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Measuring distance & motion with Lidar & Sodar Radar has been used for more than a century to detect moving or stationary objects at great distances. But sometimes you need to make precise measurements over much smaller distances – mapping a building or a crime scene, for example. Or you may want to measure wind or water currents. For these tasks, light and sound are more useful than radio waves. Hence, the invention of lidar and SODAR. D istance and motion can be measured using radio waves, light or sound. Radar (RAdio Detection And Ranging) is the most well known of such technologies, and the use of sound for sonar (SOund Navigation Ranging) on ships and submarines is also well known. In this article, we look at the use of light and sound waves for sensing technologies and how they differ from radar and sonar. Experiments with radar started in the late 19th century, but it wasn’t fully developed until the early 20th century, with rapid advances occurring between 1935 and 1945. It was used mainly to detect by Dr David 10  Silicon Chip ships and later, aircraft at great distances. Sonar developed over a similar period, and was used both for marine navigation and to detect submarines. We previously described Airborne Weather Radar in the April 2015 issue (siliconchip.com.au/Article/8449) and Broadband Marine Radar in the November 2010 issue (siliconchip.com.au/Article/343). Plus, we discussed sonar in the context of bathymetry in June 2019 (siliconchip.com. au/Article/11664). More recent developments include SODAR (SOnic Detection And Ranging) and ultrasonic ranging, both of which utilise sound waves, Maddison Australia’s electronics magazine siliconchip.com.au but they operate quite differently to sonar. You may have also heard of lidar (LIght Detection And Ranging), which uses light rather than radio waves. We’ll also briefly discuss infrasound detection, which is at the opposite end of the frequency spectrum to ultrasound. We previously discussed some uses of lidar, for Google Street View and Apple Look Around mapping, in the SILICON CHIP article on Digital Cartography in the March 2020 issue (siliconchip.com.au/Article/12577). Many autonomous ground vehicles also carry lidar units to sense their surroundings, and some such vehicles also use pre-scanned 3D maps for safe navigation. Radar vs lidar and SODAR The main differences between radar, lidar, SODAR and ultrasonic ranging are as follows: Compared to radar, SODAR and ultrasonic ranging, lidar gives much-improved object detail because of its shorter wavelength (in the hundreds of nanometres). Similarly, smaller objects can be detected, such as dust particles. Lidar and SODAR can be used to measure wind strength and direction at a distance. Lidar senses the motion of aerosol particles in the air, while SODAR is sensitive to air density differences. For example, the Windfinder AQ500 (siliconchip. com.au/link/ab2q) SODAR unit is designed for meteorological measurements. Ultrasonic ranging is superficially similar to SODAR, in that ultrasound is used to determine the range in both cases. But SODAR uses an array of microphones and sound ‘beams’, while ultrasonic ranging uses a single microphone and beam. It is often used in older autofocus cameras, and also small robots, for obstacle detection and avoidance. Radar gives a much greater detection range than lidar or SODAR. The laser beams used for lidar are readily absorbed by atmospheric particles like fog, smoke or dust, whereas Fig.1: lidar measurements taken as Apollo 15 orbited the Moon on two different orbits (numbers 15 and 22) in 1971. The lines indicate elevations and depression relative to a sphere 1738km from the centre of mass of the Moon. those hardly affect radar or SODAR. Radar detection distances are generally limited by lineof-sight considerations. Airborne radar can have a range of several hundred kilometres, while over-the-horizon radars (which reflects a beam off the ionosphere) can have a range of several thousand kilometres. One example of the latter is Australia’s Jindalee Operational Radar Network (JORN). Lidar can have a range of tens to hundreds of metres, or in extreme cases, up to about 4km. SODAR typically operates over a maximum range of about 200-2000m. Ultrasonic ranging is typically is used at distances between centimetres and a few metres. Fig.2: a lidar image of a forest. Source: Oregon State University. siliconchip.com.au Australia’s electronics magazine August 2020  11 Fig.3: a lidar-derived flood model for an area in South Carolina along the Saluda River. Source: USGS. Note that lidar will work through a glass window, but ultrasonic ranging will not, since sound waves will bounce off the glass but light waves can pass through. This was a limitation of early ultrasonic autofocus cameras such as the SX-70 (described below). Operating principles In all cases, the operating principles of radar, lidar and SODAR are essentially the same. A pulse of radio energy, light or sound waves is emitted. That pulse is reflected off an object or objects and the reflected pulse returns to the receiver. The elapsed time between emission and the detection of the reflected pulse is recorded and, in some cases, so is the frequency difference. The distance to the object is determined by multiplying the elapsed time by the speed of light or sound, and dividing the result by two. This accounts for the fact it has to travel there and back. For example, if a pulse of radio waves or light takes 3 microseconds to return to the place of emission, then the range, R = 3µs x 300,000,000m/s ÷ 2 = 450m. 300,000,000m/s is approximately the speed of light. The object’s velocity can be determined by the Doppler shift (if measured), and the angle from the transmitter/receiver can also be determined by knowing the direction of the strongest return. Lidar usually uses a single beam. It may be fixed, to measure a distance, or scanned in two or three dimensions to establish a 2D or 3D map of an area. SODAR generally uses multiple beams to develop a 2D or 3D map. In contrast, ultrasonic ranging typically uses a single beam Fig.5: a 2D (horizontal) DIAL map showing methane emissions above a landfill area. Source: Innocenti et al. (https://doi.org/10.3390/rs9090953) 12  Silicon Chip Fig.4: a photograph (left) and lidar image (right) revealing otherwise almost invisible remains from an archeological site in New England, USA. Source: Kate Johnson, University of Connecticut. to establish distance, but it is possible to move the beam to create a 2D or 3D map of an area. So why use lidar rather than a camera, as both sense visible light? A single lidar sensor can have a 360° field of view (360° cameras exist, but are composed of multiple cameras). But its main advantage is that the distance to each ‘pixel’ in the image is accurately known. Our brains are good at extracting approximate range information from a photo, but it’s very hard for a computer to do that. With a lidar image, though, it is clear to the computer exactly where each sensed object is located relative to the lidar device, as the result is a 3D ‘point cloud’. That’s much easier to use for tasks like obstacle avoidance. The point cloud can also be shown as a 2D image and rotated in place; something you can’t easily do with still images without using multiple cameras and a lot of image processing. Uses for lidar The idea of using a laser to measure distance came about in 1960, just after the laser was invented. It was then used by the US National Centre for Atmospheric Research to measure clouds. It was later used in 1971 by Apollo 15 to make topographic measurements of the Moon (Fig.1) and by Apollo 16 and 17, both in 1972. Earlier measurements with lidar were relatively simple distance measurements, or small collections of distance measurements, because of limited computer storage capabilities. But now, highly-detailed and complex 3D ‘point clouds’ representing detailed photo-like models of the environment can be produced. Fig.6: a partial photo and drawing of the Apollo 15 laser ranging retroreflector. This was the largest reflector left during the Apollo missions and is still in use. Australia’s electronics magazine siliconchip.com.au Fig.7: the NASA Clementine topographic map of the Moon from 1994. The colours indicate elevation, as shown on the scale. This data was gathered from an altitude of ~500m. Fig.8: lidar observations of Martian clouds on 3rd September 2008 from NASA’s Phoenix Mars Lander. Fall streaks are suggestive of falling of water snow (not CO2 snow). Lidar can be used from the air or space, with topography mapped as the terrain is traversed, or it can be performed at ground level, either in a fixed location or on a moving platform. Examples of the latter are Google and Apple cars making 3D maps of entire cities from a ground perspective. Airborne or ground-based lidar can be used in forestry to measure the height of trees, their rate of grown and their volume (to estimate when to harvest or for fire management purposes) – see Fig.2 overleaf. Airborne lidar can also be used to make accurate 3D maps, for example, to determine where flooding will occur (Fig.3). Lidar can be used for pollution modelling, by detecting particles in the air that are approximately the same size as the wavelength of the light used. Lidar has several uses in digital mapping and urban planning; these were described in our March 2020 article on Digital Cartography (siliconchip.com.au/Article/12577). Coastlines can be accurately mapped with lidar, and with special lidar that penetrates water calls LADS (Laser Airborne Depth Sounder), the submarine environment can also be mapped. LADS was described in our June 2019 article on sonar (siliconchip.com.au/Article/11664). Lidar is also useful for mobile phone network planning, so that line of sight locations from proposed towers can be determined. This is particularly important for 5G because of poor building and foliage signal penetration. In mineral exploration and mine management, lidar can be used for high-accuracy surveys of existing and proposed mine sites, and also to measure dust and pollutants. In archeology, lidar can be used to map ruins beneath jungle canopies, where they would otherwise be invisible, or to reveal micro-topography in other areas suggestive of buried remains (see Fig.4). Lidar can be used in architecture and building restoration to make precise models of buildings, and in the case of restorations, parts can be scanned and reproduced if necessary. It can also be used for geology; for example, to study changes in topography due to a volcanic eruption or ground movements such as landslides or avalanches. stances in the atmosphere such as pollution, or natural emissions such as from hydrocarbon deposits. The latter can be used to locate such deposits (see Fig.5). This technique was developed in the late 1970s by BP and the National Physical Laboratory in the UK. In DIAL, laser beams of two specific frequencies are emitted. One frequency is tuned to a known absorption band of a molecule of interest, and the other is at a slightly different wavelength which is not absorbed by the molecule of interest. Both beams are backscattered by atmospheric dust etc. The beam that is tuned to the absorption band will be absorbed more than the other, indicating the amount of gas of interest and its location. A map can then be drawn showing the concentration of the gas of interest as a function of range. This technique can also be used to find trace emissions of gases from hydrocarbon deposits, thus locating them, even if they are under the surface. Differential Absorption Lidar (DIAL) DIAL is a remote sensing technique and a form of lidar. It is used to determine the chemical composition of subsiliconchip.com.au Lunar laser ranging experiments On several trips to the Moon, laser retroreflectors were left behind, providing a reflective surface from which a laser could be bounced. This allows the distance from the Earth to the Moon to be measured accurately. Reflectors were placed by Apollos 11, 14 and 15 (Fig.6) and the two Soviet Lunokhod missions. All five arrays are Human echolocation Some people with visual impairments have taught themselves to echolocate similarly to bats, whales and dolphins. They use natural “passive” environmental echos while others actively produce clicks with their mouth and listen to the echos from those. Research has shown that in such people, the brain uses the visual cortex to process this information, since it is not being used for its normal function of eye vision. See the video titled “Daniel Kish: How I use sonar to navigate the world” at https://youtu.be/uH0aihGWB8U and read about the organisation he established to promote and teach this technique, World Access for the Blind at https://waftb.net Australia’s electronics magazine August 2020  13 Fig.9: the RPLIDAR A1 360° laser range scanner. still being used today to make measurements. To determine the lunar distance, a laser pulse is fired from Earth and the round trip time measured. The range is computed, based on the known speed of light. Measurements can be made with millimetre-level accuracy. When a laser is fired from Earth, the beam diameter is 6.5km on the Moon’s surface and on average, about three photons per laser pulse return to the detector on Earth. The precise calculation of the distance is not as simple as it sounds. Many variables have to be taken into account. These factors include the very slight variations of the speed of light in different parts of the atmosphere (which also have to be taken into account for satellite navigation systems) and the motion of the observing station due to tides in the Earth’s crust. The “crustal tide” due to the Moon’s gravitational pull can be as much as 384mm. Relativistic effects and many other small effects also have to be accounted for. Some facts established from the measurements are: the Moon is becoming more distant from Earth at the rate of 3.8cm per year; the Moon has a liquid core; Newton’s gravitational constant has changed less than 1 part in 100 billion in the last 50 years; and Einstein’s general theory Fig.10: this shows how the RPLIDAR A1 can scan a room in and make a 2D map of the area. of relativity is correct within the accuracy allowed by the measurements. There was a plan to install a new reflector on the Moon (called MoonLIGHT) in July 2020. This was to be placed by the MX-1E lander being built by Moon Express, but the mission was cancelled and the fate of this experiment is unknown. It would have improved the measurement accuracy by about 100 times. Lunar and Martian lidar The Moon surface has been mapped from orbit using lidar (Fig.7), and Martian cloud patterns have been observed by the Phoenix lander (Fig.8). There are also proposals by the SETI Institute to use robotic vehicles to map the surfaces of the Moon and Mars using lidar, to map interior structures such as possible caves or lava tubes. Inexpensive hobbyist or consumer lidar There are several inexpensive lidar devices available that SILICON CHIP readers may wish to use or experiment with. One example is the US$150 GARMIN LIDAR-Lite v3HP (siliconchip.com.au/link/ab2l). This has a range of 5cm to 40m, an accuracy of ±2.5cm, an update rate of more than Fig.11: a 3D map of the Jenolan Caves (near Sydney) created with the Zebedee lidar device. Source: CSIRO. See the video titled “Real science from caves to the classroom” at https://youtu.be/jt38pF_TJvY 14  Silicon Chip Australia’s electronics magazine siliconchip.com.au Fig.13: the 20-20 Ultralyte 100LR with DBC or “distance between cars” feature, showing the distance in feet on the left and time in seconds on the right, as well as speed in miles per hour. The DBC feature is used to enforce ‘tailgating’ laws. 1kHz and an I2C or PWM interface. The GARMIN LIDARLite V3 for US$130 is similar; the main difference is that the maximum update rate is lower, at 500Hz. The Seeedstudio Grove TF Mini LiDAR (siliconchip.com. au/link/ab2m) is a US$40 device with a range of 0.3m to 12m, an accuracy of 1-2% depending on range, and a UART (serial) interface. The devices mentioned above establish range only and cannot produce a two-dimensional map unless they are rotated and scanned on a mount. The Slamtec RPLIDAR A1 (see Figs.9 & 10 and www. slamtec.com/en/Lidar/A1) is a 360° laser range scanner with a sampling rate of 8kHz and a scan rate (rotation rate) of 2-10Hz, a range of 12m and an accuracy of 2mm with a serial and USB interface. It can produce a two-dimensional map and costs about US$115. Note that there are some devices marketed as “lidar” which do not use a laser but rather a regular LED, and therefore are not true lidar devices. For example, the US$60 GARMIN LIDAR-Lite V4 LED, with a 5cm to 10m range and accuracy of ±1cm to ±5cm depending on range, an update rate of around 200Hz and I2C or ANT wireless interfaces. Lidar mapping of confined spaces Lidar can be used to map the inside of caves and other enclosed spaces. If the lidar unit is stationary, then one room can be easily captured (see Fig.11). But if a “walk through” is required such as in a cave, mine or large building, a location reference is needed. It is usually not possible to use GPS as the signal does not work in such places, so the location of the lidar as it moves is determined by SLAM or Simultaneous Location and Mapping. This is where the location is determined by the use of three-axis accelerometers, which provide data about the movement of the device. siliconchip.com.au Fig.14: the Remtech PA-XS, a small SODAR unit weighing only 7kg, with a range of 400m. Lidar sensors for consumer drones Relatively inexpensive lidar devices are now available for consumer-level drones. As an example, the Livox Mid-40 LIDAR can be purchased in Australia for A$899. Lidar for crash investigation In Australia, the NSW and Victorian police forces are both known to use lidar to map vehicle crash scenes; specifically, they use the RIEGL VZ-400i, as shown on page 10. Lidar police speed enforcement Police in many countries use lidar for speed limit enforcement. One advantage of lidar over radar is that there is much less beam divergence with lidar, so theoretically, if the equipment is used correctly, it is possible to measure the speed of a specific vehicle in a stream of traffic. Speed-detecting radar, on the other hand, has difficulty in distinguishing between nearby vehicles. When used incorrectly, it has even been known to measure the speed of other objects such as windmills, aircraft and tree branches blowing in the wind! Very high levels of operator attention and training are required to ensure the accurate operation of police radar. Models of police handheld lidar used in Australia and New Zealand include the LTI TruCAM, LTI TruSpeed, LTI 20-20 Ultralyte 100 LR, LTI TruSpeed SE, LTI Ultralyte Compact, Australia’s electronics magazine August 2020  15 Fig.15: a SODAR result for Niwot Ridge in Colorado, USA, showing how the wind speed and direction vary with the height above ground level and time of day. The arrow colour indicates the wind speed while the arrow orientation shows the direction. Kustom Signals ProLaser III, Kustom Signals ProLaser 4 and Kustom Signals Pro-Lite+. See www.lasertech.com/default. aspx and https://kustomsignals.com for more details. Note that while lidar for speed enforcement is theoretically accurate (within error margins), its use in Australia has been successfully challenged, reported by the ABC at siliconchip. com.au/link/ab2n Lidar is also used by police in some areas to measure the distance between vehicles as they travel down a road (see Fig.13). SODAR SODAR is a meteorological instrument that uses sound in a similar way that lidar uses light. SODAR is generally designed to determine wind speeds as a function of height above the instrument. This type of device is also known as Fig.16: a Metek Doppler SODAR PCS.2000 with RASS temperature profiler operating at 482MHz, 915MHz or 1290MHz. This setup is used for vertical profiling of temperatures, temperature gradients and inversion layers synchronously with the SODAR wind profiling. The RASS antennae are placed on either side of the SODAR unit. The vertical range for RASS is up to 500m. 16  Silicon Chip a wind profiler (see Fig.14). They take advantage of the Doppler effect, where the frequency of an echo is altered by the motion of the object it bounces off. This is related to the effect where a moving vehicle with sirens or a horn blaring appears to change in pitch as it passes you. Apart from sound waves, wind profiler instruments can also use radar or lidar to perform measurements using the same basic principle. Applications of SODAR include: assessment of sites for wind generators, to prove there is a suitable wind speed profile throughout the height of the windmill; wind shear detection at airports; wind studies to examine dispersal of Fig.17: how RASS works. A radio beam is reflected off acoustic waves from the SODAR unit, and the backscattered signal can be used to determine the speed of sound as a function of altitude, which can be then be converted to temperature. Australia’s electronics magazine siliconchip.com.au Fig.18: lidar measurements from an aircraft over the Atlantic on 27th September 2016, testing the ALADIN Airborne Demonstrator (A2D) prototype lidar. This was used on the European Space Agency Aeolus satellite, launched on 22nd August 2018. It shows wind speed as a function of height along the flight path. Aeolus is the first satellite capable of making global wind measurements and can measure from the surface to an altitude of 30km. pollutants from smokestacks etc; and determining existing wind patterns for environmental impact studies. The ‘echo’ of the sound wave returning to a receiver from the atmosphere is known as backscatter. Backscatter can occur from substances such as atmospheric dust or rain. But due to the way SODAR operates, it generally arises from small changes in the ‘sonic refractive index’ due to the changes in wind speed or temperature. A change in wind speed of 1m/s corresponds to a change in the sonic refractive index of 0.3%; for a change in temperature of 1°C, the change is 0.17%. For radio frequency signals, the change in refractive index due to a 1°C temperature change is 1ppm (part per million) and radio waves are unaffected by changes in wind speed.Therefore, it is best to use sound to measure wind speed, as RF is very insensitive. See siliconchip.com.au/link/ab2o for more details on this. A SODAR system may be mono-static or bi-static. In a mono-static system, both the transmitted and received beam use the same ‘antenna’ (one transducer is used as both a microphone and a speaker). Backscattering is thus due to temperature fluctuations, which are carried along with the wind, enabling its speed to be determined (Fig.15). In a bi-static system, separate transmitting and receiving devices are used, and backscatter occurs from both temperature and speed fluctuations; however, all commercial SODARs are mono-static. Mono-static SODAR systems use a series of antennas pointed upward in different directions, or they may have a phased-array arrangement with the ‘beam’ electronically steered. A minimum of three beams are required to resolve the three components of wind speed, being in the x, y and z directions. More beams give better results, as with ADCP, which is discussed later. Usually, there is a vertical beam and two beams at right angles, offset from the vertical by about 15-30°. In operation, multiple transmitted pulses are backscattered (reflected) from a moving turbulent patch of air. The reflected pulses incur a Doppler shift according to the speed of the air patch, and the shift of consecutive pulses will change as the patch moves along. When the data from multiple different beams are analysed, the individual vesiliconchip.com.au Fig.19: Japan’s National Institute of Advanced Industrial Science and Technology (AIST) mounted a lidar wind profiler on a windmill to measure upwind speed and direction, for optimising the windmill’s yaw angle and blade pitch for maximum power and service life. locity components can be calculated. The sound a SODAR unit makes in operation can be heard in the video titled “Sound from SODAR wind measurements” at https://youtu.be/8HUyExuFMFI Looking at a range of typical SODAR devices such as those from Remtech, Inc (www.remtechinc.com), the audio frequency is from 1-5.5kHz with an acoustic power level from 5-150W, giving a maximum analysis altitude of 400-3000m. A single unit may use multiple frequencies. SODAR and RASS A RASS or radio acoustic sounding system may be used in conjunction with SODAR to measure the atmospheric lapse rate, which is the measure of how temperature changes with altitude. A radio signal, typically in the UHF frequency range, is directed vertically into the SODAR beam (see Figs. 16 & 17). When certain conditions are met, due to the way the acoustic beam changes the dielectric properties of the atmosphere (it causes either compression or rarefaction), this alters the amount of the radio beam which is backscattered. This provides a measure of the Doppler shift due to vertical motion of the air caused by the acoustic beam. The speed of sound in the air can be determined from this, and thus the temperature, as it alters the speed of sound. As an example of how the speed of sound varies with temperature, between -10°C and 30°C at standard sea-level atmospheric pressure, the speed of sound varies from 325m/s to 350m/s. Measurements are made at different altitudes, so the “pressure altitude” also has to be taken into account. Lidar for wind profiling Doppler lidar can also be used for wind profiling. As with SODAR, the light is backscattered, and the Doppler shift is measured to determine wind speed. Data obtained can be used to optimise windmill performance or for meteorological applications (see Figs.18 & 19). ADCP in water An equivalent device to SODAR for use in water is the acoustic doppler current profiler (ADCP). It uses the same basic principles as SODAR. The frequency range used is Australia’s electronics magazine August 2020  17 Fig.20 (above): a variety of ADCP and DVL instruments from Rowe Technologies, Inc. Note the differing numbers of transducers, as some units utilise more beams than others. typically from 38kHz to several megahertz. Figs.20, 21 & 23 show various ADCP units, while Figs.22 & 24 show how they can be used. The results are visible in Figs.25 & 26. The predecessor to the ADCP was the Doppler speed log, used to measure the speed of a ship through the water. The first commercial ADCP produced in the mid-1970s was an adaption of that system. ADCP works by sending out pulses of ultrasound which are backscattered from particles in the water column of interest. The backscattered signal yields two main pieces of information: the Doppler frequency shift, which gives information about the speed of the particle and the time delay to receive the backscattered signal, giving the range of the particle. An ADCP can also yield information about the distribution of particles in the water column, such as sediments or plankton. When the ADCP is attached to a ship or other maritime platform, the depth of the water and platform speed are also known. When the ADCP is on the seafloor, information about surface waves can be obtained. An ADCP uses two beams for horizontal measurements (2D H-ADCP) or three or more beams (3D case) to resolve water motion in two or three directions. In the 3D case, a fourth beam provides more accuracy. Additional beams Fig.21: the Teledyne RD Instruments ChannelMaster H-ADCP. It uses two beams to produce a 2D velocity profile for a water channel. Different versions can measure across a channel with a width from 20m to 300m. Such devices are often permanently mounted. can be used to make measurements at other frequencies to provide either better accuracy (high frequency) or greater range (low frequency). Three is the minimum number of beams needed for measuring the three velocity components of flow in the x, y and z directions. But the standard configuration uses four beams, as this provides redundancy plus an estimate of the measurement error. A five-beam system is a fourbeam system with an additional vertical beam for measuring waves and ice when upward-looking or depth when downward-looking. Some dual-frequency systems have seven beams; three beams per frequency plus a vertical beam, while there are also eight-beam dual-frequency systems with four beams per frequency. An ADCP can measure the flow of water current through a column. Fig.22 shows a variety of ways in which this is useful. It may be mounted horizontally, such as on the shore of a river, to measure the flow of water from shore to shore. Or it can be mounted on bridge pilings or seawalls to measure flow in streams and irrigation channels (H-ADCP). Alternatively, it may be mounted on the seafloor to look vertically through a column of water all the way to the surface, or on a ship’s hull to take measurements of current Fig.22: the variety of ways in which ACDP can be used, on mobile or fixed platforms. The direction of the multiple beams is shown. DVL refers to Doppler Velocity Logging, for measuring vehicle speed relative to the seafloor. Source: Rowe Technologies, Inc. 18  Silicon Chip Australia’s electronics magazine siliconchip.com.au Fig.24: a StreamPro ADCP attached to a small flotation device is dragged across the Boise River in the USA to measure the flow volume and speed. The velocity profile is measured continuously on the laptop computer. The device is usually connected to the computer via Bluetooth, plus the data is recorded onboard as a backup. Source: Tim Merrick, USGS. Also see the video at https:// youtu.be/E69Y3JaBIiQ Fig.23: the popular Teledyne RD Instruments StreamPro ADCP for measuring velocity and discharge in shallow streams. It is designed for measurements in water 15225cm deep and uses four beams at 2MHz. The whole system weighs just 5kg and is powered by AA cells. The transducer head overhangs the front of the float while the electronics package is in the other blue housing. flow along the path of the vessel (a transect). In H-ADCP, the instrument is set horizontally looking across a stream, irrigational channel etc at a fixed height. Current profiling is often done in two dimensions, rather than three – see Fig.25. If only a 2D slice is measured, then the total flow can be inferred by using an appropriate velocity model for rectangular, circular, trapezoidal, multi-point, or polynomial shaped channels. Relevant dimensions are entered into the measurement software. Three-dimensional ACDP readings are typically in the form of measurements for North-South, East-West and vertical flows. Ultrasonic ranging Ultrasonic or ultrasound ranging uses an ultrasonic pulse to measure the distance to an object. It can also detect if an object has moved in front of a beam. Ultrasonic ranging is used for camera autofocus systems, motion detection, robotics guidance, proximity sensing, measurement of tank liquid levels, measurement of wind speed and direction and object ranging. Parking sensors in cars are an everyday use of ultrasonic ranging. These help motorists manoeuvre vehicles without striking cars or other objects which they may not be able to see, or cannot easily estimate the distance to (Fig.28). The sensors are built into the bumpers of cars, and typically, Fig.25: measurements of the Antarctic Circumpolar Current with velocity profiles as a function of time in the N-S, E-W and vertical directions (left) with the measurement path (above). These were taken with an ADCP attached to an SD 1020 Saildrone USV (unmanned surface vehicle) at 300kHz, 90m deep. Six days of data are shown. Source: Saildrone. siliconchip.com.au Australia’s electronics magazine August 2020  19 Fig.26: typical data that can be obtained from the StreamPro. The middle image shows the measurement matrix while the measurements are at the bottom, with the flow rate indicated by colour. This 3D measurement determines the velocity profile at all depths. Source: Kyutae Lee. 20  Silicon Chip Australia’s electronics magazine siliconchip.com.au Fig.27: an ultrasonic anemometer, the Gill Instruments Ltd WindObserver II. An advantage of this type of instrument is that it contains no moving parts. there are four in the front and four in the back, although some vehicles have twelve sensors in total. Similar sensors may also be used to provide automatic parking features, for example, to measure the distance from the vehicle to the curb, or an already parked vehicle. Wind speed and direction can also be measured with an ultrasonic anemometer (see Fig.27). The time of flight of an ultrasonic pulse depends on the speed of the wind passing in front of it. With two pairs of ultrasonic sensors, the individual velocity components can be resolved to give speed and direction. Ultrasound is typically defined as sound waves with a frequency above 20kHz, which is the upper limit that any human can hear (some people have a much lower bound; it generally drops as we age). Dogs can hear up to 45kHz, cats 64kHz. Some animals such as porpoises can detect frequencies up to 160kHz. At average sea-level atmospheric pressure, 20kHz sound waves have a wavelength of 1.9cm and higher frequencies will be less than that. Ultrasound is used because it gives a more accurate range measurement due to its shorter wavelength than lower sound frequencies. Fig.29: the Polaroid SX70 camera with “Sonar” autofocus from 1978. The ultrasonic transducer is the large perforated disc above the lens. It is a valuable collector’s item today, and has a niche following. siliconchip.com.au Fig.28: a range of Bosch ultrasonic sensors for automated car parking, parking assistance and manoeuvering systems, including emergency braking. They can detect a 7.5cm “standard pole” from 15cm to 5.5m (depending on model), have a horizontal field of view of ±70°, a vertical field of view of ±35°, use frequency modulation and have dedicated ICs to make interfacing easier. The Polaroid Sonar Ranging Module In 1978, Polaroid introduced the SX-70 instant camera which featured an innovative ultrasonic rangefinding system to focus the camera automatically (see Fig.29). The technology was licensed to other users for different applications, and Polaroid built a business around the supply of this ultrasonic transducer circuit board. It was known as the 6500 Series Sonar Ranging Module (Fig.30), and it was suitable for use with a range of Polaroid transducers such as the 600 Series Instrument Grade Electrostatic Transducer (Fig.32). It was intended for use by experimenters and commercial developers alike. Its data sheet can be seen at www.robotstorehk.com/6500.pdf These modules were prized by robotics experimenters, and possibly still are, judging by the amount written about them. Some people have sourced modules from old Polaroid cameras, although the modules are not the same as those that were sold separately. There are notes (last updated 2005) on salvaging them from old cameras at www.uoxray. uoregon.edu/polamod/ Before salvaging these from old cameras, be aware of the possible value of the camera as a collector’s item – especially the SX-70! Fig.30: the Polaroid 6500 Ultrasonic Ranging Module with 600-series transducer. The scale is in inches. Note the discrete components and DIP (dual in-line package) ICs. Australia’s electronics magazine August 2020  21 Experimenting with ultrasonic distance sensors Jaycar and Altronics both sell ultrasonic sensor modules. Jaycar has Cat AU5550 (an all-in-one transmitter/receiver) and also the very popular dual HC-SR04 module, Cat XC4442. Altronics also has the HC-SR04, Cat Z6322. One interesting way to experiment with the HC-SR04 ultrasonic rangefinder module is to build the Jaycar Cat KR9292 “Duinotech Mini Smart Car Robot Kit”. The HC-SR04 module is elementary to drive, as demonstrated by our March 2016 project, the Ultrasonic Garage Parking Assistant (siliconchip.com.au/Article/9848). That was one of our first projects based on Geoff Graham’s Micromite LCD BackPack, which has built-in support for the HC-SR04 sensor module. It requires just two connections to the microcontroller: one digital output to trigger a pulse and a digital input, to determine when the echo is received. Measuring the time between one changing state and then the other tells you the distance from the front of the sensor to the closest object. The 6500 module was capable of driving a transducer such as the 600 Series at 50kHz. This provides range detection over about 2-17m, with 1% accuracy. SensComp (www.senscomp.com) bought Polaroid’s portfolio of ultrasonic ranging modules and transducers and remarkably, a modern SMT (surface mount) version of the 6500 module is still available today (Fig.31). Fig.31: the SensComp 615078LF SMT 6500 Ranging Module, a derivative of the original Polaroid 6500 module but using surface-mount components. It has the same specifications as the original Polaroid device and the parts appear to correspond directly to those shown in Fig.30. Infrasound is at the opposite end of the acoustic spectrum to ultrasound, and is defined as being acoustic frequencies less than 20Hz, the typical lower limit of human hearing. Infrasound arises in nature from some animals such as whales and elephants and natural phenomena such as earthquakes, ocean waves and aurorae. Infrasound listening arrays have been used to locate avalanches, nuclear detonations and tornadoes. The volcanic explosion of Krakatoa in 1883 was detected as small pressure fluctuations on traditional barometers around the world, as infrasonic waves circled the Earth three to four times in each direction. The low-frequency array or LOFAR is a radio astronomy observatory in the Netherlands, but the infrastructure of LOFAR is also used for sensors to perform infrasound observations. According to KNMI’s website (they are a member organisation), the observatory consists of “a temporary 80 element high density array, a permanent 30 element microbarometer array with an aperture of 100km and, at the same locations, a 20 to 30 element seismological component”. The microbarometers can be used to probe processes in the upper atmosphere above 30km and other infrasound phenomena, and also to study seismo-acoustic phenomena since seismic events are also measured at the same site. See http://siliconchip.com.au/link/ab2p Infrasound is also used by the comprehensive nucleartest-ban treaty organization (or CTBTO, of which Australia is a member) to monitor for unauthorised nuclear tests. Australia has infrasound stations located Warramunga, NT; Hobart, TAS; Shannon, WA; Cocos Islands and Davis Station, Antarctica (see Fig.33). For more information on this network, see the video titled “The Infrasound Network and how it works” at https:// youtu.be/GVWOA5pZG6o SC Fig.32: a SensCorp 604142 Series 600 Instrument Grade Ultrasonic Sensor for use with the 6500 module. This is a modern version of Polaroid’s original 600 sensor. Fig.33: the Australian infrasound monitoring station “IS03” at Davis Base, Antarctica. This is one of about 300 stations around the world maintained by CTBTO member states. Apart from infrasound, Australia monitors seismic, radionuclide and hydroacoustic phenomena to detect unauthorised nuclear tests as part of the International Monitoring System (IMS). Infrasound 22  Silicon Chip Australia’s electronics magazine siliconchip.com.au