Radar And Lidar

 

RADAR

In this article, I discuss the working of radar and Lidar and their differences.
Radar was coined in 1940 by the United States Navy as an acronym for radio detection and ranging. [1] Radar uses radio waves to determine the distance, angle, and radial velocity of objects relative to the site. To detect and track aircraft, ships, spacecraft, guided missiles, and map weather and terrain, we use radar. 
Modern radar systems use digital signal processing and machine learning. They are capable of extracting useful information from very high noise levels.
Other systems which are similar to radar make use of other parts of the electromagnetic spectrum. For example, lidar uses predominantly infrared light from lasers rather than radio waves. With the emergence of driverless vehicles, radar assists the automated platform to monitor its environment, thus preventing unwanted incidents. [2]
 
A radar system consists of:

• A transmitter produces electromagnetic waves in the radio or microwave domain.
  
• A transmitting antenna
• A receiving antenna.
• A receiver 
•  A processor to determine the properties of the objects. 
  

Working:

Radio waves (pulsed or continuous) from the transmitter reflect off the objects and return to the receiver, giving information about the objects' locations and speeds

Some uses of radar:

• Air and terrestrial traffic control
  
• Radar astronomy
  
• Air-defense systems
  
• Anti-missile systems
  
• Marine radars to locate landmarks and other ships Aircraft anti-collision systems
  
• Ocean surveillance systems
  
•  Outer space surveillance and rendezvous systems Meteorological precipitation monitoring
  
•  Radar remote sensing
  
•  Altimetry and flight control systems
  
•  Guided missile target locating systems
  
•  Self-driving cars
  
•  Ground-penetrating radar for geological observations.
  

 
History:

In 1886, German physicist Heinrich Hertz showed that radio waves could reflect from solid objects. In 1895, Alexander Popov, a physics instructor at the Imperial Russian Navy school in Kronstadt, developed an apparatus using a coherer tube for detecting distant lightning strikes.
In 1896, he added a spark-gap transmitter.
In 1897, while testing this equipment for communicating between two ships in the Baltic Sea, Popov noticed an interference beat caused by the passage of a third vessel. In his report, Popov wrote that this phenomenon would help to detect objects, but he did nothing more with this observation. [3]
The German inventor Christian Hülsmeyer was the first to use radio waves to detect the presence of distant metallic objects.
 In 1904, he demonstrated the feasibility of detecting a ship in dense fog but not its distance from the transmitter. [4] He obtained a patent [5] for his detection device in April 1904 and later a patent[10] for a related amendment for estimating the distance to the ship. He also obtained a British patent on 23 September 1904[6] for a radar, and he called it a telemobiloscope. It operated on a 50 cm wavelength and created a pulsed radar signal via a spark-gap. His system already used the classic antenna setup of horn antenna with parabolic reflector and presented to German military officials in practical tests in Cologne and Rotterdam harbor but was rejected. [7]
In 1915, Robert Watson-Watt used radio technology to warn airmen[8].
IN 1920, he made many advances using radio techniques, including the probing of the ionosphere and the detection of lightning at long distances.
Across the Atlantic in 1922, after placing a transmitter and receiver on opposite sides of the Potomac River, US Navy researchers A. Hoyt Taylor and Leo C. Young discovered that ships passing through the beam path caused the received signal to fade in and out. Taylor submitted a report suggesting that this phenomenon was useful to detect the presence of ships in low visibility, but the Navy did not immediately continue the work. Eight years later, Lawrence A. Hyland at the Naval Research Laboratory (NRL) observed similar fading effects from passing aircraft; this revelation led to a patent application[9] and a proposal for further intensive research on radio-echo signals from moving targets to take place at NRL. Similarly, in the UK, L. S. Alder took out a secret provisional patent for Naval radar in 1928. [10] W.A.S. Butement and P. E. Pollard developed a breadboard test unit operating at 50 cm (600 MHz) and using pulsed modulation, which gave successful laboratory results.
 In January 1931, the first official record of the apparatus was in the Inventions Book by the Royal Engineers.
In 1934,  researchers began developing an obstacle-locating radio apparatus.
They installed it on the ocean liner Normandie in 1935. [11]. During the same period, another invention of an experimental apparatus, RAPID. That was capable of detecting an aircraft within 3 km of a receiver. [12].
The Soviets produced their first mass production of radars RUS-1 and RUS-2 Redut in 1939.
The first Russian airborne radar, Gneiss-2, entered service in June 1943.
By the end of 1944, the production of more than 230 Gneiss-2 stations was there.
New and better radar systems emerged during 1950.
One of them is the development of a highly accurate monopulse tracking radar, capable of angular accuracy of about 0.1 milliradians. Another notable development was the klystron amplifier. Synthetic aperture and Doppler radar were also first appeared in 1950. Doppler frequency shift is the basis for police radar guns.
In 1960, the first electronically steered phased-array radar was in operation.
In 1970, the use of radar for remote sensing of the environment.
Airborne bomber radar and ballistic missile detection became feasible in the 1980s.
In the 1990s, the introduction of the Doppler weather system.
The 21st century has a further improvement in signal and data processing.

Principle

A radar system has a transmitter that emits radio waves known as radar signals in predetermined directions. When these signals contact an object, they are usually reflected or scattered in many directions. The radar signals reflected from the object towards the radar receiver are the desirable ones that make radar detection work. If the target is moving either toward or away from the transmitter, there will be a slight change in the frequency of the radio waves due to the Doppler effect.
The time it takes for the reflected wave to return to the receiver enables a computer to calculate how far away the object is. The radar receiver and transmitter are usually not in one location. The reflected radar signals captured by the receiving antenna are weak. We can use electronic amplifiers to strengthen it. Using sophisticated methods of signal processing, we can recover radar signals.
The weak absorption of radio waves by the medium through which they pass enables radar sets to detect objects at relatively long ranges. The ranges at which other electromagnetic wavelengths, such as visible light, infrared light, and ultraviolet light, are too strongly attenuated. Weather phenomena, such as fog, clouds, rain, falling snow, and sleet, that block visible light are usually transparent to radio waves. 
 
Recent developments:

Active electronically scanned arrays (AESA) are revolutionizing the performance of modern radar systems, enabling an unprecedented degree of operational flexibility. 
Advantages:

• · Superior performance
  
• · Reliability and life cycle cost. 
  

The X-Band two-dimensional sensor uses a combination of electronic scanning and mechanical rotation.
This sensor allows the radar to spotlight a geographic area of interest for long periods. It increases the detection capabilities of smaller targets, particularly in sea clutter [13]. 
 
Application:

The first use of radar was for military purposes: to locate air, ground, and sea targets.
In aviation, aircraft equipped with radar devices that warn of aircraft or other obstacles in or approaching their path

• Display weather information and give accurate altitude readings.
• Aircraft can land in fog at airports equipped with radar-assisted ground-controlled approach systems.
• Military fighter aircraft are usually fitted with air-to-air targeting radars to detect and target enemy aircraft.
•  Larger specialized military aircraft carry powerful airborne radars to observe air traffic over a region and direct fighter aircraft toward targets.[14]
• Using marine radars, we can measure the bearing and distance of ships to prevent collision with other objects.
•  To monitor and regulate ship movements in busy waters.[15]in port or harbor.
• It has become the primary tool for short-term weather forecasting and watching for severe weather conditions such as thunderstorms, tornadoes, winter storms, and precipitation.
• Geologists use specialized ground-penetrating radars to map the composition of Earth's crust.
• Police forces use radar guns to monitor vehicle speeds on the roads.
• Using smaller radar systems, we can detect human movement. For example, breathing pattern detection for sleep monitoring[16] and hand and finger gesture detection for computer interaction.[17]
• Automatic door opening, light activation, and intruder sensing are common uses.

Lidar

Lidar is a method for measuring distances using a laser on a target and measuring its reflection with a sensor. Lidar (light detection and ranging)[18] or laser imaging, detection, and ranging[19]) is a method for determining ranges by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. Lidar may operate in a fixed direction (e.g., vertical), it may scan in multiple directions (Lidar scanning or 3D laser scanning), and lidar has terrestrial, airborne, and mobile applications. [20]

Uses:

• To make high-resolution maps with applications in surveying, geodesy, geomagnetic, archaeology, geography, geology, geomorphology, seismology, forestry, atmospheric physics,[21] laser guidance, airborne laser swathe mapping (ALSM), and laser altimetry. Using lidar, we can make digital 3-D representations of areas on the Earth's surface and ocean bottom of the intertidal and near coastal zone by varying the wavelength of light. It has also been increasingly used in control and navigation for autonomous cars[22] and for the helicopter Ingenuity on its record-setting flights over the terrain of Mars. [23]

 

Lidar measurements of lunar topography made by Clementine's mission [53]

History

After the invention of the laser, the development of the first tool similar to a lidar was in 1961 at Hughes Aircraft Company. [24] It was first intended to track satellites. [25] It was called Colidar, an acronym for Coherent Light Detecting And Ranging[26]

One of the first usages of lidar was in meteorology. National Center for Atmospheric Research used it to measure clouds and pollution. [27] Hughes Aircraft Company introduced the first lidar-like system in 1961, shortly after the invention of the laser. This system combined laser-focused imaging with the ability to calculate distances by measuring the time for a signal to return using appropriate sensors and data acquisition electronics [28].

The first practical terrestrial application of a colidar system was the Colidar Mark II. It is a large rifle-like laser rangefinder produced in 1963. which had a range of 11 km and an accuracy of 4.5 m, used for military targeting. [29]  Eventually, the laser may provide an extremely sensitive detector of particular wavelengths from distant objects.

Applications:

Lidar's first applications were in meteorology, for which the National Center for Atmospheric Research used it to measure clouds and pollution. [30] The general public became aware of the accuracy and usefulness of lidar systems in 1971 during the Apollo 15 mission, when astronauts used a laser altimeter to map the surface of the Moon.

Agriculture

• Lidar is used to analyze yield rates on agricultural fields.
• Lidar helps to determine where to apply costly fertilizer. It can create a topographical map of the fields and reveal slopes and sun exposure of the farmland. 
• Lidar is now used to monitor insects in the field. It can detect the movement and behavior of individual flying insects, with identification down to sex and species.

Plant species classification:

Controlling weeds requires identifying plant species. Using 3-D lidar and machine learning, we can do it. [31] Lidar produces plant contours as a point cloud with range and reflectance values. This method is efficient because it uses a low-resolution lidar and supervised learning. It includes an easy-to-compute feature set with statistical features that are independent of the plant size.

Lidar has many uses in archaeology, including planning field campaigns, mapping features under the forest canopy, and overviewing of broad, continuous features indistinguishable from the ground. [34] Lidar can produce high-resolution datasets quickly and cheaply. 

Lidar can also help to create high-resolution digital elevation models (DEMs) of archaeological sites that can reveal micro-topography. Using the intensity of the returned lidar signal, we can detect features buried under flat vegetated surfaces such as fields, especially when mapping using the infrared spectrum. In 2016, its use in mapping ancient Maya causeways in northern Guatemala revealed 17 elevated roads linking the ancient city of El Mirador to other sites. [35]

Autonomous vehicles

Autonomous vehicles may use lidar for obstacle detection and avoidance to navigate safely through environments. Point cloud output from the lidar sensor provides the necessary data for robot software to determine where potential obstacles exist in the environment.  

Autonomous car

Object detection for transportation systems

Lidar has a significant role in the safety of transportation systems. Many electronic systems that add to driver assistance and vehicle safety are Adaptive Cruise Control (ACC), Emergency Brake Assist, and Anti-locking Brake System (ABS). All of these depend on the vehicle's environment to act autonomously or semi-autonomously. Lidar mapping and estimation achieve this.

Current lidar systems use rotating hexagonal mirrors that split the laser beam. The upper three beams apply to detect vehicles and obstacles ahead. The lower beams apply to lane markings and road features. [37]

Advantages and disadvantages of using lidar:

• · The spatial structure data 
• · We can fuse it with other sensors (namely radar).
• · We get a better picture of the vehicle environment in terms of the static and dynamic properties of the objects present.
• · To reconstruct point cloud data in poor weather conditions is difficult.
• · In heavy rain, due to rain droplets, the light pulses emitted from the lidar system partially reflected off, which adds noise to the data, called echoes[38].

Ecology and conservation

Here, I mention only some uses:

• Used for mapping natural and managed landscapes such as forests, wetlands,[97] and grasslands. 
• · Canopy heights, biomass measurements, and leaf area can all be studied using airborne lidar systems. [39]  
• · Industrial use, including Energy and Railroad
• ·  A faster way of surveying.
• ·  To generate topographic maps 
• ·  To estimate and assess the biodiversity of plants, fungi, and animals. [40]

Forestry

•    To calculate individual tree heights, crown width, and crown diameter. 
• · To estimate total plot information such as canopy volume, mean, minimum and maximum heights, vegetation cover, biomass, and carbon density. [41]
• · To map bushfires.

Geology and soil science

• ·To detect subtle topographic features such as river terraces and channel banks, glacial landforms measure the land-surface elevation beneath the vegetation canopy.
• ·  To resolve spatial derivatives of elevation
• ·  To detect elevation changes between repeat surveys. 
• · To monitor the increasing occurrence of severe rock-fall over large rock faces allegedly caused by climate change and degradation of permafrost at high altitudes. [42] (In 2005, Tour Ronde in the Mont Blanc massif became the first high alpine mountain employed to monitor the rock fall.)

Atmosphere

Uses include:

• ·  For profiling clouds
• ·  To measure winds.
• ·  To study aerosols.
• · To quantify various atmospheric components like surface pressure (by measuring the absorption of oxygen or nitrogen) and greenhouse gases (carbon dioxide and methane),
• ·  To assess photosynthesis (carbon dioxide), fires (carbon monoxide), and humidity (water vapor).

Atmospheric lidars can be ground-based, airborne, or satellite based on the type of measurement.

Law enforcement

•  The police force uses lidar speed guns to measure the speed of vehicles for speed limit enforcement purposes. [43]
•   In forensics to aid in crime scene investigations. 
•  To record the exact details of object placement, blood, and other important information for later review through scans of a scene. 
•  Using these scans, it is possible to determine bullet trajectory in cases of shootings. [44]

Mining

• · To calculate the ore volumes by periodic (monthly) scanning in areas of ore removal
• ·  For obstacle detection and avoidance for robotic mining vehicles. 

Physics and astronomy

• · To measure the distance to reflectors placed on the place.
• · In September 2008, the NASA Phoenix Lander used it to detect snow in the atmosphere of Mars. In atmospheric physics, it is used as a remote detection instrument to measure the densities of certain constituents of the middle and upper atmosphere (such as potassium, sodium, molecular nitrogen, and oxygen).
• · Lidar can also be used to measure wind speed and to provide information about the vertical distribution of aerosol particles. [45]

· 

Rock mechanics

• · To detect rock mass characterization and slope change. 
• · Using the 3-D point clouds obtained, we can extract some geomechanical properties from the rock mass.  

Some of these properties are:

· Discontinuity orientation

· Discontinuity spacing and RQD

· Discontinuity aperture

· Discontinuity persistence

· Discontinuity roughness[46]

· Water infiltration[47]

 

Thor is a laser designed to measure Earth's atmospheric conditions. The laser enters a cloud cover[48] and measures the thickness of the return light. We can measure the return light with a fiber optic aperture sensor.

Solar photovoltaic deployment optimization

· Optimizing solar photovoltaic systems at the city level by determining appropriate rooftops [49] and shading losses.

Other uses

In 2020, Apple introduced the fourth generation of iPad Pro with a lidar sensor integrated into the rear camera module, specially developed for augmented reality (AR) experiences. [50]. They included this feature in the iPhone 12 Pro lineup and subsequent Pro models. [51] On Apple devices, lidar empowers portrait mode pictures with night mode, quickens autofocus, and improves accuracy.

Significant features:

•  Lidar uses ultraviolet, visible, or near-infrared light to image objects.
• · It can target non-metallic objects, rocks, rain, chemical compounds, aerosols, clouds, and even single molecules. [52]
• · A narrow laser beam can map physical features with high resolutions (for example, an aircraft can map terrain at 30-centimeter (12 in) resolution or better). 
• · Lidar instruments fitted to aircraft and satellites carry out surveying and mapping. 
• · Suitable combinations of wavelengths allow remote mapping of atmospheric contents by identifying.[53]

To determine the distance of the object, use the equation,

d=c*t/2,

c is the speed of light, d is the distance between the detector and the object, and t is the time taken for the laser light to travel to and fro.

Radar Vs Lidar

At the basic level, radar and lidar differ because they do not use the same wavelength type of electromagnetic wave.

 

Number	Lidar	Radar
1	Uses a laser to measure distance.	Relies on radio waves.
2	It measures precise distance and detailed 3-D map.	Measures the speed and distance, excelling in long-range detection and adverse weather conditions
3	Prefers tasks requiring mapping and object recognition.	Reliable in various environmental conditions makes it indispensable in field aviation and maritime operations.
4	Uses light from pulsed laser beams with a wavelength in the near-infrared (NIR) range	Uses microwaves and operates at much longer wavelengths
5	The speed at which Lidar can record data sets is limited only by how quickly its laser can turn on and off.	The details of data depend on how frequently the system records data points and how fast it moves during flight.
6	It uses a digital camera or other imaging sensors to capture the reflected light from a laser beam. By processing the collected data, we get the point cloud for creating digital maps and other applications.	The first step is the conversion of the reflected microwaves into an image on a screen or computer monitor. Radar can not measure distance as precisely as Lidar.
7	Short range than radar systems but can provide much more precise measurements at close range.	Long range and are better at detecting objects that are far away
8	More expensive and require a clear line of sight.	Can detect objects through fog, rain, and other obstacles.
9	Lidar has higher resolution and can provide more detailed and accurate measurements than radar.	Wavelengths are large, and their results are not as good as Lidar sensors.
10	Considered to be more accurate than radar, especially at short ranges.	However, radar has the advantage of operating in a wide range of environmental conditions and at longer ranges. It is also generally less expensive than Lidar.

 

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