Recent military conflicts have demonstrated the need for close air support from precision attack platforms (e.g., the AC-130 gunship) to support ground forces. These aircraft typically use IR sensors to engage moving ground targets, such as vehicles and dismounts. Under clear conditions, targets can be easily identified and effectively engaged. In degraded environments, however, the atmosphere can inhibit traditional electro-optical sensors. Close-in air support therefore can currently only be conducted during clear weather conditions.
In many important parts of the world (such as the Korean Peninsula, Central America, Colombia, and the Balkans) clouds are present between 25 and 50% of the time. The amount of time in which US close-air-support aircraft can engage targets is therefore severely limited. In addition—even in clear weather—once targets are engaged from the AC-130, copious amounts of dust are raised. This dust—from explosions and incoming rounds—prevents targeting of adversaries from the aircraft and makes it difficult to track friendly ground forces in the area.
DARPA launched the ViSAR program, in 2013, to develop an Extremely High Frequency (EHF) targeting sensor to operate through clouds as effectively as current electro-optical and infrared (EO/IR) sensors operate in clear weather. The program’s goal is to develop a cloud-penetrating EHF sensor in a moveable gimbal that could be mounted on a variety of aerial platforms to provide high-resolution, full-motion video for engaging moving ground targets in all weather conditions—cloudy or clear.
The recent flight tests of the ViSAR sensor marked a major program milestone toward our goal, proving that we can take uninterrupted live video of targets on the ground even when flying through or above clouds,” said Bruce Wallace, program manager in DARPA’s Strategic Technology Office. “The EO/IR sensors on board the test aircraft went blank whenever clouds obscured the view, but the synthetic aperture radar tracked ground objects continuously throughout the flight.”
The recent ViSAR tests took place on a modified DC-3 aircraft that flew at low and medium altitudes, allowing researchers to collect and compare data from the ViSAR, EO, and IR sensors mounted on standard sensor gimbals. Wallace noted that cloud-penetrating radar—such as from space or other operational systems—has existed in other formats, but there has not been a synthetic aperture sensor, which can fit in a standard EO/IR sensor gimbal on aircraft and maintain frame rates fast enough to track maneuvering targets on the ground. This data will be used for targeting operations when atmospheric conditions inhibit the use of electro-optic sensors. Our radar system can be used to image the ground, even through clouds and dust, at a sufficiently high resolution and frame rate to support the engagement of maneuvering targets.
“Refining the ViSAR sensor’s visualization software to provide operators a representation they’re used to seeing is the next step in the program,” said Wallace. “We don’t want operators in the back of an aircraft to need special radar training to interpret the sensor’s data—we are working to make the visual interface as easy to interpret as existing EO/IR sensor displays.” The ViSAR program has demonstrated and continues to push technology innovations in four technical areas: Compact flyable EHF-band exciters and receivers; compact flyable EHF-band medium-power amplifier; EHF-band scene simulation; and advanced algorithms for EHF-band operation.
The next phase of the ViSAR program is to integrate the sensor into an aircraft that includes a complete battle management system, capable of real-time target engagement.
For our system, the radar must be able to create a high-resolution background, and it must permit detection and accurate location of moving targets. Radar frequency selection has therefore been a key tradeoff decision. For synthetic aperture radar (SAR) systems with a given resolution, the frame rate is proportional to the frequency. Moving to a higher frequency therefore provides a higher frame rate (and a lower latency). Moving to a higher frequency also requires a smaller antenna size. Moving to a higher frequency, however, incurs greater atmospheric losses, particularly in the presence of clouds. After assessing all these trades, we decided to operate our system in the 231.5–235GHz band. This is an approved radiolocation band (thus permission for radiation is obtainable) and it provides the desired frame rate at acceptable atmospheric losses.
The Video Synthetic Aperture Radar (ViSAR) system, consists of a 233GHZ front-end that up-converts from, and down-converts to, a back-end that is built from a conventional microwave radar instrument. The 233GHz portion is installed in a tactical gimbal, which is known as the Multi-Spectral Targeting System-B (MTS-B). The MTS-B is a 20-inch gimbal that is flown on a variety of platforms, e.g., the MQ-9 Reaper unmanned aircraft. By fitting with this gimbal, we plan to demonstrate our ViSAR system can easily be installed on tactical aircraft, such as the AC-130, , write Bruce Wallace.
Our ViSAR antenna design includes five small antenna horns, i.e., one horn for transmission and four receiving horns. There are four receivers (one for each receiving antenna horn), each with an analog-to-digital converter. We use signal-processing algorithms to process the data. We are therefore able to generate high-resolution ground-focused SAR images, which can be used to detect and relocate moving targets. The acquired images can then be mapped onto an angle/angle display, similar to the system used by AC-130 operators.
For the design and development of our prototype ViSAR system, there were no suitable existing electronics for use in the 233GHz band. We have therefore fabricated the necessary hardware ourselves, write Bruce Wallace.” We have developed 233GHz receivers, exciters, and a solid-state 1W amplifier. All these components meet the required specifications (noise figure, bandwidth, and power) over our 231.5–235GHz band. We have also started to develop a 233GHz vacuum tube amplifier, which will provide sufficient power to permit operation through the most dense of low-altitude clouds. In parallel, we have collected extremely high-frequency-band measurements on the radar cross section of terrain, dismounts, and rain. In addition, we are now designing the overall system and developing the approach for packing all of the 233GHz components into the MTS-B.”
We are currently developing a high-frequency radar—ViSAR—that can be used to image through cloud cover and provide sufficiently high resolution and frame rates to track moving targets on the ground. We plan to integrate our system in the laboratory during the fall of 2015, and then pack it into the MTS-B during the winter and spring of 2016. In the summer of 2016, we will fly the MTS-B on a DC-3 aircraft to demonstrate over-the-air real-time imaging of moving and stationary targets through clouds. We are also exploring the possibility of providing additional capabilities with our radar system. These could include the measurement of wind speeds and directions (to inform adjusting fire), performing battle damage assessments, and providing a secure short-range air-to-ground datalink.