Reflector

Abstract

A reflector that can be used to reflect radio waves emitted from an artificial satellite is provided with a structure including a plurality of reflecting bodies which are arranged at equal intervals and in the same direction along an installation surface. As a result, it is possible to provide a reflector for satellite observation, said reflector having a structure in which failures caused by wind hardly occur.

Description

Reflector

[0001] The present invention relates to a reflector used to reflect radio waves emitted from an artificial satellite.

[0002] There is known a technology for observing changes and conditions on the earth's surface by emitting radio waves from a sensor mounted on an artificial satellite and capturing the waves reflected by the earth's surface. Patent Documents 1 and 2 disclose observation methods using a synthetic aperture radar (SAR) satellite equipped with a SAR. Data obtained by the SAR satellite is used for observing, for example, topographical changes and land subsidence due to disasters, and observation of surface objects and vegetation.

[0003] JP 2004-309178 A JP 2021-114275 A

[0004] SAR satellites have the advantage of being able to observe the Earth's surface regardless of day or night or weather because they use microwaves (electromagnetic waves with wavelengths of approximately 1 mm to 1 m), which have a higher penetration ability than visible light. However, due to the reflective properties of microwaves, they have the disadvantage that reflected waves are difficult to obtain from flat structures such as roads, runways, and large infrastructure, making analysis difficult. To compensate for this weakness, efforts have been made to install reflectors (radio wave reflecting devices) at the desired observation points to ensure stable reflection of the radio waves emitted from SAR satellites.

[0005] Conventionally, for observation satellites, reflectors with a structure called a corner reflector, as shown in Patent Documents 1 and 2, have been commonly used. A corner reflector has a structure in which three right-angled isosceles triangular reflectors are combined so that they are perpendicular to each other, and has the property of accurately reflecting radio waves arriving from any direction back in the direction of their incidence. Patent Document 2 also proposes examples using fan-shaped or square reflectors, as well as examples combining multiple corner reflectors, but the basic structure is the same.

[0006] Due to their structure, corner reflectors for observation satellites inevitably have large external dimensions (especially the height dimension from the installation surface). This is because a size of at least 1 m on each side is required to ensure sufficient reflection strength for satellite observations. Therefore, corner reflectors are susceptible to wind pressure, which can cause the reflector to bend or deform, or the reflector to tilt or tip over. Because long-term, fixed-point observation is important for topographic observation, reflector failure can be fatal. Furthermore, because topographic observations using SAR satellites require the installation of numerous reflectors over an extremely wide area, it is desirable for them to be as maintenance-free as possible.

[0007] The present invention has been made in view of the above circumstances, and has as its object to provide a reflector for satellite observation that is structured to be less susceptible to breakdowns due to the influence of wind.

[0008] A reflector used to reflect radio waves emitted from a satellite may have a structure including multiple reflectors arranged at equal intervals and in the same direction along an installation surface. Because the multiple reflectors are arranged at equal intervals and in the same direction, radio waves from the satellite are incident on the multiple reflectors in the same direction and are reflected in the same direction. Since the satellite can receive the combined energy of the waves reflected by each of the multiple reflectors, the size of each reflector in the reflector of the present invention can be made smaller than that of a conventional corner reflector when attempting to obtain reflected waves of the same intensity using the reflector of the present invention and a conventional corner reflector. Therefore, by adopting the structure of the present invention, a reflector with performance equivalent to that of a corner reflector can be realized in a more compact size, and is less susceptible to wind pressure than a conventional corner reflector, resulting in a reflector with a structure less susceptible to breakdown due to wind.

[0009] The arrangement interval of the plurality of reflectors is preferably set so that the plurality of reflected waves reflected by the plurality of reflectors and reaching the satellite reinforce each other. By using a reflector with such a structure, it is possible to observe reflected waves with little energy loss and good signal-to-noise ratio.

[0010] The arrangement intervals of the plurality of reflectors may be set to satisfy the following formula: d: spacing between reflectors [m] F: central carrier frequency of radio waves [Hz] θ: elevation angle of the incident direction of radio waves relative to the installation surface [deg] C: speed of light [m / s]

[0011] By setting the placement interval in this way, the phases of the multiple reflected waves that are reflected by each of the multiple reflectors and reach the satellite are aligned, making it possible to observe reflected waves with minimal energy loss and a high signal-to-noise ratio.

[0012] The reflector may have a shape in which the width in a height direction from the installation surface is shorter than the length in a direction along the installation surface. In other words, the reflector may have a shape that is elongated in a direction along the installation surface of the reflector. By adopting such a structure, the height from the installation surface can be kept small, thereby realizing a structure that is less susceptible to the effects of wind.

[0013] The reflector may have two reflecting surfaces that intersect at a right angle, and may be arranged so that the intersection line of the two reflecting surfaces is perpendicular to the plane of incidence of the radio waves. With this structure, regardless of the elevation angle at which the radio waves are incident, the reflected waves are emitted at the same elevation angle as the incident waves. Therefore, compared to a reflector configured with a single plane, it is possible to increase the installation flexibility and robustness with respect to the elevation angle. Note that each of the two reflecting surfaces that intersect at a right angle may be a flat reflecting surface or a curved reflecting surface.

[0014] The reflector may have a reflecting surface in which the cross section at the incident surface of the radio waves is curved in an arc. With this structure, a portion of the reflected waves reflected at the reflecting surface (the radio waves reflected at a point on the reflecting surface having a normal parallel to the incident direction of the radio waves) is reflected back in the incident direction (i.e., the direction toward the satellite). Therefore, compared to a reflector configured with a single plane, it is possible to increase the installation flexibility and robustness with respect to the incident angle.

[0015] The reflector may have a reflecting surface whose cross section parallel to the installation surface is curved in an arc. With this structure, when considering the cross section parallel to the installation surface, a portion of the reflected waves reflected by the reflecting surface (radio waves reflected at points on the reflecting surface whose normals are parallel to the incident direction of the radio waves) is reflected back in the incident direction (i.e., the direction toward the satellite). Therefore, compared to a reflector configured with a single plane, it is possible to increase the installation flexibility and robustness with respect to the incident direction (i.e., incident direction) in the cross section parallel to the installation surface.

[0016] According to the present invention, it is possible to provide a reflector for satellite observation that is structured to be less susceptible to deformation, tilting, tipping, etc. due to the influence of wind.

[0017] FIG. 1 is a diagram schematically showing an observation method using a satellite and a reflector. FIG. 2 is a perspective view of a reflector. FIGS. 3A and 3B are plan views of a reflector. FIGS. 4A and 4B are side views of a reflector. FIG. 5 is a diagram explaining the placement interval of reflectors. FIGS. 6A and 6B are diagrams showing other examples of reflector structures. FIG. 7 is a diagram showing other examples of reflector structures. FIGS. 8A to 8C are diagrams showing other examples of reflector structures. FIG. 9 is a diagram showing other examples of reflector structures. FIGS. 10A and 10B are diagrams showing other examples of reflector structures. FIG. 11 is a diagram showing other examples of reflector structures.

[0018] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0019] <Observation by Satellite> Figure 1 shows a schematic diagram of an observation method using a satellite and a reflector. A reflector (radio wave reflecting device) 1 is installed in advance at a target observation point on the Earth's surface. A sensor 20 mounted on the satellite 2 emits radio waves R1 toward the Earth's surface, and the reflected waves R2 reflected by the reflector 1 are captured by the sensor 20, and information such as the intensity and phase of the received reflected waves R2 is obtained as observation data. By using the reflector 1, it is possible to receive stable reflected waves R2 and collect highly reliable observation data.

[0020] The following describes a method for observing changes at a target location using InSAR analysis (interferometric SAR analysis) by a SAR satellite as an example. The SAR satellite 2 uses a synthetic aperture radar sensor as its sensor 20. Synthetic aperture radar is a type of active sensor that irradiates a target with microwaves and obtains information about the target from the reflected waves. By combining a synthetic aperture with chirp modulation, high resolution can be achieved even with a small antenna. The observation data obtained by synthetic aperture radar is called a SAR image. In InSAR analysis, the phase difference of the received reflected waves is extracted from multiple SAR images obtained by observing the same location at different times. When the phase of the reflected waves changes between the previous observation and the current observation, this can be interpreted as a change in the distance between the SAR satellite 2 and the target location, i.e., a change in the height (subsidence or rise) of the target location. Although it depends on the performance of the synthetic aperture radar and the wavelength of the radio waves, InSAR analysis can measure the amount of change with an accuracy on the order of centimeters to millimeters.

[0021] Topographic observation using SAR satellites 2 has the advantage of being able to continuously observe wide areas at relatively low cost. Another advantage is that different objects, such as slopes, roads, and facilities, can be observed simultaneously. Furthermore, because monitoring can be carried out frequently and periodically, it is possible to grasp displacement over time and issue timely alerts when displacement above a certain level is detected. Taking advantage of these advantages, SAR satellites 2 are expected to be used for a variety of purposes, such as monitoring large-scale embankment construction sites, assessing the subsidence status of coastal areas and reclaimed land, understanding the impact of construction work on surrounding facilities and housing, and monitoring infrastructure facilities such as airports, port facilities, roads, and railways.

[0022] <Reflector> Fig. 2 shows an example of the reflector 1. Fig. 2 is a perspective view of the reflector 1.

[0023] The reflector 1 has a structure having a plurality of reflectors 10 arranged at equal intervals and facing in the same direction. In the example of Fig. 2, 12 reflectors (which may also be called reflective rods or reflective members) 10 are fixed parallel to one another on a support base 11. It is preferable that each reflector 10 is formed with the same shape, dimensions, and material so that it has the same reflection characteristics.

[0024] In the following, as shown in Fig. 2, the installation surface G to which the support base 11 is fixed is used as a reference, and the Z axis is considered to be perpendicular to the installation surface G, the X axis is along the arrangement direction of the reflectors 10, and the Y axis is along the longitudinal direction (extension direction) of the reflectors 10. In other words, the XY plane and the installation surface G are parallel. Note that the installation surface G is typically a horizontal plane, but may also be an inclined surface. The support base 11 is not an essential component, and a structure in which the reflectors 10 are directly fixed to or buried in the installation surface G, or a structure in which irregularities that function as the reflectors 10 are formed on the surface of the installation surface G, may also be used.

[0025] The reflector 10 is formed of a rod-like structure that extends elongatedly in the direction (Y direction) along the installation surface G, and its width in the height direction (Z direction) from the installation surface G is smaller than its length in the direction (Y direction) along the installation surface G. Specifically, the length of the reflector 10 in the Y direction is set to approximately 60 cm to 3 m, and the width in the Z direction is set to approximately 1 cm to 20 cm. The spacing (pitch) between adjacent reflectors 10 is set to approximately 20 cm to 50 cm. However, these dimensions are merely examples, and may be designed as appropriate depending on the wavelength of the radio waves used for observation, the orbit of the SAR satellite 2, the performance of the sensor 20, the required reflection capability, etc.

[0026] The reflector 10 in Fig. 2 has two reflecting surfaces 10a and 10b that intersect at a right angle. Each reflecting surface 10a and 10b is a horizontally long rectangular plane, and is combined so that one reflecting surface 10a is parallel to the YZ plane and the other reflecting surface 10b is parallel to the XY plane. The intersection line of the two reflecting surfaces 10a and 10b is parallel to the Y axis.

[0027] Regarding the material of the reflector 10, a metal material is preferable because it can totally reflect microwaves and has excellent rigidity. However, the reflector 10 may be formed of a material other than metal as long as the reflection intensity required for observation can be obtained and a certain level of wind resistance can be ensured. Alternatively, the reflector 10 may be made of a combination of multiple materials, such as by forming a reflective surface made of a metal film on the surface of a rod-shaped member made of a resin material.

[0028] 3A and 3B are plan views (viewed in the Z direction) of the reflector 1. The installation orientation (installation direction) of the reflector 1 will be described using FIGS. 3A and 3B. Radio waves R1 emitted from the SAR satellite 2 are incident on the reflectors 10 of the reflector 1 from diagonally above. The incident radio waves R1 are reflected by each reflector 10 and returned as reflected waves R2. (For convenience of illustration, FIG. 3A only shows the radio waves being reflected by the fourth reflector 10 from the right, but in reality, similar reflection occurs from all reflectors 10.) If the reflector 1 is installed so that the radio waves R1 are incident on the reflector 10 from a direction perpendicular to the Y axis (i.e., so that the plane of incidence of the radio waves R1 is parallel to the ZX plane), as shown in FIG. 3A, the reflected waves R2 will be reflected in a direction returning to the SAR satellite 2. 3B, if the plane of incidence of the incident wave R3 is not parallel to the ZX plane, the reflected wave R4 will be reflected in a direction deviated from the SAR satellite 2. Therefore, when installing the reflector 1, it is essential to geometrically calculate the plane of incidence (incident direction) of the radio waves based on the orbit of the SAR satellite 2 and the installation point of the reflector 1, and to accurately adjust the installation direction (installation direction) of the reflector 1 to match the incident direction.

[0029] 4A and 4B are side views (viewed in the Y direction) of the reflector 10. The independence of the reflector 10 from the elevation angle will be explained using Fig. 4A and 4B. In this specification, the elevation angle is defined as the angle between the installation surface of the reflector 1 (i.e., the XY plane) and the incident direction of the radio wave.

[0030] As shown in FIG. 4A , the reflector 10 of this embodiment has two reflecting surfaces 10a and 10b that intersect at a right angle and are oriented (oriented) so that the intersection of the two reflecting surfaces 10a and 10b is perpendicular to the plane of incidence of the radio waves. With this structure, when radio waves R1 are incident on the reflector 10 at an elevation angle θ1, the radio waves R1 are specularly reflected twice by the two reflecting surfaces 10a and 10b, and a reflected wave R2 is emitted at the same elevation angle θ1 as the incident wave R1. Therefore, the reflected wave R2 from the reflector 10 accurately returns to the SAR satellite 2. This property (the property of preserving the incident direction) is independent of the elevation angle. As an example, FIG. 4B illustrates the reflection when radio waves R1 are incident at an elevation angle θ2 that is shallower than that of FIG. 4A . It can be seen that the reflected wave R2 is emitted at the same elevation angle θ2 as the incident wave R1. Although not shown, the same applies when the incident wave R1 first strikes the reflecting surface 10b and is reflected by the reflecting surface 10b and then the reflecting surface 10a.

[0031] These properties of the reflector 1 of this embodiment allow for a high degree of freedom in installation relative to the elevation angle (i.e., less precise positioning of the elevation angle is acceptable), thereby facilitating the installation work of the reflector 1. Furthermore, because of its high robustness relative to the elevation angle, it is possible to continue observation even if the elevation angle of the reflector 10 shifts for some reason after installation or if there is a slight change in the orbit of the SAR satellite 2.

[0032] Next, a suitable setting for the arrangement interval (pitch) of the reflectors 10 will be described with reference to Fig. 5. Fig. 5 shows two reflectors 10-1 and 10-2 arranged along the installation surface (XY plane). The arrangement interval d is defined as the distance between corresponding points on the reflectors 10-1 and 10-2 in the arrangement direction of the two reflectors 10-1 and 10-2 (i.e., the X direction).

[0033] S1 denotes the path taken by radio waves R1-1 emitted from SAR satellite 2, reflected by reflector 10-1, and returning as reflected waves R2-1 to SAR satellite 2, and S2 denotes the path taken by radio waves R1-2 emitted from SAR satellite 2, reflected by reflector 10-2, and returning as reflected waves R2-2 to SAR satellite 2. If the elevation angle of the incident direction of the radio waves relative to reflectors 10-1 and 10-2 is θ, then the difference (S2-S1) between paths S2 and S1 is S2-S1=2×Δs=2d cos θ.

[0034] At this time, if the path difference (S2-S1) is an integer multiple of the wavelength of the radio wave, then due to the principle of wave superposition, the reflected waves R2-1 and R2-2 received by sensor 20 of the SAR satellite 2 will reinforce each other. In other words, when the condition 2dcosθ=mλ (m=1, 2, 3, ...) is satisfied, where λ is the wavelength of the radio wave and m is a natural number, the phases of the reflected waves R2-1 and R2-2 will match, and the reflected waves will most reinforce each other.

[0035] If the central carrier frequency of the radio wave emitted from the sensor 20 of the SAR satellite 2 is F [Hz] and the speed of light is C [m / s], then the wavelength λ is λ = C / F. Substituting this into the above equation and rearranging it for the placement interval d, the following equation is obtained. d: spacing between reflectors [m] F: central carrier frequency of radio waves [Hz] θ: elevation angle of the incident direction of radio waves relative to the installation surface [deg] C: speed of light [m / s]

[0036] If the placement interval d of all reflectors 10 of the reflector 1 is set to satisfy the above formula, the phases of the multiple reflected waves reflected by each of the multiple reflectors 10 and reaching the SAR satellite 2 will be aligned, and the multiple reflected waves will reinforce each other. This maximizes the received signal strength, enabling highly efficient observation with little energy loss. Furthermore, high received signal strength leads to an improved signal-to-noise ratio (SN), making it possible to increase the reliability of observation data.

[0037] With the reflector 1 having the above-described structure, the SAR satellite 2 can receive the combined energy of the multiple reflected waves reflected by each of the multiple reflectors 10, so even if the size of each individual reflector 10 in the Z direction (height direction) is significantly smaller than that of a conventional corner reflector, it is possible to obtain a received signal strength sufficient for observation for the entire reflector 1. Therefore, it is possible to provide a reflector 1 with performance equivalent to that of a corner reflector, but with a more compact size than a corner reflector, which is less susceptible to wind pressure than a conventional corner reflector, and as a result, it is possible to provide a reflector 1 with a structure that is less susceptible to breakdowns due to the effects of wind.

[0038] <Reflector Variations> When a linear reflector 10 such as the reflector 1 shown in FIG. 2 is used, the installation orientation of the reflector 1 must be accurately aligned with the incident direction of the radio waves R1, as described with reference to FIGS. 3A and 3B . Therefore, in actual installation work, for example, the installation location of the reflector 1 and landmarks that will serve as landmarks when facing the target direction from there are determined in advance using satellite photos, etc., and then, at the installation site, the orientation of the reflector 1 is fine-tuned based on the positions of the landmarks while looking through a scope attached to the reflector 1. This installation work has the problem of requiring a high level of skill and a considerable amount of work time. Another problem is that the above installation work cannot be applied in locations where there are no suitable landmarks or where visibility is poor and the landmarks cannot be seen.

[0039] 6A and 6B show examples of a reflector 1 that does not require adjustment of the installation orientation. As shown in Fig. 6A, a reflector 10 has a reflective surface 10a formed of a cylindrical surface and a reflective surface 10b formed in a flange shape from the lower end of the reflective surface 10a. The Z-axis cross section of the reflector 10 is L-shaped, similar to the reflector 10 in Fig. 2, but differs from that in Fig. 2 in that the cross section parallel to the installation surface (XY cross section) is curved in an arc shape. As shown in Fig. 6B, the reflector 1 has multiple reflectors 10 of different diameters arranged concentrically.

[0040] With this structure, when considered in the XY plane, a portion of the reflected waves reflected by the reflecting surfaces 10a and 10b (reflected wave R2 reflected at a point on the reflecting surface 10a having a normal parallel to the incident direction of radio wave R1) is reflected toward the SAR satellite 2. In other words, because the reflecting surface 10a is curved and its normal direction changes continuously, a reflected wave returning to the SAR satellite 2 can be obtained regardless of the incident direction of the radio wave. Due to these properties, the reflector 1 shown in FIG. 6B can be installed with increased flexibility not only in terms of the elevation angle but also in terms of the incident direction. This eliminates the need for adjustment of the installation direction, simplifying the installation of the reflector 1. Furthermore, because the reflector 10 has high robustness against incident direction, observations can be continued even if the orientation of the reflector 10 shifts for some reason after installation or if there is a slight change in the orbit of the SAR satellite 2. While the reflector 1 in Fig. 6B has a structure that covers the incident direction of 360°, it may also have a structure that corresponds to only a predetermined range of incident direction, as in the reflector 1 in Fig. 7. For example, simply using a plurality of reflectors 10 with reflective surfaces that curve in a gentle arc to cover the incident direction range of ±several degrees to ±several tens of degrees can simplify the installation work of the reflector 1 and improve robustness against incident direction.

[0041] FIGS. 8A to 8C show other structural examples of the reflector 10. The reflector 10 shown in FIGS. 8A to 8C has a cylindrical or columnar structure extending in the Y direction. While the reflector 10 shown in FIG. 2 has a structure with a reflective surface with an L-shaped cross section, the reflector 10 shown in FIGS. 8A to 8C differs in that it has a reflective surface with an arc-shaped cross section (ZX cross section) at the incident surface of the radio wave R1. With this structure, a portion of the reflected wave reflected by the reflective surface (reflected wave R2 reflected at a point on the reflective surface with a normal parallel to the incident direction of the radio wave R1) is reflected in a direction toward the SAR satellite 2. In other words, because the reflective surface is curved and its normal direction changes continuously, as shown in FIGS. 8B and 8C, a reflected wave returning to the SAR satellite 2 can be obtained regardless of the elevation angle at which the radio wave is incident. Due to these properties, the reflector 1 shown in FIG. 8A can be installed with greater flexibility in terms of the elevation angle, thereby simplifying the installation process of the reflector 1. Furthermore, because the reflector 10 has high robustness against elevation angles, it is possible to continue observation even if the elevation angle of the reflector 10 shifts for some reason after installation or if there is a slight change in the orbit of the SAR satellite 2. The reflector 10 in Fig. 8A has a structure with a cylindrical cross section, but it may also have a structure with a bow-shaped cross section that corresponds only to a predetermined range of elevation angles, as in the reflector 10 in Fig. 9. For example, simply covering an elevation angle range of ± several degrees to ± several tens of degrees can simplify the installation work of the reflector 1 and improve robustness against elevation angles.

[0042] 10A shows an example of a structure in which the reflector 10 is tilted with respect to the installation surface. In the reflector 10 having an L-shaped cross section, the two reflecting surfaces 10a, 10b are preferably the same size, and the reflector 10 is preferably installed tilted so that the incident angle α of the radio waves on each of the reflecting surfaces 10a, 10b is approximately 45°. By adopting such an arrangement, all (or most) of the radio waves reflected by one reflecting surface hits the other reflecting surface and is reflected toward the SAR satellite 2, thereby achieving maximum reflection efficiency with a compact reflecting surface.

[0043] 10B shows an example in which the reflector 1 is placed on a sloped surface. A plurality of reflectors 10 are placed so as to be aligned at equal intervals along the sloped installation surface. When the installation surface is sloped, the placement interval d between the reflectors 10 can be considered as the distance parallel to the installation surface, as shown in FIG. 10B.

[0044] The reflector 1 of this embodiment is small in size in the height direction and has a plurality of reflectors arranged at equal intervals, so that its structure makes it easy to achieve rigidity and a shape that allows it to be used even in situations where other objects are placed on the reflector 1 or where a load is applied from above the reflector 1. Fig. 11 shows an example in which the reflector 1 is buried in a road 3. The reflector 1 may be buried in a position where automobiles 4 pass over it, or it may be buried on the side of the road or in a median strip. In this way, the reflector 1 of this embodiment can be installed so as not to get in the way of structures or buildings, and therefore can be used for a variety of purposes.

[0045] The above-described embodiments merely list some preferred specific examples of the reflector according to the present invention. The reflector according to the present invention is not limited to the structures and shapes of the above-described embodiments, and various structures and shapes may be adopted within the scope of the technical concept described in the claims. For example, the above-described structures and shapes may be combined with each other. Furthermore, while the above-described embodiments use a SAR satellite as an example, the reflector according to the present invention can also be used for satellites other than SAR satellites as long as they use active radio wave sensors.

[0046] 1: Reflector 2: Satellite (SAR satellite) 3: Road 4: Automobile 10: Reflector 10a, 10b: Reflecting surface 11: Support base 20: Sensor G: Installation surface R1: Radio wave (incident wave) R2: Radio wave (reflected wave)

Claims

1. A reflector used to reflect radio waves emitted from an artificial satellite, It has multiple reflectors arranged at equal intervals and in the same orientation along the installation surface, The reflector has a reflective surface whose cross-section at the incident surface of the radio waves is curved in an arc shape, and / or whose cross-section parallel to the installation surface is curved in an arc shape. Reflector.

2. The spacing between the multiple reflectors is set such that the multiple reflected waves reflected by each of the multiple reflectors and reaching the artificial satellite reinforce each other. The reflector according to claim 1.

3. The spacing between the multiple reflectors is set to satisfy the following equation: The reflector according to claim 1. [Math 1] d: Spacing between reflectors [m] F: Central carrier frequency of radio waves [Hz] θ: Elevation angle [deg] of the direction of incidence of radio waves relative to the installation surface. C: Speed ​​of light [m / s]

4. The reflector has a shape in which the width in the height direction from the installation surface is shorter than the length in the direction along the installation surface. The reflector according to claim 1.

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