A method for designing a ground penetrating radar system for small celestial body internal structure detection

Abstract

A design method for a ground-penetrating radar system for detecting the internal structure of small celestial bodies is presented. The system includes a low-frequency detection section and a high-frequency detection section. By building an electronics box, the detection frequency and bandwidth are determined according to the target detection depth and resolution requirements. The detection timing parameters and the minimum radar transmit power corresponding to the low and high frequencies are designed respectively. The low-frequency and high-frequency antennas of the ground-penetrating radar system peripherals are designed, which takes into account both deep penetration and high resolution. The system is also small in size, light in weight, and highly reliable. It can be extended to detect the internal structure of other celestial bodies in deep space exploration.

Description

Technical Field

[0001] This invention relates to a design method for a ground-penetrating radar system for detecting the internal structure of small celestial bodies, belonging to the field of microwave remote sensing. Background Technology

[0002] Asteroid exploration missions need to achieve close-range detection of near-Earth asteroids, sample return, and flyby detection of main-belt comets through a single launch, providing scientific data and real samples for cutting-edge scientific research on the origin and evolution of small celestial bodies, enabling my country to reach an internationally advanced level in the field of asteroid exploration. Currently, existing ground-penetrating radars are mainly used on the ground and were not designed with the requirements of deep space exploration missions in terms of weight, volume, reliability, data rate, etc., so they cannot effectively detect the internal structure of asteroids. Summary of the Invention

[0003] The technical problem solved by this invention is that, in the current technology, ground-penetrating radar cannot meet the requirements of deep space exploration missions in terms of radar weight, small size, high reliability, and low data rate. Therefore, a design method for a ground-penetrating radar system for detecting the internal structure of small celestial bodies is proposed.

[0004] The present invention solves the above-mentioned technical problem through the following technical solution:

[0005] A design method for a ground-penetrating radar system for detecting the internal structure of small celestial bodies, which involves building and laying out an electronics box, and determining the detection frequency and bandwidth based on the target detection depth and detection resolution requirements;

[0006] Low-frequency antennas and high-frequency antennas were designed according to the detection frequency and bandwidth, and mounted on the electronics box.

[0007] Determine the detection scenario and design the detection timing parameters;

[0008] Define the radar penetration equation and determine the radar transmission power based on the radar penetration equation;

[0009] Based on the target detection requirements, the penetration depth requirements for different frequency bands are determined, the minimum radar transmit power for low frequency and high frequency is calculated, and the low frequency antenna and high frequency antenna are set up by combining the detection timing parameters and the minimum radar transmit power.

[0010] The electronics box houses digital units, signal processors, transceiver channels, solid-state amplifiers, and local oscillator modules. The relative layout of the electronics box, low-frequency antenna, and high-frequency antenna is adjusted according to the length of the RF cable connecting the electronics box, low-frequency antenna, and high-frequency antenna to ensure that the RF cable length is minimized.

[0011] The low-frequency antenna is an orthogonally placed symmetrical dipole antenna with four auxiliary dipoles loaded outside to increase the operating bandwidth of the low-frequency antenna. The low-frequency antenna is a deployable antenna, and the deployment drive mechanism adopts a tape measure spring hinge. The inner dipole of the low-frequency antenna adopts a tape measure-like structure and is fixed by a dipole support. During the low-frequency antenna retraction process, the dipole is wound around the dipole support and locked and fixed by a locking and releasing device. During the low-frequency antenna deployment process, the locking and releasing device is unlocked, and the dipole is deployed into place by the elastic force. The locking and releasing device adopts a fused wire locking and releasing device.

[0012] The high-frequency antenna is a Vivaldi gradient slot planar antenna, which is connected to the internal modules of the electronics box through a low-pass filter for transmitting and receiving signals in different frequency bands.

[0013] The detection frequency includes low-frequency detection frequency, high-frequency detection frequency, and the method for determining the low-frequency detection frequency and bandwidth is as follows:

[0014] The relationship between the attenuation constant of the electromagnetic wave signal in the target celestial medium and the detection frequency is determined, and the loss tangent of the medium is determined based on the absolute permeability of the target celestial medium.

[0015] Based on the relationship between the attenuation constant and the detection frequency, as well as the loss tangent, the required frequency and bandwidth for low-frequency detection are adjusted to ensure that both the detection frequency and the loss tangent are minimized within the target range of the detection mission.

[0016] The high-frequency detection frequency, high-frequency and low-frequency detection bandwidth are all determined according to the resolution requirements of the corresponding detection task.

[0017] The method for designing detection timing parameters is as follows:

[0018] Determine the parameters of the step frequency signal to be transmitted before target detection;

[0019] The step frequency signal parameters include the step frequency interval Δf, pulse width T, and pulse repetition interval T. r Pulse number N, receiving time window T w Sampling interval T s :

[0020] Determine the target's orbital altitude, and based on the step frequency signal parameters, target medium, and target altitude, determine the low-frequency detection timing sequence and high-frequency detection timing sequence.

[0021] In the stepped frequency signal parameters, the stepped frequency interval Δf is designed based on the surface parameters of different levels of the target celestial body; the pulse width T is determined based on the echo aliasing requirements of different levels of the target celestial body and the orbital altitude of the detection radar; the pulse repetition interval T rThe pulse number N is determined based on the echo time window requirements of different levels of the target celestial body's surface and the orbital altitude of the detection radar; the receiving time window T is determined based on the spatial sampling rate requirements of the target celestial body's surface observation. w The sampling interval T is determined based on the pulse width T, the penetration propagation time of the target celestial body's subsurface, the target celestial body's rotation direction, and the echo signal required by the synthetic aperture program. s Determined based on the theoretical sampling rate of the detection radar.

[0022] The radar transmit power is determined to meet the signal-to-noise ratio requirement of the subsurface echo of the target celestial body, and the specific radar penetration equation is as follows:

[0023]

[0024] In the formula, P t The minimum radar transmit power is given by α, where α is the attenuation factor of the medium, and R is the minimum transmit power. in R is the one-way propagation distance in the medium; σ is the distance between the target and the antenna in the corresponding frequency band; B is the target RCS; λ is the system bandwidth; G is the wavelength. t G r For transmit and receive antenna gain; G p For processing gain; T is the noise temperature; F is the noise temperature. n denoted as receiver noise figure; L as receiver insertion loss; k as Boltzmann constant; and SNR detection threshold as SNR. thr .

[0025] The signal-to-noise ratio (SNR) detection threshold thr Based on the target detection requirements, and provided that the signal-to-noise ratio of the target celestial body echo meets the signal-to-noise ratio detection threshold, the minimum radar transmit power for the low-frequency or high-frequency detection channel is calculated using the signal-to-noise ratio detection threshold, in order to complete the setup of the low-frequency and high-frequency antennas.

[0026] The advantages of this invention compared to the prior art are:

[0027] This invention provides a design method for a ground-penetrating radar system for detecting the internal structure of small celestial bodies. Addressing the requirements of deep penetration and high resolution in radar detection, it proposes a system design concept combining low-frequency and high-frequency channels. By having the low-frequency and high-frequency channels work in a time-division multiplexing manner, the requirements of deep penetration and high resolution are achieved at minimal cost. Simultaneously, considering the constraint of a relatively low overall data transmission rate, the design incorporates a reduced data rate and a lightweight integrated design to meet the stringent weight and size requirements of the entire platform. The use of a stepped-frequency radar system can significantly reduce the data rate, and the system is simpler than time-domain systems, facilitating miniaturization and lightweight design. Attached Figure Description

[0028] Figure 1 A block diagram of a ground-penetrating radar system provided for the invention;

[0029] Figure 2 A schematic diagram illustrating the variation of the loss tangent of deep-space planetary material with frequency, provided for the invention.

[0030] Figure 3 Schematic diagram of different slant distances of surface targets provided for the invention; Detailed Implementation

[0031] A design method for a ground-penetrating radar system for detecting the internal structure of small celestial bodies is presented. The system includes a low-frequency detection section and a high-frequency detection section. By building an electronics box, the detection frequency and bandwidth are determined according to the target detection depth and resolution requirements. The detection timing parameters and the minimum radar transmit power corresponding to the low and high frequencies are designed respectively. The low-frequency and high-frequency antennas of the ground-penetrating radar system peripherals are designed, which takes into account both deep penetration and high resolution. The system is also small in size, light in weight, and highly reliable. It can be extended to detect the internal structure of other celestial bodies in deep space exploration.

[0032] The design method for a ground-penetrating radar system includes the following specific steps:

[0033] Construct and lay out the electronics box, and determine the detection frequency and bandwidth according to the target detection depth and detection resolution requirements;

[0034] Low-frequency antennas and high-frequency antennas were designed according to the detection frequency and bandwidth, and mounted on the electronics box.

[0035] Determine the detection scenario and design the detection timing parameters;

[0036] Define the radar penetration equation and determine the radar transmission power based on the radar penetration equation;

[0037] Based on the target detection requirements, the penetration depth requirements for different frequency bands are determined, the minimum radar transmit power for low frequency and high frequency is calculated, and the low frequency antenna and high frequency antenna are set up by combining the detection timing parameters and the minimum radar transmit power.

[0038] The electronics box houses digital units, signal processors, transceiver channels, solid-state amplifiers, and local oscillator modules. The relative layout of the electronics box, low-frequency antenna, and high-frequency antenna is adjusted according to the length of the RF cable connecting them to ensure the shortest possible RF cable length.

[0039] The low-frequency antenna uses a symmetrical dipole antenna orthogonally placed, with four auxiliary dipoles loaded outside the symmetrical dipole to increase the operating bandwidth of the low-frequency antenna. The low-frequency antenna is a deployable antenna, and the deployment drive mechanism uses a tape measure spring hinge. The dipole inside the low-frequency antenna adopts a tape measure-like structure and is fixed by a dipole support. During the low-frequency antenna retraction process, the dipole is wound around the dipole support and locked and fixed by a locking and releasing device. During the low-frequency antenna deployment process, the locking and releasing device is unlocked, and the dipole is deployed into place by the elastic force. The locking and releasing device is a fused wire locking and releasing device.

[0040] The high-frequency antenna is a Vivaldi gradient slot planar antenna, which is connected to the internal modules of the electronics box through a low-pass filter for transmitting and receiving signals in different frequency bands.

[0041] The detection frequency includes low-frequency detection frequency, high-frequency detection frequency, and the method for determining the low-frequency detection frequency and bandwidth is as follows:

[0042] The relationship between the attenuation constant of the electromagnetic wave signal in the target celestial medium and the detection frequency is determined, and the loss tangent of the medium is determined based on the absolute permeability of the target celestial medium.

[0043] Based on the relationship between the attenuation constant and the detection frequency, as well as the loss tangent, the required frequency and bandwidth for low-frequency detection are adjusted to ensure that both the detection frequency and the loss tangent are minimized within the target range of the detection mission.

[0044] The high-frequency detection frequency, high-frequency and low-frequency detection bandwidth are all determined according to the resolution requirements of the corresponding detection mission.

[0045] The method for designing detection timing parameters is as follows:

[0046] Determine the parameters of the step frequency signal to be transmitted before target detection;

[0047] The step frequency signal parameters include the step frequency interval Δf, pulse width T, and pulse repetition interval T. r Pulse number N, receiving time window T w Sampling interval T s :

[0048] Determine the target's orbital altitude, and based on the step frequency signal parameters, target medium, and target altitude, determine the low-frequency detection timing sequence and high-frequency detection timing sequence.

[0049] In the stepped frequency signal parameters, the stepped frequency interval Δf is designed according to the surface parameters of different levels of the target celestial body; the pulse width T is determined according to the echo aliasing requirements of different levels of the target celestial body and the orbital altitude of the detection radar; the pulse repetition interval T rThe pulse number N is determined based on the echo time window requirements of different levels of the target celestial body's surface and the orbital altitude of the detection radar; the receiving time window T is determined based on the spatial sampling rate requirements of the target celestial body's surface observation. w The sampling interval T is determined based on the pulse width T, the penetration propagation time of the target celestial body's subsurface, the target celestial body's rotation direction, and the echo signal required by the synthetic aperture program. s Determined based on the theoretical sampling rate of the detection radar.

[0050] The radar transmit power is determined to meet the signal-to-noise ratio (SNR) requirements of the subsurface echo from the target celestial body, and the SNR detection threshold is determined accordingly. thr Based on the target detection requirements, and provided that the signal-to-noise ratio of the target celestial body echo meets the signal-to-noise ratio detection threshold, the minimum radar transmit power for the low-frequency or high-frequency detection channel is calculated using the signal-to-noise ratio detection threshold, in order to complete the setup of the low-frequency and high-frequency antennas.

[0051] The following description, in conjunction with the accompanying drawings and preferred embodiments, provides further details:

[0052] In the current embodiment, the target detection mission requirements for the target celestial body are as follows:

[0053] (1) Low-frequency detection

[0054] It can penetrate 2016HO3 to detect and image the entire internal structure in order to determine whether it is a rubble pile or a monolithic structure.

[0055] Imaging observations were conducted 300m below the surface of 311P to directly observe the deep structure and material properties;

[0056] The subsurface distance resolution is less than 5m, which is used to distinguish internal boulders from layered structures;

[0057] By employing circular polarization emission and dual polarization reception, the circular polarization ratio can be accurately measured to study surface roughness and the distribution of internal stones.

[0058] (2) High-frequency detection

[0059] Imaging observations were conducted 5m below the surface of 2016HO3 to detect its shallow surface structure and material properties.

[0060] Imaging observations were conducted up to 30 m below the surface of 311P to enable refined direct observation of the structure and properties of shallow materials.

[0061] The subsurface resolution is less than 0.25m to ensure imaging observation of shallow surface structures and debris.

[0062] To address the above characteristics and for missions involving the exploration of the internal structures of small celestial bodies, the current embodiment proposes a system design combining low-frequency and high-frequency channels to meet the mission requirements of deep penetration and high resolution for the radar. By having the low-frequency and high-frequency channels work in a time-division multiplexing manner, the mission requirements of deep penetration and high resolution are achieved at minimal cost. Simultaneously, as a payload for deep space exploration missions, the constraint of a relatively low overall data transmission rate must be considered, necessitating a reduction in the data rate during the design phase. Furthermore, a lightweight, integrated design is required to meet the stringent weight and size requirements of the entire platform. Therefore, a stepped-frequency radar system was chosen, which can significantly reduce the data rate, and the system is simpler than time-domain systems, facilitating miniaturization and lightweight design.

[0063] The block diagram of the detection radar system is as follows: Figure 1 As shown, to achieve miniaturization and integration of the detection radar, a design approach was adopted that integrates the power distributor, signal processor, transmitting channel, receiving channel, solid-state amplifier, and local oscillator into a single unit, namely the electronics box. Both the low-frequency and high-frequency antennas are shared transmitting and receiving antennas, and their layout on the device is as close as possible to shorten the length of the RF cable, reducing weight while also achieving low insertion loss in the system.

[0064] The low-frequency antenna of the detection radar adopts a symmetrical dipole antenna design, achieving dual-polarized beam coverage through two pairs of orthogonally placed dipoles. Four auxiliary dipoles are added to increase the antenna's operating bandwidth. To meet the overall envelope requirements, the low-frequency antenna is designed as a deployable antenna. To simplify the design and reduce the weight of the deployment mechanism, a tape measure spring hinge is used as the drive mechanism for deployment. The dipoles also adopt a tape measure-like structure; when retracted, the dipoles are wound around the dipole support and locked in place by locking straps. After the antenna is unlocked, the dipoles unfold into place under their own elastic force. The locking and releasing device uses a fused wire locking and releasing device. The biggest advantages of this device are low cost, low unlocking impact, light weight, and reusability, meeting the requirements of miniaturized design for the detection radar.

[0065] To achieve ultra-wideband performance, the high-frequency antenna for detection radar employs a Vivaldi tapered slot planar antenna, which features end-fire characteristics and an ultra-wide operating bandwidth. A Vivaldi antenna is a planar, horn-like structure, typically consisting of a slotted radiating aperture that gradually widens. It uses various types of slot lines, usually exponential curves. The fan-shaped opening formed by the relatively distant separation of the exponential curves constitutes its primary operating region, i.e., the main area for receiving or radiating electromagnetic waves. Depending on the frequency band, the area for transmitting and receiving electromagnetic waves shifts throughout the slotted radiating aperture, thereby achieving its ultra-wideband performance.

[0066] The detection process is as follows:

[0067] A. Select the radar system frequency and bandwidth based on the detection depth and depth resolution requirements.

[0068] For a specific target, the operating frequency of an asteroid detection radar is one of the main factors affecting the subsurface detection depth. Generally, the higher the frequency, the worse the penetration capability; as the frequency increases, the propagation loss of electromagnetic waves in the medium increases exponentially. The relationship between medium attenuation and radar frequency is as follows:

[0069]

[0070] In the formula, α is the attenuation constant, ω is the angular frequency of the electromagnetic wave, μ is the absolute permeability of the medium, and μ = μ0μ r μ0 = 1.26 × 10 -6 H / m is the absolute permeability in free space. Generally, for most soils, most of the underground medium is non-magnetic, so we take μ = μ0. tanδ is the loss tangent of the medium.

[0071] The operating frequency of asteroid radar should be selected under conditions where attenuation is minimal within the internal medium of asteroids and main-belt comets. Asteroid 2016HO3 is primarily composed of soil and rock. Soil and rock exhibit different attenuations at different frequencies. Referring to existing measurements of lunar soil and rocks, the loss tangent of deep-space planetary materials such as soil and rocks is as follows: Figure 2 As shown;

[0072] It can be seen that the loss tangent of rocks is generally greater than that of soil, and the attenuation of rocky material in asteroids is the main reason for the decay inside asteroids. The loss tangent of rocks is at a relatively low level between 100MHz and 1GHz.

[0073] Asteroid detection radar has two mission requirements: deep penetration and high resolution. For deep penetration mode, low-frequency bands should be selected as much as possible to reduce medium attenuation, while for high-resolution mode, high-frequency bands should be selected as much as possible to increase the operating bandwidth.

[0074] Low-frequency channels increase penetration depth

[0075] The loss tangent of the dielectric and the frequency together determine the dielectric attenuation:

[0076] 1. At the same frequency, the smaller the dielectric loss tangent, the smaller the dielectric attenuation;

[0077] 2. For different frequencies, when the dielectric loss tangent is constant, the smaller the frequency, the smaller the dielectric attenuation.

[0078] Therefore, the deep penetration mode should select a frequency as low as possible and a loss tangent as small as possible. The loss tangent of rock is close to its minimum at 200MHz, and the attenuation factor of water ice also reaches its minimum at 200MHz. Taking into account the attenuation of soil, rock, and water ice, the center frequency of the low-frequency channel of the asteroid detection radar is selected between 100 and 200MHz to achieve a deeper penetration depth.

[0079] High-frequency channels improve resolution

[0080] Decimeter-level resolution requires a bandwidth of over 1 GHz. Asteroid detection radars have a high-frequency center frequency of less than 1 GHz and a bandwidth greater than 1 GHz.

[0081] The operating bandwidth depends on the resolution requirement; the higher the resolution, the wider the operating bandwidth is required. The relationship between bandwidth B and resolution Δd is as follows:

[0082]

[0083] In the formula, ε r is the relative permittivity of the shallow layer.

[0084] Taking into account the requirements for detection depth and resolution, the small celestial body detection radar is determined to have a dual-channel working mode with low-frequency and high-frequency channels. The low-frequency channel has a center frequency of 150MHz and a bandwidth of 40MHz, while the high-frequency channel has a center frequency of 900MHz and a bandwidth of 1200MHz.

[0085] B. Design the radar antenna based on the radar's operating frequency and bandwidth.

[0086] Because the low-frequency channel of the detection radar is 150MHz±20MHz and the high-frequency channel is 900MHz±600MHz, if one antenna is to cover the entire low-frequency and high-frequency channel range, the antenna needs to operate at a frequency of 130MHz~1200MHz, which is close to 10 octaves. It is difficult to achieve an antenna that meets this specification in engineering. Therefore, two antennas are chosen, one for the low-frequency channel and one for the high-frequency channel.

[0087] The low-frequency channel operates at 150MHz ± 20MHz. Considering the stringent requirements of deep space exploration regarding weight, size, and reliability, a symmetrical dipole antenna was used in the design. Two pairs of orthogonally placed dipoles achieve dual-polarized beam coverage, and four auxiliary dipoles are added to increase the antenna's operating bandwidth. To meet the overall envelope requirements, the low-frequency antenna is designed as a deployable antenna. To simplify the design and reduce the weight of the deployment mechanism, a tape measure spring hinge is used as the drive mechanism for deployment. The dipoles also adopt a tape measure-like structure; when retracted, the dipoles are wound around the dipole support and locked in place by a locking strap. After the antenna is unlocked, the dipoles unfold into place under their own elastic force. The locking and releasing device uses a fused wire locking and releasing mechanism. The biggest advantages of this device are low cost, minimal unlocking impact, light weight, and reusability, meeting the requirements of miniaturized design for the detection radar.

[0088] The high-frequency channel operates at 900MHz ± 600MHz, therefore, a Vivaldi antenna was considered for implementation in the design. The Vivaldi antenna, proposed by Gibson in 1979, is an exponentially tapered slot antenna. As an ultra-wideband traveling-wave slot antenna, its gradually widening slot structure, forming a horn shape, is the main body for radiating or receiving energy. Different regions of the slot radiate or receive electromagnetic waves at different frequencies. The Vivaldi antenna features end-firing, wide bandwidth, high gain, and symmetrical pattern, meeting the requirements of high-frequency channels for detection radar, offering both bandwidth and high gain.

[0089] C. Design timing parameters for the detection radar system based on the detection scenario.

[0090] The asteroid detection radar transmits stepped frequency signals, among which key waveform parameters include the stepped frequency interval Δf, pulse width T, and pulse repetition interval T. r Pulse number N, receiving time window T w Sampling interval T s :

[0091] Step frequency interval Δf

[0092] The rough surfaces and subsurfaces of asteroids will produce diffuse scattering, resulting in broadening of surface and subsurface echoes. Echo broadening should fall within the time-domain extension period of the stepped frequency signal to avoid range aliasing.

[0093]

[0094] Different slant distances of surface targets, such as Figure 3 As shown;

[0095] Pulse width T

[0096] To prevent surface echoes from aliasing with subsurface echoes, the pulse width T and step frequency Δf must satisfy the following conditions:

[0097]

[0098] In addition, when the system uses the same antenna for both transmission and reception, the pulse width T is also limited by the orbital height H of the detection radar.

[0099] Pulse repetition interval T r

[0100] To ensure that surface and subsurface echoes from asteroids are received within a single echo window, the pulse repetition interval T... r Limited by orbital altitude and subsurface exploration depth requirements.

[0101] Pulse synthesis time NT r

[0102] Due to the asteroid's rotation, if NT r The value is relatively large, causing the azimuth displacement of the edge point corresponding to a single measurement to exceed λ / 2. This means that the spatial sampling rate for observing the celestial surface will exceed λ / 2, which does not satisfy the spatial sampling theorem required for imaging asteroids.

[0103] Receive time window T w

[0104] The receive window should contain at least the following elements:

[0105] 1. Pulse width T;

[0106] 2. Subsurface depth penetration propagation time;

[0107] 3. Non-nadir surface and subsurface echoes within the angle required for synthetic aperture imaging in the asteroid's rotation direction;

[0108] Sampling interval T s

[0109] Subsurface echoes may broaden after propagating through the medium, so the sampling rate should be appropriately increased based on the theoretical sampling rate.

[0110] Receive gate start position

[0111] The radar opens its receiving gate before the echo arrives, based on the orbital altitude information.

[0112] Taking the scenario of asteroid exploration at an orbital altitude of 600m as an example, the timing parameters for the low-frequency channel and the high-frequency channel are shown in Table 1 below.

[0113] Table 1 Waveform parameters measured by asteroid exploration radar

[0114]

[0115] D. Determine the radar transmit power based on the ground-penetrating radar equations.

[0116] According to the surface penetration radar equation, in order to ensure that the signal-to-noise ratio (SNR) of the asteroid's subsurface echo reaches the SNR detection threshold... thr Minimum transmit power P t As shown below:

[0117]

[0118] α—the attenuation factor of the medium, see Equation 1 for details;

[0119] R in —The one-way propagation distance in the medium;

[0120] R—the distance between the target and the antenna;

[0121] σ — Target RCS;

[0122] B – System bandwidth;

[0123] λ — wavelength;

[0124] G t G r —Transmit and receive antenna gain;

[0125] G p —Process gain;

[0126] T—Noise temperature;

[0127] F n —Receiver noise figure;

[0128] L—Receiver insertion loss;

[0129] k — Boltzmann constant.

[0130] Taking a certain asteroid as an example, the attenuation of the internal medium of the asteroid is shown in Table 2. Assuming a relative permittivity of 9 and a loss tangent of 0.01, the attenuation of the internal medium of the asteroid is calculated based on the attenuation constant α:

[0131] Table 2. Attenuation of the internal medium of asteroids

[0132]

[0133] Based on the detection requirements of the detection radar, taking a low-frequency channel penetration depth of 50m and a high-frequency channel penetration depth of 5m as examples, the minimum transmission power is calculated according to the radar equation, as shown in Table 3.

[0134] Table 3 Minimum Transmit Power of Detection Radar

[0135]

[0136] Considering a certain margin, the system's transmit power can be designed to be 40dBm for the low-frequency channel and 20dBm for the high-frequency channel.

[0137] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications to the technical solutions of the present invention by utilizing the methods and techniques disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall fall within the protection scope of the technical solutions of the present invention.

[0138] The contents not described in detail in this specification are common knowledge to those skilled in the art.

Claims

1. A design method for a ground-penetrating radar system for detecting the internal structure of small celestial bodies, characterized in that: Construct and lay out the electronics box, and determine the detection frequency and bandwidth according to the target detection depth and detection resolution requirements; Low-frequency antennas and high-frequency antennas were designed according to the detection frequency and bandwidth, and mounted on the electronics box. Determine the detection scenario and design the detection timing parameters; Define the radar penetration equation and determine the radar transmission power based on the radar penetration equation; Based on the target detection requirements, the penetration depth requirements for different frequency bands are determined, the minimum radar transmit power for low frequency and the minimum radar transmit power for high frequency are calculated, and the low frequency antenna and high frequency antenna settings are completed by combining the detection timing parameters and the minimum radar transmit power. The electronics box contains digital units, signal processors, transceiver channels, solid-state amplifiers, and local oscillator modules. The relative layout of the electronics box, low-frequency antenna, and high-frequency antenna is adjusted according to the length of the RF cable connecting the electronics box, low-frequency antenna, and high-frequency antenna to ensure that the RF cable length is minimized. The low-frequency antenna adopts a symmetrical dipole antenna orthogonally placed, with four auxiliary dipoles loaded outside the symmetrical dipole to increase the working bandwidth of the low-frequency antenna; the low-frequency antenna is a deployable antenna, the deployment drive mechanism adopts a tape measure spring hinge, and the inner dipole of the low-frequency antenna adopts a tape measure-like structure and is fixed by a dipole support. During the low-frequency antenna retraction process, the vibrator is wound around the vibrator support and locked in place by the locking and releasing device; during the low-frequency antenna unfolding process, the locking and releasing device is unlocked, and the vibrator unfolds into place by the elastic force; the locking and releasing device adopts a fused wire locking and releasing device. The high-frequency antenna is a Vivaldi gradient slot planar antenna, which is connected to the internal modules of the electronics box through a low-pass filter for transmitting and receiving signals in different frequency bands.

2. The design method of a ground-penetrating radar system for detecting the internal structure of small celestial bodies according to claim 1, characterized in that: The detection frequency includes low-frequency detection frequency, high-frequency detection frequency, and the method for determining the low-frequency detection frequency and bandwidth is as follows: The relationship between the attenuation constant of the electromagnetic wave signal in the target celestial medium and the detection frequency is determined, and the loss tangent of the medium is determined based on the absolute permeability of the target celestial medium. Based on the relationship between the attenuation constant and the detection frequency, as well as the loss tangent, the required frequency and bandwidth for low-frequency detection are adjusted to ensure that both the detection frequency and the loss tangent are minimized within the target range of the detection mission.

3. The design method of a ground-penetrating radar system for detecting the internal structure of small celestial bodies according to claim 2, characterized in that: The high-frequency detection frequency, high-frequency and low-frequency detection bandwidth are all determined according to the resolution requirements of the corresponding detection task.

4. The design method of a ground-penetrating radar system for detecting the internal structure of small celestial bodies according to claim 3, characterized in that: The method for designing detection timing parameters is as follows: Determine the parameters of the step frequency signal to be transmitted before target detection; Step frequency signal parameters include step frequency interval Pulse width T, pulse repetition interval Pulse number N, receiving time window Sampling interval : Determine the target's orbital altitude, and based on the step frequency signal parameters, target medium, and target altitude, determine the low-frequency detection timing sequence and high-frequency detection timing sequence.

5. The design method of a ground-penetrating radar system for detecting the internal structure of small celestial bodies according to claim 4, characterized in that: Among the step frequency signal parameters, the step frequency interval... Designed based on the surface parameters of the target celestial body at different levels; The pulse width T is determined based on the echo aliasing requirements of different levels of the target celestial body's surface and the orbital altitude of the detection radar; the pulse repetition interval The pulse number N is determined based on the echo time window requirements of different levels of the target celestial body's surface and the orbital altitude of the detection radar; the receiving time window is determined based on the spatial sampling rate requirements of the target celestial body's surface observation. The sampling interval is determined based on the pulse width T, the penetration propagation time of the target celestial body's subsurface, the target celestial body's rotation direction, and the echo signal required by the synthetic aperture program. Determined based on the theoretical sampling rate of the detection radar.

6. The design method of a ground-penetrating radar system for detecting the internal structure of small celestial bodies according to claim 5, characterized in that: The radar transmit power is determined to meet the signal-to-noise ratio requirement of the subsurface echo of the target celestial body, and the specific radar penetration equation is as follows: In the formula, For minimum radar transmit power, The attenuation factor of the medium, This refers to the one-way propagation distance in the medium. The distance between the target and the corresponding frequency band antenna; For target RCS; B For system bandwidth; Wavelength; , For the gain of the transmitting and receiving antennas; To process gain; T Noise temperature; The receiver noise figure; L For receiver insertion loss; k Where is the Boltzmann constant, and the signal-to-noise ratio detection threshold is... .

7. The design method of a ground-penetrating radar system for detecting the internal structure of small celestial bodies according to claim 6, characterized in that: The signal-to-noise ratio detection threshold Based on the target detection requirements, and provided that the signal-to-noise ratio of the target celestial body echo meets the signal-to-noise ratio detection threshold, the minimum radar transmit power for the low-frequency or high-frequency detection channel is calculated using the signal-to-noise ratio detection threshold, in order to complete the setup of the low-frequency and high-frequency antennas.

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