1. Field of the Invention
This invention relates generally to a passive millimeter-wave aircraft imaging and landing system and, more particularly, to an aircraft landing system incorporating a passive millimeter-wave camera positioned on the ground relative to the aircraft landing strip to provide an image of the airplanes approaching the landing strip, and to transmit signals of the image to the aircraft and the control tower.
2. Discussion of the Related Art
Different guidance systems are known in the art for directing aircraft along a glidepath to a runway for landing. Certain weather conditions, such as clouds, rain, fog, smoke, etc., decrease or completely prevent the pilot of the aircraft from visualizing the runway and to monitor the aircraft's approach along the landing glidepath. The type of guidance system, and the number of backup guidance systems for redundancy purposes, determines the various weather standards the particular aircraft can land in, and the aircrafts lowest safe position relative to the ground. Certain known guidance systems allow the aircraft to land in varying degrees of degraded weather conditions, and levels of redundancy of back-up guidance systems even allow the aircraft to land in zero visibility conditions.
One type of aircraft landing system known in the art uses a ground control approach including a radar system having scanning antennas located near the runway that scan the aircraft approach path in both azimuth and elevation. Reflected radar waves from the aircraft are detected and displayed on a monitor in a control facility in both the elevation and azimuth relative to the desired landing path. From this display, a ground controller instructs the pilot of the aircraft by voice transmissions of the aircraft's position relative to the desired glide path to allow the pilots to adjust the aircraft position for a proper landing. Although generally successful, radar based systems are limited in extremely low visibility conditions, and require essentially continuous access to a voice communications channel for communication between a control facility and the pilot.
An instrument landing system (ILS) is another type of known controlled approach aircraft landing system that includes antennas positioned near the runway that radiate localizer and glidepath beams to provide left-right and up-down guidance direction. Receivers in the aircraft receive and convert the localizer and glidepath beams into signals that define vertical and horizontal pointers that provide an indication of the aircraft's position relative to the desired landing path. Pointer deflection from a center position indicates the direction in which the pilot must fly the aircraft to the desired landing path. Current ILS that include triple redundancy allow the aircraft to land in zero visibility conditions.
Another controlled landing technique known in the art utilizes a microwave landing system that provides a number of acceptable landing paths. The microwave landing system also uses antennas positioned near the runway that scan the aircraft approach region, and microwave receiving equipment in the aircraft for decoding the transmitted information and converting it into an instrument display. This displayed image is significantly different from what the pilot sees when making a visual approach.
Millimeter-wave imaging systems that generate images of a scene by detecting millimeter-wave radiation (30-300 GHz) offer significant advantages over other types of imaging systems that provide imaging by detecting visible light, infrared radiation, microwave radiation, etc. These advantages generally relate to the fact that millimeter-wave radiation can penetrate low visibility and obscured atmospheric conditions caused by many factors, such as clouds, fog, haze, rain, dust, smoke, sandstorms, etc., without significant attenuation, as would occur with other types of radiation mentioned above. More particularly, certain propagation frequency windows in the millimeter-wave length spectrum, such as W-band wavelengths at about 89-94 GHz, is not significantly absorbed by the atmosphere.
Millimeter-wave imaging systems that use a focal plane imaging array to detect the millimeter-wave radiation and image a scene are known in the art. In these types of systems, the individual receivers that make up the array each include its own millimeter-wave antenna and detector. An array interface multiplexer is provided that multiplexes the electrical signals from each of the receivers to a processing system. A millimeter-wave focal plane imaging array of this type is disclosed in U.S. Pat. No. 5,438,336 issued to Lee et al. titled "Focal plane Imaging Array With Internal Calibration Source." In this patent, an optical lens focuses millimeter-wave radiation collected from a scene onto an array of pixel element receivers positioned in the focal plane of the lens. Each pixel element receiver includes an antenna that receives the millimeter-wave radiation, a low noise amplifier that amplifies the received millimeter-wave signal, a bandpass filter that filters the received signal to only pass millimeter-wave radiation of a predetermined wavelength, and a diode integration detector that detects the millimeter-wave radiation and generates an electrical signal therefrom. The signal from each of the diode detectors is then sent to an array interface unit that multiplexes the electrical signals to a central processing unit to be displayed on a suitable display unit. Each pixel element receiver includes a calibration circuit to provide a background reference signal to the detector. Other types of focal plane imaging arrays include separate detecting pixel elements are also known in the art.
Active millimeter-wave aircraft landing systems are known in the art. U.S. Pat. No. 4,940,986 discloses one such landing system for low visibility conditions. The system includes providing a number of millimeter-wave sources at or near the runway that radiate millimeter-wave beams along the runway. A millimeter-wave camera positioned on the aircraft receives the millimeter-wave radiation to create an image of the millimeter-wave sources corresponding to the landing runway.
Passive millimeter-wave landing systems are also known in the art that are able to generate an image of the aircraft landing area in low visibility conditions without the need to provide millimeter-wave radiation sources. In the known passive millimeter-wave landing systems, the millimeter-wave camera is also positioned on the aircraft, and provides an image of the scene at the landing area. However, because of the particular wavelengths and resolution requirements of such a system, the antenna associated with the passive millimeter-wave landing systems are significantly large, and the entire system is space intensive. Therefore, the size of the aircraft limits its ability to carry the known passive millimeter-wave landing hardware. Smaller aircraft generally are not able to carry such equipment.
It is anticipated that future, adverse weather, precision landing systems will be based on the global positioning satellite (GPS) system. Currently, an experimental use of a GPS system is being used on commercial airlines, but this use is limited to fair weather conditions. GPS systems are well known positioning systems based on satellite detection of high frequency signals that give the precise location of the receiver. In this type of system, GPS receivers will give the exact location of an aircraft, relative to a landing runway and/or other aircraft in the area. Fundamentally, the GPS precision landing system will be used to provide glidepaths for pilots to follow for landings at airports throughout the world.
This anticipated GPS precision landing system is a "single thread" system in that it lacks a redundant backup. Also, current GPS systems are unreliable because random errors are experienced that could lead to landing accidents if the system were used alone for these types of landings. Thus, some form of redundancy is desired to augment the anticipated GPS landing systems.
It is an object of the present invention to provide a passive millimeter-wave imaging system as part of an aircraft landing system that has a particular application as a backup to a GPS landing system and does not suffer from some of the drawbacks mentioned above.