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Sandia National Laboratories’ (Albuquerque, NM) Radar Intelligence, Surveillance and Reconnaissance (ISR) systems enable a new product paradigm in radar capabilities and modalities. With the ability to shrink sensor size, increase resolution, raise image quality, and advance realtime onboard processing, Sandia has been producing next-generation systems for nearly three decades. Sandia specializes in the full system design of Synthetic Aperture Radar (SAR), Ground Moving Target Indicator (GMTI), target recognition, and other sensor systems for the Department of Defense, other government agencies, and industry partners.

Since 1997, Sandia radars have been used to address critical military problems in all geographic Unified Combatant Commands (COCOMs). Sandia delivers complete solutions including mission planning and Concept of Operations (CONOPS) development, hardware development, algorithm development, platform integration, and Processing, Exploitation and Dissemination (PED).

High-impact, multimode SAR and GMTI systems are reconfigurable for multiple missions and designed for newer plug-and-play systems. As a Federally Funded Research and Development Center (FFRDC), Sandia develops and deploys next-generation pathfinder solutions.

What is Synthetic Aperture Radar?

Environmental monitoring, earth-resource mapping, and military systems require broad-area imaging at high resolutions. Often, this imagery must be acquired at night or during inclement weather. Synthetic Aperture Radar provides such a capability. SAR systems take advantage of the long-range propagation characteristics of radar signals and the complex information processing capability of modern digital electronics to provide high-resolution imagery. SAR complements photographic and other optical imaging capabilities because it is not limited by the time of day or atmospheric conditions and because of the unique responses of terrain and cultural targets to radar frequencies.

SAR technology has provided terrain structural information to geologists for mineral exploration, oil spill boundaries on water to environmentalists, sea state and ice hazard maps to navigators, and reconnaissance and targeting information to military operations. There are many other applications for this technology. Some of these, particularly civilian, have not yet been adequately explored because lower-cost electronics are just beginning to make SAR technology economical for smaller scale uses.

How Does SAR Work?

Figure 1. An airborne SAR imaging perpendicular to the aircraft velocity.

Consider an airborne SAR imaging perpendicular to the aircraft velocity as shown in Figure 1. Typically, SAR produces a two-dimensional (2D) image. One dimension in the image is called range (or cross track) and is a measure of the line-of-sight distance from the radar to the target. Range measurement and resolution are achieved in SAR in the same manner as most other radars. Range is determined by measuring the time from transmission of a pulse to receiving the echo from a target and, in the simplest SAR, range resolution is determined by the transmitted pulse width, i.e. narrow pulses yield fine range resolution.

The other dimension is called azimuth (or along track) and is perpendicular to range. SAR’s ability to produce relatively fine azimuth resolution differentiates it from other radars. To obtain fine azimuth resolution, a physically large antenna is needed to focus the transmitted and received energy into a sharp beam. The sharpness of the beam defines the azimuth resolution. Similarly, optical systems, such as telescopes, require large apertures (mirrors or lenses that are analogous to the radar antenna) to obtain fine imaging resolution.

Figure 2. Interferometric Synthetic Aperture Radar (IFSAR) mosaic of the greater Washington, DC area from Sandia’s Rapid Terrain Visualization (RTV) program.

Since SAR is much lower in frequency than optical systems, even moderate SAR resolutions require an antenna too large to be practically carried by an airborne platform; antenna lengths several hundred meters long are often required. However, airborne radar can collect data while flying this distance, and then process the data as if it came from a physically long antenna. The distance the aircraft flies in synthesizing the antenna is known as the synthetic aperture. A narrow synthetic beam width results from the relatively long synthetic aperture, which yields finer resolution than is possible from a smaller physical antenna.

Achieving fine azimuth resolution may also be described from a Doppler processing viewpoint. A target's position along the flight path determines the Doppler frequency of its echoes – targets ahead of the aircraft produce a positive Doppler offset, while targets behind the aircraft produce a negative offset. As the aircraft flies a distance (the synthetic aperture), echoes are resolved into a number of Doppler frequencies. The target's Doppler frequency determines its azimuth position.

Transmitting short pulses to provide range resolution is generally not practical. Typically, longer pulses with wide-bandwidth modulation are transmitted, which complicate the range processing but decrease the peak power requirements on the transmitter. For even moderate azimuth resolutions, a target's range to each location on the synthetic aperture changes along the synthetic aperture. The energy reflected from the target must be “mathematically focused” to compensate for the range dependence across the aperture prior to image formation. Additionally, for fine-resolution systems, the range and azimuth processing are coupled (dependent on each other) which also greatly increases the computational processing.

Interferometric Synthetic Aperture Radar (IFSAR) utilizes two or more SAR images collected from slightly different grazing angles to yield a topographic map of the scene. The SAR images are coherently compared or “interfered” to ascertain the phase differences between image pixels, creating an interferogram. This allows target scene topographic height information to be calculated. Sandia has fielded both two-pass and single-pass, multi-phase center antenna, real-time IFSAR systems.

Summary

Sandia has a broad range of engineering, testing, and analysis capabilities for Radar Intelligence, Surveillance and Reconnaissance (ISR) systems. With the ability to shrink sensor size, increase resolution, raise image quality, and advance real-time onboard processing, Sandia capabilities span multimode, real-time, and high-resolution radars.

With the goal to increase decision superiority through enhanced understanding, Sandia's radar systems incorporate processing and analysis onboard the aircraft in order to quickly and efficiently provide the analyst with relevant data.

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