Guide
According to a test plan, a gimbal long-wave infrared (LWIR) imaging system prototype was installed on the U.S. Navy's MZ-3A airship, and high-definition thermal mapping was performed over Baltimore (a port city in the United States). The gimbal step-and-gaze imaging technology used has a small area array with a pixel width of 320×256 and a pitch of 30μm. The Quantum Well Infrared Detector (QWIP) camera uses a 50mm lens, and the effect is three times magnified. The QWIP data collected from the airship platform forms a high-quality LWIR thermal image with a ground sampling distance (GSD) of 60 cm. The geographic space of the aerial LWIR image collected by the gimbal system is very accurate and can be quickly handed over to the GIS mapping application.
1 Introduction
In the past ten years, many modern floating air balloon and airship remote sensing systems that are lighter than air (LTA) have been proposed, developed, tested and reported. The various LTA systems deployed, whether tethered or mobile, near the ground or high-altitude, flying at near static or dynamic low speed, have their own unique capabilities, which can provide cost-effective long-term imaging for high-resolution imaging. Sailing missions. These capabilities make the LTA platform suitable for a variety of remote sensing applications, including near-stationary and persistent area coverage, environmental monitoring, scientific expeditions, and ISR (intelligence surveillance and reconnaissance) support.
Recent LTA remote sensing work includes a new generation of "system of systems" technology. To integrate the system into the LTA platform, there are still some difficulties, such as 1) to optimize the well-designed system to suit the LTA flight envelope, the specified application requirements and the size, weight and power (SWaP) limitations of the special payload 2) To optimize the dedicated software to meet the requirements of LTA instrument system control, data collection, precise registration, pre-processing and distribution. These technical problems will affect the cost, plan, and thus the overall feasibility of the LTA main system.
In the second half of 2013, researchers from NGIT, GIA, AGC, and NASA jointly launched a work to provide a practical and efficient LTA imaging system solution for a research airship of the US Navy for city-level thermal remote sensing applications. The purpose of this work is to verify 1) the use of gimbal airship imaging solutions for city-level mapping while providing high resolution and large area coverage; 2) a prototype of a combat gimbal LWIR imaging instrument system. This article reports the results of this work.
2Airship platform and instrument
The LTA platform used in this work is the U.S. Navy’s MZ-3A airship. It is a manned airship with a length of 178 feet, two 180-horsepower engines, a maximum cruising speed of 50 nautical miles (93 km/h), and a payload of 2500. Lbs (1100 kg). The MZ-3A airship can stay in the air for more than 12 hours, and its cruising altitude is between close to the ground and 9,500 feet (2900 meters).
A prototype of the Gimbal Airship Imaging System (GAIS) has been designed and successfully integrated on the Navy’s MZ-3A airship. The prototype integrates a small area array, 320×256 pixels, 30μm pitch LWIR QWIP (Quantum Well Infrared Detector) camera for thermal imaging. The camera has a 50mm f/2.0 lens mounted on an eGimbalTM gimbal. It also integrates a 29MP electro-optical (EO) camera, while performing fixed lowest point indication color imaging. Use a portable small area array computer to run the imaging sensor, GPS/INS and universal motion control components. Customized software has been developed for instrument control, data collection and direct sensor geolocation. Figure 1 shows the gimbal and sensor payload on the airship platform.
It has also developed a gimbal step-and-gaze technology to obtain multiple ground-wide, geospatially accurate high-resolution LWIR aerial images. In particular, the 50mm lens QWIP camera on the MZ-3A airship is equipped with a three-step track crossing staring (ATS) function. The 3-step trajectory cross-gaze exposure can produce 10% overlap in the step-gaze image, achieving a total field of view of approximately 30°. When flying at an altitude of 3000 feet above the ground, the ground sampling distance (GSD) of the LWIR imaging device is approximately 60 cm.
The exposure of the photoelectric camera is synchronized with the QWIP sensor. The photoelectric camera uses a 300 mm lens to provide a 2 cm GSD grayscale image at a height of 3000 feet above the ground. However, the load capacity of the prototype gimbal is limited to the 3-step ATS LWIR or the 5-step ATS EO. When the LWIR system is installed in the gimbal, the photoelectric system can only be installed at the lowest point behind the GAIS system.
Figure 1 The gimbal LWIR camera and fixed photoelectric sensor installed in the camera pod on the U.S. Navy MZ-3A airship
Figure 2a is a mosaic of QWIP. The figure shows a set of LWIR/EO image frames taken along the ground track. The gimbal LWIR 3-step ATS imaging technology expands the thermal image width to 498 meters, making it much wider than the 114-meter width provided by the fixed installation of the photoelectric system. Figures 2b and 2c show examples of 2 cm GSD taken by a photoelectric system. Note that people can be distinguished by their thumbs.
Figure 2 a) A group of LWIR/photoelectric images taken along a ground track; b) enlarged image; c) further enlarged image
3 LWIR Baltimore city drawing
From September 24 to October 2, 2013, a large amount of data was collected with the GAIS system on the U.S. Navy airship MZ-3A over Baltimore. The thermal infrared raw image provided by GAIS is as good as the ground coverage map compatible with Google Maps. Using accurate GPS/INS measurement results to fuse these two sets of data, this set of aerial images can be quickly integrated into various geographic information systems (GIS).
The ground coverage image of GAIS is used for instant LWIR display to ensure the required data coverage. Figure 3a shows an area of ​​8.63 kilometers wide and 4.67 kilometers high. It consists of 15 airship routes and approximately 7,500 QWIP sensor image frames. The figure also shows the layered map stitched by the GAISLWIR quick view. The average altitude of the flight test was 3000 feet. The duration of each flight is approximately 15 minutes. The net drawing time of these 15 flight lines is about 4 hours, which shows that the GAIS system based on the MZ-3A airship can perform efficient city-level drawing at a speed of at least 50 square kilometers/day, and the image quality is very high.
The original GAISLWIR image is stored in a 16-bit digital format, which includes the onboard thermometer data embedded with the temperature measurement data of the sensor. The original LWIR image can also be further processed for advanced applications. Figure 3b is an example of our preliminary post-processing on the original data. This figure is an enlarged view of the small area in the blue box in Figure 3a, showing the details of the inner harbor area of ​​Baltimore. By estimation, the surface temperature of 99% of the pixels is between 10 and 42°C, and the digital format of the original image has been re-adjusted to fit the estimated temperature range. After re-adjustment, the image of Fig. 3b is stitched, which includes the original exposure map of 147 frames of 3-step ATS flying from east to west. Figure 3c is a pseudo-color image with a special color index to make it easier to see the temperature difference on the sea surface. Figure 3d is the pseudo-color image of Figure 3b in order to better distinguish the temperature difference of the ground target. The processed images in Figures 3b to 3d illustrate the space and signal quality of the gimbal QWIP camera imaging system; especially Figure 3c shows that the QWIP camera has good radiation resolution and can distinguish the temperature difference between the inner harbor and the water surface.
Figure 3 Data sampling of LWIR thermal image mapping of gimbal taken over downtown Baltimore and the Inner Harbor
4 Discussion
For remote sensing imagers, a reasonably designed imaging gimbal with fast response and accurate geographic space is a performance multiplier. It can not only expand the field of view of the imaging system on the LTA platform (as shown by the Baltimore airship test), but also Can provide motion compensation and flexible viewing angles for more applications.
The flight mechanics of the airship is particularly suitable for gimbal applications. Its slower airspeed and stable platform enable the universal imaging system to achieve faster and wider coverage with fewer flight lines. In addition, a suitable gimbal can further enhance the stability of the platform and eliminate dynamic blur. This is very important for the high resolution of LWIR/EO. For example, for LWIR, GSD is below 30 cm; Photoelectric, GSD is below 1 cm.
When selecting an imager for gimbal aerial imaging/mapping applications, the maximum resolution of a large area focal plane array (FPA) based on the number of pixels may not be the most appropriate consideration. In simple terms, the factors to be considered are the wafer size limitation of the LWIR detector material, the availability of optical components and the SWaP limitation, as well as the diffraction limit of the pixel size related to the wavelength of interest (such as 10, 15μm or longer). . Step gaze can effectively compensate for the resolution limitations of some small FPA systems. This is especially true for the latest generation of low-SWaP LWIR imagers, which usually have faster frame rates/data throughput, high signal quality, and a variety of lenses and lens designs. This smaller sensor is more suitable and practical for the universal structure.
For the LWIR spectral region, the capability of the geospatial gimbal is particularly important. The author of this article compares the LWIR mapping of Lawrence City (the lowest point installed on the Cessna aircraft is: NASA’s QWIP imager: 1024×1024 pixels, f/2.5 100mm lens, pixel pitch 19.5μm) and Baltimore’s LWIR Drawing (installed on the airship is: NASA's gimbal imager: pixels 320×256, lens f/2.0 50mm, pixel pitch 30μm) 60 cm GSD data results found that the latter's spatial imaging is clearer.
5 Conclusion
The recent city mapping of Baltimore and Maryland with the gimbal-type LWIR imaging system on the MZ-3A airship proved the following results: 1) Using a customized imaging system (such as a GAIS prototype), low-speed aircraft can be reliably used Make a drawing platform; 2) The gimbal-step gaze work (GSSO) method developed for this test is a practical and efficient method that can achieve high-resolution imaging and wide field of view coverage at the same time; 3) Use GSSO, a small The area array, fast frame rate LWIR QWIP imager is optimized for use in city-level high-resolution thermal imaging.
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