In the realm of remote sensing technologies, Light Detection and Ranging (LIDAR) has emerged as a game-changer for detecting and mapping targets obscured by dense foliage. Whether deployed on manned or unmanned airborne platforms, LIDAR’s ability to penetrate vegetation can reveal previously hidden objects and provide critical insights in various fields, from military reconnaissance to environmental monitoring and disaster response. This article delves into the applications and advantages of LIDAR on manned and unmanned airborne platforms for detecting targets concealed beneath dense foliage.
Challenge of detecting under Foliage
Situation awareness and accurate target identification are critical requirements both for security forces engaged in counterterrorism operations. Ground, airborne and space-borne radars and EO sensors have proved to be of great utility in detection, tracking, and imaging of vehicles to high speed fighter aircrafts, locating mortars and artillery, over the horizon capability and all weather long range surveillance.
However theses sensors are limited in capability to detect targets concealed in the foliage. Conventional forces and terrorists / insurgents have exploited this weakness by employing camouflage, concealment and deception tactics like hiding in camouflage net and forests since long.
Understanding LIDAR Technology:
LIDAR operates on the principle of emitting laser pulses towards a target and measuring the time taken for the reflected signal to return. By analyzing the return signal, LIDAR systems can create detailed 3D representations of objects and terrain features, including those obscured by vegetation cover. Traditional optical sensors struggle to penetrate foliage, but LIDAR can capture information even beneath the canopy.
LIDARs are rapidly gaining maturity as very capable sensors for number of applications such as imaging through clouds, vegetation and camouflage, 3 D mapping and terrain visualization, navigation and obstacle avoidance for robotics as well as weapon guidance.
LIDAR wavelength is much shorter than microwave, hence it is considered incapable of penetrating forest canopy. However even in high density forests there are gaps including gaps inside single tree crown and gaps in between different tree canopies. If portion of laser beam is able to penetrate the gap and hit the target and return to the detector, the LIDAR is able to detect the target hidden under foliage.
LIDAR on Manned Airborne Platforms:
LIDAR (Light Detection and Ranging) is an active optical remote sensing sensor, that when flown aboard rotary or fixed-wing aircraft, through illumination of the target scene with laser pulses can acquire x, y, and z coordinates of both manmade and naturally occurring targets and generate 3D models of them.
Manned aircraft equipped with LIDAR systems have long been utilized in various industries, including forestry, infrastructure planning, and urban mapping. When it comes to detecting targets under dense foliage, manned platforms offer the advantage of carrying larger and more powerful LIDAR systems, capable of acquiring high-resolution data over vast areas. This makes them ideal for large-scale environmental surveys and disaster response efforts.
Applications on Manned Platforms:
- Forest Monitoring: Manned airborne LIDAR is extensively used for monitoring forest health, estimating biomass, and identifying potential hazards like diseased trees or insect infestations, all of which can be hidden beneath thick vegetation.
- Archaeological Exploration: In archaeology, LIDAR-equipped aircraft can reveal ancient ruins and archaeological sites hidden in dense jungles or heavily vegetated landscapes, providing valuable information to researchers and conservationists.
- Infrastructure Inspection: LIDAR on manned platforms aids in inspecting power lines, pipelines, and other critical infrastructure obscured by foliage, facilitating early detection of potential vulnerabilities.
For deeper understanding of Airborne LIDARs and applications please visit: Airborne LiDAR Technologies: Advancing Remote Sensing in Environmental Monitoring, Security, and Beyond
LIDAR on Unmanned Airborne Platforms (UAVs):
Unmanned Aerial Vehicles (UAVs) have revolutionized remote sensing by offering flexibility, cost-effectiveness, and the ability to access hard-to-reach areas. UAV-based LIDAR systems have proven particularly valuable for detecting targets hidden beneath foliage due to their maneuverability and adaptability.
Applications on UAV Platforms:
- Precision Agriculture: UAV-based LIDAR enables farmers to monitor crop health and assess vegetation density, aiding in precision agriculture practices and optimizing irrigation and fertilization.
- Environmental Conservation: UAVs equipped with LIDAR can survey dense forests and wildlife habitats, providing crucial data for biodiversity assessment and conservation efforts.
- Disaster Response: In the aftermath of natural disasters, UAV-based LIDAR can quickly assess damaged areas, helping emergency responders identify blocked roads and infrastructure hidden under debris and vegetation.
Advantages of UAV-based LIDAR:
- Flexibility and Versatility: UAVs equipped with LIDAR can access challenging terrains, such as steep mountains or dense jungles, which may be difficult or dangerous for manned aircraft.
- Rapid Deployment: UAVs can be deployed swiftly for on-demand data acquisition, making them ideal for real-time monitoring and urgent disaster response.
- Cost-Effectiveness: Compared to manned flights, UAV-based LIDAR missions are more cost-effective, allowing for more frequent and targeted data collection.
Successful Foliage penetrating LIDAR developments
Now, researchers at the Naval Research Laboratory in Washington, DC, have developed LIDAR based on gated digital holography, to give LiDAR an enhanced ability to see through obscuring elements like foliage and netting.“see” through minor obstructions for genuine obstacles.
A team at MIT’s Photonic Microsystems Group have integrated LIDAR systems onto a single microchip that can be mass produced in commercial CMOS foundries yielding a potential on-chip LIDAR system cost of about $10 each. Instead of a mechanical rotation system, optical phased arrays with multiple phase controlled antennas emitting arbitrary beam patterns may make devices more robust.
Naval Research Laboratory has developed a foliage penetrating LIDAR
Researchers at Washington, DC’s Naval Research Laboratory have developed a LIDAR and a new methodology based on gated digital holography, to give LiDAR an enhanced ability to see through obscuring elements like foliage and netting.
“This was an attempt to address one of the problems with something called foliage-penetrating LiDAR,” Paul Lebow of the Naval Research Laboratory said. “You can actually use it to detect three-dimensional images behind an obscuration such as a tree canopy… You can illuminate using LiDAR through the leaves and get enough light coming back through to be able to recreate a three-dimensional, topographic view of what’s going on beneath.”
However, over the years, LiDAR measurements have been limited to wherever light can penetrate. With surfaces hidden behind obstructions, the original light gets thrown away and the camera is unable to detect or only receives a limited signal, providing minimal readings.
“We have been working with a process called optical phase conjugation for quite some time and it dawned on us that we might be able to use that process to essentially project a laser beam through the openings of the leaves and be able to see through a partial obscuration,” Lebow said. “It was something that until maybe the last five years was not viable just because the technology wasn’t really there.”
The stuff we had done about 20 years ago involved using a nonlinear optical material and was a difficult process. Now everything can be done using digital holography and computer generated holograms, which is what we do.” This new system uses a specially designed laser that alone took a year and a half to develop, but was a necessary component according to Lebow and his colleague, Abbie Watnik, who is also at the Naval Research Laboratory and another of the work’s authors.
Phase conjugation is a fascinating phenomenon with very unusual characteristics and properties. It operates somewhat like holography, but it is a dynamic hologram, whose “holographic plate” is defined by interfering wave fronts in a nonlinear optical medium, rather than etched as a static pattern on a glass plate. Optical phase conjugation is a non-linear optical process, which is capable of time reversing the scattering process, and healing the distortions in a wave front.
“We send one laser beam out to the target and then it returns, and at the exact same time that return [beam] hits the detector, we interfere it locally with another laser beam,” Watnik said. The researchers needed to design the laser system to ensure the camera captures absolute coherence when the laser beams interfere with one another.
Using a pulsed laser with pulse widths of several nanoseconds, and gated measurements with similar time resolution, the holographic system selectively blocks the earliest-to-arrive light reflecting obscurations. The camera then only measures light coming back from the partially hidden surface below.
“We’ve done this earlier using a CW (continuous-wave) laser as a demo, but now we’re using a pulsed laser and a very fast gated sensor that can turn on at the appropriate time to basically only let us respond to the light coming from where we want it to come from, from the target,” Lebow said. “The laser is designed so that the time difference between the local reference pulse and the signal pulse that comes back from the target is completely adjustable to accommodate distances from a few feet to several kilometers.”
“We were able to verify what our computer model says using our real data – matching it to what we actually see using the spatial light modulator, so I think that was an interesting verification of our results,” Watnik said.
Watnik and Lebow, along with their research team, hope to continue with the project and make the adaptations to their prototype necessary to making the foliage-penetrating LiDAR system field-ready.
DARPA’s Modular Optical Aperture Building Blocks (MOABB)
MIT’s Photonic Microsystems Group is trying to take these large, expensive, mechanical lidar systems and integrate them on a microchip that can be mass produced in commercial CMOS foundries. At MIT our lidar on a chip work first began with the development of 300-mm silicon photonics, making their potential production cost on the order of $10 each at production volumes of millions of units per year.
MIT’s current on-chip LIDAR system can detect objects at ranges of up to 2 meters, though they hope within a year to achieve a 10-meter range. The minimum range is approximately 5 cm, and the team has demonstrated centimeter longitudinal resolution and expect 3-cm lateral resolution at 2 meters. There is a clear development path towards LIDAR on a chip technology that can reach a 100-meter range, with the possibility of going even farther.
DARPA had launched Modular Optical Aperture Building Blocks (MOABB) program with the aim to develop the advanced technologies it will take to build ultracompact light detection and ranging (LIDAR) systems, which use light to image objects and their motions in the same way that RADAR systems use radio waves.
The first phase of the program calls for researchers to develop the fundamental devices that will underlie the new LIDAR concept: speck-sized light-emitting and light-detecting cells capable of being readily integrated into larger arrays using typical semiconductor manufacturing processes. Phase 2 and Phase 3 of the project call for the integration of these cells into a 1 cm2 array and a 10 cm2 array comprising upwards of 100 and 10,000 unit cells, respectively.
With an integration of digital, electronic, optical and radiofrequency elements on a variety of combined semiconductor materials, the final 10-cm aperture LIDAR surface has the potential to be the most complex electronic-photonic circuit ever constructed, according to an anticipated Broad Agency
One of the most coveted applications that could emerge from the envisioned program, which could extend for five years with up to $58 million in funding, is foliage-penetrating imagers for spotting hidden threats under dense forests such as sniper or a tank
“You would be able to fly a MOABB-enabled helicopter or drone low over a lush forest canopy” and “it could instantaneously give you the range and velocity of everything up to a football field’s distance away with the resolution of a camera. And with accompanying visualization tools, he added, “you would feel like you are on the ground with nothing blocking your vision,” DARPA program manager Joshua Conway said.
Other potential applications include collision avoidance systems for small unmanned aerial vehicles (UAVs) maneuvering in tight indoor spaces, precision motor control for robotic limbs and fingers, high-capacity light-based communications and data-transfer systems, and sophisticated gaming or training modules in which LIDARs would open up new worlds of immersive experience just as GPS and motion-sensing accelerometers have done in today’s systems.
“Every machine that interacts with the 3D world—whether it is a manufacturing robot, UAV, car, or smartphone—could have a chip- or wafer-scale LIDAR on it,” Conway said.
US Army searches industry for state-of-the-art imaging LIDAR for tactical mapping from UAVs
Officials of the Army Contracting Command have issued a source-sought notice (W909MY-17-R-A006) for the LIDAR Payload for Manned and Unmanned Airborne Platforms. The LIDAR system shall be capable of supporting mission planning (also referred to as tactical mapping), concealed target detection (Multi-Aspect Foliage Penetration [FOPEN]) and mapping missions.
Of particular interest is LIDAR technologies mature enough for an advanced technology demonstrator prototype. The imaging sensor must have a pointing and stabilization unit such as a stabilized turret that houses the sensor optics, focal planes, and supporting electronics. The data storage and sensor processing has computer, data storage, sensor-processing algorithms, and imaging sensor command, control software.
The LIDAR shall have a variable swath from 200 meters or less to 1 km or more. To achieve the final product, the government will allow over-sampling and resampling. The LIDAR should be capable of reaching slant ranges of 25,000 feet. Range resolution, (ability to separate objects in range with a minimal number of laser pulses) shall be under one meter and range precision (statistical distribution of a number of measurements over a fixed range) shall be under 10 cm. Field of regard of the system shall be at least plus or minus 35 degrees in-track and 45 degrees cross-track.
The primary desire for this payload is for an unmanned airborne platform. As such, weight and aerodynamic drag (represented primarily by diameters of the front cross-section) are major concerns.
The LIDAR system shall consist of multiple Line Replaceable Units (LRUs) including but not limited to an Imaging Sensor Unit (ISU) and Storage and Processing Unit (SPU). The ISU consists of a pointing and stabilization unit (e.g. stabilized turret) that houses the sensor optics, focal planes and supporting electronics. Essentially everything required to operate the LIDAR sensor except for data storage and processing. If the ISU requires an external electronics box, the external box must be included in the ISU weight calculations.
The SPU refers to the storage and processing computer as well as the processing/ exploitation algorithms running on the hardware. The SPU will also host the ISU’s command, control and status software. This RFI is separated into two portions, a Vendor may optionally respond to one of the two sections or both sections. The first section requests information on the ISU; responses to this section are limited to 25 pages. The second section requests information on the SPU; responses to this section are limited to 20 pages.
LIDAR system’s PSU shall store and process the ISU’s imagery. The government desires automated feature extraction (AFE) and/or aided target recognition (AiTR) algorithms and transmission of a low-resolution orthographic projection of the human activity layer (HAL) annotated with anomaly detections. The Vendor, in response to the processing solution, should discuss any AFE and AiTR algorithms they currently possess. Examples include anomaly detection, segmentation, void detection, plane detection, or any algorithm or tool to minimize analysts’ workload and facilitate operation over limited bandwidth communications (<= 2 Mbps)
Commercially Available Foliage penetrating LIDARs
There are a number of commercial LIDARs that are designed for use on manned and unmanned airborne platforms. These LIDARs typically use a wavelength of 1064nm, which is well-suited for foliage penetration.
Some of the most popular commercial LIDARs for foliage penetration include:
- Velodyne Puck: The Velodyne Puck is a small, lightweight LIDAR that is designed for use on unmanned aerial vehicles (UAVs). It has a range of up to 100 meters and can penetrate foliage up to 5 meters thick.
- LidarLite V3: The LidarLite V3 is a small, affordable LIDAR that is also designed for use on UAVs. It has a range of up to 40 meters and can penetrate foliage up to 2 meters thick.
- RIEGL VZ-10: The RIEGL VZ-10 is a larger, more powerful LIDAR that is designed for use on manned aircraft. It has a range of up to 1000 meters and can penetrate foliage up to 20 meters thick.
These are just a few of the many commercial LIDARs that are available for foliage penetration. The best LIDAR for your needs will depend on the specific application you are using it for.
In addition to the commercial LIDARs listed above, there are a number of research-grade LIDARs that are also being developed for foliage penetration. These LIDARs use a variety of techniques to improve foliage penetration, such as using multiple wavelengths or using adaptive optics.
As LIDAR technology continues to develop, it is likely that we will see even more powerful and versatile LIDARs that are capable of penetrating even thicker foliage. This will make LIDAR an even more valuable tool for a variety of applications, such as military surveillance, search and rescue, and mapping.
LIDAR on manned and unmanned airborne platforms has unlocked a new realm of possibilities for detecting targets concealed beneath dense foliage. Whether used for environmental monitoring, disaster response, or archaeological exploration, LIDAR’s ability to penetrate vegetation offers valuable insights that were previously hidden from view. As technology continues to advance, the fusion of LIDAR with airborne platforms promises to shape a more efficient, sustainable, and secure future for various industries and applications.