Electronic Military & Defense magazine was developed for engineers, program managers, project managers, and those involved in the design and development of electronic and electro-optic systems for military, defense, and aerospace applications.
Issue link: https://electronicsmilitarydefense.epubxp.com/i/78772
Trends (AGL). So the payload designer is driven by the require- ment to deliver a certain instantaneous field of view (IFOV), or the angle subtended by a pixel, and thus delivers a desired ground sample distance (GSD) from the operating AGL. The AGL is determined by a number of factors. First is the optimum operating altitude for an aircraft. The smallest UAVs operate at hundreds of feet AGL. Some of these aircraft are stealthy, some are not. Other small UAVs operate at AGLs of a few thousand feet. In some cases, this is where operating doctrine allows unmanned aircraft to operate. In many cases, the altitude is determined by the point where the aircraft is visibly imperceptible from the ground, and there is low or no auditory signature. Ideally, all aircraft would be invisible, inaudible, have enough video zoom to count fingers, be perfectly stable, and operate day and night. Some would even argue for these attributes to be maintained while imaging through clouds. A fundamental fact of airborne imaging is that as fields of view get narrower and narrower, lenses get larger and heavier, and the requirement for precision stabilization and pointing increases. It is a simple calcula- tion to estimate the stability requirement, or disturbance rejection requirement, for a given GSD and exposure time. For small UAVs, payload engineers need to design around lightweight, long effective focal length (EFL) cam- eras, thereby creating lightweight, effective disturbance rejection, vibration isolation, and pointing systems around these advanced cameras. Next, there is the desired video quality, usually attributable to GSD. As of late, the scale defined by the National Image Interpretability Rating Scale, or NIIRS, has defined the ground dis- tance a pixel must cover. NIIRS scales are subjec- tive image inter- pretability and are independent of slant range. Simply put, the higher the Currently, payload engineers are struggling with the conflicting requirements of high NIIRS value and fixed, limited payload mass. One can image very small objects from very great distances given an unlimited mass bud- get. A home astronomy telescope provides very high- resolution imagery of objects at many kilometers, but it is just not possible to lift a large diameter, heavy telescope, and then fly it to a target and loiter for hours. The pay- load engineer needs to find the best possible combination of optic and sensor to deliver the desired video result. Imaging Payload Considerations Three payload turret designs are discussed here: 1) high- performance, cost-effective EO visible imaging payloads; 2) high-performance, thermal imaging payloads; and 3) multichannel imaging payloads. The drivers in payload performance can be reduced to simple camera perfor- mance criteria and achievable stability per unit mass, volume, and power (see Figure 3). EO EO, or visible imaging, payloads have been dominated by simple block camera configurations available from large commercial suppliers. These suppliers have developed imaging modules for consumer, handheld camcorders and widely deployed video security systems. The overwhelm- ing volume of units produced allows payload engineers to design around a volume-manufactured, tightly integrated, well-defined imaging unit. The cost to develop a custom camera system of equivalent performance is prohibitive for the marginal benefit in pixels, sensitivity, or EFL. Figure 3: Current capability of advanced stabilized EO UAV payloads. Narrow field of view image represents standard definition (SD) imagery with a 0.30 degree HFOV. All images captured at 3,000 meters using Hood Technology's Alticam 09 EO. NIIRS rating, from a given slant range, the greater the zoom, or the smaller the IFOV, or the smaller the GSD. From there, optical geometry will determine the sensor and effective focal length required to achieve a given GSD, and a pre- dicted NIIRS rating (see Figure 2 on prior page). The primary challenge of the payload engineer is deter- mining the best, most rational, available camera technol- ogy and designing it into the stabilized turret. They sum the mass of the stabilized imaging system, determine the power required, ensure it will fly without too much surface drag, and deliver the best video possible to the ground. Challenges arise when the delivered ISR video data needs to provide more zoom. 42 Electronic Military & Defense ■ www.vertmarkets.com/electronics These com- mercial-off-the- shelf (COTS) visible imagers typically deliver NTSC video with a well-defined zoom range. With a given zoom, or FOV, we can accurately calcu- late GSD with a given AGL and slant range. A survey of typical EO pay- loads or EO channels will find a remarkable similarity in optical performance — due primarily to the supply of common COTS imaging modules (see Figure 3). Newer EO imaging payloads combine COTS imaging modules for wide FOV imaging, high-definition cam- eras, and customized optical systems. The latest pay- loads provide GSD values of 1.0 cm from 4,200 ft. slant range. Combining the traditional block cameras side by side with customized imagers and optics, payload engineers have developed a dual sensor with HFOVs ranging from 54 degrees to 0.29 degrees, extending the NIIRS rating to 9+.