AU2015261635A1

AU2015261635A1 - Optimized architecture of a secondary ground station for generating a secondary sbas signal in an sbas system and method of generating a secondary sbas signal

Info

Publication number: AU2015261635A1
Application number: AU2015261635A
Authority: AU
Inventors: Hanaa ALBITAR; Bernard Charlot; Mathieu Raimondi; Lionel Ries
Original assignee: Thales SA
Current assignee: Thales SA
Priority date: 2014-11-26
Filing date: 2015-11-26
Publication date: 2016-06-09
Anticipated expiration: 2035-11-26
Also published as: FR3029045A1; AU2015261635B2; BR102015029458A2; EP3026460A1; BR102015029458B1; FR3029045B1

Abstract

Optimized architecture of a secondary ground station for generating a secondary SBAS signal in an SBAS system and method of generating a secondary SBAS signal A secondary NLES station comprises a generator (122) of a secondary SBAS signal in a secondary band including no equipment liable to impact on the generation of existing SoL level SBAS signals by an NLES-G2 first station. 10 The secondary NLES station comprises a receiver subsystem (106; 120, 122, 124), configured to receive on a downlink in the secondary SBAS band the secondary SBAS signal from the transmitter subsystem (106) retransmitted by an associated SBAS satellite (10). The receiver subsystem (106) is configured to verify the 15 synchronization and the consistency of the code relative to the carrier of the secondary SBAS signal received via a loop for correction of the phase of the carrier and of the phase of the code of the secondary SBAS signal of the signal transmitted by the transmitter subsystem (104). 20 Figure 2 C'j C C'j co coo C\-I (.0 000w (.0 wC:) o U- 00 0 c'o c'J Ir +

Description

Optimized architecture of a secondary ground station for generating a secondary SBAS signal in an SBAS system and method of generating a secondary SBAS signai

The present invention concerns the architecture of a secondary ground station for generating a secondary SBAS signal in a satellite-based augmentation system (SBAS). The secondary ground station for generating a secondary SBAS signal is configured to transmit in band E5b or in band E8 a secondary signal supporting non-critical secondary services, i.e. services not subject to the vital SoL (Safety of Life) security requirements of a critica! navigation service, and not to disturb critical services already certified and transmitted in bands L1/L5.

The present invention also concerns a method of generating a secondary SBAS signal implemented by a secondary ground station.

One SBAS system is the European Geostationary Navigation Overlay System (EGNOS) presently in service providing a single-frequency service in band L1 of SoL security level.

In the near future, the single-frequency SoL EGNOS service in band L1 will be replaced by a critical SoL level two-frequency (L1/L5) EGNOS service. To this end, the space segment has been reconfigured to be able to process an E5 channel having a bandwidth equai to 50 MHz and in which the lower part of the band is allocated to the L5 signai.

The band E5b remaining available in this E5 channel could therefore be used to transmit an EGNOS secondary signal for its own applications not subject to the requirements of an SoL security level service, but also not impacting on the SoL level service already tested and certified and supported by the L1/L5 two-frequency signai.

At present, the EGNOS system that provides an L1 single-frequency SoL level critical service utilizes a set of first Navigation Land Earth Stations (NLES) each configured to generate a single-frequency SoL signal in band L1 and broadcast it to end users via GEO (GEostationary Orbit) type payloads functioning on the Inmarsat 3, Inmarsat 4 and Artemis geostationary satellites. The uplink signal corresponding to the L1 signal transmitted by the first NLES stations to the geostationary satellites is retransmitted by said satellites to the first NLES stations and then processed by these same first NLES stations in order to be synchronized to the time of the EGNOS system, referred to as the EGNOS Network Time (ENT). Supplementing the first NLES stations, the ground segment of the EGNOS system includes Ranging And Integrity Monitoring Stations (RIMS) and a mission control centre (MCC). A number of EGNOS subsystems are being updated to implement the L1/L5 two-frequency SoL EGNOS service. More particularly, the L1 and L5 signals will be generated by a new version of the first NLES station, of a so-called second generation and hereinafter denoted NLES-G2. The space segment is also being updated to enable the implementation of the L1/L5 two-frequency SoL service. Each of the Astra 4B and Astra 5B geostationary satellites recently launched carries an L1/L5 two-frequency payload with approximately 50 MHz of bandwidth available for the E5 signal. It is intended that the Astra 4B satellite will replace the Artemis satellite currently at its end of life.

The L5 signal that must be used in the next L1/L5 version of EGNOS supplementing the LI signal occupies approximately 24 MHz of the Sower part of the band allocated to the E5 channel of the Astra 4B and 5B payloads.

The remaining upper part of the band of the E5 channel that is not used by the second generation L1/L5 two-frequency EGNOS system, denoted E5b hereinafter, offers the possibility of transmitting new services that will require additional correction data relative to that broadcast by the SoL signals.

However, broadcasting a new SBAS signal that shares the same EGNOS payload by activating the E5b channel of the payioad will be possible oniy if this activation entails no risk of uncontrolled impact on the performance of any SoL level critical EGNOS service, i.e. a service certified by the International Civil Aviation Organization (ICAO) in terms of integrity, availability, continuity and accuracy.

When transmitting a new SBAS signal independent of the existing SoL level signals and not impacting on these existing SoL signals is possible, the new signal is referred to as a secondary signal.

Following the example of the generation of SoL critical level L1/L5 signals, the paper by H. Al Bitar et a!., entitled “Augmenting EGNOS with an E5b Channel”, published in ION 2014, proposes that the E5b secondary signal be generated by an NLES type ground station given that the E5b signal will share the payload or payloads relaying the EGNOS SoL L1 and L5 signals.

The paper by H. Al Bitar summarizes the main functions implemented by an NLES station in the case of processing an SoL level SBAS signal, namely: .- generating the augmentation signal using pseudo-random noise (PRN) codes and particular data messages; .- synchronizing the signal transmitted at the output of the phase centre of the transmit antenna of the GEO satellite relative to the absolute time ENT of the system; .- verifying the consistency of the phase of the code relative to the phase of the carrier at the level of the phase centre of the transmit antenna.

The same paper proposes two NLES station architectures for generating the E5b secondary signal.

In accordance with a first architecture, a single NLES station is configured to generate an L1/L5/E5b three-frequency signal. This NLES station may be an augmented version of the first L1/L5 two-frequency station of the second generation (NLES-G2).

However, the first architecture has the major disadvantage of leading to the design of new hardware and new software, the long and costly implementation of a new ICAO procedure for qualification and certification of the NLES-G2 station itself and other impacted EGNOS subsystems, the writing of the corresponding documentation and an obligatory level of DAL B/C software implementation. Also, in this first architecture, the SoL security level L1/L5 EGNOS signals are weakly isolated relative to the new E5b signal and the risks of coupling between these signals are too numerous, which rules out considering the ESb signal generated by such an architecture as a secondary signal.

In accordance with a second architecture aiming to remedy the disadvantages described above, an second NLES station is configured to generate only the ESb secondary SBAS signal, and is independent of the classic NLES-G2 first station, configured and certified to generate the SoL security level EGNOS L1/L5 two-frequency signal· The second NLES station consists in a control computer followed by an ESb signal generator and a radio-frequency adapter to the uplink RF band. The second NLES station is weakly coupled to the NLES-G2 first station to receive therefrom piloting data and output data of the L1/L5 long-loop algorithm, as well as a 10 MHz local reference clock signal, and a signal generating one pulse per second (1 PPS), derived from the local clock signal.

In addition to avoiding new certification of an SoL context, this second architecture is simpler to implement because it does not utilize, or utilizes very little and in a controlled manner, equipments relating to the SoL level L1/L5 services, and provides better isolation between the two stations in the event of failure of one of them.

According to the aforementioned paper, if this architecture does not make it possible to generate L5 and E5b signals that are sufficiently mutually consistent and induces too many E5b signal synchronization errors to provide a ranging service, this minimalist second architecture for generating a secondary SBAS signal is nevertheless suitable for the provision of data dissemination services having classic continuity and availability requirements.

Now, data dissemination service continuity and availability problems have been encountered in the form of more or less frequent interruptions of service.

The technical problem is to determine an architecture of a second ground station for generating a secondary SBAS signal intended to support data dissemination services that is independent of equipments already certified for generating the SoL level SBAS signals, which is minimalistic in terms of the amount of equipment used, and which guarantees sufficient continuity and availability for data dissemination secondary services.

To this end, the invention consists in a secondary NLES station for generating a secondary signal of a satellite-based augmentation system (SBAS) intended to support data dissemination services, comprising a subsystem for transmitting the secondary SBAS signal in an uplink radiofrequency (RF) band including a generator of the secondary SBAS signal in a secondary SBAS band and a first uplink RF output port; and including no equipment liable to impact the generation of existing SoL level SBAS signals by an NLES-G2 first station; characterised in that the secondary NLES station includes a receiver subsystem, configured to receive in the secondary SBAS band at a second downlink RF input port a secondary SBAS signal from the transmitter subsystem retransmitted by an associated SBAS satellite and to verify the synchronization and the consistency of the code with respect to the carrier of the secondary SBAS signal received via a loop for correction of the phase of the carrier and of the phase of the code of the secondary SBAS signal of the signal transmitted by the transmitter subsystem.

In accordance with particular embodiments, the ground station has one or more of the following features: .- the receiver subsystem comprises a single-frequency receiver tuned to receive selectively the secondary SBAS signal in the secondary band, or a two-frequency receiver tuned to receive selectively the secondary SBAS signal in the secondary band and an SoL level SBAS signal in band L5, 24 MHz wide and centred on the frequency of 1176.45 MHz; .- the secondary band is the E5b channel band 24 MHz wide and centred on the frequency of 1207.14 MHz, or the E6 channel band 24 MHz wide and centred on the frequency of 1278.75 MHz; .- the transmitter subsystem includes an output filter selectively tuned to a band 20 MHz wide centred on the centre frequency of an uplink RF channel corresponding, apart from a transposition frequency, to the channel of the secondary SBAS signal generated by the transmitter subsystem; .- the receiver subsystem comprises a single-frequency receiver tuned to receive selectively the secondary SBAS signal in the secondary band, and the secondary NLES station further comprises a third, interface port configured to receive piloting data, output data generated by a long-loop algorithm coming from an NLES-G2 first station for transmitting a two-frequency L1/L5 signal of SoL security level in bands L1 and L5, band L1 being 24 MHz wide and centred on the frequency of 1575.42 MHz; a fourth, interface port configured to receive a clock signal sent by a local reference clock of the NLES-G2 first station; and a fifth, interface port configured to receive a pulse signal derived from the clock signal and repeated periodically at a frequency lower than that of the clock; .- at least the third, interface port is protected by a firewall, each of the third, fourth and fifth ports preferably being protected by a firewall so as to provide an incoming unidirectional link in the secondary NLES station; .- the receiver subsystem includes a control electronic computer, connected to the single-frequency receiver, and the control computer is configured to determine a first corrected phase command of the code and a second corrected phase command of the carrier to be applied to the secondary SBAS signal generated by the transmitter subsystem, the first corrected phase command of the code being determined from a first delay combining the effect of the geometrical distance separating a receive antenna of the SBAS satellite and a ground transmit antenna, shared by the secondary NLES station and the NLES-G2 first station and the effect of the ionospheric propagation delay of the L5 signal; a second ionospheric propagation delay of the uplink channel corresponding to a transposition into the uplink of the L5 signal; a third delay representative of the uplink Sagnac effect of the L5 signal; a fourth delay and a fifth delay respectively representative of the transit times of the secondary SBAS signal In the payload of the satellite and the uplink equipments of the secondary NLES station; and a piloting time difference of the NLES-G2 first station generating the two-frequency L1/L5 signal equal to the time difference between the local time of the NLES-G2 first station and the SBAS network time ENT; and the second corrected phase command of the carrier being determined from the first corrected phase command of the code of the secondary SBAS signal generated, the second ionospheric propagation delay of the uplink channel corresponding to a transposition of the L5 signal onto the uplink, and the sum of a first phase difference error caused by the payload of the associated SBAS satellite and a second phase difference error caused by uplink equipments of the secondary NLES station on the transmission of the secondary SBAS signal; the first, second, third delays by way of output data of the L1/L5 long-loop algorithm of the NLES-G2 station, and the piloting time difference of the NLES-G2 ground station by way of piloting command are delivered by the third, interface port; and the control computer is configured to estimate the sum of the first and second phase difference errors of the secondary SBAS signal from a first current measurement of the phase of the code and a second current measurement of the phase of the carrier of the secondary SBAS signal supplied by the single-frequency receiver, the first current phase command of the code and the second phase command of the carrier currently applied to the secondary SBAS signal and the second ionospheric delay; .- the first corrected phase command of the code of the secondary SBAS signal transmitted is equal to minus the difference between a first term equal to the sum of the first delay, the second delay multiplied by the square of the ratio of the carrier of the L5 signal to the carrier of the secondary SBAS signal, the third delay, the fourth delay, the fifth delay and a second term equal to the piloting time difference of the NLES-G2 first station generating the L5 signal; and the second corrected phase command of the carrier of the secondary SBAS signal is equal to the difference between the first corrected phase command of the code of the secondary SBAS signal, and the difference between the sum of the first and second phase difference errors of the secondary SBAS signal estimated from the first and second current measurements of the phase of the code and of the carrier of the secondary SBAS signal supplied by the single-frequency receiver and twice the second ionospheric delay multiplied by the square of the ratio of the carrier of the 15 signal to the uplink carrier of the secondary SBAS signal; .- the receiver subsystem comprises a two-frequency receiver tuned to receive selectively the secondary SBAS signal in the secondary band and an SBAS signal in band L5 24 MHz wide and centred on the frequency of 1176.45 MHz, the secondary NLES station comprising a local reference clock and a unit for generating a pulse signal derived from the clock signal and repeated periodically at a frequency lower than that of the clock, and the secondary NLES station has no interface ports enabling transmission of any signal liable to interfere with the generation of the SoL L1 and L2 signals by an NLES-G2 first station; .- the receiver subsystem comprises a two-frequency receiver tuned to receive selectively the secondary SBAS signal in the secondary band and an SoL level SBAS signal transmitted by an NLES-G2 first station in band L5 24 MHz wide and centred on the frequency of 1176.45 MHz, the secondary NLES station further comprising a fourth, interface port, configured to receive a clock signal transmitted by a local reference clock of an NLES-G2 first station for transmitting a SoL level two-frequency L1/L5 signal in bands L1 and L5, band L1 being 24 MHz wide and centred on the frequency of 1575.42 MHz; and a fifth, interface port, configured to receive a pulsed signal derived from the clock signal and repeated periodically at a frequency lower than that of the clock, and the secondary ground station having no third, interface port configured to receive piloting data or output data generated by a long-loop algorithm from a two-frequency L1/L5 signal coming from the NLES-G2 first station; .- each of the fourth and fifth ports is protected by a firewall to provide a different incoming unidirectional link at each of the interface ports; .- the receiver subsystem includes an electronic control computer connected to the two-frequency receiver and the electronic control computer is configured to determine a first corrected phase command of the code and a second corrected phase command of the carrier to be applied to the secondary SBAS signal generated by the transmitter subsystem, the first corrected phase command of the code being determined from a first delay combining the effect of the geometric distance separating the receive antenna of the SBAS satellite and the ground antenna transmitting the secondary SBAS signal shared by the secondary IMLES station and the NLES-G2 first station, and the effect of the tropospheric propagation delay on the secondary SBAS signal, a second ionospheric propagation delay of the secondary SBAS channel; a third delay representative of the uplink Sagnac effect of the secondary SBAS signal; a fourth delay and a fifth delay respectively representative of the transit times of the secondary SBAS signal in the payload and the uplink equipments of the secondary ground station; and a piloting time difference of the secondary IMLES station generating the secondary SBAS signal equal to the time difference between the local time of the secondary SBAS station and the SBAS network time ENT; and the second corrected phase command of the carrier of the secondary SBAS signal is determined from the first corrected phase command of the code of the secondary SBAS signal generated; the second ionospheric propagation delay of the secondary SBAS signal, and the sum of a first phase difference error caused by the payload of the SBAS satellite and a second phase difference error caused by uplink equipments of the secondary ground station on the transmission of the secondary SBAS signal; and the electronic control computer is configured to estimate the first and second delays and the piloting time difference of the secondary IMLES station generating the secondary SBAS signal and the sum of the first and second phase difference errors of the secondary SBAS signal from first and second current measurements of the phase of the code and of the carrier of the secondary SBAS signal received by the two-frequency receiver; and the first current phase command of the code and of the second current phase command of the carrier applied to the SBAS signal generated; and the fourth and fifth delays; and a sixth delay and a seventh delay respectively representative of the transit times of the secondary SBAS signal and the L5 signal in the downlink equipments of the SBAS ground station; and an eighth delay LA representative of a lever arm effect defined as the difference between the distance separating the uplink antennas from the transmit antenna shared by the secondary SBAS station to the shared receive antenna of the satellite and the distance separating the downlink antennas from the shared transmit antenna of the satellite to the shared receive antenna of the SBAS ground station; the fourth, fifth, sixth and seventh delays are predetermined; and the third delay is calculated by the computer separately from the estimation; .- the first corrected phase command of the code of the secondary SBAS signal generated is equal to minus the difference between a first term equal to the sum of the first delay, the second delay, the third delay, the fourth delay and the fifth delay and a second term equal to the piloting time difference of the secondary NLES station; and the second corrected phase command of the carrier of the secondary SBAS signal is equal to the difference between the first corrected phase command of the code of the secondary SBAS signal, and the difference between the estimated sum of the first and second phase difference errors of the secondary SBAS signal and twice the second ionospheric delay multiplied by the square of the ratio of the carrier of the downlink secondary SBAS signal to the uplink carrier of the secondary SBAS signal; .- the uplink RF band of the secondary SBAS signal is included in the set of bands C and Ku.

The invention also consists in a method of generation by a secondary NLES station of a secondary SBAS signal intended to support navigation data dissemination services, the secondary NLES station including no equipment that can impact on the generation of existing SoL security level SBAS signals by an NLES-G2 first station, comprising a single-frequency transmitter subsystem for transmitting the secondary SBAS signal in an uplink RF band, the transmitter subsystem itself including a generator of the secondary SBAS signal in a secondary band identical to the downlink band of the secondary SBAS signal, characterized in that the generation method comprises the steps consisting in: .- in a first initialization step, a first current phase command of the code and a second current phase command of the carrier of a secondary SBAS signal are respectively set to a first initial value and a second initial value; then - in a second step, navigation services dissemination data, the first current phase command of the code and the second current phase command of the carrier of the secondary SBAS signal are supplied to the generator of the secondary SBAS signal; then .- in a third step the secondary SBAS signal is generated by the secondary signal generator from navigation services dissemination data, the first current phase command of the code and the second current phase command of the carrier in a downlink secondary band; then ,- in a fourth step, a receiver of a receiver subsystem of the secondary NLES station receives the secondary SBAS signal in the downlink secondary band and performs a first current measurement of the phase of the code and a second current measurement of the phase of the carrier of the secondary SBAS signal; then .- in a fifth step, a control computer determines a first corrected phase command of the code and a second corrected phase command of the carrier to be applied to the secondary signal generated from at least the first current phase command of the code, the second current phase command of the carrier, the first measurement of the phase of the code and the second measurement of the phase of the carrier of the secondary SBAS signal; then . - in a looping sixth step, the first current phase command of the code and the second current phase command of the carrier are respectively set to the first corrected phase command of the code and the second corrected phase command of the carrier of the secondary SBAS signal generated; then .- the second, third, fourth, fifth, sixth steps are repeated successively until a decision to halt the process intervenes.

In accordance with particular embodiments, the method has one or more of the following features: .- the receiver of the receiver subsystem is a single-frequency receiver tuned to receive selectively the secondary SBAS signal in the secondary band, and the method comprises: * a supplementary seventh step, immediately before or after the fourth step, in which the control computer receives via a third, interface of the secondary NLES station piloting data concerning an NLES-G2 first station and output data generated by a long-loop algorithm from an SoL signal of the same NLES-G2 first station; and * an eighth step replacing the fifth step in which the control computer determines a first corrected phase command of the code and a second corrected phase command of the carrier to be applied to the secondary signal generated, the first corrected phase command of the code being a function of the piloting data and output data of the long-loop algorithm of the L5 signal supplied by the NLES-G2 first station, and the second corrected phase command of the carrier being a function of the first current phase command of the code, the second current phase command of the carrier of the secondary SBAS signal, the first current measurement of the phase of the code and the second current measurement of the phase of the carrier of the secondary SBAS signal and output data of the long-loop algorithm of the 15 signal; and the second, third, fourth, seventh, eighth, sixth steps are repeated successively until a decision to halt the process intervenes.; .- the receiver of the receiver subsystem is a two-frequency receiver tuned to receive selectively the secondary SBAS signal in the secondary band and the L5 signal transmitted by the NLES-G2 first station and retransmitted by the same satellite; and the method comprises: * a supplementary ninth step immediately before or after or in parallel with the fourth step in which the two-frequency receiver receives the L5 signal in the downlink secondary L5 band and performs a first current measurement of the phase of the code and a second current measurement of the phase of the carrier of the L5 signal; and * a tenth step replacing the fifth step in which the control computer determines a first corrected phase command of the code and a second corrected phase command of the carrier to be applied to the secondary signal generated, .- the first corrected phase command of the code of the secondary SBAS signal generated being a function of the first current phase command of the code and of the second current phase command of the carrier of the secondary SBAS signal, the first current measurement of the phase of the code and the second current measurement of the phase of the carrier of the secondary SBAS signal, the first current measurement of the phase of the code and the second current measurement of the phase of the carrier of the L1 signal transmitted by the NLES-G2 first station, and .- the second corrected phase command of the carrier of the secondary SBAS signal generated being a function of the first current phase command of the code and of the second current phase command of the carrier of the secondary SBAS signal, the first current measurement of the phase of the code and the second current measurement of the phase of the carrier of the secondary SBAS signal, the first current measurement of the phase of the code and the second current measurement of the phase of the carrier of the L1 signal transmitted by the NLES-G2 first station; and the second, third, fourth, seventh, eighth, sixth steps are repeated successively until a decision to halt the process intervenes.

The invention further consists in a computer product or program comprising a set of instructions configured to execute one or more of the steps of the method defined above when they are loaded into an executed by the control computer of the secondary NLES station defined above.

The invention will be better understood on reading the following description of a number of embodiments given by way of example only and with reference to the drawings in which: - Figure 1 is a view of a general architecture of an SBAS system in accordance with the invention for providing a critical SoL level L1/L5 two-frequency SBAS service and a single-frequency secondary SBAS service; - Figure 2 is a view of the architecture of an second NLES station in accordance with a first embodiment of the invention configured to generate a secondary SBAS signal by being incorporated into the system from Figure 1; - Figure 3 is a view of the architecture of an second NLES station in accordance with a second embodiment of the invention configured to generate a secondary SBAS signal by being incorporated into the system from Figure 1; - Figure 4 is a view of the architecture of an second NLES station in accordance with a third embodiment of the invention configured to generate a secondary SBAS signal by being incorporated into the system from Figure 1; - Figure 5 is a general flowchart of a method in accordance with the invention of generating a secondary SBAS signal; - Figure 6 is a flowchart of a first embodiment of the generation method from Figure 5 when the architecture of the second NLES station is the architecture of the first embodiment of the second NLES station shown in Figure 2; - Figure 7 is a flowchart of a second embodiment of the generation method from Figure 5 when the architecture of the second NLES station is the architecture of the second or third embodiment of the second NLES station shown in Figure 3 or Figure 4.

According to Figure 1, a satellite-based augmentation system (SBAS) 2, here a system of EGNOS type, comprises a ground segment 4, represented here only by its infrastructure component, omitting the user terminals component, and a space segment 6.

The space segment 6 Is a combination 8 of at least one satellite, here placed in geostationary orbit in accordance with different orbital positions, a single satellite 10 being represented in Figure 1 for simplicity.

The satellite 10 includes a transparent payload 12 having an antenna 14 for receiving an uplink 16, an antenna 18 for transmitting a downlink 20, and a transparent repeater 22 connected between the receive antenna 14 and the transmit antenna 18.

The receive antenna 14 is configured to receive in the same uplink band, for example band C, a C1/C5a/C5b three-frequency signal or a combination of three different signals each transmitted in a different channel C1, CSa, C5b respectively corresponding to the L1, L5 and E5b signals of the downlink 20 with the necessary frequency transposition.

The L1, L5 and E5b channels have respective centre frequencies of 1575.42 MHz, 1176.45 MHz and 1207.14 MHz, and each has a bandwidth equal to 24 MHz.

The transpositions from the 11, L5, E5b channels to the uplink C1, C5a, C5b channels can use different transposition frequencies as a function of the chosen frequency plan. Alternatively, a single transposition frequency may be chosen, which will simplify the generator of the reference frequencies used for these transpositions onboard the satellite.

According to the simplified diagram in Figure 1, a transmitter subsystem 24 of a transmit channel of the transparent repeater 22 comprises a bandpass filter 26 filtering all of the bands C1, C5a, C5b, followed by a subsystem for transposition to a lower frequency with narrow filtering of one of the channels and amplification, diagrammatically represented by the cascading of a multiplexer 28, a high-rejection channel band-pass filter 30 and an amplifier 32 in the downlink channel band. The transmitter subsystem 24 also includes an output filter 34 after the transposition subsystem.

The ground segment 4 conventionally comprises a combination of ranging and integrity monitoring stations (RIMS) 38 and a mission control centre (MCC) 40.

The ground segment 4 also comprises a combination 42 of at least one pair of NLES stations, a single pair 44 of matched stations being represented in Figure 1 for simplicity.

The pair 44 of NLES stations includes an NLES-G2 first station 46 for generating an SoL security level L1/L5 two-frequency signal and a secondary second NLES station 48 for generating a single-frequency secondary signal, here in band E5b.

The first and second NLES stations 46, 48 share the same ground transmit antenna 50, configured to transmit an uplink C1/C5a/C5b three-frequency signal to the associated visible satellite 10 and the same ground receive antenna 52 configured to receive a downlink L1/L5/E5b three-frequency signal from the satellite 10.

The ground transmit and receive antennas 50, 52 are installed at the same radiofrequency (RF) site 54 and on the upstream side of this RF site 54 the C1/G5a/C5b three-frequency signal transmitted is the result of the multiplexing by an RF output multiplexer 56 of an uplink C1/C5a two-frequency signal transmitted by the NLES-G2 first station 46 and an uplink CSb signal transmitted by the secondary ESb second NLES station 48.

The received downlink L1/L5/E5b three-frequency signal is divided into identical first and second RF signals respectively routed to the NLES-G2 first station 46 and to the E5b second NLES station 48.

The NLES-G2 first station 46 conventionally includes a transmitter subsystem 60, a receiver subsystem 62, a 10 MHz local reference clock 64 shared by the transmitter and receiver subsystems 60, 62, and a generator 66 of a pulse repeated once per second synchronized to the reference clock 64.

The transmitter subsystem 60 includes a control computer 70, a generator 72 of the L1/L5 two-frequency signal corresponding to the uplink C1/C5a two-frequency signal and an RF adapter unit 74 configured to transpose the L1 and L5 signals generated by the generator 72 of the L1 and L5 signals into the C1 and C5a uplink RF signals.

The receiver subsystem 62 includes the RF adapter unit 74, a two-frequency receiver 76 for receiving the downlink L1/L5 two-frequency signal and the control computer 70 for processing the downlink L1/L5 two-frequency signal received by the L1/L5 two-frequency receiver.

The control computer 70, shared by the transmitter subsystem 60 and the receiver subsystem 62, is configured to synchronize the SoL L1 and L5 signals generated to the EGNOS Network Time (ENT) and to maintain the phase consistency between the code and the carrier of the L1 signal, on the one hand, and the phase consistency between the code and the carrier of the L5 signal, on the other hand.

To this end, the control computer 70 executes a long-loop algorithm (LLA) implemented in a computer module 82 employing a Kalman filter 84 and a control algorithm 86.

Through their design and their ICAO certification the components and the software constituting the NLES-G2 first station 46 confer on the L1 and L5 signals generated an SoL security level.

Alternatively, the E5b channel is replaced by an E6 channel of the same width and having a centre frequency equal to 1278.75 MHz.

According to Figure 2 and in accordance with a first embodiment 102 of the secondary second NLES station 48 for generating and transmitting on the uplink a secondary SBAS signal, the secondary second NLES station 102 comprises a transmitter system 104 for transmitting the secondary SBAS signal in an uplink RF band, here band C, a receiver subsystem 106, a first port in the form of an uplink RF output port 108, a second port in the form of a downlink RF input port 110, a third port in the form of an interface port 112, a fourth port in the form of an interface port 114 and a fifth port in the form of an interface port 116.

The secondary second NLES station 102 includes no equipments liable to impact on the generation of existing SoL security level SBAS signals, i.e. the L1 and L5 signals.

The uplink RF output first port 108, connected to the output RF multiplexer 56, is configured to deliver the uplink C5b signal corresponding to the downlink E5b secondary SBAS signal.

The downlink RF input second port 110, connected to the ground receive antenna 52, is configured to receive the downlink L1/L5/E5b three-frequency signal.

The interface third port 112 is configured to receive piloting data and output data generated by a long-loop algorithm coming from the NLES-G2 first station 46 for transmitting the SoL Sevei L1/L5 two-frequency signal in bands L1 and L5.

Here the interface third port 112 is protected by a firewall 118 so as to provide a unidirectional link from the NLES-G2 first station 46 to the secondary second NLES station 102 and thereby to prevent a disturbing signal feeding back into the NLES-G2 first station 46,

Alternatively, the interface third, fourth and fifth ports 112, 114, 116 are each protected by a firewall.

The interface fourth port 114 is configured to receive the dock signal transmitted by the local reference clock of the NLES-G2 first station 46.

The interface fifth port 116 is configured to receive the pulse signal derived from the clock signal and repeated periodically at a frequency lower than that of the clock, here 1 Hz.

The transmitter subsystem 104 for transmitting the secondary SBAS signal is configured to provide navigation data dissemination services via the secondary SBAS signal transmitted.

The transmitter subsystem 104 for transmitting the secondary SBAS signal includes a control computer 120, a generator 122 for generating the single-frequency secondary SBAS signal in the secondary band, here band E5b, and corresponding to the uplink C5b single-frequency signal, and an RF adapter unit 124 configured to transpose the secondary Eb5 signal generated by the generator 122 into the C5b uplink RF signal.

The RF adapter unit 124 further comprises an output filter 126 selectively tuned to a band 20 MHz wide around the centre frequency of the C5b channel in order to prevent the transmission to the satellite 10 of disturbing signals in the C1 and C5a channels.

The receiver subsystem 106 is configured to receive in the secondary SBAS band at the downlink RF input second port 110 a secondary SBAS signal from the transmitter subsystem 104 retransmitted by the satellite and to verify the synchronization and the consistency of the code relative to the carrier of the secondary SBAS signal received via a loop for correcting the phase of the carrier and the phase of the code of the secondary SBAS signal of the signal transmitted by the first transmitter subsystem 104.

The receiver subsystem 106 includes the RF adapter unit 124, a singlefrequency receiver 132 for receiving the downlink E5b single-frequency secondary signal and the control computer 120 for processing the downlink E5b single-frequency signal received by the single-frequency receiver 132 for receiving the E5b downlink signal.

The control computer 120, shared by the transmitter subsystem 104 and the receiver subsystem 106, is configured to synchronize to the EGNOS Network Time (ENT) the secondary SBAS signal generated and to maintain the phase consistency between the code and the carrier of the E5b signal.

The electronic control computer 120 is configured to compute the respective phase commands to be applied to the current phase of the code and to the current phase of the carrier of the secondary SBAS signal of the signal transmitted by the first transmitter subsystem 104.

The control computer 120 is configured to determine and to apply a first corrected phase command of the code of the E5b secondary SBAS signal, designated RCE5b and generated from: .- a first delay, designated SRL5 + tropoL5, combining the effect of the geometrical distance separating the receive antenna of the SBAS satellite and the transmit antenna shared by the second NLES station and the L1/L5 NLES-G2 first station, and the effect of the tropospheric propagation delay of the C5a channel, .- a ionospheric propagation second delay of the uplink channel corresponding to a transposition of the L5 signal onto the uplink, designated ionocsa, .- a third delay by the NLES-G2 first station, representative of the uplink Sagnac effect of the L5 signal and designated SagnacUpL5; ,- a fourth delay and a fifth delay, respectively representative of the transit times of the secondary SBAS signal in the payload and the uplink equipments of the secondary ground station corresponding to the E5b signal and respectively designated HWPLE_band HWNLESE5bup ; .- a piloting time difference of the NLES-G2 ground station for generating the 15 signal equal to the time difference between the local time of the NLES-G2 first station and the SBAS network time ENT designated S0NLESls.

The control computer 120 is configured to determine and to apply a second corrected phase command of the carrier of the secondary SBAS signal generated from: .- the first corrected phase command RCE5b of the code of the E5b secondary SBAS signal generated; .- the ionospheric propagation second delay ionoC5a of the uplink channel corresponding to a transposition of the L5 signal onto the uplink, and .- the sum <pDI +φ.ΙΙΚ of a first phase difference error <pD. caused by the payload of the SBAS satellite and a second phase difference error φΝ1 ESE bcaused by uplink equipments of the secondary ground station 102 on the transmission of the E5b secondary SBAS signal.

The first, second, third delays, being output data of the L1/L5 longloop algorithm of the NLES-G2 first station 46, and the piloting time difference of the NLES-G2 ground station, being a piloting command, are delivered by the control computer 70 of the NLES-G2 first station 46 to the interface third port 112 of the secondary second NLES station 102.

The control computer 120 of the secondary second NLES station 102 is configured to estimate the sum cpPLEi.b + cpNLEsF5bup0f the first and second phase difference errors of the secondary SBAS signal from the current measurements of the phase of the code and of the carrier of the secondary SBAS signal, respectively designated PRE5band ADREsb and supplied by the single-frequency receiver 132, the first current phase command of the code and the second current phase command of the carrier, applied to the secondary SBAS signal generated and respectively designated RCactuelE5b and PCactueiE5b, and the second delay ionoC5a .

According to Figure 3 and in accordance with a second embodiment 202 of the second NLES station 48 for generating and transmitting on the uplink a secondary SBAS signal, the architecture of the second NLES station 202 is derived from the architecture of the second NLES station 102.

The secondary second NLES station 202 differs from the second NLES station 102 from Figure 2 in that: .- the single-frequency receiver 132 for receiving the E5b single-frequency secondary SBAS signal from the receiver subsystem 106 is replaced by a two-frequency receiver 232 tuned to receive selectively the downlink secondary SBAS signal in the secondary band, here band E5b, and the SoL level L5 signal transmitted by the NLES-G2 first station 46 and retransmitted by the satellite 10; and .- the second NLES station 202 does not include the interface third, fourth and fifth ports 112, 114, 116 and the corresponding links with the NLES-G2 first station; .- the second NLES station 202 includes a 10 MHz local reference clock 252 and a unit 262 for generating a pulse signal derived from the clock signal and repeated periodically at a frequency lower than that of the clock, here 1 Hz; and ,- the control computer 120 is replaced by an electronic computer 280 with exactly the same hardware architecture but configured for different processing taking into account the absence of piloting data and LLA data concerning the L5 signal delivered by the NLES-G2 first station 46 and based on current measurements of the phase of the code and of the carrier of the received E5b secondary SBAS signal, on the one hand, and on current measurements of the phase of the code and of the carrier of the L5 signal, on the other hand.

The control computer 280, shared by the transmitter and receiver subsystems, is configured to synchronize to the EGNOS Network Time ENT the E5b secondary SBAS signal generated and to maintain the phase consistency between the code and the carrier of the E5b signal.

The electronic control computer 280 is configured to compute the respective corrective phase commands to be applied to the phase of the code and to the phase of the carrier of the secondary SBAS signal generated by the secondary SBAS signa! generator of the first transmitter subsystem. The control computer 2S0 is configured to determine and to apply a first corrected phase command of the code of the E5b secondary SBAS signal, designated KCE5b and generated from: .- a first delay, designated SRE5b + tropoE5b, combining the effect of the geometrical distance separating the receive antenna of the SBAS satellite and the transmit antenna shared by the secondary second NLES station 202 and the L1/L5 NLES-G2 first station 46, and the effect of the tropospheric propagation delay of the C5b channel, .- an ionospheric propagation second delay of the E5b secondary signal, designated ionoE5b and linked to the ionospheric propagation delay ionoC5b 2 of the C5b channel by the relation iono C5b = ionoE5b x (; in which FcarrierE5bnomjna! designates the centre frequency of the downlink E5b secondary signal and Fcarriercst,^^,, designates the centre frequency of the C5b uplink signal corresponding to the E5b secondary signal generated by the E5b second NLES station 202; .- a third delay, representative of the uplink Sagnac effect of the E5b signal and designated SagnacUpE5b ; .- a fourth delay and a fifth delay, respectively representative of the transit times of the secondary SBAS signal in the payload and the uplink equipments of the secondary ground station corresponding to the E5b signal and respectively designated HWPLESband HWNLESESbup ; .- a piloting time difference of the secondary second NLES station 202 generating the E5b secondary SBAS signal equal to the time difference between the local time of the secondary second NLES station 202 and the SBAS network time (ENT) and designated S0NLESesb.

The control computer 280 is configured to determine and to apply a second corrected phase command of the carrier of the secondary SBAS signal generated from: .- the first corrected phase command RCE5b of the code of the E5b secondary SBAS signal generated; .- the ionospheric propagation second delay ionoE5b of the uplink channel corresponding to a transposition of the L5 signal onto the uplink, and ,- the sum cpDI -f <p.„ „„ of a first phase difference error <pD. caused by the payload of the SBAS satellite and a second phase difference error φΝ] ESr^caused by uplink equipments of the secondary second NLES station 102 on the transmission of the E5b secondary SBAS signal.

The first delay, the second delay, the piloting time difference of the secondary second NLES station 202 and the sum of the first and second phase difference errors form a state vector, denoted X, with four components.

The control computer 280 of the second NLES station 202 is configured to estimate the four components of the state vector from current measurements of the phase of the carrier and of the code of the secondary SBAS signal, respectively designated ADRE5band PRE5b, current measurements of the phase of the carrier and of the code of the secondary L5 signal, respectively designated ADRL5 and PRL5, the measurements being supplied by the L5/E5b two-frequency receiver 232, and from the current phase command of the code and the current phase command of the carrier, applied to the secondary SBAS signal generated and respectively designated RCactuelE5b and PCactuelE5b, and a set of predetermined parameters.

The control computer 280 is configured to compute the third delay, representative of the uplink Sagnac effect of the E5b signal, in a step separate from the state vector estimation step.

According to Figure 4 and in accordance with a third embodiment 302 of the second NLES station 48 for generating and transmitting on the uplink a secondary SBAS signal, the architecture of the second NLES station 302 is derived from the architecture of the secondary second NLES station 202.

The secondary second NLES station 302 differs from the second NLES station 202 from Figure 3 in that if comprises: .- a fourth port in the form of an interface port 314 configured to receive the clock signal transmitted by the local reference clock 64 of the NLES-G2 first station 46 for transmitting an SoL security level L1/L5 two-frequency signal in bands L1 and L5; and .- a fifth port in the form of an interface port 316 configured to receive the pulse signal transmitted by the unit 66 of the NLES-G2 first station and derived from the clock signal 64 and repeated periodically at a frequency lower than that of the clock.

Following the example of the secondary second NLES station 202 from Figure 3, the secondary second NLES station 302 does not include the third (interface) port 112 and does not receive piloting data and output data of the L1/L5 long-loop algorithm coming from the NLES-G2 first station 46.

The secondary second NLES station 302 is configured so that its electronic control computer 280 executes the same processing and the same phase commands in accordance with the same algorithms used for the second station 202 from Figure 3.

Each of the fourth and fifth (interface) ports 314, 316 is protected by a respective firewall 324, 326 so as to provide a different unidirectional incoming link at each of the interface ports and thereby to protect the NLES-G2 first station from feedback of any disturbing signal.

In all the architecture configurations of the second NLES station 48, 102, 202, 302 described above, the first corrected phase command of the code and the second phase command of the carrier to be applied to the generator of the secondary SBAS signal, for example E5b here, are respectively supplied by the following equations 1 and 2: RCE5b = - (sRE5b + tropoE5b + ionoC5b + SagnacUpEsb + HWPl.E5b + HWNLESESbup - SONLESes]b)

Equation 1 and PCE5b = RCE5b ~ (“2 X ionOC5b + (PpLE5b + Φνι.Ε5Ε513)

Equation 2 in which .- RCEsb designates the command of the phase of the code of the E5b secondary SBAS signal, .- PCE5b designates the command of the phase of the carrier of the E5b secondary SBAS signal, .- SRE5b designates the geometric distance between the phase centre of the receive antenna of the satellite and the phase centre of the transmit antenna of the second NLES station (ground station) generating the E5b secondary SBAS signal, .- tropoE5b designates the tropospheric delay affecting the E5b signal, .- ionoC5b designates the ionospheric delay suffered by the uplink signal corresponding to the E5b downlink signal, here centred on a centre frequency of a C band channel designated C5b, .- SagnacUpE5b designates the delay representative of the uplink Sagnac effect, this uplink effect being defined as a supplementary delay suffered by a signal propagating from the ground station transmitting the uplink signal to the satellite and caused by the rotation of the Earth during the propagation of the signal, .- HWPLe51) designates a supplementary delay equal to the transit time of the E5b signal in the payload from reception of the uplink signal, here C5b, .- HWNLESe designates a supplementary delay equal to the transit time of the equipments of the SBAS second station (ground station) and of formation of the C5b uplink signal from the generator of the E5b signal transmitted, .- φ designates a first phase difference error affecting the E5b signal and caused by equipments of the payload of the satellite, .- φΝΙΕ5ί ^ ^designates a second phase difference error affecting the E5b signal and caused by the equipments of the secondary second NLES station forming the C5b uplink, and .- S0NLESESb designates the piloting offset of the second NLES station generating the E5b secondary SBAS signal, i.e. the difference between the local reference time of the E5b second NLES station and the EGNOS Network Time (ENT).

The various parameters used in equations 1 and 2 must be determined and translated as a function of parameters or data provided by the different architectures of the secondary second NLES station and their respective associated mode of integration into the EGNOS system to compute the first and second corrected phase commands to be applied to the code and to the carrier, respectively, of the secondary SBAS signal generated.

Details of these determinations are provided as a function of the configurations used, i.e. in a first case when the E5b second NLES station includes a single-frequency receiver for receiving an E5b single-frequency signal and in a second case when the E5b second NLES station includes a two-frequency receiver for receiving an L5/E5b two-frequency signal.

In both cases, it is assumed that the piloting of the signal from the NLES-G2 first station is perfect. It is therefore assumed that the piloting of the ESb secondary SBAS signal is subject to any piloting error affecting the piloting of the SoL level L5 signal.

According to Figure 5, a method of generation 402 of a secondary SBAS signal intended to support navigation data dissemination services is implemented by an independent second NLES station and comprises a set of steps.

In a first initialization step 404, a first current phase command of the code and a second current phase command of the carrier of a secondary SBAS signal are respectively set to a first initial value and a second initial value.

In a second step 406, data to be broadcast, the first current phase command of the code and the second current phase command of the carrier of the secondary SBAS signal are supplied to the secondary SBAS signal generator.

Then, in a third step 408, the secondary SBAS signal is generated by the secondary signal generator from the data to be broadcast, the first current phase command of the code and the second current phase command of the carrier in a downlink secondary band, here band ESb.

Then, in a fourth step 410, a receiver receives the secondary SBAS signal in the downlink secondary band and performs a first current measurement of the phase of the code and a second current measurement of the phase of the carrier of the secondary SBAS signal.

In a successive fifth step 412, the control computer determines a first corrected phase command of the code and a second corrected phase command of the carrier to be applied to the secondary signal generated from at least the first current phase command of the code, the second current phase command of the carrier, the first current measurement of the phase of the code and the second current measurement of the phase of the carrier of the secondary SBAS signal.

Then, in a looping sixth step 414, the first current phase command of the code and the second current phase command of the carrier are respectively set to the first corrected phase command of the code and the second corrected phase command of the carrier of the secondary SBAS signal generated.

The second, third, fourth, fifth and sixth steps 406, 408, 410, 412, 414 are then repeated successively until a decision to halt the process intervenes. A loop for correcting the phase of the carrier and the phase of the code of the secondary SBAS signal generated by the generator of the first transmitter subsystem is therefore responsible for the synchronization and the consistency of the code relative to the carrier of the received secondary SBAS signal is therefore [assured] via a, which makes it possible to obtain satisfactory continuity and availability of the data dissemination services.

First case: E5b single-frequency receiver

In the first case of an architecture of the second NLES station using a single-frequency receiver, the piloting of the phase of the carrier and of the code of the secondary SBAS signal transmitted depends on two types of input data, namely the piloting commands of the L5 signal and the outputs of said L1/L5 long-loop algorithm delivered at the first interface port by the NLES-G2 first station and depends on current measurements of the phase of the code and of the phase of the carrier of the E5b signal received by the single-frequency receiver.

The first phase command of the code of the L5 signal to be applied to the generator of the L5 signal and supplied by the NLES-G2 first station is expressed in accordance with the following equation: RCL5 = - (SRL5 + tropoL5 + ionoC5a 4- SagnacUpL5 + HWPLl_ + HWNLESlsup - soNLESl5 )

Equation 3

The following hypotheses are made concerning the various terms of the expressions relating to the phase commands applied to the L5 and E5b signals: .- SRE5b = SRL5 = SR, because by assuming that the same antenna is used and shared for transmitting the L5 and E5b signals respectively transposed into the C5a and C5b uplink channels, the geometrical distance between the transmit antenna shared by the first NLES and second stations and the receive antenna of the satellite is the same for both signals, .- tropoE5b = tropoL5 as the tropospheric propagation delay is independent of the frequency, .- SagnacUpE5b = SagnacUpL5 = SagnacUp , because the delay representative of the Sagnac effect depends only on the geometry of the system and the two NLES stations share the same ground transmit antenna and the same satellite receive antenna for both the C5a and G5b uplink signals corresponding to the two L5 and E5b signals transmitted,

in which F and carrier i, to j?ovc>sn at

Fcamer csb normal respectively designate the centre frequencies of the C5a and C5b uplink signals, .- S0NljES{;_b = S0NLESls given that the NLES-G2 first station and the E5b second NLES station are assumed to share the same reference local clock in the single-frequency receiver E5b second NLES station architecture.

The expression for the new phase command of the code to be applied to the E5b secondary signal generated is then written:

Equation 4

Alternatively, it is assumed that HWPL|Esb = HWPLls. in this case, from equations 1 and 3, the difference between the expressions of the phase commands of the codes of the L5 and E5b signals lead to the following equation 4bis:

Equation 4bis

In this variant, it is therefore assumed that only the phase command of the code of the L5 signal and the ionospheric delay of the C5a uplink signal are supplied as output data of the L1/L5 long-loop algorithm by the NLES-G2 first station at the third (interface) port 112 of the secondary second NLES station, the additional delays HWPLESb and HWNLESEsbup being predetermined by previous calibration in the factory of the payload and the downlink equipments for receiving the E5b secondary signal, or by calibration in service using additional calibration signals.

As indicated above, the measurements of the phase of the code and of the phase of the carrier of the E5b signal received by the single-frequency receiver make it possible to estimate the phase difference error of the carrier of the E5b signal and therefore to verify the phase consistency between the code and the carrier of the E5b secondary signal.

The current measurement of the phase of the code and the current measurement of the phase of the carrier of the E5b secondary signal received by the single-frequency receiver are modelled by the following equations 5 and 6:

Equation 5 and

Equation 6 in which: . ADRE5b designates the current measurement of the phase of the carrier of the E5b secondary signal received by the single-frequency receiver; . PRE5b designates the current measurement of the phase of the code of the E5b secondary signal received by the single-frequency receiver; . PCactuelE5b designates the current phase command of the carrier applied to the E5b signal transmitted and affecting the current measurement; , RCactuelE5b designates the current phase command of the code applied to the E5b signal transmitted and affecting the current measurement; . LA designates the lever arm defined as the difference of the geometrical extents of the uplink and the downlink between the satellite and the second NLES station and calculated from the positions of the receive and transmit antennas of the satellite and the positions of the antennas of the uplink and the downlink of the second NLES station, assuming that the position of the satellite is known at one second intervals; HWNLESjEt.bDwn designates a supplementary delay equal to the transit time of the equipments receiving the downlink E5b signal of the E5b second NLES station; , FcarrierE5bnomjna] designates the centre frequency of the E5b secondary signal generated on the uplink before transposition or received on the downlink by the E5b second NLES station,

It is assumed here that the possible phase difference errors or inconsistencies of the code relative to the carrier introduced by the single-frequency receiver are neglected.

Equations 5 and 6 also take into account the approximation to the effect that the Sagnac effect of the C5b signal corresponding to the uplink E5b signal and the Sagnac effect of the downlink E5b signal are equal, which is written as follows: |SagnacUpE5bj = |SagnacDwnE5b|

From equations 5 and 6, the difference between the current measurements of the phase of the code and of the carrier of the E5b secondary signal is written in accordance with the following equation:

Equation 7

The only remaining unknown, which is the phase difference error of the E5b signal caused conjointly by the payload and the uplink equipments of the E5b second NLES station, i.e. the sum of the terms φρι and (PNLEsEsb is determined from equation 7. Once the phase difference error has been determined from equation 7, the second corrected phase command to be applied to the phase of the carrier of the E5b secondary signal generated on transmission is determined from equation 2, and is written:

Equation 8

According to Figure 6, a first particular embodiment 502 of the Figure 5 general method 402 concerns the use of an E5b single-frequency receiver in the secondary second NLES station, i.e. the first embodiment 102 of the secondary second NLES station shown in Figure 2.

The method 502 is derived from the general method 402 and comprises in the identical steps 404, 406, 408, 410, 414.

The method 502 differs from the general method 402 in that it comprises a supplementary seventh step 510 immediately before or after the fourth step 410 and in that the fifth step 412 is replaced by an eighth step 512.

In the seventh step 510, the computer receives via the third, Interface 112 piloting data concerning an NLES-G2 first station and output data generated by a long-loop algorithm from an SoL signal from the same NLES-G2 first station 46.

In the eighth step 512, the control computer determines a first corrected phase command of the code and a second corrected phase command of the carrier to be applied to the secondary signal generated.

The first corrected phase command of the code is a function of the piloting data and the output data of the L5 signal long-loop algorithm determined by equation 4 or 4bis.

The second corrected phase command of the carrier is a function of the first current phase command of the code, the second current phase command of the carrier of the secondary SBAS signal, the first current measurement of the phase of the code and the second current measurement of the phase of the carrier of the secondary SBAS signal and output data of the L5 signal long-loop algorithm.

The second, third, fourth, seventh, eighth and sixth steps 408, 408, 410, 510, 512, 414 are successively repeated until a decision to halt the process intervenes.

Second case: L5/E5b two-frequency receiver

If the second NLES station includes an L5/E5b two-frequency receiver, the first NLES station generating an SoL security level L1/L5 two-frequency signal and the secondary second NLES station generating an ESb single-frequency secondary signal are completely separate. No parameter of the L1/L5 NLES-G2 first station is transmitted by the L1/L5 NLES-G2 first station to the second station generating the ESb secondary signal. The first and secondary second NLES stations each have their own local reference clock with no link between them. According to this configuration, the piloting of the phase of the code and of the carrier of the ESb secondary signal depends only on phase measurements of the code and of the carrier of the ESb secondary signal received by the two-frequency receiver and measurements of the phase of the code and of the carrier of the L5 SoL level signal received by the two-frequency receiver. The E5b secondary signal received on the downlink and the L5 SoL level signal received on the downlink respectively correspond to the ESb secondary signal generated by the secondary ESb second NLES station and the SoL level L5 signal generated by the NLES-G2 first station. The current measurements of the phase are effected by the two-frequency receiver and processed by the control computer 280.

The expressions for the first corrected phase command of the code and the second corrected phase command of the carrier to be applied to the generator of the secondary, here ESb, SBAS signal are respectively the same as those from equations 1 and 2 above and repeated below: RCESb = (sRE5b + tropoEsb + ionoC5b + SagnacUpE5b + HWPLes1j + HWNLESEsbup - S0NLESEsb)

Equation 1 and PCE5b “ RCE5b “ (~2 X ionOC5b + CPpLE5b + ΦνΕΕ5Εγι13)

Equation 2

State variables to be estimated form a state vector denoted X that is written as follows:

Equation 9 in which: .- a first state variable is the sum of a first term SRE5b designating the geometrical distance between the phase centre of the receive antenna of the satellite and the phase centre of the transmit antenna of the second NLES station (ground station) generating the E5b secondary SBAS signal, and a second term tropoE5b , designating the tropospheric delay affecting the E5b signal; ,- a second state variable is the ionospheric delay ionoE5b suffered by the downlink signal and linked to the ionospheric delay ionoC5b of the uplink C5b signal by the relation

in which

FcarrierE5bnominal designates the centre frequency of the downlink E5b secondary signal and FcarrierC5bnominal designates the centre frequency of the uplink C5b signal corresponding to the E5b secondary signal generated by the E5b second NLES station, .- a third state variable is a phase difference error cpEr,b equal to the sum cPpiE,b + ΦΝι EsErb p of a term ΦριΕ^ designating a first phase difference error affecting the E5b signal and caused by the payload equipments of the satellite and a second term 9NLESe designating a second phase difference error affecting the E5b signal and caused by the equipments of the second NLES station forming the C5b uplink; and .- a fourth state variabie is the piloting offset S()NLESEsb of the secondary second NLES station generating the E5b secondary SBAS signal, i.e. the difference between the local reference time of the secondary second NLES station and the EGNOS Network Time ENT, this piloting offset being different from that of the L1/L5 NLES-G2 first station and having to be estimated because the two NLES stations do not share the same local reference clock or other information.

Following the example of the configuration using a single-frequency receiver the current measurements of the phase of the code and of the carrier of the E5b secondary signal obey the same modelling equations 5 and 6 that are repeated below.

and

Where phase measurements of the L5 signal by the two-frequency receiver are concerned, it is assumed that the L1/L5 NLES-G2 first station piloting is perfect. On this assumption, the current measurements of the phase of the code and of the carrier of the L5 signal by the two-frequency receiver can be modelled by the following equations 10 and 11:

PRls = SRE5b + tropoE5b + ionoL5 + HWNLESL5Dwn - SONLESj_.sb + LA

Equation 10 and

ADRls — SRE5b + tropoE5b — ionoL5 + HWNLESls!Jw!) — SONLESEgb + LA

Equation 11 in which HWNLES[cDwn designates a supplementary delay equal to the transit time of the downlink L5 signal receiving equipments of the E5b second NLES station.

All the above equations assume that in practice, i.e. realistically, the downlink equipments of the second NLES station do not introduce any phase inconsistency between the code and the carrier of the signals received by the two-frequency receiver. A set or system of observables forms an observable vector Z that Is written in the form of a column vector:

Equation 12 in which the first, second, third and fourth observables are arranged from top to bottom. A measurements matrix is written:

Equation 13 in which

and

The states vector X, the observable vector Z and the measurement matrix satisfy the following equation:

HxX = Z Equation 14 in which X is the unknown vector.

The matrix H can be inverted and is written:

in which, for j varying from 1 to 4, β|3 and β]4 are not equal to zero.

Once the unknowns of X have been determined, there are deduced from equations 1 and 2:

Equation 15 and

Equation 16

In accordance with a variant of the architecture of the second embodiment, following the example of the second embodiment, the secondary second NLES station includes a two-frequency receiver and differs from that second embodiment in that it shares with the L1/L5 NLES-G2 first station its 10 MHz local reference clock and its generation unit. In this case, the same equations as those for the second embodiment are applicable and the method described above is not modified. In this variant, the complexity and cost of the hardware of the second station have been reduced to the detriment of an increase in the security risk.

According to Figure 7, a second particular embodiment 602 of the Figure 5 general method 402 concerns the use of an L5/E5b two-frequency receiver in the secondary second NLES station, i.e. the second and third embodiments 202 and 302 of the second station shown in Figure 3 and Figure 4.

The method step 602 is derived from the general method 402 and comprises the identical steps 404, 406, 408, 410, 414.

The method 602 differs from the general method 402 in that it comprises a supplementary ninth step 610 immediately before or after or in parallel with the fourth step 410 and in that the fifth step 412 is replaced by a tenth step 612.

In the ninth step 610, the two-frequency receiver receives the L5 signal in the downlink L5 secondary band and performs a first current measurement of the phase of the code and a second current measurement of the phase of the carrier of the L5 signal.

In the tenth step 612, the control computer 280 determines a first corrected phase command of the code and a second corrected phase command of the carrier to be applied to the secondary signal generated.

The first corrected phase command of the code of the secondary SBAS signal generated is a function of the first current phase command of the code and of the second current phase command of the carrier of the secondary SBAS signal, the first current measurement of the phase of the code and the second current measurement of the phase of the carrier of the secondary SBAS signal, the first current measurement of the phase of the code and the second current measurement of the phase of the carrier of the L1 signal transmitted by the NLES-G2 first station, in accordance with equations 9 and 12 to 15.

The second corrected phase command of the carrier of the secondary SBAS signal generated is a function of the first current phase command of the code and of the second current phase command of the carrier of the secondary SBAS signal, the first current measurement of the phase of the code and the second current measurement of the phase of the carrier of the secondary SBAS signal, the first current measurement of the phase of the code and the second current measurement of the phase of the carrier of the L5 signal transmitted by the NLES-G2 first station, in accordance with equations 9 and 12 to 16.

The second, third, fourth, seventh, eighth and sixth steps 406, 408, 410, 610, 612, 414 are then repeated successively until a decision to halt the process intervenes.

The preferred solution is the architecture of the secondary second NLES station 202 in accordance with the second embodiment shown in Figure 2. This preferred architecture is based on arriving at a compromise between, firstly sufficient separation and isolation between the NLES-G2 first station and the second NLES station to qualify it as secondary, secondly sufficient control of the synchronization and the phase consistency of the code relative to the carrier of the secondary SBAS signal to be sure of good performance in terms of GNSS data dissemination services continuity and availability, and thirdly a minimum amount of complex equipment.

The secondary second NLES station is coupled in a unidirectionai manner with the NLES-G2 first station and includes a single-frequency receiver tuned to receive selectively in band E5b.

The preferred solution makes it possible to guarantee the absence of impact on L1/L5 services because of a minimum number of and complete control in terms of security of the coupling ports. It also makes it possible to guarantee: .- good performance in terms of service continuity and availability thanks to the E5b receiver and the coupling, making it possible to guarantee the synchronization and the consistency of the code relative to the carrier of the E5b signal; .- a complexity and a cost of hardware lower than those of a classic L1/L5 NLES through eliminating hardware such as in particular the clock, the calibration units, the integrity module; .- a complexity and a cost of software lower than those of a classic NLES by reason of the absence of piloting complex algorithm implementation; .- a development not subject to SoL constraints, in particular to the DAL D0178B level requirements, by reason of the independence of the E5b NLES relative to the L1/L5 NLES, which leads to a reduced development cost.

Nevertheless, the architectures of the second and third embodiments shown in Figures 3 and 4 are also beneficial if reinforcement of the separation and the isolation between the NLES-G2 first station and the secondary second NLES station is required.

Note that all the architectures and the algorithms described above in the situation where the secondary band used is band E5b remain valid if the secondary band used is band E6. It suffices for this to replace all the expressions “5b” included in the descriptive text and the equations set out above with the expression “6”; for example “E5b” will be replaced by Έ6” and “C5b” will be replaced by “C8”.

The geostationary satellites may be replaced by satellites in other types of orbit, for example HEO or MEO orbits.

Claims

1. Secondary NLES station for generating a secondary signal of a satellite-based augmentation system SBAS intended to support data dissemination services, comprising .- a subsystem (104) for transmitting the secondary SBAS signal in an uplink radiofrequency RF band (16) including a generator (122) of the secondary SBAS signal in a secondary SBAS band and a first uplink RF output port (108); and .- a receiver subsystem (106), configured to receive in the secondary SBAS band at a second downlink (18) RF input port (110) a secondary SBAS signal from the transmitter subsystem (104) retransmitted by an associated SBAS satellite (10) and to verify the synchronization and the consistency of the code with respect to the carrier of the secondary SBAS signal received via a loop for correction of the phase of the carrier and of the phase of the code of the secondary SBAS signal of the signal transmitted by the transmitter subsystem (104); the NLES station including no existing SoL level equipment able to impact on the generation of SBAS signals by a first station NLES-G2 and being characterized in that the transmitter subsystem (104) includes an output filter (126), selectively tuned over a band of 20 MHz width entered on the centre frequency of an uplink RF channel corresponding, apart from a transposition frequency, to the secondary SBAS signal channel generated by the transmitter subsystem (104).

2. Secondary NLES station according to Claim 1 for transmitting a secondary SBAS signal, wherein the receiver subsystem (106) comprises a single-frequency receiver (132) tuned to receive selectively the secondary SBAS signal in the secondary band, or a two-frequency receiver (232) tuned to receive selectively the secondary SBAS signal in the secondary band and an SoL level SBAS signal in the band L5, 24 MHz wide and centred on the frequency of 1176.45 MHz.

3. Secondary NLES station according to either one of Claims 1 and 2 for transmitting a secondary SBAS signal, wherein the secondary band is the E5b channel band 24 MHz wide and centred on the frequency of 1207.14 MHz, or the E6 channel band 24 MHz wide and centred on the frequency of 1278.75 MHz.

4. Secondary NLES station according to any one of Claims 1 to 3 for transmitting a secondary SBAS signal, wherein the receiver subsystem (106) comprises a single-frequency receiver (132) tuned to receive selectively the secondary SBAS signal in the secondary band, and further comprising a third interface port (112), configured to receive piloting data, output data generated by a long-loop algorithm coming from an NLES-G2 first station (46) for transmitting a two-frequency L1/L5 signal of SoL security level in bands L1 and L5, band L1 being 24 MHz wide and centred on the frequency of 1575.42 MHz; a fourth interface port (114), configured to receive a dock signal sent by a local reference clock (64) of the NLES-G2 first station (46); and a fifth interface port (116), configured to receive a pulse signal derived from the clock signal and repeated periodically at a frequency Sower than that of the clock.

5. Secondary NLES station according to Claim 4 for transmitting a secondary SBAS signal, wherein at least the third, interface port (112) is protected by a firewall (118), each of the third, fourth and fifth ports (112, 114, 116) preferably being protected by a firewall so as to provide an incoming unidirectional link in the secondary NLES station.

6. Secondary NLES station according to either one of Claims 4 or 5 for transmitting a secondary SBAS signal, wherein the receiver subsystem (106) includes a control electronic computer (120), connected to the singlefrequency receiver (132), and the control computer (120) is configured to determine a first corrected phase command of the code and a second corrected phase command of the carrier to be applied to the secondary SBAS signal generated by the transmitter subsystem (104), the first corrected phase command of the code being determined from a first delay combining the effect of the geometrical distance separating a receive antenna (14) of the SBAS satellite (10) and a ground transmit antenna (50), shared by the secondary NLES station and the NLES-G2 first station (46) and the effect of the ionospheric propagation delay of the L5 signal; a second ionospheric propagation delay of the uplink channel corresponding to a transposition into the uplink of the L5 signal; a third delay representative of the uplink Sagnac effect of the L5 signal; a fourth delay and a fifth delay respectively representative of the transit times of the secondary SBAS signal in the payload (12) of the satellite (10) and the uplink equipments of the secondary NLES station; and a piloting time difference of the NLES-G2 first station generating the two-frequency L1/L5 signal equal to the time difference between the local time of the NLES-G2 first station and the SBAS network time ENT; and the second corrected phase command of the carrier being determined from the first corrected phase command of the code of the secondary SBAS signal generated, the second ionospheric propagation delay of the upiink channel corresponding to a transposition of the L5 signal onto the uplink, and the sum of a first phase difference error caused by the payload (12) of the associated SBAS satellite (10) and a second phase difference error caused by uplink equipments of the secondary NLES station on the transmission of the secondary SBAS signal; the first, second, third delays by way of output data of the L1/L5 long-loop algorithm of the IMLES-G2 station, and the piloting time difference of the NLES-G2 ground station by way of piloting command are delivered by the third, interface port; and the control computer (120) is configured to estimate the sum of the first and second phase difference errors of the secondary SBAS signal from a first current measurement of the phase of the code and a second current measurement of the phase of the carrier of the secondary SBAS signal supplied by the single-frequency receiver, the first current phase command of the code and the second phase command of the carrier currently applied to the secondary SBAS signal and the second ionospheric delay.

7. Secondary NLES station according to Claim 6 for transmitting a secondary SBAS signal, wherein .- the first corrected phase command of the code of the secondary SBAS signal transmitted is equal to minus the difference between a first term equal to the sum of the first delay, the second delay multiplied by the square of the ratio of the carrier of the L5 signal to the carrier of the secondary SBAS signal, the third delay, the fourth delay, the fifth delay and a second term equal to the piloting time difference of the NLES-G2 first station (46) generating the L5 signal; and .- the second corrected phase command of the carrier of the secondary SBAS signal is equal to the difference between the first corrected phase command of the code of the secondary SBAS signal, and the difference between the sum of the first and second phase difference errors of the secondary SBAS signal estimated from the first and second current measurements of the phase of the code and of the carrier of the secondary SBAS signal supplied by the single-frequency receiver and twice the second ionospheric delay multiplied by the square of the ratio of the carrier of the L5 signal to the uplink carrier of the secondary SBAS signal.

8. Secondary NLES station according to any one of Claims 1 to 3 for transmitting a secondary SBAS signal, wherein the receiver subsystem (106) comprises a two-frequency receiver (232) tuned to receive selectively the secondary SBAS signal in the secondary band and an SBAS signal in band L5 24 MHz wide and centred on the frequency of 1176.45 MHz, the secondary NLES station comprising a local reference clock (252) and a unit (262) for generating a pulse signal derived from the clock signal and repeated periodically at a frequency lower than that of the clock (252), and the secondary NLES station having no interface ports enabling transmission of any signal liable to interfere with the generation of the SoL L1 and L5 signals by an NLES-G2 first station.

9. Secondary NLES station according to any one of Claims 1 to 3 for transmitting a secondary SBAS signal, wherein the receiver subsystem (106) comprises a two-frequency receiver tuned to receive selectively the secondary SBAS signal in the secondary band and an Sol level SBAS signal transmitted by an NLES-G2 first station in band L5 24 MHz wide and centred on the frequency of 1176.45 MHz, the secondary NLES station further comprising a fourth interface port (314), configured to receive a clock signal transmitted by a local reference clock (64) of an NLES-G2 first station (46) for transmitting a SoL level two-frequency signal in bands L1 and L5, band L1 being 24 MHz wide and centred on the frequency of 1575.42 MHz; and a fifth interface port (316), configured to receive a pulsed signal derived from the dock signal and repeated periodically at a frequency lower than that of the clock (64), and the secondary ground station having no third, interface port configured to receive piloting data or output data generated by a long-loop algorithm from a two-frequency L1/L5 signal coming from the NLES-G2 first station (46).

10. Secondary NLES station according to Claims for transmitting a secondary SBAS signal, wherein each of the fourth and fifth ports (314, 316) is protected by a firewall (324, 326) to provide a different incoming unidirectional link at each of the interface ports (314, 316).

11. Secondary NLES station according to any one of Claims 8 to 10 for transmitting a secondary SBAS signal, wherein the receiver subsystem (106) includes an electronic control computer (280) connected to the two-frequency receiver (232), and the electronic control computer (280) is configured to determine a first corrected phase command of the code and a second corrected phase command of the carrier to be applied to the secondary SBAS signal generated by the transmitter subsystem (104), the first corrected phase command of the code being determined from a first delay combining the effect of the geometric distance separating the receive antenna of the SBAS satellite and the ground antenna (50) transmitting the secondary SBAS signal shared by the secondary NLES station and the NLES-G2 first station (46), and the effect of the tropospheric propagation delay on the secondary SBAS signal, a second ionospheric propagation delay of the secondary SBAS channel; a third delay representative of the uplink Sagnac effect of the secondary SBAS signal; a fourth delay and a fifth delay respectively representative of the transit times of the secondary SBAS signal in the payload and the uplink equipments of the secondary ground station; and a piloting time difference of the secondary NLES station generating the secondary SBAS signal equal to the time difference between the local time of the secondary NLES station and the SBAS network time ENT; and the second corrected phase command of the carrier of the secondary SBAS signal is determined from the first corrected phase command of the code of the secondary SBAS signal generated; the second ionospheric propagation delay of the secondary SBAS signal, and the sum of a first phase difference error caused by the payload of the SBAS satellite and a second phase difference error caused by uplink equipments of the secondary ground station on the transmission of the secondary SBAS signal; and the electronic control computer (280) is configured to estimate the first and second delays and the piloting time difference of the secondary NLES station generating the secondary SBAS signal and the sum of the first and second phase difference errors of the secondary SBAS signal from first and second current measurements of the phase of the code and of the carrier of the secondary SBAS signal received by the two-frequency receiver (232); and the first current phase command of the code and of the second current phase command of the carrier applied to the SBAS signal generated; and the fourth and fifth delays; and a sixth delay and a seventh delay respectively representative of the transit times of the secondary SBAS signal and the L5 signal in the downlink equipments of the SBAS ground station; and an eighth delay LA representative of a lever arm effect defined as the difference between the distance separating the uplink antennas from the transmit antenna shared by the secondary SBAS station to the shared receive antenna of the satellite and the distance separating the downlink antennas from the shared transmit antenna of the satellite to the shared receive antenna of the SBAS ground station; the fourth, fifth, sixth and seventh delays are predetermined; and the third delay is calculated by the computer separately from the estimation.

12. Secondary NLES station according to Claim 11 for transmitting a secondary SBAS signal, wherein .- the first corrected phase command of the code of the secondary SBAS signal generated is equal to minus the difference between a first term equal to the sum of the first delay, the second delay, the third delay, the fourth delay and the fifth delay and a second term equal to the piloting time difference of the secondary NLES station; and .- the second corrected phase command of the carrier of the secondary SBAS signal is equal to the difference between the first corrected phase command of the code of the secondary SBAS signal, and the difference between the estimated sum of the first and second phase difference errors of the secondary SBAS signal and twice the second ionospheric delay multiplied by the square of the ratio of the carrier of the downlink secondary SBAS signal to the uplink carrier of the secondary SBAS signal.

13. Secondary NLES station according to any one of Claims 1 to 12 for transmitting a secondary SBAS signal, wherein the uplink RF band of the secondary SBAS signal is in the set of bands C and Ku.

14. Method of generation by a secondary NLES station of a secondary SBAS signal intended to support navigation data dissemination services, the secondary NLES station including no equipment that can impact on the generation of existing SoL security level SBAS signals by an NLES-G2 first station, comprising a single-frequency transmitter subsystem for transmitting the secondary SBAS signal in an uplink RF band, the transmitter subsystem itself including a generator of the secondary SBAS signal in a secondary band identical to the downlink band of the secondary SBAS signal, characterized in that the generation method comprises the steps consisting in: .- in a first initialization step (404), a first current phase command of the code and a second current phase command of the carrier of a secondary SBAS signal are respectively set to a first initial value and a second initial value; then .- in a second step (406), navigation services dissemination data, the first current phase command of the code and the second current phase command of the carrier of the secondary SBAS signal are supplied to the generator of the secondary SBAS signal; then - in a third step (408) the secondary SBAS signal is generated by the secondary signal generator from navigation services dissemination data, the first current phase command of the code and the second current phase command of the carrier, and filtered by an output filter (126) selectively tuned to a band 20 MHz wide centred on the centre frequency of an uplink RF channel corresponding, apart from a transposition frequency, to the channel of the secondary SBAS signal, generated by the transmitter subsystem (104); then .- in a fourth step (410), a receiver of a receiver subsystem of the secondary NLES station receives the secondary SBAS signal in the downlink secondary band and performs a first current measurement of the phase of the code and a second current measurement of the phase of the carrier of the secondary SBAS signal; then .- in a fifth step (412), a control computer determines a first corrected phase command of the code and a second corrected phase command of the carrier to be applied to the secondary signal generated from at least the first current phase command of the code, the second current phase command of the carrier, the first measurement of the phase of the code and the second measurement of the phase of the carrier of the secondary SBAS signal; then . - in a looping sixth step (414), the first current phase command of the code and the second current phase command of the carrier are respectively set to the first corrected phase command of the code and the second corrected phase command of the carrier of the secondary SBAS signal generated; then .- the second, third, fourth, fifth, sixth steps (406, 408, 410, 412, 414) are repeated successively until a decision to halt the process intervenes,

15. Method according to Claim 14 of generating a secondary SBAS signal, wherein the receiver (132) of the receiver subsystem is a singlefrequency receiver tuned to receive selectively the secondary SBAS signal in the secondary band, and comprising a supplementary seventh step (510), immediately before or after the fourth step (410), in which the control computer (120) receives via a third, interface (112) of the secondary NLES station (102) piloting data concerning an NLES-G2 first station (46) and output data generated by a long-loop algorithm from an SoL signal of the same NLES-G2 first station (46); and comprising an eighth step (512) replacing the fifth step (412) in which the control computer (120) determines a first corrected phase command of the code and a second corrected phase command of the carrier to be applied to the secondary signal generated, .- the first corrected phase command of the code being a function of the piloting data and output data of the long-loop algorithm of the L5 signal supplied by the NLES-G2 first station (46), and .- the second corrected phase command of the carrier being a function of the first current phase command of the code, the second current phase command of the carrier of the secondary SBAS signal, the first current measurement of the phase of the code and the second current measurement of the phase of the carrier of the secondary SBAS signal and output data of the long-loop algorithm of the L5 signal; and wherein the second, third, fourth, seventh, eighth, sixth steps (406, 408, 410, 510, 512, 414) are repeated successively until a decision to halt the process intervenes.

16. Method according to Claim 14 of generating a secondary SBAS signal, wherein the receiver (232) of the receiver subsystem is a two-frequency receiver tuned to receive selectively the secondary SBAS signal in the secondary band and the L5 signal transmitted by the NLES-G2 first station (46) and retransmitted by the same satellite (10); and comprising a supplementary ninth step (610) immediately before or after or in parallel with the fourth step (410) in which the two-frequency receiver (232) receives the L5 signal in the downlink secondary L5 band and performs a first current measurement of the phase of the code and a second current measurement of the phase of the carrier of the L5 signal; and comprising a tenth step (612) replacing the fifth step (412) in which the control computer (280) determines a first corrected phase command of the code and a second corrected phase command of the carrier to be applied to the secondary signal generated, .- the first corrected phase command of the code of the secondary SBAS signal generated being a function of the first current phase command of the code and of the second current phase command of the carrier of the secondary SBAS signal, the first current measurement of the phase of the code and the second current measurement of the phase of the carrier of the secondary SBAS signal, the first current measurement of the phase of the code and the second current measurement of the phase of the carrier of the L5 signal transmitted by the NLES-G2 first station, and .- the second corrected phase command of the carrier of the secondary SBAS signal generated being a function of the first current phase command of the code and of the second current phase command of the carrier of the secondary SBAS signal, the first current measurement of the phase of the code and the second current measurement of the phase of the carrier of the secondary SBAS signal, the first current measurement of the phase of the code and the second current measurement of the phase of the carrier of the L5 signal transmitted by the NLES-G2 first station; and wherein the second, third, fourth, seventh, eighth, sixth steps (406, 408, 410, 610, 612, 414) are repeated successively until a decision to halt the process intervenes.

17. Computer product or program comprising a set of instructions configured to execute one or more of the steps of the method defined in accordance with any one of Claims 14 to 16 when they are loaded into an executed by the control computer (120, 280) of the secondary NLES station defined in accordance with any one of Claims 1 to 13, 14.