HK1051573B

HK1051573B - Radio frequency signal receiver with means for correcting the radiofrequency signal receiver with means for correcting the effects of multipath signals, and method for activating the receiver

Info

Publication number: HK1051573B
Application number: HK03103811.8A
Authority: HK
Inventors: Oesch Yves; Montheard Anne; Farine Pierre-Andre
Original assignee: Asulab S. A.
Priority date: 2001-06-08
Filing date: 2003-05-28
Publication date: 2008-10-31

Description

Radio frequency signal receiver with means for correcting the effects of multipath signals and method for starting said receiver

Technical Field

The present invention relates to a radio frequency signal receiver, in particular of the GPS type, with means for correcting the effects of multipath signals. The invention also relates to a method for starting or setting the operation of a receiver.

Background

A receiver of a radio frequency signal modulated by a transmission source specific code comprises receiving and shaping means. These devices allow frequency translation of the radio frequency signal to provide an intermediate signal.

The receiver also comprises a correlation stage consisting of several correlation channels for receiving the intermediate signal. Each channel has a correlator in which the intermediate signals are correlated. The correlation with at least two replicas of the visible transmission source specific code to be searched and tracked, which replicas are always in phase, is achieved by at least one control loop of the correlator when the channel is being used. The correlator comprises means for integrating the correlated signal, providing at the end of each integration period a first magnitude of the autocorrelation function of the early signal and a second magnitude of the autocorrelation function of the late signal. In the transmission source tracking mode, the first amplitude value and the second amplitude value are substantially the same.

The receiver also comprises microprocessor means connected to the correlation stage for processing data extracted from the radio frequency signal after correlation.

If the receiver is a GPS receiver, the data extracted from the radio frequency signals, in particular GPS messages, pseudoranges and doppler frequencies, are used to calculate position, velocity and time (hours).

The radio frequency signal receiver of the invention can of course also be used in satellite navigation systems of the GLONASS or GALILEO type. Likewise, the receiver may be used in a mobile telephone network, such as a CDMA (code division multiple access) type network. In this case, the transmission source is no longer a satellite but a basic cell of the telephone network, and the processed data relates to an audible or recognizable message, or a navigation message.

Currently, 24 satellites are placed in orbits in 6 orbital planes approximately 20,200 kilometers above the earth's surface, each orbital plane being 55 ° off-set with respect to the equator. The time it takes for a complete satellite revolution before returning to the same position on an orbit above the earth is approximately 12 hours. The distribution of satellites in orbit enables GPS receivers on earth to receive GPS signals from at least four satellites in view for determining position, velocity and local time.

In civilian use, each satellite in orbit transmits a radio frequency signal consisting of a 1.57542GHz carrier frequency with a 1.023MHz pseudo-random PRN code unique to each satellite and a 50Hz GPS message modulated. The GPS messages contain ephemeris and almanac data for the sending satellites, which are particularly useful for computing X, Y, Z position, velocity and time-related data.

The PRN (pseudo random noise) codes, in particular gold code type codes, are different from each other for each satellite. The gold code is a digital signal consisting of 1023 chips that are repeated every millisecond. The repetition period is also defined by the term "time period (epoch)" in the gold code. It is noted that a chip takes a value of 1 or 0 for one bit. However, a chip (a term applied in the GPS technology) is to be distinguished from one bit used to define a data unit.

The gold code defining the 32 satellite identification numbers has the characteristic of being orthogonal. The correlation results in a value close to 0 by their correlation with each other. This feature thus allows several radio frequency signals generated simultaneously from several satellites and transmitted at the same frequency to be processed independently in multiple channels in the same GPS receiver.

Currently, in some everyday activities, GPS receivers, portable or particularly integrated in vehicles, are used to make navigation data available to the user. These data are particularly useful for locating, finding objects and knowing orientation. In addition, portable GPS receivers tend to be of smaller size so that they can be integrated into objects such as in a cellular phone or a watch, which can be easily transported by an individual. However, since they are powered by small-sized batteries or accumulators, it is generally desirable to minimize the energy consumption of the receiver.

It is worth noting that the GPS receiver needs to receive radio frequency signals transmitted by at least four visible satellites in order to determine its position and time-related data in detail. The receiver is also able to receive almanac data and almanac data specific to each satellite by tracking one of the visible satellites individually with a radar.

Fig. 1 shows schematically a GPS receiver 1 with an antenna 2 for receiving radio frequency signals. In order to be able to determine its position, velocity and time-related data, the above-mentioned GPS receiver 1 must receive signals SV1 to SV4 from at least four visible satellites S1 to S4. However, when the above-described receiver 1 is used in a place surrounded by many obstacles, such as in a building B of a city, some of the radio frequency signals SV1 'and SV 3' received by the receiver 1 are sometimes reflected by these obstacles. These signals SV1 'and SV 3', which are reflected and combined with the direct signals SV1 and SV3 from the same transmission source, cause a bias in the data extracted from the signal set received by the receiver. These deviations have an effect, inter alia, on calculating the position of the receiver.

For a terrestrial navigation receiver, the phase offset due to multipath signals may be greater than or equal to 150ns, corresponding to a calculated position of 45 m. Typically, the nominal deviation is within the 30ns limit, which corresponds to a deviation of approximately 9m of the calculated position. Even if the phenomenon of such multipath signals is clearly solved, these deviations are often difficult to completely eliminate. Some embodiments have been proposed to minimize the effects of these multipath signals.

In particular, the Novatel (Novatel) patent application No. WO95/14937 discloses a pseudo-random noise encoded radio frequency signal receiver with means for compensating for distortion due to multipath signals. To do this, the receiver contains several associated channels, each aimed at simultaneously capturing a particular satellite. The autocorrelation apparatus for each channel comprises several correlators, each correlator receiving the phase of an internally generated replica of the pseudo-random code, which is offset with respect to the other replicas associated with the intermediate signal. An output signal power level estimator for each correlator in the channel is provided to remove the effects of multipath signals. The phase deviation between each replica is less than 0.2 chips, which requires a very high fixed frequency for each replica.

The main disadvantage of such a receiver is that each channel has many correlators to acquire and track the phase of a particular visible satellite. The large number of components required in forming the channel correlation stage therefore results in a high power consumption, which does not allow the receiver to be integrated on a portable object containing a low capacity energy source.

In the same context, U.S. patent No.5,966,403 to teleflex Limited discloses a spread spectrum radio receiver that also includes means for minimizing the effects of multipath signals. This document provides two alternative embodiments. In a first variant, a uniform or non-uniform signal weighting function is used to correlate the early and late replicas with the intermediate signal. The microprocessor means receives some of the correlated and weighted signals and closes the carrier and code control loop. These microprocessor devices undertake the task of estimating signal distortions due to multipath signals and minimizing these distortions.

In a second variant, the two correlated channels of the receiver are used in parallel to track the same satellite, which transmits a signal that deviates from its orbit. The second channel is used to minimize distortion due to multipath signals by the microprocessor device. In order for the microprocessor to be able to evaluate the distortion due to the multipath signals, a phase delay is imposed to produce early and late replicas of each channel.

In view of the foregoing, one disadvantage of these embodiments is essentially the complexity of the respective channel structure used to minimize the effects of multipath signals. In addition, a large-scale microprocessor is used for all synchronized tasks. The complexity also leads to high power consumption, which prevents integration of such receivers on small-sized portable objects with low-capacity energy sources.

Disclosure of Invention

It is an object of the present invention to provide a radio frequency signal receiver capable of correcting the effects of multipath signals while limiting the number of components necessary for the receiver and reducing its power consumption so as to overcome the disadvantages of the prior art receivers. So that the receiver can be adapted to portable objects of small size.

It is another object of the present invention to use those unused channels in the receiver that have the same structure as the channels that allow the effects of multipath signals to be corrected.

This and other objects are achieved by a receiver as described herein, which is characterized in that at least one second unused channel is provided by microprocessor means, which is placed in parallel with at least one first operating channel for searching and/or tracking the same visual transmission source, and that, when the microprocessor detects the presence of multipath signals in the first operating channel, the microprocessor means controls the second channel to produce a replica of a particular code associated with the intermediate signal, so that the integrating means of the second channel provide the maximum magnitude of the autocorrelation function between a first and a second magnitude of the autocorrelation function of the first channel.

An advantage of the receiver of the present invention is that the problem of calculation errors due to the effect of multiple radio frequency signals, each channel containing the same number of components, is avoided by using the same channel and co-operating with microprocessor means. When the receiver is operating, not all relevant channels are used, because the number of visible transmission sources, in particular visible satellites, is smaller than the number of relevant channels in the receiver. This means that a certain number of channels remain unused. As a result, these channels, which are defined as unused, can be connected in parallel with the operating channel to allow the microprocessor means to correct for deviations due to multipath signals. It is noted that the microprocessor means requires at least four associated channels to be used in a particular visible satellite tracking mode so that it can calculate position, velocity and time related data. Thus, when a multipath signal is detected in one of the operating channels, only one unused channel may be provided in parallel with the operating channel.

The above-mentioned receiver must be capable of being integrated in an object that contains a low amount of energy and that can be easily carried, which forces a reduction in the number of components in each relevant channel. Furthermore, the management of all synchronization tasks must be able to be implemented in a simplified manner, in particular in each of the associated channels independently of the microprocessor means. The microprocessor means assists the unused channels to search for the maximum amplitude of the autocorrelation function, typically only after the presence of a multipath signal has been detected in the operating channel. This results in less data transfer between the microprocessor means and the operating channel, which reduces the power consumption of the receiver.

Another advantage of the receiver of the invention is that the parameters normally used by the first channel are transmitted by the microprocessor means to a second channel connected in parallel to the first channel. The second channel works with the microprocessor means to find the maximum amplitude of the autocorrelation function more quickly. Even if the second channel is connected for stability reasons, the first channel remains in use because of the possibility that the multipath signal will disappear. In this case the second channel is deactivated and the first channel provides the microprocessor means with data, in particular operations for calculating position, velocity and time-related data of the GPS receiver.

The phase offset between the maximum magnitude of the autocorrelation function provided by the second channel and the magnitude of the first autocorrelation function of the incipient signal of the first channel may be stored. The offset in phase may be introduced as an additional parameter to the second unused channel, which will be connected in parallel with the first operating channel in the future.

Typically, the microprocessor means contains memory means in which data about the satellite positions, the satellite specific codes and the satellites are visible to a receiver on land when the receiver is activated. The receiver can thus determine which satellites are visible when setting the associated channel selected to operate.

This and other objects are also achieved in accordance with the method for correcting for receiver multipath signal effects, the method comprising a first set of steps comprising:

-configuring and switching a number of first channels so that each channel searches and tracks a specific transmission source;

-phase shifting the early and late replicas of each first operating channel specific code associated with the intermediate signal until the first and second magnitudes of the autocorrelation function are equal;

-storing the relative amplitudes of the early signal and the late signal and the corresponding phase shifts during a search and/or tracking phase, said method being characterized in that it further comprises a second set of steps:

-for each first operating channel, using the autocorrelation function magnitude and the stored corresponding phase offset during the search and/or tracking phase, to calculate a first slope of the autocorrelation function at a first magnitude point of the early signal and a second slope of the autocorrelation function at a second magnitude point of the late signal when the channel is in the transmission source tracking mode;

-configuring and switching at least one second unused channel placed in parallel with the first operating channel if the two calculated slopes differ significantly in absolute value or if a change is found in a first amplitude of the early signal or in a second amplitude of the late signal in the tracking mode;

-phase shifting one of the replicas of the code of the second channel under instruction of the microprocessor means until the integrating means of the second channel provides a maximum magnitude of the autocorrelation function between the first and second magnitudes of the autocorrelation function of the first channel, so that the microprocessor means can extract data from the radio frequency signal of the second channel while correcting for the effects of multipath signals.

Drawings

These objects, advantages and features of the radio frequency signal receiver and the method of starting up the receiver described above are apparent in the following description of embodiments with reference to the accompanying drawings, in which:

as mentioned above, fig. 1 presents a radio-frequency signal receiver of the GPS type for acquiring signals from at least four satellites, wherein the signals from two satellites deviate from their orbit due to obstacles;

figure 2 schematically illustrates parts of a radio frequency signal receiver according to the invention;

figure 3 schematically illustrates the components of a correlator in one channel of the correlation stage of the receiver according to the invention;

fig. 4a and 4b illustrate, respectively, a curve of the autocorrelation function and the intermediate signal defining the autocorrelation function in correlation with a phase shifted replica;

fig. 5a illustrates a graph of an autocorrelation function, the amplitude of the signal associated with an early replica being equal to the amplitude of the signal associated with a late replica during the tracking phase;

figure 5b illustrates a graph of the autocorrelation function obtained by subtracting the early and late components;

fig. 6a and 6b illustrate plots of the autocorrelation function in the case of a multipath signal, in which the amplitude of the signal associated with the early replica is equal to the amplitude of the signal associated with the late replica during the tracking phase, but with a phase shift with respect to the signal without multipath;

fig. 6c illustrates a graph of the correlation function obtained by subtracting the early and late components in the case of a multipath signal;

fig. 7 illustrates a flow chart of the steps of a method for starting up a receiver, irrespective of whether a multipath signal is detected or not.

Detailed Description

In the following description, some components of a radio frequency signal receiver, in particular of a GPS type receiver, are mentioned only in a simplified manner, these components being well known to those skilled in the art. The receiver described herein is preferably a GPS receiver. However it may be used in the GLONASS or GALILEO navigation system or any other navigation system or in a mobile phone network.

As shown in fig. 1, the radio frequency signals SV1 to SV4 received by the antenna 2 of the GPS receiver are transmitted by four visible satellites S1 to S4. The signals SV1 to SV4 of these four satellites are necessary for the above-mentioned GPS receiver 1 to be able to extract all the information used to calculate its position, velocity and/or time-related data. However, on their paths, certain radio frequency signals SV1 'and SV 3' may be reflected by various obstacles such as building B. These deviating signals SV1 'and SV 3' may corrupt the detection of the direct signals SV1 and SV3 received by the receiver. Thus, the associated channels in searching and tracking the phases of the satellites S1 and S3 are subject to multipath signals, which results in bias in the position calculation. As will be described in the following description, at least one channel defined as unused is placed in parallel with each of the operating channels of the tracking satellites S1 and S3 to correct for the effects of multipath signals.

The GPS receiver is preferably adapted to a portable object, such as a watch, in order to provide position, velocity and local time data required by the person wearing the watch. Since the watch has a small size battery or cells, the power consumption during operation of the GPS receiver must be as small as possible.

Of course, the GPS receiver is also applicable to other small-sized portable objects such as portable telephones, which are equipped with small-sized storage batteries or batteries.

The GPS receiver 1 is schematically illustrated in figure 2. It comprises receiving and shaping means with frequency conversion of the radio-frequency signal 3 supplied by the antenna 2 for generating an intermediate signal IF, a correlation stage 7 consisting of 12 channels 7' for receiving the intermediate signal IF, a data transmission bus 10 connecting each channel to a respective buffer register 11, and finally a data bus 13 connecting each buffer register to microprocessor means 12.

The intermediate signal IF is preferably in complex form and is provided by the shaping means 3 at a frequency of about 400kHz from a part of the in-phase signal I and a part of the two-phase signal Q. The complex intermediate signal IF is characterized in fig. 2 by a thick line perpendicular to the diagonal line, which defines 2 bits.

The number of channels 7' available in the receiver 1 is more than the maximum number of satellites visible anywhere on the earth in order to maintain a certain number of unused channels. These unused channels are intended to be in parallel with the operating channel when the microprocessor detects the presence of multipath signals in the normally operating channel. The effects of multipath signals, and the connection of unused channels, will be described hereinafter with particular reference to figures 3 to 6.

In general, in the receiving device 3, the first electronic circuit 4' first converts a radio frequency signal having a frequency of 1.57542GHz to a frequency of, for example, 179 MHz. The second electronic circuit IF4 "then performs a double conversion, converting the GPS signal first to a frequency of 4.76MHz and then finally to a frequency of, for example, 400kHz by sampling at 4.36 MHz. The intermediate complex signal IF sampled and quantized at a frequency of approximately 400kHz is supplied to the channel 7' of the correlation stage 7.

For the frequency conversion operation, the clock signal generator 5 forms part of the radio frequency signal receiving and shaping device 3. Such a generator is provided with, for example, a quartz oscillator, not shown, scaled at a frequency of about 17.6 MHz. Two clock signals CLK and CLK16 are provided to the correlation stage 7 and the microprocessor means 12 in particular for clocking all the operations of these components. The first clock frequency CLK has a value of 4.36MHz and the second clock frequency is set at its 1/16, i.e. 272.5kHz, which is employed in most of the relevant stages in order to save energy consumption.

It is noted that it is conceivable to obtain the clock signal CLK16 by placing a divider in the relevant stage instead of integrating it with the clock signal generator 5 in the receiving means 3.

The signal generated by the second circuit 4 "in these examples gives a signal of different polarity (+1 or-1). This polarity must be considered for the demodulation operation of the GPS signal in the receiver. In an alternative embodiment the second circuit 4 "is able to generate a signal (+ 3; + 1; -1; -3) distributed over two output bits for the in-phase part as well as for the two-phase part.

In the example of the GPS receiver of the invention, an intermediate signal IF with a 1-bit quantised carrier frequency is supplied to the correlation stage, even though such quantisation produces a signal-to-noise ratio (SNR) penalty of about 3 dB.

The register 11 of each channel can receive configuration data or parameters from the microprocessor means. Each channel can transmit data through registers relating to GPS messages, the status of the PRN codes, frequency increments associated with doppler effects, pseudoranges, relative magnitude values with phase offsets and other data after correlation and tracking of a particular satellite.

The buffer register 11 is composed of several registers, such as a command register and a status register, NCO (numerically controlled oscillator) registers for each channel, a pseudo range register, an energy register, an offset register and an increment register for carrier and code, and a test register. It is noted that these registers can accumulate data for use during satellite acquisition and tracking during the relevant phase without having to be automatically transferred to the microprocessor.

In an alternative embodiment, a single block of registers 11 may be envisaged for all channels 7' of the relevant stage, given that some of the data placed in the register cells is common to each channel.

Each channel 7' of the correlation stage 7 comprises a correlator 8 and a controller 9 set to operate by dedicated means, in particular a signal processing algorithm for acquiring satellite signals and tracking the satellites detected by the channel.

The controller 9 of each channel comprises in particular a storage unit, an arithmetic unit, a data bit synchronization unit, a correlator control unit and an interrupt unit, which are not visible in the drawing. The memory unit is in particular constituted by a RAM memory for storing transient data. RAM memory is distributed in a regular or irregular structure. The arithmetic unit performs addition, subtraction, multiplication, division and shift operations.

Thus, in normal operation, all acquisition and tracking operations for the detected satellites are performed autonomously in each channel of the relevant stage. These tasks are performed in a bit-parallel structure, where the computation of multiple bits is performed in one clock pulse. The digital signal is 1kHz, which allows the carrier frequency signal and the control loop PRN code to be processed autonomously at a less significant frequency rate. When the channel has tracked one satellite, the circuit will synchronize the GPS data stream for use in subsequent calculations.

In this way, data transfer with the microprocessor means 12 no longer takes place during all relevant steps. Only the correlation results, in particular GPS messages at a frequency of 50Hz, of each channel 7' of the correlation stage are transmitted to the microprocessor. This results in a significant reduction in current consumption. However, when the microprocessor means must connect an unused channel in parallel with a normally operating channel to correct for multipath signal effects, the synchronization of the unused channel is performed by the microprocessor means. But by means of the microprocessor device some parameters of the working channel can be transferred quickly to configure the unused channel without any detriment to the power consumption of the receiver.

Accordingly, microprocessor 12 preferably comprises an 8-bit CoolRISC816 microprocessor, available from EM Microelectronic-Marin, Switzerland. This microprocessor is clocked by a 4.36MHz clock signal. The microprocessor means 12 also comprise memory means, not shown, in which information about the position of the above-mentioned satellites, their gold codes and those that can be received by a GPS receiver on the land are stored.

During all satellite search and tracking procedures, the operating channel 7' sends interrupt signals INT1 and INT2 to the microprocessor to tell the microprocessor the data it can extract. Upon receiving an interrupt signal, the microprocessor typically must go through all channels to find out from which channel the data can be extracted. For example, the data is related to configuration parameters, GPS messages, the status of the PRN code, frequency increments due to doppler effects, pseudoranges, modes for interrupting the receiving device, the status of the integration counter, and other information.

Since several interrupt signals INT1 to INT12 can be generated at the same time, the microprocessor means 12 also comprise a priority decoder for operating the channel 7'. The microprocessor can then directly access the priority channel on which the interrupt signal is sent according to the determined priority order.

In another embodiment, not shown, the priority decoder may also be integrated in the correlation stage.

A single semiconductor substrate may contain the entire associated stage with registers, priority decoders, microprocessors, and possibly a portion of a clock signal generator.

When the receiver 1 is set in operation, several channels 7' of the correlation stage 7 are configured by the microprocessor means 12. Each channel is also structured in such a way as to introduce therein different parameters related to the carrier frequency and the PRN code of the particular satellite to be searched and tracked. In the normal mode of operation, the configuration of each channel is different in order to search for and track their respective satellites. Since the operating channel can only track the visible satellites, some unused channels are left.

Fig. 3 shows a correlator 8 with a PRN code control loop in one part and a carrier control loop in another part. The correlators 8 are identical in each correlation channel 7' of the correlation stage 7, but their configuration is different in each channel. As described with reference to fig. 2, in normal operation, each channel performs all synchronization tasks to search or track a particular visible satellite independently of the microprocessor. This simplifies the manufacture of the receiver and thus allows to reduce its power consumption.

For more details on the various components of the correlator, the reader is referred to in chapter 5 of the book "understanding GPS principles and applications", and in particular fig. 5.8 and 5.13. The book was written by Phillip Ward, edited by Elliott D.Kaplan (Artech House Press, 1996) with publication number ISBN 0-89006-.

Referring to fig. 3, an intermediate signal IF represented by a thick line defining the intersection of oblique lines of 2 bits in the figure is a complex signal (I + iQ) composed of a 1-bit in-phase signal component and a 1-bit two-phase signal component Q. The intermediate signal IF has been sampled and quantized and first passed through the first carrier mixer 20. A mixer or multiplier 21 multiplies the signal IF by the cosine of the internally generated carrier replica minus I times its sine to extract the in-phase signal I from the complex signal, and a mixer or multiplier 22 multiplies the signal IF by the minus sine of the internally generated carrier replica minus I times the remaining sine to extract the two-phase signal Q from the complex signal.

These Sin and Cos signals are derived from block 45 of the Cos/Sin table of replica signals. The purpose of the first step in the first mixer 20 is to extract the carrier frequency from the signal carrying the GPS message.

After this operation, the equivalent PRN code of the signal from the satellite to be acquired must be found in the working or switching channel, in which the PRN code corresponding to the desired satellite is generated. To do this, the in-phase and two-phase signals are passed through a second mixer 23 to correlate the signals I and Q with the early and late replicas of the PRN code to obtain four correlated signals. In each channel of the correlation stage, only the early and late replicas are maintained without regard to the replica on time. This minimizes the number of related components. However, by removing the on-time component from the code control loop, a signal-to-noise ratio loss of about 2.5dB can be seen.

Mixer or multiplier 24 receives signal I and early replica signal E from 2-bit register 36 and provides a correlated early in-phase signal. A mixer or multiplier 25 receives the signal I and the late replica signal L from the register 36 and provides a correlated late in-phase signal. A mixer or multiplier 26 receives the two-phase signal Q and the early signal E and provides an associated early two-phase signal. Finally a mixer or multiplier 27 receives the signal Q and the late replica signal L and provides a late two phase signal. In an embodiment of the invention the drift or offset of the early replica signal E and the late replica signal L is half a chip, which means that the drift amount of the central real-time component P is 1/4 chips. For simplicity, the multiplier may be fabricated, for example, with exclusive or (XOR) logic gates.

The four correlation signals each enter an integration device formed by an integration counter 28, 29, 30, 31 as wavefront elements. At the end of each integration period, these integration counters supply a binary output word I represented on 10 bits_ES，I_LS，Q_ESAnd Q_LS. These binary words define the magnitude of the autocorrelation function as particularly illustrated in fig. 4 a. These amplitudes are typically normalized to the amplitude of the correlation signal. In the satellite search mode, some of the amplitude and phase offsets for the early correlation signal and the late correlation signal are stored in a memory device for use by the microprocessor device in calculating the slope at the autocorrelation function amplitude points of the early and late correlation signals. Based on the slope calculation, the microprocessor means can detect whether the operating channel is affected by the effect of multipath signals.

The integral counter is defined to be able to count to 1023, which is equal to the number of chips of the PRN code. Each integration counter 28, 29, 30, 31 of each microprocessor means selected channel is configured in the initial phase of the search to provide a complete set of binary words L every millisecond_ES，I_LS，Q_ESAnd Q_LS。

In the loop following these integrators, all operations take place in a bit-parallel structure, the signal frequency being 1kHz, and only the 8 most significant bits are used in the other part of the digital signal processing chain in order to eliminate part of the noise in the useful signal to be demodulated.

Binary output word L characterized in the figure by a thick line intersecting the oblique line defining 8 bits_ES，I_LS，Q_ESAnd Q_LSInto the code loop discriminator 32 and the code loop filter 33. The code loop discriminator executes the calculation signal I_ES，I_LS，Q_ESAnd Q_LSOperation of the energy. The accumulation of the values is performed in the discriminator over a certain number N of integration cycles, for example 16 cycles.

In the present invention, the discriminator is irrelevant and is of the delay locked loop type (DLL). In this discriminator, subtraction is performed between the square of the amplitude of the early signal and the square of the amplitude of the late signal. However, any type of discriminator may be used if the early signal amplitude and the late signal amplitude are substantially equal. The reader is referred to chapter v entitled "GPS receiver" in the book "global positioning system and applications", authored by a.j.van diendenck, 1996 of the american academy of aerospace.

In such a discriminator, the correction is generated from the carrier loop, since not only the doppler effect is felt at the carrier frequency, but also in the PRN code modulated at the carrier frequency, during the satellite transmission of the signal. Bringing the carrier into the code loop discriminator corresponds to dividing the carrier offset increment by 1540.

Depending on the result of the filtering by the discriminator, a phase increment is imposed by a 28-bit NCO oscillator on the PRN code generator 35 to send the PRN code bit sequence to the register 36 to generate a new correlation. The frequency resolution of such a 28-bit NCO is 16mHz (for a clock frequency of 4.36 mHz).

The controller processes the various results of the loop so that it can adjust the acquisition and tracking operations. Once there is a synchronization and tracking of the desired satellite, value I_ESAnd I_LSIt is introduced into a demodulation unit 50 which is able to supply a 50Hz per bit data message to the microprocessor means via a data input output register. In addition to this message the microprocessor means can obtain information about pseudoranges, especially those introduced in buffer registers, in order to calculate X, Y, Z position, velocity and accurate local time.

Any of the components introduced above will not be described in detail as they are part of the general knowledge of a person skilled in the art.

Using the signal I in an adder 37_ESAnd I_LSTo generate a signal I_PSUsing the signal Q in the adder 38_ESAnd Q_LSTo produce a signal Q_PSSignal I_PSAnd Q_PSAre characterized by 8 bits. These binary words are introduced into the carrier loop discriminator 42 (envelope detection) at a frequency of 1kHz to calculate the energy of the signal followed by the carrier loop filter 43. The discriminator is in particular constituted by an 8-bit multiplier and a 20-bit accumulator. It is of the frequency and phase locked loop type.

Averaging is performed in the frequency discriminator to increase the reliability and accuracy of the carrier tracking loop. The accumulation provided in the discriminator lasts N cycles, for example 16 cycles, which corresponds to 16 ms. The microprocessor device also imposes a signal S_TCTo the unused channels in the discriminator 42 for parallel connection with the selected channel.

Based on the result of the discriminator and after passing through the filter, the 24-bit NCO oscillator receive frequency increment (bin) of the carrier 44 is used to correct the carrier frequency replica. This 24-bit NCO has a frequency resolution of about 260 mHz.

During tracking, both methods of control or overriding of the code and carrier are synchronized, although the carrier tracking loop can only be updated after the presence of a satellite signal is determined.

It will be appreciated that during the transmission of radio frequency signals from a satellite, the doppler effect has an effect on both the carrier frequency and the PRN code of the signal, which means that the code and carrier control loops are connected to each other to achieve better accuracy in the receiver for the PRN code phase and carrier frequency corrections.

The phase of the PRN code replica is delayed in time, such as by one chip pitch, during each correlation epoch in the search phase in order to find the satellite phase offset. Once the satellite is found in the tracking phase, code adjustments occur at a spacing of 0.05 to 0.1 chips. In addition, the carrier frequency, which contains the doppler effect, must be corrected, which occurs in the carrier control loop. Besides the doppler effect, the lack of accuracy of the internal oscillator and the ionospheric effects must also be taken into account. These deviations, corrected in the code and carrier loops, correspond to a frequency offset of 7.5 kHz.

When no multipath signal is detected, the synchronization task is performed synchronously in each operating channel. If the microprocessor means detects the effect of a multipath signal in the first operating channel, a second unused channel is connected in parallel with the first channel in order to find the maximum amplitude of the autocorrelation function. In this case the code and carrier control loops of the above channels can no longer be used, since the use of a second unused channel in tracking mode no longer requires the acquisition of equivalent amplitudes of the autocorrelation functions of the early and late signals.

The unused channel must be looped through microprocessor means for phase shifting operations of one of the early or late replicas in order to find the maximum magnitude of the autocorrelation function between the two magnitudes of the first channel. To do this, two interruption units 45 and 47 are placed in each control loop for all channels. When the microprocessor means detects the presence of a multi-path signal in the first channel, the second unused channel receives a Sc command from the device to interrupt both control loops.

Fig. 4a and 4b illustrate, on the one hand, the autocorrelation function and, on the other hand, the specific code intermediate signals associated with the replica of the code.

The autocorrelation function in fig. 4a illustrates the correlation results of two identical rectangular pulse signals, but with a phase shift from each other. The associated logic signal is on the one hand the intermediate signal of the pseudo-random code and on the other hand a replica of the pseudo-random code generated in the receiver channel. The general formula for the autocorrelation function is as follows:

R(t)＝∫f(τ)f(t+τ)dτ

wherein when the absolute value of τ is less than or equal to T_CAt/2, f (τ) is equal to the amplitude A of the rectangular signal to be correlated, otherwise it is 0. Since the pseudo-random code frequency is 1023MHz, T_CRepresenting one chip corresponding to 977.5 ns. Since the signal is a rectangular signal, this function r (t) can be defined only by the following formula:

R(t)＝A²T_c(1-|t|/T_c) |t|≤T_c

r (t) ═ 0 other

When there is no interference from multipath signal effects, the shape of the autocorrelation function is triangular, and the absolute value of the slope on each side of the apex or peak of the function is equal. The amplitude a of the signal has a value of 1, which means that the amplitude defined as normalized is 1 at the vertex of the above function when the signals to be correlated are strictly in phase.

In fig. 4b, the replica (0) with a phase offset t of 0 is strictly in phase with the extracted signal (intermediate signal), which gives a maximum amplitude equal to 1. With respect to the phase offset t of the extracted signal, which is 1/2 chip replica (1), the amplitude is given equal to 1/2. Finally, with respect to the phase offset t of the extracted signal, which is 1 chip replica (2), the amplitude is given equal to 0.

The GPS receiver of the present invention generates two replicas of the early and late pseudorandom signals, each of which is associated with an intermediate signal. The phase offset between the two replicas is 1/2 chips. Fig. 5a illustrates an autocorrelation function in which the amplitudes of the early and late signals of correlation are expressed in a visual satellite tracking mode. The initial magnitude adjusted in tracking mode is-1/4 chips offset from the vertex of the autocorrelation function and the late magnitude adjusted in tracking mode is 1/4 chips offset from the vertex of the autocorrelation function.

In normal operation, the amplitudes of the early and late signals of the operating channel are equal in the visual satellite tracking mode. These amplitudes never occur at the maximum of the autocorrelation function. The discriminator performs subtraction of the early signal amplitude and the late signal amplitude at each integration period. By subtraction, the discriminator can provide a more accurate code correction increment.

When the amplitudes are equal, the result of the subtraction in the discriminator results in a value of 0, which is necessary to enter the tracking mode. The correlation function of these amplitude deviations in the discriminator is illustrated in fig. 5 b.

It is noted that the autocorrelation function shown in fig. 5a is not an ideal triangle because 2MHz band pass filtering is performed in the receiving and shaping means, which removes some lobes from the power spectrum. So that the autocorrelation function is a rounded portion at the top rather than a sharp peak.

When a radio frequency signal deviates in its path due to an obstacle, the signal received by the receiver is a multipath signal. These signals are added to the radio frequency signals generated directly from the transmitting satellites being tracked. These effects interfere with the calculation of the receiver position if one channel is in the tracking mode of the satellite.

Figure 6a illustrates the autocorrelation function of a direct signal and a multipath signal received by a receiver. As can be generally seen, the autocorrelation function of the multipath signal is shifted to the right of the autocorrelation function of the direct signal. In addition, the maximum amplitude of the autocorrelation function of the multipath signal is less than the maximum amplitude of the autocorrelation function of the direct signal.

In fig. 6a, the multi-way autocorrelation function is constructive, i.e. the maximum amplitude is positive. It is also possible to produce autocorrelation functions for multipath signals that have a negative maximum amplitude. In this case, the signal becomes a destructive multipath signal.

Figure 6b illustrates the resulting autocorrelation function obtained at the output of the receiver integrator when multipath signals are present. This resulting function is a superposition of the two autocorrelation functions shown in fig. 6 a.

The channel operating the satellite used to search for and track the signal off-orbit is set to have the same magnitude of the autocorrelation function for the early correlation signal E1 and the late correlation signal L1, but phase shifted by a relative to the top of the function. In the case of constructive multipath signals, the two equal amplitudes in the tracking mode are greater than the amplitude of the operating channel that is not affected by the multipath signal. Since the apex P1 of the autocorrelation function is generally at a constant phase offset between the two early and late replicas, the multipath signal produces a phase offset represented by the offset Δ.

In fig. 6b, this offset Δ is approximately 1/8 chips, which corresponds to a deviation of 35m of the position calculated by the microprocessor device.

The autocorrelation function of the early and late signal amplitude subtraction E1-L1 obtained in a discriminator with multipath signal effects is illustrated in fig. 6 c.

To understand how the receiver corrects for the effects of multipath signals, reference is made to fig. 7, which illustrates a flow chart of the steps of a method of starting up the receiver or setting the operation of the latter. It is noted that the microprocessor means should normally be provided with at least four channels for tracking the satellites in view in order to calculate the position of the receiver. However, for simplicity, only one selected channel is described in the steps of the method with reference to fig. 7.

In step 100, a first channel is selected by the microprocessor means for searching and tracking the visible satellites. The first channel searches for visible satellites while correcting the carrier and code replica associated with the intermediate signal in the carrier and code control loop.

In the search phase, amplitude verification of the early and late signal autocorrelation functions is performed at the output of the integration means in step 101. If the magnitudes are not equal, a phase offset of the pseudo-random PRN code is generated in step 102. Ideally, the code replica is offset by one chip during the search phase.

During all the search phases of the satellite, the amplitude values, and the corresponding phase offsets, are stored at the output of the integrating means.

Once the amplitudes E1 and L1 of the first channel are equal, slope calculations at points E1 and L1 are performed by the microprocessor in step 103. The calculated slopes are compared in step 104. If the slope P is_E1And P_L1Substantially equal in absolute value, the first channel is unaffected by the multipath signal. This channel, adjusted in step 105, provides accurate data to the microprocessor device for use in, among other things, calculating X, Y, Z the position.

Multipath signals may occur when the receiver is moving, even if the microprocessor means does not detect the presence of multipath signals in the first channel. In order to ensure that the first channel is not affected by the multipath signals in the tracking mode, detection of the amplitude of the autocorrelation function of the early and late signals is also performed.

As long as no change in the amplitude E1 is seen in step 106, the first channel still provides an accurate signal to the microprocessor means without the effects of multipath signals. If, on the other hand, a change occurs in the amplitude E1, the microprocessor means configures and switches in step 107 a second unused channel, which is set to operate in parallel with the first channel. The second channel is also switched if the absolute values of the slopes of the first channel calculated in step 104 are significantly different.

The second channel is configured by the microprocessor means using the stored parameters of the first channel. This allows in-phase steering of the code replica of the second channel in step 108 for a fast search for the maximum amplitude E2. The maximum amplitude E2 is located between two amplitudes E1 and L1 in the first channel tracking mode. The control loop of the second channel cannot be used directly since the second channel has to find the maximum amplitude of the initial code replica or the late code replica. The microprocessor means is therefore responsible for conquering the second channel to find the vertices of the autocorrelation function by linear regression methods or by newton-simpson optimization algorithms. Thus, the two interrupting components imposed on the second channel have an instruction to open their control loops.

As long as the amplitude E2 is not maximal, a code phase shift of one of the code replicas of the second channel is performed in step 109. In these search operations for all the maximum amplitudes, the amplitude E2 of the second channel, which lies between the two amplitudes E1 and L1, and the corresponding phase deviation are stored.

When the maximum magnitude E2 is found in step 108, a calculation of the slope of each side of the autocorrelation function vertex is performed using the stored magnitudes in step 110. If the slope P is_2AVAnd P_2APIs significantly different, only the second channel, arranged in maximum amplitude, provides accurate data to the microprocessor means in step 113, particularly for use in position calculation. The amplitude of the second channel is continuously detected.

If the slope P is_2AVAnd P_2APAlmost the same in absolute value, which means that a multipath signal does not exist. In this case, the second channel is stopped in step 112. The microprocessor means is thus able to retrieve data from the first channel again because the second channel is not stopped when the first channel is switched.

Since the above described receiver is to be applied in portable objects of reduced size, such as watches or mobile phones, the separate unused channel is preferably switched in parallel with one of the operating channels when the microprocessor means detects the presence of a multipath signal in the above-mentioned operating channel. As described above, only at least four channels need to be initially selected, each channel tracking a particular visible satellite.

Typically, the second channel is switched over the first channel only in the mode of visual satellite tracking. However, since the parameters and phase offsets of each channel are stored in the storage means, the second channel can be switched in parallel with the first channel even if the first channel is in the search mode. The microprocessor means knows whether the operating channel can be affected by multipath signals.

From the description given above, a person skilled in the art can devise many variants of said receiver, in particular of the GPS type, without departing from the scope of the invention as defined by the claims.

Claims

1. A receiver for a GPS-type radio frequency signal modulated by a transmission source specific code, the receiver comprising:

-receiving and shaping means (3) with frequency conversion of the radio frequency signal for generating an intermediate frequency signal (IF);

-a correlation stage (7) for receiving the intermediate frequency signal is formed by several correlation channels (7'), each channel being provided with a correlator (8) for correlating, when the channel is in use, the intermediate frequency signal with at least one early replica and at least one late replica of a specific code of a visual transmission source to be searched and tracked in a control loop of at least one of the correlators, the correlator comprising integrating means (28, 29, 30, 31) for correlating the signal, at the end of each determined integration period providing a first magnitude of an autocorrelation function of the early signal and a second magnitude of an autocorrelation function of the late signal, the first magnitude and the second magnitude being kept equal in a transmission source tracking mode;

-microprocessor means (12) connected to the correlation stage for processing the data extracted from the radio frequency signals after correlation, said receiver being characterized in that at least one second unused channel is placed in parallel with at least one first operating channel by the microprocessor means for searching and/or tracking the same visible transmission source, the microprocessor means controlling the second channel to generate a replica of a specific code associated with the intermediate frequency signal when the microprocessor detects the presence of multiple radio frequency signals in the first operating channel, so that the integrating means of the second channel provide the maximum amplitude of the autocorrelation function between the first and second amplitudes of the autocorrelation function of the first channel.

2. A receiver as claimed in claim 1, c h a r a c t e r i z e d in that during the search and/or tracking phase of the visually transmitting source, at least some of the amplitudes of the autocorrelation function and the corresponding phase offsets provided by the integrating means are stored in storage means for causing the microprocessor means to calculate a first slope of the autocorrelation function at a first amplitude point of the early signal and a second slope of the autocorrelation function at a second amplitude point of the late signal, said microprocessor means detecting the presence of multiple radio frequency signals in the first operating channel when the first slope and the second slope are different in the tracking mode when the first and second amplitudes are equal.

3. A receiver as claimed in claim 1, characterised in that in the visual transmission source tracking mode when the microprocessor means (12) detects a change in the amplitude of the autocorrelation function of the early and/or late correlation signals, a second unused channel is placed and configured in parallel with the first operating channel.

4. A receiver according to claim 1, characterized in that it is arranged to receive radio frequency signals transmitted by satellites, wherein said correlation stage (7) comprises a greater number of correlation channels (7') than the number of visible satellites, so that at least one second unused channel, connected in parallel with the first active channel, can be switched for searching and/or tracking the same visible transmission source.

5. Receiver according to claim 4, characterized in that the number of channels (7') is greater than or equal to 12.

6. A receiver as claimed in any one of claims 1, 4 and 5, characterised in that when the microprocessor means detects the presence of a multipath signal in each of the first channels, several second unused channels are arranged to be switched, each of which is connected in parallel with one of the first operating channels.

7. A receiver as claimed in claim 1, arranged to receive radio frequency signals transmitted by satellites, the integrating means being arranged to operate on channels during an integration period corresponding to the repetition period of the transmission of satellite specific codes during the visible satellite search and/or tracking phase.

8. A receiver as claimed in claim 1, characterized in that a set of data input and output registers (11) is arranged as an interface between the correlation stage (7) and the microprocessor means (12) for receiving data transmitted by the microprocessor to the correlation stage and data supplied to the microprocessor by the correlation stage.

9. A receiver as claimed in claim 2, characterised in that each channel comprises storage means for storing the amplitude of the autocorrelation function and the corresponding phase offset.

10. A receiver as claimed in claim 2, characterized in that the memory means form part of the microprocessor means (12).

11. A receiver as claimed in claim 1, characterized in that, when the channels (7') are normally set in operation, a controller (9) containing a digital signal processing algorithm in each channel is associated with the correlator (8) to allow all synchronisation tasks for searching and tracking satellites to be performed autonomously independently of the microprocessor means (12).

12. A receiver as claimed in claim 1, characterized in that it is arranged to receive a radio frequency signal with a carrier frequency transmitted by a satellite, each channel receiving a complex intermediate frequency signal consisting of an in-phase component (I) and a quadrature-phase signal component (Q), and in that each correlator of a channel comprises:

-a first mixer (20) for correlating the in-phase signal component with a first carrier frequency replica and for correlating the quadrature-phase signal component with a second carrier frequency replica shifted by 90 ° compared to the first carrier frequency replica;

-a second mixer (23) for correlating the output in-phase signal of the first mixer with the first early specific code replica and the second late specific code replica, and for correlating the quadrature phase output signal of the first mixer with the first early replica and the second late replica;

and in that four integration counters (28, 29, 30, 31) of the integrating means of each channel receive the correlated output signals from the second mixer to provide four magnitudes (I) of the autocorrelation function, respectively_ES，I_LS，Q_ESAnd Q_LS)。

13. A receiver as claimed in claim 1 or 12, characterised in that the early signal has a phase offset of half a chip relative to the late signal.

14. A receiver as claimed in claim 12, characterized in that, after the integration counter, each correlator (8) of the channel (7') comprises, in the code control loop: a code loop discriminator (32) which performs subtraction of each magnitude of the autocorrelation function of the early and late signals to provide a code replica with a code correction amount in a discriminator period which is N times the integration period, where N is an integer; a code loop filter (33); a code generator coupled to a 2-bit register for transferring the initial and late replicas of the source specific code to the second multiplier stage (23); and in the carrier control loop, a carrier loop discriminator (42); a carrier loop filter (43); a second numerically controlled oscillator (44); and means (45) for providing the first multiplier stage (20) with first and second replicas of the carrier frequency; loop interrupt means (46, 47) controlled by the microprocessor means (12) in each control loop are positioned to ensure control of a second unused channel positioned in parallel with the first operating channel when interrupt instructions are imposed on the above mentioned means of the second channel.

15. A method for correcting multipath signal effects in a receiver for a radio frequency signal modulated by a transmission source specific code, said receiver comprising:

-a correlation stage (7) for receiving the intermediate frequency signal is formed by several correlation channels (7'), each channel being provided with a correlator (8) for correlating the intermediate frequency signal, when the channel is in use, with at least one early replica and at least one late replica of a specific code of a visual transmission source to be searched and tracked in a control loop of at least one of the correlators, the correlator comprising integrating means (28, 29, 30, 31) for correlating the signal, for providing, at the end of each determined integration period, a first magnitude of an autocorrelation function of the early signal and a second magnitude of an autocorrelation function of the late signal; the first argument and the second argument remain equal in the transmission source tracking mode;

-microprocessor means (12) connected to the correlation stage for processing the data extracted from the radio frequency signals after correlation, said receiver being characterized in that at least one second unused channel is placed in parallel with at least one first operating channel by the microprocessor means for searching and/or tracking the same visible transmission source, the microprocessor means controlling the second channel to generate a replica of a specific code related to the intermediate frequency signal when the microprocessor detects the presence of multiple radio frequency signals in the first operating channel, so that the integrating means of the second channel provide a maximum magnitude of the autocorrelation function between a first and a second magnitude of the autocorrelation function of the first channel,

the method comprises a first set of steps:

-phase shifting the early and late replicas of the specific code of each operating channel associated with the intermediate frequency signal until the first and second magnitudes of the autocorrelation function are equal;

-storing the correlated amplitudes of the early signal and the late signal and the corresponding phase offsets during a search and/or tracking phase, said method being characterized in that it further comprises a second set of steps:

-for each first operating channel, using the stored autocorrelation function magnitude and corresponding phase offset during the search and/or tracking phase, calculating a first slope of the autocorrelation function at a first magnitude point of the early signal and a second slope of the autocorrelation function at a second magnitude point of the late signal when the channel is in the transmission source tracking mode;

-configuring and switching at least one second unused channel placed in parallel with the first operating channel if the absolute values of the two calculated slopes are different or if a change is found in the first amplitude of the early signal or the second amplitude of the late signal in the tracking mode;

-phase shifting one of the code replicas of the second channel under instruction of the microprocessor means until the integrating means of the second channel provides a maximum magnitude of the autocorrelation function between the first and second magnitudes of the autocorrelation function of the first channel, so that the microprocessor means can extract data from the radio frequency signal of this second channel to correct for the effects of the multipath signal.

16. A method according to claim 15, characterized by storing the amplitude of the autocorrelation function of the second channel and the corresponding phase offset until the maximum amplitude is provided by the integrating means of the second channel.

17. A method according to claim 16, characterized in that the microprocessor means calculates the slope of the change in the amplitude of the autocorrelation function before and after the maximum amplitude of the second channel, and in that the second channel is stopped if the absolute values of the slopes are equal, the microprocessor means being able to extract data from the radio frequency signal of the first channel.