Fiber-Based Laser Technology
On the Cutting-Edge of Today’s LiDAR Systems
This article takes a look at fiber-based LiDAR technology and its geospatial development spearheaded by German-based TopoSys GmbH. Currently this is the only organization manufacturing and utilizing fiber-based LiDAR for airborne survey and remote sensing applications.
By Frank Artés
Fiber-based Lidar optical principle (Image credit TopoSys, Germany). |
Broad-based Research
LiDAR (Light Detection And Ranging) is often thought of as being a fairly recent technology. Fact is that it has been in existence for almost four decades after laser light was first developed during the early 1960’s. Since then the basic technology has evolved to become a sophisticated sensing instrument used for an increasing and diverse number of applications that focus on object distance, speed, rotation or chemical composition. Fiber-based lasers are currently a hot topic in the field of on-going LiDAR research and development. There has been a growing demand for a decrease in component size, weight and manufacturing costs. Coupled with an increased requirement for a more rugged system with improved precision, this has brought about a shift in the basic LiDAR design parameters, which have stood for almost 20 years. To address these issues and expand markets within the medical, aviation safety, and airborne/spaceborne remote sensing industries, fiber technology has been integrated as the optical component designed to increase the levels of accuracy currently generated with the traditional mirror-deflection systems.The advantages of fiber technology include relatively low-cost manufacture using COTS components which produce a lightweight, compact, yet very robust laser system. Fiber technology offers optical-alignment stability which means instrument calibration need only be done once during the manufacture and production stage. Fiber-based lasers, with fusion spliced optical links between components are highly efficient, broadly tunable and for a spaceflight environment in particular, require only a low power supply.
Fiber-based Lidar scan pattern (Image credit: TopoSys, Germany). |
These attributes have attracted organizations such as the Japan Aerospace Exploration Agency (JAXA) and Honeywell. Both of them are carrying out detailed research into the use of fiber-based LiDAR systems to reveal areas of clear air turbulence (CAT) ahead of approaching commercial aircraft. The thing is that currently there is not a system available which can reliably detect this phenomenon. NASA is also heavily involved in researching the operational capabilities of fiber-laser systems where the size/weight ratio is an important criterion for deep-space missions. In addition, ONERA, the French national research institution dedicated to the study of aerospace problems is testing fiber-based LiDAR to model and measure aircraft wake vortices.
Forest canopy model (Image credit: LFG Mecklenburg-Vorpommern, Germany). |
Initial Development
During the 1980’s, Dornier GmbH began development of a fiber-based laser in an effort to produce an obstacle avoidance system for military helicopters, together with an autonomous vehicle guidance sensor. The primary goals were precision and reliability which fiber-optic technology offered. This ultimately led to the successful development of the System 1 LiDAR, which was taken over by TopoSys GmbH., a spin-off company founded in 1995. TopoSys used it as the basis for their subsequent development and production of the Falcon series of Fiber-based LiDAR systems, which became operational a year later. Dornier has continued its research into obstacle avoidance technology and now produces the HELLAS system designed to warn helicopters operating close to transmission lines of the potential for hydro cable strikes.
Colour-coded lagoon near Venice (Image credit: EU-project TIDE). |
Geospatial Utilization
For the geospatial community traditional LiDAR systems have been commercially available for little more than 20 years, primarily as a range-finding technology for ground-based survey and electronic distance measurement (EDM) instruments. Its value as a practical airborne sensing and data gathering tool only became apparent during the 1990’s with the subsequent integration of inertial navigation systems and GPS technology. This allowed precise position and orientation information to be applied to the point data to compensate for the aircraft’s movement about its three axes, roll, pitch and yaw. Now with the increased interest in fiber-based LiDAR and its broad application potential, Alexander Wiechert, Managing Director, TopoSys GmbH, comments on how the technology has developed within the geospatial community. “Today’s geospatial applications are technology-driven with image quality and data integrity forming the basic foundation on which many projects are built. Fiber-based LiDAR systems are quickly becoming a mainstream geo-technology by providing application-specific precision for many assignments. It has been recognized that high resolution laser data can provide a basis for detailed research and analysis. For example, this was seen with the Tidal Inlets Dynamics and Environment (TIDE) project, where the interaction between landforms and ecosystem structures was effectively studied using a new generation of data modeling techniques. The TopoSys LiDAR provided the high resolution data for this project and we have since become involved with a number of universities and scientific institutions from Germany, Italy, France and the UK.” The TopoSys Falcon series of LiDAR systems have taken full advantage of the attributes associated with fiber-optic technology to generate high-quality elevation data. Precise laser beam deflection, short laser pulse, fast echo detection and small viewing angle deliver an accuracy and reliability that now position their sensors as the benchmark technology for precision DEM data collection.
3D raster city model (Image credit: Vermessungsamt Mannheim). |
3D forest DSM, Billenhagen (Image credit: LFG Mecklenburg-Vorpommern, Germany). |
How Airborne LiDAR Works
An airborne LiDAR system generates a pulsed laser light beam which is projected onto a scanning mirror and reflected earthwards. Upon reaching an object, such as a building, a tree or the ground, the light pulse is reflected back to the system. The speed of light is a known constant, so by using a mathematical equation, the distance the pulsed light beam travels can be accurately calculated and effectively generates a Z (height) value for the objects it encounters. The X and Y coordinates are determined using integrated GPS and inertial measurement unit (IMU) components which together produce precise position and orientation information for each data point. The result is an accurate set of three-dimensional coordinate data. Various other factors come into play such as slant measurements, multiple return signals, return signal intensity together with variations in terrain and surface deformation however, further discussion of these topics is beyond the scope of this article.
Design Differences
The primary difference between the traditional LiDAR and the fiber-based alternative lies in the optical configuration and scanning methodology used to deflect the laser beam. The quality of the sensor system, its operational efficiency, point density and swath width are often established by the scanning method used and the resulting laser scan pattern. The scan lines are either uni-directional or bi-directional. The individual points along the scan line are produced using equal or varied angle increments at source, which can generate diverse and inconsistent point spacing on the ground.
The two standard scanning techniques are:
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Oscillating Mirror
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Rotating Polygon Mirror
Oscillating mirror
Using this technique the laser beam is deflected by the mirror as it oscillates back and forth at varying speeds across a wide range of adjustable angles. This produces a zigzag (sinusoidal) or bi-directional scan line with non-uniform point spacing, a function of the cross-track and along-track spot positioning. This random scan pattern can be particularly coarse at the swath edges where increased along-track separation takes place.
Rotating polygon mirror
In this configuration the laser beam is deflected off the individual facets of the polygon as it rotates producing a parallel or uni-directional scan line. Both mirror rotation and laser pulse rate are adjusted to correspond with aircraft speed and altitude producing a fairly uniform point density across the entire swath.
These scanning methods are more suitable to small scale mapping projects or tasks where the accuracy requirement is less stringent. Both methodologies are prone to component mechanical wear and are subject to re-calibration to ensure stated system accuracies are maintained.
Fiber-based System
The fiber-based system is quite different. The LiDAR developed by TopoSys operates with an array of 128 optical fibers in which the transmitting and receiving optics are identical, arranged in a linear pattern at the transmitter end, and a circular pattern at the receiver end. The light from the transmitter-fibers is linked to the corresponding receiver-fibers via a very small mirror rotating at high speed. The individual fibers are scanned in succession and synchronized to deliver a very precise laser beam deflection with a small viewing angle. The returning pulse is accepted by its corresponding receiver fiber ensuring absolute precision, a feature of the stable geometric orientation of the complete fiber array. A single fiber is fed directly from the transmitter to the receiver acting as a dedicated reference. Should any discrepancies arise in the measurement electronics they can be detected and immediately corrected to maintain data accuracy. In general, this methodology employs extremely small moving parts, but there are no moving parts associated with the beam deflection. Therefore, it is possible to reach scanning speeds as high as 630Hz, which is not possible with the other two scanning methods.
TopoSys’ Latest Technology
Falcon III is the company’s latest system, which generates 125,000 measurements per second in a slight swing motion. This produces a regular scan pattern with a ‘snaking effect’ which allows the point distribution to achieve optimal ground coverage. The typical characteristics are a narrow viewing angle, in this case 28 degrees, a wide beam, which translates as 0.7m ground diameter from an altitude of 1000m, and high measurement overlap. The wide beam generates multiple echoes (up to eight) from a single pulse, detecting differences in elevation of at least 1m. In addition, a full wave form recording is also available. Adjacent scan displacement is 16cm with an aircraft flying speed of 65 m/sec. The standard point density averages between four and five measurements per 1m2 and could be increased to more than 25 measurements per 1m2 by using a helicopter platform for applications such as corridor mapping. Consequently, the reliance on a single measurement to generate an accurate result is eliminated.
3D view of point cloud for powerline corridor (Image credit: TopoSys, Germany). |
Advantages
The high measurement overlap and optimal ground coverage provide a number of distinct advantages. One of the most beneficial ones is the ability to detect small linear-type objects and dramatic changes in elevation, caused by planimetric features such as retaining walls, ditches, ridges and embankments. The first and last echo from all pulses which intercept these elements, are identified. This powerful edge-detection capability eliminates the requirement to capture additional photogrammetric breaklines in order to generate a true topographic representation. Prudent data filtering will clearly identify buildings and roof lines for example, or drainage patterns and other hydrographic details. This level of accuracy has enabled a diverse application potential, such as urban planning and the production of 3D city modeling where composite cadastral and engineering data can be integrated with rendered building structures to visualize expansion plans, redevelopment projects and infrastructure networks. Similarly, archeological landforms and historic sites, not readily discernible at ground level, can be accurately identified, measured, analyzed and displayed very quickly when compared to traditional data-gathering methodologies using conventional fieldwork.
For the forest management community, fiber-based LiDAR systems enable single tree segmentation and crown area verification, trunk diameter measurements and tree center coordinates values, to be precisely determined. In areas where open foliage is limited, the wide beam, narrow viewing angle, and optimal ground coverage vastly increase the probability of canopy penetration.
Multi-sensor Systems
Since 2004, the Falcon LiDAR has been integrated with a passive RGB/NIR line scanner to produce digital RGB and CIR true ortho images, with the result that precision digital elevation models and spectral image data can be produced from a single flight. Roman Kathofer, TopoSys representative North America, explained: “Both the LiDAR and the line scanner operate simultaneously using a single, integrated inertial/GPS system using independent, sensor-specific lever-arm calculations to provide position and orientation information for both data sets. Generating accurate three-dimensional geospatial information can be a costly enterprise so maximizing airborne mission time is crucial. Until recently both data sets were often captured on separate flight missions at different altitudes, a result of the dissimilar field-of-view and area coverage of LiDAR and large-format digital cameras.”
He continues: “However, in the last few years a move towards integrating medium-format digital cameras with standard LiDAR systems has been very successful. The similarity in area coverage between both technologies has improved their compatibility. This can be seen with the TopoSys Harrier series of standard LiDARs which offer the Applanix DSS 322 digital camera system integrated with a Riegl laser scanner. Similarly Optech also promotes a LiDAR/digital camera combination with its ALTM LiDAR and Rollei camera.”
Falcon system installation in a Piper Seneca II (Image credit: TopoSys, Germany). |
True Ortho Images
The TopoSys LiDAR/line scanner configuration is also multi-sensor compatible but takes a different approach to image generation than that of a digital camera. The imagery is generated line-by-line producing a continuous strip with a swath width 1.5 times larger than the LiDAR. This increased lateral overlap allows for minimized shadowing when generating mosaics from multiple flight lines. The Falcon III multi-sensor produces what are termed true ortho images. It applies image rectification and georeferencing using intelligently-filtered LiDAR data to produce position-accurate imagery. The correct positioning of all features, such as buildings, bridges, towers and trees, is achieved by incorporating the elevation data from these features together with the terrain data to effectively eliminate object displacement. Compatibility between the fiber-based LiDAR and the line scanner can be seen in its successful use particularly for corridor surveys, where seamless, rectified image data and a precision digital surface model can highlight transmission line components and pipeline construction details with remarkable clarity.
Terrain relief image of Celtic chieftain’s settlement, Heuneburg (Image credit: Landesdenkmalamt Baden-Wurttemberg, Germany). |
What the Future Holds
Wiechert predicts an exciting future for fiber-based lasers: “We are seeing an increase in the demand for reliable and extremely precise elevation and image data, an indication that the geospatial market continues to develop in this direction. Years ago, the most common product our customers asked for was a 2m grid size elevation model. Nowadays, it is extremely rare for someone to ask for data with a lower resolution than 1m.” He continues: “While the common mirror-based technology, particularly the oscillating-mirror system, seems to be at the end of its development cycle, fiber-based technology has an enormous potential for further development. TopoSys has continuously focused on precision LiDAR, both in sensor development and as a service provider. Falcon III has been a major step forward. By integrating the precision of a fiber-based laser with the production capacity of a mirror-based system, the geospatial community can now take advantage of an unmatched functionality, which is a very exciting prospect.”
References
Katzenbeiber, R., Schnadt, K.,Unique Airborne Fiber Scanner Technique for Application-Oriented LiDAR Products (ISPRS working group VIII, 2004) Wiechert, A., Precise LiDAR Data and True-Ortho Images (Map Asia, 2004)
Frank Artés (fartes@geoinformatics.com) is a Contributing Editor of GeoInformatics. For more information visit www.toposys.comand www.cluin.org/programs/21m2/openpath/lidar/.
