Detecting Humans with SAR remote sensing technologies

Low Altitude automatic target detection with Remote Sensing using SAR technology.

Automated Target Detection

Automatic target detection (ATD), such as Detecting Humans with SAR, is very important in military applications and there are challenging problems with ground surveillance. The inability to achieve high detection rates and low false alarm rates for well-known challenges, such as the target type, weather conditions, and background clutter have put many a long range surveillance mission at risk. In addition, the target detection problem is more difficult if the surveillance area cannot be accessed directly and/or is covered without opaque foliage conditions.

Synthetic Aperture Radar (SAR)

Using the latest advances in Spaceborne or High Altitude Airborne systems, Synthetic Aperture Radar Systems technologies (SAR), are now being used on small, low altitude  UAVs to bring the remote sensing power of SAR much closer to targets, utilizing UWB radar for detection and imaging of targets in concealed environments, such as foliage penetrating radar (FOPEN), more reliably, and with much higher accuracy and finer detail than other imaging techniques.

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 (SAR) 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.

Applications

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. Other applications for this technology, particularly civilian, have not yet been adequately explored because lower cost electronics are just beginning to make SAR technology economical for smaller scale uses at altitudes much closer to points of interest.

SAR is a mode run by a conventional radar system that is moving to artificially create a representation of an extremely large antenna with the help of clever signal processing.

SAR techniques are used to achieve much greater angular antenna resolution, which can be used to generate highly detailed maps in situations and conditions when optical and infrared cameras or sensors are not adequate.

Detecting humans

Creating 2-D and 3-D images from radar reflections off the ground makes SAR ideal for identifying targets the size of a human body.

SAR relies on radio or microwaves rather than visible light and can see through haze, clouds, and even thick forest canopies.

The ATD system uses the L/X-Bands ensuring that it can penetrate through forest canopies to ground level.

SAR has become the go-to technique for planetary sciences, remote sensing intelligence, earth observation, as well as surveillance airborne missions, for accurate and persistent data collection.

‘Noisy’ cluttered environments and weather conditions

Long-range ground targets are difficult to detect in a noisy cluttered environment for which synthetic aperture radar (SAR) images or infrared (IR) images are frequently used.

SAR can measure the electromagnetic scattering property of targets under any weather and light conditions. This method is used frequently to detect distant targets because it provides strong radar cross section (RCS) values and shape information of targets referred to as RCS signatures as opposed to the thermal signatures in the IR case. While both sensors have day and night capabilities the SAR sensor has weather-independency compared to the IR sensors that are strongly affected by weather conditions.

Foliage, Ground and Flame Penetration

Synthetic aperture radars further also offer the capability for penetrating materials, which are optically opaque and thus not visible by optical or IR techniques. Low-frequency SARs may be used under certain conditions to penetrate foliage and even soil. This provides the capability for imaging targets, normally hidden by trees, brush, and other ground cover.

Detecting the human body

Electromagnetic Spectrum

The human body emits a narrow range of Infra-Red radiation and to-date IR/thermal has been the go-to sensor for detecting humans but subject to a number of limiting conditions.

Microwaves

However the size of the human body is more in the range wave lengths of Radio or Microwaves making them ideal for detection by transmitted radar energy.

Influencing factors for Microwave radiation

With Radio or Microwave detection, an object reflects a limited amount of radar energy back to the source. Factors that influence this reflection of radar energy include:

• the material of which the target is made;
• the size of the target relative to the wavelength of the illuminating radar signal;
• the absolute size of the target;
• the incident angle (angle at which the radar beam hits a particular portion of the target, which depends upon the shape of the target and its orientation to the radar source);
• the reflected angle (angle at which the reflected beam leaves the part of the target hit; it depends upon incident angle);
• the polarization of the transmitted and the received radiation with respect to the orientation of the target.

High Powered, Low Flying, Target Acquisition

Using the strong radar cross section (RCS) values of the human body and with complex SAR signal processing, it is possible to identify targets (represented by selected white dots in Figure 6) with 100% accuracy. Detecting humans with SAR is enhanced by able to locate the coordinates of the (human) target because of the low altitudes that can only be achieved low flying drones in the small UAV category.

IR/Thermal and SAR Image signatures

IR/thermal

At remote sensing altitudes, the visual thermal signatures of people, animals, hot rocks or terrain would look exactly the same, with no way of distinguishing between them.

Limiting conditions for IR/Thermal include: at night, no sunlight, no solar gain on the fabric, dry outside, cold, low wind, and heating on in the building.

SAR

Whereas with SAR, from the same perspective, the preset RCS signatures along with the radio waves used, and with complex algorithms built into signal processing software, enabled by the right learning software (AI), you will not only be able to automatically recognize a human, but with incremental values of SAR, also differentiate amongst various individuals. Recent developments in SAR technology include the ability to differentiate between individuals based on their vital signs, like rates of breathing making detecting humans with SAR a superior option.

SAR APPLICATIONS

SAR systems can be used to perform multiple applications in a variety of industries.

DETECTING HUMANS WITH SAR ON A LOW ALTITUDE UAV

The key elements of a low flying, small UAV-based SAR system include:

• Long Range Aerial Platform.
• Radar Pod.
• Synthetic Aperture Radar (SAR) electronics unit