DE3222869C1 - Device for generating artificial target marks in the image of a radar with a synthetic aperture - Google Patents

Description

Device for generating artificial target marks in the image of a radar with a synthetic aperture (SAR).

The invention relates to a device for generating artificial targets in the imaging of a radar with synthetic aperture (SAR) using an in a transparent echo pulse transmitter arranged on the ground by the SAR mapped area (Transponder).

The increasing use of synthetic aperture radar (SAR) for a wide range of tasks also increasingly requires auxiliary devices accompanying the mission. These include fixed and movable artificial ones Targets and target groups on the ground or close to the ground, marking geographical points or areas or a control or calibration of operating parameters of an overflying SAR system to serve.

For this category of artificial targets, radar reflectors of a known type in the form of triple mirrors are used. Luneberg lenses or reflectors with adjustable radar backscatter cross-section are used. For a more precise examination of the functional properties of radar systems with synthetic aperture and their calibration, however, very high defined radar backscatter cross-sections in the order of approx. 40 to 50 dB m ² with corresponding dimensions of the passive art targets are required. Depending on the operating wavelength of the SAR, these dimensions are several meters, the contour accuracy of these devices requiring frame constructions of considerable weight. As a result, the use of such art targets is expensive to transport and is hindered or impossible in different geographical positions or terrain forms.

In addition, auxiliary devices of some kind are increasingly required which additional information deliver to an SAR which, due to its measuring principle, are only indirectly accessible or inaccessible to the SAR are. These include B. the ground temperatures, wind vectors or even broader current information from a target area.

With the conventional measurement, transmission and evaluation of various soil parameters, e.g. B. of meteorological or seismic data, these are taken from the known measuring networks of the various Services with the known means of telemetry with considerable expenditure on equipment and costs only to a limited extent Scope recorded. An exact temporal and spatial correlation of these methods However, sizes with the cartographic mapping of a SAR overflight is extremely complex and mostly not feasible.

In the known method for radar-coupled

ίο Friend / enemy identification (IFF - identification friend or foe) is carried out in a similar way to air surveillance with ATC (air traffic control) transponders a coded identifier is reflected back from the detected target object via a separate frequency channel and received and decoded by electronic means separate from the radar. With the equally well-known Methods of radar jammers (jammers) are particularly common in the case of the pseudo-random frequency hopping method Broadband noise transmitters with higher power levels the radar receiver into saturation.

Such known methods or electronic means are also used as radar test and calibration methods applied and are z. B. from DE-OS 26 16 770 and US-PS 30 18 478 known. There are test devices in it for radars described which are provided with an input and an input and output connection and in which simulated with the help of a signal loop arranged between the radar transmitter and the radar receiver Moving target echo signals are generated. A signal loop is used to simulate moving target echoes A real or an artificial one between the output and input terminal of the radar receiver Fixed target echoes are converted into a moving signal or are time-delayed and Doppler-shifted radar pulses generated. However, these devices relate to laboratory or operational test procedures in which the signal path remains excluded via the radar antenna and its far field and which are signal-modulated Functions are carried out at the location of the SAR.

In a coded coherent transponder with a receiver known from DE-OS 30 00 876 and a transmitter for receiving the interrogation signal and for transmitting a coherent response signal a device is provided which is used to provide a predetermined to introduce coded complex phase shift in the coherent return signal, so that thereby a number of Doppler frequency components can be represented simultaneously. With one from the DE -OS 28 13 917 known circuit arrangement are simulated echo signals for the stability measurement of radar systems generated by means of a receiving S '/ transmitting device that picks up and simulates radar transmission pulses Emits echo pulses. The receiving / transmitting device is provided with a horn antenna, which sends the incoming radar pulses to one or more decoupling and switching devices acoustic delay line and a phase modulator conducts and with a controllable delay and frequency shift re-emits as simulated radar echo pulses. One from the DE-OS 26 12 634 known method for determining the inner Running time or delay time of a transponder, which is triggered from a remote location by a first received signal are excited and can generate a first transmission signal as a function thereof, the first reception signal detected at a first point on the inner signal path of the transponder. This is part of the first

Transmitted signal converted to the frequency of the first received signal to thereby generate a second received signal to create. The second received signal is at the first point in the inner signal path of the transponder detected, the between the detection of the first received signal and the detection of the second signal Elapsed time is measured. The transponder is characterized by antenna means, hereby connected receiving means for receiving the received signal on a first radio frequency and for generating of an output signal as a function thereof, by transmitting means coupled to the antenna means for generating a transmission signal on a second radio frequency by means of which are coupled to the receiving means Means that activate the transmission means as a function of the output signal of the receiving means and by means, coupled to the antenna means, for generating a second received signal on the first Radio frequency as a function of the first transmission signal. The transponder thereby generates a second transmission signal, which compared to the first transmission signal by internal time delay or transit time of the transponder is delayed. Next is in an article by Thomson-CSF in INTERAVIA (1979/1) page 90 describes a test procedure for radar stations, which is based on an active transponder that sends a pulse at the radar's own frequency, but shifted Phase and thus conveys a radar echo that contains all the properties of a mobile object with a fixed location having. The echo is not suppressed as a fixed character and is related to position and amplitude and pulse width defined. Finally, IEEE Transactions on Aerospace and Electronic Systems Vol. AES 3 (1967/1) page 148 a transponder for simulating a radar range using the Doppler effect and a Fixed target, and in Sea Technology 7 (1974/16) page 24 describes a sea buoy with an HF radar.

These known transponder devices and methods aim to generate echoes from one or the other several point targets, which are used for friend / foe detection, for overall testing of radar devices, for distance calibration (Transit time measurement) and to check the stability of radar devices. The transponders have no relation to the generation of areas in the image of a radar device with synthetic Aperture (SAR). Here, only the decoupling between the send and receive branches, if it is a transparent transponder (no reception suppression during the transponder transmits), using conventional transmit / receive switches (circulators) (maximum decoupling only approx. 35 dB) or by using separate antennas for the transmit and receive branches. These transponders do not allow decoupling of the order of magnitude required to generate large areas high Reflecting cross section is necessary.

A transmission of non-radar information is only possible with such transponders if the radar device is equipped with special additional devices, whereby the information is available directly on the Radar echo is modulated or transmitted via a separate channel (secondary radar transponder, ATC-TR). In both cases the information transfer is clearly recognizable in the echo spectrum, since the information transferring Echo must be distinguished from real echoes in order to be extracted by the radar accessory to be able to.

The object of the invention is to create a device with which artificial target marks in the image a synthetic aperture radar (SAR) through an area on the ground mapped in a SAR arranged transparent transponders can be generated. It should use the same electronic A high radar backscatter cross-section is generated and displayed in the SAR image in such a way that that areas of a defined shape and size cover real targets within defined areas in the SAR image and thereby block an opposing SAR from viewing these areas (camouflage). Also should the device reduced in size and weight compared to the known by orders of magnitude and be easily transportable.

According to the invention, the characterizing features of the claim are to solve the problem intended.

The advantage of the invention is that the electronic design of the transponder in one In an area of suitable size, such a high radar backscatter cross-section is generated that the natural ones Backscatter cross-sections are covered by targets located in the area and thereby an opposing one SAR makes it impossible to look into this area. It is also advantageous that the reduction achieved in size and weight an application related to satellite systems and a mobile one Use is possible.

Exemplary embodiments are described below and explained by means of sketches. It shows

Fig. La shows a satellite overflight over the surface of the earth,

F i g. 1 b shows a section according to FIG. 1 a,

F i g. 2a a device of a transponder,
2b shows a section through the resonator of the transponder,

F i g. 2c left the resonator with coupling diaphragm and right a filter curve of two resonances,

F i g. 2d the cylindrical waveguide with main arm and side arm with circuit diagram, as well as resonator according to F i g. 2a,

F i g. 2e a polarization scheme of the transponder,

F i g. 3 shows a character modulator according to FIG. 2d.

In Fig. 1 shows a flyover of a satellite 1 over the earth's surface 2, from which the effect of a device of a SAR 3 can be seen by way of example. In an image strip 4 mapped by the SAR 3 with a locally high radar backscatter cross section (e.g. mountains), the brands A and Bu Bj, Bz, Bt are due to the action of the transponder 6 from the SAR 3 into the terrain or area 5 faded in (see excerpt F i g. 1 b). The mark A describes the position of the transponder 6; the brands B \. B ₂ , B ₃ , B ^ are examples of other markers that can be generated, differ in shape and position and which describe the endpoints of vectors or the calibration points of an area through their mutual positions or through their positions in relation to a known coordinate system. The mark A arises from the fact that the transponder 6 sends back the received radar signal without effective time and frequency shift at a sufficient transmission level by using an amplifier 25 (see FIG. 2d). The positions of the marks B \ to B ^ are determined by time delays, which enable the marks to be positioned transversely to the direction of flight of the SAR 3, and by frequency shifts, which fix the positions of the marks in the direction of flight relative to the position of the transponder 6. The amount

this frequency shift is limited by an overlapping of the Doppler history in the spectral range. To a certain extent, this restriction can be circumvented by shortening the Doppler history, ie by reducing the transmission time or response time of the transponder 6 and by compensating for the energy deficit that occurs as a result. The shape of the marks A and B] to B4 is generated by suitable manipulation of the signals in the time and frequency domains, the positioning and shaping being effected by electronic means (see FIGS. 2d and 3).

-F i g. 2a presents a possible technical design of a transponder 6 in principle. The received signal £ "5 passes through the antenna 7, 8 designed as a dielectric horn antenna into a downstream, cylindrically designed waveguide 9. To reduce its dimensions as much as possible, the antenna 7, 8 consists of an internally graded dielectric horn 7, the material of which has a dielectric constant of approx. e = 2.4 and a loss tangent tan of about δ = 10 ~ ⁴ has. the arranged at the base point short excitation horn 8 forms the transition between the dielectric horn 7 and the downstream waveguide 9. the dimensions of the horn or the Antennas 7, 8 are about three times and in diameter twice the operating wavelength and result in an antenna gain of about 100 ~ 20 dBi, based on the isotropic radiator. The cylindrical waveguide 9 forming the transition forms an arm of a 3 dB- Directional coupler 10 (see FIG. 2d). Via a coupling diaphragm 11, which by its rectangular shape and alignment device has a polarization-discriminating effect, the received signal ES reaches a resonator 12 arranged at the end of the waveguide 9 (see also FIG. 2c). After a frequency preselection of the received signal ES in the resonator 12, which takes into account a (possible) SAR-side frequency hopping method in its bandwidth, it is picked up at the coaxial output 13 of the resonator 12 and transferred to a line modulator 26 arranged in the housing 21 (FIG. 3 ) for signal processing and the amplifier 25 for pure level increase. The directional coupler 10 consists of a cylindrical main arm. 14 and a side arm 14 'representing a flat rectangular waveguide, which is adapted in terms of shape to the cylindrical shape of the waveguide 9 or main arm 14 and is arranged thereon. On the end facing the antenna side, the side arm 14 'is closed off by a wave absorber 15 whose field wave resistance is matched. At the opposite end, the transmitted signal SS processed from the received signal ES is fed to the coupling diaphragm 11 via a coaxial input 18 arranged on the side arm 14 'through a resonator 19 connected to it. The selection of the resonator 19 causes the emitted frequency spectrum to be limited. The transmission signal SS prepared in this way is coupled into the side arm 14 'via a coupling diaphragm 16. From this it is coupled into the main arm 14 via coupling elements 17 in such a way that firstly its electrical vector V is orthogonal to the received signal ES and thus decoupled from it and secondly is passed through the directional coupler 10 via the main arm 14 to the antenna 7, 8 and thereby additionally is directionally decoupled from the received signal ES. The transmission signal SS to the SAR 3 is emitted via the antenna 7, 8. The antenna 7, 8 is covered by a thin-walled, cylindrical plastic casing 20 for protection against the effects of the weather. In a downstream metal housing 21 between this and the waveguide 9 electronic components 37 and the batteries 38 required for power supply are arranged. The entire transponder 6 is carried by a tripod 22 and aligned with the satellite 1.

From Fig. 2b is a section through the resonator 12 of the transponder 6 with coaxial output 13, the one thereon arranged resonator 19 with coaxial input 18, the coupling diaphragm 16 arranged therein, the coupling elements 17, the coupling diaphragm 11 and a further coupling element 23 can be seen.

Ih F i g. 2c, the resonator 12 with coupling diaphragm 11, coaxial output 13 and the coupling element 23 is shown on the left and a filter curve 24 of two resonances R] and Rz is shown on the right. The resonator 12 is used twice by the excitation of two orthogonal waves, so that a filter curve 24 is produced by the critical coupling of two resonances R 1 and i 2. The received signal ES coupled in through the coupling diaphragm 11 excites a field represented by the horizontal electrical vector V ^ with the resonance point R]. A vertical field for this purpose with the electrical vector V _e 2 with the resonance point R2 is coupled via the coupling element 23 and picked up at the coaxial output 13.

From Figure 2d, the cylindrical waveguide 10 is with Main arm 14 and secondary arm 14 'arranged thereon with resonators 12 and 19 and amplifier 25, as well as a character modulator 26 and from FIG. 2e shows a polarization scheme of the transponder 6.

The polarization orthogonality of these signals is used internally to decouple the received and re-transmitted signals ES and SS of the same frequency. Therefore, their coupling to the SAR far field can only be done by rotating the polarizations of the received (Pe) and transmitted signals (Ps) by plus or minus 45 ° against the polarization of a SAR signal Psar (see Fig. 2e). In the case of reception and transmission, this takes place with a power loss of 3 dB each. To derive the internal level balance, the coaxial outputs and inputs 13, 18 are connected by the linear amplifier 25. The visual wave of the oscillation or the stability range are achieved by the product of the directional effect of the arms 14, 14 ′ acting as directional couplers 10 and the polarization decoupling in the main arm 14. For a narrow-band design (e.g. B <20 MHz), 35 dB are used for both values. With a safety margin of 6 dB, the amplification ν of the amplifier 25 is allowed this tendency of the overall arrangement to oscillate

ν = 2 ■ 35 - 6 3 = 67 dB

amount, with + 3 dB from the power distribution ocr in the side arm 14 'between the coupling elements 17 and the wave absorber 15 result.

To generate a site marker in the SAR image, which of the dynamic range of the SAR and figure stands out safely override the side lobe of the impulse response function on also by a surrounding terrain of high average backscatter cross section, a point target with the backscatter cross section Ok of about is ο κ ■ = ■ 50 dB m ² required. The backscatter cross-section a _p one as. Radar reflector used, short-circuited in the feed point and aligned to the SAR is parabolic antenna

where gp is the gain of the antenna and A _{n is} its absorption area. With a SAR operating frequency of, for example, 5 GHz, the wavelength λ - 0.06 m and the antenna efficiency of · /// · = 0.55, a parabolic mirror with the diameter is required for this purpose

4λ ² -

= 3.03 (m)

required (op = 50 dB m). The antenna gain is

= 13845,

g _P = 41.42 (dB).

At a power density on the ground of Φ - \ 0 ~ ⁱ mW m ~ ³ , which is typically generated by the SAT, is the power it absorbs

P ₁ , = η _ρ -Α _Ρ -Φ = ηρ ■ Φ · ^ - = * 4 · ΙΟ " ³ (mW)

and the effective, related to a spherical emitter, reflected power

EiRPp = gp ■ P ₁₁ = 55.4 mW = 17.4 (dB m).

To achieve the desired effect, the SAR transponder 6 should also use the emitted power, but apply with a reserve of approx. + 6 dB. So it should be achieved

Fig. 2d on an enlarged scale and in greater detail. The character modulator 26 modulates information about the radar pulses received by him ; iuf. Both the positioning and the shape or brightness of the brands can act as information carriers to serve.

The drawing modulator 26 is via isolation amplifiers

27, 27 'integrated into the signal tract. The character modulation takes place for reasons of feasibility

ίο (according to the currently known state of the art) at a Intermediate frequency of approx. 10 MHz. From mixers

28, 28 'is the signal from the receiving frequency level to an intermediate frequency level and back transformed. The required mixing frequencies are generated in an oscillator unit 29.

A tapped delay line 30 is from the parallel processing lanes via switch 31 selectable. This enables the delay time to be preset (e.g. in four stages), which is necessary for the positioning of the brands. A fine adjustment of this delay time takes place by continuously controllable delay elements 32. A subsequent mixer 33, which by a continuously variable oscillator 34 is controlled, generates the frequency offset for the mark positioning. The signal then passes through an adjustable mark module 35 that takes the shape of the mark generated by putting them together from closely spaced individual points that look after the same Laws are positioned, which are already described in the positioning of the brand became. The last stage in the parallel processing branches is an amplifier 36 with a continuously controllable Degree of reinforcement provided. This means that the brightness of the respective brand can be set in the SAR image.

For this purpose 3 sheets of drawings

EiRP ₇ = EiRPs
(dB m).

4 = Ps \

gr

= 23

Which is from the horn and from the transponder antenna 7, 8 with the gain of g _T = 100 (20 dB) from the power density Φ = 10- ³ m ~ ² mW ocp under the polarization loss = 0.5 absorbed power

= 1.4 x 1 (T ⁵ (mW).

The necessary transmission power at the coaxial socket 18 (FIG. 2a) is then

_D A ₁ _ EiRPr _{= A}

CCr- dp- g _T

100

For this purpose, the gain ν of the amplifier must be 25

"'= Ik. ίο Ig = 57.5 (dB).

This can meet the requirements for the Richtffektimg of the R Richt couplers 10 and the arms 14, 14 'and the polarization discrimination of the coupling diaphragm 11 by 4 dB each in favor of a broadband design of the waveguide 10 can be withdrawn.

A character modulator 26 is shown in FIG

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Claims (1)

Claim: Device for generating artificial target marks in the image of a radar with synthetic aperture (SAR) by means of a transparent echo pulse transmitter (transponder) arranged on the ground in an area mapped by the SAR, characterized in that the transponder (6) has a powerful and electronic design , with which in an area (5) with a suitable edge length (Bu B ₂ , B ₃ , B ₄ ) such a high uniform radar backscatter cross section (σκ) is generated that this the natural backscatter cross sections {on) from in this area (5 ) covers existing targets and thereby blocks an opposing SAR (3) with known operating parameters from viewing the area (5).