CA2897569A1 - Method for radiometric determination of the radar cross-section of radar targets - Google Patents

Abstract

A method for radiometric calibration of the radar cross-section RCS i of a N number of radar targets T i, with i = 1, 2, ..., N, and N >= 3 is provided. At least one radar target T1 comprises a radar transmitter S1 with a transmitting antenna SA1 and a radar receiver E1 with a receiving antenna EA1, wherein the radar receiver E1 and the radar transmitter S1 operate independently of one another, a radar target T2 comprises a radar transmitter S2 with a transmitting antenna SA2, a radar receiver E2 with a receiver antenna EA2, and a unit D2, with which the radar transmitter S2, in a transponder mode TM of the radar target T2, is connected with the radar receiver E2, so that a signal S received by the radar receiver E2 is again emitted by the radar transmitter S2, and with which the radar transmitter S2, in a radar mode RM of the radar target T2, is not connected with the radar receiver E2, so that the radar transmitter S2 and the radar receiver E2 operate independently of each other.

Description

ak 02897569 2015-07-16 METHOD FOR ABSOLUTE RADIOMETRIC CALIBRATION OF THE RADAR CROSS-SECTION OF RADAR TARGETS
Technical Field The invention relates a method for absolute radiometric calibration of the radar cross-section RCS of a N number of radar targets Ti with i = 1, 2, ..., N and N
3. The invention is used in the aerospace industry for absolute radiometric calibration of the radar cross-section, in particular of active reference targets, so-called transponders.
Background Due to a constantly rising demand for remote sensing data of the earth, in particular for remote sensing data obtained with the aid of satellite-supported SAR systems such as TerraSAR-X or Sentinel-1, the quality of so-called SAR data products is becoming increasing more important. It is only when the quality of the SAR data products is guaranteed that useful information for earth observation can be derived (such as for example when observing glacier movements, the pack ice or flooding, the cutting down of tropical rain forest, the derivation of the world-wide bio mass etc.).
However, a high quality of SAR data products can only be achieved if the satellite-supported SAR systems are accurately calibrated, and since future SAR systems are to be tuned to higher accuracy, the accuracy of the reference targets required for this, and thus a sufficiently accurate calibration of these reference targets, is becoming more and more important.

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- 2 -Summary The main application of the described method initially is the absolute radiometric calibration of active reference targets, the so¨called (radar) Transponders, which are subsequently used - for the calibration of satellite-supported SAR systems. In principle the method can be used in all cases where radar cross-sections RCS of in particular active calibrating targets shall be measured. This includes for example transponders such as used for the calibration of weather radars.
In principle, there are currently three different variants of the method for determining the radar cross-section RCS of active radar targets. In the first variant, individual transponder components such as antennae and amplifiers, are measured in the laboratory followed by the calculation of the resulting radar cross-section RCS of the respective transponder. With this variant the problem lies in the high systematic measuring uncertainty of the method, which results from the multitude of individual measurements. In addition uncertainties arise from - the series connections of individual components of the transponder (antennae, converters, amplifiers etc.) after measuring, because the interfaces are not part of the original measurements.
In a second variant of the method, the transponder is measured as a radar target in a suitable RCS measuring station (inside and outside). Here the transponder is regarded as a black box, the inner workings of which are unimportant. A particular problem with this variant, apart from the usual challenges, such as the suppression of undesirable backscatter from the measuring environment, which is caused, for example, by mountings, a rotating tower etc., is the fact that this measuring system is a comparison, for which an additional reference target with a known radar cross-section RCS is required. The uncertainty, for ak 02897569 2015-07-16

- 3 -which this last-mentioned radar cross-section RCS is known, immediately puts a limit on achieving calibrations which are still accurate. In addition, with transponders with a high radar cross-section RCS and thus high amplification, an undesirable upswing may happen, if these are operated in a shielding chamber. As a result an accurate absolute radiometric calibration of the transponders in a shielding chamber is prevented.
In a third variant of the method the transponder is operated as a radar device, i.e. with an active transmitter and receiver, which is used for measuring a reference target with known radar cross-section RCS at a known distance. A problem with this variant lies in the fact that apart from the same problems as with the previous second variant, adequate filtering of the background within the time range is not possible due to the typically limited transponder band width.
In total it is true to say of the said calibration of SAR
systems that the measuring uncertainty in calibrating a reference transponder has a direct influence upon the achievable radiometric accuracy of the SAR systems to be calibrated. The more accurate the calibration standard is, the more accurately the SAR system can be calibrated.
It is the requirement of the invention to propose an improved method for absolute radiometric calibration of the radar cross-section of three or more radar calibration targets.
The invention is based on the characteristics of the independent claims. Advantageous further developments and designs are the subject of the dependent claims. Further features, possible applications and advantages of the invention are found in the description below as well as the explanation of exemplary embodiments of the invention shown in the figures.

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- 4 The requirement is met by a method for absolute radiometric calibration of the radar cross-section RCSi of a N number of radar targets Ti with i = 1, 2, ..., N and N
3. The proposed method is based on the fact that at least one radar target comprises a radar transmitter S1 with a transmitting antenna SA1 and a radar receiver El with a receiver antenna EAl, wherein the radar receiver El and radar transmitter S1 operate independently of each other. Thus the radar target Tl can be operated in a radar mode, wherein the radar target Tl can transmit radar signals and can receive radar signals independently thereof.
Further, the proposed method is based on the fact that a second radar target T1=2 exists, which comprises a radar transmitter S2 with a transmitting antenna SA2, a radar receiver E2 with a - receiver antenna EA2, and a unit D2, with which the radar transmitter S2, in a transponder mode TM of the radar target T2 is connected with the radar receiver E2, so that a signal S
received by the radar receiver E2 is again (actively) emitted by the radar transmitter S2 (almost immediately), and with which the radar transmitter S2, in a radar mode RM of the radar target T2, is not connected with the radar receiver E2, so that the radar transmitter S2 and the radar receiver E2 operate independently of each other.
Further, the proposed method is based on the fact that a third radar target T1-3 exists, which comprises a radar transmitter S3 with a transmitting antenna SA3, a radar receiver E3 with a receiver antenna EA3, and a unit D3, with which the radar transmitter S3 is connected with the radar receiver E3, so that a - signal S received by the radar receiver El is again emitted by the radar transmitter S3, or the radar target T3 is a passive radar target, which reflects an incoming signal S. A passive radar target Ti and a radar target Ti operated in transponder mode here have in common, that they reflect an incoming radar

- 5 -signal (in the first case this happens in a passive manner, in the second case in an active manner and possibly with a time delay).
- The above-mentioned properties of the at least three radar targets T1 are minimum requirements. For example all three of the radar targets T1 may be realised in such a way that they can be operated both in radar mode and in a transponder mode, and thus comprise the properties of the above-mentioned second radar target T2.
The proposed method comprises the following steps:
The method begins with the radar transmitter Sk of the radar target Tk with a transmission power PTkj sending a signal S to another radar target T. The sent signal S is received by the other radar target Tj. This radar target Tj sends or reflects the received signal S back to the radar target Tk. The signal S
emitted by the radar target Tj is received by the radar receiver 20_ Ek of the radar target Tk with the receiving power PRkj, wherein k, j e {1, 2,..., NI and k 0 j. These sequences are summarised for the following statements as a "first step" of the method.
Furthermore in a second step previously defined as the ,first step", N different pairings TkTj of radar targets Tk and T3 are executed, wherein the pairings TkTj and TjTk are assumed to be identical. The variables: transmission power PTk,j and receiving power PRIõi are recorded, respectively. Furthermore the distances Dk,3 = l(Pk -P3)1 of the radar transponders Tk and Tj, on which measuring is based, are recorded, i.e. accurately determined.
Then, in a third step, the radar transponder Tj is ascertained based on known distances Dk,3 of the radar transponders Tk and Tj during execution of the first and second step as well as based

- 6 -on ascertained ratios of PRk,j/PTk,j of the radar cross-sections RCS, on the basis of the following relationship:
GR = GT = 42 = Al_.3nr-fc =
( 1) PR = PT _____________________ k kJ
(4703 = DkJ .4 with GR: antenna gain of the receiver antenna EAk, GT: antenna gain of the transmitter antenna SAk, and 2\ wavelength of the radar signal S.
Advantageously, in performing the ,first step", the main beam direction of the radar transmitter Sk is aligned exactly with the radar target Tj or its receiving antenna. This alignment may for example be effected by means of laser measuring.
Advantageously the distances Dk,j result from the accurate detection of the positions Pk and Pj of the radar targets Tk and Tj while taking the measurements: Dk,j = I(Pk -Pj) I, wherein, respectively, measurements are taken from the phase centre of the respective antenna. Accurate detection of these positions is effected for example, by means of a differential GPS device.
Advantageously the distances DIõj may also be detected by means of a laser distance measuring means.
- It should be noted that the radar cross-section RCS i is defined in a meaningful way only for a distant field. Based on an antenna dimension D the distant field condition D applies to a distance Dkj of 2 wherein A is the wavelength of the signal S.
If the radar targets used are, for example, radar transponders with two antennae (transmitter antenna, receiver antenna), the antenna apertures of which have a diameter of 20 cm and where the antenna feed lines are separated from each other by approx.

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-7-40 cm, then this means for a D of 60 cm and a wavelength A = 5.6 - cm, that the distant field begins at distances of DIõj > 13 m.
Provided N> 3, the further radar targets Ti>3 may be realised at random, i.e. they may be passive radar targets, radar targets working as transponders, or optionally radar targets operable in the radar or transponder mode.
For absolute calibration of the radar cross-sections RCSj=1,2,3 of three generic radar targets T1=1,2,3 the proposed method merely requires that the ratio PRI,,i/PT}õi for radar target combinations, e.g. T1T2, T1T3 und T2T3, is measured for known distances D1,2, D1,3 and D2,3. This data is inserted into a linear and easily solvable equation system resulting from the equation (1) with three equations and three unknowns (radar cross-sections RCS1=1,2,3) =
- This means that determination of the radar cross-sections of N
radar targets Ti requires at least N measurements for N different pairings.
One advantageous further development of the method is characterised in that the radar target Ti comprises a unit Di, with which the radar transmitter Si in a transponder mode TM of radar target Ti is connected with the radar receiver El, so that a signal S received by the radar receiver El is again emitted by the radar transmitter Sl, and with which the radar transmitter Si in a radar mode RN of the radar target Ti is not connected with the radar receiver El, so that the radar receiver El and the radar transmitter Si operate independently of one another. In other words, the radar target Ti in this further development can be used both in transponder mode and in radar mode, so that in total two of the at least three radar targets Ti can be operated in both modes, whilst the third radar target T3 can only be operated as a transponder or represents a passive radar target.

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- 8 -One advantageous further development of the method is characterised in that the radar target T3 comprises a unit D3, with which the radar transmitter S3 in a transponder mode TM of the radar target T3 is connected with the radar receiver E3, so that a signal S received by the radar receiver E3 is again emitted by the radar transmitter S3, and with which the radar transmitter S3 in a radar mode RM of the radar target T3 is not connected with the radar receiver E3, so that the radar receiver E3 and the radar transmitter S3 operate independently of each other. This variant of the method together with the above further development includes the combination that all three radar targets Ti are operable in both modes (transponder mode and radar mode).
One advantageous further development of the method is characterised in that the entire method, i.e. from the first to the third step, is performed for different transmitting frequencies f of the radar transmitters Si or the signal S, wherein ultimately the radar cross-sections RCS j of the radar targets Tj are ascertained as frequency-dependent radar cross-sections RCSj(f). Thus as a result of the method absolute calibrated frequency-dependent radar cross-sections RCS(f) are - obtained.
Furthermore it is advantageous to perform the entire method from the first to the third step for each pairing TkTi for different distances plc,i for the radar targets Tk and Tj, wherein the measured distance-dependent ratios (PR1õj/PTIõj)(Dk,j) are utilised for the correction of multipath effects when ascertaining the radar cross-sections RCS. Interferences or inaccuracies occurring due to the multipath propagation of the signals S or due to stationary waves can thereby be compensated for to a large extent, which ultimately improves accuracy of the ascertained radar cross-sections RCS.

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- 9 -Furthermore it is advantageous to perform the entire method from the first to the third step for different polarisations P of the signal S, wherein as a result the radar cross-sections RCS j of the radar targets Ti are ascertained as being dependent on the polarisation P.
It is advantageous to repeatedly perform the entire method from the first to the third step q-times, wherein the radar cross-sections RCSi are ascertained as mean values <RCSi>g, wherein q {2, 3, 4 _}. Furthermore it is advantageous if only the ,first step" is repeatedly performed q-times for a pairing of radar - targets Tk und Tj, wherein the transmission powers PT1,,j and receiving powers PRI,,j measured during the process are averaged, and the means values: <PTI,,j>q and <PR1õj>q generated are used for ascertaining the ratio PRIõj/PT1,,j = <PRj,k>q/<PTIõj>g and for respectively ascertaining the radar cross-sections RCSj.
An advantageous further development of the method is characterised in that the units Di, in transponder mode of the radar target Tj, amplify and/or filter and/or delay over time the .
signals S received from the radar receiver Ej, before these are forwarded to the radar transmitter Si for emitting them. In particular by emitting the signals with a time delay, interference effects from multipath signals and further environment-dependent effects such as upswings can be very largely excluded.
An advantageous further development of the method is characterised in that the distances Dic,j of the radar targets Tk and Tj from each other satisfy the following condition:
(2) Dk,i > (2*D2)/A

CD, 02897569 2015-07-16

- 10 -with D: antenna diameter of the transmitter antenna SA, and A:
wavelength of the signal S. This corresponds to the distant field condition already addressed.
Advantageously the transmitting antenna SAk of the radar target Tk and the receiving antenna EAJ of the radar target Ti are aligned in a co-polar manner.
The at least three measurements of transmission power PTici and receiving power PRk,j permit an equation system to be developed, from which the radar cross-section RCS of each radar target Tõ
can be unequivocally calculated, provided the distance Dk,i between the radar targets Tk and Tj is known with sufficient accuracy.
In contrast to the known methods an additional radar target with known backscatter cross-section is not required, resulting in higher calibrating accuracies being achieved. Besides the radar targets T, are measured in their final configurations (as a black box), i.e. cable connections and corresponding internal interfaces need not be altered again after measuring, which would subsequently falsify the radar cross-section RCS. In addition radar targets Tõ, which operate as transponders with a digital delay, allow the decoupling, over time, of the transmitting and receiving of signals. Any upswings, as may happen with previous measurements of transponders in a shielding chamber, may thereby be prevented. A further advantage consists in that when taking measurements there is no need for additional high-frequency measuring equipment, such as for example a network channel analyser. This will result not only in cost savings, but also in additional measuring uncertainties being avoided. Finally the traceability of the proposed RCS calibration is limited to standards for a comparatively simple length measuring system, whilst up to now the radar cross-section RCS had to be ascertained either by a detour via a further calibrated reference ak 02897569 2015-07-16

- 11 -target or by measuring the individual components of a respective radar target. Thus the present method allows to trace the calibration back to different standards in a simple way.
The requirement of the invention is further met by a computer system with a data processing device, wherein the data processing . device is realised in such a way that a method as described above is executed on the data processing device.
In addition the requirement of the invention is met by a digital storage medium with electrically triggered control signals, wherein the control signals can interact with a programmable computer system in such a way that a method as described above is executed.
Furthermore the requirement according to the invention is met by a computer program product with a program code stored on a machine-readable medium for executing the method as described above, when the program code is executed on a data processing device.
. Finally the invention relates to a computer program with program codes for executing the method, as described above, when the program is run on the data processing device. To this end the data processing device may be designed as a random computer system known from the state of the art.
Brief Description of Drawings Further advantages, features and details are revealed in the following description, in which - possibly with reference to the drawing - at least one exemplary embodiment is described in detail. Identical, similar and/or functionally identical parts are provided with the same reference symbols.

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- 12 -In the drawing Fig. 1 shows a schematic exemplary representation of a radar target operated solely as a radar, and a radar target operated here in transponder mode and which can be switched between transponder mode and radar mode, and Fig. 2 shows a schematic flow diagram of an exemplary embodiment of the proposed method.
In the following an exemplary embodiment of the proposed method - is described in detail, in which the radar cross-sections RCS', RCS2, and RCS3 are absolutely calibrated for three radar targets T1, T2, and T3. Two of the three radar targets, that is Tl and T2, may be operated both in radar mode and in transponder mode, i.e.
they comprise a unit, which can switch between these two modes.
The radar target T3 here can be operated only as a radar, i.e.
receiving and emitting of radar signals S is effected independently of each other.
Detailed Description Fig. 1 schematically shows a radar target 201 operated only as a radar, and a radar target 220 operated here in transponder mode, and which can be switched between transponder mode and radar mode.
The radar target 201 comprises a radar transmitter 202 with an amplifier 204 (operating in the digital range), a digital-analogue converter 205 and a transmission unit 206 with a transmitting antenna 207. A signal is provided to the radar transmitter 202 at the amplifier 204, which signal is ultimately emitted via the transmitting antenna 207 as a signal S. Further the radar target comprises a radar receiver 203 with a receiving antenna 210, an amplifier 209 and an analogue-digital converter

- 13 -208. A signal S received by the radar receiver travels from the receiving antenna 210 via the shown signal path to the analogue-digital converter and is provided there for further processing.
It is easily recognisable that the radar transmitter 202 is not connected with the radar receiver 203, i.e. a signal S received by the radar receiver 203 is not provided for emission at the radar transmitter 202.
The radar target 220 comprises a radar transmitter 222 with an amplifier 225 (operating in the digital range), a digital-analogue converter 224 and a transmitting unit 223 with a transmitting antenna 226. The radar target 220 further comprises a radar receiver 221 with a relying antenna 227, an amplifier 228 and an analogue-digital converter 229. Furthermore the radar target 220 comprises a unit 230, with which the radar transmitter 222, in the transponder mode TM of radar target 220 shown, is connected with the radar receiver 221, so that a signal S
- received by the radar receiver 221 is again emitted by the radar transmitter 222, and with which the radar transmitter 222, in a radar mode RM (not shown) of the radar target 220, is not connected with the radar receiver 221, so that the radar transmitter 222 and the radar receiver 221 operate independently of each other, i.e. as radar.
The present three radar targets of the exemplary embodiment respectively correspond to the principal construction of the radar targets 201 and 220 generally described above.
For absolute radiometric calibration of the radar cross-section RCS1=1,2,3 of the above defined radar targets Ti, with i = 1, 2, 3, the measurements described below are taken for the following pairings: T3T1, T3T2 and T1T2, wherein in the first two pairings:
- T3T1, T3T2 the radar targets T1 and T2 are operated in transponder mode, so that the radar target T3 emits respective signals S to the radar targets Tl and T2 and records the respective signals =

- 14 -returned from there. With the pairing T1T2 one of the two radar targets Tl or T2 must operate in radar mode and the respectively other target must operate in transponder mode. Let it be assumed in this case that Tl operates in radar mode and T2 operates in transponder mode.
For each of these pairings: a signal S is emitted by the radar transmitter Sk of a radar targets Tk with a transmission power PTkfj, a signal S is received by the respectively other radar target Tj of the pairings, the received signal S is emitted by the other radar target Tj, and the signal S coming from the radar target Tj is received by the radar receiver Ek of the radar target Tk with a receiving power PRk,j with k, j 11, 2, NI
and k j.
10 All measurements are taken for an identical frequency of signal S, or by utilising a sinus signal. Subsequently the measurements are repeated preferably for gradually changed frequencies f of signal S, in order to ascertain the frequency-dependent radar cross-section RCSi(f).
- For each measurement the transmission power PTiõj and the receiving power PRiõj are measured on the radar target Tk operated as radar. In total therefore, following the measurements, the transmission powers present are: PTk-3,j=1r PTk=3,j=2 and PTk=1,j=2 and the receiving powers are: PRk=3,j=1, PRk=3,i=2, and PRk---1,j=2 -Furthermore, for each pairing, the distance 20 Dic,j present between the respective radar targets Tk and Tj is ascertained by means of laser range measuring.
Furthermore based on known distances Di,,j of the radar transponders Tk and Tj during the measurements, and the ascertained ratios PRk,j/PTk,i: PRk=3,j=1/PTk=3,j=1r PRk=3, j=2 Prk=3, j=2 and PRk=1,j=2/PTk=1,j=2, the radar cross-sections RCS i of radar

- 15 -transponders T1 are ascertained based on the following radar equation:
GR = GT = 22 = RCS
(1) PR =PT
k7 k4 .1 (4703 = DA-44 with GR: antenna gain of the receiving antenna EA, GT:
antenna gain of the transmitter antenna SAD, and A
wavelength of the radar signal.
This equation (1) describes the receiving power PRk,3 received " from a radar target Tk with an antenna gain GR of the receiving antenna EAk as a function of the radar cross section RCS3 of the radar target T3 at a distance Djc,j, and the transmission power PTk,j of the radar transmitter Sk with a transmitting antenna SAk comprising an antenna gain Gt, for a wavelength A of the emitted signal S.
The radar cross-section RCS3 may also be expressed as the total gain G1 (,loop gain") of the radar target T. This results in:

A:
(3) RCS µ, 42r wherein the loop gain can be typically expressed as follows: G/ =
Gs*Ge*Gr, i.e. as a product of the antenna gain Gs of the transmitting antenna SA3 of the radar target T3, the antenna gain - Gr of the receiving antenna EA] of the radar target TJ, and the gain Ge of the electrical amplification of the received signal at the radar target Tj.
Equations (2) and (3) result in:

A
(4) RCS = : ¨ (7.
tr rr

- 16 -wherein Gt, and Gr), are the gains of the transmission path / the receiving path in the radar target Tj, i.e. a combination of antenna gain and gain by electrical amplification in the respective path. The transmission path / the receiving path, with reference to fig. 1, results from the signal path in the radar receiver 221 such as the radar transmitter 222.
Equations (2) and (3) can be combined to form one equation. Here it is presumed as explained above, that the radar target Tk is operated as a radar, and the radar target Tj is operated as a radar transponder. This results in:
PRkJ

(5) RCSk = RCS
prk ______________ j (4.7)1D A-44 22 For the proposed three pairings three equations are thus obtained. These equations can be transformed by a logarithmic - transformation into a linear equation system. This is possible because all expressions are larger than zero.
For simplification, the same symbols are then used for the radar cross-section RCS, however, it is pointed out that following the logarithmic transformation: 10 log(...) they now refer to values which are indicated in decibel.
Equation (5) can then be expressed as follows:
(6) RCSA. + RCS _P1 -C
k with Pk,: ratio 10log(PRIõj/PTIõj) wherein the radar target Tk functions as radar and the radar target T] functions as a transponder, wherein the latter is measured.
-C can be expressed as:
(7) C = -201og(4/TDA )

- 17 -It is pointed out that equation (7) merely shows the tracking back of a longitudinal measurement (Dic,j) to a national standard, in order to make calibration traceable.
The linear equation system may be described in matrix form as follows:
(1 1 0\ (RCõ'S3 (8) 1 0 1 RCS P
1 = - C
0 1 1 RCS-, k=1.j=2 When inverted the result is:
7 RCS \( 3 4-.34=1 ' (9) RCS 1 = - 1 -1 1 P - C

\ 2/

Fig. 2 shows a highly schematic exemplary embodiment of the proposed method for radiometric calibration of the radar cross-section RCS i of a N number of radar targets Ti, with i = 1, 2, .., s N, and N 3, wherein at least one radar target Tl comprises a radar transmitter S1 with a transmitting antenna SA1 and a radar receiver El with a receiving antenna EAl, wherein the radar receiver El and the radar transmitter S1 operate independently of one another, a radar target T2 comprises a radar transmitter S2 with a transmitting antenna SA2, a radar receiver E2 with a receiving antenna EA2, and a unit D2, with which the radar transmitter S2, in a transponder mode TM of the radar target T2, is connected with the radar receiver E2, so that a signal S
received by the radar receiver E2 is again emitted by the radar transmitter S2, and with which the radar transmitter S2, in a radar mode RM of the radar target T2 is not connected with the radar receiver E2, so that the radar transmitter S2 and the radar ak 02897569 2015-07-16

- 18 -receiver E2 operate independently of each other, and a radar target T3 comprises a radar transmitter S3 with a transmitting antenna SA3, a radar receiver E3 with a receiver antenna EA3, and a unit D3, with which the radar transmitter S3 is connected with the radar receiver E3, so that a signal S received by the radar receiver E3 is again emitted by the radar transmitter S3, or the radar target T3 is a passive radar target, which reflects an incoming signal S. The method comprises the following steps:
In a first step 101 a signal S is emitted by the radar transmitter Sk of one of the radar targets Tk with a transmission power PT1õj, a signal S is received by another of the radar . targets Tj, the received signal S is emitted or reflected by the other radar target Tj, and a signal S emitted by the radar target Tj is received by the radar receiver Ek of the radar target Tk with a receiving power PRj,k, with k, j {1, 2, ..., NI and k j.
In a second step 102 step 101 is executed for N different pairings TkTj of radar targets Tk and Tj, wherein the pairings TkTj and TjTk are considered identical.
In a third step 103, based on known distances Dk,j of the radar transponders Tk and Tj during execution of steps 101 and 102 and based on the ascertained ratios PRi,k/PTkj, the radar cross-sections RCS i of radar transponders Ti are ascertained based on the following relationship:
G = G = 22 RCS
(1) PR .PT PT
J-11 k1 R T

(4703=Dk with GR: antenna gain of the receiver antenna EAk, GT: antenna gain of the transmitter antenna SAk, and A wavelength of the radar signal.

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- 19 -Although the invention has been illustrated and described in detail by way of preferred exemplary embodiments, the invention , is not limited by the disclosed embodiments, and other variations may be derived therefrom by the expert without deviating from the scope of the invention. It is therefore clear that a multitude of possible variations exists. It is also clear that embodiments cited as examples really only represent examples, which should not be understood in any way as limiting the protective scope, the passible applications or the configuration of the invention.
Rather, the above description and the description of the figures enable the expert to translate the exemplary embodiments into practice, wherein the expert, in the knowledge of the disclosed inventive idea, can make various changes, for example as regards the function or the arrangement of individual elements named in an exemplary embodiment, without deviating from the protective scope which is defined by the claims and their legal equivalents such as the extended explanation in the description.

Claims (10)

What is claimed is:

1. A method for radiometric calibration of the radar section RCS1 of a N number of radar targets T1, with i = 1, 2, .., N, and N>= 3, wherein at least - one radar target T1 comprises a radar transmitter S1 with a transmitting antenna SA1 and a radar receiver E1 with a receiver antenna EA1, wherein the radar receiver E1 and the radar transmitter S1 operate independently of each other, - one radar target T2 comprises a radar transmitter S2 with a transmitting antenna SA2, a radar receiver E2 with a receiver antenna EA2 and a unit D2, with which the radar transmitter S2, in a transponder mode TM of the radar target T2, is connected, so that a signal S
received by the radar receiver E2 is emitted again by the radar transmitter S2, and with which the radar transmitter S2, in a radar mode RM of the radar target T2, is not connected with the radar receiver E2, so that the radar transmitter S2 and the radar receiver E2 operate independently of each other, and - one radar target T3 comprises a radar transmitter S3 with a transmitting antenna SA3, a radar receiver E3 with a receiver antenna EA3, and a unit D3, with which the radar transmitter S3 is connected with the radar receiver E3, so that a signal S received by the radar receiver E3 is again emitted by the radar transmitter S3, or the radar target T3 is a passive radar target, which reflects an incoming signal S, with the following steps:

1.1. Emission of a signal S by the radar transmitter S k of one of the radar targets T k with a transmission power PT k,j, reception of the signal S by another of the radar targets T j, emission or reflection of the received signal S by the other radar target T j, and reception of the signal S coming from the radar target T j by the radar receiver E k of the radar target T k with a receiving power PR k,j, with k, j .epsilon. {1, 2, ..., N} and k .noteq.
j, 1.2. Execution of step 1.1. for N different pairings T k T j of radar targets T k and T j, wherein the pairings T k T j and T j T k are considered identical, 1.3. Based on known distances D k,j of the radar transponders T k and T3 during execution of steps 1.1. and 1.2 and based on ascertained ratios PR k,j/PT k,j, determination of the radar cross-sections RCS1 of radar transponders T1 based on the following relationship:
with G R: antenna gain of the receiver antenna EA k, G T:
antenna gain of the transmitter antenna SA k, and .lambda.
wavelength of the radar signal.

2.
The method according to claim 1, wherein the radar target T1 comprises a unit D1 with which the radar transmitter S1 in a transponder mode TM of the radar target T1 is connected with the radar receiver E1, so that the signal S received by the radar receiver E1 is again emitted by the radar transmitter S1 and with which the radar transmitter S1 in a radar mode RM of the radar target T1, is not connected with the radar receiver E1, so that the radar receiver E1 and the radar transmitter S1 operate independently of one another.

3. The method according to claim 1 or 2, wherein the radar target T3 comprises a unit D3, with which the radar transmitter S3, in a transponder mode TM of the radar target T3, is connected with the radar receiver E3, so that a signal S is again emitted by the radar transmitter S3, and with which the radar transmitter S3, in a radar mode RM of the radar target T3, is not connected with the radar receiver E3, so that the radar receiver E3 and the radar transmitter S3 operate independently of one another.

4. The method according to any one of claims 1 to 3, wherein the steps 1.1. to 1.3. are executed for different transmitting frequencies f of the radar transmitters SI, wherein the radar cross sections RCS j of radar targets T3 are ascertained as frequency-dependent radar cross-sections RCS J (f).

5. The method according to any one of claims 1 to 4, wherein the steps 1.1. to 1.3. for each pairing T k T j are executed for different distances D k,j of the radar targets T k and T j, wherein the measured distance-dependent ratios (PR k,j /PT k, j) D k,j) are utilised for the correction of multipath effects during determination of the radar cross-sections RCS1.

6. The method according to any one of claims 1 to 5, wherein the steps 1.1. to 1.3. are executed for different polarisations P of the signal S, wherein the radar cross-sections RCS j of radar targets T j are ascertained as polarisation-P-dependent radar cross-sections RCS J (P).

7. The method according to any one of claims 1 to 6, wherein the steps 1.1. to 1.3. are repeated q-times and the radar cross-sections RCS1 are ascertained as mean values <RCS1>q.

8. The method according to any one of claims 1 to 7, where the units D1, in transponder mode TM of the radar target T1, amplify and/or filter and/or delay over time, signals S
received by the radar receiver E1, before these signals are forwarded to the radar transmitter S1.

9. The method according to any one of claims 1 to 8, wherein the distances D k, j between the radar targets T k and T j satisfy the following condition:
(2) D k,j > (2*D2)/.lambda.
with D: Antenna diameter of the transmitting antenna SA1 .lambda.: wavelength of signal S

10. The method according to any one of claims 1 to 9, wherein the transmitting antenna SA k of the radar target T k and the receiving antenna EA j of the radar target T j are aligned in a copolar manner.