Communications: directive radio wave systems and devices (e.g. – Directive – Including a radiometer
Reexamination Certificate
2000-09-27
2002-04-23
Gregory, Bernarr E. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a radiometer
C342S02600R, C342S192000, C342S195000, C324S640000, C073S170160, C073S170260
Reexamination Certificate
active
06377207
ABSTRACT:
FIELD OF INVENTION
This invention relates to systems that can passively and remotely sense meteorological conditions from an airborne platform or from a ground station and, in particular, to a passive polarimetric microwave radiometer for detecting aircraft icing conditions.
PROBLEM
It is a problem in the field of aircraft operations that the occurrence of supercooled liquid droplets in the atmosphere presents a significant aviation hazard in that these particles are prone to instantaneous nucleation into solid water (ice) when subjected to minor mechanical perturbations. The size of the droplets and the temperature range of concern are approximately 5 to 200 microns in diameter and from approximately −30° C. to 0° C., respectively. If the supercooled liquid droplets undergo a phase transition to ice due to contact with a control, thrusting, lifting, or other external surface of either a fixed-wing or rotary-wing aircraft, the resulting surface becomes coated with ice, thus degrading the aerodynamic qualities of the surface and ultimately leading to reduced lift and possible stall of the aircraft. Accrued ice on external surfaces of the aircraft reduces aircraft performance, including: limits climb and ceiling capabilities, reduces airspeed, increases fuel consumption, reduces control, and reduces range. Icing also chokes engine inlets, fuel and other vents; coats radio antennas, which reduces transmission range; and obscures vision by coating windscreens and sensor optics and/or radomes. The aircraft icing hazard is especially serious for rotary wing aircraft because of the large volume of air swept by the rotor blades, the varying angle of attack of the advancing and retreating blades (as the blades rotate they tend to collect ice around the entire airfoil), and the criticality of the airfoil shape to maintaining laminar flow, and hence control and lift.
The size and temperature of water droplets determine the likelihood of ice formation. Large droplets are less common than medium-sized or small droplets because they tend to spontaneously nucleate at supercooled temperatures, and thus comprise a relatively small fraction of total supercooled cloud liquid water occurrences. Small droplets are also of less significance since they are more likely to be carried around the aircraft surface by laminar boundary airflow. Small droplets are also less likely to nucleate upon impact with aircraft surfaces due to the effects of surface tension in maintaining their spherical shape. Temperatures colder than ~−30° C. typically cause spontaneous nucleation, wherein a rapid conversion of the water into crystalline ice takes place. Ice crystals are less of a danger since they do not adhere to aircraft surfaces as readily as nucleating supercooled liquid. Therefore, clouds comprised solely of ice crystals are not an icing hazard. Liquid water droplets can be present along with ice crystals (a so-called mixed phase condition), and are generally depleted over time by contact with ice crystals, self-glaciation, evaporation, or precipitation processes. In the mixed-phase condition, however, the liquid droplets are necessarily supercooled, and thus present an icing hazard. Such mixed phase conditions are common within convection, a condition that is often also a source of moderate to extreme turbulence. At temperatures above freezing, water droplets remain liquid upon impact with aircraft surfaces and are rapidly shed by the slipstream.
Aircraft icing has been determined to be the cause of many aviation accidents, and can be avoided if the presence of supercooled droplets in the path of an aircraft can be determined at least a few nautical miles ahead of the aircraft. Typical flight times required for evasive maneuvering range from ~15 seconds (for helicopters) to a minute (for large jet aircraft). Given the typical velocities of jet aircraft (~240-550 knots), such an icing determination would be valuable at distances out to ~10 nautical miles ahead of the aircraft. Moreover, any instrumentation installed for icing detection should be simple (to be reliable), low cost (to facilitate installation on a large fleet of regional carrier and general aviation aircraft), reliable (requiring little maintenance and calibration), unambiguous in warning, and require little or no interpretation by the air crew at a time when their cockpit work load is high.
One existing icing detection system is disclosed in U.S. Pat. No. 5,028,929, wherein a forward-looking airborne radar system is used for detection of supercooled liquid droplets using a dual-frequency radar scheme. The return signals of the two radar frequencies are processed by calculating a calculus derivative of the difference in attenuation between the two radar frequencies over various radar ranges to determine the liquid water density in the atmosphere at that radar range. Suitable radar frequency pairs used in this system are X-band and Ka-band. The dual-frequency radar system is active in that it requires a powerful pulsed transmitter with associated range gating electronics. Accordingly, the dual-frequency radar system is heavy, requires a significant amount of power, is costly, and is more prone to component failure than a passive system. Matched antenna gain patterns are also critical to the accuracy of the radar measurements. Moreover, the dual-frequency radar system emits signals that are potentially detectable and trackable, thus placing military aircraft operating in a hostile environment and using the dual-frequency radar system under increased threat of detection and enemy fire. However, the dual-frequency radar system has the advantage of providing more precise range information on the distance of a supercooled liquid cell from the aircraft than a passive system.
U.S. Pat. No. 5,526,676 discloses a passive microwave radiometer utilizing a tunable frequency synthesizer as a local oscillator. Included in this microwave radiometer is a method of utilizing the varying attenuation of radio signals across an atmospheric line feature to vary the range or distance to features of interest. This microwave radiometer further measures the ranging of liquid water, the intensity of emission of which varies approximately as frequency squared, by measuring the skew of a line shape profile due to the greater enhancement of emission of the high frequency side of the line relative to the low frequency side of the line. These methodologies are pertinent to measuring the range or distance to aircraft icing conditions.
The article written by I. A. Tarabukin and G. G. Shchukin, entitled “Detection of Possible Aircraft Icing in Clouds by Passive-Active Radars”, was published in the Proceedings of the Specialist Meeting on Microwave Radiometry and Remote Sensing Applications, a NOAA publication, pages 381-385, June 1992. This paper describes experimental results suggesting that icing conditions can be detected in many instances using a combination of passive and active systems. The active system (radar) determines the presence and location of clouds, and the passive system (radiometer) refines the measurement of the amount of liquid water in the cloud by measuring unpolarized emission (the first Stokes parameter) at a shallow angle above the horizon. This detection scheme requires the simultaneous use of both a passive and active sensor, and thus requires significantly more instrumentation than the system proposed herein.
A passive icing avoidance system was also described in the MIAS project to operate at 37 and 89 GHz. This system relies on extremely narrow antenna beams looking forward at two horizontal angles to detect the presence of clouds by their characteristic brightness (first Stokes parameter). It is similar to the method of Tarabukin and Shchukin above. The unpolarized emission signature of these methods can be ambiguous for some meteorological conditions, however. This is shown by icing conditions and non-icing conditions having similar emission signatures as a function of elevation angle. The system does not provide a means of distinguish
Gasiewski Albin J.
Solheim Fredrick S.
Burdick Harold A.
Gregory Bernarr E.
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