Method and apparatus for measuring atmospheric temperature

Land vehicles – Wheeled – Occupant propelled type

Reexamination Certificate

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C250S339010

Reexamination Certificate

active

06409198

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for the measurement of atmospheric air temperature; and, more specifically, to passive, remote atmospheric air temperature sensing.
2. Description of the Related Art
Atmospheric air temperature sensing and measurements are critical to the performance and safety of aircraft. As performance requirements of modern aircraft increase, the need for more accurate air temperature sensing, with more resolution and higher data gathering rates, becomes more critical. The temperature of the air through which the aircraft is moving impacts its aerodynamics, engine performance, and the environmental control.
Air temperature is also important to flight safety. There are numerous atmospheric disturbances and flight hazards associated with different types of temperature conditions. These include microbursts, clear air turbulence, and icing. Therefore, more accurate and/or faster air temperature measurements, especially when detecting the air temperature ahead of modern aircraft, is important. Warning of temperature variations and clear air turbulence for supersonic aircraft, such as the proposed High Speed Civil Transport, is particularly critical. Temperature variations and clear air turbulence can cause, among other problems, a phenomenon known as “engine unstart,” a momentarily loss of engine power. The air temperature must therefore be measured at some distance in front of the aircraft, thereby potentially providing a warning of clear air turbulence and temperature variations and allowing the engines to be adjusted to prevent the unstart occurrence. At supersonic speeds this is difficult.
Air temperature measurements must also be accurate enough to yield good thermal profiles at supersonic speeds which then can be used to timely detect rapid thermal anomalies indicative of clear air turbulence. However, this is difficult to do with traditional technology, such as a thermistor in the airstream, since the aereodynamic heating from the airflow heats the probe above the actual air temperature by a few degrees Celsius for low speed aircraft and up to hundreds of degrees for supersonic aircraft. Therefore, small variations in air craft velocity at supersonic speeds causes atmospheric temperatures measurements to vary greatly.
Beyond the data requirements of modern aircraft, there are also new configuration requirements which impact the fundamental measurement problem. An example is military aircraft having low radar profiles, known as stealth aircraft. This configuration requires that the temperature probe also have a low radar profile. The stealth requirement, thus, cannot be met by traditional physical probes which must extend out beyond the boundary layer, ahead of the aircraft, to sample air undisturbed by the aircraft.
In order to solve the problems associated with conventional immersion type thermometers which physically extend into the air, remote sensors have been developed. They are basically of two types: active and passive. Active systems send out, or emit, a signal that is then, for example, reflected to a detector. Optical systems, such as laser-based (lidar) systems are examples of active systems. These systems, however, suffer from many problems, two of which are particularly significant. First, because they are active, signal power requirements are substantial. Second, and most significantly, active systems are traceable by optical detection systems destroying the stealth aspect of their use.
Passive systems do not emit electromagnetic radiation signals, but register naturally occurring emissions from, for example, heated gas molecules. Since, the temperature of the air, as a gas, is related to the storage of energy by molecules in the gas, as the temperature increases, the radiation emitted by gas molecules also increases. Thus, for example, if one picks nitrogen, carbon dioxide or oxygen, which are three of the most prevalent gas molecules in air, and measures the increase and decrease of radiation by those molecules versus the directly measured temperature of a known reference body, a correlation can be established. By measuring the radiation with a passive detector proximate the aircraft's skin, one can then determine the temperature of the air through which the craft is passing.
“Radiometers” are passive instruments which measure the magnitude of radiation at various wavelengths by passive radiant gas thermometry. Broad spectrum radiometers are able to measure radiation over a number of wavelengths, while narrow spectrum radiometers focus on a very narrow range and maybe even a particular wavelength from a particular element or compound. Airborne, remote sensing radiometers, for the determination of atmospheric temperature from aircraft, hold great promise as the instrument of choice for highly accurate, high speed, stealth atmospheric temperature measurement.
Passive radiant gas thermometry holds several important advantages over conventional airborne thermometer techniques. Radiometry based air temperature measurement starts with the passive collection of radiated emissions from the atmosphere proximate the aircraft. The system can “see” beyond the thermal boundary layer and measure infrared radiation some distance in front of the aircraft. The intensity of this radiation is measured at selected wavelengths which are correlated with the absorption bands of particular gases in the atmosphere. For example, U.S. Pat. No. 4,394,575, issued to Nelson, describes a radiometer which measures infrared emissions from the atmosphere at 4.3 &mgr;m, which are centered on an absorption band of atmospheric carbon dioxide (C0
2
). The measured intensity of the selected wavelengths of infrared radiation from the atmosphere is then calibrated against radiation emitted from a “blackbody” source having a known temperature and emissivity, in order to calculate the static air temperature. Here the static air temperature is taken to be the true air temperature undisturbed by the presence of any aircraft.
Radiometry based air temperature measurements have several proven advantages over conventional airborne thermometer techniques. First, passive remote sensing radiometers are not adversely affected by aerodynamic heating. Thus, no corrections for aircraft velocity, aircraft attitude (sideslip or yaw) or atmospheric pressure are required. In this manner radiometers provide a “direct” measurement of the air temperature. Second, radiometers are not affected by convective cooling resulting from sensor wetting. In hydrometeor clouds, conventional thermometers can erroneously read several degrees Celsius below the actual atmospheric temperature, due to the sensor probe becoming wet. Third, radiometers can accurately measure the static air temperature beyond the aircraft thermal boundary layer. Such layers can be very thick (several meters) for high speed aircraft, precluding the use of a conventional thermistor entirely. Radiometers are also not adversely affected by shock fronts associated with supersonic flight. Fourth, radiometers typically receive atmospheric thermal radiation through a small optical window normally mounted along the aircraft outer mold-line. An atmosphere-to-sensor interface conformal to the aircraft skin is advantageous to high-speed, high performance and low radar cross-section aircraft. Since the window is optically very transparent (low absorption), window heating from the surrounding skin and atmosphere does not adversely affect the radiometrically measured air temperature.
Radiometry based temperature measurement is ideally suited for high-speed, high-altitude aircraft. Fast and accurate air temperature information are used by these advanced aircraft for optimizing engine efficiency and calculating flight parameters, as well as for warning of approaching atmospheric conditions. Traditional temperature measurement technology, such as a thermistor that extends from the aircraft into the airstream, are inaccurate when the aerodynamic heating from the very rapid ai

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