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Abstract
Passive millimeter wave (PMMW) imagers are increasingly becoming practical sensor candidates for target detection tasks. This paper is devoted to the ship detection by land-based W band passive polarized imager. The radiation characteristics of sky, sea surface and the atmospheric absorption characteristics of sea area near Qingdao are calculated based on sounding data. And the W band radiation characteristic of painted metal is measured. Then, the experiments of detection for sea surface ship are carried out in different ranges including 2,5, and 15 kilometers. The results show that land-based PMMW imager can detect and locate ship target from sea surface more than tens of kilometers away in thick fog, and polarization selection has influence on detection performance.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
PMMW imaging technology obtains the information of target scene through the passive detection of naturally occurring millimeter wave radiation of the scene [1]. Due to the differences in material and temperature, many objects have different gray levels in millimeter wave radiation image, the PMMW detectors can receive the energies from the objects within the scene and, by observing the differences in their intensities then construct images of them. Then the classification or clustering of the target in the scene can be discriminated from the images. PMMW imaging technology has been widely used in radio astronomy [2,3], remote sensing [4,5] and security checking [6,7]. At the same time, PMMW imaging technology also gains great attention in the fields of remote target detection [8–12] and medical diagnosis [13].
PMMW imagers do not launch electromagnetic energy actively, and they act like “thermal” sensors, imaging the radiation in millimeter wave bands emitted from the scene. Compared to active sensors, PMMW sensors offer the following advantages to image a target: with the ability of silent detection which imaging without inadvertently giving away the sensor location, eliminating specular reflections and extreme dynamic ranges of active systems, and avoidance of interference with communications or radar systems [9].
Some areas in the frequency domain are less vulnerable to atmospheric attenuation than others, and the millimeter wave region is one such region. Visual or infrared scene energy that would otherwise be attenuated by atmospheric low visibility conditions can still be received and displayed by a millimeter wave system [14]. In fact. millimeter wave energy is attenuated millions of times less in clouds, log, smoke, snow, and sandstorms than visual or infrared energy [15]. This is the critical advantage of passive millimeter wave imagery. Moreover, millimeter wave imagery is marginally affected by sun or artificial illumination. In other words, PMMW imaging sensors provide visual-like images of objects under low visibility conditions which blind visual and infrared sensors.
PMMW imaging sensors have been in development since the 1980s, and they have achieved video rate sensing about a decade ago [9]. Recently, the application of passive millimeter wave imaging to target detection has incurred some discussions. Ship detection is vitally important for a wide range of applications, including illegal smuggling detection, traffic surveillance and management, and military target detection and tracking. In comparison with SARs, ship detection by the PMMW imager has no speckle issue and slightly suffers from sea clutter [1].
Recently, many researchers are interested in ship detect utilizing microwave/millimeter-wave radiation, and carried out some simulation and experimental researches. A model for scene simulation which describes the recognized phenomenology of PMMW imaging in land, sea, and air scenarios was presented by Neil [16]. Several typical ships along with the sea surface background were simulated in his works, and the feasibility of W band radiometer in detecting various kinds of ships on sea surface is verified.
In the works of E.J.Boettcher et.al, some common small watercrafts at different aspects were imaged by PMMW sensors working at Ka and W bands. Metal and fiberglass are the main materials of the watercrafts using in their experiments and the difficulty criteria are calculated of the task to identify small watercrafts by PMMW images [9]. But in their works, PMMW images were obtained at a close range, so the advantages of the millimeter wave are not highlighted in the experiments. Similar ship imaging tests have been reported in [17] and [18].
In Lu’s works, the radiation images of a ship which are observed by airborne platform at microwave and millimeter-wave bands were simulated [12]. In addition, they also used an airborne synthetic aperture radiometer to image a ship at the altitude of nearly 1000 m. The experiments have been carried out in sunny and cloudy respectively with the sensor working at X band. It is proved that passive microwave sensor can detect the sear surface ship in cloudy weather. Furthermore, the authors proposed a ship target monitoring and tracking algorithm based on passive radiation images [19]. Furthermore, some PMMW imaging experiments for ships on the sea based on helicopter platform has been reported [1].
Tang’s works imaged a boat (a plastic boat was coated with metal foil) under the horizontal polarization and vertical polarization respectively in 94 GHz, and the brightness temperature images have been used to generate polarized mixed image as the basis of identifying ship target [20]. In his experiments, the boat can be clearly distinguished from the background in either the polarized light temperature images or the polarized mixed image. However, Tang's experiments were only a few dozen meters away from observation, and the observation angle was relatively large. In additional, the boat which as the target is a flat structure resembling a cargo ship, which is not representative in practical applications.
In our previous works, W-band polarized images were used to perfectly distinguish metal and water in the same scene, which also verified the possibility of detection and identification of large ships on the sea based on polarized radiation images [21].
In this paper, several long-range detection experiments for military ships are carried out and the warships on the sea-surface is imaged by a land-based W-band dual polarization scanning imaging radiometer in different distances (namely 2km, 10 km and 15 km). The radiant brightness temperature and transmission coefficient of the sky and sea surface at the experiment sites are calculated based on the radiosonde data. The radiation temperature of the metal coated with the anticorrosive coating are measured and compared with the metal plate in the same posture. The relationships between detection range and target background brightness, temperature difference and effective radiation area are calculated and plotted. Experiment results show that the W-band passive imaging system has good penetration in atmospheric low visibility conditions and can realize the detection of ship targets more than ten kilometers away even when the target is completely obscured by the thick fog in the photographs.
2. Passive millimeter wave radiation characteristics
All objects with temperature higher than 0 K will emit energy in the form of electromagnetic wave. The radiant energy is distributed across the electromagnetic spectrum for solid and liquid targets. Similar to an infrared camera (which detects infrared radiation), PMMW camera receive the radiation in millimeter-wave band [15], and the difference of radiation intensity in the field of view forms a millimeter-wave image. The observed radiation intensity is related to the target material, structure and other factors, as well as the ambient radiation, observation angle, and observation polarization. For an opaque target, according to the law of Kirchhoff, the emissivity equals to the absorption rate when the target is in thermal equilibrium, and the mathematical relationship between emissivity and reflectivity is listed as follows:
where, e is the emissivity and ρ represents the directional-hemispheric reflectivity. For smooth surface, the ρ is equal to specular reflectivity. In actual observation, the radiant energy arriving at the detector not only emitted from the target but also from the radiant of ambient environment reflected by the target surface. Then, the apparent brightness temperature of a target can be expressed as2.1 Downward radiation of sky
For metal materials, their own emissivity is approximately 0 in W band, then the observation brightness temperature displaying the radiation from ambient background. While the ships on the sea surface mainly reflect the radiation from cold sky. The sky radiation mainly comes from the water vapor in the atmosphere and the radiation of some gas molecules. In this paper, the downward radiation of sky in Qingdao is calculated based on the sounding data that is supported by the university of Wyoming [22]. There, the radiation is calculated by using the sounding data for March 1, 2019, and the main data are listed in Table 1. The calculation is lagging behind the experiments.
Table 1. The main atmosphere data of QingDao for March 1, 2019
As can be seen from Table 1, the atmospheric temperature, pressure, humidity and other parameters are closely related to the altitude. Therefore, according to the accuracy of sounding data, the atmosphere can be stratified. It is assumed that the atmosphere in a layer is evenly distributed, and the attenuation rate and refraction index remain unchanged. The radiant brightness temperature of each layer is calculated. The radiation of each layer is integrated along the path, so as to obtain the radiant brightness temperature of the sky downward.
The downward radiation of the atmosphere can be calculated by
Fig. 1. Downward brightness temperature of sky calculated by radiosonde date. See Data File 1 for underlying values.
2.2 Radiation characteristic of sea surface
The radiation characteristic of sea surface is an important factor for evaluating of ship target radiation detection, because it will directly affect the radiation contrast and thus determine the detection performance. The radiation of sea surface depends on sea surface salinity (SSS), sea surface temperature (SST), wavelength and observation elements which include observation angle, polarization state and etc. In addition, the wind field can cause changes in the sea surface, thus affecting the radiant brightness temperature of the sea surface. The emissivity and apparent radiation brightness temperature of experimental sea area are calculated by using the sounding data and the seawater radiation calculation model. The calculated results are plotted in Fig. 2, where, the value of SSS is 32 psu and the value of SST is 281 K in the calculation which are obtained through measurements.
Fig. 2. The emissivity and apparent brightness temperature of sea surface in 94GHz. (a) Emissivity in 94 GHz, (b) Apparent brightness temperature in 94 GHz. See Data File 2, Data File 3, Data File 4,and Data File 5 for underlying values.
For calm sea surface, the apparent brightness temperature can be solved by
2.3 Radiation characteristics of ship materials
The emissivity of the material is relatively stable, and the emissivity of metal is close to 0 in millimeter wave band [1]. The feature makes the metal targets usually to have obvious contrast with the background. Many large ships are made of metal, in order to resist the corrosion of seawater, nati-corrosion paint is applied on the surface of ship. To determine the radiation characteristics of metal hull coated with anticorrosive coating, two metal plates are prepared and anticorrosive is painted on one of them. The bright temperature image of the two was obtained by using a W-band imaging radiometer. The visible image and the imaging result are shown in Fig. 3.
Fig. 3. Comparison of brightness temperature image for metal plate with coating and without coating in 3mm band. (a) Visible image. (b) Radiation image. See Data File 6 for underlying values.
As shown in the left of Fig. 3, an aluminum plate and an aluminum plate with yellow plaint are placed together. The plates have the size of 500 mm×500 mm. The right of Fig. 3 is the brightness temperature image of the two plates, and the bare metal and the coated metal show a similar intensity of radiation in brightness temperature. To give the quantitative differences between the two radiated brightness temperatures, the exact brightness temperature values were extracted from the brightness temperature image as shown in Fig. 4.
Figure 4 shows the data in twentieth row of the brightness temperature image in Fig. 3, and the abscissa represents the nth pixel form the left to right. The results in Fig. 4 intuitively reveal the differences of brightness temperature between metal plates with coating and without coating under the same observation angle, which is about 1.1K at the most significant point.
The results indicated that the anticorrosive coating is almost completely transparent in W band. Of course, this result is related to the coating thickness. This means that the radiation and scattering properties of the ship surface elements are similar to those of metals.
From Fig. 2 and Fig. 4, it can be predicted that the painted metallic ship with a “cold” characteristic may have a high contrast with the sea background in W band.
3. Analysis for detection range
Regardless of the effect of antenna efficiency, the radiation energy received by the antenna is the integral of the antenna pattern after the attenuation of the scene radiation through the transmission path. Therefore, the antenna temperature can be calculated as follows:
In this paper, the atmospheric absorption coefficients of several typical elevations near sea level in millimeter wave bands are calculated based on the data in Table 1. The results are plotted in Fig. 5. The typical atmospheric windows in the millimeter wave band are shown in Fig. 5 clearly.
In Fig. 5, The four curves at the bottom are the atmospheric absorption coefficients at different altitudes which are given by combining the winter water vapor content calculated by the MPM93 model with the corresponding temperature and pressure data in the experiments. The four curves at the top are the atmospheric absorption coefficients calculated by using the water vapor content in the monitoring data. Figure 5 shows an obvious conclusion that the atmospheric absorption coefficients calculated from the strategy data are higher, which can be explained by the higher water vapor content in the sea compared to the land. Therefore, the calculation results by monitoring data seem to be more reasonable.
At different altitudes, absorption coefficients corresponding to four typical millimeter wave window frequency points (35 GHz, 94 GHz, 140 GHz, 220GHz) are listed in Table 2.
Table 2. Absorption coefficient of atmosphere for four window frequencies in millimeter wave band
From Fig. 5 and Table 2, the atmospheric absorption coefficient decreases gradually with increasing altitude at the same frequency. This is because the closer you get to sea level, the higher the concentration of water vapor and oxygen, and this is the core influencing factor of atmospheric absorption in millimeter wave band. In addition, another obvious rule can be easily seen, as the window frequency increases, the absorption coefficient is also higher and higher.
For target detection using radiometer, the maximum detectable range of a radiometer can be calculated by
Based on Eq. (4), the relations of Rmax and ΔTT and S are plotted in Fig. 6. In the legends of Fig. 6, ΔTT equals to ΔTT and the unit is Kelvin, S equals to S and the unit is square meter. The results in the figure are based on following prerequisites: ΔTsys is 0.4 K; α is equal to 3; ΩA≈(π/180)2·dpθdpφ, where dpθ and dpφ represent the degree resolution of dimensions θ and φ, which are equal to 0.4°. An obvious conclusion from Fig. 6 is that the maximum detection distance will increase as the brightness temperature difference between the target radiation and the background radiation or the effective radiation area increases when the other parameters being the same. S is related to the ship type and observation direction. For large military ships such as destroyers and aircraft carriers, S may reach to tens of thousands of square meters at certain viewing angles and will provide the possibility of detection range for tens or even hundreds of kilometers.
Fig. 6. (a) The Rmax depends on ΔTT, (b) The Rmax depends on S. See Data File 8 and Data File 9 for underlying values.
4. Experiments
4.1 Experiments scene
As shown in Fig. 7, a dual-polarization radiation imager [23] working in W band is used in our experiments, and the imager have a fully polarized Cassegrain antenna with a size of 600 mm. The radiation energy is received by the antenna and then split in two by a polarization separator. Hence, the imager can simultaneously acquire dual-polarization radiation images by scanning the scene within the set scope. The main specifications of the imager are listed in Table 3. A 3D turntable weighing up to 50 kg can achieve scanning efficiently and accurately, and reduce the vibration of the system in the working process. The turntable is controlled by computer, which can realize the scanning in pitch direction and azimuth direction, as well as the antenna and receiver rotation. The integration time of each pixel is 5 ms and the scanning interval can be installed and it is set as 0.1° in our experiments.
Fig. 7. 3mm dual-polarization imaging radiometer. (a) The imager comprises a turntable and a radiometer. (b) The Cassegrain antenna of the imager. (c) Two computers control the turntable and the radiometer respectively.
Table 3. Main specifications of the dual-polarization imaging radiometer
The experiments were carried out on mainland coast and island respectively. The first imaging experiment was carried out by setting up the imager on the ashore flat ground and facing the target on the sea, and the main objective is to obtain millimeter-wave radiation images of the sea surface at the front horizontally looking. Under this circumstance, the main radiation of the ship comes from the buildings and equipment on board. Because the tilt of the structure makes the wall reflecting the cold sky radiation under the horizontal observation line of sight, these areas have obvious brightness temperature difference with the background radiation from the sea surface. The experiment scene is shown in Fig. 8(a).
Fig. 8. Experimental scene. (a) Experimental scene and scene of mainland coast experiments. (b) Schematic diagram of island experiments.
The second set of measurements are the down-looking measurement, and larger ERA can be obtained in this mode compared with horizontal detection. Furthermore, this mode can reduce some of the sea-wave's occlusion on the target. In the second set of experiments, the imaging radiometer was set up on top of a mountain on the LinShan island in the Yellow Sea, and this observation point is about 40 nautical miles from the last one. The radiometer was installed on a sightseeing platform and the altitude is about 496 m. This location allows the radiometer to easily cover a wider view filed of the sea and the imaging of ship at different distances can be realized. The radiation of the ship is mainly composed of the deck and the building at close range. With the increase of the distance between the receiver and the ship, the effective radiation area of the deck in the observation direction decreases gradually. The target in our experiments is a conventional warship with a longitudinal dimension of about 100 meters and a transverse dimension of about 15 meters, and is imaged at different distances. The schematic diagram of experimental scene is described in Fig. 8(b).
4.2 Results
The millimeter wave dual-polarized radiation images are obtained by scanning the target scenes at different distances with the above imager. Figure 9 shows the scene’s visible image and W-band radiation images of the first experiments. There, the distance between the ship and the imager is about 2 km. From Fig. 9(b) and Fig. 9(c), the ship can be clearly recognized in horizontally polarized image or vertically polarized image, and on the visual effect, the contrast is even more significant than that of the photograph. Furthermore, due to that the radiation from the ship is reflected on the sea, the reflection of the ship can be clearly seen in the radiation images.
Fig. 9. Imaging results for scene with the target distance of 2 km. (a) Visible image. (b) Vertically polarized radiation image. (c) Horizontally polarized radiation image. See Data File 10 and Data File 11 for underlying values.
The imaging results of second set experiments are shown in Fig. 10 and Fig. 11. Figure 10 shows the results with the target range is about 5 km. As shown in Fig. 10(a), the target and even the entire sea surface were covered in dense fog in visible image which is taken from the observation platform. It is next to impossible to capture the target through the visible image, but in the radiation image, the target (which are marked by a red box) can still be found clearly both horizontal polarization and vertical polarization.
Fig. 10. Imaging results for scene with the target distance of 5 km. (a) Visible image. (b) Vertically polarized radiation image. (c) Horizontally polarized radiation image. See Data File 12 and Data File 13 for underlying values.
Fig. 11. Imaging results for scene with the target distance of 15 km. (a) Vertically polarized radiation image. (b) Horizontally polarized radiation image. See Data File 14 and Data File 15 for underlying values.
The imaging results of the target range is 15 km away are shown in Fig. 11. In this case, the target is completely invisible in the visible image as same as Fig. 10(a), but the target can still be clearly locked in the radiation images.
In order to quantitatively evaluate the radiation brightness temperature difference between the target and the background at different observation distances, some typical row data containing target information are extracted and converted to the brightness temperature values by calibration. Corresponding to the radiation images in Figs. 9–11, the brightness temperature data are plotted in Figs. 12–14. From Figs. 12–14, the brightness temperature of the target and background at different imaging distances can be obtained clearly. When the distance is 2 km, the average background measured brightness temperature (MBT) is about 230 K under horizontal polarization and about 235 K under vertical polarization. The MBTs of the target region are between 180 K and 210 K since the data of different pixels come from different region of the ship. The MBT difference between ship and sea surface (MBTDSS) up to 50 K under both horizontal and vertical polarization. When the distance is 5 km, the mean background MBTs are about 218.5 K and 208 K, and the MBTs of the target region are about 213 K and 202 K under vertical and horizontal polarization respectively. The MBTDSS is about 5∼6 K. When the distance is 15 km, the average MBT of background is about 217 K under horizontal polarization and about 222 K under vertical polarization. The minimum MBTs of the target region are about 212 K and 219 K under horizontal polarization and vertical polarization respectively.
Fig. 12. Brightness temperature of target and background with the distance is 2 km. (a) Vertical polarization. (b) Horizonal polarization.
Fig. 13. Brightness temperature of target and background with the distance is 5 km. (a) Vertical polarization. (b) Horizonal polarization.
Fig. 14. Brightness temperature of target and background with the distance is 15 km. (a) Vertical polarization. (b) Horizonal polarization.
As mentioned above, the apparent radiation of the target, the apparent radiation of the sea surface and the loss and radiation of the observation path are all the important affect factors for the MBT. With the increase of imaging distance, the MBTDSS gradually decreases, and the number of pixels in target area gradually decreases. Due to the limitation of conditions, further imaging experiments have not been carried out. However, the quantitative results of brightness temperature difference show that, the land-based W-band imaging radiometer can achieve a detection distance longer than 15km, perhaps up to 20 km or more for medium-sized ships. with the improvement of the performance of imager, or the increase of the target radiation area, or the increase of the viewing angle, or the better weather conditions, the detection range will be further improved.
5. Conclusions and discussions
To test the ship detection based on land-based PMMW radiometer, this paper utilizes the sounding data and the measured data to analyze the sea surface radiation, the downward radiation characteristics of the sea surface atmosphere in the W band, and the transmission characteristics of the sea surface atmosphere in the millimeter wave band. At the same time, the metal plate with anticorrosive coats was prepared, and the radiation brightness temperature is measured and compared with bare metal in W band. These results are valuable for sea surface target detection and related research.
The imaging experiments of ship targets on sea surface at different distances are carried out by using W band dual polarization imaging radiometer. Horizontally polarized radiation images and vertically polarized radiation images of target and background are obtained when the target distance is 2 km, 5 km, and 15 km, respectively. To our limited knowledge, this is perhaps the longest-range imaging experiments of land-based W-band radiometer imaging of a ship target that have been reported. The results show that when the target distance is 2 km, the target contour is clearly visible; when the target distance is 5 km and 15 km, the target is no longer visible in the optical image while it is still clearly visible in the radiation image. There is still a bright temperature difference of about 3 K at 15 km, which will provide the possibility for the realization of further detection. Meanwhile, it is also proved that PMMW imager can find long-distance targets in thick fog. Furthermore, the theoretical and experimental analysis in this paper also shows the polarization difference in the actual detection, which also provides reference for the polarization consideration in the detector design.
At present, we are collecting sea surface radiation data under different weather conditions. In the future, we will also measure the radiation characteristics of the sea surface environment in different seasons, and carry out the ship detection test based on the shipboard polarized imaging radiometer. Furthermore, sea surface target monitoring based on polarized image fusion is a research topic of great research value.
Funding
China Postdoctoral Science Foundation (2020M682412); National Natural Science Foundation of China (61871438).
Acknowledgements
The authors would like to thank the editor and reviewers for their precious advice.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying values of Fig. 1, Fig. 2(a), Fig. 2(b), Fig. 3(b), Fig. 4, Fig. 6(a), Fig. 6(b), Fig. 9(b), Fig. 9(c), Fig. 10(b), Fig. 10(c), Fig. 11(b), Fig. 11(c)are available in Data File 1–Data File 15.
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