate of said corresponding train position associated with a railway route and a corresponding time offset associated with navigation signals received from said three satellites based on said first and second coordinates of said corresponding train position associated with a railway route and data locations corresponding to said three satellites, said third coordinate corresponding to a curvilinear abscissa associated with road routes; and the corresponding average error associated with the second coordinate of said corresponding position of a train tied to a railway route, based on the first and third coordinates of said corresponding position of a train tied to a railway route, corresponding calculated time offset and location data corresponding to said three satellites; and an appropriate level of protection based on the corresponding average error so that the maximum error associated with said corresponding train position associated with the railway route is less than said corresponding protection level. 3. The satellite terminal according to claim 2, in which:
the first coordinate of each calculated position of the train, tied to the railway route, corresponds to the first reference axis, vertical relative to the surface of the Earth; and
the second and third coordinates of each calculated position of the train tied to the railway route correspond, respectively, to the second reference axis and the third reference axis, which are mutually perpendicular and are in a plane tangent to the Earth's surface. 4. The satellite terminal according to claim 2 or 3, further configured to calculate for each set of three satellites from which the navigation signals are received:
the corresponding variance associated with the corresponding average error based on a predetermined probability distribution; and
appropriate protection level based on a multiple of the corresponding variance. 5. Satellite terminal according to any one of paragraphs. 1-3, configured to select a set of three satellites for which the minimum level of protection was calculated. 6. Satellite terminal according to any one of paragraphs. 1-3, made with the possibility of:
calculating for each set of three satellites from which the navigation signals are received, a corresponding accuracy reduction index based on the corresponding train position associated with the railway route and location data corresponding to the three satellites, and a corresponding reliability index based on the corresponding accuracy reduction index, and appropriate level of protection; and
selection of a set of three satellites based on calculated reliability indices. 7. A system for determining the location of trains designed with the possibility of installation on board a train (33), comprising a satellite terminal (334) according to any preceding paragraph and configured to:
receiving from the odometer mounted on board the train (33), a current position estimate transmitted by said odometer;
receiving the exact positions of the train (33) from the alarm system (311) installed along the railway route;
if it receives the exact position of the train (33) from the alarm system (311), supplying, as the current position of the train (33), said exact position and correcting the current position estimate supplied by the odometer based on said exact position;
if it does not accept any exact position of the train (33) from the alarm system (311), and the satellite terminal (334) determines the current position of the train (33) along the railway route, associated with a confidence level that satisfies the predetermined safety conditions on the railway the road, supplying, as the current position of the train (33), the current position determined by the satellite terminal (334), and correcting the current position estimate transmitted by the odometer based on said current position determined about a satellite terminal (334);
if it does not accept any exact position of the train (33) from the alarm system (311), and the satellite terminal (334) determines the current position of the train (33) along the railway route, associated with a confidence level that does not satisfy the predetermined safety conditions for railway, filing as the current position of the train (33) the current position estimate transmitted by the odometer; and
if it does not accept any exact position of the train (33) from the alarm system (311), and the satellite terminal (334) does not determine any current position of the train (33) along the railway route, supply it as the current position of the train (33) the current position estimate reported by the odometer.

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