BR102018004960A2 - MOBILE TARGET MOBILE TARGET MONITORING SYSTEM FOR EARTH RADAR - Google Patents

Abstract

mobile radar targeting system in a dense land radar forest region capable of detecting moving targets moving on the ground or water surface where the attenuation of signals in the p band is lower than that observed under clear space conditions, the propagation in the vertical plane of radar signals (54) being confined between soil (56) and foliage (52). The possible interference of radio emissions from third party equipment in uhf over the signals received by the radar is suppressed by subtracting, in the frequency domain, the bands (51) of the interfering range signals. after range compression (34) the same removal process is performed in the azimuth direction (38), followed by compression (45) to achieve high azimuth angular resolution. detection (46) and display (47) follows. The antenna may be fixed, illuminating a sector, or eccentricly rotated such that a synthetic aperture may be formed. To identify such bands with greater sensitivity, the system ceases to transmit the radar signal every m pulses, keeping the system in receive mode for a period equal to that employed in capturing signals reflected by the target in response to pulses emitted by the radar. The signal received under these conditions is due to interference only, containing neither the echo of the target nor the clutter due to irregularities of the propagating medium. once the frequency range of such interference is identified, it is replaced by zero in the frequency spectrum of the radar signal containing the target echo.

Description

MOBILE TARGET MONITORING SYSTEM IN DENSE FOREST REGION FOR TERRESTRIAL RADAR

Field of the invention [001] This invention relates to the technique of detecting moving targets by means of radar and, more particularly, the detection of such targets located in regions of dense forest, such as, for example, the Amazon rainforest.

Summary of the state of the art [002] The detection of moving targets is essential in monitoring land and river regions in order to curb illegal activities such as drug trafficking, weapons, people and cargo theft, among others. These objectives are mainly aimed at detecting road and fluivial vehicles through systems that use fixed, mobile or transportable radars.

[003] Despite the fact that airborne or satellite radars present a wide coverage, their usefulness is limited in the detection of targets that move on the surface at low speeds, in the order of a few meters per second. In addition, in regions of dense forest, as found in the equatorial zone, the echo of the target can be masked by the irregular return (“clutter”) caused by the multiple reflections and refractions of vegetation.

[004] The obstacles that the closed forest presents to the propagation of the electromagnetic signals require the use of the P band for this application. It happens, however, that the P band comprises the UHF communications band, widely used in this region. Such communication signals, which are also

Petition 870180020176, of 03/13/2018, p. 11/30 / 11 captured by the radar antenna, can have such a power that they produce the blocking of the low noise amplifier in the reception stage (“low noise amplifier in the front end”), with the consequent interruption of the system's operation.

Objectives of the invention [005] In view of the above, it is a first objective of the invention to provide a radar system with the ability to detect moving targets moving on the surface of the ground or water.

[006] Another objective is the provision of a radar system that can detect moving targets in places covered by dense vegetation, such as the equatorial forest, even in the presence of irregular echoes caused by reflections and refractions in the forest. Its operation within the forest is also planned, that is, both the radar and the targets may be inside the forest or not.

[007] Another objective is to provide a radar system that is immune to interference from UHF communication signals.

Brief description of the invention [008] The aforementioned objectives, as well as others, are achieved by the invention by providing a radar operating in the P band, installed on the ground, equipped with means of canceling the interfering effects of UHF communication signals.

[009] According to another characteristic of the invention, said concelation is provided by subtracting, from the echo of the return signal, one or more bands of

Petition 870180020176, of 03/13/2018, p. 12/30 / 11 interfering signals.

[0010] According to another characteristic of the invention, the extraction of the echo from the target inserted in a non-uniform return is provided by the autocorrelation of the return signals corresponding to successive scans.

[0011] According to another characteristic of the invention the resolution of the target location in azimuth is increased by means of the synthetic opening technique (SAR) and / or interferometry.

Description of the figures [0012] The other characteristics and advantages of the invention will be more evident through the description of a preferred embodiment, given by way of example and not limiting, and the figures that refer to it, in which:

[0013] Figure 1 exemplifies the use of the invention in monitoring a target through dense forest.

[0014] Figure 2 is a flow chart of the attenuator adjustment of the input signal.

[0015] Figure 3 is a block diagram of the system of the invention.

[0016] Figure 4 exemplifies the spread of the radar signal in a dense forest.

[0017] Figure 5 presents the detailed block diagram of the elimination of interference in azimuth.

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4/11

Detailed description of the invention [0018] The present invention takes advantage of the fact that the propagation of electromagnetic radiation of certain frequencies in dense forests is less attenuated than that observed in free space conditions.

[0019] In effect, in the free space an electromagnetic wave propagates according to a sphere, the relationship between the powers received and transmitted by a radar system given by the expression:

Where:

Pt is the power of the transmitter;

P _r is the received power;

G is the gain of the transmitting / receiving antenna;

R is the distance between the radar and the target;

K is a constant.

[0020] In the case where the radar signal crosses a dense forest, its propagation in the vertical plane is confined between the soil 56 and the foliage 52, as exemplified in Fig. 3, where the propagation comprises successive reflections, being the attenuation of the radar signal proportional to (1 / R) and no longer to (1 / R ² ) as in the case of spherical propagation. However, it occurs that the signal suffers an additional attenuation due to the losses in the reflections in the soil and in the crowns, as well as the fact of propagating among the vegetation; consequently, the relationship between the received and transmitted powers is as follows:

K · P _t G ²

D ’= ----- í ---- Petition 870180020176, of 03/13/2018, p. 14/30 / 11 [0021] The above expression is valid for radar signals in the P band, particularly in the frequency range between 420 and 450 MHz.

[0022] There are two problems related to the radars used in dense forest and operating in the P band. The first one is related to the fact that several communications services operate in UHF, such as, for example, radio amateurs, whose range of 70cm covers the spectrum ranging from 430MHz to 440MHz. The radiation from this service can interfere with the radar signals, adding to them and, eventually, creating blockage through the saturation of the amplifier stages of the system, mainly the saturation of the low noise amplifier 23. Such blocking is avoided through the adjustment strategy of the variable attenuator 21 described later and shown in Fig.

2.

[0023] A second problem refers to the fact that propagation within the forest is subject to multiple reflections and refractions due to irregularities in the propagating medium, here called “clutter”, making the return signal captured by the antenna not only have the return signal from the moving target as well as the return signal from the “clutter”. The signature of the “clutter” appears as an apparently random noise. The intensity of the “clutter” is almost always greater than that of the moving target, and can be more than 50 dB more intense. The system now proposed comprises means that make it possible to extract the echo from the moving target of the “clutter” as will be detailed below.

[0024] The system of the invention is designed to detect only moving targets, here called moving targets. The targets can be aimed directly at the radar, they can be inside the forest or they can be outdoors and have a forest between them and the proposed system. The proposed system can be located inside or outside the forest.

Petition 870180020176, of 03/13/2018, p. 15/30 / 11 [0025] The proposed system has two sets of antennas, 19 and 19a, as shown in figure 3. Antenna 19 is used for transmission and reception and antenna 19a is used only for reception. There is a horizontal baseline between the two antennas, whose dimension is in the order of magnitude of 1 m and is defined according to the required azimuth angular precision.

[0026] There are two basic configurations of the proposed system, where the antennas are immobilized or travel in a circular motion. The configuration where the antennas travel in a circular motion is documented in patent application BR 10 2012 003900 1 for the Sentir-M20 radar, whose teachings are now fully incorporated. In the immobile configuration, the antennas illuminate only one sector, which has an azimuth opening of approximately 100 degrees. For complete azimuth coverage, it is necessary to implement 4 antenna panels, where each antenna covers a 90 degree sector. The advantage of this solution is the absence of moving parts, increasing the availability of the system. The configuration where they travel in a circular motion, requires only an antenna panel, presented a lower cost, but less availability than the solution without moving parts.

[0027] According to Fig. 3, which shows the block diagram of the proposed system, the system comprises the radar signal generator 16 which, in the exemplary embodiment described here, generates pulses of 20μs with a repetition frequency of 5kHz, in the bands 360-370MHz and 420-430Mhz. Such pulses are of two types: the first 48 is a chirp signal between the limit frequencies of the bands (360-370MHz and 420-450Mhz). The second is a noise pulse 49, also included between said limit frequencies. The choice between one or the other depends on the circumstances, such as the greater or lesser possibility of detection by an eventual enemy. This signal is amplified by the amplifier

Petition 870180020176, of 03/13/2018, p. 16/30 / 11 linear 17 and routed to antenna 19 through circulator 18.

[0028] The signal reflected by the moving target and the "clutter" returns to both antenna 19 and antenna 19a. Antenna 19 is used for both transmission and reception. Antenna 19a for reception only. There is a horizontal baseline between the antennas 19 and 19a, in such a way that they receive the same signal, but with different phases, enabling the precise measurement of the azimuth angle.

[0029] Considering that the radar working environment receives external interference from other transmitters, antennas 19 and 19a will receive them, in addition to the signals of the moving target and the "clutter". If the intensity of the interference is high, the first amplifier in the reception chain, the so-called low noise amplifier 23, or 23 a in the case of antenna 19a, can go into saturation. To enable its operation in the linear range, the signals from antennas 19 and 19a are attenuated with the attenuators 21 and 21a respectively. The attenuator adjustment procedure is shown in FIG. 3. The amplifier's input signal strength is measured and the respective attenuator is adjusted, in such a way that it does not saturate and always operates in the linear range. This procedure is done separately for amplifiers 23 and 23a. Even when operating in the linear range, the intensity of interference can still prevent detection of moving targets.

[0030] For this, there are two phases of interference detection and its respective elimination. The first phase occurs before range or range compression. The second phase occurs after the range compression, but before the azimuth compression. The processes for eliminating

Petition 870180020176, of 03/13/2018, p. 17/30 / 11 interference are performed independently for signals from antennas 19 and 19a.

[0031] The return signal flow from the moving target travels the paths of the two receivers equally and simultaneously. The return signal from the moving target is added to the clutter signal, thermal noise and external interference, and enters through antennas 19 and 19a. Then it passes through the circulator 18 and arrives at the variable attenuator 21. For simplicity of description, only the chain of the variable attenuator 21 until the azimuth compression input 45 will be described, since the respective antenna chain 19a is identical. The attenuator adjusts the signal strength from the moving target, the clutter and the interference; limiters 22 and 22a prevent amplifiers 23 and 23a from being damaged. The output of amplifiers 23 and 23a, which work in the linear region, is converted into a digital signal through analog-to-digital converters 24 and 24a, each of which performs N sampling in range or range.

[0032] In the description that follows the modules that process the signals captured by the antenna 19 are mentioned, however, it is understood that the same steps are valid for the signals coming from the antenna 19a.

[0033] The digital signal provided by the analog-to-digital converter (A / D) 24 is then converted to the frequency domain by the Fourier transform module 53 and proceeds to the range or range interference elimination module 30.

[0034] The range or range interference elimination module works with two modes of radar operation, for better detection and elimination of interference. The radar operating sequence comprises

Petition 870180020176, of 03/13/2018, p. 18/30 / 11

M cycles, with a radar pulse being transmitted in each cycle and where the return signal (echo) of each pulse is sampled in N samples. However, with each M cycle the radar does not transmit any pulse. In this way there are M-1 return signals with echoes from the moving target, the “clutter” and external interference and an M-th return signal with only the interference signal, because for this return there was no signal transmission. The spectrum of the return line of N points, or spectrum in range of the return signal where the aforementioned transmission did not occur, will reveal all lasting interference from the environment. The range spectrum of the other M-1 lines of N points, will reveal the intermittent interferences 51, however with less sensitivity. As the interferences have a spectral band much smaller than that of the radar, the spectrum of the N point line, sampled by the Analog / Digital converter 24, directly reveals the presence of the interferences, as they appear as “towers” 29 in the frequency spectrum of the interference only signal. Elimination or blocking is done by replacing the interference towers with “zeros”, canceling the presence of all of them. The towers 29 detected through the spectrum corresponding to the line without transmission are blocked on all the next M lines. In addition, for each of the M-1 lines, towers 51 corresponding to the intermittent interference of each line will be replaced by zeros.

[0035] Right after the blocking process of the range interferences, the range compression follows. As the first FFT has already been performed, it is now sufficient to perform the multiplication with the reference function in the frequency domain and the inverse Fourier transform to complete the range compression.

[0036] The azimuth interference blocking process follows after the range compression performed on module 34. Block 35 is a buffer of

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10/11 signal forwarding to block 38 that creates the array 41 of range versus azimuth with Μ x N points. In this matrix, the return signal for each of the M pulses is represented by the instantaneous amplitude of the N samples of that signal arranged in the horizontal direction, that is, on the x axis. It obeys the same strategy as the range-locked transmission. However, the block is made in the direction orthogonal to the range, that is, in the direction of the y axis or azimuth. Thus, after acquiring M returns, N FFTs M points are made, thus obtaining N azimuth spectra with M points, corresponding to a complete radar operation sequence.

[0037] Then the respective towers 57 are identified and blocked. The elimination or blocking of interferences is done by replacing the interference towers with “zeros”, thereby canceling the presence of all of them. Due to the fact that in each of the N spectra the position of the towers can be different, the elimination of interference must be done individually for each of the N spectra. In other words, the towers 57 detected in each of the N azimuth spectra will only be used for their own removal. Thus, there will be N azimuth spectra with N different interference removal masks.

[0038] After eliminating the interference from antennas 19 and 19a simultaneously with the range compression, we have the signal only with the information of the moving targets and the "clutter", added by the thermal noise. The azimuth angle information of the target is obtained by compressing the azimuth 45. In case the antennas are rotating, the synthetic opening process is added.

[0039] Detection 46 eliminates the signal from the "clutter" in the frequency domain through a frequency threshold, which eliminates all objects

Petition 870180020176, of 03/13/2018, p. 20/30 / 11 without or with little movement through a Doppler frequency threshold. It also eliminates thermal noise in the frequency domain through the use of the CFAR (Constant False Alarm Rate) technique based on a minimum signal strength threshold, where the thermal noise is located mainly below that threshold. The detection output contains moving targets, which are shown on the 47 radar screen.

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Claims (13)

1. MOBILE TARGET MONITORING SYSTEM IN A DENSE FOREST REGION BY TERRESTRIAL RADAR with the ability to detect moving targets moving on the surface of the ground or water, where at least part of the radar signal path comprises dense forest, characterized by the fact that the attenuation of the signals, lower than that observed in free space conditions, is provided by the operation in the P band, where the propagation in the vertical plane of said signals (54) comprises successive reflections confined between the soil (56) and the foliage (52 ). 2. MOBILE TARGET MONITORING SYSTEM IN A DENSE FOREST REGION BY TERRESTRIAL RADAR capable of detecting moving targets moving on the ground or water surface, comprising a radar operating in the P band installed on the ground, characterized by the fact that the cancellation the interfering effects of UHF communication signals on the echo of the return signal, be provided by subtracting, in the frequency domain, the bands of the interfering signals (51, 57). 3. MOBILE TARGET MONITORING SYSTEM IN A DENSE FOREST REGION BY TERRESTRIAL RADAR according to claim 2, characterized by the fact that the extraction of the target echo inserted in a non-uniform return is provided by the autocorrelation of the return signals corresponding to successive scans. 4. MOBILE TARGET MONITORING SYSTEM IN A DENSE FOREST REGION FOR TERRESTRIAL RADAR Petition 870180020176, of 03/13/2018, p. 22/30 2/4 according to any one of claims 1, 2 or 3, characterized in that the resolution of the target location in azimuth is increased by means of the synthetic opening technique (SAR). 5. SYSTEM OF MONITORING MOBILE TARGETS IN REGION OF DENSE FOREST BY TERRESTRIAL RADAR according to any one of claims 1, 2 or 3, characterized by the fact that the resolution of the target location in azimuth is increased by means of the interferometry technique. 6. SYSTEM OF MONITORING OF MOBILE TARGETS IN REGION OF DENSE FOREST BY TERRESTRIAL RADAR according to any one of the claims 1, 2 or 3, characterized by the fact that it uses the transmission of pulses type chirp (48), within a determined frequencies. 7. SYSTEM FOR MONITORING MOVABLE TARGETS IN REGION OF DENSE FOREST BY TERRESTRIAL RADAR according to any one of claims 1, 2 or 3, characterized by the fact that it uses the transmission of noise pulses (49), within a given band of frequencies. 8. SYSTEM OF MONITORING OF MOBILE TARGETS IN REGION OF DENSE FOREST BY TERRESTRIAL RADAR according to any of claims 6 or 7, characterized by the fact that said frequency band is between the limits of 360MHz and 370MHz. Petition 870180020176, of 03/13/2018, p. 23/30 3/4 9. SYSTEM OF MONITORING OF MOBILE TARGETS IN REGION OF DENSE FOREST BY TERRESTRIAL RADAR according to any of claims 6 or 7, characterized by the fact that said frequency band is between the limits of 420MHz and 450MHz. 10. MOBILE TARGET MONITORING SYSTEM IN A DENSE FOREST REGION BY TERRESTRIAL RADAR according to claim 2, characterized by the fact that the elimination of range interference in a radar operation sequence consisting of M transmission and reception cycles, understanding the steps of: - transmit radar pulses (48, 49) in M-1 cycles, interrupting the transmission in M-th cycle; - receive the return signals corresponding to each of the M cycles, sampling each signal in N samples; - identify the interfering signals (51) in the return signal of the M-cycle; - replace in the frequency spectrum of the return signals said inferent signals with zeros. 11. SYSTEM OF MONITORING MOBILE TARGETS IN REGION OF DENSE FOREST BY TERRESTRIAL RADAR according to claim 2, characterized by the fact that the elimination of interfering signals in azimuth present in the M-1 return cycles of the radar signals comprising the target echo, understand the steps of: - create a matrix (41) of range versus azimuth with M x N points, where the return signal captured in each of the M cycles is represented by the instantaneous amplitude of the N samples of that signal arranged in the direction of the Petition 870180020176, of 03/13/2018, p. 24/30 4 x 4 axis; - perform N FFT's of M points by sampling in the direction of the y axis; - identify the interfering signals (57) in the frequency spectra of each of the M-1 return signals; - eliminate interference in each of the M-1 return signals by replacing the frequency bands occupied by said interfering signals in each frequency spectrum with zeros. 12. MOBILE TARGET MONITORING SYSTEM IN A DENSE FOREST REGION BY TERRESTRIAL RADAR according to claim 1, characterized by the fact that the elimination of the “clutter”, produced by the multiple reflections and refractions due to the irregularities of the propagating medium, is provided through a Doppler frequency threshold in the frequency domain. 13. SYSTEM OF MONITORING MOBILE TARGETS IN REGION OF DENSE FOREST BY TERRESTRIAL RADAR according to claim 1, characterized by the fact that the elimination of thermal noise is carried out in the frequency domain using the CFAR (Constant False Alarm Rate) technique based on minimum threshold of signal strength.