1. Field of the invention
The present invention relates to processing apparatus for clear air turbulence detection and, more particularly, to apparatus for processing the temperature measurement signals from a dual beam radiometer so as to provide a turbulence warning with a high detection probability and a low false alarm rate.
2. Description of the Prior Art
Atmospheric turbulence is an environmental condition which effects both passenger safety and comfort and may be physically destructive to aircraft. Most atmospheric turbulence is located at the edge of jet streams and in mountain waves in the atmosphere, although turbulence is also associated with jet streams overrunning convection cold fronts and cold or warm vertical currents between thunderstorms. Since most of such turbulence is not in or adjacent to visible convection activity and is not visible on airplane weather radar, pilots usually have no indication prior to encountering such turbulence.
The National Transportation Safety Board conducted a special study on non-fatal passenger and flight attendant injuries associated with air carrier accidents from 1968 through 1971 caused by turbulence, evasive maneuvers, and self-initiated injuries (personal carelessness). Of the 79 accidents that were investigated in the study, 23 were attibuted to clear air turbulence. The study also revealed that a major operation factor that directly effected both the probability and the severity of injuries was the amount of warning time the pilot was able to provide to flight attendants and passengers. In other words, if the pilot has sufficient warning time to check passenger seat belts, prepare the cabin and galley for turbulence, secure himself in his seat, and, in some cases, take evasive action to avoid the most intensive patches of turbulence, both the probability and the severity of injuries will be substantially reduced.
With the above in mind, there have been several techniques investigated for detecting atmospheric turbulence. However, no system to date has successfully provided the long range capability of detecting atmospheric turbulence with a sufficiently high detection probability and a sufficiently low false alarm rate to justify an operation system.
A Survey of Clear Air Turbulence Detection Methods was presented by Paul W. Kadlec at the USSR/US Aeronautical Technology Symposium co-sponsored by the American Institute of Aeronautics and Astronautics and the USSR State Committee for Science and Technology in Moscow in July, 1973. As reported by Mr. Kadlec, ground based systems include multi-wavelength radars with steerable antennas utilized to observe fluctuations in water vapor and temperature that occur in stratified stable layers in the clear atmosphere and the use of optical or lasar radars to observe the structure of the atmosphere or to identify aerosol concentrations in conjunction with areas of turbulence. Programs to develop an airborne system have concentrated primarily on the remote measurement of atmospheric temperature gradients and aerosol motion as possible indicators of clear air turbulence.
The most promising detection technique for an airborne detector utilizes a passive infrared radiometer which senses atmospheric turbulence at distances averaging up to 32 miles ahead of an aircraft. Flight tests with infrared systems have been conducted for many years utilizing both government and commercial aircraft. Such systems measure the infrared energy emitted in certain spectral bands. Since carbon dioxide (CO.sub.2) is known to maintain very nearly constant proportions throughout the troposphere and the lower stratosphere, radiometers have been designed to measure its emission/absorption properties. The strong carbon dioxide absorption band centered at 15 microns in the near infrared spectrum has been selected for investigation in all airborne infrared sensors.
Atmospheric data recorded over hundreds of thousands of miles of airline transport routes and studies of dozens of cases of turbulence have indicated that there are infrared signals generated in the proximity of turbulence. These infrared signals are caused by horizontal temperature gradients ahead of the aircraft and vertical temperature lapse rates. Turbulence has been found to be associated with torn cirrostratus clouds when a jet stream overran a cold front at approximately 90.degree. in direction and south of a deep low pressure cell. In such a case, clear air openings, approximately 8 miles wide, in dense cirrostratus clouds appeared approximately 24 to 32 miles from moderate turbulence. These cloud openings are found on the west wide of the turbulence on an eastbound aircraft with a thermal wave on the east side. Where turbulence occurred, there was a radiation change associated with it. On the polar side of a jet stream core, the major turbulence is found above and below the core and both thermal horizontal waves and vertical lapse changes occur. On the other hand, flying through the polar side of a jet stream at the core level, the major thermal signal is a vertical lapse change on the inside of the jet stream and not a horizontal wave.
It therefore becomes obvious that a reliable infrared passive system to warn a pilot in advance of penetration of turbulence must be able to detect horizontal thermal waves, vertical thermal lapse changes, and openings in dense cirrostratus clouds. However, flight tests conducted into and out of clouds at constant temperature have also determined that changes in radiation occur when going into and out of clouds and that this signal must be reduced from the system output to be compatible with the thermal horizontal wave and to provide an anomaly threshold signal with a high rate of detection and low false alarm rate.
The results of flight tests have indicated that the cloud to clear air difference in voltage at 13.8 microns wavelength is twice that at 14.15 microns. Both of these beam outputs were tested at the same calibration against a black body reference. Therefore, two beams from 14.15 and 13.8 microns can be combined with their sensitivities adjusted for equal ice effect. Then, when the beam output voltages are subtracted, the result will be a thermal signal of the atmosphere with reduced ice effect from clouds.
My prior U.S. Pat. No. 3,735,136 issued May 22, 1973, for Dual Color Radiometer Method Selectively Discriminating Against a Radiant Energy Emissivity Characteristic of a Preselected Atmospheric State teaches that by compensatorily gain-changing one of the beam output voltages before combining the two beam output voltages there will be provided a resultant signal with reduced ice effect from clouds. On the other hand, my prior U.S. Pat. No. 3,780,293 issued Dec. 18, 1973, for Dual Beam Radiometer for Clear Air Turbulence Measurement teaches that by adjusting the spectral band pass of one beam and its reference relative to the other beam and its reference, the ice effect may be reduced to a level sufficient to prevent false alarms when flying from clear air to cloud, or vice versa. What remains to be accomplished and is accomplished hereby is the processing of the output of such a dual beam radiometer with reduced ice effect in such a manner as to detect horizontal thermal waves, vertical thermal lapse changes, and openings in dense cirrostratus clouds to provide a turbulence warning signal with a sufficiently high rate of detection and a sufficiently low false alarm rate as to be operational for use in both military and commercial aircraft.