Previously, we learned about the principles and characteristics of several major ranging/distance sensors . Among them, laser ranging sensors have seen tremendous development since their inception due to their strong anti-interference capabilities and high accuracy, and have played a significant role in various industries.
Shortly after the world’s first ruby laser was invented in 1960, laser ranging technology, with its main function of precision ranging, was born.
After more than 50 years of development, its progress can be broadly summarized in two aspects: first, the application of various new technologies and equipment to improve ranging accuracy and the amount of observation data; and second, the improvement of the automation level of the ranging system to reduce the consumption of manpower and material resources. Specifically:
1. In terms of ranging accuracy, it has gradually improved from the initial meter level to the decimeter level and then to the centimeter level. Currently, the most advanced stations in the world can achieve ranging accuracy at the millimeter level.
2. In terms of ranging capability, the initial maximum range of 1,000-2,000 km has increased to 20,000 km, and even 36,000 km. The realization of laser lunar ranging has brought the ranging capability to 380,000 km.
3. In terms of ranging frequency, it has evolved from once per second initially to 1000-2000 times per second currently, and higher frequency laser ranging (such as 10kHz ranging) is also being tested.
4. Regarding ranging wavelengths, monochromatic ranging systems are still widely used, although some stations are also using dual-color/multi-color laser ranging systems. Dual-wavelength laser ranging systems no longer require corrections based on atmospheric physical parameters and atmospheric models; the measurement results themselves can correct for atmospheric delays, thus achieving better data accuracy.
5. In terms of automation, it has evolved from the initial manual visual tracking to today's computer-controlled automatic tracking.
Applications of laser ranging sensors in space exploration
Space laser ranging technology plays a crucial role in monitoring continental plate movement, crustal deformation, Earth's rotation, improving the Earth's gravitational field and gravitational constant, determining the patterns of Earth and ocean tidal changes, and monitoring space debris. It is also one of the main methods for high-precision satellite positioning observations, boasting the highest single-point sampling accuracy among modern positioning observation methods, and is one of the technical means supporting the International Earth Rotation and Reference Frame Service (IERS). Satellite laser ranging technology has achieved significant success in the following applications and has broad application prospects.
Precise measurement of the orbit of laser satellites
The orbital accuracy determined using satellite laser observation data can reach 1 cm for a 3-day arc segment; for radial orbit determination, the accuracy can reach 2 to 2.5 cm.
Precise determination of Earth's gravitational field model and its time-varying properties
In studying the changes in the position of the Earth's center of mass, laser technology has determined the most accurate Earth gravitational constant GM to date, with a value of GM = 398600.4415 km³/s². Using laser satellites with different orbital inclinations and altitudes, the Earth's gravitational field model was precisely measured, and the seasonal variations of the lower-order spherical harmonic coefficients of the Earth's gravitational field were determined. Furthermore, the periodic variations in the position of the Earth's center of mass, including seasonal and interannual variations, were also derived, with the latest value being J² = -2.6 × 10⁻¹¹/year (epoch 1986.0 ). The changes in the Earth's gravitational field reflect the complex movements and interactions within the Earth's interior and its various spheres (including the ocean, atmosphere, groundwater, and ice sheets), and are of significant research value.
Precise measurement of Earth's rotation parameters
The Earth's rotation parameters (ERPs) define the Earth's rotational axis and its directional motion over time, as well as its rotational speed within a celestial reference frame. The ERPs include polar motion and length of day (LOD) variations. The accuracy of the Earth's polar motion components (XP, YP) measured using laser diffraction techniques has reached 0.1–0.2 mas ; the accuracy of the length of day (LOD) measurement has reached 0.1 ms.
Monitoring global tectonic plate movement
Long-term laser observation data allows for the precise determination of the geocentric coordinates of ground stations, and the high-precision calculation of these coordinates makes it possible to monitor plate tectonics. Data obtained using laser ranging technology has been used to estimate the station velocities and rates of change of inter-station baselines for over 40 stations. If a station is located on a rigid part of a plate, its station velocity represents the plate's motion. The relative motion between plates can be calculated using the rate of change of the station baseline and the station velocity.
High-precision sea level and ice sheet topography measurement
The combined application of laser ranging technology with other space technologies (such as GNSS, radar altimeters, SAR, etc.) may enable the measurement of sea level and ice sheet topography with millimeter-level accuracy.
Space debris orbit determination and monitoring
Laser ranging technology can be used to accurately measure space debris and determine its orbital position, providing precise orbital information for space debris monitoring and space collision early warning systems.
Applications of laser rangefinders in the military field
Lightweight portable pulse laser rangefinder
Lightweight portable pulsed laser rangefinders include handheld models for infantry and artillery reconnaissance, as well as dual-purpose laser rangefinders—target designators—for forward reconnaissance and forward air control (FAC). Systems for these applications require high mobility, repetitiveness, small size, battery power, reliability, maintainability, and low cost per unit.
In modern warfare, operations have evolved from independent infantry and artillery combat to joint operations involving special forces comprised of infantry, artillery, and marines. Weapon systems have also evolved from simple artillery pieces like field guns and anti-aircraft guns to multi-functional, integrated high-tech systems. Consequently, laser rangefinders have progressed from portable, handheld devices with single ranging functions to day/night observation devices combining laser ranging and infrared aiming, as well as laser-infrared target designators that integrate laser ranging, target designation, and infrared aiming.
Ground-based vehicle-mounted pulse laser rangefinder
Ground-based vehicle-mounted pulsed laser rangefinders include those for tanks, infantry fighting vehicles (IFVs), fire control systems, air defense systems, artillery or missile guidance fire control systems, and currently developing ground-based vehicle-mounted laser rangefinders—target designators. Their main technical specifications include: maximum range of 4-10 km, ranging accuracy of ±5-10 m, target resolution of approximately 20 m, repetition frequency of 0.1-1 Hz, and beam divergence angle of 0.4-1 mrad.
The application of laser rangefinders in tank fire control systems is to provide over-elevation correction information for ballistic trajectories, azimuth correction information caused by headwinds or target movement, and distance information. Infantry fighting vehicles primarily use laser rangefinders to measure whether targets are within range of anti-tank missiles, and secondarily for gun and cannon fire control and target sorting.
Anti-aircraft artillery and missile defense pulse laser rangefinder
Pulse laser rangefinders for air defense, as well as pulse laser rangefinders for air defense of infantry fighting vehicles with self-protection measures, should operate in accordance with the requirements of the fire control system and combat system, providing stable tracking and range information for high-speed maneuvering targets in the air within the range and range rate, in order to counter the threats of armed helicopters, stealth aircraft, cruise missiles, and anti-radiation missiles.
This requires the laser rangefinder to provide a relatively high data rate (high laser pulse rate) and a fairly high distance accuracy, such as a maximum range of 4~20km, a ranging accuracy of ± 2.5 ~5m, a repetition frequency of 6~20Hz, and a beam divergence angle of 0.5 ~ 2.5mrad .
Airborne pulsed laser rangefinder
Airborne pulse laser rangefinders can be used to equip armed helicopters with missile command guidance and fixed-wing aircraft to block support electro-optical aircraft and intercept aircraft and missile attacks.
The main technical performance characteristics of airborne pulsed laser rangefinders are: long range, high ranging accuracy, high repetition frequency, and small beam divergence angle. At the same time, the airborne equipment should be small in size, light in weight, and compatible with aviation indicators.
Therefore, lasers must use highly efficient circulating liquids as coolers to meet the requirements of high operating rates; otherwise, gas or mixed gas pressurization cooling must be used.
Shipborne pulse laser rangefinder
The development of shipborne pulsed laser rangefinders follows that of lightweight portable, vehicle-mounted, and air defense laser rangefinders, and it includes two main categories: surface shipborne and submarine periscopes.
In terms of technical performance indicators, surface-mounted pulsed laser rangefinders are the same as vehicle-mounted fire control and air defense laser rangefinders. In terms of environmental use, they must be able to adapt to the harsh requirements of shipboard air and sea surface as well as sea salt spray, while the requirements for size, weight, electrical efficiency, maintenance capabilities and cost are not demanding.
Therefore, unique shipboard applications are emerging in naval vessels currently equipped with conventional fire control and air defense systems, such as shielding (silent radar) carrier-based aircraft recovery and integrating with infrared thermal imaging, television, and other tracking systems to monitor and track aerial targets around the clock. These applications have a very broad prospect.
Submarine periscope pulse laser rangefinders currently employ two combination methods. The first method integrates the laser rangefinder, image intensifier, and thermal imager within the periscope, while the distance display, trigger button, and other components are mounted above or near the operator's hand.
Its advantage is that the laser loss in the transmission optical path is small, but the beam drifts and it is not easy to capture the target; the second type installs the above three parts at the bottom of the periscope, and the installation, debugging and disassembly of the whole system are very convenient, but the laser beam using this method has to pass through a 12m long periscope tube and 15 to 20 lenses, resulting in greater energy loss.
Yungao Pulse Laser Rangefinder
An instrument that uses a pulsed laser rangefinder to measure the vertical height of clouds is called a cloud height laser rangefinder. These rangefinders are primarily used to measure cloud height at airports, and can also be used to measure cloud height at satellite launch sites, providing safe meteorological data for aircraft takeoffs and landings or satellite launches.
These pulsed laser rangefinders can provide reliable meteorological data for the safety of forward military bases, airports, or military satellite launch sites (at close range), making them indispensable instruments in modern warfare. If used to provide safe meteorological data for aircraft take-off and landing or satellite launches at large international airports, small commercial and civilian airports, and civilian communication satellite launch sites (at close range), it will generate huge economic and social benefits for national economic development and enhancing international reputation.
Applications of laser ranging sensors in intelligent transportation
Some potential applications of laser ranging technology in IoT-based intelligent transportation include: laser speed sensors, vehicle collision avoidance systems, traffic flow monitoring, vehicle model mapping, vehicle and pedestrian violation monitoring, and other applications in precision monitoring and measurement.
Car collision avoidance detector
Generally, most existing automotive collision avoidance systems use laser rangefinders to identify the distance between target vehicles in front of or behind the vehicle in a non-contact manner. When the distance between the vehicles is less than a predetermined safe distance, the collision avoidance system will apply emergency braking, issue a warning to the driver, or make an immediate judgment and response based on factors such as the target vehicle's speed, distance, braking distance, and response time. This can significantly reduce traffic accidents. Its advantages are even more pronounced when used on highways.
Traffic flow monitoring and vehicle outline mapping
This method of use typically involves mounting the laser on a gantry at a highway or important intersection. The laser emitter and receiver are perpendicular to the ground and pointed downwards, aiming at the middle of a lane. When a vehicle passes by, the laser rangefinder can output the relative change in the measured distance value in real time, thereby depicting the outline of the measured vehicle.
This measurement method generally requires a ranging range of less than 30 meters and a relatively high laser ranging rate, typically 100 Hz.
This is highly effective for monitoring critical road sections, capable of distinguishing between various vehicle types, and achieving a sampling rate of 10 centimeters per point for vehicle height scanning. It can also distinguish vehicle height restrictions, length restrictions, and vehicle type in real time, and output results quickly.
When no vehicles are present, the laser rangefinder measures a constant distance, which is the distance from the laser rangefinder to the ground. When a vehicle passes under the laser rangefinder, the distance value changes. When the distance value returns to a constant value, it is considered that a vehicle has passed. Based on this method, we can monitor the traffic flow through certain road sections.
The commonly used method is to statistically average traffic flow over a period of time, which involves a large estimation component. Video statistics methods also face many difficulties in practical applications. Therefore, laser ranging statistical methods provide a feasible solution for traffic flow statistics.
Vehicle and pedestrian violation monitoring
Since the laser rangefinder's beam is not a substantial obstacle, it does not impede the normal operation of traffic when used to monitor the road surface.
Therefore, in some road sections where parking is prohibited or pedestrians and vehicles are not allowed to pass, a laser beam is used to emit a fixed beam parallel to the road surface at a certain height or to scan at a certain angle. When a vehicle illegally parks or runs a red light, or a pedestrian illegally crosses a guardrail, the laser ranging distance value changes, which can trigger an alarm or warning.
This type of application doesn't require a very wide beam, but it generally requires a relatively long ranging distance to ensure a protection distance for a certain road segment. Intelligent traffic violation monitoring systems constructed in this way will find widespread application in the Internet of Things (IoT) for transportation.
Laser speed sensor
Laser rangefinders are the earliest form of laser ranging technology used in traffic management, and due to their superior performance, they have gradually become more widely used in practice. Laser rangefinders work by performing two laser ranging measurements on a target object at specific time intervals, obtaining the distance change of the target object within that time interval, and thus determining the object's speed.
Laser speed measuring instruments are divided into two types: fixed and mobile. Fixed ones are generally fixed on the roadside or on a gantry, facing oncoming vehicles at a relatively small angle. They are usually measured by the reflection of the license plate. The measurement accuracy is relatively high, reaching ±1 km/h, and the speed measurement range can reach 250 km/h. The distance measurement range does not need to be too large in this application, generally 80 to 100 meters is sufficient.
Mobile laser speed measuring instruments have relatively high operational requirements. Generally, the beam divergence angle must be greater than 3mrad. Given the principle of laser speed measurement, the laser beam must be aimed at the plane reflection point perpendicular to the laser beam. Since the vehicle is in motion, the vehicle body is not large, and the speed measurement takes a certain amount of time, it can only be used for temporary speed measurement and evidence collection.
Laser rangefinders are easy to use for speed measurement and evidence collection because of their small beam divergence angle. Unlike radar Doppler speedometers, which cannot identify the specific speeding vehicle when measuring in multi-lane traffic, laser speed sensors emit near-infrared light waves, making them undetectable by radar detectors, electronic dog devices, etc., and they are less susceptible to interference from radar clutter in urban areas.
In the currently booming field of autonomous driving, laser ranging sensors are also playing a significant role.
A striking feature of Google's self-driving cars is the rotating laser rangefinder mounted on their roofs. This rangefinder emits 64 laser beams to help the car identify potential hazards on the road. The lasers are high-intensity and can calculate the distance to objects within a 200-meter range, thereby creating an environmental model.
According to Sebatian Thrun, the chief engineer in charge of the autonomous vehicle project, the core of the entire system is the Velodyne 64-beam laser rangefinder on the roof of the vehicle. This device emits 64 laser beams in all directions while rotating at high speed. The laser beams hit surrounding objects and return, allowing the distance between the vehicle and the surrounding objects to be calculated.
The computer system then uses this distance data to create a detailed 3D terrain map, which is then combined with a high-resolution map to generate different data models for use by the onboard computer system.
Summarize
In fact, the applications of laser rangefinders extend far beyond those mentioned above. They are also widely used in various fields such as power, water conservancy, communications, environment, construction, geology, police, fire fighting, blasting, navigation, railway, counter-terrorism/military, agriculture, forestry, real estate, and leisure/outdoor sports.
Currently, laser ranging technology is increasingly developing towards miniaturization, simpler structure, higher precision, and wider applicability. Especially with the advancement of digital processing technology, laser ranging technology will become even more sophisticated. For example, the introduction of advanced background noise reduction and triangulation techniques will enable laser ranging sensors to operate better in more complex environments. We believe that with technological advancements, laser ranging sensors and laser ranging technology will find increasingly wider applications.
