WO2019173886A1

WO2019173886A1 - Land radar mobile target tracking system in dense forest region

Info

Publication number: WO2019173886A1
Application number: PCT/BR2019/000009
Authority: WO
Inventors: Edson Cesar Dos Reis; João Roberto MOREIRA NETO
Original assignee: Embraer SA
Current assignee: Embraer SA
Priority date: 2018-03-13
Filing date: 2019-03-12
Publication date: 2019-09-19
Anticipated expiration: 2020-09-13
Also published as: BR102018004960A2

Abstract

A land radar mobile target tracking system in dense forest region capable of detecting mobile targets moving on land or water surface, where the attenuation of the P-band signals, is lesser than that noted under open space conditions, the propagation in the vertical plane of the radar signals (54) being confined between the ground (56) and the foliage (52). Any radio interference from third party equipment in UHF over the signals received by the radar, is suppressed by subtraction, in the frequency domain, of the bands (51) of the interfering signals within range. After compression within range (34) the same process of removal is carried out towards the azimuth (38), followed by compression (45) for obtaining high azimuth angle resolution. Detection (46) and visualization (47) follow. The antenna can be fixed, lighting a sector, or eccentric rotary, such that a synthetic aperture can be formed. To identify said bands with greater sensitivity, the system stops transmitting the radar signal upon every M pulses, maintaining the system in reception mode for a period equal to the one used to capture the signals reflected by the target in response to the pulses emitted by the radar. The signal received under these conditions is solely due to the interferences, not containing the echo from the target nor the clutter due to the irregularities of the propagating means. Once the frequency range of said interference is identified, it is substituted by zero in the frequency spectrum of the radar signal containing the echo from the target.

Description

LAND RADAR MOBILE TARGET TRACKING SYSTEM IN DENSE FOREST REGION

Field of the Invention

[001] The present invention refers to the art of detecting mobile targets by means of radar and, more particularly, to the detection of said targets situated in dense forest regions, such as, for example, the Amazon rainforest .

Summary of the state of the art

[002] Detecting mobile targets is crucial in the tracking of land regions and waterways to order to curb unlawful activities such as the trafficking of drugs, firearms, people and theft of cargo, and others. Said objectives are mainly designed to detect road and waterway vehicles by means of systems that use fixed, mobile or transportable radars.

[003] Although the airborne or satellite radars present major coverage scope, their use is limited to detecting targets that move on the surface at low speeds, in the order of a few meters per second. Additionally, in the dense jungle regions, such as found in the equatorial zone, the echo from the target may be masked by clutter originating from the multiple reflections and refractions from the vegetation .

[004] The obstacles presented by dense vegetation to the propagation of the electro-magnetic signals require the use of the P-band for this application. However, it so happens that the P-range comprises the UHF communication band, generally used in this region. Said communication signals, which are also picked up by the radar antenna, may have a strength such that they produce o blockage do low noise amplifier in the front end, with the consequent interruption of the working of the system.

Objectives of the invention [005] In light of that expounded, it is a first objective of the invention to provide a radar system capable of detecting mobile targets moving on land or water surface.

[006] Another objective is to provide a radar system that can detect mobile targets in places covered by dense vegetation, such as a equatorial rainforest, even in the presence of irregular echoes caused by reflections and refractions in the forest. Operation inside the forest is also foreseen, that is, both the radar and the targets may inside or outside the forest.

[007] Yet another objective is to provide a radar system that is immune to the interferences of UHF communication signals.

Brief summary of the invention

[008] The objectives cited, in addition to others, are achieved by the invention by providing a radar operating in P- band, installed on the ground, endowed with means to cancel the interfering effects of the UHF communication signals.

[009] According to another characteristic of the invention, said cancellation is provided by the subtraction, from the echo of the feedback signal, of one or more bands of interfering signals.

[0010] According to another characteristic of the invention of the invention, extracting the echo from the target included in non-uniform feedback is provided by the autocorrelation of the feedback signals corresponding to sweeps .

[0011] According to another characteristic of the invention, the resolution of the location of the azimuth target is enhanced by the synthetic aperture technique (SAR) and/or interferometry.

Description of the drawings

[0012] The other characteristics and advantages of the invention will become more obvious from the description of a preferred embodiment, given as a non-limiting example, and from the related drawings, wherein:

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

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

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

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

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

Detailed description of the invention

[0018] The present invention takes advantage of the fact that propagation of electro-magnetic radiations of certain frequencies in dense forests undergoes lesser attenuation than that noted under open space conditions.

[0019] Indeed, in the open space an electromagnetic wave propagates according to a sphere, the ratio between the strengths received and transmitted by a radar system being given by the expression:

K- Pt · G²

Pr = -

R⁴

wherein :

P_t is a strength of the transmitter;

P_r is a strength received;

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 cases where the radar signal traverses a dense forest, its propagation on the vertical plane is confined between the ground 56 and the foliage 52, as exemplified in Fig. 3, where the propagation comprises successive reflections, the attenuation of the radar signal being proportional to (1/R) and no longer to (1/R²) as in the case of spherical propagation. However, it so happens that the signal undergoes further attenuation due to the losses in the reflections on the ground and in the treetops, and also the fact of propagating in the midst of vegetation; consequently, the ratio between the received and transmitted strengths is hereafter as follows:

K· P_t · G²

_P5_r = -

R⁴

[0021] The above expression is valid for the radar signals in 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 that operate in P-band. The first relates to the fact that diverse communication services operate in UHF, such as, for example, radio amateurs, the 70cm range of which covers the spectrum that goes from 430MHz to

440MHz. The radiation from this service may interfere with the radar signals, additionally and potentially creating a blockage by saturating the amplifying stages of the system, chiefly by saturating the low noise amplifier 23. Such blockage is avoidable by the strategy of adjusting the variable attenuator 21 described ahead and presented in Fig. 2.

[0023] A second problem refers to the fact that the propagation inside the forest is subject to multiple reflections and refractions due to the irregularities of the propagating means, referred to herein as clutter, meaning the feedback signal captured by the antenna not only has the feedback signal from the mobile target but also the feedback signal from the clutter. The signature of the clutter presents itself as apparently random noise. The intensity of the clutter is almost always greater than that of the mobile target, and may be more than 50 dB more intense. The system now proposed comprises means that allow the echo to be extracted from the mobile target of the clutter as explained in detail ahead.

[0024] The system of the invention is devised for detecting moving targets only, referred to here as mobile targets. The targets may be directly aimed at the radar, may be inside the forest or may be in the open air and have a forest between them and the proposed system. The proposed system may be located inside or outside the forest.

[0025] The proposed system has two sets of antennas, 19 and 19a, as presented in figure 3. Antenna 19 is used for transmitting and receiving and antenna 19a is used for receiving only. There is a horizontal baseline between the two antennas, the size of which is in the order of magnitude of 1 m and is defined according to the required accuracy of the azimuth angle.

[0026] There are two basic arrangements of the proposed system, where the antennas are immobilized or follow a circular movement. The arrangement where the antennas follow a circular movement is documented in patent application BR 10 2012 003900 1 of the radar Sentir-M20, the teachings of which are now incorporated in their entirety. In the mobile arrangement, the antennas light just one sector, which has an azimuth aperture of approximately 100 degrees. For complete azimuth coverage, it is necessary to implement 4 panels of antennas, where each antenna covers a sector of 90 degrees. The advantage of this solution is the non-existence of mobile parts, increasing the availability of the system. The arrangement where they follow a circular movement only requires one panel of antennas, having a lower cost, but a lower availability than the solution without mobile 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 now described, generates pulses of 20^As with repetition frequency of 5kHz, in the bands of 360-370MHz and 420-430Mhz. Said pulses are of two types: the first 48 is a signal chirp between the frequency limits of the bands (360-370MHz and 420- 450Mhz) . The second is a noise pulse 49, also comprised between said frequency limits. The choice between one or the other depends on the circumstances, such as the greater or lesser possibility of detection by a potential enemy. This signal is amplified by the linear amplifier 17 and forwarded to the antenna 19 through the circulator 18.

[0028] The signal reflected by the mobile target and by the clutter return both to antenna 19 and antenna 19a. Antenna 19 is used both for transmitting and receiving. Antenna 19a for receiving only. There is a horizontal baseline between antennas 19 and 19a, such that they receive the same signal, but with different phases, assuring the feasibility of the accurate measure of the azimuth angle.

[0029] Considering that the work environment of the radar receives external interferences from other transmitters, antennas 19 and 19a will receive them, besides the signals from the mobile target and from the clutter. Should the intensity of the interference be high, the first amplifier of the reception chain, the so-called low noise amplifier 23, or 23a in the case of antenna 19a, it may become saturated. To assure a feasible operation in the linear range, the signals coming from antennas 19 and 19a are attenuated with the attenuators 21 and 21a respectively. The procedure of adjusting the attenuators is presented in Fig. 3. The intensity of the input signal from the amplifier is measured and the respective attenuator is adjusted, such that it does not saturate and always operates in the linear range. This procedure is carried out separately for amplifiers 23 and 23a. Even when they are operating in the linear range, the intensity of the interference may still prevent the mobile targets from being detected.

[0030] For this, there are two phases of detecting interference and the respective elimination thereof. The first phase occurs prior to compression within range or scope. The second phase occurs after compression within range, but prior to azimuth compression. The interference elimination processes are carried out independently for the signals coming from antennas 19 and 19a.

[0031] The flow of the feedback signal from the mobile target follows the paths of the two receivers equally and simultaneously. The feedback signal from the mobile target is added to the signal from the clutter, to the thermal noise and to the external interference, and enters through antennas 19 and 19a. Thereafter it passes through the circulator 18 and arrives at the variable attenuator 21. For the simplicity of the description, only the chain of the variable attenuator 21 up to the entry of the azimuth compression 45 will be described, since the respective chain of antenna 19a is identical. The attenuator adjusts the intensity of the signal coming from the mobile target, of the clutter and of the interference; the limitators 22 and 22a prevent the amplifiers 23 and 23a from being damaged. The output of the amplifiers 23 and 23a, which work in the linear region, is converted into a digital signal through the analogical-digital converters 24 and 24a, each one of which executes N samplings within range or scope.

[0032] The description that follows cites the modules that process the signals picked up by the antenna 19, but it is understood that the same steps are valid for the signals coming from antenna 19a.

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

[0034] The interference elimination module within range or scope works with two operating modes of the radar, for enhanced detection and elimination of the interference. The operating sequence of the radar comprises M cycles, a radar pulse being transmitted in each cycle and where the feedback signal (echo) of each pulse is sampled in N samples. However, with each M cycle the radar does not transmit any pulse at all. Therefore, there are M-l feedback signals with echoes from the mobile target, from the clutter and from the external interference and an M-th feedback signal having just the interference signal, because no signal transmission occurred for this feedback. The spectrum of the line of feedback of N points, or spectrum within range of the feedback signal where said transmission did not occur, will reveal all the long-lasting interferences of the environment. The spectrum within range of the other M-l lines of N points, will reveal the intermittent interferences 51, but with lower sensitivity. As the interferences have a much smaller spectrum band than that of the radar, the spectrum of the line of N points, sampled by the Analogical/Digital converter 24, directly reveals the presence of the interferences, as they appear as "towers" 29 in the frequency spectrum of the interference signal only. The elimination or blockage is done by substituting the interference towers for "zeros", cancelling the presence of them all. The towers 29 detected by the spectrum corresponding to the line without transmission are blocked in all the following M lines. Additionally, for each of the M-l lines, the towers 51 corresponding to the intermittent interferences of each line will be substituted for zeros.

[0035] Soon after the blockage process of the interferences within range, the compression within range follows. Since the first FFT has already been carried out, it is now sufficient to perform the multiplication with the function of reference in the frequency domain and the inverse Fourier transform to conclude the compression within range.

[0036] The process of blocking the azimuth interference follows after the compression within range is performed in the module 34. The block 35 is the forwarding buffer of the signal for the block 38 that creates the range matrix 41 versus azimuth with M x N points. In this matrix, the feedback signal from each one of the M pulses is represented by the instantaneous amplitude of the N samples of this signal disposed horizontally, that is in the x axis. It follows the same strategy of the blockage within range with transmission. However, the blockage is made orthogonally to that of the range, that is, towards the y axis or azimuth. Therefore, after acquiring M feedback, N FFTs M points are carried out, whereby obtaining azimuth N spectrums with M points, corresponding to a full operating sequence of the radar .

[0037] Thereafter, the respective towers 57 are identified and blocked. The elimination or blockage of the interferences is carried out by substituting the interference towers for "zeros", whereby cancelling the presence of them all. Due to the fact that in each one of the N spectrums the position of the towers may be different, interferences must be eliminated individually for each one of the N spectrums. In other words, the towers 57 detected in each of the azimuth N spectrums will only be used for their own removal. Accordingly, there will be azimuth N spectrums with different interference removal N masks.

[0038] After eliminating the interferences from the antennas 19 and 19a simultaneously with the compression within range, we only have the signal with the information from the mobile targets and from the clutter, added by the thermal noise . The information from the azimuth angle of the target is obtained by azimuth compression 45. If the antennas are rotary, the synthetic aperture process is added.

[0039] The detection 46 eliminates the signal from the clutter in the frequency domain by a frequency threshold, which eliminates all the objects without or with little movement by way of a Doppler frequency threshold. It also eliminates the thermal noise in the frequency domain by using the CFAR (Constant False Alarm Rate) technique based on a minimum intensity threshold of the signal, where the thermal noise is located mainly below this threshold. The output of the detection contains the mobile targets, which are presented on the radar screen 47.

Claims

1. A LAND RADAR MOBILE TARGET TRACKING SYSTEM IN DENSE FOREST REGION capable of detecting mobile targets moving on land or water surface, where at least a part of the path of the radar signals comprises dense forest, characterized in that the attenuation of the signals, lesser than that noted under open space conditions, is provided by operating in P- band, where the propagation on the vertical plane of said signals (54) comprises successive reflections confined between the ground (56) and the foliage (52) .

2. A LAND RADAR MOBILE TARGET TRACKING SYSTEM IN DENSE FOREST REGION capable of detecting mobile targets moving on land or water surface, comprising a radar operating in P- band installed on the ground, characterized in that cancelling the interfering effects of the UHF communication signals no echo of the feedback signal, is provided by subtracting, in the frequency domain, the bands of the interfering signals (51, 57) .

3. The LAND RADAR MOBILE TARGET TRACKING SYSTEM IN DENSE FOREST REGION according to claim 2, characterized in that extracting the echo from the target included in a non- uniform feedback is provided by the autocorrelation of the feedback signals corresponding to successive sweeps.

4. The LAND RADAR MOBILE TARGET TRACKING SYSTEM IN DENSE FOREST REGION according to any one of claims 1, 2 or 3, characterized in that the resolution of the location of the azimuth target is enhanced by the synthetic aperture technique (SAR) .

5. The LAND RADAR MOBILE TARGET TRACKING SYSTEM IN DENSE FOREST REGION according to any one of claims 1, 2 or 3, characterized in that the resolution of the location of the azimuth target is enhanced by the interferometry technique.

6. The LAND RADAR MOBILE TARGET TRACKING SYSTEM IN DENSE FOREST REGION according to any one of claims 1, 2 or 3, characterized in that it uses the transmission of chirp-type pulses (48), within a given frequency band.

7. The LAND RADAR MOBILE TARGET TRACKING SYSTEM IN DENSE FOREST REGION according to any one of claims 1, 2 or 3, characterized in that it uses the transmission of noise pulses (49), within a given frequency band.

8. The LAND RADAR MOBILE TARGET TRACKING SYSTEM IN DENSE FOREST REGION according to any one of claims 6 or 7, characterized in that said frequency band is comprised between the limits of 360MHz and 370MHz.

9. The LAND RADAR MOBILE TARGET TRACKING SYSTEM IN DENSE FOREST REGION according to any one of claims 6 or 7, characterized in that said frequency band is comprised between the limits of 420MHz and 450MHz.

10. The LAND RADAR MOBILE TARGET TRACKING SYSTEM IN DENSE FOREST REGION according to claim 2, characterized in that the elimination of the interference within range in an radar operating sequence consisting of M cycles of transmission and reception, comprises the steps of:

- transmitting radar pulses (48, 49) in M-l cycles, interrupting the transmission in the M-th cycle;

- receiving the feedback signals corresponding to each one of the M cycles, sampling each signal in N samples;

- identifying in the feedback signal of the M-th cycle the interfering signals (51) ;

substituting in the frequency spectrum of the feedback signals said interfering signals for zeros.

11. The LAND RADAR MOBILE TARGET TRACKING SYSTEM IN DENSE FOREST REGION according to claim 2, characterized in that the elimination of the azimuth interfering signals present in the M-l feedback cycles from the radar signals comprising the echo of the target, comprises the steps of: - creating a range matrix (41) versus azimuth with M x N points, where the feedback signal captured in each one of the M cycles is represented by the instantaneous amplitude of the N samples of this signal disposed towards the x axis;

- performing N FFT ' s of M points by sampling towards the y axis;

identifying the interfering signals (57) in the frequency spectrums of each one of the M-l feedback signals;

- eliminating the interferences in each one of the M-l feedback signals substituting the frequency ranges occupied by said interfering signals for zeros in each frequency spectrum.

12. The LAND RADAR MOBILE TARGET TRACKING SYSTEM IN DENSE FOREST REGION according to claim 1, characterized in that the elimination of the clutter, produced by the multiple reflections and refractions due to the irregularities of the propagating means, is provided by way of a Doppler frequency threshold in the frequency domain.

13. The LAND RADAR MOBILE TARGET TRACKING SYSTEM IN DENSE FOREST REGION according to claim 1, characterized in that the thermal noise is eliminated in the frequency domain by using the CFAR (Constant False Alarm Rate) technique based on a minimum intensity threshold of the signal.