Open Access
Add to BibSonomy
Abstract
This paper presents an integrated imaging system of optical camera and synthetic aperture radar (SAR). The system can realize 400 nm–900 nm visible and near infrared band and 35 GHz microwave Ka band dual-band imaging. Compared with the single band imaging system, the observation ability and environmental adaptability of the integrated imaging system have been significantly improved. The optical camera shares a common front system with the synthetic aperture radar. After simulation, the average modulation transfer function (MTF) of 50 line pairs per millimeter (lp/mm) of the optical subsystem is 0.47. In addition, a principle prototype with a pupil diameter of 210 mm was developed to verify the performance of synthetic aperture radar antennas. After the experimental test, the SAR radiation pattern simulation results are in good conformity with the measured results, which are in line with the expected results.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Synthetic aperture radar (SAR) and optical camera are the two most commonly used remote sensing devices, which are widely used in military reconnaissance, natural disaster rescue, forest and ocean observation and other fields. The optical camera has the ability of high-resolution imaging, and its image color is rich, which can truly reveal the color characteristics of the target, which is in line with human visual habits, and is easy to observe and identify the target; However, optical cameras are susceptible to light conditions and weather and other factors, and its observation ability is limited in nights and rainy days. The synthetic aperture radar has the advantages of all-day and all-weather observation, and is not restricted by factors such as illumination and climatic conditions; However, synthetic aperture radar can only reflect the electromagnetic scattering characteristics of the target in a specific band, and its image cannot reflect the color characteristics of the target, which is inconsistent with the visual habits of the human eye; In addition, its observation ability will be limited in the complex electromagnetic environment [1–2]. Each imaging system has its inherent limitations, and if optical cameras can be combined with synthetic aperture radar to achieve the functions of both devices in one device, their observation capability will be greatly enhanced. The integrated imaging system can give full play to the advantages of the two, make up for each other's shortcomings to a certain extent, meet the needs of all-day and all-weather intelligence perception and information acquisition ability of the observed area, and improve the environmental adaptability of the equipment. In addition, the target feature information presented by different observation bands is also different. In the case of good climate and environmental conditions, the integrated system can realize the visible light and microwave dual band simultaneous imaging of the target area, effectively enhance the observation ability of the equipment, improve the accuracy of observation, and reduce the misjudgment rate.
Most synthetic aperture radar (SAR) currently uses mesh antennas, often covering eight bands from P-band (frequency 230 MHz–1000 MHz, wavelength 1300 mm–300 mm) to Ka-band (frequency 27 GHz–40 GHz, wavelength 11.11 mm–7.5 mm). In order to improve microwave reflectivity and reduce energy loss, the mesh spacing of mesh antenna will be reduced with the increase of selected band frequency, The denser the grid, the higher the microwave reflectivity of the antenna. However, when the frequency of the working band is higher than that of the X-band (frequency 8 GHz–12 GHz, wavelength 37.5 mm–25 mm), the mesh antenna will be difficult to meet the high-quality imaging requirements of the SAR system. In order to achieve better imaging effect, it is necessary to use the solid reflector antenna. Since the surface accuracy of the optical mirror is much higher than that of the SAR reflector antenna, it is a good choice to use optical mirror as radar antenna [3–4]. Using the reflector of optical camera as the antenna of synthetic aperture radar (SAR) can not only enhance the image quality of SAR, but also save the manufacturing cost of the antenna and the emission cost of the equipment.
In summary, the integrated imaging system of SAR and optical camera can switch observation bands according to different scenes, which not only improves the observation ability of the equipment, but also enhances its environmental adaptability. The use of optical camera reflectors as radar antennas can reduce the overall size of the equipment and reduce costs while maintaining high accuracy. Therefore, the integrated imaging equipment of SAR and optical camera has important research significance and application prospect.
2. Integrated design principles and selection
Synthetic aperture radar and optical camera are quite different in many aspects such as imaging principle, system structure, design requirements, processing and adjustment. For example, synthetic aperture radar is an active imaging device that acquires ground object information by transmitting microwave pulses and receiving target scattered echoes [5–10]. The antenna subsystem of SAR is composed of feed arrays, reflector antennas, and waveguides. In order to reduce the number of reflections, the antenna subsystem usually adopts the design of a single reflector antenna. The reflector antenna is parabolic, and the focal lens ratio is generally less than 1. A small focal lens ratio is conducive to beam energy collection and reduces energy scattering. The optical camera is a passive imaging device that directly receives the sunlight reflected by the target for imaging. In order to reduce the volume of the camera and reduce the chromatic aberration, the high-resolution optical remote sensing camera usually adopts reflective or catadioptric optical system. The reflector surface type mostly adopts quadric or aspherical. The F number of mirror (corresponding to the focal lens ratio of SAR system) is generally greater than 0.7. The smaller the F number, the steeper the edge of the reflector and the more difficult it is to process, which is not conducive to the control of surface accuracy.
The differences between the two systems lead to many contradictions in the integrated design, which also makes the integrated design have more constraints. Therefore, in the process of integrated system design, we need to consider the system characteristics and design requirements of the two kinds of equipment, and comprehensively consider various factors, in order to give a reasonable and feasible design scheme.
2.1 Integrated design classification
The integrated design is generally divided into two schemes: coaxial scheme and off-axis scheme. The conceptual diagram of coaxial design scheme is shown in Fig. 1. The SAR system adopts the center feed structure of single reflector antenna, and the optical system adopts the coaxial catadioptric structure. The visible light and microwave share the same optical primary mirror. The optical secondary mirror can transmit the microwave and reflect the visible light. The microwave feed array and thermal control system are installed on the back of the secondary mirror. The visible light is reflected by the secondary mirror and then imaged by the rear optical system. A band separation device for reflecting microwave and transmitting visible light is set at the central opening of the primary mirror to improve the microwave reflectance [5–7].
The coaxial design scheme has a simple and compact structure. The optical primary and secondary mirrors are all rotationally symmetrical, which is easy for mirror processing, inspection and equipment installation and adjustment, and the cost is relatively low. The biggest problem with coaxial design is the existence of central obstruction. In order to obtain sufficient power, SAR systems usually use multiple feeds to form a feed array, which will block the central beam. For center feed structure, the closer to the center of the antenna, the greater the microwave energy density, and the greater the energy loss caused by central obstruction. Therefore, the center feed design is more suitable for the case where the feed array is small.
The off-axis scheme is shown in Fig. 2, which corresponds to the offset feed design of SAR system. Microwave and visible light share the primary and secondary mirrors of the optical camera. The two bands are separated at the third mirror. The third mirror uses a microwave-permeable material substrate and is coated with a film that reflects visible band. The microwave passes through the third mirror to the feed, and the visible light is reflected to the rear optical system to complete the imaging.
The off-axis scheme can effectively solve the central obstruction problem of the coaxial scheme, and greatly reduce the energy loss of the system. The SAR system uses a dual-reflector antenna design, which improves the freedom of system design. However, the structure of the off-axis scheme is relatively complex, and the mirror is non-rotationally symmetric, so the difficulty and cost of processing, inspection, and adjustment are higher than the center feed scheme. The offset feed design is more suitable for situations where the feed array is large and the central obstruction is large, and its cost is relatively high.
Both schemes have their own advantages and disadvantages. The specific design should be based on the actual use environment requirements combined with technical indicators to select the appropriate scheme.
2.2 Integrated design scheme
Based on the Low Earth Orbit (LEO) satellite platform, an integrated design scheme of optical camera and synthetic aperture radar is proposed in this paper. The design diagram is shown in Fig. 3.
The integrated system consists of two parts. The first half is an afocal system, including optical primary mirror and secondary mirror, which mainly serves to compress the beam. The primary and secondary mirrors of the system are parabolic, and the visible light and microwave share the primary and secondary mirrors [11].
This integrated design scheme is mainly based on the following considerations:
- 1) This design scheme is based on the off-axis design scheme, and its main purpose is to avoid the center obscuration problem, which can largely reduce the energy loss of the SAR system, and the off-axis scheme has less limitation on the feed array size. Ka-band feedhorns are usually smaller in size and weaker in power, so its feed module usually consists of multiple feed horns in the form of an array. The wavefront emitted by a single feed horn is spherical, but when multiple feed horns form a feed array, the central part of the wave front of the emitted microwave can be approximated as a plane wave, which has good directivity.
- 2) The band separation device of the traditional off-axis integrated design adopts a solution that transmits microwaves and reflects visible light. This device achieves band separation by plating a layer of visible light band high reflection coating on a high microwave transmittance material substrate. At present, the materials with high microwave transmittance are mainly plastic materials, such as high-density foam plastics, polytetrafluoroethylene, etc., but such materials have poor stability and are not suitable for mirror substrates. Quartz glass has good hardness and stability and is suitable for mirror substrates, and has relatively high microwave transmittance. However, Boron trioxide(B2O3), Barium oxide (BaO), Sodium oxide (Na2O) and other substances are mixed in the production of optical glass, which will have a certain impact on microwave transmittance. In addition, high reflection coating is usually made of low refractive index material Silicon dioxide (SiO2) and high refractive index material Titanium Dioxide (TiO2) or Tantalum pentoxide (Ta2O5) or Hafnium oxide (HfO2) interleaved plating, while high refractive index coating materials are often metal oxides with high dielectric constant and low microwave transmission rate, resulting in the system energy cannot meet the synthetic aperture radar imaging requirements. The band separation device of the traditional off-axis integrated design scheme is difficult to achieve high-efficiency band separation. Therefore, based on the traditional design scheme, we changed to use a band separation device that reflects microwaves and transmits visible light. The visible light band has a higher transmittance in quartz glass. The anti-reflection coating on the glass surface can make the visible light band transmittance higher than 99%, the energy loss is negligible.
- 3) This design adopts a separation device scheme that transmits visible light and reflects microwaves, so the band separation device is equivalent to tilted parallel lithographic glass in the visible light path. Due to the convergent or divergent beam passing through the tilted parallel lithographic glass, nonlinear astigmatism will occur, resulting in a sharp decline in image quality, and correction of nonlinear astigmatism will also increase the system complexity. The afocal design of the first half of the system can make the visible light wave band emerge in parallel, which can effectively avoid the generation of nonlinear astigmatism. In addition, the dual-antenna structure of SAR can reduce the system's requirements for the antenna focal lens ratio, avoid the contradiction between the optical camera and the SAR in the F number of the main mirror (corresponding to the SAR antenna focal lens ratio), and the system design freedom is also higher.
Compared with the traditional coaxial design scheme, the integrated design scheme in this paper is more complex in structure, but it solves the problem of central obscuration and has higher energy efficiency; the traditional off-axis design scheme is simple and compact in structure and does not have the problem of central obscuration, but it is difficult to find a substrate material that meets both high microwave transmittance and optical mirror processing requirements, and the microwave transmittance of existing optical coating materials and mirror substrate materials cannot achieve the energy requirements of the SAR system imaging in this paper. The integrated design scheme in this paper is designed on the basis of the traditional off-axis design scheme, and its band separation device is more efficient and the scheme is reasonable and feasible.
3. Optical system calculation and analysis
When the orbit altitude of the satellite is determined, the focal length of the system can be calculated according to the resolution of the system and the pixel size of the detector. It should be noted that since the working mode of SAR is squint observation, the optical camera is also squint observation. The oblique viewing angle θ is generally between 20° and 60°. The actual observation height H′ can be calculated according to the equation H′=H/cos θ, where H is Orbital altitude. The initial structure of the optical system can be calculated by using the design parameters such as the focal length, field of view, and pupil diameter of the optical system in combination with the design requirements.
In this paper, the coaxial reflection system is used as the initial structure of the optical system. On this basis, the off-axis deviation pupil operation is performed and the design is further optimized. The first half of the initial structure is an afocal system, including the primary and secondary mirror. The beams enter and exit in parallel without aberration. After the beam passes through the primary and secondary mirror, the beam width is reduced by n times. In order to ensure that the secondary mirror has enough area to collect microwave energy, the size of the secondary mirror must be at least equivalent to the size of the feed array. Therefore, when the size of the primary mirror is determined, the value of n mainly depends on the size of the feed array. The feed array size is related to the beam width (here the beam width is the angle between the two half-power points, -3 dB points, of the SAR beam, independent of the other beam widths in the text), the scanning angle, and the efficiency of the back-end TR (transceiver amplifier assembly). It should be noted that when the beam width is reduced by n times, the field of view of the rear optical system will increase by n times, and the design difficulty will also increase; when the value of n is reduced, the size of the secondary mirror and the feed array will increase, the volume of the optical system will increase accordingly. Therefore, the selection of the value of n should comprehensively consider factors such as the design requirements of the SAR system, the overall size of the system and the design difficulty of the rear optical system [11].
The second half of the initial structure is a coaxial three mirror system, which has seven free variables: e12, e22, e32, β1, β2, α1, α2. Among them, e12, e22 and e32 represent the quadric characteristics of three reflecting surfaces, β1 is the magnification of the secondary mirror, β2 is the magnification of the third mirror, α1 is the obstruction ratio of the secondary mirror to the primary mirror, α2 is the obstruction ratio of the third mirror to the secondary mirror. Figure 4 is the schematic diagram of the initial structure of the three-mirror system.
Through the parameters α1, α2, β1, and β2, the light direction of the three-mirror system and the system structure type can be basically determined, combined with the system focal length f, the surface parameters of the system and the distance between the mirrors can be calculated by Eq. (2)-Eq. (4):
Among them, R1, R2 and R3 are the vertex curvature radii of primary mirror, secondary mirror and third mirror respectively, d1 is the distance from the primary mirror to the secondary mirror, d2 is the distance from the secondary mirror to the third mirror, and l3’ is the distance from the third mirror to the image plane. This design uses a one-time imaging system. There is no image surface between the primary and the secondary mirror, and the light beam emitted by the secondary mirror is a convergent light beam, so 0<α1<1, α2>0, β1<0, β2<0.
After the system structure parameters are determined, the quadric coefficients of the three mirrors can be calculated according to the aberration elimination condition of Eq. (5)–Eq. (8). The three-mirror optical system has seven free parameters, which can simultaneously eliminate four kinds of aberrations: spherical aberration, coma, astigmatism, field curvature. The aberration elimination condition are as follows:
S I=0,
S II=0,
S III=0,
S IV=0,
Through the above aberration elimination conditions, the quadric coefficients -e12, -e22 and-e32 of the primary mirror, secondary mirror, and third mirror can be determined. At this point, the parameters of the initial structure have been determined and can be brought into the design software for further optimization.
4. System design results
4.1 Optical system design
In this paper, the simulation design of the integrated system is carried out based on the 617 km orbit platform. The optical observation band is 400 nm–900 nm, covering visible light and near infrared, the ground observation width is greater than 10 km, the detector pixel size is 10 um × 10 um, and the oblique Angle of view is 30°. After calculation, the optical system design parameters are shown in Table 1:
Table 1. Optical system design parameters
The design result of the optical system is shown in Fig. 5. The system consists of five mirrors and a band separation device. Since the separation device reflects microwaves and transmits visible light, K9 glass is used as the substrate, which can be approximated as parallel flat glass in the optical system.
Since the optical system is symmetric in the X-axis direction and asymmetric in the Y-axis direction, nine points A (0.355°, 0.05°), B (0.18°, 0.05°), C (0°, 0.05°), D (0.355, 0°), E (0.18°, 0°), F (0°, 0°), G (0.355°, -0.05°), H (0.18°, -0.05°), I (0°, -0.05°) on the image plane are selected to evaluate the imaging quality of the system in the visible band. Figure 6(a) and Fig. 6(b) shows the system's modulation transfer function (MTF) curve. It can be seen from the figure that the system's MTF curve at 400–900 nm is close to the diffraction limit, and the average MTF value at 50 lp/mm basically reaches 0.47. Figure 6(c) is the image point diagram of the system. From the figure, it can be seen that the image points of each field of view of the system are smaller than the Avery disk. Figure 6 (d) is the system's full field of view distortion grid. According to the data provided in the Code V distortion grid, the maximum Tan Dist% is 0.51% and the maximum Rad Dist% is 0.9%. The image quality is good, as we show in Code 1 [12].
Fig. 6. Image quality evaluation of optical subsystem: (a), (b) modulation transfer function; (c) Airy disk and image point diagram; (d) Full field of view distortion grid.
4.2 Synthetic aperture radar system design
The antenna subsystem of synthetic aperture radar is composed of feed array, reflector antenna, separation device, etc. It works in the Ka band and adopts confocal dual reflector antenna solution. The center frequency of the antenna is 35 GHz and the bandwidth is 0.6 GHz. The SAR antenna reflector is shared with the optical primary and secondary mirrors. The diameter of the optical camera pupil is 2.6 m, so the aperture diameter of the primary mirror of the antenna is 2.7 m, and the focal length of the primary mirror is 11.14 m. It uses offset feed to form a fixed beam feed array with 8 elements in azimuth and 16 elements in elevation, a total of 128 elements, and the size of the feed array is 64 mm × 128 mm. The antenna subsystem schematic diagram is shown in the Fig. 7 [13–14].
The design azimuth beam width is 0.27°, and the elevation beam width is 0.73°. 128 beam positions are designed within the range of incidence angle of 10°–50°, each beam position is 7 km wide, and adjacent beam positions overlap by 50%. The azimuth ambiguity is better than -20 dB, the range ambiguity is better than -22 dB, and the Noise Equivalent Sigma Zero (NESZ) is better than -16 dB. The main performance curves of each wave position are shown in the Fig. 8.
Fig. 8. SAR system image quality evaluation: (a) Nesz of each wave position; (b) Range ambiguity to signal ratio (RASR) of each wave; (c) Normalized pattern of elevation; (d) Normalized pattern of azimuth.
5. Principle prototype test
The dual-parabolic structure is rarely used in the SAR antenna system, and its beamforming ability, antenna gain and other performances need to be further verified. In order to verify the engineering feasibility of the integrated design in this paper, we developed a scaled-down principle prototype to test the radiation pattern of the dual-parabolic antenna structure and the antenna gain. In addition, the mirror substrate and the band separation device test piece were processed separately, and the reflection and transmission characteristics of the two kinds of test pieces in the visible light and microwave bands were tested.
The principle prototype is scaled down on the basis of the SAR sub-system design. It is composed of a primary mirror, a secondary mirror, a band separation device, a reflector antenna and a feed source. The system structure and electromagnetic wave propagation path are shown in the Fig. 9. The diameter of the primary mirror is 210 mm, and the focal length is 857 mm. The diameter of the secondary mirror is 70 mm, and the focal length is 285.7 mm. The working frequency of SAR is 35 GHz, the frequency bandwidth is ±0.3 GHz, and the principle prototype uses a single horn for power feeding. The electromagnetic wave is emitted from the feed horn and reflected by the reflective surface of the feed module to the band separation device, and then propagated to the outside space after being amplified by the primary and secondary mirrors.
Fig. 9. Schematic diagram of the principle prototype structure and electromagnetic wave propagation path.
In order to ensure that the electromagnetic waves emitted by the feed horn can be emitted to the outside space to the maximum, the size of the feed reflection surface needs to be matched with the size of the secondary mirror of the principle prototype, so the design of the feed reflective surface diameter D = 74 mm, the focal diameter ratio of the reflection surface F/D = 0.6, focal length F = 44.4 mm. The working frequency of the radar is 35 GHz, and the feed horn selects standard waveguide BJ-320 for feeding. The feed module is shown in the Fig. 10.
The primary and secondary mirrors of the principle prototype are made of aluminum-based silicon carbide material, with a nickel layer of 50 μm on the surface. In order to facilitate the test, an aluminum-based silicon carbide nickel-plated test piece was made, and its reflection performance in the visible light band and microwave band was tested. The test results are shown in Fig. 11 and Fig. 12. The reflectance of the aluminum-based silicon carbide mirror test piece in the Ka-band is greater than 98%, and the visible light band average reflectance is approximately equal to 96%. The hood and front panel of the principle prototype need to use quartz/cyanate resin composite material, which not only has low dielectric constant and dielectric loss tangent value, high microwave transmittance, but also has excellent mechanical properties.
The band separation device adopts a crack-template based metallic mesh coated glass window. A conductive copper (Cu) film with a thickness of 200 nm and average metal wire width of 3-5 μm was deposited on one side of the glass substrate microwave shielding. The metallic mesh coated glass window shows highly homogeneous light transmittance and high microwave reflectance. As shown in Fig. 13 and Fig. 14, the average reflectivity of the test piece in the microwave Ka band of 33 GHz–37 GHz is greater than 94%, and the transmittance of the test piece in the visible light band of 400 nm–800 nm is 86% [15–19]. The band separation device test piece is not coated with an optical anti-reflection coating, and the optical anti-reflection coating also has a certain ability to reflect microwaves. Therefore, the visible light band transmittance and microwave band reflectivity of the band separation device may be further improved.
Figure 15 shows the actual samples of the aluminum-based silicon carbide nickel-plated substrate test piece (left) and the metallic mesh band separation device test piece (right).
Fig. 15. Nickel-plated mirror substrate test piece (left) and band separation device test piece (right).
Figure 16 is the structure diagram of the principle prototype, and the Fig. 17 shows the experimental principle prototype after assembly.
The radiation pattern of the dual-antenna structure is simulated by CST software, and the radiation pattern of the principle prototype is tested in a microwave anechoic chamber, and the simulation results and test results of the normalized radiation pattern are obtained. As shown in Fig. 18, in the elevation and azimuth directions, the simulation and test results are in good agreement, the main lobe basically coincide, and the expected results are achieved.
Fig. 18. Comparison of normalized radiation pattern simulation results and test results of the principle prototype.
The microwave space loss value Gloss can be calculated according to Eq. (9):
In the Eq. (9), Gsource uses a standard gain horn to calibrate the gain of the feed horn plus the feed reflective surface, and Gant uses a standard gain horn to calibrate the gain of the common aperture antenna. A set of data is obtained through simulation, and a set of data is obtained through experimental testing. The experimental and simulation data are shown in Table 2. The microwave space loss value is -0.98 dB, and the microwave loss meets the design requirements.
Table 2. Microwave loss statistics
6. Conclusion
This paper proposes an integrated system design of optical camera and synthetic aperture radar, which realizes the integration of the two most commonly used imaging equipment, so that the observation equipment has stronger observation ability and better environmental adaptability. The integrated system is improved on the basis of the off-axis (offset feed) scheme, which can effectively avoid the energy loss caused by the center obstruction. The primary and secondary mirrors adopt a double parabolic structure to avoid the nonlinear astigmatism of the rear optical system. The primary and secondary mirrors of the system use the aluminum-based silicon carbide substrate nickel-plated solution, which has high visible band and microwave band reflectivity; the band separation device adopts the glass-based metallic mesh solution, which reflects microwaves and transmits visible light. Through software simulation analysis, both subsystems achieve good imaging results. The beamforming ability and antenna gain of the dual parabolic antenna structure are verified by testing the principle prototype, and the expected effect is achieved.
Disclosures
The authors declare no conflicts of interest.
Data availability
The raw data from this work is available as Code 1 [12].
References
1. L. H. Hughes, N. Merkle, T. Bürgmann, S. Auer, and M. Schmitt, “Deep learning for SAR-optical image matching,” Proc. IEEE 169, 4877–4880 (2019). [CrossRef]
2. S. Fu, F. Xu, and Y.-Q. Jin, “Translating SAR to optical images for assisted interpretation,” arXiv preprint arXiv:1901.03749 (2019).
3. E. Arnieri, L. Boccia, G. Amendola, S. Glisic, C. Mao, S. Gao, T. Rommel, P. Penkala, M. Krstic, and U. Yodprasit, “An integrated radar tile for digital beamforming X-/Ka-band synthetic aperture radar instruments,” IEEE Trans. Microwave Theory Tech. 67(3), 1197–1206 (2019). [CrossRef]
4. W. D. Williams, M. Collins, D. M. Boroson, J. Lesh, A. Biswas, R. Orr, L. Schuchman, and O. S. Sands, “RF and optical communications: A comparison of high data rate returns from deep space in the 2020 timeframe,” NASA (John H. Glenn Research Center2007).
5. G. Gilbreath, M. Rupar, C. Font, and R. Romeo, “Combined Aperture for radio and optical communications for deep space links,” NRL Review, 162–164 (2011).
6. K. B. Fielhauer, B. G. Boone, and D. E. Raible, “Concurrent system engineering and risk reduction for dual-band (RF/optical) spacecraft communications,” Proc. IEEE, 1–7 (2012).
7. J. R. Charles, D. J. Hoppe, and A. Sehic, “Hybrid RF/optical communication terminal with spherical primary optics for optical reception,” Proc. IEEE, 171–179 (2011).
8. D. Hoppe, J. Charles, S. Piazzolla, F. Amoozegar, M. Britcliffe, and H. Hemmati, “Integrated RF/optical ground station technology challenges,” Interplanetary Network Progress Report 42, 1–38 (2010).
9. A. Biswas, D. Boroson, and B. Edwards, “Mars laser communication demonstration: what it would have been,” Proc. SPIE 6105, 610502 (2006). [CrossRef]
10. V. Vilnrotter, D. Hoppe, B. Moision, and J. Charles, “Optical communications performance of hybrid 34-meter microwave antennas,” Proc. IEEE, 1–13 (2010).
11. X. Fan, G. Zou, Y. Qiu, Z. Pang, H. Zhao, Q. Chen, Y. Pan, and H. Yuan, “Optical design of the visible telescope for the SVOM mission,” Appl. Opt. 59(10), 3049–3057 (2020). [CrossRef]
12. R. Li, “Code V design file,” figshare (2021), https://doi.org/10.6084/m9.figshare.16811047.
13. D. Bekers, S. Jacobs, R. Bolt, S. Monni, D. Fortini, C. Germani, P. Capece, and G. Toso, “Design of a Ka-band sparse array antenna for spaceborne SAR applications,” Proc. IEEE, 117–120 (2017).
14. M. Zhou, M. F. Palvig, S. B. Sørensen, J. R. de Lasson, D. M. Alvarez, M. Notter, and D. Schobert, “Design of Ka-band reflectarray antennas for high resolution SAR instrument,” Proc. IEEE, 1–5 (2020).
15. Q. I. Liangfei, C. Zhu, Y. Yang, J. Huang, L. I. Jia, S. Zhao, and W. Song, “Progress on metal mesh transparent conductive coatings and its applications,” Mater. Rev. 9, 31–36062 (2015).
16. J. Van De Groep, P. Spinelli, and A. Polman, “Transparent conducting silver nanowire networks,” Nano Lett. 12(6), 3138–3144 (2012). [CrossRef]
17. K. Lee and J. Ahn, “Substrate effects on the transmittance of 1D metal grid transparent electrodes,” Opt. Express 22(16), 19021–19028 (2014). [CrossRef]
18. A. Voronin, Y. Fadeev, I. Govorun, I. Podshivalov, M. Simunin, I. Tambasov, D. Karpova, T. Smolyarova, A. Lukyanenko, and A. Karacharov, “Cu–Ag and Ni–Ag meshes based on cracked template as efficient transparent electromagnetic shielding coating with excellent mechanical performance,” J Mater Sci 56, 1–22 (2021). [CrossRef]
19. Y. Han, J. Lin, Y. Liu, H. Fu, Y. Ma, P. Jin, and J. Tan, “Crackle template based metallic mesh with highly homogeneous light transmission for high-performance transparent EMI shielding,” Sci. Rep. 6(1), 1–11 (2016). [CrossRef]
