Method for measuring and calibrating outfield airborne double-station radar

Abstract

The invention relates to the technical field of double-station radar measurement and calibration, in particular to an outfield airborne double-station radar measurement and calibration method, which comprises the following steps of: receiving a signal of a radar transmitting antenna by selecting active calibration equipment, amplifying the signal and forwarding the amplified signal to a radar receiving antenna; and the first aircraft provided with the radar transmitting antenna and the second aircraft provided with the radar receiving antenna respectively fly according to the measurement calibration working scene, and the corresponding indexes are measured. The corresponding relation between specific indexes and link gain of active calibration equipment and the RCS value of the passive standard body is defined, background clutters during calibration can be restrained, and the problem that the RCS value of the passive standard body is remarkably changed due to double-flight-platform double-station angle errors in airborne double-station measurement is solved. The precision of airborne double-station radar measurement data is improved, the same active calibration device can replace passive standard bodies of various specifications, external field application is light and flexible, and the method has important engineering application value.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of bistatic radar measurement calibration, and particularly relates to a method for calibrating airborne bistatic radar measurement in an external field. BACKGROUND

[0002] An airborne bistatic radar measurement system can obtain electromagnetic scattering characteristic data of ground and sea targets under different bistatic angle conditions, and is increasingly widely concerned. The extensive application of accurate ground and sea attack weapons makes the airborne bistatic radar measurement play an increasingly important role in the research on ground and sea target characteristics. Meanwhile, because the transmitting and receiving are placed on different flight platforms, different flight trajectories form different flight configurations, which can obtain scattering information of ground objects irradiated and received at different angles, and have great research value for studying the scattering characteristics of targets.

[0003] Calibration is an essential step in airborne bistatic radar measurement, and its accuracy is directly related to the accuracy of airborne bistatic measurement data.

[0004] At present, simulation calibration is mostly used for airborne bistatic radar, and the calibration accuracy is relatively poor. With the development of technology, the requirement for the calibration accuracy of airborne bistatic radar is higher and higher, and simulation calibration cannot meet the requirement. However, because of the need for coordination of airspace and higher technical accumulation requirement, there is less research on external calibration.

[0005] A Chinese patent application with the publication number CN117647784A discloses a bistatic ground-air dynamic RCS calibration method, which calibrates the coefficients of a bistatic transmitting radar and a bistatic receiving radar based on a rotor unmanned aerial vehicle platform, obtains the K value coefficient of the bistatic radar, and provides a standard basis for calculating the bistatic RCS data of a target. The calibration method is not limited by the baseline distance of the bistatic radar, has strong operability, and has good adaptability to natural environment. The present application calibrates the bistatic transmitting radar and the receiving radar respectively, the relative distance of the standard metal ball is relatively short, and the pitch angle is high, which can effectively avoid the influence of ground clutter, and has good calibration quality. The present application solves the technical problem that the current requirement for the bistatic angle is less than a certain angle, the distance limit of the baseline is relatively high, it is difficult to meet the requirements of pre-calibration and post-calibration, and the practicality is greatly limited. The airborne bistatic radar calibration using the method has the following technical problems:

[0006] 1) the method is not aimed at calibrating the scene for receiving radar and transmitting radar to capture the respective standard metal ball by optical gun sight, and the so-called calibration is to calibrate the system error of the bistatic radar in the measurement process, so the calibration and measurement should be completed in the measurement time period, rather than in different time periods, because the system error of the radar system in different time periods may be different, so accurate calibration cannot be achieved. In addition, the receiving radar in the bistatic radar only bears the receiving function and does not transmit signals, so the obtained K2 is not the actual calibration coefficient K2 of the receiving radar in the bistatic radar. Moreover, after the transmitting radar and the receiving radar in the bistatic radar form a bistatic radar, a new calibration coefficient is brought, which cannot be determined by the calibration coefficient formula of the transmitting radar and the receiving radar respectively.

[0007] 2) the metal ball cannot be measured downward, because the size of the metal ball itself is small and may be submerged in clutter, and in addition, there is no theoretical value of the double-station angle of the metal ball.

[0008] Therefore, the above-mentioned bistatic ground-to-air dynamic RCS calibration method cannot well achieve the accurate calibration problem of the airborne bistatic radar.

[0009] And using a passive standard body for calibration is seriously interfered by background clutter, and the passive standard body that meets the calibration condition has a large area and a heavy weight, and the double-station angle error of the two flight platforms will bring a significant change in the RCS value of the standard body, which cannot meet the precision requirement of the calibration data of the airborne bistatic radar measurement.

[0010] Therefore, there is an urgent need for a method and device for calibrating airborne bistatic radar measurement in the field. SUMMARY

[0011] The purpose of the present application is to provide a method for calibrating airborne bistatic radar measurement in the field, which solves the problems of passive standard body interference by background clutter and significant change in the RCS value of the passive standard body caused by the double-station angle error of the double flight platform, and realizes the accurate calibration of airborne bistatic radar in the field.

[0012] In order to achieve the above-mentioned purpose, the present application provides a method for calibrating airborne bistatic radar measurement in the field, comprising the following steps:

[0013] An active calibration device is selected to receive the signal of the transmitting antenna of the receiving radar and transmit the amplified signal to the receiving antenna of the radar;

[0014] The first aircraft with the radar transmitting antenna and the second aircraft with the radar receiving antenna fly according to the measurement calibration working scene respectively, and measure the following indexes:

[0015] the respective flight altitudes, flight speeds, radar transmitting antenna beam widths, angles of radar transmitting antenna beam pointing to the ground, radar transmitting antenna operating parameters, transmitting powers, antenna gains, main-to-side lobe ratios of the antennas, radar receiving antenna beam widths, angles of radar receiving antenna beam pointing to the ground, radar receiving antenna operating parameters, receiving powers, antenna gains, main-to-side lobe ratios of the antennas;

[0016] determining the active calibration device receiving power range, link gain and device delay time;

[0017] determining the conversion relationship between the active calibration device link gain and the corresponding equivalent passive standard body RCS value according to the radar equation;

[0018] determining the RCS of the airborne bistatic measurement data according to the radar equation and using the relative comparison method.

[0019] Optionally, the selected active calibration device satisfies the following conditions:

[0020] The azimuth and elevation 3db beam widths of the transmitting antenna and the receiving antenna of the active calibration device are both greater than 40°.

[0021] Optionally, the azimuth and elevation angles of the transmitting antenna and the receiving antenna of the selected active calibration device can be adjusted and detected.

[0022] Optionally, the selected active calibration device satisfies the following conditions:

[0023] (a) coherent with the bistatic radar signal, the retransmitted signal maintains phase and waveform consistency;

[0024] (b) fixed gain amplification is performed on the received signal, so that the device is equivalent to a constant RCS target.

[0025] Optionally, the determination of the active calibration device receiving power range, link gain and device delay time includes the following steps:

[0026] According to the airborne bistatic radar measurement calibration working scene, the farthest and nearest distances of the radar transmitting antenna to the active calibration device main lobe are calculated, so as to calculate the active calibration device receiving power range;

[0027] According to the receiving power index and the device equivalent passive standard body magnitude, the link gain of the active calibration device is determined.

[0028] According to the length of the clutter echo in the transmitting antenna main lobe in the measurement radar calibration working scene, the delay time is determined.

[0029] Optionally, the conversion relationship between the active calibration device link gain and the corresponding equivalent passive standard body RCS value is determined according to the radar equation, including the following steps:

[0030] The transmitting power of the airborne bistatic radar measurement is P t , the transmitting antenna gain is G t , the receiving antenna receiving effective area is A e , assuming that the equivalent passive standard body RCS is σ, the distance from the radar transmitting antenna to the target is R t , the distance from the target to the radar receiving antenna is R r , the bistatic measurement radar transmitting signal is reflected to the passive standard body and then to the bistatic measurement radar receiving antenna, then the radar receiving power P r1 is

[0031]

[0032] The passive standard body is replaced by an active calibration device, the bistatic measurement radar wavelength is λ, the active calibration device receiving antenna gain is G dr , and the receiving effective area is A d :

[0033]

[0034] The actual link overall gain is G n , the radar transmitting signal is reflected to the active calibration device, the active calibration device transmitting antenna gain is G dt , and the radar receiving power P r2 is:

[0035]

[0036] Let P r1 =P r2 , then:

[0037]

[0038] Further, we get:

[0039]

[0040] Taking the logarithm operation:

[0041] (σ) dBsm =(G n ) dB +(G dr ) dB -(G dt ) dB +20log(λ)-10log(4π) (6)。

[0042] Alternatively, according to the radar equation, the relative comparison method is applied to determine the RCS of the measurement data, comprising the following steps:

[0043] According to the radar equations, the radar-received target echo power can be expressed as a function of several parameters, including the target RCS, the transmitting system, the receiving system, and the propagation path. In airborne bistatic measurements, the bistatic RCS is defined as:

[0044]

[0045] Where, p r It is the received radar target echo power; p t It is the radar transmission power; G t It is the transmit antenna gain; G r λ is the receiver antenna gain; L is the radar wavelength; t It is the loss factor of the transmission system; L r It is the receiving system loss factor; L p It is the polarization loss factor; L m It is the loss factor along the propagation path; R t R is the distance from the radar transmitting antenna to the target. r It is the distance from the target to the radar receiving antenna; let:

[0046]

[0047] Then we can obtain:

[0048]

[0049] Apply RCS as (σ) dBsm The active calibration equipment calibrates the airborne bistatic radar measurements, letting (σ) dBsm =σ0, then:

[0050]

[0051] In the formula, k0 is the constant k of the active calibration device; p r0 To receive the echo power from the active calibration equipment; R t0 R is the distance from the radar transmitting antenna to the active calibration equipment. r0 It is the distance from the active calibration equipment to the radar receiving antenna; L m0 This represents the loss factor along the propagation path at that time.

[0052] During the measurement period, the airborne bistatic radar system operated smoothly, atmospheric propagation was stable, and the constants k = k0 and L were constant. m =L m0 The RCS of the target being measured is calculated using the following formula:

[0053]

[0054] Expressed in terms of receiver output voltage:

[0055]

[0056] Optionally, a minimum input threshold signal and a delay time of the active calibration device are set, when the signal received by the active calibration device is greater than the minimum input threshold signal, it is a valid signal, the valid signal is amplified and forwarded after being delayed for the set delay time.

[0057] Optionally, the minimum input threshold signal of the active calibration device is determined according to the transmission power of the airborne bistatic radar transmission antenna, the transmission antenna gain and the farthest distance from the transmission antenna to the active calibration device.

[0058] Optionally, according to the maximum coverage length and width of the transmission antenna beam during the calibration of the bistatic radar, the active calibration device delay time is greater than the sum of the bistatic measurement radar transmission pulse width and the clutter broadening.

[0059] The above technical scheme of the present application has the following advantages:

[0060] The method for measuring and calibrating the field airborne bistatic radar provided by the present application uses an active calibration device to receive the signal of the radar transmission antenna and forwards the amplified signal to the radar receiving antenna, the first aircraft installed with the radar transmission antenna and the second aircraft installed with the radar receiving antenna fly according to the measurement and calibration working scene respectively, and the corresponding indicators are measured. The correspondence between the specific indicators of the active calibration device, the link gain and the passive standard body RCS value is determined, the background clutter during calibration is suppressed, the problem of significant change of the passive standard body RCS value caused by the bistatic angle error of the double flight platforms in the airborne bistatic measurement is solved. The precision of the airborne bistatic radar measurement data is improved, the same active calibration device can replace passive standard bodies of multiple specifications, the field application is light and flexible, and has important engineering application value. BRIEF DESCRIPTION OF DRAWINGS

[0061] The drawings of the present application are provided for illustrative purposes only, and the proportions and quantities of the components in the drawings may not be consistent with the actual products.

[0062] Figure 1 is a schematic diagram of airborne bistatic radar measurement and calibration in the embodiment of the present application;

[0063] Figure 2 is a flowchart of airborne bistatic radar measurement and calibration in the embodiment of the present application. DETAILED DESCRIPTION

[0064] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort belong to the scope of protection of the present application.

[0065] In the airborne bistatic radar measurement process, the bistatic angle is affected by factors such as air speed and flight error, and there is an error between the actual flight bistatic angle and the designed bistatic angle. The bistatic angle error will cause the RCS of the traditional passive calibration body to change dramatically. The present application uses an active calibration device to receive the radar signal after fixed amplification and then transmits it to the radar receiving antenna of the bistatic radar. As long as the receiving antenna and the transmitting antenna of the active calibration device have a wide enough beam, the problem of the RCS of the traditional passive calibration body changing dramatically due to the bistatic angle error can be solved. At the same time, the background clutter during calibration can be suppressed, the accuracy of the airborne bistatic radar measurement data is improved, and the same active calibration device can replace passive standard bodies of multiple specifications, which has important engineering application value.

[0066] The present application will be further described below through embodiments:

[0067] As shown in Figure 1 and Figure 2 , an embodiment of the present application for field calibration of airborne bistatic radar measurement using an active calibration device includes the following steps:

[0068] S1: Select an active calibration device to receive the signal of the radar transmitting antenna and retransmit it to the radar receiving antenna after amplification;

[0069] S2: The first aircraft with the radar transmitting antenna and the second aircraft with the radar receiving antenna respectively fly according to the measurement and calibration working scene, and measure the following indicators:

[0070] The flight height and speed of the first aircraft and the second aircraft, the beam width of the radar transmitting antenna, the angle between the beam direction of the radar transmitting antenna and the ground, the working parameters of the radar transmitting antenna, the transmitting power, the antenna gain, the main-to-side lobe ratio of the antenna, the beam width of the radar receiving antenna, the angle between the beam direction of the radar receiving antenna and the ground, the working parameters of the radar receiving antenna, the receiving power, the antenna gain, and the main-to-side lobe ratio of the antenna;

[0071] S3: Determine the receiving power range, link gain and device delay time of the active calibration device;

[0072] S4: Determine the conversion relationship between the link gain of the active calibration device and the equivalent passive standard body RCS value according to the radar equation;

[0073] S5: According to the radar equation, the relative comparison method is applied to determine the RCS of the airborne bistatic measurement data.

[0074] In this embodiment, the active calibration device is used for field calibration of airborne bistatic radar measurement, the reflection echo of the active calibration device in the main lobe of the bistatic measurement radar transmitting antenna is determined, the target detected by the sidelobe of the bistatic measurement radar transmitting antenna is prevented, and the measurement error is caused; the echo and clutter of the active calibration device in the main lobe of the bistatic measurement radar transmitting antenna can be effectively distinguished in distance and power, so as to suppress the clutter and improve the calibration accuracy. The problem that the RCS value of the passive standard body changes significantly due to the bistatic angle error of the double flight platforms in the airborne bistatic measurement is solved. The accuracy of the airborne bistatic radar measurement data is improved, and the same active calibration device can replace passive standard bodies of various specifications, and the field application is light and flexible, which has important engineering application value.

[0075] In an example, the selected active calibration device satisfies the following conditions:

[0076] The 3db beam width of the transmitting antenna and the receiving antenna of the active calibration device in the azimuth direction and the elevation direction is greater than 40°, and the 3db beam with sufficient width can better solve the problem that the RCS value of the passive standard body changes significantly due to the bistatic angle error of the double flight platforms in the airborne bistatic measurement, and generally the bistatic angle error of about 10° can still avoid the significant change of the RCS value of the passive standard body.

[0077] In an example, the azimuth angle and the elevation angle of the transmitting antenna and the receiving antenna of the selected active calibration device can be adjusted and detected (the adjusted angle can be known), which can meet the calibration under different bistatic angles. It is worth noting that the adjustment of the azimuth angle and the elevation angle of the transmitting antenna and the receiving antenna of the active calibration device can be realized by using the existing structure, and details are not repeated here. The active calibration device generally includes a transmitting antenna, a receiving antenna and a fixed amplification diagram, and the structure is prior art, and details are not repeated here.

[0078] In an example, the selected active calibration device satisfies the following conditions:

[0079] (a) coherent with the bistatic radar signal, the retransmitted signal maintains phase and waveform consistency;

[0080] (b) the received signal is amplified with fixed gain, so that the device is equivalent to a constant RCS target.

[0081] In an example, the receiving power range, link gain and device delay time of the active calibration device are determined, including the following steps:

[0082] According to the airborne bistatic radar measurement calibration work scene, the farthest and nearest distance of the radar transmitting antenna reaching the active calibration device main lobe is calculated, so as to calculate the active calibration device receiving power range;

[0083] According to the receiving power index, the device equivalent passive standard body level, the link gain of the active calibration device is determined;

[0084] According to the length of the clutter echo in the main lobe of the transmitting antenna in the measurement radar calibration work scene, the delay time is determined.

[0085] In this embodiment, the minimum input threshold signal and the delay time of the active calibration device are set, and when the signal received by the active calibration device is greater than the minimum input threshold signal, it is an effective signal, which is amplified and delayed for a set delay time and then forwarded.

[0086] Among them, according to the transmitting power of the airborne bistatic radar transmitting antenna, the transmitting antenna gain and the farthest distance from the transmitting antenna to the active calibration device, the minimum input threshold signal of the active calibration device is determined.

[0087] Among them, according to the maximum coverage length and width of the transmitting antenna beam during the calibration of the bistatic radar, the clutter spreading time in the main lobe of the bistatic measurement radar transmitting antenna is calculated, and the delay time of the active calibration device is greater than the sum of the bistatic measurement radar transmitting pulse width and the clutter spreading.

[0088] The receiving power of the active calibration device and the transmitting power p t of the airborne bistatic measurement radar t , the transmitting antenna gain G t and the space transmission loss Los are related,

[0089] Los=32.44+20*log(D)+20*log(F);

[0090] Where: D is the transmission distance, unit m; F is the working frequency, unit GHz; according to the airborne bistatic radar measurement calibration work scene, the farthest and nearest distance D of the bistatic measurement radar transmitting antenna reaching the active calibration device main lobe is calculated, so as to calculate the active calibration device receiving power Pin range: Pin=P t +G t -Los. According to the receiving power index of the active calibration device, the equivalent passive standard body RCS value size, the link gain of the active calibration device is determined. According to the length of the clutter echo in the main lobe in the airborne bistatic radar measurement calibration work scene, the signal delay time is determined.

[0091] In an example, the conversion relationship between the link gain of the active calibration device and the equivalent passive standard body RCS value is determined according to the radar equation, including the following steps:

[0092] The transmitting power of the airborne bistatic radar measurement is P t , the transmitting antenna gain is G t , the receiving effective area of the receiving antenna is A e , the equivalent passive standard body RCS is σ, the distance from the radar transmitting antenna to the target is R t , the distance from the target to the radar receiving antenna is R r , the bistatic measurement radar transmitting signal is reflected to the passive standard body and then to the bistatic measurement radar receiving antenna, and the radar receiving power P r1 is

[0093]

[0094] The passive standard body is replaced by an active calibration device, the bistatic measurement radar wavelength is λ, the receiving antenna gain of the active calibration device is G dr , and the receiving effective area is A d :

[0095]

[0096] The actual link overall gain is G n , the radar transmitting signal is reflected to the active calibration device, the transmitting antenna gain of the active calibration device is G dt , and the radar receiving power P r2 is:

[0097]

[0098] Let P r1＝ P r2 , then:

[0099]

[0100] Further, we have:

[0101]

[0102] Taking the logarithm:

[0103] (σ) dBsm = (G n ) dB + (G dr ) dB - (G dt ) dB + 20log(λ) - 10log(4π) (6).

[0104] So far, the technical indicators of the active calibration device are determined. Because the active calibration device and the passive standard body have the same working scene, the reflected echo power of the airborne bistatic measurement radar transmitted signal irradiated to the passive standard body and the reflected echo power of the active calibration device are the same, and the RCS value (σ) of the active calibration device is calculated dBsm and the link gain (G n ) dB , the receiving antenna gain (G dr ) dB , the transmitting antenna gain (G dt ) dB and the wavelength λ.

[0105] In an example, according to the radar equation, the relative comparison method is applied to determine the RCS of the measurement data, including the following steps:

[0106] According to the radar equation, the radar received target echo power can be expressed as a function of target RCS, transmitting system, receiving system and several parameters of the propagation path. When airborne bistatic measurement is performed, the bistatic RCS is defined as:

[0107]

[0108] Where p r is the received radar target echo power; p t is the radar transmitting power; G t is the transmitting antenna gain; G r is the receiving antenna gain; λ is the radar wavelength; L t is the transmitting system loss factor; L r is the receiving system loss factor; L p is the polarization loss factor; L m is the loss factor on the propagation path; R t is the distance from the radar transmitting antenna to the target, R r is the distance from the target to the radar receiving antenna; let:

[0109]

[0110] Then the following can be obtained:

[0111]

[0112] The active calibration device with RCS (σ) dBsm is used to calibrate the airborne bistatic radar measurement, let (σ) dBsm = σ0, then:

[0113]

[0114] In the formula, k0 is a constant k of the active calibration device; p r0 is the echo power of the active calibration device; R t0 is the distance from the radar transmitting antenna to the active calibration device, R r0 is the distance from the active calibration device to the radar receiving antenna; L m0 is a loss factor on the propagation path at the time;

[0115] During the measurement time period, the airborne bistatic radar system works smoothly, the atmospheric propagation is stable, the constant k = k0, L m = L m0 The RCS of the measured target is calculated by the following formula:

[0116]

[0117] It is expressed in the form of receiver output voltage:

[0118]

[0119] So far, the calibration method for airborne bistatic radar measurement using the active calibration device is determined.

[0120] According to the radar equation, the relative comparison method is used, the receiver output voltage value when measuring the target by the airborne bistatic radar and the receiver output voltage value when measuring the active calibration device, and the distance between the airborne radar transmitting antenna and the measured target, the distance between the airborne radar transmitting antenna and the active calibration device, the distance between the measured target and the airborne radar receiving antenna, and the distance between the active calibration device and the airborne radar receiving antenna are used to calculate the RCS of the measured target.

[0121] It should be noted that in the present application, the measurement calibration working scene is determined, the active calibration device receiving power range, link gain and device delay time are determined, and the conversion relationship between the active calibration device link gain and the corresponding equivalent passive standard body RCS value is determined according to the radar equation, the relative comparison method is used to determine the RCS of the bistatic measurement data, and the module devices used are all prior art, which will not be described here.

[0122] The parts not described in detail in the present application are common knowledge in the art or prior art, which will not be described here.

[0123] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: not every example contains only one independent technical solution, in the absence of scheme conflict, each technical feature mentioned in each example can be combined in any way to form other embodiments that can be understood by those skilled in the art.

[0124] Moreover, the technical solutions described in the foregoing embodiments are modified, or some of the technical features are replaced with equivalent replacements, without departing from the scope of the present application, and the essence of the corresponding technical solutions does not deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for measuring and calibrating an external field airborne bistatic radar, characterized in that, The method comprises the following steps: An active calibration device is selected to receive signals of a radar transmitting antenna and to retransmit the signals after amplification to a radar receiving antenna; A first aircraft with the radar transmitting antenna and a second aircraft with the radar receiving antenna are respectively flown in a measurement calibration working scene, and the following indexes are measured: Respective flight heights and flight speeds of the first aircraft and the second aircraft, a beam width of the radar transmitting antenna, an included angle between a beam direction of the radar transmitting antenna and the ground, working parameters of the radar transmitting antenna, a transmitting power, an antenna gain, a main-to-side lobe ratio of the antenna, a beam width of the radar receiving antenna, an included angle between a beam direction of the radar receiving antenna and the ground, working parameters of the radar receiving antenna, a receiving power, an antenna gain, and a main-to-side lobe ratio of the antenna; A receiving power range, a link gain and a device delay time of the active calibration device are determined; A conversion relationship between the link gain of the active calibration device and an equivalent passive standard body RCS value is determined according to a radar equation; An RCS of the airborne bistatic measurement data is determined according to the radar equation and a relative comparison method.

2. The method according to claim 1, wherein: The selected active calibration device satisfies the following conditions: The 3db beam widths of the transmitting antenna and the receiving antenna of the active calibration device in the azimuth direction and the elevation direction are both greater than 40°.

3. The method according to claim 1, wherein: The azimuth angle and the elevation angle of the transmitting antenna and the receiving antenna of the selected active calibration device can be adjusted and detected.

4. The method according to claim 1, wherein: The selected active calibration device satisfies the following conditions: (a) coherent with the bistatic radar signal, the retransmitted signal maintains phase and waveform consistency; (b) the received signal is amplified with a fixed gain, so that the device is equivalent to a constant RCS target.

5. The method according to claim 1, wherein: The receiving power range, the link gain and the device delay time of the active calibration device are determined, comprising the following steps: According to the airborne bistatic radar measurement calibration working scene, the farthest and nearest distances of the main lobe of the radar transmitting antenna reaching the active calibration device are calculated, so as to calculate the receiving power range of the active calibration device; According to the received power index and the equivalent passive standard body magnitude of the device, the link gain of the active calibration device is determined; According to the length of the clutter echo in the main lobe of the transmitting antenna in the measurement radar calibration working scene, the delay time is determined.

6. The method according to claim 1, wherein: The conversion relationship between the link gain of the active calibration device and the equivalent passive standard body RCS value is determined according to the radar equation, comprising the following steps: The transmitting power of the airborne bistatic radar measurement is P t , the transmitting antenna gain is G t , the receiving effective area of the receiving antenna is A e , the equivalent passive standard body RCS is σ, the distance from the radar transmitting antenna to the target is R t , the distance from the target to the radar receiving antenna is R r , the bistatic measurement radar transmitting signal is reflected to the passive standard body and then to the bistatic measurement radar receiving antenna, and the radar receiving power P r1 is Using an active calibration device to replace the passive standard body, the bistatic measurement radar wavelength is λ, the active calibration device receiving antenna gain is G dr , and the receiving effective area is A d : The actual link overall gain is G n The radar transmitting signal irradiates to the active calibration device and reflects the echo, and the active calibration device transmitting antenna gain is G dt The radar receiving power P r2 is: Let P r1 = P r2 , then: Further, logarithmic operation is performed:

7. The method according to claim 1, wherein: (σ) dBsm = (G n ) dB + (G dr ) dB - (G dt ) dB + 20 log (λ) - 10 log (4π) (6). The RCS of the measurement data is determined according to the radar equation and the relative comparison method, comprising the following steps: According to the radar equation, the received target echo power of the radar can be expressed as a function of the target RCS, the transmitting system, the receiving system and the propagation path parameters. When the airborne bistatic measurement is performed, the bistatic RCS is defined as: Then, the following equation can be obtained: where p r is the received radar target return power; p t is the radar transmit power; G t is the transmit antenna gain; G r is the receive antenna gain; l is the radar wavelength; L t is the transmit system loss factor; L r is the receive system loss factor; L p is the polarization loss factor; L m is the loss factor on the propagation path; R t is the distance from the radar transmit antenna to the target, L r is the distance from the target to the radar receive antenna; let: The receiver output voltage is expressed as: The airborne bistatic radar measurements are calibrated with the active calibration device of RCS = (σ) dBsm dBsm = σ0, then:​ where k0 is a constant k of the active calibration device; p r0 is the received echo power of the active calibration device; R t0 is the distance of the radar transmit antenna to the active calibration device, R r0 is the distance of the active calibration device to the radar receive antenna; L m0 is the loss factor on the propagation path at that time; During the measurement period, the airborne bistatic radar system worked smoothly, the atmospheric propagation was stable, and the constant k = k0, L m = L m0 The RCS of the measured target was calculated by the following formula:

8. The method according to claim 1, wherein: ​ The minimum input threshold signal and delay time of the active calibration device are set, when the signal received by the active calibration device is greater than the minimum input threshold signal, it is an effective signal, the effective signal is amplified and forwarded after being delayed for the set delay time.

9. The method of claim 8, wherein: The minimum input threshold signal of the active calibration device is determined according to the transmitting power of the transmitting antenna of the airborne bistatic radar, the gain of the transmitting antenna and the farthest distance from the transmitting antenna to the active calibration device.

10. The method of claim 8, wherein: According to the maximum coverage length and width of the transmitting antenna beam during the calibration of the bistatic radar, the active calibration device delay time is greater than the sum of the transmitting pulse width and the clutter spread of the bistatic measuring radar.

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