New sensors enhance border security
Selecting the right electro-optical (EO) sensor technology for security systems can be challenging. Different characteristics and performance levels are required depending on whether the system must detect, track, and identify individuals and/or detect contraband and hidden objects. Visible, near-, mid- and far-infrared (IR) as well as ultraviolet (UV) sensors can be used individually or in combination to perform specific security applications (see Figures 1 and 2). When using EO/IR sensors, system designers need to consider the cost/benefit trade offs are for both point protection and area surveillance applications. After identifying the specific type of security need, designers can select from various EO/IR sensors and combine them with other types, such as radar, seismic, and acoustic sensors. Table 1 lists sensors that are suited for various security applications.
Visible/near-IR sensors, in high-resolution color formats, provide the most ‘life-like’ information and are typically used in combination with IR sensors for night-vision applications. Depending on the filters used, near-IR sensors can also enable the ability to see in twilight conditions.
Often the first choice for cost-driven applications, shortwave IR sensors are typically used in night-vision goggles that are based on image intensification. Their disadvantages include lack of situational awareness and depth-perception, and they require illumination (which can be achieved with IR illuminators).
Mid- and long-wave IR sensors are often used interchangeably. Generally, mid-wave sensors are suited for use in high humidity (maritime) applications, and cooled versions provide high performance. Uncooled PbSe sensors are currently under development and promise good performance at a reduced cost. Long-wave IR sensors are available in both cooled (high performance) and uncooled (low cost/moderate performance) formats.
UV sensors can cue other sensors and personnel to the presence of gunfire and explosions. Featuring a wide field of view, which is sometimes more than 90° for a single device, these sensors are widely available.
System specifications will vary in priority depending on the specific security needs. Key performance trade offs include area of coverage, coverage rate, target types, resolution capability, size, and cost.
Area coverage is also known as the field of regard (FOR) and refers to the the total area to be covered at any one time both in terms of azimuth and elevation.
Coverage rate describes how often a specific point within the FOR must be revisited in a given time. This aspect of system design has powerful consequences on sensor selection as well as overall system architecture and cost.
Examples of target types include individuals or vehicles. The target's characteristics impact what sensor to choose, especially in terms of resolution. For instance, if vehicles need to be detected rather than people, then resolution can be traded for larger FOR.
Resolution capability can refer to detection, recognition, or identification. It is typically defined in EO/IR sensors by Johnson's Criteria, which sets resolution requirements to detect, recognize, or identify (DRI) targets. As an example, detection is when a system determines that something significant is present, but does not specify if it is a stationary vehicle or small building. Recognition means that the system has determined the general type of object, such as a vehicle. Identification reveals what specific type of object it is, such as a pickup truck or a sedan. Standard DRI criteria exists for different classes of targets, such as people, with specific resolution requirements based on target size. For example, a security system's DRI specifications could be: detection of man-sized targets at 3500m, recognition of man-sized targets at 2200m, and detection of vehicle-sized targets at 7500m.
Size is a key factor, and is often considered in combination with weight and power (SWaP). Finally, any cost analysis must include the expenses for the entire system, including life cycle costs and system maintenance.
Before selecting the optimal sensors, it is necessary to determine the specific security tasking type (see Table 2). Typical implementations include: event detection, perimeter detection, intrusion detection, and threat identification.
Event detection applications, often referred to as ‘bell ringer’ or ‘trip-wire’ events, cue the system to a general area. (Examples of events include gunshots or explosions.) These applications require very sensitive sensors with short reaction times and limited identification capability: they sensors simply need to alert system operators to the fact that something significant has occurred, when and where it happened, and the general nature of the event.
Perimeter detection requires comprehensive coverage of a defined perimeter (see Figure 3). These applications can include border or shoreline areas as well as airspace. The wide area of coverage usually means that threats are detected, but not necessarily recognized or identified. These systems require follow-up by other sensors or security forces.
Intrusion detection targets a specific area of defense within a well-defined area. Sensors used in these applications are typically highly sensitive, trading off FOR for sensitivity (see Figure 4).
Threat identification sensors are often first cued by other sensors to a specific point. These devices must offer high resolution capability, which is often achieved at the expense of speed, FOR, and/or sensitivity.
In order to take full advantage of the qualities of specific EO/IR sensors, it is imperative to identify the type of security need, determine system requirements, and then review available sensors to assess design trade offs.
Robert McDaniel is the vice president of US Defense and Electro Optics at Kollsman Inc. He has more than 20 years of experience in the design and development of a variety of electro-optical systems for aircraft self protection, thermal imaging, and fire control applications.
