Equipment and methods for measuring the ranging capability of laser altimeters/rangefinders

Equipment and methods for measuring the ranging capability of laser altimeters/rangefinders
Laser altimeter and laser rangefinder (hereinafter referred to as laser altimeter) are composed of laser transmitting system and laser receiving system, and their performance largely refers to their distance measurement capability, i.e. maximum range. Especially for long-distance laser altimeter (more than 20km), the maximum range index is particularly important. Due to a variety of factors, it is not practical to measure a target at the maximum range distance on the ground, and the accuracy of the results is not high.
The extinction coefficient method is one of the traditional laboratory methods for determining the maximum range of laser altimeters. Although this method also needs to work in the field, but it is not the same as the field measurement. The specific implementation process of the extinction coefficient method is as follows: place a diffuse reflecting plate at a distance of 0.5km from the laser altimeter, turn on the laser, and the emitted laser hits the diffuse reflecting plate. A filter is placed in front of the receiving system to reduce the laser energy reaching the receiving system. The transmittance of the filter is pre-set and is adjusted slightly during operation. The transmittance rate at which the laser altimeter has a correct distance reading is its sensitivity. The maximum range can be obtained from the sensitivity. When calculating the maximum range, it is also necessary to know the atmospheric visibility at the time of measurement. The extinction coefficient method is a simple method of measuring the maximum range of a laser altimeter. However, since this method can only be realized in the field, it requires high requirements for the experimental site. At the same time, it is greatly affected by the atmospheric conditions, so it is difficult to obtain a high degree of accuracy.
The method of fiber-optic simulation of target distance can simulate targets at different distances through the cyclic transmission of laser light in the optical fiber to achieve the measurement of different laser altimeters. Because the diameter of the laser beam is much larger than the diameter of the optical fiber, it is necessary to design an optical system to couple the laser beam, so this type of detection equipment optical path is complex, and the optical axis is difficult to align, at the same time, it is more difficult to determine the delay capacity of the optical fiber of the method, you need to use a laser simulator. Simulation of the laser is used to generate simulated echoes. This is done by changing the performance of the internal components of the laser simulator through a combination of hardware and software to obtain a simulated laser beam with adjustable performance parameters, simulating the effect of the external environment on the emitted laser beam during actual ranging. The delay circuit controls the delay between the laser simulator and the emitted laser to simulate different target distances.

 

The technical problem to be solved is to overcome the above mentioned difficulties of the existing technology and provide an equipment and method for measuring the maximum range of the rangefinding capability of a laser altimeter/rangefinder, which should be simple, and the measurement method should be simple and effective.
The maximum range of a laser altimeter is relative to certain conditions. Under different atmospheric conditions, the maximum range obtained by ranging targets with different surface characteristics is not the same. Therefore, not only is the laser energy different for ranging targets with the same surface characteristics at different distances, but the laser energy required for ranging targets with different surface characteristics at the same distance is also different. Corresponding to these different ranging conditions, the laser altimeter receiving system, the minimum detectable power of the echo detector is unchanged. That is, the minimum echo energy to which the receiving system responds is a certain value. Therefore, according to the laser altimeter to reach the minimum detectable power value corresponding to the value of each parameter, you can deduce the maximum range of the laser altimeter distance measurement.
Measurement of laser altimeter range-finding capability of the device, characterized by its composition is: along the laser altimeter laser transmitter system issued by the laser beam forward direction in turn, including a mirror, reflector, attenuator, analog target, in the direction of the transmission of the mirror is equipped with an energy meter; mirror and the beam into 45 °, the reflector and the beam into 45 °, and the role of the two is to be the laser beam issued by the laser altimeter refraction The role of the two is to fold the laser beam from the laser altimeter, so that it is coaxial with the laser altimeter receiving system, and waveform display system.

Said attenuator comprises an attenuator holder and an attenuator set comprising a plurality of attenuators, the attenuator holder having a plurality of attenuator sockets for such attenuators to be inserted.
Said simulation target is a reflective panel with known surface reflectivity.
The following steps are included:
①. Docking the laser altimeter with the measurement equipment, so that the angle between the mirror and the optical axis of the laser transmission system of the laser altimeter is 45°, and the angle between the mirror and the optical axis of the laser reception system of the laser altimeter is also 45°, and ensure that the laser beam reflected by the mirror is coaxial with the optical axis of the laser reception system. The angle between the total reflector and the optical axis of the laser receiving system of the laser altimeter is also 45°;
②. Set up a simulated target at a certain distance Rs in the forward direction of the laser beam;
③. Set the energy of a single pulse of the emitted laser beam to a certain value, which is monitored by an energy meter as Wo; ④.
④. Increase the attenuator attenuator piece, at the same time by the laser rangefinder laser receiving system to monitor the laser return digital signal. digital signal, when the return digital signal is 0, write down the transmittance rate of the attenuator To; ⑤.
⑤. Calculate the maximum range of the laser altimeter using the following formula:

Laser rangefinder

In the formula:

Wts=Woto- Minimum detectable single-pulse laser energy of the laser altimeter
Wo- energy meter detected by the emitted single pulse laser energy
To- Transmission rate of the attenuator when the return digital signal is 0
Wt- maximum single-pulse laser energy emitted by the laser altimeter
Ptar- Surface reflectance of the target
Ptars-simulated surface reflectance of the target
α- Angle of incidence of the laser on the surface of the target
as- angle of incidence of the laser on the surface of the simulated target
t²a- atmospheric two-way transmittance from the laser altimeter to the measured target
T²as- atmospheric two-way transmittance from the laser altimeter to the simulated target
Rs - distance from the laser altimeter to the simulated target
Advantages: simple measurement equipment, simple measurement method; transmissive mirror 1 adopts half-reflective half-transparent sheet, the energy meter can realize real-time measurement of the laser emission energy; the use of transmitting the laser two times reflective folding, to ensure that the laser emission and reception of coaxial, indoor experiments, not affected by the external environment; the use of diffuse reflection simulation of the target, the real simulation of the actual ranging, the target of the laser reflection.
Illustration:

FIG. 1 is a schematic diagram of the apparatus of the present invention for measuring the ranging capability of a laser altimeter and its measurement state.

Laser rangefinder
FIG. 2 is a schematic diagram of the structure of the attenuator of the present invention.

Laser rangefinder
Specific implementation:
First of all, please refer to Figure 1, Figure 2, Figure 1 is the present invention to measure the laser altimeter distance measurement ability of the device and its measurement state schematic diagram, Figure 2 is the structure of the attenuator schematic diagram. As can be seen from the figure, the equipment of the present invention for measuring the distance measuring capability of the laser altimeter includes: transmissive mirror 1, reflector 2, energy meter 3, attenuator 4, analog target 5, and waveform display system 6. The transmissive mirror 1 and the reflector 2 convert the laser emitted from the laser transmitting system 71 of the laser altimeter 7 to be coaxial with the laser receiving system 72. The transmissive mirror 1 is a semi-transparent and semi-reflective sheet, and the reflector 2 is a fully reflective sheet. An energy meter 3 is used to measure the energy of the laser beam emitted by the laser emission system 71. The attenuator 4 comprises a holder 41 and an attenuator sheet set 42. The whole attenuator 4 is used to attenuate the energy of the emitted laser. The attenuator set 42 consists of attenuator sheets 421 with different transmittance rates, and by changing the composition of the attenuator set 42, different transmittance rates of the attenuator 4 can be obtained. The simulated target 5 is a reflective plate with a known surface reflectance Pars, and the reflections generated by the incident laser follow the Lambert's Law of Reflection, which can be used to simulate the diffuse reflections generated by the target when ranging the real target by the laser altimeter. The waveform display system 6 is implemented using an oscilloscope to monitor the return signal. See Figure 1.
The method of measuring the maximum range of the laser altimeter using the above equipment includes the following steps:
A. Dock the laser altimeter 7 to the measuring device so that the angle between the transmissive mirror 1 and the optical axis of the laser transmitting system 71 is 45°, and the angle between the reflecting mirror 2 and the optical axis of the laser receiving system 72 is also 45.
The angle between mirror 2 and laser receiving system 72 is also 45°. This ensures that the laser beam reflected by mirror 2 is in the same optical axis as the laser receiving system 72;
B. Set the simulated target 5 in the optical path, and adjust the distance between the simulated target 5 and the laser altimeter 7 according to the need, the distance is more than 20m less than 50m is appropriate, and use the traditional method to measure the simulated target 5 and the laser altimeter 7 between the exact distance Rs;.
C. Set the laser energy emitted by the laser transmitting system 71 to a certain value Wo, change the composition of the attenuator 4 to obtain different transmission rates, use the waveform display system 6 to display the echo digital signals obtained by the laser receiving system 72, and when the digital echo signal disappears, record the corresponding transmission rate of the attenuator 4 To and the energy value Wo shown in the energy meter 3, i.e., when measuring the distance of the simulated target 5 at the given distance Rs, the desired transmission rate will be recorded, which will be used to measure the distance between the simulated target 5 and the laser altimeter 7. This is the laser energy Wo required for ranging an analog target 5 at a given distance Rs. At this point, the corresponding echo detector power is the minimum detectable power WoTo of the laser altimeter.
D. Calculate the maximum range of the laser altimeter 7 under the given conditions from the correspondence between the maximum range and each parameter.
The maximum range of the laser altimeter is given by the following equation:
Laser rangefinder

In the formula:
Wts=Woto- Minimum detectable single-pulse laser energy of the laser altimeter
Wo- energy meter (3) detected the emission of a single pulse of laser energy
To- transmittance of the attenuator (4) when the return digital signal is 0
Wt- maximum single pulse laser energy emitted by the laser altimeter (7)
Ptar- surface reflectivity of the target
Pars-simulated surface reflectance of the target (5)
α-angle of incidence of the laser on the surface of the target
as-angle of incidence of the laser on the surface of the simulated target (5)
T²a-Atmospheric two-way transmittance from the laser altimeter (7) to the measured target
T²as-atmospheric two-way transmittance from the laser altimeter (7) to the simulated target (5)
Rs - distance from the laser altimeter (7) to the simulated target (5)

RELATED ARTICLES

Leave a comment

Your email address will not be published. Required fields are marked *

Please note, comments must be approved before they are published