TY - JOUR
AU - Wilson, David
AB - Remote sensing has been an integral and growing technology for managing natural resources. One of the chief impediments to land managers has been the relatively high cost that is associated with high-resolution imagery acquired by aircraft and satellites. Legislation and other new developments in the United States will substantially increase the use of unmanned aerial systems (UASs) as remote sensing platforms. There are also recent technological developments that have made platforms and sensors available at more reasonable prices. These changes will probably reduce the cost of high-resolution imagery and promote remote sensing applications for natural resource management. We describe current developments in the operation and potential technology of UASs within the United States, present a recent UAS flight in which real-time video imagery of a forested area was captured, and discuss the potential for future UAS applications. unmanned aerial system, UAS, remote sensing, imagery Remote sensing has provided strong capabilities to help manage natural resources (Pitt et al. 1997). Chief among these capabilities is the ability to capture resource measurements quickly across broad landscapes. Remote sensing platforms include satellite-, airborne-, and ground-based vehicles. There are nontrivial costs involved in remote sensing natural resource applications, despite the potential advantages to land managers (US Department of Agriculture Forest Service 2004). In the case of remote sensing across broad landscapes, one of the prohibitive costs has been the expense involved in supporting aerial platforms that enable high spatial resolution of landscapes. Manned aerial platforms (airplanes or helicopters) that support high-resolution imagery require, at a minimum, a pilot and an aircraft capable of supporting significant payloads and also pose risks for pilots. Pilot risks constrain the ability of platforms to fly close to the ground, which in turn limits the potential resolution of imagery. In addition, cloud cover and weather patterns can obscure sensors from being able to see ground cover because most aircraft must fly at significant heights (Hunt et al. 2008). Recent legislation and other developments in the United States have the potential to encourage the use of unmanned aerial systems (UASs) as remote sensing platforms (Davis 2008). In addition, recent technological developments in the sophistication of sensors and communications have spurred capabilities in capturing imagery and getting this imagery into the hands of decisionmakers (Barnhart et al. 2011). These developments will have significant impacts on the affordability and versatility of remote sensing applications for natural resource management (Beard et al. 2005, Kim et al. 2006). Our objectives are to describe ongoing changes in the operation and technology of UASs within the United States, discuss how these changes might affect remote sensing opportunities, and describe a recent UAS flight in which real-time video imagery of a forested area was captured. Background The Federal Aviation Administration (FAA) oversees and manages airborne platform flights within the United States. The FAA manages the National Airspace, and all commercial and recreational air traffic must operate under FAA regulations within this space. An exception to FAA oversight is “hobbyist” applications that involve UASs that are operated for noncommercial applications, within line of sight of a ground-based observer during flight and are conducted within 400 ft above ground level (AGL) (FAA 1981). UAS applications that seek to operate outside of hobbyist concentrations must be conducted within military-restricted airspace or have received FAA approval under a Certificate of Authorization (COA). A COA application must currently be hosted by a public institution, make reference to a specific platform and geographic area of operation, and detail the conditions under which a flight may proceed (FAA 2008). The COA application also requires a line-of-sight operation between observers and airborne platform, safety procedures should the platform deviate from expectations, and, typically, a pilot certified to fly a manned aircraft as the pilot in command of the UAS. There are two significant developments in the United States that will promote UAS applications. The first of these is that the US Congress has directed the FAA to integrate UASs into the National Airspace by 2015 (FAA Modernization and Reform Act of 2012, §332, 49 U.S.C.). Although details of how that was to be accomplished were not provided, it is anticipated that this directive would eventually result in UASs being allowed to operate in areas and under the same policies that currently regulate aircraft operations within the United States (Dalamagkidis et al. 2011). UAS integration presents a number of technological challenges, the most highly discussed are issues regarding navigation and communication (Joint Planning and Development Office 2012). In response to this directive, the FAA has announced a Screening Information Request in which up to six national test sites for UAS applications would be selected (FAA 2013). Successful applicants are expected to have significant flexibility in conducting UAS operations within their jurisdictions. The second development is that the FAA is expected to issue a request for proposal (RFP) for a UAS Center of Excellence (CoE). The FAA has previously issued CoE RFP competitions and has awarded CoE designations for Commercial Space Transportation, General Aviation, and Airport Technology. The CoE designates have been tasked with providing research results to the FAA concerning topics that were of interest to flight considerations. A broad consortium of universities, with Mississippi State University as the lead institution, has formed a coalition in response to this expected RFP. Researchers in other countries have used UASs for remote sensing applications with greater frequency because many countries do not have the same airspace restrictions as those imposed by the FAA (Dalamagkidis et al. 2011, Everaerts 2008). As a result, US applications have lagged in comparison; however, researchers have used UASs for applications including forest firefighting and monitoring (US Department of Agriculture), agricultural management and inventory, and law enforcement. Potential UAS Applications The potential applications of firefighting and monitoring are perhaps an obvious application of UASs (Ambrosia et al. 2011), but further development in this area could be rotary-wing platforms (helicopters) that permit sustained remote sensing of burning landscapes (Wing et al. 2013). This sustained remote sensing capability could provide a timely response to fire severity and spread. With this information, fire responders could decide where best to allocate containment efforts and could also provide timely warnings to communities that are at fire risk. These timely warnings could help affected residents to leave their properties, should conditions warrant, and thereby alleviate risk to lives. In addition, fire response crews could be removed more efficiently from fire areas that posed an unnecessary risk to their presence. Previous UAS research has considered the application of high-resolution (1.5 in.) imagery for determining forest canopy structure (Getzin et al. 2012). Rango et al. (2009) considered the use of a mini-UAS for measuring and monitoring rangeland. Detecting and monitoring vegetation disease outbreak and spread is also an area in which UASs could provide valuable support. Vegetative disease often affects the moisture content of leaves and needles. Moisture content can be remotely indicated by sensors that operate with sensitivity to reflectance in the infrared spectrum. Some aerial surveying relies on convenience sampling, which implies that surveys are conducted when aircraft and pilots are available. In addition, some current aerial surveillance for disease presence and spread relies on visual observations of pilots and passengers, with follow-up surveys to be conducted by field crews (Johnson and Wittwer 2008). UAS applications in which aerial platforms that are capable of high-resolution imagery able to sense vegetative response have the potential for efficiencies in this monitoring activity. Management and Policy Implications Significant administrative and technological UAS developments that have the potential to transform how remote sensing can contribute to land management in the United States and abroad are underway. One potential transformation is reducing the cost of acquiring high-resolution imagery. Another potential transformation is an increase in the flexibility and repeatability in which remotely sensed imagery is collected. These ongoing developments will lead to land managers and decisionmakers having more timely access to information from remotely sensed imagery. It is also likely that a growing number of land managers and decisionmakers will consider the use of remotely sensed data for their information needs. There will be social and technical challenges in this maturation that are sure to involve navigation, safety, and privacy issues. However, if there is one lesson that can be gleaned from this nation's aeronautic history, it is that these difficult challenges can only be answered by facilitating increased research and innovation in the burgeoning UAS industry. Although uncertainty remains in the present development of UAS protocols and capabilities, there remains little doubt that the skies above us will have a different look to them in the future. Another potential use for UASs in natural resource management is with law enforcement activities. Previous research has considered very small UASs, which are capable of highly detailed ground observations (Helble and Cameron 2007). Previous studies have documented (Wing and Tynon 2006, 2008) the spread of what was once understood to be urban-centric crime to forest areas. Public recreation lands, such as the national forests, national parks, and other areas, are increasingly the locales of criminal activities. These activities include assaults, vandalism including setting fires, and drug production operations. On the national forestlands, law enforcement officers are reported to have patrol areas that exceed 500,000 acres (Wing and Tynon 2006). The extent of these area patrols makes it nearly impossible for law enforcement officers to be present in all areas where they are needed. In some cases, the presence of a law enforcement officer has resulted in lethal consequences. UAS law enforcement applications have the potential to enable responders to monitor a large area and to make more informed decisions as to their presence. Part of this decision support capability involves officers deciding whether a situation requires a more concentrated response, with a goal of minimizing risk. In addition, UASs and particularly those that are powered by electrical batteries can approach target areas in a manner that is more stealthy than terrestrial vehicles. This capability may provide significant advantage in detecting and responding to illegal activities. Some recent studies have applied UASs for wildlife research (Watts et al. 2010) and search and rescue (SAR) assistance (Goodrich et al. 2009). Habitat surveys are another discipline that could benefit from UAS involvement (Watts et al. 2012, Vermeulen et al. 2013). Many habitat surveys include installing transmitters on species such as bird or fish. The transmitters often have a limited range in which a receiver can be located to receive a transmission. When common VHF and UHF transmitters are used, a researcher must be located within this range to receive the transmission (Hulbert 2001). This condition often results in received transmissions being the consequence of convenience sampling, which means that received habitat preferences are influenced by when a researcher is present. One of the strengths of a UAS is the ability to fly a predetermined path, and the ability to fly this path with repeatability (Laliberte et al. 2010). This sampling capacity has the potential to increase the applicability of habitat sampling results. One of the most promising niches for UAS technology is in SAR. Researchers at Brigham Young University have been experimenting with UAS applications in SAR (Goodrich et al. 2008). The focus of their work is automatic search and detection. They decided on fixed wing UASs with a gimbaled electro-optical camera due to longer flight endurances than rotary wing UASs. The experimental platform was a flying wing design weighing 4 pounds. It was equipped with three-axis gyroscopes, three-axis accelerometers, global positioning systems (GPS), and barometric pressure sensors. The UAS was programmed to conduct a spiral search with the gimbaled camera based on generalized contour search algorithms that have been proven to provide the most efficient search methods with the highest probability of detection. Field demonstrations were successful in detecting missing persons who were both fixed and mobile in a mountainous forest. Their work identified detection limitations based on the resolution of the camera and the altitude of the aircraft. They found that an ideal search altitude was 200 to 350 ft, above which a human was not recognizable and below which cross-winds were too erratic for stable flight. The study highlighted several key issues that tend to be common to UAS integration in any operation—the need for advanced planning and a clear process for providing feedback to field-based personnel. The major advantage UASs offer SAR is that it removes the personnel safety risk of low-altitude manned flights, improves detection probability because it adds automatic detection capability, and increases search efficiency by providing the ability to search in multiple directions at a rate significantly faster than that of personnel on foot. Whereas collected imagery can be viewed immediately, image processing for distinct spectral signatures would require digital image processing capability and at least a slight delay. Kudo et al. (2012) used a remotely controlled (RC) helicopter to estimate chum salmon populations on the Moheji River in Japan. They flew a 1.6-cu in. gas-powered RC helicopter equipped with an antivibration device and a Canon SLR camera at an altitude approximately 100 ft above the water. Digital image analysis techniques were used to extract the salmon silhouettes from the images. The success of the study was that the results provided statistical evidence that the standardized aerial count was positively correlated to the standardized number of salmon seine-netted between the weir and estuary. The authors concluded that the visual requirements of an aerial study limit application of this technology to shallow rivers and streams with low turbidity that are not overgrown with vegetation. Two additional technology limitations were also identified but could be overcome using UASs. First was the requirement to have personnel on the river bank to the control the aircraft and a separate photographer to take pictures. This limited the stretch of river that could be surveyed to one that could support the ground station. A UAS could fly a predetermined route using GPS sensors and take pictures at specified time intervals, increasing the flexibility of ground station placement. Second, because the aircraft could not fly at a steady altitude, rulers had to be placed on the river banks to scale images. A UAS with a digital elevation model and altitude sensor would be able to fly a more steady altitude; furthermore, the addition of an inertial navigation unit and GPS allows for image georectification. UAS technology also has application in situations in which high spatial resolution is necessary, but the flight profile is too dangerous to fly. In 2011, the US Fish and Wildlife Service with the cooperation of the US Geological Survey conducted a sandhill crane population count at the Monte Vista National Wildlife Refuge in Colorado (Hutt 2011, Owen 2011). To conduct the count, the team needed equipment that could see well in low light, had the resolution to distinguish individual birds, and was quiet enough to reduce flush risk. For this study, the team used a hand-launched 4-lb RQ-11A Raven UAS equipped with a thermal camera positioned at nadir. It flew a grid pattern over a roosting sandhill crane flock at an elevation of 200 ft AGL during morning twilight at 6:30 am. The imagery was mosaicked and imagery analysis, using manual extraction, automated segmentation, and feature extraction, was used to count the number of individuals. The results were very promising, showing only a 4.6% difference from trained ground crew observations (2,567 compared with 2,692). Successes included a safe low-elevation flight, the ability to observe without flushing, and the use of thermal imagery for counting live subjects. The only significant issue encountered during the study was the 5-month COA approval process and the fact that the FAA limited operations to civil daylight hours, disapproving the initially requested nighttime operation. One distinct advantage that UAS platforms may have over manned flights is the ability to fly lower and slower. This increases the opportunity for high-resolution imagery (Table 1). The definition of high resolution is subjective, but we use the minimum amount of resolution that we believe is necessary to delineate tree species (<3.28 ft) as our basis for defining high resolution. Although this minimal required resolution for tree species delineation has been established by previous research (Hill and Leckie 1999), it is clear that increased resolution leads to improvement in other tree structural measurements that go well beyond species determination (Pouliot et al. 2002). A number of cameras are available for UAS applications at a range of prices. Ground sample distance is the measure of the length that each side of an image's pixel would cover on the ground (Comer et al. 1998) and is a proxy for image resolution. Small format digital cameras are capable of very high-resolution imagery (Table 1). Table 1. Potential UAS camera characteristics. Ground sample distance is based on a 400-ft height AGL. MSRP, manufacturer's suggested retail price. View Large Table 1. Potential UAS camera characteristics. Ground sample distance is based on a 400-ft height AGL. MSRP, manufacturer's suggested retail price. View Large One potential disadvantage that UAS platforms may have with respect to manned aircraft, which tend to be larger, is increased impacts from turbulence and other platform disturbances. Koh and Wich (2012) describe some approaches to minimizing disturbance impacts. One strategy is to set cameras to automatic metering, which allows the camera to measure and react to available light in the selection of a shutter speed. Another strategy involves a vibration dampening system constructed from low-cost packing foam or, in the absence of this material, a sponge. Case Study Oregon State University (OSU) recently applied for and received acceptance by the FAA as a public institution with the recognition as a COA applicant. OSU submitted an initial COA application to fly a Maveric UAS (Prioria, Gainesville, FL) over a portion of the OSU McDonald-Dunn Research Forest. The COA is good for a period of 1 year, with the possibility of a second year. The Maveric is a compact UAS with bendable wings that fits within what appears to most natural resource professionals to be a map tube (Figure 1). The Maveric has a cruising speed of 30 mph and is capable of traveling as fast as 63 mph. The Maveric has been flown to altitudes of 1,500 ft AGL and has a typical flight time between 45 and 90 minutes, depending on flight conditions. It is launched by being either thrown into the air or projected from a launch tube. Figure 1. View largeDownload slide Maveric UAS. Figure 1. View largeDownload slide Maveric UAS. OSU conducted its first flight in October 2012. The Maveric has an onboard GPS that it uses to track its position and a data transmitter so that it can relay information to a ground control station. A second transmitter enables a video link with a ground station (Figure 2). A fixed camera is mounted in the nose of the aircraft and cannot be redirected. A retractable gimbaled camera is mounted in the payload bay, which permits the direction of the camera to be controlled from the ground. The ground control station was a ruggedized notebook computer that was able to digitally communicate with the Maveric, including being able to receive video imagery in real time. Camera options include both thermal and shortwave infrared capabilities. Figure 2. View largeDownload slide Ground control station. Figure 2. View largeDownload slide Ground control station. A preconfigured flight pattern can also be uploaded to the aircraft, or an operator can guide the flight pattern through manual means. Should contact between the aircraft and ground station be lost, a built-in routine will direct the Maveric back to a predetermined location. A certified pilot launched the Maveric by starting its electrically powered engine and hurling the platform into the air. Once airborne, the Maveric instantaneously transmitted to a ground control station the color video imagery that was being collected by its side-looking video sensor. The imagery was instantly displayed on a laptop monitor, and the shape of individual trees and branches could be easily discerned. Georeferencing information was also transferred to the ground control station, which could enable georeferencing of the video, if desired. The pilot directed the Maveric to fly over a field containing research plots and adjacent to a forest road. The road location was also clearly observable in the imagery. Given our interest in other forest structures, the pilot also flew over a portion of the recreational trail network in McDonald Forest, and foot paths were clearly visible (Figure 3). The camera used for this flight captured imagery at a resolution of approximately 3 ft. A higher-resolution camera that is capable of capturing imagery at much greater resolution (< 3 in) is available (Figure 4). Figure 3. View largeDownload slide Recreational trail imagery from the Maveric UAS. Figure 3. View largeDownload slide Recreational trail imagery from the Maveric UAS. Figure 4. View largeDownload slide High-resolution (<3 in.) imagery of forest agricultural land interface taken from the Maveric UAS. Figure 4. View largeDownload slide High-resolution (<3 in.) imagery of forest agricultural land interface taken from the Maveric UAS. After approximately 30 minutes, the pilot redirected the Maveric back to the launch area and landed the platform on the surrounding grass field. Discussion The flexibility of UAS operations and subsequent flights allows land or resource managers to view areas of their choosing and to return to areas that they judge to be worthy of repeat imagery. In addition, repeat flights to monitor seasonal changes in land resources or reaction to disease and other threats would be advantageous. UASs offer the potential for reduced logistical planning compared with that for manned flights. After the current paradigm progresses beyond FAA COA limitations, it will be very convenient for a two-man crew to unload a 70-lb system (groundstation and unmanned air vehicle) to provide efficient and agile field monitoring capabilities. It can be difficult to find a pilot and aircraft to fly dull, dirty, and dangerous sorties at prices resource managers can afford. Such difficulties are further compounded by rapidly changing weather and inflexible flight plans. In contrast, small rapidly deployable systems like the Maveric can safely fly under cloud occlusions. These advantages translate into several capabilities that would be valuable for forest management: The ability to fly over large swaths of forest to identify and characterize the extent of disturbances such as blowdown and landslides. UASs can potentially do this safer, cheaper, and faster than a ground crew, especially when the area is small and isolated. The ability to assess relative productivity and density. In the case of more heterogeneous forest stands, UAS imagery allows accurate characterization of site conditions (e.g., species of commercial interest, 80% wetland, high slope grade, and opening encroachment). It can also assist with strategic road placement and location of failed fill slopes. The ability to provide targeted and repeated coverage. Rather than mapping the whole forest periodically, UASs offer the capability to take higher frequency photos of an area of interest. For example, a properly equipped UAS can spend a few hours capturing high-resolution imagery over a 6-square mile area during leaf-on and then repeat this flight during leaf-off. The ability to have recourse. For example, if a UAS survey reveals a stressed patch of trees, immediately conducting a second flight with a multiband or hyperspectral sensor may help characterize the nature of the stress (e.g., drought versus insect versus nutrient deficiency). The ability to detect and isolate drug-growing operations without risking the safety of ground personnel. The use of short-wave infrared sensors to assist in identifying illegal campsites and assist with after-dark SAR. The ability to have data ready for immediate analysis. Some UAS platforms by design facilitate near-real-time integration into GIS, which immensely increases the value of these platforms over traditional aerial systems. These systems are not without their downsides. 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TI - Eyes in the Sky: Remote Sensing Technology Development Using Small Unmanned Aircraft Systems
JF - Journal of Forestry
DO - 10.5849/jof.12-117
DA - 2013-09-01
UR - https://www.deepdyve.com/lp/springer-journals/eyes-in-the-sky-remote-sensing-technology-development-using-small-0yhGP0NFei
SP - 341
EP - 347
VL - 111
IS - 5
DP - DeepDyve
ER -