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 Thermal Thermal imaging presents a sig- nificant challenge in small UAV payloads. Anecdotally, more ISR missions occur at night than dur- ing the day. EO payloads have tre- mendous performance, but they are blind at night, thus the perva- siveness of thermal imaging in the UAV payload world. Infrared (IR) pixels are larger; the optics are also larger, less flex- ible, and heavier; and integration Figure 4: Sample test imagers representing the relative size and weight of MWIR (left) and LWIR (right) assemblies deliv- ering <100 µrad IFOV performance necessary for quality ISR from typical, small UAVs. times tend to be longer than the exposure times for a typical EO imager in daylight — the longer the integra- tion time, the greater the stability requirement. The natu- ral instinct for the payload designer is to select a small, uncooled camera. Uncooled cameras are plentiful, inex- pensive, lightweight, low power, and small — a payload designer's dream — until you consider the optics required for uncooled imagers. All uncooled imaging occurs at f/1.8 or lower; the lower the f/#, the larger and heavier the optic. The longest conceivable EFL for an f/1.5 optic in a small (20 kg) UAV would likely be 100 to 150 mm. For a slant range of 4,200 ft., this would translate into a FOV of greater than 4 deg. At this FOV, for standard definition (SD) imagers, one could suggest acceptable performance. But a number of other criteria make this a false suggestion. First, uncooled zoom optics are generally massive. Only recently have manufacturers introduced lightweight zoom optics for uncooled sensors. Uncooled sensors have long reset times (equivalent to exposure times), thus increasing the stability and disturbance rejection requirement. Given a camera that weighs less than 40 grams, an optic can still weigh greater than 1,000 grams. In Figure 4, a photograph of two different imaging test fixtures for MWIR (left) and LWIR (right) are displayed. Both are continuous zoom optics including the latest sensor tech- nology. The MWIR can deliver 55 µrad IFOV, while the much larger LWIR delivers 77 µrad IFOV. The entire MWIR assem- bly weighs nominally 1,100 grams, while the LWIR assembly weighs 4,350 grams. For a target IFOV of <100 µrad for rea- sonable ISR quality, the LWIR is nearly impossible to fly on a 20 kg UAV, simply because of size and weight, while the MWIR delivers better IFOV, shorter integration times, more sensitivity, and much lower mass and volume, despite the requirement of a complex Stirling-cycle cooler. At a point, there is a clear benefit to cooled midwave infra- red (MWIR) sensors. Intuitively, one would think a cooler, a cold shield, and all the associated electronics for a cooled integrated Dewar cooler assembly (IDCA) would be preclud- ed from a small UAV payload because of the sensor's starting mass. Just the sensor, the IDCA, and no electronics or optics, costs nominally 400 grams. This starting mass is decreasing as new sensor technologies are introduced. The real benefit 44 Electronic Military & Defense ■ www.vertmarkets.com/electronics is realized at the optic. With f/#s as high as f/5.5, suddenly the payload engineer has EFLs approaching 300 mm at a mass of nominally 450 grams. Add electronics at about 40 grams, and a cooled MWIR camera can be considered at around 1,200 grams delivering NIIRS 7 or bet- ter from 1,300 meter slant range. Intelligent optical design keeps phys- ical geometries within reason. Short integration times allowable with highly sensitive cooled sensors, and reasonable power requirements on the order of 6 to 8 W, add to an effective sensor with better performance and lower mass, even with the mechanical cooler, when compared with longwave infrared (LWIR) uncooled solutions. Multichannel Most airborne imaging payloads have more than one imaging channel. For example, designers often combine EO/IR for imaging, a laser pointer (LP), and perhaps a laser rangefinder (LRF). More channels can be considered: shortwave infrared (SWIR), low-light television (LLTV), laser markers, and laser spot trackers. Even more exotic sensors are around the corner — third-generation IR with multicolor pixels, small flash light detection and ranging (LIDAR), and hyperspectral imagers. The same design considerations apply. What is the slant range? What is the ground sample size? What is the mass budget, volume budget, and power budget (especially important when considering advanced laser applications)? Multichannel payloads for small UAVs will also need to address the issue of surface area available for windows. A reasonable multichannel payload will have limited channels on small UAVs. The most common configuration will include EO/IR/LP/LRF. The EO, LP, and LRF can share windows. The IR commonly has its own window, silicon (Si) in the case of MWIR. Multichannel payloads will be dominated by the weight of the IR system, which in turn will likely be dominated by the weight of its optic. On the power side, the inclusion of the newest laser marker tech- nology will impart a new power draw that requires trade- offs. Multichannel payloads for the STUAS-class airframes start at nominally 3.3 kg and increase from there. In conclusion, we've reviewed the challenges that are present when considering various payload options for small unmanned aircraft systems. The goal is to design and provide payloads that deliver the least mass, volume, and power draw per IFOV unit, which will result in the longest duration and the most cost-effective ISR missions possible. Chris Johnston has 25 years of experience in infrared imaging technology and is VP Infrared Projects at HoodTech Vision, Inc., and is president of Sierra-Olympic Technologies, a distributor of IR imaging technologies.