WO2012009780A1

WO2012009780A1 - System and method for detecting and locating punctiform, distributed targets

Info

Publication number: WO2012009780A1
Application number: PCT/BR2011/000244
Authority: WO
Inventors: Joao Roberto Moreira Neto
Original assignee: Orbisat Da Amazónia Industria E Aerolevantamento S/A
Priority date: 2010-07-22
Filing date: 2011-07-22
Publication date: 2012-01-26
Also published as: BRPI1002542A2

Abstract

System and method for detecting and locating punctiform, distributed targets, which comprises two non-directional stationary antennas with high range resolution using a coherence test (Γ) for locating differentiated targets (A, B), one of said stationary antennas being a transmitter/receiver (TR-N, TR-L, TR-S, TR-O, B1, C1, D1, A2) and the other being only a receiver (RA-N, RB-N, RA-L, RB-L, RA-S, RB-S, RA-O, RB-O). Antennas (30) with 120 degrees of azimuth opening (31), directed every 90 degrees, are preferably used.

Description

TARGET DETECTION AND LOCATION SYSTEM AND METHOD

POINTS AND DISTRIBUTED

Field of the invention

The present invention relates to a ground radar, composed of a set of fixed antennas, which measures the range, azimuth, elevation and velocity of point and distributed targets.

Description of the prior art

Usually known radar equipment such as exemplified in Figures 1a, 1b and 1c use rotary antennas 11 to scan the desired coverage area. Such rotary antennas emit a directional beam 16 of radio waves which, upon reaching a solid obstacle 17, are reflected by it and picked up back on the same antenna. Such effect is treated by special circuits 14 allowing the visualization 15 of the contour and size of the object hit, as well as its location and distance from the antenna. In the case of moving objects, the system also allows to capture information about its path and speed.

To fully sweep a large coverage area such antennas 11 continuously rotate 360 ° about their vertical axis 12. They are equipped with motors 13 and control mechanisms with complex mechanics that, in addition to allowing movement, ensure electrical contacts. operation of the radio equipment and its connection to the antenna waveguides and feeders.

When the targets are at different altitudes, it is necessary to provide mechanisms that modify the beam elevation angle. This is achieved by providing a hoisting motor 18 which, as shown in Fig. 1-c, moves the antenna 1 second as indicated by the arrow, so as to vary the vertical angle of the beam, which in one direction 19 detects target 20 and in another direction 21 detects target 22.

The mechanical complexity of such an arrangement makes the system more expensive and increases vulnerability to failure and enemy attacks in the case of military equipment involved in armed conflict. In order to minimize such deficiencies static antennas have been developed which do not require movement and are especially suitable for military activities. Such antennas are provided with numerous element-shaped feeders spaced out on a cylindrical or polyhedral surface. Said feeders are actuated sequentially through a switching circuit thus obtaining the same sweeping effect of the rotating antennas.

However, to achieve a result equivalent to rotary antennas, the static antenna needs a large number of elements that is greater the higher the reading accuracy desired for the radar equipment. Since each element is actually a complete radio transceiver such technology results in even higher cost antennas than rotary antennas making their widespread use economically unfeasible.

In this regard, the development of lower cost or compatible static antennas with conventional rotary antennas is attractive as long as the accuracy and reliability requirements are not affected. Objectives of the invention

In view of the above, it is the first object of the present invention to provide a new type of low cost radar.

It is the second object of the invention to provide a new type of radar which, while having low cost, is capable of making range, azimuth, elevation and velocity measurements of point or distributed targets.

It is a further object of the invention to provide a new type of fixed radar that completely eliminates the mechanical complexity of conventional radar.

Brief Description of the Invention

The stated objectives, as well as others, are achieved by the invention by providing a new type of radar composed of non-directional fixed antennas, each antenna providing coverage of a given vertical and horizontal angle.

According to another feature of the invention, said radar has a minimized and optimized set of fixed antennas.

According to another feature of the invention, the horiontal coverage of 360 degrees is obtained with a small number of fixed antennas.

According to another feature of the invention, said radar allows to accurately calculate the range, azimuth, elevation and velocity of point or distributed targets.

Advantageously, all the mechanical complexity of conventional mobile radars has been eliminated.

According to yet another feature of the invention, said radar uses a sophisticated computational base to perform the necessary calculations in detecting said targets.

Brief Description of the Figures

The features and advantages of the present invention will be better understood by describing a preferred embodiment, given by way of illustration and not limitation, and the related figures, in which:

Figure 1 shows the block diagram of the components of a conventional system (figure 1a) as well as simplified horizontal (figure 1-b) and side (figure 1-c) views.

Figure 2a illustrates the horizontal coverage angle of an isolated antenna of the type used in the invention compared to the coverage angle of a known antenna as well as the main array of 360 ° horizontal coverage fixed antennas, Figure 2. -B.

Figure 3 illustrates an example of an application of the invention to a typical airborne ground radar system.

Figure 4 illustrates an example of application of the invention to a typical embedded marine radar system.

Figure 5 shows an illustrative diagram of the triangulation of two different targets effected by two fixed antennas as performed by the invention.

Figure 6 shows an illustrative diagram of signal processing by emitting pulses emitted by transmitting antennas and captured by transmitting and receiving antennas.

Figures 7a and 7b illustrate the process flow chart.

Figure 8 illustrates the application for a weather radar.

Detailed Description of the Invention The features and advantages of the present invention will be better understood by describing a preferred embodiment, given by way of illustration and not limitation, thus:

The present invention is applicable to weather radar; land, sea and air surveillance radars and also in maritime navigation radars.

The patent application radar system of this patent is capable of making a two-dimensional (range and azimuth) or three-dimensional (range, azimuth and elevation) representation of point and distributed targets.

Point targets are isolated targets. As an example we have the aircraft in flight.

Distributed targets, on the other hand, are exemplified by the area around us: fields, urban areas, seas, lakes, and forests.

Considering first the case of two-dimensional radars, exemplified by marine radars, said radar is composed of a first set of fixed transceiver antennas and a second set of receiver-only antennas. The antenna sets and are interconnected to a dedicated electronic module that computes the information received from said antennas and displays the result on a monitor screen.

As shown in Fig. 2-a, the basic element of the system is an antenna 30 whose radiation diagram 31 covers an angle 32 of approximately 120 degrees in azimuth. For comparison purposes only, the radiation diagram 33 of a conventional antenna commonly used in known radar systems covers a substantially smaller angle 34. In the proposed system, the array 35 of fixed antennas is implemented by four antennas, each of which has a diagram of approximately 120 degrees in azimuth, resulting in the radiation diagram 36, which illuminates the entire sector in question, ie. 360 degrees as shown in Fig. 2-b.

As shown in Figure 3, in a terrestrial radar system the four transceiver antennas that make up array 32 are mounted on a pole or tower and designated as TR-N, TR-L, TR-s and TR-O, facing, respectively to the north, east, south and west. The four antennae of the receiver assembly are mounted on different poles or brackets at a convenient distance from the transceiver antennas to form baselines β long enough to allow good triangulation of the targets to be detected. The transceiver and receiver antennas work together, two by two, as exemplified in Figure 3. Thus, for example, in this figure, the baseline βΑ is the distance between the south-facing TR-S transceiver antenna of the center assembly, and the RA-S receiving antenna, also facing south. The same principle applies to antenna assemblies mounted on a vessel, as shown in Fig. 4, where the baseline βΒ is the distance between the transceiver antenna B1 and the receiving antenna. B2, both facing starboard.

Said target triangulation is illustrated in Figure 5. Two antennas 41 and 42 working together can accurately track two targets A and B by azimuth triangulation of these targets. Antenna 41 which is transceiver emits radio signal pulses which, upon reaching targets A and B, are reflected by them back to the same antenna 41 and also to antenna 42 which is only receiver.

Thus we have signal Sx received by antenna 41 (path riA) and signal Sx 'received by antenna 42 (path r ₂ A). Due to the differences between the paths, both signals are present in the respective antennas with different return times.

Thus, the angle Θ, shown in figure 5b, is precisely determined because we have the values Sx, Sx 'and β. Where β is the distance or baseline between antennas 1 and 2.

Through the computational analysis of the interferometry of the differences in the time it takes the transmitted signal to reach target A and return to both antennas 1 and 2, performed by the electronics module computer (not shown in this figure), we determine the azimuth of the target. The one that can be viewed on the monitor screen. The process is described in more detail as follows.

Cp = d is considered. (-4π / λ), that is, a departure from target d in range causes a phase change of d. (-4π / λ). Considering the geometry of Fig. 5, the triangle formed between antenna 1, antenna 2 and target A is given by: Γ | Α, r ₂ A and β. β is precisely known as it can be measured physically. The precise difference between Γ | Α and r ₂ A, here called ΔΓ, is determined by interferometry as follows:

(i) the signal is received

of the antenna 2.

φ1 = (-4π / λ). ria and q> 2 = (-4π / λ). r2a

check whether S1 and S2 are from the same target by calculating the coherence Γ of 4d.

ii) If the consistency is low Γ ~ 0, this pair of signals is ignored.

If the coherence is high Γ ~ 1, the phase is calculated:

Δ φ = arctan (S1 - S2).

iii) Developing:

S1 S2 = A1 and ^{i <p1} . Α2 ^ ^φ2

S1 S2 = A1. A2. β · * ^{1 -} ^

Δ φ = arctan (A1. A2. And ^ ^{1 "ç2)} )

Δ φ = φ1 - φ2 from where

Δ φ = (-4π / λ) (ria - r2a) = (-4π / λ). ΔΓ

Thus, by measuring Δφ through the tangent arc of the multiplication of S1 by the conjugate complex of S2, the difference ΔΓ between P | A and ₂ A is precisely calculated.

ΔΓ = - (λ / 4ττ). Δ φ

Constructing the triangle with β, ΓιΑ er ₂ A = A + ΔΓ

iv) With the three precise sides of the triangle the angle Ga, which is the azimuth measurement, is calculated with the cosine law

The detection of the horizontal position is then given by the range and the azimuth angle ΘΑ. This required a horizontal baseline. With riA and ΘΑ determine the position of target A having known the position of antenna 1.

Detection of the vertical position is done with the same method as the horizontal position determination. This requires a vertical baseline. The vertical elevation angle is calculated in the same way as the horizontal azimuth. Knowing the height of antenna 1 and ria calculates the height of target A.

Speed is determined by the following steps:

(i) Doppler frequency measurement fd by spectral analysis of the target signal.

(i) frequency to speed conversion with

Vr = fd. λ / 2

Where Vr is the radial velocity of the target.

Figures 7A and 7B constitute the block diagram of the localization algorithm. We consider that signals S1 and S2 are being sampled correctly, such that the coherence and phase measurements can be made precisely.

A two-dimensional signal matrix is generated for S1 and S2, because for both:

(i) the return signal is range sampled and has a resolution given by õr = cT / 2.

ii) several pulses are transmitted and their returns in range sampled. Thus we have several return signal lines that form the second dimension.

Regarding the resolution provided by the system can work with 300 MHz bandwidth, which provides a pulse of 3.3 Ns. This pulse provides a resolution in range of 50cm. Depending on the application the resolution may be larger or smaller than 50cm.

Fig. 8 illustrates the application for a weather radar where antennas point north, south, east and west.

In terrestrial radar application there is no need for height determination. There is thus the same block diagram as Fig. 8.

In navigation radar application, the system block diagram is the same as in Fig. 8.

Comparing in the prior art, the advantages of the invention are apparent as follows: • Weather radar

E. Technique: Azimuth-controlled mobile antennas with elevation close to 2 meters. For example: a 2m diameter satellite dish.

Innovation: 4 transmitter / receiver antennas and 8 receiver-only antennas, all fixed and with dimensions close to 10 cm X 10 cm. Horizontal baseline close to 30m and vertical baseline close to 30m. Thus we have for a range of 20Km: õazimute = (0.5m / 30m) ..20.000 = 333m

Elevation = (0.5m / 30m). 20,000 = 333m

which corresponds to beam widths of:

9a = (333m / 20,000m). 57 = 1 ^o

Ge = (333m / 20,000m). 57 = 1 ^o

The great advantage of the innovation is that 12 small fixed antennas replace a large, 2m diameter mobile antenna with equivalent resolution.

• Ground radar:

E. Technique: 1 m mobile azimuth antenna.

Innovation: 4 transmit / receive antennas, and 4 only receive antennas. Only 15m horizontal baseline gives:

azimuth = (0.5m / 15m). 20,000 = 666m

2 corresponding to ^the beam width equivalent to 1m of the antenna beam. ·.

• Marine Navigation Radar:

E. Technique: 0.5m azimuth mobile antenna.

Innovation: the same implementation as the previous example, with 7.5m baseline, producing .de · 4 ^ixe beam equal to that of 0.5m antenna.

As shown in figure 5, we can have simultaneously in the coverage area of the antennas more than one target, in the case illustrated the targets A and B.

It is essential that the system can differentiate these targets from other. This objective is achieved by analyzing the coherence between signals received from different targets. Signals from different targets appear on the same receiver antenna with different phases.

The signal received from target A appears as:

Sx = Tx and ^{i <px}

Already the signal coming from target B appears as:

Sy zr Ty eW

Therefore the signal phases are different. Dividing the conjugate by the normalized of these signals, we have:

r _ J <Sx _* Sx *> * <Sy _* Sy *>

And calculating the interferometry between these phases:

9interf = <^ ^ y *>

Thus, we have the identification and determination of the position of different targets within the coverage area of the antenna system.

Therefore, azimuth resolution is achieved by interferometric processing of two or more antennas with horizontal baselines. This system is suitable for marine radars where target azimuth determination is sufficient. Figure 4 illustrates the typical arrangement of antennas on a boat capable of detecting the position and velocity of punctual and distributed targets on the water surface.

Target speed information is obtained by measuring the doppler frequency of a sequence of return signals from the same target.

In the case of aeronautical or weather radar it is also necessary to determine the elevation of the target in relation to the ground. This requires an additional set of receiver antennas mounted on the same masts as the first set already described. Such additional antennas are fitted to the masts at a convenient vertical distance to form a vertical baseline with said antennas of the first set. Thus, using the same calculation methodology, the system is now able to determine target elevation. Summing up:

• Range resolution is achieved by pulse or CW modulation.

• Azimuth resolution is achieved by interferometric processing of two or more antennas with horizontal baselines.

• Elevation resolution is achieved by interferometric processing of two or more antennas with vertical baselines.

• Target speed information is obtained by measuring the doppler frequency of a sequence of return signals from the same target.

While the invention has been described on the basis of some preferred exemplary embodiments, those skilled in the art may make modifications within the basic inventive concept, particularly in relation to the shape and dimensions of the antenna. Accordingly, the invention is defined by the following set of claims.

Claims

1. DISTRIBUTED POINT DETECTION AND LOCATION SYSTEM, characterized in that it comprises at least two non-directional fixed antennas with high range resolution using coherence testing to locate differentiated targets.

SYSTEM according to claim 1, characterized in that one of said fixed antennas is transmitter and receiver (TR-N, TR-L, TR-S, TR-O, B1, C1, D1, A2), and the other being receiver only (RA-N, RB-N, RA-L, RB-L, RA-S, RB-S, RA-O, RB-O).

A system according to claim 2, characterized in that said antennas (30) have an aperture (31) of approximately 120 degrees in azimuth.

SYSTEM according to claim 3, characterized in that it comprises a set (32) of 4 transmitter / receiver antennas (TR-N, TR-L, TR-S, TR-O) directed every 90 degrees. and 4 receiving antennas (RA-N, RB-N, RA-L, RB-L, RA-S, RB-S, RA-O, RB-O) equally directed every 90 degrees, with distanced receiving antennas being said respective transmitter / receiver antennas.

SYSTEM according to Claim 4, characterized in that the distances between each transmitting / receiving antenna and the corresponding receiving antenna constitute the baseline (β) for calculating the location of the detected targets.

Method of detection and location of point and distributed targets using the system defined by claims 1 to 5, characterized in that it is the azimuth of the target calculated by the interferometric method which uses the return signals (r- | A, Γ2Α). , P | B, r ₂ B) of said target captured by both antennas (41, 42) pointed in a given direction.

Method according to claim 6, characterized in that the coherence (Γ) of the signals reflected two by two by the targets is verified to identify them.

Method according to Claim 6 or 7, characterized in that the target velocity (Vr) is determined by the Doppler frequency measurement (f _d ) by means of a spectral analysis of the reflected signal.