Satellite payload for augmentation systems
The present invention relates to the field of augmentation systems, for
augmenting the integrity and accuracy and robustness of satellite navigation
5 systems.
The invention relates more particularly to the uplink between a number
of navigation land earth stations and a satellite dedicated to the transmission
of augmentation data. The subject of the invention is notably a digital
navigation payload that is semi-transparent to such a satellite.
10 Hereinafter, the following acronyms, well known in the field, will be
used. The satellite navigation and positioning systems are generally
designated by GNSS systems (Global Navigation Satellite Systems). The
performance augmentation systems are called SBAS systems (Satellite
Based Augmentation Systems). The ground stations suitable for transmitting
15 augmentation data to the satellite are called NILES stations (Navigation Land
Earth Stations). The ground stations suitable for receiving satellite signals
comprising augmentation data and for performing measurements on these
signals are commonly called RIMS stations (Ranging & Integrity Monitoring
Stations).
20
The known SBAS augmentation systems make it possible to deliver in
real time corrections to the GNSS receivers in order, notably, to increase the
accuracy of the geo-localization that is performed. They also make it possible
to broadcast information used to improve the integrity of the service supplied
25 by the system. Generally, the corrections and other information generated
and broadcast by such systems are called augmentation data and are
transmitted in the form of augmentation messages directly in the navigation
signal.
To produce and broadcast such data the SBAS systems generally
30 consist of RIMS ground stations which permanently measure the GNSS
signals transmitted by the navigation satellites, processing centres which
2
receive these measurements and generate the augmentation messages and
NLES ground stations which transmit these messages by the GNSS
navigation signal to an SBAS augmentation satellite which serves as a relay
by retransmitting the received signal to the GNSS receivers. In such a
5 system, the payload of an SBAS satellite is said to be transparent, which
means that no processing resulting in a modification of the content of the
user received signal is performed onboard the satellite.
Such systems have limitations regarding the availability and the
1o continuity of operation that they offer which is guaranteed only at the cost of
additional complexity of the system, notably through a redundancy of some
equipment items.
Since the payload of an SBAS satellite is of transparent type, it does
not allow simultaneous access to its resources. The implementation of hot
15 redundancy between a nominal NLES ground station and a redundant
standby NLES station is thus not possible because the SBAS satellite is
capable only of receiving and retransmitting a signal at nominal power
transmitted by a single NLES station called master or nominal station.
The expression hot redundancy is used with reference to a system for
20 which at least two NLES ground stations can transmit a signal simultaneously
over the uplink channel of the SBAS satellite. By contrast, the term cold
redundancy is used when at least two stations are available for the
transmission of the navigation signal to the SBAS satellite but they do not
transmit simultaneously. The principle of cold redundancy is applied to the
25 known SBAS systems. When a failure of the nominal NLES station is
detected, the standby NLES station, which is not active by default, is started
up in order to handle the switching of stations and the continuation of service.
The time needed to start up the standby station results in a loss of continuity
and of consequential service interruption, which may exceed a minute. This
30 interruption time is also due to the following processing operations, which are
necessary for re-establishing the link: detection of the fault, switch over to the
3
redundant NLES station, stabilization of the station servo control loops,
acquisition of the required integrity level.
Another problem associated with the transparent aspect of the SBAS
5 satellite relates to the integrity of the navigation message received by this
satellite. The known solutions implement an integrity check on the signals in
the NILES ground stations. This check is performed by comparing the
navigation signal transmitted on the uplink channel with the signal transmitted
by the SBAS satellite on the downlink channel, which is picked up by the
1o NLES stations.
The detection of a possible scrambling or misrepresentation of the
navigation signals is performed on the ground and results either in the
broadcasting of a specific alert message which is not instantaneous, or a
cessation of transmission from the NLES station. There is no possible way of
15 preventing the broadcasting of a misrepresented signal on the SBAS satellite
itself, except by switching off the payload via a remote control link from the
ground. This type of operation can lead to a loss of availability for the GNSS
receivers which have to wait for the satellite to transmit a new valid signal.
20 A third problem lies in the use, for the transmission of the navigation
signals transmitted by the NLES stations, of multiple frequency bands. For
example, the GPS systems can use three frequency sub-bands in band L,
namely the bands L1, L2 and L5 for various uses. Similarly, the European
Galileo system provides for the use of four frequency sub-bands.
25 Furthermore, the operational maintenance of the system may require the
transmission of test channels.
The transmission of the signals over the uplink between a NILES
station and the satellite is done conventionally according to an FDMA-type
frequency plan and on a single polarization, that is to say that each signal is
30 transmitted in the frequency band (L1,L2,L5, etc.) which corresponds to it.
The multiplicity of the channels can therefore result in a very significant
4
spectral occupancy, and lead to an increase in the complexity of the ground
stations and of the payload.
Onboard the satellite, the payload provides a number of processing
channels suited to each frequency band. The transmission of the signals on
5 at least two distinct frequency bands leads to a differential dispersion in gain
and phase between the navigation channels relative to these different
frequencies. In practice, the propagation channel leads to different
disturbances (noise, impact of the ionosphere) according to the transmission
frequency. The signals must therefore be corrected in amplitude, in delay and
1o in phase to' compensate these differential errors. Furthermore, difficulties in
pairing and calibrating the channels between them result in poor
simultaneous management of signals transmitted on two or more distinct
frequency bands, which results in a performance degradation for the user of
the system.
15
The present invention aims notably to overcome the abovementioned
limitations of the known SBAS systems by proposing a payload for an SBAS
satellite that is digital and semi-transparent, that is to say which still allows for
the satellite mission of transparently relaying navigation and augmentation
20 messages to be fulfilled while introducing certain specific processing
operations on the satellite that make it possible to improve the security, the
service continuity, the control of the integrity checking and the radio
performance.
25 To this end, the subject of the invention is a payload for augmentation
satellite comprising an input channel suitable for receiving navigation signals
transmitted by at least one navigation land earth station in a first frequency
band and a plurality of output channels, each suitable for broadcasting
navigation signals in a frequency band different from said first band and from
30 other broadcasting bands, characterized in that it also comprises a navigation
5
processor suitable for implementing the following operations for each of said
received signals:
• from a first set of spreading codes each associated with a
navigation land earth station and for each of said codes,
5 unspreading the signal in phase in order to extract a pilot signal,
® authenticating said pilot signal and deducing from it the station
transmitting said signal,
• if the authentication is negative, blocking said navigation signal,
• measuring the signal-to-noise and interference ratio affecting said
10 pilot signal,
• retaining, from the navigation signals received, the one that
exhibits the highest signal-to-noise and interference ratio and for
which the authentication is positive, said retained signal being
called nominal signal, the station transmitting the nominal signal
15 being called nominal station, the other transmitting stations being
called redundant stations,
• from a second set of spreading codes each associated with a type
of navigation signal intended to be transmitted on one of the
broadcasting frequency bands and for each of said codes,
20 unspreading the nominal navigation signal in quadrature,
® tran_smitting said nominal navigation. signal in the broadcasting
frequency band associated with the spreading code used.
According to a particular aspect of the invention, said pilot signal is
25 first demodulated according to a "Cyclic Code Shift Keying" type modulation,
the authentication of the pilot signal being performed by identifying the code
associated with said modulation.
Another subject of the invention is a payload for augmentation satellite
comprising an input channel suitable for receiving navigation signals
30 transmitted by at least one navigation land earth station in a first frequency
band and a plurality of output channels, each suitable for broadcasting
6
navigation signals in a frequency band different from said first band and from
other broadcasting bands, characterized in that it also comprises a navigation
processor suitable for implementing the following operations for each of said
received signals:
5 • demodulating said received signal according to a "Cyclic Code
Shift Keying" type modulation,
® authenticating said received signal by identifying the code
associated with said modulation and deducing from it the station
transmitting said signal,
10 ® if the authentication is negative, blocking said navigation signal,
® measuring the signal-to-noise and interference ratio affecting said
received signal,
® retaining, from the navigation signals received, the one that
exhibits the highest signal-to-noise and interference ratio and for
15 which the authentication is positive, said retained signal being
called nominal signal, the station transmitting the nominal signal
being called nominal station, the other transmitting stations being
called redundant stations,
® from a set of spreading codes each associated with a type of
20 navigation signal intended to be transmitted on one of the
broadcasting frequency bands and for each of said codes,
unspreading the nominal navigation signal in quadrature,
® transmitting said nominal navigation signal in the broadcasting
frequency band associated with the spreading code used.
25 According to a particular aspect of the invention, when the signal-tonoise
and interference ratio measured on said nominal signal decreases
below a predetermined threshold, the new nominal signal retained is the one
which exhibits the highest signal-to-noise and interference ratio.
In a variant embodiment, the payload according to the invention also
30 comprises a return channel suitable for broadcasting, in said first frequency
band, at least one service signal to at least one navigation land earth station,
7
said service signal comprising at least the measurement of the signal-tonoise
and interference ratio of at least the nominal signal, said service signal
being suitable for implementing a servocontrol of the transmission power of
said redundant stations to the transmission power of the nominal station.
5 According to a particular aspect of the invention, said service signal
also comprises the measurements of the signal-to-noise and interference
ratio of the navigation signals received by the satellite and transmitted by all
the transmitting stations.
According to a particular aspect of the invention, said service signal
1o also comprises a measurement of the time offsets between the reception, by
the satellite, of the nominal signal on the one hand and of the signals
transmitted by the redundant navigation land earth stations, said service
signal being suitable for implementing a time synchronization between said
stations.
15 According to a particular aspect of the invention, said return channel is
also suitable for broadcasting, in said first frequency band, the nominal
navigation signal.
According to a particular aspect of the invention, said first frequency
band is the band C or Ku and said broadcasting frequency bands are at least
20 the bands L1 and L5.
According to a particular aspect of the invention, said spreading codes
are Walsh codes.
Another subject of the invention is an augmentation satellite
comprising a payload according to the invention, suitable for receiving a
25 navigation signal over an uplink in a first frequency band and rebroadcasting
said signal over a downlink in a plurality of broadcasting frequency bands.
Another subject of the invention is a navigation land earth station
suitable for receiving an augmentation message and for generating a
navigation signal containing said message, said navigation signal being
30 spectrally spread using a first spreading code associated with its
broadcasting frequency band, said navigation signal being transmitted over
8
an uplink in a first frequency band different from the broadcasting frequency
band.
According to a particular aspect of the navigation land earth station
according to the invention, said navigation signal is added in quadrature to a
5 pilot signal that is spectrally spread using a second spreading code
associated with said navigation land earth station.
According to a particular aspect of the navigation land earth station
according to the invention, a "Cyclic Code Shift Keying" type modulation is
first applied to said navigation signal or to said pilot signal.
10 According to a particular aspect of the navigation land earth station
according to the invention, the polarization of the transmitted navigation
signal is different according to the broadcasting frequency band.
Another subject of the invention is an augmentation system
comprising:
15 ® at least one observation station suitable for receiving a
radionavigation signal transmitted by at least one radionavigation
satellite and for performing measurements on s?id signal,
• at least one processing centre suitable for receiving said
measurements transmitted by at least one measurement station
20 and for generating, from said measurements, at least one
augmentation message,
• a plurality of navigation land earth stations according to the
invention,
® at least one augmentation satellite according to the invention.
25 In a particular embodiment, the augmentation system according to the
invention is suitable for implementing a switching of navigation land earth
stations of hot redundancy type.
In a particular embodiment, the polarization of the transmitted
navigation signal is different between two redundant navigation land earth
30 stations.
9
In a particular embodiment, on reception of a service signal
transmitted by said augmentation satellite and comprising at least the
measurement of the signal-to-noise and interference ratio of at least said
nominal signal, said navigation land earth stations implement a servocontrol
5 of their transmission power to the transmission power of the nominal station.
In a particular embodiment, on reception of said service signal, said
navigation land earth stations implement a time synchronization of their
respective transmissions.
Other features and advantages of the invention will become apparent
1o from the following description in light of the appended drawings which
represent:
- Figure 1, a block diagram of the architecture of an SBAS system
according to the prior art,
- Figure 2, a functional block diagram of the payload of an SBAS
15 satellite according to the prior art,
- Figure 3, a diagram illustrating the generation of the uplink signal
between an NLES station and an SBAS satellite, according to the
prior art,
- Figure 4a, a diagram illustrating the generation of navigation
20 signals by an NLES station according to the invention,
- Figure 4b, a variant embodiment of Figure 4a,
- Figure 5a, a functional block diagram of the payload of an SBAS
satellite according to the invention, in a first embodiment,
- Figure 5b, a functional block diagram of the payload of an SBAS
25 satellite according to the invention, in a second embodiment,
- Figure 6, a block diagram of the architecture of an SBAS system
according to the invention, in the second embodiment of the
payload,
- Figure 7, a functional block diagram of the navigation processor
30 according to the invention.
10
Figure 1 schematically represents, in a block diagram, the overall
architecture of an SBAS system according to the prior art. Such a system is
suitable for producing augmentation data from measurements performed on
the navigation signals 101 transmitted by a plurality of radionavigation
5 satellites NAV. The measurements and data originating from the
radiohavigation satellites NAV are collected by a plurality of observation
stations RIMS then transmitted 102, at a given rate, to a plurality of
processing centres CPF. The latter produce, from the measurements
received, an estimation of the differential corrections to be applied to the
1o radionavigation signal then generate augmentation messages, which include
these corrections and are then transmitted 103 to a navigation land earth
station NILES. The NILES station receives the augmentation messages and
transmits them 104 to an augmentation satellite SAT to then be broadcast
105 to the users U and to the ground stations RIMS, NILES. The
15 augmentation messages are, to this end, integrated, in a way similar to the
navigation messages, in a navigation signal compatible with the GNSS
system. A navigation signal, generated by,an NILES station then broadcast by
a satellite SAT, therefore contains both navigation messages and
augmentation messages which can be exploited by the GNSS receivers to
20 improve their locating performance. The augmentation satellite SAT may be
a geostationary satellite or a high earth orbit HEO satellite or even a satellite
in inclined geostationary orbit of IGSO (Inclined Geosynchronous Satellite
Orbit) type. The NLES station performs an integrity check on the messages
received, transmitted by the processing centres CPF, with the messages
25 broadcast by the augmentation satellite SAT over the downlink. The signal
retransmitted by the augmentation satellite SAT is also received by the RIMS
stations. The processing centre CPF can address a second redundant NLES
station, in the event of failure of the main NLES station, but this redundancy
is of cold type, that is to say that the redundant NLES station is off when the
30 main station is transmitting. The uplink channel between an NLES station
and an augmentation satellite SAT operates in band Ku or in band C. The
11
downlink channel between an augmentation satellite SAT and an RIMS
station or a user U operates in band L for the transmission of the navigation
signal.
Furthermore, some known augmentation systems such as the
5 European EGNOS system, implement a return channel in band Ku or in band
C between the augmentation satellite SAT and the NLES ground stations
allowing for a bi-frequency estimation (Ku or C and L) of the physical
parameters of the satellite link. This return channel is used only to implement
a servo control of the transmission time of the satellite through the use of a
10 long loop. In order to exactly synchronize the transmission time of the
navigation signal by the augmentation satellite SAT, the NILES ground
stations advance or delay the transmission of their signal over the uplink
channel in order to synchronize the satellite with a fixed clock.
When there is no return channel available, the temporal servo control
15 can also be produced through the navigation signal transmitted to the users
in band L, but with a PRN code specific to the test procedures and which will
therefore be unseen by the users.
The diagram of Figure 1 represents only one unit for each entity that
20 the system comprises, but this in reality comprises a number of augmentation
satellites SAT, a number of RIMS observation stations, a number of
processing centres CPF and a number of NILES transmission stations. As a
general rule, two NLES transmission stations (nominal and redundant) are
used for an augmentation satellite.
25 When an NLES station is defective, a switch over is performed to the
redundant NILES station, which results in an interruption of the signal
transmitted over the uplink channel, lasting a few seconds. The reestablishing
of the transmission of an usable signal results in an additional
delay of several tens of seconds and the complete restoration of the system
30 takes around a minute. This loss of service continuity particularly impacts on
the applications with high availability demand such as the aeronautical
12
applications. It results, for the users in sight of a single augmentation
satellite, in a total loss of service and, for the others, the need to switch to
another satellite. Furthermore, even on resumption of service, the integrity
checking loop which is used to guarantee the integrity of the messages
5 transmitted, is not immediately operational as long as the ground stations
have not locked on again to the new signal transmitted by the satellite. The
monitoring of the integrity of the system is not then effective on the downlink
and a misrepresentation of this downlink may impact a large regional area
without the system being able to detect it.
10
Figure 2 schematically represents , in a block diagram, the main
functions of the payload of an SBAS augmentation satellite according to the
prior art . The signals transmitted by the NLES station are received on an
input 201 , suitable for receiving signals in band C or in band Ku, amplified by
15 a low- noise amplifier 202, then are directed , via a distributor 203, to a
frequency conversion channel in one of the sub -bands of the band L used by
the GNSS system . In the,diagram of Figure 2 , two conversion channels are
represented , making it possible , respectively, to convert the signal into band
L1 or into band L5 for retransmission to the users of the GNSS system.
20 A conversion channel comprises a first frequency transposition device
204,214 to an intermediate frequency , a second frequency transposition
device 205 , 215 to a frequency in band L1 or L5, a reference local oscillator
209, an amplifier 206,216 , a band pass filter 207 ,217 and an output 208,218
suitable for transmitting signals in band L1 or L5.
25 Such a payload is said to be transparent because it performs no
specific processing on the received signal other than the processing needed
for the frequency transposition to the users ' band . Thus, the signals received
by the satellite are always retransmitted to the ground, with no check , at this
level, as to their integrity . Furthermore , such a payload consists only of
3o analogue circuits and the adaptation to two user frequency bands entails
duplicating the frequency conversion channel , which presents drawbacks.
13
The two channels have to be paired in amplitude, phase and differential
delays.
Figure 3 schematically represents the generation of the signal
5 transmitted over the uplink channel between an NILES station of the prior art
and a satellite SAT. The signal to be transmitted 301,302 is first of all
converted into frequency L1 or L5 depending on the frequency band of the
final user. Two local oscillators IF1,IF2 are used for this purpose. The
resulting signal complies with an FDMA-type frequency plan and it is then
10 converted into band C or Ku through the intermediary of a third local
oscillator IF. The frequency bandwidth of the navigation signal is therefore
proportional to the number of augmentation signals to be transmitted.
The aim of the invention is notably to overcome the drawbacks of the
15 known systems described with the help of Figures 1, 2 and 3.
Figure 4a represents, in a diagram, the modification, according to the
invention, of the signal transmitted by an NLES station to an augmentation
satellite SAT. To simplify the description, only the case of two navigation
20 signals is presented, said signals containing augmentation messages to be
transmitted over two distinct frequency bands L1,L5.
Each navigation signal to be transmitted 301,302 is spread by a
specific spreading code C1,C5, for example a Walsh code, so that each
signal 401,402 occupies the same spectral bandwidth. The function of this
25 code is to unambiguously identify the transmission channel associated with
the frequency band L1 or L5. The duly spread signals 401,402 are then
transmitted on the same central frequency, in accordance with the CDMA
access method. In particular, the spreading codes used are mutually
orthogonal so as to allow for a simultaneous transmission of signals over the
30 uplink channel emanating from a number of NILES stations. The navigation
signal 403 obtained in this way has added to it a pilot signal, phase-shifted by
14
90°, and also spread via a specific spreading code CO which serves to
identify the transmitting NLES station and to authenticate the navigation
signal. The authentication method used can be any known method that
makes it possible to ensure an identification of the received signal. Examples
5 that can in particular be cited are the authentication methods described in the
following patent applications: FR2921528 relating to a method providing the
means for recognizing the origin and/or the content of an RF signal or
US2009/0179743, entitled "Pseudo-random authentication code altering
scheme for a transponder and a base station" or even US 2005/0041955,
1o entitled "Authentification of data in a digital transmission system". So-called
"watermarking" techniques, suitable for the satellite links, can also be used.
The resulting signal is finally transposed into band C or Ku through a local
oscillator IF to be transmitted over the uplink channel to the augmentation
satellite SAT.
15 By modifying the generation of the navigation signal as indicated in
Figure 4a, the invention makes it possible notably to eliminate the differential
errors induced by the propagation channels associated with the different
uplink frequency bands L1,L5. The introduction of the pilot signal makes it
possible both to produce an authentication, onboard the satellite, of the
20 transmitted signal and to augment the signal-to-noise ratio estimation
efficiency. Furthermore, the problems of synchronization between the two
channels are also eliminated since the signals previously intended for
transmission in band L1 or L5 are now spectrally coded and transmitted on
one and the same frequency. Finally, the use of a spreading code CO specific
25 to each transmitting NILES station makes it possible to implement a
redundancy between stations of hot type as will be explained later.
In a variant embodiment of the invention illustrated in Figure 4b, a
CCSK (Cyclic Code Shift Keying) type modulation is applied previously to the
navigation signals in band L1 and L5 before the step of spectral spreading
30 using the Walsh codes C1,C5.
15
The CCSK modulation technique is well known to those skilled in the
art, for example from the publication "Cyclic Code Shift Keying: A Low
Probability of Intercept Communication Technique, IEEE Transactions on
Aerospace and Electronic Systems, Vol. 39, No 3, July 2003". It consists in
5 using a single Csk spreading code of PN (Pseudo Noise) type to modulate
each navigation signal by shifting the sequence of the code by a
predetermined number of symbols to produce the modulation of each signal.
This variant embodiment makes it possible to eliminate the pilot signal
10 and to produce the authentication of the NLES station directly by identifying
the CCSK codes applied to the navigation signals. A single CCSK code is
used for all the transmitting NLES stations by each time shifting the
sequence of the code by a predetermined number of symbols before
producing the modulation of the signal.
15 In another variant embodiment of the invention (not represented), the
pilot signal can be retained but over modulated by a CCSK code in order to
improve the efficiency in detecting and therefore authenticating the different
NLES stations through the good properties of the CCSK modulation selfcorrelation
function.
20
In another variant of the invention, different polarizations are employed
for the transmission of the signals in band L1 and L5. For example, a right
polarization is used for one of the signals and a left polarization for the other.
In this way, the cross-correlation between the two signals is limited which
25 makes it possible to further improve the performance while keeping a low
differential dispersion between the signals.
In another variant of the invention, the same polarizations are
employed for the transmission of the signals in band L1 and L5 of one and
the same NILES station but different polarizations are used for two different
3o redundant NLES stations. This scenario also encompasses the case where a
given NILES station transmits only signals in band L1 and another redundant
16
NLES station transmits only signals in band L5 with a different polarization
from the signals in band L1 transmitted by the first station.
Figure 5a schematically represents, in a block diagram, the main
5 functions of the payload of an SBAS augmentation satellite according to the
invention. The elements that are common with the payload according to the
prior art described in Figure 2 are numbered with the same references.
The frequency transposition devices 204,205,214,215 for each
channel are eliminated and replaced by a single navigation processor 501
1o which receives the navigation signal transmitted by one or more NILES
stations and transmits it to one of the amplification and filtering channels for
broadcasting to the users according to the frequency band L1,L5 identified by
the associated spreading code C1,C5.
15 Figure 5b schematically represents a variant embodiment of the
payload according to the invention. In this variant, a specific return channel to
the NLES station or stations on the ground is implemented through an
amplifier 502 and a band pass filter 504. A second band pass filter 503 is
necessary to separate the signal received by an NLES station from the signal
20 retransmitted to this same station. This return channel operates in band Ku or
C just like the uplink channel to the augmentation satellite.
As mentioned above, some augmentation systems, like the European
EGNOS system, already implement a specific return channel for the
augmentation satellites, this channel being used solely for the temporal servo
25 control of the satellite on a time base linked to the ground stations.
In the variant embodiment of the invention now being described, the
return channel of the augmentation satellite also makes it possible to transmit
certain information to the NLES ground stations in order to further improve
the service continuity upon a switchover between two transmitting stations. In
30 particular, a measurement of the signal-to-noise plus interference ratio SNIR
of the uplink is performed onboard the satellite then retransmitted to the
17
NLES stations via this return channel. A relative synchronization between
nominal station and redundant station is also provided.
Figure 6 illustrates the overall operation of an SBAS system according
5 to the embodiment of the invention for which a return channel 611 is
implemented between the augmentation satellite 600 and the navigation land
earth stations 601,602. From the information transmitted by the satellite, the
earth stations 601,602 establish a synchronization link 612 between them.
This link 612 is used notably to ensure the mutual time synchronization of the
1o stations.
Figure 7 describes a functional block diagram of the navigation
processor 501 according to the invention.
The analogue signal received at the input of the processor 501 is
15 digitally converted via an analogue/digital converter 701. The digital signal
obtained is supplied as input for a first pilot signal processing module 702.
This module 702 provides a synchronization and a demodulation of the pilot
signal from the spreading codes referenced for each NILES station in a
memory 703. The demodulated pilot signal is authenticated, according to the
20 selected prior art authentication method, via an authentication module 706.
The ratio between the signal and the combination of the noise and of the
interference, SNIR, is measured via a measurement module 707.
In parallel, the signal in quadrature, corresponding to the navigation
signal transmitted by different NILES stations, is transmitted to a
25 demultiplexer 704 which associates the received navigation signal with an
identifier of an NILES station according to the reference spreading code CO of
said station. The navigation signal associated with the identifier of the
transmitting NLES station is then transmitted to a module 705 which is
notably responsible for authorizing the retransmission of the signal to the
30 users. Based on the authentication result produced by the authentication
18
module 706, the signal may possibly be blocked in the case where this result
is negative.
In the case where a pilot signal is not used (case not represented in
Figure 7), the authentication of the transmitting NILES station is produced
5 directly on the CCSK codes applied to the navigation signals. The
measurement of the signal-to-noise plus interference ratio SNIR of the uplink
is also performed directly on the navigation signals and not on the pilot
signal.
If the authentication is positive, the signal-to-noise and interference
10 ratio SNIR measurements obtained for the different NLES stations are
compared and only the signal transmitted by the station benefiting from the
best link budget is retained to be retransmitted to the ground. When the best
SNIR ratio is obtained for a signal transmitted by a redundant station, which
is therefore not the optimal signal actually retransmitted, the payload
15 according to the invention performs a station switchover by selecting the new
optimal signal as the one transmitted by the redundant station exhibiting the
highest SNIR ratio. Thus, the switchover is performed transparently onboard
the satellite, without causing any service interruption. The decision to
switchover between two transmitting NILES stations is made by the
20 navigation processor 501 when the SNIR ratio measured on the nominal
signal decreases below a predetermined threshold.
In the case where a return channel is available (embodiments of the
invention described in Figures 5b and 6), the signal-to-noise and interference
25 ratio SNIR measurements are retransmitted to the NILES stations on the
ground via the available return channel. To this end, said measurements are
transmitted by a modulated signal, for example modulated using a BPSK
modulation, and on a different frequency to that used to transmit the
navigation signal. An FDM-type frequency plan is used,
30 The return channel according to the invention therefore allows, on the
one hand, for the transmission, on a first frequency band, of the navigation
19
signal, both nominal and redundant, for the purposes of time servocontrolling
the signal transmitted by the satellite, and, on the other hand, for the
transmission, on a second frequency band, of a service signal conveying the
SNIR ratio information for each identified transmitting NLES station. The
5 module 705 is, for this purpose, responsible for generating the service signal
conveying the SNIR ratio information estimated by the module 707. The
return channel is implemented in the form of at least one oversampling
module 708 and one digital/analogue converter 709.
Furthermore, when a switchover is performed onboard the satellite,
10 the choice of the nominal NILES station can also be retransmitted via this
service signal in order to inform the ground stations that a switchover has
taken place.
When the NLES stations 601,602 receive the service signal
15 transmitted by the augmentation satellite 600, they adjust their transmission
power so that each station transmits a signal that is servocontrolled in gain or
amplitude on the optimal signal, in other words the one transmitted by the
nominal NILES station. Thus, all the signals transmitted over the uplink
channel from the augmentation satellite exhibit an equivalent power, which
20 further facilitates the transparent switchover between stations, onboard the
satellite, when the nominal station fails.
To produce this power servocontrol, only the measurement of the
SNIR ratio of the nominal signal is needed. In a variant embodiment of the
invention, all the SNIR ratio measurements are transmitted by the satellite, to
25 enable the redundant NLES stations to estimate the quality of their link.
In another variant embodiment of the invention, in addition to the
measurement of the SNIR ratio, the payload of the satellite performs a
measurement of the time offset between the different signals received on the
30 uplink channel, originating from the nominal NLES station on the one hand
and from the redundant NLES stations on the other hand. The signal from the
20
nominal station is identified by virtue of the spreading code of the pilot signal
transmitted as explained previously or directly from the identification of the
CCSK codes of the navigation signals themselves depending on the variant
of the invention that is selected. A measurement of the time offset between
5 the instant of reception of the nominal signal and the instant of reception of
each of the signals transmitted by the redundant NLES stations is performed.
This offset measurement is also transmitted in the service signal via the
satellite's return channel. This information is received by the NLES ground
stations which can then provide a time synchronization, via a terrestrial link
10 612, for their respective transmitters, in order to synchronize their
transmissions on the uplink channel.
This synchronization further improves the service continuity in the
event of a station switchover performed by the payload.
The invention aims to synchronize in time and in power all the signals
15 transmitted by the different NLES stations on the uplink channel in order to
implement a so-called hot redundancy.
Furthermore, each station, on reception of the navigation signal
transmitted over the return channel, performs an integrity check by
20 comparing the augmentation message received with the one previously
transmitted. If this check is negative, the station stops transmitting. The
satellite will no longer receive any navigation signal transmitted by this station
and can perform a switchover to the redundant station that benefits from the
best link budget.
25
Once the optimal navigation signal has been authenticated and
selected from the different signals transmitted by the NLES stations, it is then
transmitted to a second signal processing module 710 which performs the
demodulation of the navigation signal according to its associated spreading
30 code 711, and/or its polarization component. Specifically, a signal
unspreading operation, based on the known spreading code. is performed.
21
Each spreading code corresponds to a frequency band, in band L, used to
transmit the navigation signals to the users. Once the frequency band is
identified, for example the band L1 or L5, the navigation signal is
retransmitted to the users of the system via a frequency conversion channel
5 comprising at least one oversampling module 713 and one digital/analogue
converter 714. In the case where no pilot signal is used, the authentication of
the transmitting NILES station is performed jointly with the CCSK
demodulation.
10 Furthermore, the signal processing module 710 also makes it possible
to apply corrections, in amplitude, phase and/or delay, to the signals based
on one or more calibration tables 712. These calibration tables are completed
on the basis on temperature-related measurements performed on the
payload and that make it possible to identify imbalances in amplitude, delay
15 and phase, that are differential between each band L transmission channel.
These tables are generated from measurements performed before the
commissioning of the satellite. The corrections are then updated according to
the temperature of the equipment in service, measured elsewhere by a
temperature measuring device. These corrections allow for an accurate
20 balancing of the differential transmission paths.
An alternative way to produce the differential correction of the different
band L transmission channels consists in using one or more redundant
channels in transmission mode which indirectly make it possible to save the
25 signal transmitted then measure the drifts between the saved transmitted
signal and the received signal.
A particular use of the invention consists in using the nominal NLES
station to broadcast operational signals in band L1 and L5 and a redundant
NLES station to transmit test signals used to validate the system in order to
30 test some of its functionalities or perform a qualification of the system.
22
In this case, the different spreading codes C1,C5,CO used are specific
to the test signals.
The test signals used can be transmitted by the same NLES station
used to transmit the nominal navigation signals, with one and the same
5 antenna polarization or with a cross polarization relative to the navigation
signals. The test signals can also be transmitted by an NLES station
specifically dedicated to the tests.
The invention notably offers the advantage of retaining the primary
1o mission of the augmentation satellite, that is to say relaying the augmentation
messages to the users, while improving the service continuity in the event of
a switchover between redundant NLES stations. It also makes it possible to
improve the authentication of the signals transmitted and to fight against any
misrepresentations and it also improves the performance of the system when
15 different frequency bands L1,L5 are used to transmit the navigation signals.
23

CLAIMS
1. Payload for augmentation satellite (600) comprising an input channel
5 (201,202,503) suitable for receiving navigation signals transmitted by at
least one navigation land earth station (NLES) in a first frequency band
and a plurality of output channels (206,207,208,216,217,218), each
suitable for broadcasting navigation signals in a frequency band different
from said first band and from other broadcasting bands, characterized in
10 that it also comprises a navigation processor (501) suitable for
implementing the following operations for each of said received signals:
® from a first set of spreading codes (703) each associated with a
navigation land earth station (NLES) and for each of said codes,
unspreading (702) the signal in phase in order to extract a pilot
15 signal,
• authenticating (706) said pilot signal and deducing from it the
station (NLES) transmitting said signal,
® if the authentication (706) is negative, blocking said navigation
signal,
20 ® measuring (707) the signal-to-noise and interference ratio (SNIR)
affecting said pilot signal,
® retaining, from the navigation signals received, the one that
exhibits the highest signal-to-noise and interference ratio and for
which the authentication is positive, said retained signal being
25 called nominal signal, the station (NLES) transmitting the nominal
signal being called nominal station, the other transmitting stations
(NLES) being called redundant stations,
® from a second set of spreading codes (711) each associated with a
type of navigation signal intended to be transmitted on one of the
30 broadcasting frequency bands and for each of said codes,
unspreading (710) the nominal navigation signal in quadrature,
24
® transmitting (713,714) said nominal navigation signal in the
broadcasting frequency band associated with the spreading code
used.
5 2. Payload for augmentation satellite (600) according to Claim 1, in which
said pilot signal is first demodulated according to a "Cyclic Code Shift
Keying" type modulation, the authentication of the pilot signal being
performed by identifying the code associated with said modulation.
10 3. Payload for augmentation satellite (600) comprising an input channel
(201,202,503) suitable for receiving navigation signals transmitted by at
least one navigation land earth station (NLES) in a first frequency band
and a plurality of output channels (206,207,208,216,217,218), each
suitable for broadcasting navigation signals in a frequency band different
15 from said first band and from other broadcasting bands, characterized in
that it also comprises a navigation processor (501) suitable for
implementing the following qperations for each of said received signals:
• demodulating said received signal according to a "Cyclic Code
Shift Keying" type modulation,
20 • authenticating (706) said received signal by identifying the code
associated with said modulation and deducing from it the station
(NLES) transmitting said signal,
• if the authentication (706) is negative, blocking said navigation
signal,
25 • measuring (707) the signal-to-noise and interference ratio (SNIR)
affecting said received signal,
• retaining, from the navigation signals received, the one that
exhibits the highest signal-to-noise and interference ratio and for
which the authentication is positive, said retained signal being
30 called nominal signal, the station (NLES) transmitting the nominal
25
signal being called nominal station, the other transmitting stations
(NILES) being called redundant stations,
® from a set of spreading codes (711) each associated with a type of
navigation signal intended to be transmitted on one of the
5 broadcasting frequency bands and for each of said codes,
unspreading (710) the nominal navigation signal in quadrature,
® transmitting (713,714) said nominal navigation signal in the
broadcasting frequency band associated with the spreading code
used.
10
4. Payload for augmentation satellite (600) according to any of the preceding
claims, characterized in that, when the signal-to-noise and interference
ratio (SNIR) measured on said nominal signal decreases below a
predetermined threshold, the new nominal signal retained is the one
15 which exhibits the highest signal-to-noise and interference ratio.
5. Payload for augmentation satellite (600) according to any of the preceding
claims, characterized in that it also comprises a return channel (502,504)
suitable for broadcasting, in said first frequency band, at least one service
20 signal to at least one navigation land earth station (NILES), said service
signal comprising at least the measurement of the signal-to-noise and
interference-ratio (SNIR) of at least the nominal-signal, said service signal
being suitable for implementing a servocontrol of the transmission power
of said redundant stations (NLES) to the transmission power of the
25 nominal station (NLES).
6. Payload for augmentation satellite (600) according to Claim 5,
characterized in that said service signal also comprises the
measurements of the signal-to-noise and interference ratio (SNIR) of the
30 navigation signals received by the satellite (600) and transmitted by all the
transmitting stations (NLES).
26
7. Payload for augmentation satellite (600) according to Claim 5 or 6,
characterized in that said service signal also comprises a measurement of
the time offsets between the reception, by the satellite (600), of the
nominal signal on the one hand and of the signals transmitted by the
redundant navigation land earth stations (NLES), said service signal
being suitable for implementing a time synchronization between said
stations (NLES).
10 8. Payload for augmentation satellite (600) according to any of Claims 5, 6
and 7, characterized in that said return channel (502,504) is also suitable
for broadcasting, in said first frequency band, the nominal navigation
signal.
15 9. Payload for augmentation satellite (600) according to any of the preceding
claims, characterized in that said first frequency band is the band C or Ku
and said broadcasting frequency bands are at least the bands L1 and L5.
10. Payload for augmentation satellite (600) according to any of the preceding
20 claims, characterized in that said spreading codes are Walsh codes.
11. Augmentation satellite (600) comprising a payload according to any of the
preceding claims, suitable for receiving a navigation signal over an uplink
in a first frequency band and rebroadcasting said signal over a downlink
25 in a plurality of broadcasting frequency bands.
12. Navigation land earth station (NLES) suitable for receiving an
augmentation message and for generating a navigation signal containing
said message , said navigation signal being spectrally spread using a first
30 spreading code (C1,C5) associated with its broadcasting frequency band
27
(L1,L5), said navigation signal being transmitted over an uplink in a first
frequency band different from the broadcasting frequency band (L1,L5).
13. Navigation land earth station (NLES) according to Claim 12 , in which said
5 navigation signal is added in quadrature to a pilot signal that is spectrally
spread using a second spreading code (CO) associated with said
navigation land earth station (NLES).
14. Navigation land earth station (NLES) according to any of Claims 12 and
10 13, in which a "Cyclic Code Shift Keying" type modulation is first applied
to said navigation signal or to said pilot signal.
15. Navigation land earth station (NLES) according to any of Claims 12 to 14,
in which the polarization of the transmitted navigation signal is different
15 according to the broadcasting frequency band (L1,L5)
16. Augmentation system comprising:
® at least one observation station (RIMS) suitable for receiving a
radio navigation signal transmitted by at least one radio navigation
20 satellite (NAV) and for performing measurements on said signal,
® at least one processing centre (CPF) suitable for receiving said
measurements transmitted by at least one measurement station
(RIMS) and for generating, from-said measurements,- at least one
augmentation message,
25 a plurality of navigation land earth stations (NLES) according to
any of Claims 12 to 15,
® at least one augmentation satellite (600) according to Claim 11.
17.Augmentation system according to Claim 16, characterized in that it is
30 suitable for implementing a switching of navigation land earth stations
(NLES) of hot redundancy type.
28
18.Augmentation system according to any of Claims 16 and 17, in which the
polarization of the transmitted navigation signal is different between two
redundant navigation land earth stations (NLES).
5 19.Augmentation system according to any of Claims 16 to 18, characterized
in that, on reception of a service signal transmitted by said augmentation
satellite (600) and comprising at least the measurement of the signal-tonoise
and interference ratio (SNIR) of at least said nominal signal, said
navigation land earth stations (NLES) implement a servocontrol of their
10 transmission power to the transmission power of the nominal station.
20.Augmentation system according to any of Claim 16 to 19, characterized in
that, on reception of said service signal, said navigation land earth
stations (NLES) implement a time synchronization of their respective
15 transmissions.