1. Field of Invention
This invention relates to methods and devices for providing a flight crew of an equipped aircraft with timely traffic advice regarding nearby aircraft equipped with operating strobe-type anticollision lights. More specifically, the present invention pertains to methods and devices for detecting anticollision, strobe-like emissions despite background noise and other light pulses which would tend to activate a traffic alert alarm despite the absence of nearby aircraft.
2. Prior Art
Numerous devices and methods have been employed over aviation history to assist in the visual detection of nearby aircraft. The use of anticollision strobe lights mounted on the exterior fuselage and wing tips has long been part of standard aircraft equipment for enhancing the ability of flight personnel to see approaching aircraft.
It is obviously desirable to facilitate early detection of such anticollision strobe lights so that a pilot may take evasive action well in advance of danger. In daylight hours, however, visual identification of a momentary pulse of light generated at substantial distance is very difficult. Accordingly, scanning devices have been developed to assist in identifying the occurrence of strobe flashes, as well as the approximate azimuth of the strobe light with respect to the aircraft. U. S. Pat. Nos. 3,551,676 and 4,527,158 by Runnels describe the general use of optics and detectors which endeavor not only to identify the strobe pulse of a nearby aircraft, but at the same time provide filtering or blocking of background noise and stray pulses which may generate an alarm signal when in fact aircraft are not in the vicinity. The latter Runnels patent suggests the use of multiple detectors of photosensitive composition as part of a scanning system. Strobe pulse signals are isolated from background noise by use of a bandpass filter which is tuned to the range of frequencies associated with strobe light devices. In addition, a threshold adjustment enables blocking of much of the background noise gathered by the respective detectors.
U. S. Pat. No. 4,724,312 by Snapper also discloses a detection system embodying forms of filters for eliminating spurious and background signals. In addition, Snapper and Runnels provide a blocking circuit for disabling the detector system during the emittance of pulse radiation from strobes mounted on the monitoring aircraft. These disclosed devices also provide some orientation indication as to the azimuth of the pulse detected. This directional orientation is basically achieved by determining which of a plurality of detectors received the strobe radiation. The azimuth orientation is based on the relative position of that detector on the exterior of the aircraft.
Although the use of passive detectors enhances the ability of a flight crew to detect approaching aircraft having anticollision strobe lights, lack of system efficiency and limited effective range have inhibited commercial acceptance in the small aircraft industry. Such systems may be contrasted with active systems wherein a radar signal or some other form of radiation is emitted by the monitoring aircraft, with subsequent detection of a reflected signal as the basis of detection. In such instances, the detectors are tuned specifically to the frequency of radiation which was emitted, making identification of the reflected signal much less complex.
In a passive system such as the present invention, detectors are not tuned to a specific frequency because strobe light frequencies vary, not only in wave length and amplitude, but in pulse rate. Therefore, the system must be prepared to detect a variety of signals which generally conform to a strobe-like flash falling within the frequency range of approximately 300 nm to 1,000 nm with pulse durations of 150 microseconds to 600 microseconds. Pulse repetition rates further vary between 40 and 120 pulses per minute. Accordingly, the challenge of developing an effective passive system for detecting strobe lights is developing hardware and electrical and logic circuitry which can differentiate among these numerous variables for incoming pulse signals.
A frequent problem with passive strobe detectors has been to develop techniques which isolate true strobe pulses from other signals which have the appearance of a pulse of light similar to that which might be generated by a strobe source. If one designs the range of detection too narrow, true strobe pulses which fall outside the narrow definition may not be identified. For example, hazy or cloudy conditions may affect the detected signal such that it falls outside the narrow range defined as a desired strobe signal. On the other hand, if one broadens the parameters of the system to cover the full range of potential strobe emissions under all weather conditions, the detector system will not only identify such actual strobe pulses, but will trigger false alarms based on reflections of light from ground vehicles and structures, aircraft reflections and changing background such as white clouds against blue sky.
Past efforts to resolve this dichotomy have generally focused on enhancing the accuracy of detection of a true strobe pulse, to thereby enable distinguishing this pulse from background noise or other similar forms of radiation. Similarly, prior art techniques for enhancing such passive detection systems have concentrated on more accurate definition of the spurious signals which could trigger an alarm condition. The system is than provided with blocking circuits or filters to delete or ignore those specific signals. For example, the Runnels and Snapper approach of disabling the detector during emission of the strobe of the monitoring aircraft is representative of the philosophy of more accurately defining the unwanted signal which may be detected, with subsequent screening of those signals to avoid an erroneous alarm.
It has now become apparent to the present inventors that the traditional design strategy of attempting to more accurately specify parameters of (i) the strobe pulse to be detected or (ii) the spurious signal to be screened, filtered or blocked is simply not compatible with the "real" world. Too many variables exist in pulse amplitude, width and pulse rate to enable careful refinement of a sufficiently narrow bandpass capable of retrieving only true pulse signals. With respect to spurious signal definition, there are simply too many predictable and unpredictable forms of light pulse to enable development of a catalog of spurious signal forms which the detection system must ignore. Furthermore, some spurious signals may actually have the form and appearance of a true strobe pulse. A system which deletes such a signal form from detection may in fact delete a true pulse in error.
What is needed, therefore, is a passive detection system which is capable of identifying substantially all pulse-type signals similar to an anticollision strobe signal and isolating only those signals which are true strobe pulse signals. Just as important, this system needs to be capable of operating with respect to signals detected at a range of as much as 3 nautical miles, as compared to the more limited ranges of approximately 1 mile for prior art systems.