Spherical Sensor Configurations (SSC) have been designed for detecting and tracking signals in three dimensions. The Spherical Sensor Configurations offer distinct advantages over contemporary imaging systems, significantly enhancing three-dimensional (3D) situational awareness. Sensor systems utilizing a spherical geometry as a foundation (Figure 1) can be designed for a variety of applications.
A singular ring of sensors provides a basic device that can monitor sources in a two-dimensional plane. These systems calculate extremely accurate angular directions to signal sources. More sophisticated systems with full spherical sensor placement are being designed for monitoring multiple targets in any spatial orientation. Information generated from these systems can be integrated with contemporary imaging systems for further target identification. Full spherical SSC systems offer a 4 Pi steradian Field of Regard (FOR).
The underlying concept of the innovation is based upon fundamental properties of signal transmission and its reception on a spherical surface. The technology exploits the principle that a source of light will illuminate one hemisphere of a spherical object (similar to the Earth/Sun system). The significance of this concept lies in the consistent and mathematically predictable position of the illuminated hemisphere relative to the source. Harnessing this concept, a spherical receiver can be constructed with strategically placed sensors to determine the position of the illuminated hemisphere and, subsequently, the 3D direction of the source. Spherical Sensor Configurations offer distinct advantages over contemporary imaging systems in monitoring a wide FOR. SSC systems overcome imaging Field of View (FOV) limitations, enabling a single sensor system to view targets in all directions, significantly enhancing 3D situational awareness.
Spherical Detection Systems (SDS) are being designed for detecting and tracking infrared (IR) heat signatures, primarily in the 3 - 5 and 8 - 12-um thermal imaging bands. Current prototype development is in the 8 - 12-um band for detecting and tracking human and vehicle infrared (IR) targets. Analysis is being performed in the 3 - 5-um band for the detection of hotter temperature targets including missiles, RPGs, highspeed vehicles, and arms fire. The SDS can simultaneously track multiple targets in any spatial orientation, making it the ideal sensor system for Infrared Search and Track (IRST). The SDS can be fitted with sensors that can sample into the megahertz and analyzed with an onboard DSP for ultra-high-speed threat detection and tracking. The system offers continuous, passive sensing in a mechanically passive package with low power consumption. The wide FOR and high-speed sensing is well suited for capturing the location of initial launches or other incoming threats.
A Two-Dimensional Prototype
The 2D prototype was designed with 30 IR sensors mounted on a 4"-diameter ring. The sensors have a 100° FOV and are each separated angularly by 12°. The sensors receive light in the 350 - 1150 nanometer spectral range. The sampling rate is variable up to 10 kHz. Sensors directly facing an IR source produce a maximum response from the source relative to the other sensors on the ring. As the angle of the sensor relative to the source increases, the response from the sensor decreases. The sensor results can be plotted in a histogram fashion with the x-axis representing the angle of the sensor on the ring and the y-axis representing sensor response in Volts. The histogram takes on the shape of a Gaussian function.
The center of the peak represents the strongest sensor response and therefore the direction of the source. The sensor data is then fitted with a peak as shown in the display. The maximum of the peak is calculated and the corresponding x-value is obtained to determine the direction of the source. Using this mathematical technique, incoming sensor data is analyzed to determine the direction of the source with better than 0.01° (.174 mrad) of accuracy. Current advancements in circuit design and peak-fitting algorithms should significantly increase the accuracy.
In addition to determining the maximum of the peak, the peak width can be measured to determine the width of a source. The peak width is directly proportional to the angle subtended by a source. The current prototype uses a sensor with a very wide FOV, which allows for about three independent sources to be tracked. By increasing the density of sensors and decreasing the field of view of the sensor, more sources can be tracked simultaneously.
In designing the prototype, a Fresnel lens was used with a 30° FOV allowing for the tracking of more independent sources. Currently, the system uses 2M thermopile sensors to detect human IR in the 8 - 12-um thermal imaging band. Using a combination of a thin Fresnel lens and the 2M detector, field tests have obtained ranges of 100+ feet under average conditions (23°C). The data acquisition and control system is based on a PC- 104 system running XPembedded with two 16-channel data acquisition cards for the analog input. The control program operates at 10 Hz with a similar display to the VIS-NIR prototype. The unit provides a robust method for locating thermal targets for ground- or marinebased applications.
The 2D Human IR System (Figure 2) can be utilized on a ground-based vehicle, tripod, or as a pole-mounted system for detecting and tracking human IR targets. Networked systems can be integrated for wide-area surveillance. Lower-power field-programmable gate array (FPGA) systems are being developed for remote deployment.