Three-dimensional laser radar for perimeter security
The threat of terrorism at sensitive facilities makes maintaining awareness of unusual activity around their perimeters increasingly important. Terrorist attacks such as the bombing of the USS Cole and the sabotage of a French oil tanker in Yemen might have been avoided using a warning system capable of detecting and tracking the motion of intruders on the ground or in the water. Early identification and location of potential ground and marine threats is vital for taking appropriate countermeasures to stop such attacks. The Goodrich Laser Perimeter Awareness System (LPAS, US Patent 6,985,212)1 is a laser radar capable of providing advanced warning of intruders approaching the perimeter of a facility. Laser radar is similar to conventional radar, except that a scanning laser beam, rather than radio waves, illuminates the area of interest. The LPAS detects laser light reflected from an object and computes its range from the total amount of time required for the light to travel to the object and return to the sensor. The LPAS can detect multiple walkers at ranges from 20–250m and vehicles the size of cars or small boats up to approximately one kilometer away.
The LPAS laser beam is invisible to the unaided eye. The (1.55μm) laser wavelength lies in the near-infrared band, outside the detection band of CCD(charge-coupled device) cameras and standard night vision equipment, and does not interfere with video-surveillance systems. Light at this wavelength is unable to penetrate the eye and damage the retina. The rapid scanning speed of the laser beam and its low average intensity ensure that it cannot injure humans or animals, regardless of their distance from the sensor. Also, the LPAS is not a source of significant radio frequency emissions, an important consideration for deploying the system in urban or military environments.
The narrow width of the laser beam enables the LPAS to measure both the bearing and elevation angles of an object and tabulate its three-dimensional coordinates. Conventional radar systems with larger beam widths and spatial side lobes measure only the range and bearing of an object, not its elevation. In addition, conventional radar side lobes often create multiple reflections from objects at close range, especially when the radar scans over water, resulting in multi-path returns and high false alarm rates. The LPAS complements conventional radar because it works best at short ranges and in cluttered environments where conventional radar does not always provide reliable detection.
The LPAS is composed of three modules: a scan head, an electro-optical module, and a laser. The scan head mounts outdoors, along the perimeter of a facility. The electro-optical module and laser reside in a temperature and humidity-controlled environment. Fiber optic and electrical cables connect the three modules. The LPAS laser is an erbium-doped fiber laser, the same rugged type often used in long-haul fiber telecommunication systems. The laser emits very short pulses of light at pulse repetition rates between 40 and 100kHz. The light exits a fiber within the scan head and passes through a lens that collimates the beam to a full-angle divergence of 2 milliradians.
A second lens collects the reflected light pulse (a laser echo) and launches it into a separate fiber that carries the light to an Indium Gallium Arsenide avalanche photodiode within the electro-optical module. The propagating laser beam and the laser echo reflect from a common mirror that scans in elevation. The mirror oscillates at 85–90Hz, while the scan head itself sweeps back and forth slowly (0.3–2Hz) across the azimuth field of regard. Typically, the elevation field of regard is 10°, while the field of regard for the azimuth scan is 180°. The time for one full scan of the entire field of regard is 2.5 seconds, although all of these parameters are adjustable for the particular terrain in which the LPAS functions.
The system detects intruders after first generating a background clutter map of the terrain. The map is composed of laser echoes from objects within the LPAS field of regard. After storing the clutter map into memory, it continues to collect laser echoes and search for intrusions against this three-dimensional map in real time. Laser echoes that do not correspond to those in the clutter map within a specified tolerance trigger an intruder alert at an appropriate interface. This is a display screen with intruder icons overlaid on an aerial photo (as shown in Figure 1), an audible alarm, or a network linking multiple sensors together. The alert contains the range, bearing, and elevation coordinates of the intruder, as well as a time stamp.
The additional elevation data is a key feature of the LPAS. The user configures the system to ignore laser echoes from zones where traffic is expected, such as roads and walkways. Exclusion zones are common to other perimeter sensors: however, with the addition of elevation data, the zones are more local and do not create blind spots in areas where detection is still needed. For example, the LPAS can exclude echoes from vehicle traffic below an overpass while still detecting intruders on top of it.
When the LPAS locates an intruder, it can automatically cue another sensor, such as a CCD camera or thermal imager (shown in 2), to slew its field of view to the intruder and allow security personnel to assess the threat level. For this function, the elevation angle provided by LPAS is critical. A human intruder at 250m range subtends an angle less than 0.5°in elevation. If a standard video camera is to provide a detailed image of the intruder, its elevation field of view should be far less than the 10° field of regard of the LPAS. Range and bearing information alone would be insufficient to aim the camera directly at the intruder.
The Goodrich LPAS is capable of detecting and tracking intruders at ranges of 20m to greater than one kilometer, depending on the size and reflectance of the intruder. The high degree of spatial resolution and the data processing algorithms allow the LPAS to accurately locate intruders in range, bearing, and elevation. A simple visual display of the data on a map or aerial photo, combined with automated slewing of other sensors (such as visible and IR cameras), alerts security personnel of potential threats. The LPAS can function as a stand-alone system, or it can be integrated into a larger security network.2
Goodrich has deployed these systems at various high-value asset facilities to automate the detection and surveillance of critical infrastructure. This persistent24 × 7 surveillance maintains force readiness and alertness.
