Field of the invention
The present invention refers to the technical field of coherent radars, to be used, for example, installed on vessels or on the coast for maritime surveillance. Radar equipments for navigation and for monitoring of the marine environment are called indifferently marine radar or navigation radar. Other uses of the present invention may be, for example, for coherent radars installed on road vehicles with the function of collision avoidance and/or of control of the vehicle.
State of the art
The localization with electromagnetic waves of vessels and in general of mobile objects or of obstacles, for monitoring and navigation purposes is an ancient technique that dates back to the beginning of the twentieth century. It is well known that radar is the main system used to detect and locate objects at distances that may be very large (hundreds of km). This type of equipment illuminates objects with electromagnetic signals of adequate power and analyzes the return signal (radar echo). Through over a half century vessels use radar equipment for the navigation in conditions of limited visibility (fog, night time) and the location of surface targets. Practical use has shown that such apparatuses are essential for the detection and localization of vessels and obstacles of various kinds, as well as for a safe navigation, particularly at night and/or with fog. Similar radar equipments are also used to monitor the vessels from coastal locations, primarily for the management of sea traffic. These applications are called Vessel Traffic Service (VTS) or Vessel Traffic Management Information System (VTMIS).
A general framework of this equipment and of the systems using it is presented, in two volumes:
- "Target Detection by Marine Radar" by J. N. Briggs, IEE Radar, Sonar and Navigation Series 16, the Institute of Electrical Engineers, London, 2004;
- "Radar and ARPA Manual" by A. Bole, B. Dineley, A. Wall, Elsevier, Second Edition, 2005.
The navigation radar equipments with a magnetron transmitter are today the most widely used. They operate with a typical pulse duration ranging from a minimum varying between 0,055 ps and 0,08 ps (with a nominal resolution in distance
between 8m and 12m) to a maximum of the order of ps, for example 0.9 s. The values of the pulse repetition frequency (pulses/s) and of the duration of the pulse (s) depend on the setting of the full scale and their product is kept roughly constant in such a way as to respect the constraint of the duty cycle of the transmitter, of the order of a few units in ten thousand: the transceiver transmits for a fractional part (so-called duty cycle) equal to 0,0002 - 0,0007 (order of magnitude)of the time, and receives for the remaining part of the time.
Recently, for the navigation radar market new solutions have been proposed based on solid state transmitters. They operate with peak power values typically between 170 and 250 W. These values are much less than the ones of magnetron transmitters, and the duration of the pulse are much greater. Furthermore, they have the values of the duty cycle of the order of 10-13 %, i.e. from 200 to 500 times the typical values for magnetron radars. It has to be noticed that when comparing the "solid state" technique and the "magnetron" technique the energy emitted on the target remains substantially unchanged. As a matter of fact, the lower peak power of the pulse is compensated by its greater length, in addition to the gain due to the coherent integration of the pulses in the time of illumination. The mutual interference is a well-known problem of navigation radar. This interference is due to the large number of naval units equipped with these equipments, and the fact that they operate, in order to comply with the international regulations, in some, relatively narrow, frequency bands. The equipments in the S-band (mainly used in the presence of rain and, together with the X-band, by medium-large vessels) operate from 3020 to 3080MHz. Instead the equipments in the X-band, widely used also for small vessels, operate from 9300 to 9500MHz. The interference problem has led to some international standards, related to navigation radar, which have increased the cost of the equipment without a direct benefit for the individual user. It is a regulation issued in 2003 for the "reduction of out-of-band emissions and the sidelobes of the spectrum" and another one in 2006 for the "reduction of spurious emissions". Until today the interference effects in marine radars are mitigated just exploiting the limited "duty cycle" of interfering radars. In fact, it is exploited the fact that the pulse repetition frequency is normally different from radar equipment to radar equipment.
Therefore, even in the presence of a large number of radars in the visibility region, with the times of emission being diversified, the disturbing radar pulses are in positions such as to be recognized as such. Therefore they can then be deleted and, anyway, they are not integrated in the dwell time processing.
The said remedy will no longer be valid when numerous solid-state maritime radars will be installed and, as already shown, will have values of the duty cycle 200 to 500 times larger than the current one. Therefore there is the need to realize a new type of radar capable of overcoming the above drawbacks and, more in general, to notably improve the compatibility of several radar in the same operating scenario.
Summary of the invention
A primary scope of the present invention is to provide a radar of the coherent type capable of transmitting pseudo-random waveforms. Said waveforms, of the noisy type, are not predictable from an external observer, and may be conveniently generated using numerical techniques, also known as digital.
Another scope of the invention is to provide a process of synthesis of such pseudo-random waveforms in a practically unlimited number. By attributing each of these waveforms to different radar, the effect of mutual disturbance is reduced by two or three orders of magnitude.
In this description, the pseudo-random waveforms, transmitted by the radars of the present invention, are defined as "orthogonal" when their function of mutual correlation is close to zero, i.e. has values of at least two or three orders of magnitude less than in the autocorrelation of each of the said waveforms.
Therefore, the present invention aims to achieve the scopes discussed above, by providing a coherent radar that, according to claim 1 , comprises
- a mass memory adapted for recording thereon at least one selection of sequences of first numerical pairs in phase and quadrature I, Q representing, and allowing the generation of analog signals falling within the range of working frequencies of said radar and suitable for being transmitted by said radar;
- a coherent transceiver comprising
a coherent modulator, adapted to receive said at least one selection of the sequences of first numerical pairs I, Q, read by the mass memory, and generate
respective analog signals; a power amplifier for amplifying said analog signals; at least one antenna for transmitting said analog signals and receiving analog signals reflected by one or more radar targets; a coherent demodulator adapted to treat said reflected analog signals and to generate digital signals, representative of said reflected analog signals, in the shape of sequences of second numerical pairs Γ, Q';
- a digital correlation receiver for calculating the function of mutual correlation between said selection of sequences of first numerical pairs I, Q and said sequences of second numerical pairs Γ, Q', suitable for detecting the radar targets and determining the distance thereof,
- a digital generator of pseudo-random waveforms in the shape of sequences of said first numerical pairs in phase and in quadrature I, Q, said digital generator comprising, in a sequence:
- a processor for generating the sequences of first numerical pairs I, Q;
- a low-pass filter adapted to linearly transform said sequences of first numerical pairs I, Q for defining a power spectrum of said sequences of first numerical pairs such as to make them representative of an analog signal contained in the range of working frequencies of the radar, said low-pass filter providing in output sequences of third numerical pairs in phase and in quadrature li, Qi;
- zero memory non linear (ZMNL) transformation means of said sequences of third numerical pairs l-i , Qi for producing sequences of fourth numerical pairs l2, Q2 and sending said sequences of fourth numerical pairs l2, Q2 again to said low-pass filter until condition n<N exists, where n is the number of current iterations and N is a default value sufficient to reach a convergence of the iterative process, or sending for n=N said sequences of fourth numerical pairs l2, Q2 to a recording and selection block;
- said recording and selection block being adapted to record and select a subset of said sequences of fourth numerical pairs l2, Q2 defining the at least one selection of said sequences of first numerical pairs I, Q.
A second scope of the present invention provides a process for the synthesis of pseudo-random waveforms, which are transmitted by the said radar. This synthesis process comprises, according to claim 10, the following steps:
- generating pseudo-random waveforms in the form of sequences of first numerical pairs in phase and quadrature I, Q by means of a processor;
- linear transformation of said sequences of the numerical first pairs I, Q by means of a low-pass filter to define a power spectrum of said sequences of first numerical pairs such as to make them representative of an analog signal contained in the range of working frequencies of the radar, said low-pass filter providing in output sequences of third numerical pairs in phase and quadrature l1 f Qi ;
- zero-memory non-linear transformation (Z NL) of said sequences of third numerical pairs , Qi to produce sequences of fourth numerical pairs l2, Q2;
- sending said sequences of fourth numerical pairsl2, Q2 to said low-pass filter until condition n<N exists, where n is the number of current iterations and N is a default value sufficient to reach a convergence of the iterative process, or for n=N sending said sequences of fourth numerical pairs l2, Q2 to a recording and selection block;
- recording said sequences of fourth numerical pairs l2, Q2 and selecting, by means of said recording and selection block, a subset of said sequences of fourth numerical pairs l2, Q2 that defines at least one selection of said sequences of the first numerical pairs I, Q which represent, and allow to generate, analog signals falling within the range of working frequencies of said radar and suitable for being transmitted by the radar itself.
As just introduced, the problems described above are overcome, in the present invention, using pseudo-random waveforms for the radar transmission. The related concept is known as that of the one of Noise Radar. Noise radars emits pseudo- random signals, which are very difficult to intercept. Therefore, in military applications it is difficult to locate these radars and to interfere with their operation. In summary, the present invention relates to a radar system of new design that, along with other concepts, uses the concept of Noise Radar already known for military applications, and broadens its capabilities and applications. In the present invention, the main purpose of the use of pseudo-random waveforms is not to make difficult the interception of the radar: in fact, the interception has no real practical interest in the field of civilian radar. Vice versa, the aim of this invention is
6 to minimize the mutual interference of the numerous radars present in water or in road traffic by exploiting the mutual orthogonality of these waveforms.
In the previous discussion it has been shown that, once the transition - already in its start-up phase - will be completed from the magnetron technology to that of the solid state transmitter, the interfering phenomena will have a hundreds of times greater relevance. It has been shown that this fact will make it difficult, if not impossible, to use the maritime solid state radars in environments of dense traffic, that are precisely those in which the radar is most needed. To those skilled in the art it is known that in a radar system the interfering signals of different radars can be separated, and possibly neutralized, with one or more of these known techniques: (a) by means of filters, by exploiting the difference in frequency (more exactly, the different band occupation); (b) by exploiting space diversity using strongly directive antennas and very low sidelobes, or with a filtering at the antenna level by array processing;(c) by exploiting the difference in the emission time. The solution (c), as explained above, it is commonly practiced in maritime radars that, now equipped with magnetron, transmit very short pulses (with a duration typically less than one microsecond). However, the said solution will not prove effective enough in the case of solid state radars, with pulses hundreds of times longer. The solution (a)is of limited benefit because of the narrow range of frequencies available, as already shown. Finally, the solution (b), typical of military radar, it is not suitable for the specific application for obvious reasons of complexity and cost.
In the context of the present invention, then, a fourth solution is used, based on the orthogonality of the pseudo-random signals transmitted by the new radar family of the present invention. It is well known that a noise (obtainable for example by filtering the thermal noise), which occupies a range of frequencies (band) B and has a duration T, has an autocorrelation function of duration of 2T and a peak at zero delay whose amplitude is proportional to the product BT and whose width, or decorrelation time, is 1/B. The random sidelobes out of the area of the peak have average amplitude - low with respect to the peak - which decreases with the distance from the peak. Noise segments of duration T, with T»1/B, taken at disjoint times realize waveforms whose mutual correlation is
much lower than the autocorrelation of each of them, and is tending to zero when T tends to infinity. It is therefore possible to generate a substantially unlimited number of waveforms conceptually orthogonal in the sense mentioned above. By attributing each waveform to a different radar, the effect of mutual interference is reduced by two or three orders of magnitude.
In a preferred embodiment of the present invention, the pseudo-random waveforms are suitably generated and recorded on a mass memory, for example a hard disk. The number of such pseudo-random waveforms varies with the particular application of the coherent radar of this invention. This number can easily reach values of the order of tens or hundreds of billions (largely compatible with the normal capacity of hard disks). With these numbers, the probability that two radars by chance are transmitting the same sequence in a synchronous manner is the order of one out of ten millions or even less.
Advantageously, the radar and the process of this invention permit to obtain:
- A peak-to-sidelobes ratio of the autocorrelation function sufficiently high (of the order of 30-35 dB, or even better, depending on the particular configuration and application).This fact permits to avoid, or at least to drastically reduce, the masking of small objects close to a main target;
- A mutual correlation sufficiently lower (20-25 dB or even better) than the peak of the autocorrelation. This makes it possible to bound the disturbance due to other radar present in operating theater at tolerable value, not higher than the present magnetron-based radar;
- A peak factor (the ratio of peak to average power, or equivalently between peak amplitude and mean amplitude) near or equal to the unit. This feature permits to best exploit the maximum capacity of the solid state transmitter, making it working mainly in the saturation regime. This is particularly advantageous for detection and location of targets at a great distance with the subsequent transmission of long pulses.
The digital waveforms generator of the radar, which is part of the present invention, works recursively. First, sequences of uncorrelated pseudo-random numbers are generated. These are then filtered in a low-pass way to obtain a spectral density such as to occupy a defined range of frequencies. Then a zero-
memory non-linear transformation is applied to make the peak factor equal to the unit or close to it, in order to better use the available transmission power. The said process is iterated for a predetermined number of times, adapted to reach an adequate degree of convergence .successively, the sequence of numerical pairs is passed to a recording and selection block.
The waveforms to be transmitted, contained in the memory of the radar, are previously selected in the said recording and selection block, in order to obtain a peak-to-sidelobes ratio of the autocorrelation function of preferably at least 35 dB as well as the minimum possible value for the peak-to-sidelobes ratio in the function of mutual correlation, indicatively less than 25 dB below the peak of the autocorrelation, minimizing the interference between radars operating in the same environment. Said numerical values can also vary significantly depending on the specific operating requirements and of the characteristics (such as the BT, bandwidth by emission time, product) of the particular embodiment of the coherent radar, always remaining in the context of the present invention, as it is described in the following Claims.
For example the waveforms to be transmitted can be selected in the said recording and selection block in order to obtain a peak-to-sidelobes of the autocorrelation function of at least 30 dB and possible values in the function of mutual correlation at least 20 dB below the peak of the autocorrelation.
The dependent Claims describe preferred embodiments of the invention.
Brief description of the Figures
Further features and advantages of the invention will become apparent in the frame of the detailed description of a preferred, but not exclusive, embodiment of a coherent radar. Said coherent radar is shown, by way of example but not of limitation, with the aid of the accompanying drawing tables, or Figures, wherein: Figure 1 shows a general scheme of the radar of the present invention, with illustration of the main signals exchanged in it;
Figure 2 shows a diagram of a coherent modulator, part of the present invention; Figure 3 shows a diagram of a coherent demodulator, part of the present invention;
Figure 4 shows a diagram of the generation of waveforms that are transmitted in
the context of the present invention.
Detailed description of a preferred embodiment of the invention
A first embodiment of the coherent radar according to the present invention is here presented with reference to the Figures. This radar, which is the object of the invention, comprises:
- A digital generator 1 of pseudo-random waveforms in the form of sequences of first numerical pairs I, Q (the letters I, Q indicate "in phase" and "quadrature", respectively);
-A mass memory 2, for example, a hard drive, capable of recording on it at least a selection of these sequences of first numerical pairs I, Q that represent, and permit to generate, analog signals falling in the range of the operating frequencies of said radar and are suited to the transmission by said radar;
- A coherent transceiver comprising in turn
- A coherent modulator 3, adapted to receive the said at least one selection of the sequences of first numerical pairs I, Q, read from the mass memory
2, and to generate the related analog signals;
- A power amplifier 4 to amplify said analog signals;
- At least one antenna 6 able to transmit said analog signals (after their transit in a duplexer 5 which is provided only when the same antenna is used both to transmit and to receive) and receive the analog signals reflected by one or more radar targets;
- A coherent demodulator 7 adapted to receive said analog signals reflected (also called radar echo) and to generate digital signals, representative of said analog reflected signals, in the form of sequences of second numerical pairs Γ, Q';
- A frequency synthesizer 8 able to generate the reference frequencies fc, fs (commonly called frequency of the COHO and of the STALO, respectively) to be used by the coherent modulator 3 and the coherent demodulator 7;
- A receiver 9 of the digital correlation type to calculate the correlation function between said selection of sequences of the first numerical first pairs I, Q and said sequences of second numerical pairs Γ, Q', said receiver 9 being suitable to detect the radar targets and determine their distance. Said receiver 9 performs
substantially a digital correlation between the pseudo-random transmitted waveforms and the pseudo-random received waveforms, in order to calculate the correlation function between the electromagnetic echo caused by radar targets and a reference provided by the replication of the transmitted waveform, transmitted on the target itself.
An alternative embodiment of the part made up by the blocks numbered from 1 to 9 in Figure 1 can be convenient in some cases where, for its particular technical implementation, the transmitting chain, consisting of the coherent modulator 3 and the power amplifier 4 in cascade, significantly distorts the signal. In this alternative embodiment, the said sequence of the first numerical pairs I, Q, instead of being sent directly from the mass memory 2 to the receiver 9 as a reference to calculate the said mutual correlation function, is obtained as follows. The output signal from the mass memory 2 is passed into the coherent modulator 3 and into the power amplifier 4 (and possibly also into the duplexer 5 which, however, normally has no any significant effect). The analog signal represented by said sequence of the first numerical pairs I, Q, taken out from the power amplifier 4 (or equivalently from the duplexer 5) by means of a directional coupler (not shown) during the downtime of the radar, is sent to the coherent demodulator 7. During the downtime of the radar, the coherent demodulator 7 presents at its output the said sequence I, Q, which is stored and sent to the input of the digital correlation receiver 9, where the correlation with the said sequence Γ, Q', is calculated as shown above. Herein, downtime of the radar refers to the time intervals, in which there is no useful reflected analog signals or radar echo. The distortions introduced by the transmitting chain are therefore compensated in the correlation process. Whenever these distortions are negligible, the two implementations above presented for the set of blocks 1 -9 are completely equivalent.
When, in a given environment of vessel traffic, or road traffic or similar, a plurality of radars according to the present invention is present, the signals transmitted by said radars are pseudo-random, time-varying and different from radar to radar, and designed in such a way as to avoid the possible mutual interference between said plurality of radars operating in the same environment.
Advantageously, the digital generatorl of pseudo-random waveforms, that in a preferred embodiment may not be part of the radar, i.e., may be a separate element outside the radar, comprises in succession:
- A processor 32 to generate the sequences of the first numerical first pairs I, Q with the well-known digital methods for the generation of uncorrelated pseudorandom numbers;
- A low-pass filter 33 which linearly transform said sequences of the first numerical pairs I, Q defining a power spectrum of these sequences of the first numerical pairs, considered downstream of said processing, such as to make them representative of an analog signal whose spectrum is contained in the range of the operating frequencies of the said radar. Said low-pass filter 33 provides at its output sequences of third numerical pairs in-phase and quadrature l1 p Qi ;
- Means 34 for applying a zero-memory non-linear transformation (ZMNL: Zero Memory Non Linearity) of said sequences of third numerical pairs \ -\ , Qi to produce sequences of fourth numerical pairs l2, Q.2, preferably characterized by a peak factor near or equal to 1 , and sending these sequences of fourth numerical pairs l2, Q.2 again at the said low-pass filter 33 until there is the condition n<N, where n is the current number of iterations (the parameter n, which is equal to the unity in the first iteration, is incremented by the unity at each iteration step) and N is a default value sufficient to reach a convergence of the iterative process, or send for n=N said sequences of fourth numerical pairs I2, Q.2to a recording and selection block 36;
- Said recording and selection block 36 being adapted to record and select a subset of these sequences of fourth numerical pairs l2, Q2 characterized by values established by the radar designer, preferably - but not necessarily - of at least 35 dB for the peak sidelobes ratio of the autocorrelation function as well as with a smallest possible value of the mutual correlation function, indicatively less than 25 dB below the peak of the autocorrelation function. Said subset of these sequences of fourth numerical pairs I2, Q2 defines, then, the result of said at least one selection of sequences of the first numerical pairs I, Q.
In an alternative embodiment, which is intended to provide a lesser degree of saturation in amplitude (and, therefore, a greater linearity), the said zero-memory
non-linear transformation is applied only to those elements of the said sequence , Q-i whose envelope or amplitude M =( 2+ Qi2)1/2statistically exceeds a predetermined value by the designer. If the excess is not the case, then it is set l2= and Q2 = Qi , which is equivalent, in the said case of a sufficiently low envelope, not to apply the ZMNL transformation.
Downstream of the digital correlation receiver 9, or simply digital correlator, Doppler filters 10 are provided to separate the radar targets between them and from environmental disturbances, at different radial speeds. On each Doppler filter 10 is applied a system of thresholds 1 adapted to detect the radar targets of interest.
The overall layout of the coherent radar of this invention is shown in Figure 1. The numerical sequences generated from the digital generator 1 are recorded in the mass memory 2, in the form of "in phase" and "quadrature" samples i.e. as pairs I, Q in the representation well known to those skilled in digital processing of signals and in radar. These sequences of numerical pairs I, Q, read from the memory 2, are sent to the coherent modulator 3 whose output, which is the analog signal to be transmitted, is sent to the power amplifier 4 and, in a first preferred embodiment that uses the same antenna 6 for both transmitting and receiving, to the duplexer 5 and finally to the said antenna 6.lna second embodiment (not shown) the duplexer is not provided but there are two antennas: a first transmitting antenna and a second receiving antenna.
In the receiving process, the echo of the target, i.e. the reflected signal, picked up by the antenna 6, is sent to the coherent demodulator 7.The demodulator 7 and the modulator 3, as it is well known to those skilled in the art, use reference frequencies whose number is two (fs, fc) in the preferred embodiment, that are generated by the synthesizer 8, which comprises a high stable oscillator and some suitable circuits for frequency multiplication and sum. The output of the coherent demodulator 7 consists of digital signals, representative of the said analog reflected signals, in the form of a sequence of second numerical pairs Γ, Q' that is sent to the digital correlator 9. Therein are performed the calculation of the correlation of the sequences of pairs , Q' with the sequences of pairs I, Q read from the memory 2. As it is known to those skilled in radar, the correlation
produces a filtering matched to the transmitted waveform and permits to detect the targets and to determine their distance. The digital correlator output is sent to a block provided with Doppler filters 10 where filtering takes place. Said Doppler filtering separates the targets at different radial speeds, including the null speed. On each Doppler filter 10 a system of thresholds 1 1 is applied, which permits to detect the targets of interest in the presence of noise and disturbances, creating the so-called "target reports" that are sent to the radar data processor and subsequently, in the form of radar tracks, to the users.
The coherent modulator 3 is shown, in a preferred embodiment, in Figure 2. Its preferred configuration is the double conversion super-heterodyne with two reference oscillators: the STALO at frequency fs and the COHO at frequency fc. The transmission frequency of the radar is, in the embodiment described herein, the sum (fs+fc) of the said frequencies fs and fc.
The numerical sequences I, Q read from the memory 2 are converted into analog form by the pair of Digital to Analogue Converters (D/A) 12, 13 and translated to the frequency fc by means of the pair of mixers 14, 15. Onto the mixer 14 comes the reference frequency fc coming from the synthesizer 8, while onto the mixer 15 comes the said reference frequency fc shifted by ninety degrees thanks to its transit in the block 16. The two signals at the frequency fc, thus obtained, are summed in the block 17 and the result is filtered in the block 18 to substantially eliminate the frequency components which, in transmission, may lie outside the range of the operating frequencies of the radar. The mixer 19 translates the resulting signal to the transmit frequency (fs + fc) of the radar. After the filtering by the block 20, in order to eliminate the unwanted frequency components outside the range of the operating frequencies of the radar, the signal at the transmission frequency (fs+ fc) is sent to the amplifier 4 for the power amplification and the subsequent transmission.
In an alternative embodiment, functionally equivalent, the transmission frequency of the radar can be the difference (fs - fc of the frequencies fs and fc.
The coherent demodulator 7 is shown, in a first preferred embodiment, in Figure 3. The analog signal received from the duplexer 5 or directly from the receiving antenna, is amplified with a low-noise amplifier 21 , shifted to the frequency fs with
the mixer 22, filtered by the band-pass filter 23, amplified by the amplifier 24 and translated the video frequency (i.e., on a band centered at zero frequency) by means of the pair of mixers 25 and 26. On the mixer 25 comes the reference at frequency fc, from the synthesizer 8, while on the mixer 26 comes the reference at frequency fc shifted by ninety degrees thanks to its transit in block 27. Using the low-pass filters 28 and 29 two video signals, substantially free of components that are out of range of the operating frequencies of the radar, are obtained. The outputs of said blocks 28 and 29, after the conversion from analog to digital respectively in the converters 30 and 31 , constitute the said sequences of numerical pairs Γ and Q' that are sent to the digital correlator 9.
In a second alternative embodiment (not shown), capable of providing the same result as regards the outputs, the coherent demodulator contains only one mixer and there is only one analog to digital converter operating with a higher conversion rate (at least twice) of the one of converters 30, 31 belonging to the first embodiment described above. A suited processing, well-known to those skilled in the processing of radar signals, and based on the Hilbert filter, transforms the output of the said single analog-digital converter into the sequences of numerical pairs and Q' that are sent to the digital correlator 9.
In the following, with reference to Figure 4, the process of synthesis of digital pseudo-random waveforms according to the present invention is described i.e the process for the generation of the waveforms transmitted by the coherent radar of the present invention.
The said process includes the following basic steps:
- Generation of pseudo-random waveforms in the form of sequences of first uncorrelated numerical pairs I, Q, in-phase and quadrature by the block 32;
- Linear transformation of said sequences of the first numerical pairs I, Q, by means of the low-pass filter 33 to define a power spectrum of these sequences of the first numerical pairs so as to render them representative of an analog signal whose spectrum is contained in the range of the operating frequencies of the radar, said low-pass filter 33 providing output sequences of third numerical pairs l-i , Qi , in-phase and quadrature;
- Zero-memory non-linear transformation (ZMNL) of said sequences of third numerical pairs l-i, Qito produce sequences of fourth numerical pairs l2, Q2 preferably characterized by a peak factor near or equal to 1 ;
- Sending said sequences of fourth numerical pairs l2, C?2to said low-pass filter 33 until there is the condition n<N, where n is the current number of iterations and N is a default value sufficient to reach a convergence of the iterative process, or for n=N sending said sequences of fourth numerical pairs l2, Q2to the recording and selection block 36;
- Recording said sequences of fourth numerical pairs l2, Q2 and selection, by means of said recording and selection block 36, of a subset of these sequences of fourth numerical pairs l2, Q2, preferably - but not necessarily - characterized by values of at least 35 dB of the peak/sidelobes ratio of the autocorrelation function and values of the mutual correlation function less than at least 25 dB with respect to the peak of the autocorrelation function, said subset of these sequences of fourth numerical pairs l2, Q2 defining the selection of those sequences of the first numerical pairs I, Q representing, and permitting to generate, analog signals falling in the range of operating frequencies of the radar and suitable for the transmission by the radar itself.
The generation of sequences of independent pseudo- random Gaussian numbers or variables, made by the digital processor 32 with one of the many algorithms known to the experts, provides the in phase-quadrature sequences I, Q, to be sent to the low-pass digital filter 33, which defines the shape and the optimum width B of the power spectrum of the sequence.
In order to make better use the power of the transmitter, made up by the power amplifier 4,the sequences , Qi at the output of the low-pass filter 33 are treated in a Zero Memory Non Linear(ZMNL) block 34, which provides a suited zero-memory non-linear transformation.
A preferred embodiment of the said of ZMNL transformation of the sequences of third numerical pairs , Qi is effected by imposing an amplitude equal to a predetermined constant value to each complex digital signal component and Qi . This preferred embodiment achieved this transformation by dividing each value of I1 and Qi by the corresponding envelope M= (l1 2+Q 2) 2, obtaining
l2 and Q2 =a Q1/M, being "a" a multiplicative factor defined in the project. Other methods for the ZMNL processing, also effective, are, however, possible, for example:
- Methods that use computationally simple algorithms to perform the square root and the division operations,
- Methods that apply the said division by M only to those values that exceed a given amplitude threshold, that is, in order to provide a lesser degree of amplitude saturation, in those alternative methods said nonlinear transformation without memory is applied only if the amplitude
exceeds a predetermined design value, while in the opposite case it is set l2 = hand 0.2 = Q- ,
- Methods that use some alternative non-linear functions different from the division by the square root of the envelope, said functions being aimed to achieve a saturation that for some of these functions may be "smooth", i.e. different from the "hard" saturation just described.
The output from the non-linear transformation without memory, block 34, referred as a sequence of fourth numerical pairs I2, Q2 preferably characterized by a peak factor near or equal to 1 , is treated according to the decision of the block 35. In particular, when the number n of iterations is greater than the unity but less than a predefined value N, (this predefined value being sufficient to reach a substantial convergence of the iterative method), then the sequence , Q2 is sent to the input of the low-pass filter 33 (thus replacing the output of the block 32, which is used only if n=1 ), otherwise the sequence l2, Q2 is sent to the recording and selection block 36.
In said block 36 all sequences l2, Q2 whose autocorrelation functions and mutual correlation are unsatisfactory for the purposes of the coherent radar of the present invention (too high sidelobes for the autocorrelation, too high values for the mutual correlation) are discarded. Vice versa the sequences l2, Q2 that are satisfactory are recorded on the mass memory 2. In particular, are selected and recorded on the mass memory 2 the sequences of fourth numerical pairs l2, Q2 characterized by values of the peak/sidelobes ratio of the autocorrelation function exceeding a minimum acceptable value, defined in the design stage as a function of the specific application. A subsequent analysis, always performed inside the said
block 36, is made at the level of pairs of sequences l2, Q2 in order to eliminate possible sequences whose mutual correlation function is unsatisfactory for the purposes of the coherent radar of the present invention (too high peaks, too high average value). In particular, on the mass memory 2 are selected and recorded sequences of fourth numerical pairs I2, Ckcharacterized by values of the of mutual correlation function, as compared to the peak of the autocorrelation function, below a minimum acceptable value, defined in the design phase as a function of the specific application.
A possible realization of the said low-pass filtering, is performed, by way of example only, in the frequency domain using a Fast Fourier Transform, with a step in the frequency domain that allows to define the preferred shape of the power spectral density and, thus, of the autocorrelation function (for example, the Gaussian one) and that an inverse Fast Fourier Transformation is suited to come back in the time domain.
The synthesis process, as explained and illustrated above, advantageously allows to maximize the ratio of the average amplitude and the peak amplitude of the waveforms, at the same time, the transmitted spectrum being made low enough in those frequency ranges of the electromagnetic spectrum that can be used by other users so as not to create disturbance to them. The said synthesis process can be realized within the radar of the present invention by the digital pseudo-random waveforms generator 1 as described above, integrated in said radar, or, can be implemented off-line by a separate digital generator, outside the said radar.
Among the possible applications of the coherent radar of this invention there are:
- Use of the radar for naval applications and/or navigation. It can be installed either on vessels or on the coast or in the framework of the control of harbor traffic;
- Use of a radar on road vehicles, or on the vehicles dedicated to the airport service for the purposes of collision avoidance and/or for the control of the vehicle, preferably through the coupling of the vehicle speed to that of another vehicle that precedes it.
The coherent radar, which is the object of the present invention, can be conveniently implemented both by means of the "pulse compression" technique
(pulse compression radar) and by means of the "continuous wave" technique (continuous wave radar).