Data server used in a system for supplying augmentation data for the satellite navigation signals

The invention relates to a data server (40) used in a system (10) for supplying complementary data, so-called augmentation data, for satellite navigation signals. The inventive server (40) is especially adapted to be used with elements that are compatible with those used in EGNOS technology (European Geostationary Navigation Overlay Service). Said system (10) for supplying augmentation data for the satellite navigation signals comprises at least one calculator (20) for the determination of said augmentation data, said augmentation data being determined from data transmitted by at least one receiving station (S01. . . SON) receiving navigation information sent by means of at least one satellite. Said server (40) comprises a first inlet (401) for receiving the augmentation data transmitted by the calculator (20), a first outlet (402) for transmitting the augmentation data towards at least one user (U01. . . UOK), and a second outlet (403) for re-emitting the augmentation data towards the calculator (20) with a delay that is pre-determined in relation to the reception at the first inlet (401).

The present invention relates to a data server used in a system for supplying complementary data, known as augmentation data, for satellite navigation signals. The server of the invention is more particularly adapted to be produced with elements compatible with those used in the European Geostationary Navigation Overlay Service (EGNOS) technology.

The data supplied by Global Navigation Satellite Systems (GNSS) of the Global Positioning System (GPS) or GLONASS type are greatly enhanced by the use of the Satellite-Based Augmentation Systems (SBAS) technology. This technology provides further correction by satellite to the accuracy of the GPS, thereby very significantly augmenting the accuracy of position measurements. It also guarantees integrity and availability. SBAS satellite positioning accuracy augmentation systems are divided into three areas: EGNOS for Europe, WAAS for North America and MSAS for Japan.

FIG. 1is a diagram of a GPS satellite navigation system1using the EGNOS technology.

In the case of EGNOS, the receiver stations S1to SN are Remote Integrity and Monitoring Stations (RIMS) of which there are currently34.

The computer2is of the Central Process Facility (CPF) type. For clarity, only one computer2is shown, but there may equally well be several computers2. Thus for the EGNOS, there are in reality five CPF providing redundancy in the event of a computation difference or equipment failure; at any given time, the five CPF are operating continuously. However, at any given time, only one CPF, called the active CPF, is supplying data.

The transmission station4is a Navigation Land Earth Station (NLES) that transmits the data from the active CPF, which it selects at intervals of one second from the five CPF, as a function of the indication as to the quality of the computations that the latter announce.

The geostationary satellite5is of the Inmarsat III or ARTEMIS type.

In concrete terms, the set3of GPS satellites sends position data to the receiver stations S1to SN. The latter transmit the data to the computer2in particular. There, the data, called augmentation data, is computed by the computer2. The augmentation data enables the following functions to be provided, for example:differential basic corrections: broadcasting of ephemerides and clock corrections relative to the set3of GPS satellites,differential precise corrections: broadcasting of ionosphere corrections relating to the set3of GPS satellites,integrity (see below).

The augmentation data is combined before it is sent to the transmitter station4that transmits the data to the geostationary satellite5.

The geostationary satellite5redistributes the augmentation data to the users U1to UK, who also receive navigation signals from the set3of GPS satellites. The navigation signals combined with the augmentation data enable a user to determine his position with enhanced accuracy.

The augmentation data must be supplied by the computer2with a certain integrity, i.e. a capacity to supply augmentation data indicating to users with a high probability that the augmentation data is reliable and usable, thereby inducing a high level of security, compatible with the quantified data determined by the civil aviation authorities.

To transmit the integrity data, the computer2needs to receive in return and in real time all of the data that it transmits continuously to the users. If this is not the case, the computer declares itself non-integrated. Thus the computer itself verifies the data that it sends. In the case of EGNOS, and as shown inFIG. 1, the CPF computer2receives the data that it sends itself via the geostationary satellite5which retransmits the data to the CPF2via the RIMS receiver stations S1to SN. The path of the data therefore corresponds to a loop6, called the integrity loop. The geostationary satellite5, which transmits in the L band (1.6/1.4 GHz), therefore has a two-fold function, firstly, transmitting the augmentation data to the users and, secondly, providing the integrity loop6.

However, a solution of the above kind using a geostationary satellite to provide the integrity loop is subject to certain constraints or difficulties.

The cost of a geostationary satellite is prohibitive.

A certain number of countries wishing to evaluate the service gain by using an EGNOS type SBAS system have no geostationary satellite available.

A solution of the above kind imposes the broadcasting of augmentation data to users via the geostationary satellite and not via other broadcasting means.

The present invention aims to provide a system for supplying complementary data, called augmentation data, for satellite navigation signals, said system offering the opportunity to broadcast augmentation data by various broadcasting means without modifying or interfering with the augmentation data computer, the integrity of the system being assured without using a geostationary satellite or without having to receive what a geostationary navigation satellite broadcasts to retransmit it differently, and therefore with a delay.

To this end the present invention proposes a data server used in a system for supplying complementary data, called augmentation data, for satellite navigation signals, called user signals, said system including at least one computer for determining said augmentation data, which is determined from data transmitted by at least one receiver station receiving navigation information sent by at least one satellite, said server being characterized in that it has:a first input for receiving said augmentation data transmitted by said computer,a first output for sending said augmentation data to at least one user,a second output for retransmitting said augmentation data to said computer with a predetermined time-delay relative to reception at said first input.

Thanks to the invention, the augmentation data server provides the integrity loop. From its second output, the server feeds augmentation data back to the computer by simulating a transmission delay such as would have been induced by the presence of a geostationary satellite. As a result, the computer operates in the same way as with a geostationary satellite, but without the presence of the latter being necessary, and does not necessitate any modification compared to the versions qualified (and where applicable certified) by the various players.

Moreover, the first output of the server transmits the augmentation data to various broadcasting means such as the INTERNET, for example. The system of the invention therefore derives the augmentation data from a Satellite-Based Augmentation Systems (SBAS) type system, to supply it to a user without modifying the SBAS computer.

Said server advantageously has a third output for retransmitting at least part of said augmentation data to said computer at the same time as sending said augmentation data to the user via said first output.

Said server advantageously has a second input for receiving information data coming from at least one user.

Said server advantageously includes means for particularizing said augmentation data sent via said first output as a function of said information data coming from at least one user.

It is particularly advantageous if said server is assigned an available geostationary satellite identification number.

Said server is advantageously assigned a virtual receiver station number.

In one embodiment, said augmentation data is determined from data transmitted by a plurality of receiver stations, said server having a third input for receiving data transmitted by one of said receiver stations.

The invention also provides a system for supplying complementary data, called augmentation data, for satellite navigation signals, called user signals, said system including at least one computer for determining said augmentation data, which is determined from data transmitted by at least one receiver station receiving navigation information sent by at least one satellite, said system being characterized in that it includes at least one data server according to the invention.

One advantageous embodiment of said system includes a plurality of computers for determining said augmentation data, said augmentation data server including means for selecting a computer from said plurality of computers, said second output of said server retransmitting said augmentation data received from said selected computer to said plurality of computers with a predetermined time-delay relative to the reception of said augmentation data.

Said augmentation data retransmitted to said plurality of computers advantageously includes an identifier of said selected computer.

Said selection is advantageously repeated cyclically on each reception of said augmentation data by said server.

One particularly advantageous embodiment of said system includes at least one active first augmentation data server and one redundant second augmentation data server, said computer transmitting said augmentation data to said first input of said active server, and not transmitting said augmentation data to said first input of said redundant server, and said computer including means for inverting the roles of said first and second servers, said second server becoming the active server and said first server becoming the redundant server.

Said means for reversing the roles of said first and second servers are advantageously commanded cyclically on each sending of said augmentation data.

Said system includes broadcasting means connected to said first output of said server to broadcast said augmentation data to the users.

Said broadcasting means advantageously consist of the Internet.

One particularly advantageous embodiment of said system includes routing and broadcasting means, said augmentation data being determined from data transmitted by a plurality of receiver stations and then routed and broadcast to said computer by said routing and broadcasting means, said augmentation data retransmitted by said server being also routed and broadcast to said computer by said routing and broadcasting means.

Said system advantageously includes a plurality of augmentation data servers.

Other features and advantages of the present invention will become apparent in the following description of illustrative and nonlimiting embodiments of the invention.

In all the figures, common elements carry the same reference numbers.

FIG. 1has already been described with reference to the prior art.

The following description of the three embodiments shown inFIGS. 1 to 3relates to the EGNOS technology, but may be adapted to other technologies such as the WAAS technology and the MSAS technology.

FIG. 2is a diagram of a system10for supplying augmentation data for satellite navigation signals in accordance with a first embodiment of the invention.

In the case of EGNOS, the receiver stations S01to SON are Remote Integrity and Monitoring Stations (RIMS), of which there are currently34.

The computer20is of the Central Process Facility (CPF) type.

The system10broadcasts navigation information using its satellites30. That information is received by the N receiver stations S01to SON.

Each of those stations S01to SON transmits every second the data received (GPS navigation message, etc.) and measured (pseudo-distances, etc.) to the central processing facility (CPF) type computer20as shown by the arrow F3aas well as (optionally) to the third input406of the server40, as shown by the arrow F3b.The data transmitted by the receiver stations S01to SON is routed and broadcast as shown by the arrows F3aand F3bby the routing and broadcasting means70. The data is in a first message format adapted to the EGNOS technology. A plurality of messages of the above type are transmitted every second by each station to the computer20.

The computer20determines the navigation corrections to be applied and the associated integrity information and transmits them to the first input401of the server40as shown by the arrow F4. This data is referred to as augmentation data hereinafter.

The computer20considers the server40to be a virtual NLES type transmission station to a virtual geostationary satellite. Note that an identification number (PRN) of that virtual satellite is used, selected from those not reserved for other purposes (the reservation table is managed in accordance with the appendix of the RTCA standardization document MOPS D0229).

The augmentation data is transmitted to the server40like a set of messages using a second message format adapted to the EGNOS technology.

The server40receives these messages first, extracts the navigation overlay frame (NOF) navigation method contained in the received message and transmits the navigation message corresponding to the augmentation data from its first output402to the broadcasting means50.

The broadcasting means50broadcast the message to the users U01to UOK. The broadcasting means50consist of the Internet, for example. This transmission generally necessitates encapsulation of the NOF message in a message, joining to it the elements necessary for the transmission protocol layers used. Note, however, that other broadcasting means may equally be used.

The users therefore receive this NOF message as well as GPS signals and can use both kinds of information to compute a corrected GPS navigation solution and verify its integrity using the corrections contained in the NOF messages received over time.

To assure correct operation of the computer20, it is important for the computer20to return the NOF message transmitted to the users U01to UON.

This is assured by simulating a transmission delay as would be caused by a geostationary satellite. This is assured by the server40which sends from its second output403to the computer20, as shown by the arrow F7, the NOF message transmitted to users encapsulated in a message using a third message format adapted to the EGNOS technology. The NOF message is sent back with a predetermined time-delay relative to the reception at the first input401of the augmentation data from the computer20. That time-delay is equal to 1150 milliseconds, for example, starting from reception of the augmentation data from the computer20. The NOF message sent back passes in transit through the routing and broadcasting means70which are responsible for transmitting the message to the computer20as shown by the arrow F3a.

This therefore simulates the reception of the NOF message by a virtual RIMS receiver station receiving data only from the server40corresponding to a virtual geostationary satellite. The integrity loop necessary for correct operation of the computer20is provided by the path F4-F7-F3a.

Note that there may equally be used instead a real (i.e. not virtual) RIMS receiver station number, corresponding to one of the receiver stations S01to SON. In this case, the server40receives information from that real station as shown by the arrow F3band mixes the received information with the NOF message before sending the combination to the computer20.

Another loop that may be used assures fast return to the computer20of the NOF message transmitted to users. This is assured by the server40which sends to the computer20, as shown by the arrow F5, from its third output404, and immediately after sending the NOF message to the users, the same NOF message encapsulated in a message using a fourth format adapted to the EGNOS technology.

The server40can take another data stream into account: the arrow F6ccorresponds to a return to the second input405of the server40of information coming from a user or a user group. This stream authorized by the invention does not exist in the diagram of the prior art augmentation system using a geostationary satellite. The stream may optionally be used by the server40for two purposes:Modifying its behavior in relation to users and therefore the subsequent content of the messages transmitted as shown by the arrow F6a,Modifying its behavior in relation to the computer20and therefore the subsequent content of messages transmitted as shown by the arrows F7and F5.

The server40includes software means for implementing the data exchange algorithm.

One example of the above kind of algorithm is given below:

Repeat for Each Second GPS Number n, Denoted Sn

Step 1: Before Sn+150 milliseconds, encapsulate the NOF(Sn-1) in a message to the appropriate format, and send it to the computer20.

Step 2: Receive the per-second augmentation data Sn.

Step 3: Extract the NOF(Sn) message from said augmentation data and store it for the next cycle.

Step 4: Encapsulate the NOF(Sn) in a message with the format adopted to the broadcast interface used (for example the Internet) and send it to users.

Step 5: Encapsulate the NOF(Sn) in a message with the appropriate format and send it to the computer20(note that the EGNOS implementation also entails sending back the NOF messages of the three preceding cycles).

NOF(Sn) and NOF(Sn-1) respectively denote the NOF messages corresponding to the augmentation data received at the input401of the server40at the seconds Snand Sn-1and transmitted by the computer20.

FIG. 3is a diagram of a system100for supplying augmentation data for satellite navigation signals in accordance with a second embodiment of the invention.

The system100differs from the system shown inFIG. 2only in that it includes a plurality of n computers201to20n.

For clarity, the GPS satellites, the receiver stations, the broadcasting means and the users are not shown.

The computers201to20nreceive at intervals of one second navigation data coming from receiver stations as shown by the arrows F3a.lto F3a.n.

After processing by the computers201to20n,augmentation data is transmitted by the computers201to20nto the server40as shown by the arrows F4.1to F4.n,respectively.

The server40includes means for selecting an NOF message corresponding to the augmentation data received from a selected computer. After encapsulation in an appropriate format, the NOF message is sent to users and sent back to the set of computers201to20nwith a predetermined time-delay, as shown by the arrow F7.

The algorithm providing the selection means may be based on the presence of, for example:an integrity flag in the messages received from the computers, indicating if the sender of the message considers the computer to be integrated,a flag in the messages received from the computers indicating if the computer considers itself to have been selected by the server40,of a quality of service (QoS) value in the messages received from the computers.

Fast return of the NOF message transmitted to the users to the computers201to20nis equally possible, as shown by the arrows F5.1to F5.n,respectively.

FIG. 4is a diagram of a system101for supplying augmentation data for satellite navigation signals conforming to a third embodiment of the invention.

The system101differs from the system shown inFIG. 2only in that it includes a plurality k of servers41to4kof the invention. The k servers41to4kare grouped into pairs, for example: this type of system is useful in particular for enhancing system availability and continuity.

Consider here the example of only one pair, i.e. k=2.

Each server41and42of the pair receives at intervals of one second messages from the computer20as shown by the arrows F4.1and F4.2, respectively.

Only one of the two servers41(called the active server) of the pair receives from the computer20messages containing the NOF message to be transmitted to the users. The other server42(which is called the redundant server) receives a message containing no NOF message.

Each NOF message is retransmitted to the computer20as shown by the arrow F7.1k.

In each one-second cycle, the computer may decide to switch between the two servers41and42of the pair to activate that which was previously not activated.

To function in this way, each server must incorporate an algorithm that manages the change from an active mode to a redundant mode.

The fast returns shown by the arrows F5.1and F5.2are used to communicate the active or redundant mode of the server to the computer20.

Of course, the invention is not limited to the embodiments that have just been described.

In particular, there may equally be envisaged a system including at one and the same time a plurality of computers and a plurality of servers.

Moreover, the invention is not limited to the EGNOS technology and may be transposed to other technologies such as the WAAS and MSAS technologies.

Finally, referring toFIG. 1, the server40has been described as able to receive information from a real station as shown by the arrow F3band able to mix that received information with the NOF message before sending the combination to the computer20. The information received from a real receiver station may also be transmitted directly to the users from the first output402of the server40.