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Aerial imaging with manned aircraft for precision agriculture.

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Over the last two decades, numerous commercial and custom-built airborne imaging systems have been developed and deployed for diverse remote sensing applications, including precision agriculture. More recently, unmanned aircraft systems (UAS) have emerged as a versatile and cost-effective platform for airborne remote sensing. These systems fill a gap in spatial resolution in remote sensing between ground-based and manned aircraft-based platforms. However, the safety concerns of commercial pilots and, in particular, aerial applicators and other pilots operating in low-level airspace need to be addressed before the widespread use of UAS for commercial applications. Meanwhile, conventional manned aircraft, including thousands of agricultural aircraft in the U.S., provide a readily available and versatile platform for airborne remote sensing.

Aerial applicators are highly trained pilots who use aircraft to apply crop production and protection materials. If the aircraft are also equipped with imaging systems, they can be used to monitor crop growing conditions, detect crop pests (weeds, diseases, and insect damage), and assess the performance and efficacy of ground and aerial application treatments. This additional imaging capability will increase the usefulness of manned aircraft and help aerial applicators generate additional revenue with remote sensing services.

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The commercial availability of high-resolution satellite imaging systems (GeoEye-1, World View-2, and WorldView-3) in recent years has provided new opportunities for remote sensing applications in agriculture. Nevertheless, airborne imaging systems still offer advantages over satellite imagery due to their relatively low cost, high spatial resolution, easy deployment, and real-time or near-real-time availability of imagery for visual assessment and processing. More importantly, satellite imagery cannot always be acquired for a desired target area at a specified time due to satellite orbits, weather conditions, and competition with other customers for the same time slot.

Most of today's airborne imaging systems are designed for use on aircraft equipped with camera ports for research and commercial applications, such as the Cessna 206. These systems typically include multiple scientific-grade cameras equipped with different filters to obtain three or four spectral bands in the blue, green, red, and near-infrared (NIR) regions of the spectrum. True-color images are created with the red, green, and blue bands, while color-infrared (CIR) images are produced with the NIR, red, and green bands. Some imaging systems can capture mid-infrared and far-infrared (thermal) images, while others can capture hyperspectral images from dozens to hundreds of spectral bands in the visible to thermal regions of the spectrum.

Recent advances in imaging technology have made consumer-grade digital cameras an attractive option for remote sensing applications due to their low cost, small size, compact data storage, and ease of use. Consumer-grade cameras are fitted with either a charge-coupled device (CCD) sensor or a complementary metal-oxide-semiconductor (CMOS) sensor. These cameras typically use a Bayer color filter mosaic to obtain true-color RGB images with a single sensor. Consequently, consumer-grade digital cameras have been increasingly used for remote sensing applications.

However, consumer-grade cameras only provide the three broad visible bands. If NIR images are needed for image analysis or calculation of vegetation indices, such as the normalized difference vegetation index (NDVI), filtering techniques can be used to allow a second RGB camera to capture NIR images or to convert the RGB camera to capture CIR images. With an RGB camera and a converted NIR camera, both RGB and CIR images can be captured simultaneously, but alignment between the two images is necessary.

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The Aerial Application Technology Research Unit at the USDA Agricultural Research Service's Southern Plains Agricultural Research Center in College Station, Texas, has devoted considerable effort to the development and evaluation of airborne imaging systems as part of our research program. Currently, we have a suite of airborne multispectral and hyperspectral imaging systems for monitoring crop conditions, creating prescription maps, and assessing the performance of precision ground and aerial applications.

Multispectral imaging

Our multispectral imaging system consists of four high-resolution CCD cameras and a ruggedized PC equipped with a frame grabber and image acquisition software. The cameras are sensitive in the 400 to 1000 nm spectral range and provide 2048 x 2048 active pixels with 12-bit data depth. They are equipped with blue (430-470 nm), green (530-570 nm), red (630-670 nm), and NIR (810-850 nm) bandpass interference filters, respectively, but have the flexibility to change filters for desired wavelengths and bandwidths. The cameras are arranged in a quad configuration and attached to adjustable mounts that facilitate aligning the cameras horizontally, vertically, and rotationally.

The image acquisition software allows the synchronized black-and-white band images from the cameras to be viewed on the computer monitor in three modes: one band image at a time, a normal color composite, or a CIR composite. Images can be captured at altitudes of 305 to 3048 m (1000 to 10,000 ft) above ground level to achieve pixel sizes of 0.1 to 1.0 m. If the flight altitude doubles, the pixel size will double and the ground coverage will quadruple.

This multispectral system has been extensively used for monitoring and mapping of crop diseases and weeds and for assessing the performance and efficacy of site-specific chemical applications. In particular, the system has been used to monitor cotton root rot infection in south and central Texas since 2010, and the imagery has been used to create prescription maps for site-specific fungicide applications. As this disease tends to occur in the same general areas within fields in recurring years, site-specific application of Topguard Terra Fungicide only to the infected areas is more effective and economical than uniform application.

Hyperspectral imaging

Our hyperspectral imaging system consists of a Headwall HyperSpec VNIR E-Series imaging spectrometer, an integrated GPS/inertia navigation system, and a hyperspectral data processing unit. The spectrometer can capture 16-bit images with up to 923 spectral bands and a swath of 1600 pixels in the wavelength range of 380 to 1000 nm. At 305 m (1000 ft) above ground level, the hyperspectral camera covers a swath of 220 m (720 ft) with a pixel size of 12 cm. The hyperspectral imaging system has been used to distinguish different plant species with similar spectral signatures. The imagery from the system is compared with imagery from consumer-grade cameras for crop identification and pest detection.

Thermal camera

The thermal camera is a FLIR model SC640 thermal imaging camera that is sensitive in the 7.5 to 13 m spectral range. It captures 14-bit thermal images with a 640 x 480 pixel array. The camera also captures visible RGB images with a 2048 x 1536 pixel array. Temperatures can be measured from -40[degrees]C to 1500[degrees]C. At 305 m (1000 ft) above ground level, the thermal camera covers a ground area of 130 x 97 m (426 x 318 ft) with a pixel size of 20 cm, while the RGB images cover a ground area of 280 x 210 m (920 x 690 ft) with a pixel size of 14 cm. Because stressed plants tend to have higher canopy temperatures, the thermal camera has been used to map crop diseases and assess irrigation uniformity.

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Consumer-grade cameras

More recently, we have assembled two multispectral imaging systems using consumer-grade cameras. One system consists of two Canon EOS D5 Mark III cameras, and the other system includes two Nikon D90 cameras. The Canon cameras have a larger pixel array of 5784 x 3861, compared to the 4288 x 2848 pixel array of the Nikon cameras. In each system, one camera captures normal RGB color images, while the other camera has been modified to obtain NIR images. The RGB camera is also equipped with a GPS receiver to allow the images to be geotagged and a video monitor to view live images.

A remote control is used to trigger both cameras simultaneously. Images are stored in memory cards as 14-bit RAW files and 8-bit JPEG files. Each system can be attached to our Cessna 206 or Air Tractor 402 B for capturing images at altitudes of 152 to 3048 m (500 to 10,000 ft). At 305 m (1000 ft) above ground level, the Canon cameras cover a ground area of 550 x 366 m (1800 x 1200 ft) with a pixel size of 10 cm, while the Nikon cameras cover a ground area of approximately 300 x 200 m (1000 x 660 ft) with a pixel size of 7 cm. Flight altitude can be adjusted based on the desired pixel size and ground coverage.

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Image mosaicking

If a single image cannot cover the area of interest with the required pixel size, then multiple images can be taken along one or more flight lines. For example, to map a 6.4 x 12.8 km (4 x 8 mile) cropping area near College Station, Texas, two Nikon D90 cameras (one RGB and one NIR) were mounted on the Air Tractor 402B. The Nikon cameras were flown at 1524 m (5000 ft) above ground level along 11 flight lines spaced 610 m (2000 ft) apart. With a ground speed of 240 kph (150 mph) and an imaging interval of 6 s, a total of 418 pairs of geotagged RGB and NIR images were acquired with a side overlap of 60% and a forward overlap of 56%. The images were processed using Pix4D software to generate georeferenced RGB and NIR orthomosaics, a 3D surface model, and a NDVI image for the cropping area.

These imaging systems have been used for a variety of research and practical applications in agriculture. For general-purpose applications where broadband multispectral images are needed, two-camera systems can be used for both small fields and large areas. If narrow-band or high spectral resolution images are needed, the four-camera and hyperspectral systems can be selected. The hyperspectral system can identify optimal narrow bands or combinations of bands for a specific application, and the four cameras can then be filtered for the selected bands to optimize data acquisition. The thermal camera can be used in conjunction with any other system when thermal imagery is needed.

As consumer-grade cameras are being increasingly used for remote sensing, more research is needed to evaluate these types of cameras and compare them with more sophisticated multispectral and hyperspectral imaging systems for precision agriculture and other applications. Imagery acquired from different platforms (i.e., UAS, manned aircraft, and satellites) should also be evaluated for its suitability and effectiveness for practical applications.

ASABE member Chenghai Yang, Research Agricultural Engineer, USDA Agricultural Research Service, Aerial Application Technology Research Unit, College Station, Texas, USA, chenghai.yang@ars.usda.gov.

Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA.
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Author:Yang, Chenghai
Publication:Resource: Engineering & Technology for a Sustainable World
Date:Jul 1, 2016
Words:1775
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