Engineering Tactical, Real-Time, Multi-Mode Imaging Radar Systems
Sandia National Laboratories' Airborne Intelligence, Surveillance and Reconnaissance Radar Systems enable a new product paradigm in radar capabilities and modalities. With the ability to shrink sensor sizes, increase resolution, raise image quality, and advance near real-time on-board processing, Sandia has been dedicated to producing exquisite radars for over two decades. Now, in addition to the 13 current state-of-the-art Synthetic Aperture Radar (SAR) modes, VideoSAR has been developed to help detect, recognize, and identify action in low-to-no-visibility conditions.
Algorithms and advanced processing techniques allow Sandia to manifest data into various modes for surveillance and reconnaissance, ultimately presented in one package ready for flight on manned and unmanned aircraft vehicle (UAV). Sandia continues to advance the next generation of SAR with highly integrated, miniaturized, and fully mission-capable radar sensors to impact tactical surveillance and reconnaissance mission space.
Sandia's radar sensor systems leverage developments in small, low-power, embedded processors, along with custom electronics miniaturization, extreme system integration and intelligent application of COTS interfaces and form factors. The key to the next generation's small, compact design is the co-location and integration of radar functionality, allowing for multi-platform UAVs. Sandia capitalizes on multi-mode devices with on-board processing for different functions, utilizing various modes in different situations.
Sandia's Synthetic Aperture Radar (SAR) provides wide-area imaging at a fine resolution with minimum constraints on atmospheric conditions, processed on-board and in near real-time. These world-class SAR systems provide an expansive data view ready to be packaged and utilized on UAVs:
Spotlight SAR - This mode increases resolution significantly by nature of steering the radar beam towards the target for a longer time, providing a longer exposure through synthetic aperture. Spotlight SAR mode of operation is usually at the expense of spatial coverage, but in conjunction with other modes, Sandia is able to use spotlight SAR on missions to track movement and changes in environment. SpotDwell - SpotDwell is the point-and-shoot mode of the Sandia radars. The radar operator need only specify the coordinates of the scene reference point and the radar does the rest. The coordinates of the scene reference point (SRP) can even be specified by clicking on the map from within the operator interface. In this mode, the radar will form images of the SRP until imaging geometry limits are exceeded.
Circle - While it is called the "circle" mode, to the radar, the circle consists of a series of straight-line segments which form a polygon. Only one image or patch is formed for each side of the polygon and the aperture is always centered on the respective side.Stripmap SAR - The conventional SAR strip-mapping mode assumes a fixed pointing direction of the radar antenna broadside to the platform track. A stripmap is an image formed parallel to the aircraft ground track that can be as long as desired by the user. Resolution of the stripmap image is limited by the azimuth size of the SAR antenna.
Tracking Mode SAR - Similar to stripmap, this imagery is produced along a line that is oriented in an arbitrary direction relative to the aircraft flight path.
UHF - Sandia operated a VHF / UHF synthetic aperture radar capable of operating over the band of 125 MHz to 950 MHz. This SAR was carried by the Sandia Twin Otter aircraft. Data was collected at ranges of 2 to 10 km. It could process images in near real-time to resolutions as fine as 1.5 meters, but also collected data that could be processed post-flight to finer resolutions. This radar was used for a variety of foliage penetration and ground penetration experiments.
Interferometry (IFSAR) - Interferometric synthetic aperture radar (IFSAR) allows extraction of height information that can be used to render 3-D topographic views of a SAR scene.Rapid Terrain Visualization (RTV) - This radar provides the ability to generate highly accurate map products in near real-time, including digital elevation models (DEMs), orthorectified SAR images, as well as a measure of the data quality. The DEMs can be produced according to the draft standards for DTED Level III or DTED Level IV, though RTV's accuracies exceed those standards. The system is capable of an absolute height accuracy of 1-meter LE90 (90% of the data within the map is within 3 feet of its true position), and the absolute ground position accuracy is 2-meters CE90 (90% of the data within the map are within a 6-foot radius circle in north/east of their true position).X-Band - A range of frequencies often used for radars that spans from 8 GHz to 12 GHz.
Coherent Change Detection (CCD) - This is a sensitive technique that identifies subtle differences that occur in a ground scene between two SAR passes. To detect whether or not a change has occurred, two images are taken of the same scene, but at different times. These images are then geometrically registered so that the pixels align in each image. After the images are registered, they are cross-correlated. Where a change has not occurred between the imaging passes, correlation remains high. If a change has occurred correlation will be low. Of course, targets that are not fixed or rigid, such as trees blowing in the wind, will naturally decorrelate and show as having "changed." While this technique is useful for detecting change, it does not measure direction or the magnitude of change.Polarimetry - Polarimetry is a technique in which the radar is designed to illuminate a scene with a known polarized (often vertical and/or horizontal) electromagnetic field, and collects the backscattered responses in two orthogonal polarizations (often vertical (V) and horizontal (H)). For example: One pulse may be transmitted in vertical polarization and then responses in both vertical and horizontal polarization are recorded. Subsequently, an additional pulse may be transmitted in horizontal polarization and responses are collected in both vertical and horizontal polarization. Alternating transmit and receive polarizations such as this can be collected throughout a synthetic aperture producing a collection of 4 images - one for each of the possible transmit/receive combinations (VV, VH, HV, HH). Use of the polarization scattering information from such a collections leads to greater understanding of objects in the scene of interest.Near Real-Time Processing - Raw radar data is processed into images and/or exploitation products at the rate data comes into the radar. Sandia radar makes images available as fast as the airplane flies, but with relatively small latency.
Wide Area Surveillance (WAS) - Wide-area surveillance (WAS) mode is a sequence of two or more Stripmap mode SAR passes to produce an image of an area that is larger than possible in a single imaging pass. This mode is used to rapidly produce current imagery of a large area.
Sandia's Ground Moving Target Indication (GMTI) enable target tracking through all ranges of motion, reliably detecting, locating, and tracking vehicles in near real-time. Dismount Moving Target Indication (DMTI), an extension of GMTI, are capable of detecting individual foot traffic within a small area.
Endo-Clutter GMTI -
Endo-clutter Ground Moving Target Indication is the detection of slow moving targets that compete with stationary radar clutter. This capability generally requires a more sophisticated antenna.
Ground Moving Target Indication (GMTI) - Radar-based detection and tracking of moving surface objects in an area of interest that inform situational awareness.
On-Board Analysis: Processing, Exploitation and Dissemination (PED)
Sandia's cutting-edge radar sensors incorporate modalities highly advantageous for surveillance and reconnaissance in wide-area mapping.
VideoSAR - Offers continuous collection and processing of phase history data, enabling observation of slow-moving targets even during inclement weather conditions. Time between images is user-selectable and is independent of aperture length.Velocity Independent Continuous Tracking (VICTR) - VICTR is capable of detecting and tracking "movers" through all ranges of velocity, including when an object comes to a stop.Coherent Change Detection (CCD) - This is a sensitive technique that identifies subtle differences that occur in a ground scene between two SAR passes. To detect whether or not a change has occurred, two images are taken of the same scene, but at different times. These images are then geometrically registered so that the pixels align in each image. After the images are registered, they are cross-correlated. Where a change has not occurred between the imaging passes, correlation remains high. If a change has occurred correlation will be low. Of course, targets that are not fixed or rigid, such as trees blowing in the wind, will naturally decorrelate and show as having "changed." While this technique is useful for detecting change, it does not measure direction or the magnitude of change.
Operations Support and Maintenance
Airborne ISR at Sandia National Laboratories is dedicated to enabling the analyst. Processing, exploitation, and dissemination (PED) of high-quality, real-time imagery enables better informed decision-making. Sandia continues to pursue improvements and extensions to its radar sensor design architectures through advanced imaging, tracking, and exploitation algorithms.
Sandia airborne SAR systems have been deployed worldwide for near real-time surveillance by every military command. High-resolution terrain elevation data (3 m sampling) has been created on 3 continents, mapping some areas that have never been mapped before. Our work has had significant impact in the lives of people around the world.