Laser beam strikes against commercial aircraft have become increasingly common in recent years, with nearly 6,000 reported to the US Federal Aviation Authority in 2018. A laser beam strike poses a serious threat to pilots and aircraft. To reduce the increasing frequency of attacks, the ground-based Laser Aircraft Strike Suppression Optical System (LASSOS) has been developed by researchers from the MIT Lincoln Laboratory.
During a laser strike on an aircraft, the extent of the impact on the pilots’ visibility is largely dependent on altitude, the laser wavelength, power and beam divergence. The human eye is very sensitive to green wavelengths of light, which means that laser strikes involving green light, particularly after dark, have a large detrimental impact on visibility.
A legal green laser pointer, with wavelength 532 nm and power up to 5 mW, is indistinguishable from background lights above 3500m. However, at lower altitudes it can cause a serious glare hazard (350m) and potential flash blindness (110m). Moreover, devices with higher powers drastically increase the distance at which these altitude thresholds are met. For example, for a 500 mW laser, flash blindness, glare and distraction hazards occur at altitudes of ~1100m, 3500m and 11000m respectively. Importantly, lasers with a power of 500 mW or greater can be a distraction hazard at or near aircraft cruising altitudes and can cause a glare hazard or flash blindness throughout the approach to landing. Therefore, an effective strategy must be in place to protect aircraft from potential laser attacks.
Preventative measures for laser strikes can include using interference filters on aircraft windows or pilot goggles. Typically, the filters only exclude one or a narrow range of wavelengths of light, but this can distort the way exterior colors are perceived within the cockpit. The costs of implementing this technology on aircraft can also be high. Moreover, filter efficacy is angle dependent, meaning the filter’s effectiveness will vary depending on the angle at which laser light strikes the window.
Due to the many challenges associated with implementing effective protections against laser strikes on aircraft themselves, a ground-based solution to locate the source of the laser light and inform the appropriate law enforcement authority has been developed by researchers from the MIT Lincoln Laboratory.
The ground-based LASSOS works on the fundamental premise that as a laser beam is shone upwards into the sky, the laser light is scattered by aerosols and particulate matter prevalent in the atmosphere. The scattered light is detected by highly sensitive Electron Multiplying Charge Coupled Devices (EMCCDs) which are used to image the scattered light from different viewpoints. A schematic of the detector configuration in the LASSOS is shown in Figure 1.
The LASSOS sensor includes an off-axis star tracker, which is used for accurate calibration of sensor attitude. The laser line filter is comprised of a broad bandpass filter (10 – 18 nm) and a narrow pass filter (0.5 – 4 nm), which excludes background light from the sky. The optical path also includes a lens which is coupled to the iXon EMCCD detector, capable of single photon sensitivity.
The use of multiple detectors, in the configuration shown in Figure 1, placed at multiple locations can be used to extract a streak geometry of the scattered laser light path, as shown schematically in Figure 2. Data from multiple sensors can be transferred to a central location and the intersection plane of the individual streaks can be used to determine the location of the laser strike.
Accurate sensor positioning in location and attitude is essential to optimize the effectiveness of LASSOS. The positioning of the sensors for optimal coverage relies on reaching a compromise on the field of view of the detector. For example, a narrow field of view improves geolocation accuracy, but a wide field of view allows for a large geographical area to be monitored. Therefore, the most effective approach involves targeted monitoring of likely sites where strikes may occur. For example, a moving mount could be used for automated tracking of incoming and/or outgoing aircraft.
The LASSOS data collection and analysis procedure is outlined in the schematic in Figure 3. Stage 1 comprises signal detection and characterization from the two EMCCDs. Stage 2 incorporates signal synthesis, geolocation and alerting law enforcement, in addition to generating appropriate information post-event to assist with prosecution.
Image processing is performed during signal detection and characterization in Stage 1. First, the signal-to-noise ratio is improved using a combination of image summing, background subtraction, pixel re-binning, spatial or temporal filtering, and thresholding. A Hough transformation is applied to processed images to identify potential laser strikes from line segments in the image. A series of simple checks are in place to identify whether the detected line streak is, in fact, a potential laser strike. For example, line segments shorter than a certain threshold length or horizontal line segments are immediately excluded.
Once relevant line segments have been identified, they are further processed using a weighted-least-squares linear regression algorithm. The center of each pixel has a pre-defined, pre-calibrated local azimuth and elevation, which is based on the calibration of the entire sensor array in its current field of view. The two outermost points of the line segment are recorded as spherical coordinates. These points, together with the sensor location, form a unique plane. In Stage 2, the intersection of the unique planes from each sensor can be determined using planar geometry. The intersection of the plane geometries from each LASSOS sensor detected in Stage 1 can be used to identify the location of the laser source.
Every image collected by the detector has an associated timestamp. Accurate timing information is integral to correlate the laser strike path with associated aircraft flight paths. This information assists with the prosecution of perpetrators by providing post-event reconstruction of events, acting as evidence that a suspect was attempting to target an aircraft. Without this information it is difficult to secure the successful prosecution of suspects.
“These sensors can provide persistent, automated protection for a high-risk volume of airspace, such as a final approach path, by quickly locating the origin of a laser strike and transmitting the coordinates to local law enforcement,” said Dr Tom Reynolds, Leader of the Air Traffic Control Systems Group. “This technology will enable law enforcement to launch a rapid and targeted response to a laser strike event, greatly increasing their chance of apprehending and prosecuting perpetrators.”
Overall, LASSOS can be used to identify the location of the perpetrator of the laser strike and assist with rapid arrest and subsequent prosecution. Thus, LASSOS can also act as a deterrent for potential offenders in the future. Moreover, the technology, software and associated procedures developed within the LASSOS sensor platform have numerous potential applications, including guarding against laser strikes for targets other than aircraft.
Reference E. Tomlinson, R. Westhoff, T. Reynolds and B. Saar, Aircraft Laser Strike Geolocation System , 2017, Paper for the 17th AIAA Aviation 2017 Conference.