FR2811834A1 - Satellite navigation system for aircraft has reference station using coding circuits with pseudo-random in spreading code - Google Patents

Abstract

The satellite navigation system for aircraft has a reference station on the ground for a satellite array. The reference station has radio-electrical coding circuits to the differential receivers mounted on the aircraft. The coding system operates using pseudo random changes in the pseudo-binary spreading code for the communication band.

Description

DIFFERENTIAL SATELLITE NAVIGATION SYSTEM WITH

ENCRYPTED DATA LINK

  The present invention relates to differential satellite navigation systems such as, for example, that known under the

  Anglo-Saxon name of Differential Global Positioning System.

  The GPS satellite navigation system was designed in the 1970s under the leadership of the United States Department of Defense. It is made up of a constellation of twenty-four traveling satellites, fixed satellite control stations placed on the ground and navigation receivers fitted to the mobiles to be steered. The traveling satellites have precisely known orbits and positions in their orbits and emit continuously, signals containing information on the positions of the satellites and the system clock, and allowing a user to be in relation to them , in distances and in relative speeds, and therefore to be located in relation to the terrestrial globe by triangulation operations. One of the peculiarities of the GPS satellite navigation system is to have two levels of accessibility: a first level with free access but allowing only a location of low precision, of the order of thirty meters and a second level controlled access requiring approval from the US Department of Defense (DOD), but allowing

  much more precise location, of the order of ten meters.

  The civil uses are too numerous to be able to be treated with an approval procedure without this procedure losing its vocation of limitation of access. However, some of them, such as guiding aircraft during the landing phase, have precision requirements which are not satisfied with the first level of open access of the system.

GPS navigation.

  The difference in location accuracy and speed measurement between the two access levels of the GPS navigation system is due, among other things, to an intentional limitation of the accuracy with which the system clock can be recovered on reception. This limitation is mainly manifested by systematic errors in position and speed measurements which are common to all navigation receivers operating in the same area and which are designated in the art by differential errors. As an aircraft guidance during the landing phase requires a high accuracy of localization, only locally, in the vicinity of the runway entrance, we quickly thought of improving locally the accuracy of a navigation system receiver by GPS satellites using the first level with free access by eliminating from its measurements the differential errors measured elsewhere by an auxiliary GPS navigation receiver placed on the ground, in a fixed reference station, in a place whose geographic coordinates are known with precision, and transmitted to the navigation receivers placed on board mobiles operating in the vicinity, by radio link. This technique, also used in the context of other radionavigation systems such as the LORAN system, is

  known, in the context of GPS, under the name of "differential GPS".

  It allows to reach locally the desired precision of ten meters. It has the advantage of correcting not only the differential errors due to the voluntary inaccuracy of the clock of the GPS satellites but also those due to the intangibles affecting the conditions of

  propagation of radio signals from satellites.

  The implementation of the differential GPS technique involves the presence, at the level of the mobile equipped with the GPS receiver, of equipment making it possible to receive information on the estimation of the differential errors transmitted from a reference ground station by radio. . This radio reception equipment can be

  shared by the GPS receiver, with other mobile equipment.

  The use, at the level of a navigation receiver of a mobile, of the estimation of the differential errors made by a reference ground station essentially results in a modification of the software implemented by the processor of this navigation receiver , to correct the pseudo-distance and pseudo-speed measurements before using them

  to draw, by triangulation, the position and the velocity vector of the mobile.

  The guidance of an aircraft in the terminal landing phase requires not only great positioning accuracy on the part of the guidance system but also the availability and integrity which are lacking in current navigation satellite navigation systems. In fact, the latter suffer from temporary losses in availability due to the fact that the constellation of satellites visible from a receiver may temporarily take on a geometry unfit for operation.

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  triangulation. They also suffer from a lack of integrity to the extent that a receiver can receive faulty location signals from one or more satellites of the navigation system, the time for the control stations to notice the problem. and turn off the satellites in question. To address these problems of availability and integrity of scrolling satellite navigation systems which make them unsuitable for guiding aircraft when approaching a landing field, even in the absence of a lack of positioning accuracy, it has been proposed by the American civil aviation regulatory authority known by the acronym FAA (Federal Aviation Administration in English) to add to these satellite navigation systems additional capacity for positioning accuracy (differential technique), availability and monitoring of the integrity of the location signals of the satellites of the navigation system by implementing, in addition to the network of moving satellites of a navigation system , a network of ground monitoring stations and one or more geostationary satellites. This improvement in navigation satellite systems is known by the name WAAS from the Anglo-Saxon expression "Wide Area Augmentation System". It is made up of ground monitoring stations which are spread over a large area of coverage and which locally measure the differential errors (common mode errors on pseudo-distances and pseudo-speeds) and the apparent integrity of the location signal. each visible satellite of the navigation system (s), a data center which receives all the measurements from ground monitoring stations and use them to derive correction data, and a terrestrial navigation station which receives this correction data and communicates it to the navigation receivers of the mobiles operating in the area. The terrestrial navigation station uses an INMARSAT geostationary satellite to communicate its information to navigation receivers on board mobiles. More specifically, it uses as a transmission medium to transmit its information to mobile navigation receivers, a radio signal having a shape allowing the geostationary satellite INMARSAT to pass as one of the satellites of the navigation system although it does not

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  call, on board this geostationary INMARSAT satellite, only to a C-band L-band transponder or transponder. The information transmitted enables the navigation receiver of a mobile to operate in differential mode and to reject the measurements deduced from the location signals traveling satellites whose integrity is reported to be insufficient while the particular form of the signal retransmitted by the geostationary satellite INMARSAT allows the navigation receiver of a mobile, to use the INMARSAT satellite in the same way as a traveling satellite of the satellite navigation system. This adds an additional satellite to the constellation of visible scrolling satellites from which the mobile satellite navigation receiver performs its location by triangulation and reduces, in a large proportion, transient losses

  availability of the satellite navigation system.

  Other systems equivalent to the WAAS system are in the process of being developed, such as the EGNOS system for Europe or the MSAS system for Japan. These differential navigation systems, more or less sophisticated, all have the disadvantage of full free access in their area of influence, giving a user, whatever the nature of their intentions, a very precise guidance possibility. , a disadvantage that the US State Department wanted to avoid with the GPS system by implementing an accreditation to access precision guidance. This problem can be solved, as in the case of simple GPS, by encryption of the information transmitted to the navigation receivers fitted to the mobiles, this encryption then relating to the information in

from the ground station.

  A radio transmission signal to or from a satellite is most of the time a spread band signal of DSSS or CDMA type, acronyms drawn from the English expressions "Direct Sequence Spread Spectrum" and "Code Division Multiple Access "which, because of its wide band, has the advantage of being relatively insensitive to parasitic radio signals in narrow band and which is naturally coded since it results from successive modulations of a carrier by a pseudo-random binary code and by the data to be transmitted, the code

  with a speed significantly higher than that of the data.

  One solution, to ensure the confidentiality of the uplink radio transmission link of a differential navigation system, would consist in using, for frequency spreading of the latter, a specific non-public pseudo-random binary code. This solution is difficult to implement with a satellite navigation system such as GPS, which uses pseudo-random binary spreading codes in separate frequency bands to identify the navigation satellites because the pseudo-random binary codes used must have between them orthogonality properties which greatly limit the possibilities of choice of codes and therefore the robustness of the encryption that they can provide. In addition, it requires the addition to on-board navigation receivers of a local pseudo-random binary code generator dedicated to the encryption of the data transmission link, which does not lend itself easily to a purely software implementation due to the importance of the speed of the frequency band spreading code and which implies adaptations

  relatively large hardware and software.

  Another solution, which avoids the disadvantages of the first, consists in using, for the uplink radio transmission of a differential navigation system, a public pseudo-random binary code but encrypted by an over-modulation by means of a second pseudo-random binary code non-public debit intermediate between that of the public pseudo-random binary code ensuring the spread of the frequency band and that of the transmitted data, in the manner of what is done in the context of

  Simple GPS for the level of access requiring accreditation.

  The object of the present invention is another way of encrypting the uplink radio link of a differential satellite navigation system also operating on a public pseudo-random binary code used for bandwidth of the link, which gives great freedom. in encryption and requires only a few hardware modifications from a satellite navigation receiver to take encryption into account. It relates to a differential satellite navigation system comprising:

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  - a constellation of navigation satellites generating location signals occupying frequency spread bands, - at least one reference ground station measuring the differential errors made by a satellite navigation receiver occupying its location and using the location signals of the visible satellites of the constellation, - at least one differential satellite navigation receiver on board a mobile operating in the area of the reference ground station using the location signals of the visible satellites of the constellation and information on the differential errors measured at the reference ground station transmitted by encrypted spread band radio link, and - said encrypted spread band radio link rising from said reference ground station to said differential receivers on board mobiles of which spread in frequency band results from modulation by means of a

pseudo-random binary code.

  said differential navigation system being characterized in that it includes means for encrypting the radio link up from said reference ground station to said differential navigation receivers on board mobiles operating by means of pseudo-random jumps in the unwinding of the pseudo-random binary band spreading code

of the link.

  Advantageously, the band spreading code of the uplink radio link from the reference ground station to the differential navigation receivers is generated locally on transmission as on reception by means of a multi-stage shift register with serial and parallel inputs, looped back on itself by a network of logic gates, the encryption resulting from a periodic repositioning of the stages of the register

  offset to values imposed by a pseudo- number generator

  random. Advantageously, the band spreading code of the uplink radio link from the reference ground station to the receivers

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  navigation differentials is generated locally on transmission as on reception by means of several shift registers with several stages with serial and parallel inputs, looped back on themselves by networks of logic gates and by means of a combinational logic circuit combining the output signals of the shift registers, the encryption resulting from a periodic reorganization of the combinational logic circuit according to

  configurations imposed by a pseudo-random number generator.

  Advantageously, the band spreading code of the uplink radio link from the reference ground station to the differential navigation receivers is generated locally at transmission as at reception by means of several shift registers with several input stages. series and parallel, looped back on themselves by networks of logic gates and by means of a combinational logic circuit combining the output signals of the shift registers, the encryption resulting from a periodic repositioning of the stages of the shift register and d '' a periodic reorganization of the combinational logic circuit according to

  configurations imposed by a pseudo-random number generator.

  Advantageously, the encrypted spread-band radio link ascending from said reference ground station to said differential receivers on board mobiles occupies a frequency band different from those used by navigation satellites, differential on-board navigation receivers comprising at input , an omnidirectional antenna suitable for receiving the signal from this radio link and a frequency transposition stage reducing the signal from this radio link in a frequency band occupied by the location signals of the navigation satellites so that it can be processed , within the on-board differential navigation receivers, by the

  stages already processing the location signals of the navigation satellites.

  Advantageously, the encrypted spread-band radio link ascending from said reference ground station to said differential receivers on board mobiles is located in band C of the

  radio frequency spectrum.

  Advantageously, the encrypted spread-band radio link ascending from said reference ground station to said differential receivers on board mobiles is relayed by a repeater arranged on an intermediate platform such as an aircraft, a balloon or

a satellite.

  Advantageously, the encrypted spread-band radio link ascending from said reference ground station to said differential receivers on board mobiles is relayed by a repeater placed

  aboard a geostationary satellite.

  Advantageously, the encrypted spread-band radio link ascending from said reference ground station to said differential receivers on board mobiles is, from the ground station, in C band and relayed by a C-band L-band repeater placed at on board a satellite. Other characteristics and advantages of the invention will emerge

  of the description below of an embodiment given by way of example.

  This description will be made with reference to the drawing in which:

  - a figure 1 represents, diagrammatically, a differential satellite navigation system, - a figure 2 represents, diagrammatically, a pseudo-random binary spread spectrum code generator used in a satellite navigation system, - a figure 3 represents, diagrammatically, a generator of encrypted pseudo-random binary code according to the invention, usable for the uplink radio link of a differential system of navigation by satellites, and - a figure 4 represents, diagrammatically , an example of an adaptation assembly to be placed at the input of a navigation receiver to enable it to process the signal of an uplink radio link from a reference ground station and to be able to operate in differential mode As can be see it in figure 1, a satellite navigation system comprises: - a set of navigation satellites 1 which transmit location signals,

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  - satellite navigation receivers 2 on board mobiles 3 which are guided by the location signals of the navigation satellites 1, and - a reference station 4 fixed to the ground 5 which transmits signals for correcting differential errors in the direction receivers

  navigation satellite 2 on board mobiles 3.

  The set of navigation satellites 1 is part of a network of navigation satellites which have orbits and positions on their orbits adjusted so as to achieve at all times a mesh as regular as possible around the earth's surface, this to have, above any point on the globe and at any time, a visible constellation of at least four satellites of known positions from which it

  is possible to localize by triangulation.

  The location is based on a measurement of the propagation times of the signals emitted by the navigation satellites which involves the use of a reference clock when transmitting and knowing when to receive this reference clock. Reference clocks on board navigation satellites are very precise atomic clocks

  synchronized with each other to simplify the design of receivers.

  On the receiver side, the clock is a quartz clock much simpler than an atomic hordoge but also much less precise. This implies, on reception, a significant clock bias which must be taken into account for

  achieve propagation time measurements of acceptable accuracy.

  This taking into account the bias of the clock of the receiver is done by considering it as an unknown to be resolved in the triangulation in the same way as the three geographical coordinates of longitude, latitude and altitude. To solve this system with four unknowns, triangulation

  claims the minimum presence of four navigation satellites and not three.

  One of the best known satellite navigation systems is the GPS system of the Anglo-Saxon "Ground Positioning System" which implements a network of 24 navigation satellites arranged in six orbital planes with four navigation satellites per plan . Each navigation satellite emits a radio localization signal, the shape of which allows, when received by a user, to know, on the one hand, its transmission delay and therefore the apparent distance between the satellite and the place of

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  reception of its signal and, on the other hand, the Doppler effect which affects it and therefore the speed of the relative movement between the satellite and the receiver. These propagation time and Doppler effect measurements are based on a high-precision system clock on which all the navigation satellites are synchronized. In addition to its location properties due to its shape, the signal emitted by each navigation satellite of the GPS system contains information on the orbits and the positions and speeds on these orbits of all the satellites to allow a receiver to determine, by triangulation, its position and its velocity vector. Plus 1 o specifically, a navigation satellite of the GPS system transmits on two carrier frequencies, one called L1 at the frequency of 1575.42 MHz and the other called L2 at the frequency of 1227.6 MHz of the CDMA type signals. (Code Division Multiple Access in Anglo-Saxon language), which are of use

  conventional in radiocommunication by satellite.

  At the carrier frequency L1, a GPS satellite transmits a signal resulting from a quadrature phase modulation of this carrier by two pseudo-random binary codes for spreading the frequency spectrum and by navigation data. One of the codes known as the C / A code has a bit rate of 1.023 MHz. The other code called code P has a bit rate of 10.23 MHz. The data

  have a flow rate of 50 Hz.

  At the carrier frequency L2, a GPS satellite emits a signal resulting from a biphase modulation of this carrier by the pseudo-random binary code for spreading the frequency spectrum P at the rate of 10.23

  MHZ and by navigation data at a rate of 50 Hz.

  All the navigation satellites of the GPS system transmit on the frequencies L1 and L2 but with spread spectrum codes of frequency C / A and P specific for each navigation satellite which make it possible to identify them and which have been selected for their low cross-correlation rate. The measurement of the propagation times of these signals comes down to the measurement of the delays affecting reception in their spread spectrum codes with respect to their transmission instants by the navigation satellites while the Doppler effect measurement comes back to measuring the apparent frequency offsets of their carriers relative to their values

resignation.

  il 2811834 The use of the two carrier frequencies L1 and L2 allows an appreciation in reception of the effect of the ionosphere on the speed of

  propagation of satellite signals.

  The P code having a flow 10 times higher than the C / A code allows a delay measurement 10 times more precise than the C / A code. On the other hand, its access can be prohibited by encryption which consists of over-modulation by a non-public pseudo-random binary code Y at a speed of 500 KHz. It is

then called code P (Y).

  The availability of the C / A code is general and not exclusive. It corresponds to a first level of standard service, which is free to access but of low precision known as SPS (Standard Positioning Service in Anglo-Saxon language) as far as the regularity of the clock is also artificially affected to limit this precision. The availability of the code P (Y) with that of the way in which the regularity of the clock is affected corresponds to a second level of high precision service called PPS (Precision Positioning Service in Anglo-Saxon language) which requires an authorization of the part of the US Department of Defense DOD and which is not available for large-scale civilian use such as guidance

of civil aircraft.

  The lack of guidance precision achievable from the first standard service level of the GPS system makes it unsuitable for use as is, for guiding aircraft in a large number of flight phases including the approach to landing. However, as this lack of precision results in positioning errors which are largely common to all satellite navigation receivers on board mobiles operating in the same vicinity, it is possible to reduce it locally, in a more or less area restricted, by measuring the positioning errors made by a satellite navigation receiver placed on the ground, in said zone, in a place whose geographic coordinates are known with precision, and by communicating these error measurements to the on-board navigation receivers at edge of mobiles moving in said zone so that they take it into account. We then speak of a differential navigation system. This is the case of the navigation system represented in FIG. 1 which includes a

  fixed reference station 4 placed on the ground 5.

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  This fixed reference station 4 is placed on the ground, at a location whose geographic coordinates are known with precision. It contains: - a fixed satellite navigation receiver or at least its first stages making it possible to extract signals from the navigation satellites 1 from measurements of pseudo-distances and pseudo-speeds or relative distances and relative speeds between the location of the receiver, here the reference fixed station 4, and the various navigation satellites 1 visible from the receiver,

  - a computer center determining the real values of the pseudo-

  distances and pseudo-speeds that the fixed navigation satellite receiver should display, this evaluation being made on the basis of the knowledge of the geographical positions of the receiver and of the satellites at the instants of the measurements of the fixed navigation receiver, and developing the values of the differential errors or deviations separating the pseudo-distance and pseudo-speed measurements from their real values, and - a radio transmission center communicating the values of the differential errors to the satellite navigation receivers 2 on board mobiles operating in a more mobile neighborhood or less close to the fixed station of reference 4.

  To improve its coverage area and avoid the effects of masking by relief, the radio transmission center of the fixed reference station 4 passes through a relay placed on a remote platform with clear vision. The relay platform is here carried by a geostationary satellite 6 but it can also be carried by an aircraft, a tethered balloon or even simply by a pylon. The geostationary satellite 6 is either a simple telecommunications satellite without special equipment for aid to navigation, or a satellite specialized in aid for navigation as

an INMARSAT satellite.

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  As the use of the satellite navigation system in differential mode brings a substantial improvement to the accuracy of navigation, in the area covered by the reference fixed station 4, the problem arises again in this area. free access to a very precise guidance possibility for any user, whatever the nature of their intentions, a problem that the US State Department wanted to avoid with the GPS system by implementing an accreditation

  to access the high-precision PPS service level.

  It is proposed here to solve this problem by particular encryption of the uplink radio link from the reference ground station 4 in the direction of the satellite navigation receivers 2 on board mobiles 3, which is particularly well suited to the usual configuration. of these receptors and only requires, for its implementation

  work, only minor modifications.

  The proposed radio link uses a CDMA type radio signal, like most transmission radio signals used with satellites, resulting from phase modulation of a carrier, by a pseudo-random binary spread spectrum code. public type, such as a P code with a bit rate of 10.23 MHz, but encrypted by means of periodic hops at a rate of 500 Hz and

  non-public pseudo-random lengths.

  Figure 2 is a block diagram of a pseudo-random binary spread spectrum code generator. This pseudo-random binary code generator is made up of: - several shift registers 10a, 10Ob, 10Oc, 10Od, with several stages, serial and parallel inputs and outputs, delivering pseudo-random binary code components on their serial output, - combinations 11a, 11b, 11c, lld of logic gates often of the "or exclusive" type looping the serial and parallel outputs of the shift registers 10a, 10b, 10c, 10d to their serial input, - a sequential combinational logic circuit 12 assembling the different components of pseudo-random binary code

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  generated by the shift registers 1 Oa, 1 Ob, 1 Oc, 1 Od in a final pseudo-random binary code, - from a memory register 13 of initial state connected to the parallel inputs of the stages of the different shift registers 10a, 10b, 10c, 10d and memorizing the states taken by these shift registers 10a, 10b, 10c, 10d at the start of the period of the pseudo-random binary code, - a clock 14 for clocking the shift registers Oa, 10Ob, 10Oc , 10Od, and of the sequential combinational logic circuit 12, which delivers a HCHIRP clock signal and which, in a GPS navigation receiver, is slaved to the clock of the navigation system, and - a control circuit for initialization 15 used to control the parallel loading command of the shift registers 10a, 10b, 10c, 10d and the reset command of the sequential combinational logic circuit 12 in order to cause a loading of the initialization values in the shelves es shift registers 10Oa, 10Ob, 10Oc, 10Od and in the sequential combinational logic circuit 12 each time

  We want to replace ourselves with the pseudo- binary code

  random. In a satellite navigation receiver 2, the initialization control circuit 15 is synchronized with the instant of initialization of the pseudo-random binary code ensuring the spread of spectrum of the received signal

  being processed in order to locally obtain a pseudo- binary code

  random spreading in phase with that of the received signal, which allows despreading of this signal to access the data it carries and which informs, by its delay on the transmitted pseudo-random binary code, on the

  propagation time of the received signal.

  As is well known, the looping in more or less of the parallel and serial outputs of a shift register on its serial input via a combination of logic gates allows, when it receives a timing signal HCHIRP of a clock 13 to cause it to generate a periodic series of binary states which is entirely defined by the combination of logic gates ensuring the loopback and whose period can reach a very large value, up to NN, N being the number of stages of the register, giving it the characteristics of a practically random sequence. A single suitably looped shift register already makes it possible to generate a pseudorandom binary sequence. The use of several looped shift registers whose output signals are combined by a sequential combinational logic circuit taking successively different states corresponding to different configurations makes it possible to obtain a pseudo-random binary code of length greater than that one could obtain by means of a single shift register having a number of stages equivalent to the sum of the stages of all the shift registers used. For more details on the bit-random binary sequences, the way of generating them and their uses for the spreading of spectrum of the signals of navigation satellites, one can refer to the book of ED Kaplan entitled "Understanding GPS, principles and applications "appeared to

  Artech House Publishers 1996.

  Figure 3 is a block diagram of a code generator

  pseudo-random binary encrypted by periodic hopping of pseudo- lengths

random.

  Identically found in this FIG. 3, as in FIG. 2, the shift registers 10a, 10b, 10c, 10d with serial and parallel inputs and outputs with their parallel and serial outputs looped back to their serial input by the through a combination 1 1a, 11b, 1 1c, 1 1d of logic gates as well as the clock 14 delivering the HCHIRP signal for timing of the

  shift registers 10a, 10Ob, 10Oc, 10d.

  The sequential combinational logic circuit combining the pseudo-random code components available on the serial outputs of the shift registers 10a, 10b, O10c, 10d is modified in 12 'to allow it to be prepositioned, not only on its initial state corresponding to the start of the binary pseudo-random code but also on intermediate states. It now presents a prepositioning input and a prepositioning command like the shift registers 10a,

  Ob, 10Oc, 10Od instead of a simple initialization command.

  16 d2811834 The initial state memory register 13 is replaced by initial state memory registers 13 'in order to store a collection of initial states corresponding to the different states taken by the stages of the shift registers 10a, 10Ob, 10Oc , 10Od and by the combinational logic circuit at the start of a period of pseudo-random binary code and at various intermediate stages during a period of pseudo-random binary code which is taken as the destination of the encryption hops. These initial state memory registers 13 ′ comprise an initial state selection command controlled by a pseudo-random number generator 16 lo operating at a rate sub-multiple of that of the shift registers.

  1 Oa, 1 Ob, 1 Oc, 1 Od taken from the clock HCHIRp signal 14.

  The initialization control circuit remains in a modified form 15 ′ and not only provides the loading command no longer acts on the loading command in parallel of the stages of the shift register 10Oa, 10Ob, 10Oc, 10Od which also serves as repositioning command of the sequential combinational logic circuit 12 ′ but also an initialization command of the binary number generator

pseudo-random 16.

  When such an encrypted pseudo-random binary code generator is used in a GPS satellite navigation receiver to locally generate the encrypted pseudo-random binary code ensuring the spread of spectrum of a signal received during processing, I ' clock 14 ensuring the timing of the shift registers 10Oa, 10Ob, 10Oc, 10Od, of the sequential combinational logic circuit 12 'and of the pseudo-random binary number generator 16 is slaved to that of the navigation system while initialization circuit 15' is synchronized with the initialization instant of the encrypted pseudo-random binary code ensuring the spread of the spectrum of the signal received during processing in order to allow the despreading of this

  signal to access the data it carries.

  One way to synchronize the encrypted spread code initialization circuits of a transmitter and a receiver is to transmit a synchronization signal. Another is to agree in advance on the precise start time of the spreading code by taking as a reference a system clock known to both the transmitter and the receiver. The latter method is preferred in the present case of a

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  satellite navigation that operates from a system clock

  recovered both in transmission and in reception.

  Of course, the generator of the encrypted spreading code that comes

  to be described in relation to FIG. 3 can undergo various simplifications.

  The encryption jumps can be obtained by playing only on a repositioning of the sequential combinational logic circuit and by not intervening on the shift registers providing the components of pseudo-random binary code. Conversely, they can also be obtained by playing only on the repositioning of the shift registers providing the components of pseudo-random binary code without intervening on the sequential combinational logic circuit. They can even be obtained by playing only on a partial repositioning of the

  shift registers providing the binary-pseudo- code components

  random. The spreading code to be encrypted can also be generated only by means of a single shift register looped back on itself. In this case, the

  12 'sequential combinational logic circuit disappears.

  In all these variants, the capacity of the memory registers 13 '

  is adapted to the data to be stored.

  As shown in Figure 1, the uplink of the reference ground station (4 Figure 1) towards the navigation receivers (2 Figure 1) is relayed by a so-called telecommunications satellite (6 Figure 1) in order to avoid masking problems due to the relief and of being distributed over a large area of the earth's surface surrounding the reference ground station (4 figure 1). This relay involves a change in carrier frequency at the satellite level to avoid coupling problems between the transmitting and receiving antennas of the satellite, which can be used to pass in L band in the sensitivity range of the stages of input of GPS navigation receivers and allow reception and demodulation of the uplink signal by the antenna and the usual input stages of GPS navigation receivers but this requires the presence, on board the communication satellite, of specific equipment which must have been

  planned from the design of the satellite.

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  When the geostationary telecommunication satellite has not been provided with specific equipment dedicated to satellite navigation, the uplink may nevertheless pass through one of its repeaters. It is then carried out in band C, which is the frequency band usually allocated to the repeaters of telecommunication satellites, with spreading of frequency band compatible with the bandwidth of the repeater used. As the C band is not usually received by GPS navigation receivers, it is necessary to equip the navigation receivers (2 figure 1) with an omnidirectional lo multiband antenna (C band and L band) and a stage d input adaptation for adapting the uplink signal received in C-band to the processing capacities of their demodulation stage and their following stages. To allow the GPS navigation receivers equipping the mobiles to process the signal of the uplink of the reference ground station at a lower cost, the signal of this uplink is given the characteristics of a GPS navigation satellite signal at the carrier frequency L2, except that it is spread in the frequency band by a P code encrypted by periodic hopping of random amplitudes and that it reaches the receivers in the C band. Thus, after capture by a C band antenna and transposition at the frequency L2, the uplink signal can be applied to the usual input stages of GPS receivers which treat it as a navigation satellite signal and identify it by its P code encrypted by periodic jumps of random amplitudes, when they are authorized to use such a code, i.e. when they are aware of this code which

  can be provided to them by the authority managing the reference ground station.

  The choice of the P code used by the uplink can be done among the P codes used by GPS systems, its encryption not disturbing its weak intercorrelation with the other P codes. Figure 4 gives a possible example of an assembly of adaptation to be placed at the input of a GPS navigation receiver to allow it to also process the uplink signal and operate in differential GPS. We can see in this figure 4, a patch antenna, omnidirectional reception, with radiating elements 20 tuned

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  on the carrier frequency L1, others 21 on the carrier frequency L2 and others finally 22 on the carrier frequency in band C of the uplink at its output from the repeater of the telecommunications satellite (6 FIG. 1). The radiating elements 20 of the reception antenna tuned to the carrier frequency L1 used by the GPS navigation satellites (2 FIG. 1) are connected to the input of a preamplifier 40 preceded and followed by band filters 30, 50 limiting the noise. The radiating elements 21 of the reception antenna tuned to the carrier frequency L2 also used by the GPS navigation satellites (2 FIG. 1) are connected to the input of a preamplifier 41 preceded and followed by band filters 31, 51 limiting noise. The radiating elements 22 of the reception antenna tuned to the carrier frequency in band C used by the uplink at its output from the repeater of the communication satellite 6 are connected to the input of a frequency preamplifier-changer 42 bringing the uplink signal at carrier frequency L2, this frequency changer preamplifier 42 being preceded and followed by filters

band 32, 52 limiting noise.

  The outputs of the three preamplification channels are applied to the actual input of the GPS receiver 60, one, that processing signals received at carrier frequency L1 directly, the other two processing signals received at frequency L2 and the signal of the uplink received in band C and transposed to the frequency L2, by means of a switch 70 controlled by the GPS receiver 60 which is provided with additional software enabling it to use in addition signals from the navigation satellites 1 at carrier frequencies L1 and L2, the signal from the

uplink.

  The use of a P code for spreading the uplink frequency spectrum allows spreading of the uplink over a bandwidth of the order of 20 MHz since the P code has a bit rate of 10, 23 MHz. The encryption of the P code by periodic jumps of random amplitudes is advantageously carried out at a jump rate of the order of 500 KHz intermediate between the bit rate of the P code and that of the data transmitted by the

  ground link which can be of the order of 500 Bauds.

  Of course, it is also possible to spread the uplink signal with an encrypted C / A code, but the spread obtained is less since the speed of the C / A code is ten times lower than that of a P code. Indeed, we pass from a spread over a band with a width of 20 MHz to a spread over a band with a width of 2 MHz which is less

  robust against radio interference lines.

  It is also possible to use a telecommunication satellite provided with specific equipment dedicated to satellite navigation and more specifically a C-band L-band repeater bringing the uplink signal to the carrier frequency L1 or L2, this which then avoids having to use an adaptation assembly at the input of the satellite navigation receivers equipping the mobiles operating in the vicinity of the reference ground station. One can even provide, as in the case of a WAAS, EGNOS or MSAS system, to slave the phases of the encryption spreading code and of the carrier used at the output of the repeater of the telecommunications satellite for the uplink, so to give this signal the appearance of a navigation satellite signal in the eyes of the navigation receivers fitted to the mobiles and thus use the

  communication as an additional navigation satellite.

21 d2811834

Claims (16)

  1. Differential satellite navigation system comprising: - a constellation of navigation satellites (1) generating location signals (L1, L2) occupying frequency spread bands, - at least one reference ground station (4) measuring differential errors committed by a satellite navigation receiver occupying its location and using the location signals (L1, L2) of visible satellites (1) of the constellation, - at least one differential satellite navigation receiver (2), on board on board a mobile (3) operating in the area of the reference ground station (4), using the location signals of the visible satellites (1) of the constellation and information on the differential errors measured at the level of the reference ground station (4) transmitted by spread band encrypted radio link, and - said spread band encrypted radio link rising from said reference ground station (4) to said differential receivers (2) on board mobiles (3), whose spreading in frequency band results from modulation   by means of a pseudo-random binary code.   said differential navigation system being characterized in that it comprises means for encrypting the radio link up from said reference ground station (4) to said differential navigation receivers (2) on board mobiles (3) operating by means of pseudo-random jumps in the course of the pseudo-random binary code   band spreading of the link.   2. The system as claimed in claim 1, in which the frequency band spreading code used for the uplink radio link from the reference ground station to the differential navigation receivers is generated locally at transmission as at reception at by means of a shift register (10) with several stages with serial and parallel inputs and outputs, looped back on itself by a network (11) of logic gates, characterized in that the encryption results from a periodic repositioning of the stages from the shift register to values imposed by   a pseudo-random number generator (16).   3. The system as claimed in claim 1, in which the frequency band spreading code used for the uplink radio link from the reference ground station to the differential navigation receivers is generated locally at transmission as at reception at lo by means of several shift registers (10a, 10b, 10c, 10d) with several stages with serial and parallel inputs and outputs, looped back on themselves by door networks (1 a, 1 b, 1 c, 1 d) logic and by means of a combinational logic circuit (12 ') combining the output signals of the shift registers (10a, 10b, 10c, 10d), characterized in that the encryption results from a periodic reorganization of the combinational logic circuit ( 12 ') according to   configurations imposed by a pseudo- number generator random (16).   4. The system as claimed in claim 1, in which the frequency band spreading code used for the uplink radio link from the reference ground station to the differential navigation receivers is generated locally at transmission as at reception at by means of several shift registers (10a, 10b, 10c, 10d) with several stages with serial and parallel inputs and outputs, looped back on themselves by networks (11a, 1b, 1 lc, 1 ld) of logic gates and by means of a combinational logic circuit (12), characterized in that the encryption results from a periodic repositioning of the stages of the shift registers (10a, 1Ob,   Oc, 10d) to values imposed by a pseudo- number generator random (16).   5. The system as claimed in claim 1, in which the frequency band spreading code used for the uplink radio link from the reference ground station to the differential navigation receivers is generated locally at transmission as at reception at using several shift registers (10a, 10b, 10c, 10d) with several 23 2811834   stages with serial and parallel inputs and outputs, looped back on themselves by networks (l11a, 1b, 1c, 1 ld) of logic gates and by means of a combinational logic circuit (12), characterized in that encryption results on the one hand from a periodic repositioning of the stages of the shift registers (10a, 10b, 10c, 10d) at values imposed by a pseudo-random number generator (16) and on the other hand, from a periodic reorganization of the combinational logic circuit (12 ') according to configurations imposed by said   pseudo-random number generator (16).   6. System according to claim 1, characterized in that the encrypted spread-band radio link ascending from said reference ground station (4) to said differential receivers (2) on board mobiles (3) occupies a frequency band different from those used by navigation satellites (1), differential on-board navigation receivers (2) comprising as input an omnidirectional antenna (20, 21, 22) suitable for receiving the signal from this uplink radio link and a stage frequency transposition (42) bringing the signal of this uplink radio link in a frequency band occupied by the location signals of the navigation satellites (1) so that it can be processed, within the differential receivers (2) of on-board navigation, by the stages already processing the location signals of navigation satellites (1).   7. The system as claimed in claim 6, characterized in that the encrypted spread band radio link ascending from said reference ground station (4) to said differential receivers (2) on board mobiles (3) is located in the band C of the radio frequency spectrum. 8. System according to claim 1, characterized in that the encrypted spread-band radio link ascending from said reference ground station (4) to said differential receivers (2) on board mobiles (3) is relayed by a relay placed on board a platform (6)   remote from the reference ground station.   9. The system as claimed in claim 8, characterized in that the encrypted spread-band radio link ascending from said reference ground station (4) to said differential receivers (2) on board mobiles (3) is relayed by a relay placed at a remote platform from the reference ground station (6), consisting of a satellite (6). 10. The system as claimed in claim 8, characterized in that the encrypted spread-band radio link ascending from said reference ground station (4) to said differential receivers (2) on board mobile lo (3) is relayed by a relay placed at a remote platform from the   reference ground station (6), consisting of an aircraft.   11. System according to claim 8, characterized in that the encrypted spread band radio link rising from said reference ground station 1 5 (4) to said differential receivers (2) on board mobiles (3) is relayed by a relay placed at a remote platform from the   reference ground station (6), consisting of a balloon.   12. System according to claim 8, characterized in that the encrypted spread-band radio link ascending from said reference ground station (4) to said differential receivers (2) on board mobiles (3) is relayed by a relay placed at a remote platform from the   reference ground station (6), consisting of a tethered balloon.   13. The system as claimed in claim 8, characterized in that the encrypted spread band radio link ascending from said reference ground station (4) to said differential receivers (2) on board mobiles (3) is relayed by a relay placed at a remote platform from the   reference ground station (6) and supported by a pylon.   14. The system as claimed in claim 9, characterized in that the encrypted spread band radio link ascending from said reference ground station (4) to said differential receivers (2) on board mobiles (3) is relayed by a repeater placed on board a satellite geostationary (6).   15. System according to claim 7, characterized in that the encrypted spread band radio link ascending from said reference ground station (4) to said differential receivers (2) on board mobiles (3) is relayed by a repeater C-band L-band placed on board a satellite (6). 16. The system as claimed in claim 7, characterized in that the encrypted spread band radio link rising from said reference or ground station (4) to said differential receivers (2) on board mobiles (3) is relayed by a C-band L band repeater placed on board a geostationary satellite (6).