Devin Mangus 2017-02-22 23:42:38
Agricultural producers, researchers, and service providers have been bombarded with articles, journal postings, conferences, and even primetime television shows on the revolutionary changes provided by small unmanned aerial systems (sUAS), also known as unmanned aerial vehicles (UAVs) or drones, in the agricultural industry. Some say this technology is a fad that will pass, while others say that it provides—and will continue to provide— real benefits to its users. When it comes to solutions in agriculture, no single technology works the same for all users. All agricultural operations are as unique as the people who are committed to providing food, fuel, and fiber for the U.S. and the world. This article is arranged as answers to some basic questions to help the reader better understand sUAS-based thermal imaging in agriculture, including the capabilities of the technology and the opportunities for further development. A key point to remember is that producers will continue to adapt new technologies that fit their needs and discover situations where the current capabilities are limited, while researchers and product developers will continue to expand the existing technology’s capabilities and provide solutions that were not previously possible. Why is thermal imaging important for agriculture? In the midst of recent droughts, greater water demand, and conservative water allocations, the irrigated acreage in the U.S. has increased to the point where demand for irrigation water is quickly exceeding sustainable supplies. As a result, irrigation management increasingly depends on site-specific plans that increase water use efficiency. Of the many types of crop stress, water stress is the most common and has the greatest effect on crop yield. Crop yield is particularly affected by water stress during critical crop growth stages, and the severity of water stress depends on the timing and duration of irrigation. Therefore, the quantity and placement of irrigation water are critical. The traditional methods used to detect crop water stress, based on sampling plants in the field, are destructive, labor intensive, and subject to placement error. In addition, they can’t capture samples across a large area, and they can’t be easily automated, which limits their adaptability. In addition to traditional methods, thermal sensing can detect crop water stress because thermal infrared energy can be measured remotely, it requires less labor, and the method is non-destructive. Plants close their stomata—the openings in the leaf surface— during periods of water stress, which reduces transpiration and increases the leaf temperature. Crop water stress can be detected by measuring the canopy temperature with manual or mounted infrared thermometers (IRTs). However, while these measurements are quick and easy to perform, IRTs are single-point instruments, and the measurements are therefore local. As an alternative to IRTs, thermal cameras can monitor crop temperatures over a larger area using thermal imagery, or thermography. Thermal imaging was once limited by slow processing speeds, large memory requirements, and high hardware costs. Over the last decade, as the military declassified the technology for civilian use, the hardware costs have decreased with an increase in functionality and ease of use. As a result, thermography is being rediscovered for use in precision agriculture. The new systems have also automated the image capture and analysis process. These automated thermal imaging systems can capture crop temperature with high spatial resolution (e.g., 1 cm per pixel) and high temporal resolution, which are both essential for monitoring subtle changes in crops. What types of thermal sensing platforms are there? Current satellite and ground-based thermography platforms have limited use in agriculture because producers need to cover entire fields with frequent revisit times. To meet these requirements, thermal cameras are mounted on piloted aircraft or sUAS, which can provide high-resolution images on demand. Airborne imaging systems have great potential for thermography because they can cover larger areas than ground-based platforms at higher resolutions than satellite-based platforms. These spatial resolutions (up to 2 m) can be used to assess crop water stress, aid in breeding programs, and conduct irrigation maintenance. However, a trade-off exists between coverage area and the measurable crop characteristics. Even with piloted aircraft, the spatial resolution—in ground pixels—of thermal cameras is limited. Operating expenses, fuel limitations, pilot fatigue, infrequent revisit times, and the complexities involved in obtaining useful imagery also limit widespread use of piloted aircraft for thermal imaging. Meanwhile, the technical capabilities and regulatory progress of sUAS have increased interest in unmanned systems for thermal imaging. The sUAS industry is rapidly becoming a complement to satellites and piloted aircraft for farm management. What are the advantages of sUAS for thermal imaging? Producers are adopting sUAS because low-altitude flight provides high-definition images and on-demand response times with low investment costs. Thermal imaging with sUAS is a way to assess crop health over a larger area than is possible with manual or single-point systems, at a fraction of the cost of piloted aircraft. Thermal cameras on sUAS typically provide sub-meter (0.10 to 2 m) spatial resolution and have flexible revisit times for whole-field temperature mapping. Another advantage of sUAS is their ability to fly at low speeds (30 kph) under manual control or on autopilot control with predetermined flight routes. Autopilot allows easier operation and can cover areas that were not previously accessible because of distance, time, or terrain. Currently, many off-the-shelf sUAS include autopilot control, cost-effective telemetry, and automated image processing systems. A variety of sUAS configurations are available for agricultural use. Each configuration has different flight dynamics for specific applications. In addition, the potential to meet the needs of non-agricultural users has given thermal camera manufacturers an incentive to develop cameras for agricultural conditions. What are the limitations of sUAS for thermal imaging? Although low-altitude flight increases thermography’s ability to measure crop health, sUAS are subject to the same atmospheric effects as ground, satellite, and piloted aircraft imaging. In orchards, sparse canopies make thermal measurements difficult unless the spatial resolution is finer than 2 m. Evenly spaced canopies, which are typical of small crops, reduce this problem. Field scouting and manual processing are still needed with sUAS to interpret the measured crop response and implement management zones. These on-the-ground measurements provide a method of verifying the accuracy and precision of the thermography. In-field measurements are also needed to determine the environmental conditions—such as wind speed and direction, air temperature, humidity, and solar intensity— that can affect crop water stress measurements. Unfortunately, knowledge is still limited regarding the performance of sUAS-based thermal imaging in agricultural fields. This lack of knowledge is mostly due to the high cost of self-cooled thermal cameras, which made measurement of crop temperature uneconomical for commercial agriculture. However, the recent introduction of cameras that don’t require cooling has allowed new thermal imaging systems to be smaller and lighter and consume less power, which extends the operating time at a fraction of the cost of the previous self-cooled cameras. What developments are still needed? Thermography has been marketed to producers because of its ability to capture images across an entire field in a short time. However, several barriers still limit the use of thermal imaging with sUAS, including image resolution, image capture and transfer, practical agricultural experience, image processing software, and cost. In particular, crop monitoring requires images with higher spatial and temporal resolutions. Studies of thermography systems in agriculture have been limited because of the expense, unfamiliarity, operating restrictions, system complexity, and the lack of proven durability in agricultural conditions. Because of these factors, thermography has been restricted to laboratories, greenhouses, and only intermittent use in prolonged field studies. Reductions in the cost, size, and weight of sUAS thermography systems will offset these limitations. Until then, thermography can be a complementary technology to other sensing methods. What is the key take-away? Thermal imaging, especially using sUAS, can be used to monitor the canopy temperature—and thus the water stress—of crops that are exposed to drought. Thermal imaging also gives producers a viable technology for applying crop inputs more accurately based on incremental crop needs. However, thermography has been less studied than other types of imaging. Further developments are needed to meet the specific requirements of agricultural producers. In the meantime, this technology is already providing benefits. Given the urgent need to manage water resources for sustainability, technologies like sUAS-based thermal imaging will help us soar above our crops to feed, clothe, and fuel the world. ASABE member Devin Mangus, Research Assistant, Department of Biological and Agricultural Engineering, Kansas State University, Manhattan, USA, MangusDevinL@johndeere.com.
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