Altitude measurement system and altitude measurement method

Abstract

The aircraft's altitude is acquired with high precision, even during takeoff and landing. [Solution] The aircraft is equipped with a flight altimeter which includes at least one of a barometric altimeter and a GPS altimeter, a laser altimeter which is equipped with the aircraft, and a control unit which switches which of the measured values ​​of the flight altimeter and the measured value of the laser altimeter to use when the aircraft is descending, based on the operating conditions.

Description

Technical Field

[0006] ,

[0001] The present disclosure relates to an altitude measurement system and an altitude measurement method.

Background Art

[0002] Conventionally, an automatic landing system for landing a vertical takeoff and landing aircraft on a landing target point provided on a ship has been known (see, for example, Patent Document 1). The automatic landing system for the vertical takeoff and landing aircraft of Patent Document 1 controls the vertical takeoff and landing aircraft so that the relative position between the vertical takeoff and landing aircraft and the landing target point becomes zero.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] When the vertical takeoff and landing aircraft lands on the landing target point, the vertical takeoff and landing aircraft uses an altimeter such as a barometric altimeter or a laser altimeter to calculate the relative position in the vertical direction between the vertical takeoff and landing aircraft and the landing target point, and performs a flight operation so that the relative position becomes zero. However, with a barometric altimeter, it may be difficult to accurately measure the altitude from the landing target point when the position in the vertical direction changes between takeoff and landing due to changes in air pressure or the like.

[0005] Therefore, an object of the present disclosure is to provide an altitude measurement system and an altitude measurement method capable of accurately acquiring the altitude of an aircraft even during takeoff and landing.

Means for Solving the Problems

[0006] The altitude measurement system of this disclosure comprises: an altimeter provided on an aircraft and including at least one of a barometric altimeter and a GPS altimeter; a laser altimeter provided on the aircraft; and a control unit that, when the aircraft is descending, switches which of the measured values ​​of the altimeter and the laser altimeter to use based on the operating conditions.

[0007] The altitude measurement method of this disclosure is performed by an altitude measurement system that, during aircraft descent, switches between using the measurement value of a flight altimeter, which includes at least one of a barometric altimeter and a GPS altimeter, and the measurement value of a laser altimeter, based on the operating conditions. [Effects of the Invention]

[0008] According to this disclosure, the aircraft's altitude can be obtained with high precision even during takeoff and landing. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a schematic diagram showing an example of an advanced measurement system according to this embodiment. [Figure 2] Figure 2 is an explanatory diagram illustrating the landing procedure of an aircraft. [Figure 3] Figure 3 is a block diagram related to the determination of whether or not the laser altimeter can be used. [Figure 4] Figure 4 is a block diagram relating to advanced correction processing. [Modes for carrying out the invention]

[0010] Embodiments relating to this disclosure will be described in detail below with reference to the drawings. However, these embodiments do not limit this disclosure. Furthermore, some components in the embodiments described below are substituted or substantially identical to those easily substituted by those skilled in the art. Moreover, the components described below can be combined as appropriate, and if there are multiple embodiments, each embodiment can be combined.

[0011] [This Circumstance] Figure 1 is a schematic diagram showing an example of the altitude measurement system according to this embodiment. Figure 2 is an explanatory diagram showing the landing operation of an aircraft. Figure 3 is a block diagram relating to the determination of whether or not the laser altimeter can be used. Figure 4 is a block diagram relating to the altitude correction process. The altitude measurement system 100 is a system that measures the altitude between the aircraft 1 and the target landing point where the aircraft 1 will land.

[0012] Aircraft 1 is a rotary-wing aircraft (e.g., a helicopter, drone, or other vertical take-off and landing aircraft). In this embodiment, aircraft 1 is an unmanned aerial vehicle (UAV). Aircraft 1 can be any aircraft capable of moving forward, backward, sideways, turning, and hovering, and may be a manned aircraft. Aircraft 1 is equipped with an altitude measurement system 100, and its flight is controlled based on altitude information acquired by the altitude measurement system 100.

[0013] As shown in Figure 2, the target landing point is located on the ship 5. Therefore, the aircraft 1 lands (docks) on the ship 5, which is a moving object on the water. However, the target landing point is not limited to the ship 5; it may also be located on a vehicle or other moving object on land, or on stationary equipment or the ground.

[0014] The altitude measurement system 100 according to this embodiment acquires the altitude between the aircraft 1 and the target landing point on the ship 5. As shown in Figure 1, the altitude measurement system 100 is configured as a system installed on the aircraft 1. First, the ship 5 will be described with reference to Figures 1 and 2.

[0015] (ship) As shown in Figure 1, the vessel 5 is equipped with a navigation system 70, a data transmission device 80, and an operation display unit 90. The vessel 5 is also equipped with a marker 7 that serves as a target for the aircraft 1 when it lands (doses) on the vessel.

[0016] The navigation device 70 is, for example, an inertial navigation system (INS: Inertial Navigation System) (registered trademark), and acquires the attitude angles in the pitch and roll directions of the ship 5, the heading, the speed, the acceleration, the position coordinates in the earth coordinate system, and the like. In the present embodiment, the navigation device 70 is described by applying it to an inertial navigation device, but it is not particularly limited, and any navigation device 70 may be used. Further, the navigation device 70 is an inertial navigation device including a GPS (Global Positioning System) in order to improve the measurement accuracy of the position in the present embodiment. In the present embodiment, it is described by applying it to an inertial navigation device including a GPS, but it is not particularly limited to the GPS, and any device that can measure the position accurately may be used. For example, a device using a quasi-zenith satellite system may be used, or if the navigation device 70 alone can measure the position accurately, a configuration omitting the GPS or the like may be used. Further, the navigation device 70 may acquire at least a part of various data by a sensor.

[0017] The data transmission device 80 exchanges various signals with the data transmission device 40 mounted on the aircraft 1 by wireless communication.

[0018] The operation display unit 90 is a user interface for an operator on board the ship 5 to grasp the control status and input various instructions. The instructions input by the operation display unit 90 are transmitted from the data transmission device 80 to the data transmission device 40. Further, the control status of the aircraft 1 is transmitted from the data transmission device 40 to the data transmission device 80. That is, the data transmission device 40 and the data transmission device 80 can perform two-way communication.

[0019] The marker 7 is provided on the deck of the ship 5. The marker 7 is, for example, an AR marker color-coded in two colors, black and white, and serves as a marker for the aircraft 1 to capture the position of the target landing point.

[0020] (Aircraft) Next, the aircraft 1 will be described. As shown in FIG. 1, the aircraft 1 includes a camera 10, a navigation device 20, a control unit 30, and a data transmission device 40.

[0021] The camera 10 is a photographing device mounted on the aircraft 1 via a gimbal (not shown). The camera 10 may be a monocular camera, a compound eye camera, an infrared camera, etc., as long as it can photograph the marker 7. The camera 10 is provided for photographing the marker 7 provided at the target landing point from the aircraft 1. The camera 10 is capable of adjusting the photographing direction via a gimbal (not shown).

[0022] Similar to the navigation device 70, the navigation device 20 is, for example, an inertial navigation device including GPS. Note that the navigation device 20 may also be an inertial navigation device including GPS or the like, or an inertial navigation device excluding GPS or the like, and is not particularly limited. The navigation device 20 including GPS acquires the attitude angles in the pitch direction and roll direction of the aircraft 1, the nose azimuth, the airframe speed, the airframe acceleration, and the position coordinates in the earth coordinate system of the aircraft 1, etc. Note that the navigation device 20 may have an attitude angle sensor that detects the attitude angle of the aircraft 1, a speed detection sensor that detects the airframe speed of the aircraft 1, an acceleration detection sensor that detects the airframe acceleration of the aircraft 1, and a sensor that detects the nose azimuth of the aircraft 1. The navigation device 20 outputs the acquired attitude angle, airframe speed, airframe acceleration, and position coordinates of the aircraft 1 to the control unit 30.

[0023] The control unit 30 includes, for example, an integrated circuit such as a CPU (Central Processing Unit). The control unit 30 has an image processing unit 32, a guidance calculation unit 34, and a flight control unit 36.

[0024] The image processing unit 32 performs image processing on the image photographed by the camera 10 to detect the center position of the marker 7. The image processing unit 32 outputs the calculated center position of the marker 7 to the guidance calculation unit 34.

[0025] The guidance calculation unit 34 calculates the control variables for aircraft 1 to guide aircraft 1 to the target landing point. The control variables are used to adjust the aircraft's speed, attitude angle, rate of change of attitude angle, etc. In order to calculate the control variables, the guidance calculation unit 34 calculates the relative coordinate position between aircraft 1 and the target landing point. Specifically, the guidance calculation unit 34 calculates the relative position between aircraft 1 and the target landing point, and the relative altitude between aircraft 1 and the target landing point as relative coordinate positions. The guidance calculation unit 34 also calculates the relative speed between aircraft 1 and the target landing point, etc. The relative position is the distance between aircraft 1 and the target landing point of ship 5 in the horizontal direction. The relative altitude is the distance between aircraft 1 and the target landing point of ship 5 in the vertical direction. Based on the altitude of aircraft 1 detected by the altitude sensor 25, the guidance calculation unit 34 calculates the relative altitude to the landing target point.

[0026] Furthermore, the guidance calculation unit 34 calculates the relative speed between the aircraft 1 and the target landing point. More specifically, the guidance calculation unit 34 calculates the relative speed between the aircraft 1 and the target landing point based on the relative position and the aircraft speed.

[0027] The guidance calculation unit 34 then calculates the control quantity using feedback control (e.g., PID control) based on the relative position, relative altitude, relative velocity, and aircraft acceleration. Note that the feedback control is not limited to PID control, but may also be P control, PI control, PD control, etc. The guidance calculation unit 34 outputs the calculated control quantity to the flight control unit 36.

[0028] As shown in Figure 2, in calculating such control quantities, the guidance calculation unit 34 controls the aircraft 1 in multiple control modes in order to guide the aircraft 1 to the target landing site and land it. The multiple control modes include an approach mode, a hovering mode which includes a high-altitude hovering mode and a low-altitude hovering mode, and a landing mode.

[0029] The approach mode is a mode in which, based on a command from the ship 5, the aircraft 1 approaches the deck of the ship 5 and hovers over the target landing point marker 7. The high-altitude hovering mode is a mode in which the aircraft 1 acquires the marker 7 on the deck with the camera 10 and hovers. The low-altitude hovering mode is a mode in which the aircraft 1 descends and hovers at a lower altitude than the high-altitude hovering mode. In other words, the low-altitude hovering mode consists of a descent mode in which the aircraft descends from the high-altitude hovering mode to a low altitude, and a hovering mode in which the aircraft hovers at a low altitude after the descent. The landing mode is a mode in which the aircraft 1 lands at the target landing point.

[0030] The flight control unit 36 ​​controls each component of the aircraft 1 according to the control values ​​calculated by the guidance calculation unit 34 to make the aircraft 1 fly. The flight control unit 36 ​​controls the blade pitch angle, rotation speed, etc. of each rotor according to the control values ​​and adjusts the aircraft's speed, attitude angle, rate of change of attitude angle, etc. As a result, the aircraft 1 is guided to the target landing point. In this embodiment, the guidance calculation unit 34 is described as a separate functional unit from the flight control unit 36, but the flight control unit 36 ​​and the guidance calculation unit 34 may be an integrated functional unit. That is, the flight control unit 36 ​​may perform the processing of the guidance calculation unit 34.

[0031] The data transmission device 40, like the data transmission device 80 mentioned above, is included in the advanced measurement system 100 described later, and exchanges various signals with the data transmission device 80 mounted on the ship 5 via wireless communication.

[0032] The altitude sensor 25 is described below. The altitude sensor 25 includes a flight altimeter 25a, which includes at least one of a barometric altimeter and a GPS altimeter, and a laser altimeter 25b. The barometric altimeter measures altitude based on atmospheric pressure. The GPS altimeter measures altitude based on position coordinates in the Earth coordinate system. The laser altimeter 25b measures altitude by emitting a laser and receiving the reflected laser wave. The flight altimeter 25a and the laser altimeter 25b are switched according to the flight mode and flight conditions of the aircraft 1.

[0033] (Advanced measurement system) Next, the switching control of the altitude measurement system 100 will be described with reference to Figure 3. The control circuit shown in Figure 3 is incorporated into, for example, the induction calculation unit 34, and when the usable flag for the laser altimeter 25b is raised based on the usage conditions, switching control from the flight altimeter 25a to the laser altimeter 25b is executed. In this switching control, the system switches which of the measured values ​​of the flight altimeter 25a and the laser altimeter 25b to use, based on the usage conditions. At this time, it is possible to switch only the use of the measured values ​​while performing measurements with both the flight altimeter 25a and the laser altimeter 25b, or it is possible to switch the use of the measured values ​​by switching the execution of measurements with both the flight altimeter 25a and the laser altimeter 25b. In the following description of the switching control, it will simply be referred to as switching the use of the flight altimeter 25a and the laser altimeter 25b.

[0034] The induction calculation unit 34 includes a first AND circuit 51, an OR circuit 52, and a second AND circuit 53 as circuits for determining the operating conditions.

[0035] The first AND gate 51 outputs an output signal to raise the usable flag if the altitude measured by the flight altimeter 25a is lower than a preset threshold altitude, and the intensity of the reflected wave acquired by the laser altimeter 25b is higher than the threshold.

[0036] The OR circuit 52 outputs an output signal to raise the usable flag when the altitude measured by the laser altimeter 25b is lower than a preset altitude, or when an output signal from the first AND circuit 51 is input.

[0037] The second AND circuit 53 raises the usable flag when aircraft 1 is in low-altitude hovering mode and an output signal from OR circuit 52 is input.

[0038] In addition to the above circuit, the circuit enclosed by the dotted line in Figure 3 may be further added. This circuit determines whether the relative position between the aircraft and the target landing point is within a predetermined range, and includes a first threshold circuit 55, a second threshold circuit 56, a third AND circuit 57, and a timer 58.

[0039] The first threshold circuit 55 receives the relative position of the aircraft 1 and the target landing site in the X direction of the horizontal plane as input, and outputs an output signal to raise the usable flag if the relative position in the X direction is within a threshold (e.g., ±2.0m).

[0040] The second threshold circuit 56 receives the relative position between the aircraft 1 and the target landing site in the Y direction of the horizontal plane as input, and outputs an output signal to raise the usable flag if the relative position in the Y direction is within a threshold (e.g., ±2.0m).

[0041] The third AND gate 57 outputs an output signal to raise the usable flag when it receives the output signal from the first threshold gate 55 and the output signal from the second threshold gate 56.

[0042] Timer 58 outputs an output signal to raise the usable flag if an output signal is continuously input from the third AND circuit 57 for a predetermined time (for example, 0.5 seconds). Note that the circuit enclosed by the dotted line in Figure 3 may be omitted.

[0043] In the circuit shown in Figure 3, when the available flag is raised, the induction calculation unit 34 switches to using the laser altimeter 25b in the circuit shown in Figure 4 and calculates a control amount based on the altitude in the vertical direction. In the circuit shown in Figure 4, PID control and the like are executed so that the difference (ΔALT) obtained by subtracting the relative altitude between the aircraft 1 and the target landing point from the target altitude in low-altitude hovering mode becomes zero.

[0044] The circuit shown in Figure 4 includes a first changeover switch 61, a subtractor 62, a second changeover switch 63, an integrator 64, an adder 65, a limiter 66, a third changeover switch 67, and a memory unit 68.

[0045] The first toggle switch 61 is a switch that switches between the flight altimeter 25a and the laser altimeter 25b. When the available flag for the laser altimeter 25b is raised, the first toggle switch 61 switches to the laser altimeter 25b side. The altitude measured by the flight altimeter 25a is referred to as altitude (1), and the altitude measured by the laser altimeter 25b is referred to as altitude (2).

[0046] The subtractor 62 is a circuit that subtracts the relative altitude input from the first toggle switch 61 from the target altitude input from the limiter 66. The subtractor 62 outputs the difference (ΔALT) between the target altitude and the relative altitude to the control unit that performs PID control, etc.

[0047] The second toggle switch 63 is connected when the low-altitude hovering mode is in operation, and outputs the rate of descent of aircraft 1, that is, the rate of change in altitude when aircraft 1 descends.

[0048] The integrator 64 receives the initial target altitude for the high-altitude hovering mode, as well as the descent rate from the second toggle switch 63. The integrator 64 outputs the target altitude as the initial target altitude minus the descent altitude accumulated based on the descent rate.

[0049] The adder 65 adds the correction value output from the memory unit 68 to the target altitude output from the integrator 64, and outputs the corrected target altitude.

[0050] Limiter 66 has a target altitude set as a lower limit for low-altitude hovering mode, and outputs a corrected target altitude that is above the lower limit so that the corrected target altitude does not fall below the lower limit.

[0051] The third toggle switch 67 is connected when the laser altimeter 25b is in use, and outputs a correction value when the laser altimeter 25b is in use. The correction value is the difference between the altitude (2) of the laser altimeter 25b and the altitude (1) of the flight altimeter 25a, and this difference is the value obtained the last time the laser altimeter 25b was switched on.

[0052] The memory unit 68 stores the correction value. The correction value to be stored is the difference acquired when switching to the laser altimeter 25b.

[0053] In the circuit shown in Figure 4, the difference in altitude when switching from the flight altimeter 25a to the laser altimeter 25b is added to the target altitude output from the integrator 64 in the adder 65, thereby correcting the target altitude.

[0054] (Altitude measurement method) Referring again to Figure 3, the altitude measurement method performed by the altitude measurement system 100 according to this embodiment will be described. In the altitude measurement method, the use of the flight altimeter 25a and the laser altimeter 25b is switched based on the usage conditions of the altitude sensor 25. In the altitude measurement method, the control unit 30 (guidance calculation unit 34) determines in the first AND circuit 51 whether the usage conditions are met, namely that the altitude measured by the flight altimeter 25a is lower than a preset altitude and the reflection intensity acquired by the laser altimeter 25b is higher than a threshold. If the control unit 30 determines in the first AND circuit 51 that the usage conditions are met, it then determines in the OR circuit 52 whether the altitude measured by the laser altimeter 25b is lower than a preset decision altitude, or whether the usage conditions in the first AND circuit 51 are met. The decision altitude is the altitude at which the decision is made whether or not to land. In the OR circuit 52, the condition for use was whether the altitude measured by the laser altimeter 25b was lower than a preset decision altitude (threshold). However, instead of this condition, the condition for use may be whether the barometric altitude or GPS altitude has reached the decision altitude. When the control unit 30 determines that the conditions for use are met in the OR circuit 52, it determines in the second AND circuit 53 whether the conditions for use are met, namely that the aircraft 1 is in low-altitude hovering mode and that the conditions for use in the OR circuit 52 are met. When the control unit 30 determines that the conditions for use are met in the second AND circuit 53, it outputs a flag indicating that the laser altimeter 25b is usable and switches to using the laser altimeter 25b. In the second AND circuit 53, the AND condition was that the aircraft 1 is in low-altitude hovering mode, but this does not have to be an AND condition. In other words, when the aircraft 1 is in low-altitude hovering mode, the use of the laser altimeter 25b was permitted, but this is not limited to that.

[0055] As described above, the altitude measurement system 100 and altitude measurement method described in this embodiment can be understood, for example, as follows.

[0056] The first embodiment of the altitude measurement system 100 includes a flight altimeter 25a provided on the aircraft 1 and including at least one of a barometric altimeter and a GPS altimeter, a laser altimeter 25b provided on the aircraft 1, and a control unit 30 that, when the aircraft 1 is descending, switches which of the measured values ​​of the flight altimeter 25a and the laser altimeter 25b to use based on the operating conditions.

[0057] With this configuration, the measured values ​​of the flight altimeter 25a or the laser altimeter 25b can be switched and used based on the operating conditions, allowing for the use of more appropriate measured values ​​and enabling high-precision altitude measurement according to the operating conditions.

[0058] In a second embodiment, in the altitude measurement system 100 according to the first embodiment, the control unit 30 switches to using the measured value of the laser altimeter 25b if the altitude measured by the laser altimeter 25b is lower than a preset decision altitude.

[0059] With this configuration, if the altitude measured by the laser altimeter 25b is lower than the decision altitude, the altitude can be measured with high accuracy by switching to the more accurate laser altimeter 25b.

[0060] In a third embodiment, in the altitude measurement system 100 according to the first or second embodiment, the control unit 30 switches to using the measured value of the laser altimeter 25b if, as a usage condition, the altitude measured by the flight altimeter 25a is lower than a preset altitude and the reflectance intensity acquired by the laser altimeter 25b is higher than a threshold.

[0061] With this configuration, if the altitude measured by the flight altimeter 25a is lower than the set altitude, and the reflectance intensity of the laser altimeter 25b is higher than the threshold, that is, if altitude measurement is possible using the laser altimeter 25b, the altitude can be measured with high accuracy by switching to and using the laser altimeter 25b.

[0062] In a fourth embodiment, in the altitude measurement system 100 according to the first embodiment, the control unit 30 allows switching to the use of the measured value of the laser altimeter 25b when the aircraft 1 enters a predetermined flight mode, as a usage condition.

[0063] With this configuration, the laser altimeter 25b can be used when the aircraft 1 is in a predetermined flight mode, that is, a flight mode that requires high-precision altitude measurement.

[0064] In a fifth embodiment, in the altitude measurement system 100 according to the first or second embodiment, the control unit 30 allows switching to the use of the measured value of the laser altimeter 25b when the relative position between the aircraft 1 landing at the target landing point and the target landing point is within a predetermined range.

[0065] With this configuration, the laser altimeter 25b can be used when the aircraft 1 is within a predetermined range including the target landing site. Therefore, it is possible to suppress the use of the laser altimeter 25b in the sea surface far from the target landing site.

[0066] In a sixth embodiment, in the altitude measurement system 100 according to any one of the third to fourth embodiments, the control unit 30 corrects the difference in altitude when switching between using the measured value of the flight altimeter 25a and the measured value of the laser altimeter 25b.

[0067] With this configuration, when switching from the flight altimeter 25a to the laser altimeter 25b, any difference in altitude can be corrected, allowing for more accurate altitude measurement.

[0068] The altitude measurement method according to the seventh embodiment is performed by an altitude measurement system 100 that, when the aircraft 1 is descending, switches between using the measurement value of a flight altimeter 25a, which includes at least one of a barometric altimeter and a GPS altimeter, and the measurement value of a laser altimeter 25b, based on the operating conditions.

[0069] With this configuration, if the altitude measured by the laser altimeter 25b is lower than the set altitude, the altitude can be measured with high accuracy by switching to the laser altimeter 25b. Also, if the altitude measured by the flight altimeter 25a is lower than the set altitude, and the reflectance intensity of the laser altimeter 25b is higher than the threshold, that is, if altitude measurement by the laser altimeter 25b is possible, the altitude can be measured with high accuracy by switching to the laser altimeter 25b. [Explanation of Symbols]

[0070] 1 aircraft 5 Ships 7 Marker 10 Cameras 20 Navigation equipment 25 Advanced Sensors 25a flight altimeter 25b Laser Altimeter 30 Control Unit 32 Image Processing Unit 34 Guidance calculation section 36 Flight Control Unit 40 Data transmission device 51 The first AND gate 52 OR circuit 53. The second AND gate 55 First threshold circuit 56. Second threshold circuit 57. The third AND gate 58 timers 61 First changeover switch 62 Subtractors 63 Second changeover switch 64 Integrator 65 Adder 66 Limiter 67 Third toggle switch 68 Memory section 70 Navigation equipment 80 Data transmission device 90 Operation display section 100 Advanced Measurement Systems

Claims

1. An aircraft is equipped with a flight altimeter that includes at least one of a barometric altimeter and a GPS altimeter, A laser altimeter installed on the aforementioned aircraft, An altitude measurement system comprising: a control unit that, during the descent of the aforementioned aircraft, switches, based on the operating conditions, which measurement value to use from the measurement value of the flight altimeter and the measurement value of the laser altimeter.

2. The control unit, as the usage conditions, The altitude measurement system according to claim 1, wherein if the altitude measured by the laser altimeter is lower than a preset decision altitude, the system switches to using the measured value of the laser altimeter.

3. The control unit, as the usage conditions, The altitude measurement system according to claim 1, which switches to using the measured value of the laser altimeter when the altitude measured by the flight altimeter is lower than a preset altitude and the reflectance acquired by the laser altimeter is higher than a threshold value.

4. The control unit, as the usage conditions, The altitude measurement system according to claim 1, which allows switching to the use of the measured value of the laser altimeter when the aircraft enters a predetermined flight mode.

5. The control unit, as the usage conditions, The altitude measurement system according to claim 1, which allows switching to the use of the laser altimeter's measurement value when the relative position between the aircraft landing at the target landing point and the target landing point is within a predetermined range.

6. The control unit, The altitude measurement system according to claim 1, which corrects the difference in altitude when switching between the measurement value of the flight altimeter and the measurement value of the laser altimeter.

7. An altitude measurement method performed by an altitude measurement system that, during aircraft descent, switches between using the measurement value of a flight altimeter (which includes at least one of a barometric altimeter and a GPS altimeter) and the measurement value of a laser altimeter, based on the operating conditions.

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