High-altitude unmanned aerial vehicle for Earth observation

Abstract

Synthetic aperture radar (SAR) is a type of imaging technology that can be used for a wide range of applications, including Earth observation, mapping, object tracking, change detection (e.g., Arctic ice), natural disaster monitoring, and many other uses. A high-altitude unmanned aerial vehicle for performing synthetic aperture radar (SAR) imaging comprises a fuselage, wings attached to the fuselage to provide lift to the aircraft, one or more electric propellers to provide thrust to the aircraft, a SAR payload for SAR imaging the Earth's surface as the aircraft flies, solar cells to generate power to drive one or more electric propellers and operate the SAR payload, and one or more batteries to store the electrical energy generated by the solar cells.

Description

[Technical Field]

[0001] This disclosure relates to radar imaging, and more particularly to high-altitude unmanned aerial vehicles for Earth observation. [Background technology]

[0002] Synthetic aperture radar (SAR) is a type of imaging technology that can be used for a wide range of applications, including Earth observation, mapping, object tracking, change detection (e.g., Arctic ice), natural disaster monitoring, and many other uses. A SAR image is a type of image created by transmitting a radar signal, receiving the reflected and scattered radar signals, and processing the returned signals to form an image. In contrast, optical imaging is a passive technique that captures images by receiving light reflected or emitted from an object. On the other hand, SAR technology is an active technique, as it relies on transmitting radar signals rather than relying on sunlight or other light sources. SAR technology has a major advantage over optical imaging in that it can capture images at night and under cloudy or other adverse weather conditions. However, image formation using SAR technology is generally more complex, and generating an image generally requires considerable signal processing of the returned echoes.

[0003] More specifically, SAR images are acquired from moving transceivers, such as transceivers that make up part of a satellite. In conventional radar, the spatial resolution of the image produced by measuring the reflection of the radar signal is directly proportional to the bandwidth of the radar signal and inversely proportional to the dimensions of the antenna used to transmit and receive the radar signal. In other words, the larger the antenna, the finer the detail. This means that the length and height of the antenna required to take high-resolution images with conventional radar are not practical, especially for aerial use.

[0004] In contrast, SAR images are captured using a "synthetic aperture." A smaller, more practical antenna is used on a moving platform, and a series of measurements of the reflected radar signal are taken. By combining these measurements, a much larger antenna is simulated. This is achieved by leveraging the Doppler effect created by the moving SAR platform. As a result, the resolution of the SAR image corresponds to the resolution of a conventional radar image taken using an antenna larger than the one used to capture the SAR image.

[0005] It is well known that SAR technology is installed on orbiting satellites for the purpose of taking images of target locations on the Earth's surface. However, one of the problems with using orbiting satellites for SAR imaging is that the satellite can only image the target location discontinuously because it has a line of sight to the target location for only a short time in each orbit. This disclosure presents a solution to this problem. [Overview of the Initiative]

[0006] According to a first aspect of the present disclosure, a high-altitude unmanned aerial vehicle for performing SAR imaging is provided, comprising: a fuselage; wings connected to the fuselage for providing lift to the aircraft; one or more electric propellers for providing thrust to the aircraft; a SAR payload for synthetic aperture radar (SAR) imaging of the Earth's surface as the aircraft flies; solar cells for driving one or more electric propellers and generating power to operate the SAR payload; and one or more batteries for storing the electrical energy generated by the solar cells.

[0007] The SAR payload is K a It may be configured to operate in a band.

[0008] The SAR payload may be configured to monitor the side with a look angle of 10° or more.

[0009] One or more batteries may include lithium-ion batteries.

[0010] The SAR payload may include at least one transmitter for transmitting SAR radar waves and at least one receiver for receiving reflections of SAR radar waves. The at least one transmitter may be located in a module separate from the module in which the at least one receiver is located.

[0011] At least one transmitter may be located in the nose of the aircraft, and at least one receiver may be located in at least one of the wings or tail of the aircraft.

[0012] The SAR payload may include at least one transmitter for transmitting SAR radar waves and at least one receiver for receiving reflections of SAR radar waves. The at least one transmitter and at least one receiver may be located in the nose of the aircraft.

[0013] The aircraft may further include a radio communication module that enables the aircraft to be remotely controlled using communications from a satellite or ground control station.

[0014] The aircraft may further include one or more vertical surfaces. At least one of the solar cells may be located on one or more vertical surfaces.

[0015] One or more vertical surfaces may include one or more surfaces of the aircraft's tail fin and one or more surfaces of the tip of at least one wing of the aircraft.

[0016] The aircraft may be configured to fly at an altitude of 50,000 to 80,000 feet.

[0017] The aircraft may further include a tail fin attached to the fuselage, which has one or more elevators for controlling the aircraft's pitch.

[0018] The tail may further include a rudder for controlling the yaw of the aircraft.

[0019] According to a further aspect of the present disclosure, there is provided a method of performing SAR imaging, including flying an unmanned aircraft having a fuselage, a wing connected to the fuselage for providing lift to the aircraft, one or more electric propellers for providing thrust to the aircraft, a SAR payload, a solar panel for generating power to drive the one or more electric propellers and operate the SAR payload, and one or more batteries for storing electrical energy generated by the solar panel, above a target location on the surface of the Earth, and performing synthetic aperture radar (SAR) imaging of the target location using the SAR payload.

[0020] Performing SAR imaging using the SAR payload may include operating the SAR payload in the K a band.

[0021] Flying the unmanned aircraft may include ascending the unmanned aircraft to a first altitude during the day to obtain potential energy, and descending the unmanned aircraft to a second altitude lower than the first altitude using the obtained potential energy.

[0022] Descending the unmanned aircraft to the second altitude may be performed at night.

[0023] Descending the unmanned aircraft to the second altitude may start from sunset.

[0024] Ascending the unmanned aircraft to the first altitude may include flying the aircraft at the second altitude and waiting for the battery to be charged beyond a threshold or for the solar power generation ratio of the solar panel to exceed the power consumption ratio while flying the aircraft at the second altitude, and ascending the aircraft to the first altitude when the battery is charged beyond the threshold or when the solar power generation ratio of the solar panel exceeds the power consumption ratio.

[0025] Flying the aircraft may further include descending the aircraft to a second altitude at sunset.

[0026] Descending the aircraft to a second altitude may include operating the propellers at a lower power setting than the cruising power setting.

[0027] This summary does not necessarily cover all embodiments. Other embodiments, features, and advantages will become apparent to those skilled in the art by examining the descriptions of the specific embodiments below. [Brief explanation of the drawing]

[0028] The embodiments of this disclosure will be described in detail below, in conjunction with the attached drawings.

[0029] [Figure 1] This is a schematic diagram of an unmanned aerial vehicle for performing SAR imaging according to one embodiment of the present disclosure. [Figure 2] Figure 1 is a top view of an unmanned aerial vehicle according to one embodiment of the present disclosure. [Figure 3A-3B] These are a cross-sectional view and a perspective view of a SAR payload according to one embodiment of the present disclosure. [Figure 4] This is a plot showing the altitude of an aircraft as a function of time according to one embodiment of the present disclosure. [Figure 5] This is a plot showing the state of a battery as a function of time according to one embodiment of the present disclosure. [Figure 6] Figure 1 shows an unmanned aerial vehicle performing SAR imaging according to one embodiment of the present disclosure. [Figure 7] This is a flowchart illustrating one example of a method for flying an unmanned aerial vehicle to perform SAR imaging. [Modes for carrying out the invention]

[0030] This disclosure aims to provide a novel high-altitude unmanned aerial vehicle for Earth observation. Various embodiments of this disclosure are described below, but this disclosure is not limited to these embodiments, and modifications of these embodiments may be included within the scope of the appended claims.

[0031] High-altitude platforms (HAPS), sometimes called "high-altitude pseudo-satellites," are vehicles designed to operate at high altitudes within the Earth's atmosphere, rather than in space. Examples of such vehicles include balloons, airships, or aircraft.

[0032] Generally, embodiments of the present disclosure relate to high-altitude unmanned aerial vehicles for performing Earth observations using synthetic aperture radar (SAR) imaging. The aircraft comprises a fuselage, wings attached to the fuselage for providing lift to the aircraft, one or more electric propellers for providing thrust to the aircraft, a SAR payload for SAR imaging the Earth's surface as the aircraft flies, solar panels for driving one or more electric propellers and generating power to operate the SAR payload, and one or more batteries for storing the electrical energy generated by the solar panels.

[0033] Such aircraft may be aircraft capable of long-endurance flight. For example, an aircraft may operate at an altitude of at least 50,000 feet, and according to some embodiments, it may operate in the altitude range of 50,000 to 80,000 feet or 60,000 to 80,000 feet. For example, an aircraft can conserve energy by increasing its altitude during the day to store potential energy and using that potential energy to decrease its altitude at night when there is no sunlight. According to some embodiments, a long-endurance aircraft is an aircraft capable of operating in the atmosphere for at least 24 hours, or at least 36 hours, or at least 48 hours, or at least 72 hours, or at least one week.

[0034] Advantageously, the unmanned aerial vehicle (UAV) can orbit above a predetermined target point on the Earth's surface for extended periods (e.g., days, weeks, or even months), and the UAV can continuously monitor the target point using its SAR payload. The aircraft uses electric propulsion powered by solar cells and batteries and can be optimized to be as lightweight and efficient as possible, thereby maximizing the aircraft's flight time. For example, according to one embodiment, the aircraft may weigh approximately 80 kg, have a wingspan of 28 meters, and be composed mainly of composite materials. According to some embodiments, the aircraft can remain airborne indefinitely, only landing for maintenance or other reasons such as battery degradation.

[0035] To enable unmanned aerial vehicles to withstand long-duration flights, the aircraft's components may be powered by the electricity from its onboard batteries. The batteries may also be recharged using solar panels installed on the aircraft.

[0036] According to some embodiments, solar cells may be located on the outer surface of the wings or on the outer surface of other parts of the aircraft. Solar cells may be located on other structures of the aircraft. For example, energy production can be extended by positioning the solar cells to maximize vertical plane coverage at sunrise and sunset. For example, solar cells may be provided on one or more parts of the aircraft's tail, such as the vertical stabilizer, as described above. Other vertical surfaces of the aircraft to which solar cells may be attached include, for example, the fuselage (e.g., the vertical portion of the fuselage) and any vertical winglets at the wingtips.

[0037] Additionally, by equipping the unmanned aerial vehicle with a wireless communication module, the aircraft can be remotely controlled via satellite communication or communication from a ground control station.

[0038] Next, referring to Figure 1, a perspective view of a high-altitude unmanned aerial vehicle 100 according to an embodiment of this disclosure is shown.

[0039] The aircraft 100 comprises a fuselage 10, wings 12 connected to the fuselage 10, and a tail fin 22 connected to the rear of the fuselage 10. The tail fin 22 comprises a pair of horizontal stabilizers 20 equipped with elevators (not shown) for controlling the pitch of the aircraft 100, and a vertical stabilizer 18 equipped with a rudder (not shown) for controlling the yaw of the aircraft 100.

[0040] Thrust is provided by a pair of electric propellers 14 connected to the wings 12 (although only the nacelles are shown in the drawings, it will be apparent to those skilled in the art that propellers for providing thrust are fixed to the nacelles). The nose of the aircraft 100 is located in front of the aircraft 100. Inside the nose is a SAR payload 16 equipped with a SAR radar for SAR imaging of target locations on the Earth's surface while the aircraft 100 is in flight. According to some embodiments, the SAR payload 16 may be housed in different locations on the aircraft 100 or may be distributed among multiple locations on the aircraft 100.

[0041] Referring to Figure 2, a top view of the aircraft 100 is shown. A pair of battery banks of batteries 24 (such as lithium-ion batteries) are provided within the wings 12. Positioning the batteries 24 in the wings 12 reduces weight by placing the relatively heavy battery components close to the lift plane of the aircraft 100, thereby reducing the structural requirements of the aircraft 100. According to some embodiments, the batteries 24 may be housed in other locations on the aircraft 100, such as within the nacelles. The batteries 24 store electrical energy generated by solar cells, as will be further detailed below. Electrical energy from the batteries 24 may also be used to power other components of the aircraft 100, such as the propellers 14, the SAR payload 16, and radio communication modules (not shown) used to send and receive radio communications to and from the aircraft 100 from ground stations and / or orbiting satellites. Batteries containing lithium, such as lithium-ion batteries, are advantageous in that they can store a large amount of energy per unit weight. In alternative embodiments, other types of batteries with similarly high specific energy or gravimetric energy density (Wh / kg) may also be used.

[0042] The wireless communication module allows the orbiting satellite constellation to relay instructions from the ground control station to the aircraft 100 even when the aircraft 100 is outside the direct communication range of the ground station. Additionally, data captured by the SAR payload 16 may be downloaded to the control station via the orbiting satellites. Furthermore, the control station may centrally control multiple such unmanned aircraft. Also, since line of sight is not required to control the aircraft 100, once it is in the air, the areas where the aircraft 100 can fly are virtually unlimited. Moreover, relaying communications via orbiting satellites can make communication with the aircraft 100 more reliable and reduce the probability of communication link loss due to interference or jamming.

[0043] Figures 3A and 3B show the SAR payload 16 in more detail. As shown in Figure 3A, the SAR payload 16 comprises a phased array transceiver antenna 32, a power amplifier 34, and an RF connection 33 for transferring power from the power amplifier 34 to the phased array antenna 32. According to this embodiment, the SAR payload 16 is housed inside the fuselage 10 (particularly the nose of the aircraft 100). SAR radar used for Earth observation is typically lateral look. In other words, it does not look down from directly above, as in other techniques such as optical imaging. This is because the specular radar reflection from the nadir (the area directly below the SAR platform) is too bright to form a good image. Thus, SAR is performed by sending radar pulses to the side at a certain "look angle," which is the angle between a line pointing straight down from the radar towards the Earth and the direction the antenna is pointing. Figure 3A shows the look angle 36. In one example, the look angle may be greater than 10° and between 10° and 80°. In Figures 3A and 3B, an adjustable payload mount 35 is used to adjust the look angle 36 of the SAR radar waves emitted by the phased array antenna 32. In one example, the look angle 36 may be manually adjustable or automatically adjustable using a motor or other mechanism. In one example, the look angle 36 may be automatically adjusted during flight from a command from the ground or a pre-programmed mission. The payload mount 35 may also be vibrationally isolated from the fuselage 10.

[0044] Thus, and as shown in Figure 7, according to the operating method of this example of a high-altitude unmanned aerial vehicle, the aircraft can remain airborne for very long periods of time, only needing to descend, for example, for maintenance or when battery capacity is low. In this example of an aircraft, solar panels are positioned on the surface of the wings and cover a portion of the leading edge of the wings, allowing the aircraft to maximize charging by flying towards the sun when the sun is low in the sky, such as at sunrise or sunset. According to one embodiment, the aircraft may be towed and may fly, for example, to a high altitude of 18 to 25 km where the aircraft operates. During the day, the aircraft uses sunlight to charge its batteries. At night, when sunlight is unavailable, the aircraft continues to operate by relying on the energy stored in the batteries and the potential energy gained during the day. At sunrise (block 82 in Figure 7), the aircraft is at an altitude of 18 km, and the aircraft's batteries may be low in charge and nearly depleted. Therefore, at sunrise, the aircraft can maximize charging by flying horizontally towards the sun. The batteries begin charging when the rate of solar power generation exceeds the rate of power consumption. When the battery is fully charged, the aircraft may begin to climb, for example to an altitude of 25 km, or as high as possible based on the amount of available energy (Block 84 in Figure 7). Alternatively, the aircraft could begin to climb as soon as the proportion of solar power generation exceeds the proportion of power consumption, and complete the battery charging when it reaches the target altitude.

[0045] The SAR payload can be activated at any time, as long as sufficient power is being generated by the solar panels, or until the battery is recharged, to maintain the aircraft's flight at night and if there is enough energy remaining in the battery to operate the SAR payload. For example, the power consumption of the SAR payload is relatively small compared to the power required to keep the aircraft flying. When the SAR payload is not performing imaging, it consumes very little power. The duty cycle of the SAR payload can be adjusted based on the amount of sunlight available to keep the aircraft in the air indefinitely. Before sunset (block 86 in Figure 7), the aircraft can fly towards the sun again to maximize battery charging through energy generation from the solar panels. At sunset (block 88 in Figure 7), the motors are turned off or set to a low energy consumption state, and the aircraft is allowed to glide up to 18 km. Once 18 km is reached (block 80 in Figure 7), the motors are turned on again, and the aircraft may fly level until sunrise, when the cycle repeats.

[0046] In some embodiments, an aircraft may operate its motors and other functions (e.g., SAR payload) solely on battery power. However, increasing battery capacity increases the aircraft's weight, so aircraft may employ various flight strategies to reduce the need for battery capacity. For example, as will be described in more detail below, an aircraft may compensate for the required battery capacity by relying on the potential energy gained by flying at higher altitudes. Reducing battery capacity also reduces the amount of power that solar cells need to generate to charge the batteries, which can leave more power for propulsion or reduce the overall number of solar cells required.

[0047] Considering this, as shown in Figure 4, according to some embodiments, an aircraft can ascend to a higher altitude (e.g., 25 km) during the day to gain potential energy (when the solar panels provide enough energy to lift the aircraft) and then use that potential energy to descend at night after sunset (e.g., 18 km). Generally, aircraft are designed to fly with less power at higher altitudes (e.g., 60,000–80,000 feet), and therefore should be able to fly more efficiently at higher altitudes (using less power). Also, since flight efficiency does not depend solely on the efficiency of the propulsion unit or motors, turning off the motors or gliding at a low power setting (e.g., slightly below the cruising power setting) makes it more efficient to descend relatively slowly when descending to a low altitude, resulting in more time spent descending and less time spent in level flight after reaching the desired low altitude. As shown in Figure 4, using this method, an aircraft can maintain high-altitude flight for extended periods, in this case nearly three weeks.

[0048] According to one embodiment, a high-altitude unmanned aerial vehicle equipped with a SAR payload can carry 25 kg of battery with a specific energy of 400 Wh / kg. The mass-to-area ratio of the solar cells is 250 g / m². 2 The power density of the solar panel is 350W / m². 2This may also be the case. In this example, the solar cell is in sheet form (area approximately 40 square meters), 75 μm thick, mounted on the front of the wing and wrapped around part of the leading edge, maximizing solar charging while flying towards the sun. Alternatively, for example, when flying in circles, solar cells may also be mounted on the vertical surface of the tail fin to maximize charging from sunlight from the side. During level flight, approximately 1200W of power can be consumed. The aircraft flies either towards the sun or in circles. The initial climb to high altitude (18 km) can be facilitated, for example, by towing the aircraft to that altitude. With a climb efficiency of 1 / 3, the aircraft can climb up to 25 km. For descent, a glide ratio of 30 is used, which corresponds to a descent rate of 56 m / min at an air speed of 100 km / h.

[0049] As shown in Figure 5, the battery may be discharged periodically when the aircraft descends to low altitudes at night, and then recharged when the aircraft ascends to high altitudes during the day. The battery state fluctuates between full (approximately 10,000 Wh) and a depleted state with approximately 1,000 Wh remaining. In this example, the solar panels are sufficient to fully recharge the battery every day and have enough surplus power to operate the SAR payload and other functions. This is demonstrated by the fact that the battery reaches and maintains a fully charged state for a certain period each day during level flight of the aircraft. The battery is also sized to provide sufficient capacity to keep the aircraft flying at night when sunlight is unavailable, and includes, for example, a buffer of approximately 1,000 Wh that can be used to operate the SAR payload.

[0050] Referring to Figure 6, an example is shown in which the unmanned aerial vehicle 100 circles a target point 70 on the Earth's surface via a flight path 50. When the SAR payload 16 is activated to image the target point 70, the radar waves emitted from the SAR radar of the SAR payload 16 define a spotlight 60 that envelops the target point 70 as the aircraft 100 circles overhead. According to some embodiments, the width of the spotlight 60 can range from 5 to 30 km. In Figure 6, the area of ​​the spotlight is shown as circular, but it is understood that other flight patterns (such as flying in a square shape) for monitoring the target point are also possible. As shown in Figure 6, a radar beam is emitted from the antenna of the SAR payload 16 in front of the aircraft 100 in a direction approximately perpendicular to the direction of motion of the aircraft 100, and this radar beam is pointed laterally at a constant look angle. In some examples, the phased array radar may have some electronic steering that allows the radar beam to be directed slightly forward or backward from the vertical, for example, within a range of ±2° or ±10°.

[0051] The angle of incidence is the angle between the centerline 61 of the radar beam traveling from the SAR antenna to the target point and the normal to the Earth's surface at the target point. In Figure 6, the angle of incidence is shown as θ1. The angle of incidence and the look angle are often used interchangeably, but they are not necessarily the same. When the ground is flat and the curvature of the Earth is ignored, the look angle and the angle of incidence can be approximately equal. However, when the terrain is not flat, the angle of incidence and the look angle may differ. Also, as the distance from the SAR payload to the target point increases, especially at large look angles, the angle of incidence may differ from the look angle due to the curvature of the Earth.

[0052] In some embodiments, if the angle of incidence is approximately the same as the look angle in this example, the look angle and the angle of incidence θ1 may be greater than 10° and less than 80° in SAR imaging of a marine target, and less than 60° in SAR imaging of a land target. The radar beam has a near edge 62 of the angle of incidence θ2 and a far edge 63 of the angle of incidence θ3. From Figure 6, it can be seen that θ2 is smaller than θ1 and θ3 is larger than θ1. In one example, if θ1 is 60°, θ2 may be approximately 55° and θ3 may be approximately 65°. Depending on the radar configuration, other beam widths with different angles of incidence are also possible.

[0053] In some embodiments, the SAR payload 16 may be configured to perform SAR imaging using X-band frequencies (e.g., 8–12 GHz). This is a frequency band also available for SAR imaging on satellites. In other embodiments, the SAR payload 16 may be configured to perform SAR imaging using C-band (e.g., 4–8 GHz) or L-band (e.g., 1–2 GHz). The C-band, in particular, is a frequency band that has traditionally been used in SAR Earth observation satellites. The C and L bands may be useful when lower image quality is acceptable or to compensate for the generally lower electron efficiency in other frequency bands. When SAR radar operates in the Earth's atmosphere, higher frequency bands may require improved turbulence compensation.

[0054] According to some embodiments, the SAR radar is K a It is configured to emit radar waves in a band, for example, in the range of 26.5–40 GHz or 33.4–36 GHz (as regulated). Generally, K a The band is not used for SAR imaging from space because it is more susceptible to atmospheric interference, among other reasons. However, K a Operating the band's SAR payload 16 on a high-altitude unmanned aerial vehicle could be beneficial for several reasons.

[0055] For example, K aSince the band occupies a relatively high frequency range, the distance that the aircraft 100 needs to travel to form an image with a certain resolution is shorter compared to the case where the SAR radar operates in a lower frequency band. This is because the SAR antenna has to travel a certain distance relative to the ground to form the synthetic aperture that is fundamental to synthetic aperture radar. Since the aircraft 100 usually travels at an airspeed of about 100 km / h, using the K a band can significantly shorten the time until an image is formed. In contrast, a satellite operating in a low Earth orbit can travel at a speed of approximately 7.8 km / s, so the time required to form an image in a satellite-based SAR platform is not as much of an issue compared to the HAPS platform.

[0056] Also, using the K a band can relatively reduce the size of the SAR payload 16. For example, according to some embodiments, the size of the SAR radar may be 10 cm × 10 cm or smaller. This is compared to some space-based SAR radars with antennas of 1 m × several meters. According to some embodiments, in a 10 cm × 10 cm K a band phased array antenna, the antenna may have 12 × 12 antenna patches or 16 × 16 antenna patches. The resolution that SAR imaging based on the K a band can ultimately achieve can be a very small value of less than 10 cm. According to some embodiments, the SAR radar may be configured to image a 2 GHz band with a pulse repetition frequency (PRF) > 2 GHz. PRF refers to the number of times (per second) of switching on and off the transmitted radar signal. The phased array antenna may be electronically steered at an angle of, for example, ±10 degrees. Also, a device may be used to adjust the look angle of the SAR radar within the fuselage 10 or to allow for larger changes in the direction of the SAR radar or the spread of the spotlight 60.

[0057] K a A power amplifier designed for a specific frequency band may be less efficient than other power amplifiers designed for other frequency bands, therefore, K a The power output requirements for radars operating at lower frequencies can be higher than those for radars operating at lower frequencies. However, this can be compensated for at least partly because the distance the SAR payload 16 must travel to form the synthetic aperture is shorter compared to lower frequencies. This can result in shorter illumination times, and consequently, to capture the desired single or multiple images, K a This would mean operating the radar for the band for a relatively short time (e.g., a low duty cycle). While differences in illumination time may not be significant for high-speed, space-based SAR platforms, differences in illumination time of, for example, tens of seconds to over a minute can be important for relatively slow-moving HAPS platforms.

[0058] According to the embodiment of Figure 1, the transmitter and receiver of the SAR payload 16 are located in the nose of the aircraft 100. An additional receiver may be installed on the tail fin 22 and / or along the wing 12. The transmitter and receiver of the SAR radar may be combined into the same unit, or alternatively, separated into two units. Separating the transmitter and receiver into two units has the advantage of simplifying the design and separating the transmitter, which generates most of the heat, from the receiver, which should preferably be cooled. For example, the transmitter may be mounted on the fuselage 10, and the receiver may be mounted on the fuselage 10 or elsewhere on the aircraft 100, such as at the tip of the wing 12 or the tail fin 22. Separating the transmitter and receiver in this way helps with thermal management and can be beneficial for locating and detecting objects, such as detecting moving objects on the Earth's surface or assessing the elevation of the Earth's surface.

[0059] According to one embodiment, a high-altitude unmanned HAPS aircraft has a 0.1m x 0.05m K panel with eight antenna patches in a row. aThe panel features a band-wide phased array antenna. The panel has 32 power amplifiers feeding the antenna patches, each outputting 4W. With a connector loss of 0.2W and an antenna loss of 1.2W per power amplifier, each antenna patch radiates a net 2.6W. For the entire panel, the radiated power is 84.6W, and the power supplied to the phased array antenna is 122.2W, representing the peak power supplied and radiated. Depending on the duty cycle in which the phased array antenna operates, the average power radiated and the average power supplied to the antenna can be significantly reduced.

[0060] In one embodiment of SAR imaging using a high-altitude unmanned HAPS aircraft, the aircraft's airspeed is 0.028 km / s, or approximately 100 km / h, and it is flying at an altitude of 20 km. An image with an incidence angle θ1 of 30° (roughly equivalent to a look angle of 30°) can be acquired, which corresponds to a slant range distance of approximately 23 km from the aircraft to the target. The ground distance from the nadir (the point directly below the aircraft) to the target is approximately 11.5 km, and the swath width is approximately 2 km.

[0061] According to one embodiment, a K with a center frequency of 35 GHz a The band radar performs imaging. The pulse repetition frequency (PRF) is set to 140 Hz with a duty cycle of 0.3. According to this duty cycle, the average radiated power and average supplied power are approximately 25 W and 37 W, respectively. Since this is only a small fraction of the approximately 1200 W required to maintain the aircraft's level flight in this example, such power requirements can be met by a battery and solar panel system of a high-altitude unmanned aerial vehicle appropriately designed to handle this load. The system bandwidth is 1200 MHz, and a total illumination time of approximately 18 seconds is required to provide an azimuth resolution of approximately 0.20 m and a distance (cross-track) resolution of 0.25 m. This resolution corresponds to approximately 0.05 square meter cells on the ground.

[0062] In another embodiment (demonstrating the achievement of even finer resolution), imaging is performed from an altitude of 19 km with an incident angle θ1 of approximately 60°, a slant range resolution of 0.075 m, an azimuthal resolution of 0.050 m, and a ground range resolution of 0.086 m. This level of resolution is particularly useful in applications where continuous monitoring is desired, such as in cities or towns. Imaging in urban environments can become very difficult to utilize if the imaging resolution exceeds 0.5 m. Therefore, for monitoring such densely populated environments, finer resolutions of less than 0.1 m × 0.1 m are extremely useful, and a K-bandwidth of 2 GHz with a center frequency of 35 GHz is required. a This can be easily achieved by using a band SAR payload.

[0063] In this application, setting the incidence angle θ1 to 60° (±10°) offers another advantage. Satellite-operated SAR systems typically operate at great distances from ground targets, and therefore do not use large incidence angles like 60°. Such long distances can lead to poor distance and azimuthal ambiguity, significantly degrading image quality. However, using small incidence angles can lead to extreme layovers (a well-known artifact in SAR images) where ground and building reflections combine irreversibly, making them difficult to use. Low-Earth orbit satellites are more than 400 km above the Earth's surface, but aircraft are much closer to the ground than satellites. Therefore, even at high altitudes within the atmosphere, a large incidence angle θ1 (e.g., 60°) is used for high-altitude unmanned aerial vehicles. aThis problem can be solved by using band radar. In this way, ambiguity in the distance and azimuthal directions can be greatly reduced, resulting in better image quality. Using a larger incidence angle (e.g., 60°) also reduces the effects of layover, but in return increases shadowing (where a nearby object prevents the radar signal from reaching an object behind it). However, unlike the effects of layover, the effects of shadowing can be more easily compensated for by imaging the area around the target city, for example, as shown in Figure 6. A larger incidence angle also helps to increase the resolution of the ground. In this example, the nominal diameter of the spotlight focusing on an area using a high-altitude unmanned aerial vehicle as a SAR platform, with a resolution of 10 cm or less, is approximately 5 km, as shown by distance D in Figure 6.

[0064] According to some embodiments, fuel cells may be used in place of, or in addition to, solar cells to power the various components of the aircraft 100.

[0065] According to some embodiments, the unmanned aerial vehicle may not have a tail. For example, the pitch and yaw of the aircraft may be given using elevators and rudders located on other parts of the aircraft.

[0066] While the drawings are presented in the context of an unmanned aerial vehicle having wings extending from the fuselage, this disclosure also extends to other wing designs, such as blended wing designs, in which the wings can be formed more integrally with the fuselage.

[0067] In patent claims and / or specifications, the words "a" or "an" used with terms such as "equipment" or "include" may mean "one," but unless otherwise specified, they can also mean "one or more," "at least one," or "one or more." Similarly, the word "another" may mean at least two or more unless otherwise specified.

[0068] The terms “combined” and “joined” as used herein may have several different meanings depending on the context in which they are used. For example, in this specification, the terms “combined” and “joined” may, depending on the specific context, indicate that two elements or devices are directly connected to each other or are connected to each other through mechanical elements or through one or more intermediate elements or devices. The terms “and / or” as used herein, when used in relation to an enumeration of items, mean one or more of the items that make up that enumeration.

[0069] In this specification, when a number is described as "approximately" or "about," or as being "substantially" equal to a number, it means that it is within ±10% of that number.

[0070] While this disclosure has described specific embodiments, it should be understood that this disclosure is not limited to these embodiments, and that modifications, alterations, and variations of these embodiments can be carried out by those skilled in the art without departing from the scope of this disclosure.

[0071] Furthermore, any part of any aspect or embodiment described herein is intended to be implemented or combined with any part of any other aspect or embodiment described herein.

Claims

1. A high-altitude unmanned aerial vehicle for performing synthetic aperture radar (SAR) imaging, Torso and, Wings connected to the fuselage for providing lift to the aircraft, One or more electric propellers for providing thrust to the aircraft, K ａ A SAR payload configured to operate in a band for SAR imaging of the Earth's surface as the aircraft is in flight, A solar cell for driving one or more of the aforementioned electric propellers and generating power to operate the SAR payload, One or more batteries for storing electrical energy generated by the solar cell, An aircraft equipped with [the following features].

2. The aircraft according to claim 1, wherein the SAR payload is configured to monitor the side with a look angle of 10° or more.

3. The aircraft according to claim 1 or 2, wherein the one or more batteries include lithium-ion batteries.

4. The SAR payload comprises at least one transmitter for transmitting SAR radar waves and at least one receiver for receiving reflections of the SAR radar waves. The aircraft according to any one of claims 1 to 3, wherein the at least one transmitter is provided in a module separate from the module in which the at least one receiver is provided.

5. The at least one transmitter is located in the nose of the aircraft. The aircraft according to claim 4, wherein the at least one receiver is provided on at least one of the wings or tail of the aircraft.

6. The SAR payload comprises at least one transmitter for transmitting SAR radar waves and at least one receiver for receiving reflections of the SAR radar waves. The aircraft according to any one of claims 1 to 5, wherein the at least one transmitter and the at least one receiver are located in the nose of the aircraft.

7. The aircraft according to any one of claims 1 to 6, further comprising a wireless communication module for enabling remote control of the aircraft using communications from a satellite or ground control station.

8. The aircraft further includes one or more vertical surfaces, The aircraft according to any one of claims 1 to 7, wherein at least one of the solar cells is located on one or more vertical surfaces.

9. The one or more vertical surfaces are, The aircraft according to claim 8, comprising one or more surfaces of the tail fin of the aircraft and one or more surfaces of the tip of at least one of the wings of the aircraft.

10. The aircraft according to any one of claims 1 to 9, wherein the aircraft is configured to fly at an altitude of 50,000 to 80,000 feet.

11. The aircraft according to any one of claims 1 to 10, further comprising the tail fin connected to the fuselage and having one or more elevators for controlling the pitch of the aircraft.

12. The aircraft according to claim 11, wherein the tail fin further comprises a rudder for controlling the yaw of the aircraft.

13. A method for performing synthetic aperture radar (SAR) imaging, To fly an unmanned aerial vehicle over a target point on the Earth's surface, the unmanned aerial vehicle comprising a fuselage, wings connected to the fuselage for providing lift to the aircraft, one or more electric propellers for providing thrust to the aircraft, a SAR payload, solar panels for driving the one or more electric propellers and generating power to operate the SAR payload, and one or more batteries for storing the electrical energy generated by the solar panels, This includes performing SAR imaging of the target location using the SAR payload, Flying the aforementioned unmanned aircraft is, To obtain potential energy, the unmanned aerial vehicle is raised to a first altitude during the daytime, Using the obtained potential energy, the unmanned aerial vehicle is lowered to a second altitude lower than the first altitude. Methods that include...

14. The method according to claim 13, wherein the descent of the unmanned aerial vehicle to the second altitude is performed at night.

15. Performing SAR imaging using the aforementioned SAR payload is performed as described above. ａ The method according to claim 13 or 14, comprising activating the SAR payload in the band.

16. Raising the unmanned aerial vehicle to the first altitude is Flying the aircraft at the second altitude, While flying the aircraft at the second altitude, the system waits for the battery to be charged beyond a threshold, or for the proportion of solar power generation from the solar panels to exceed the proportion of power consumption. When the battery is charged beyond the threshold or when the proportion of solar power generation from the solar panel exceeds the proportion of power consumption, the aircraft is raised to the first altitude. The method according to any one of claims 13 to 15, including the method described in any one of claims 13 to 15.

17. The method according to claim 16, wherein the descent of the aircraft to the second altitude is started at sunset.

18. The method according to any one of claims 13 to 17, wherein descending the aircraft to the second altitude includes operating the propellers at a setting lower than the cruising power setting.

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