Apparatus and process for measuring the gradient of refractivity of a gas

Abstract

A differential refractometer measures the difference of refractive index of electromagnetic centimeter waves of samples of gas with high sensitivity, and can be used to measure the gradient of refractive index of electromagnetic waves in air, and more particularly, to measure radar ducting conditions low over bodies of water. The instrument uses two microwave oscillators, each with its own frequency controlling cavity, with the frequencies of the oscillators differing by a specific value. Air from each of two intakes, vertically separated a specific distance, is directed into each cavity, and the difference frequency measured for a short time, to provide a first measured difference frequency. Then, the air flow from the intakes are cross-fed to the cavities, and the difference frequency is measured for a short time to provide a second measured difference frequency. The difference of refractive index of the air flowing through the intakes is proportional to the difference between the measured difference frequencies. Subsequently, the flow of air through the cavities is alternated periodically, at a rate of about 10 Hz. The drift of the oscillators is mostly eliminated by this method since there is very little frequency drift within the short measurement time.

I claim: 
     
       1. An apparatus for measuring the vertical gradient of refractivity of a gas, comprising: a first source and a second source for providing samples of the gas, said sources being vertically separated a known distance;   two resonant cavities for receiving samples of said gas, said cavities being tuned to different frequencies;   means for directing samples of gas from each of said sources to each of said cavities;   means for determining the difference frequency for each of said cavities, with gas samples from each of said sources; and   processing means for determining the gradient of the refractive index for said gas for the separation distance of said sources.   
     
     
       2. An apparatus according to claim 1, wherein the means for directing samples of gas includes flow control means for alternately and intermittently directing gas samples from each of said sources to each of said cavities. 
     
     
       3. An apparatus according to claim 2, wherein the means for directing samples of gas includes flow passages interconnecting said sources and said cavities, and valve means in said flow passages which operate to alternately direct gas samples from each of said sources to each of said cavities. 
     
     
       4. An apparatus according to claim 2, wherein the means for directing samples of gas includes a rotatable member having flow passages which can, by rotation of said member, be placed in fluid communication between said sources and said cavities, to alternately and intermittently conduct gas samples from each of said sources to each of said cavities. 
     
     
       5. An apparatus according to claim 1, further including control means to regulate the overall operation of the apparatus. 
     
     
       6. A method for determining the vertical gradient of refractive index for a gas, comprising obtaining samples of the gas substantially simultaneously at two heights which differ by a small, known increment;   determining the refractive index of each of the two samples, including providing said gas samples to two, separate microwave oscillators, each of said oscillators having its own frequency controlling cavity, and determining the difference between the refractive indices; and   determining the gradient by dividing the difference in refractive indices by the height at which said gas samples were obtained.   
     
     
       7. A method according to claim 6, including repeating the steps of claim 9 and averaging the values of refractive index gradient to average out fluctuations. 
     
     
       8. A method according to claim 7, further including repeating the method steps of claim 6 at different heights to provide a profile of the gradient for specific heights. 
     
     
       9. A method according to claim 6, wherein the step of providing the gas samples includes alternately providing gas samples from each of said two heights, to each of said oscillator cavities. 
     
     
       10. A method according to claim 9, wherein the step of alternately providing the gas samples to the oscillator cavities includes directing the flow of said gas samples through flow passages and selectively operating control valves in said flow passages. 
     
     
       11. A method according to claim 9, wherein the step of alternately providing the gas samples includes rotating a rotary flow switching device having flow passages which can, by rotation of said device, be placed in fluid communication with gas samples from said two heights and said cavities, to alternately and intermittently switch gas samples from each of said heights to each of said cavities. 
     
     
       12. An apparatus for measuring the gradient of refractivity of air, comprising: two microwave oscillators, each having a resonant cavity as its frequency determining elements, said cavities being tuned to different frequencies;   two intakes spaced a predetermined distance apart, for providing sample air to each of said cavities;   flow control means connecting said intakes with said cavities to direct sample air from each of said intakes alternately to each of said cavities;   frequency mixing means coupled to said oscillators and providing as an output the difference between the frequencies provided by said oscillators;   frequency measuring device for receiving the output from said frequency mixing means; and   data processing means for receiving the output from said frequency measuring device, and providing as output the gradient of refractivity for the air from said intakes.   
     
     
       13. An apparatus according to claim 12, further comprising timing and control means to control operation of said flow control means, frequency measuring device and data processing means. 
     
     
       14. An apparatus according to claim 13, wherein said flow control means comprises a pump, said pump is adapted to direct said sample air from each of said intakes alternately to each of said cavities either by suction or by pressure. 
     
     
       15. An apparatus according to claim 12, further comprising means for displaying the output of said data processing means. 
     
     
       16. An apparatus according to claim 12, wherein said data processing means operates to determine the difference of the frequencies interchangeably measured in said cavities and provided by said frequency measuring device, average the results and dividing the average by the sum of the cavity frequencies and the spacing distance of said intakes, to provide as output the gradient of refractivity. 
     
     
       17. An apparatus according to claim 12, wherein said flow control means includes flow conduits for providing air flow from each of said intakes to both of said cavities; and valve means disposed in said flow conduits and operable to intermittently interchange connection of said intakes to said cavities, such that the air flow from each of said intakes is alternately directed to each of said cavities. 
     
     
       18. An apparatus according to claim 12, wherein said flow control means includes a rotatable member having flow passages which can, by rotation of said member, be placed in fluid communiction between said intakes and said cavities, to alternately conduct air samples from each of said intakes to each of said cavities. 
     
     
       19. An apparatus according to claim 18, further including motive means to rotate said rotatable member, and timing and control means to synchronize rotation of said rotatable member and to control operation of said frequency measuring device and data processing means. 
     
     
       20. An apparatus according to claim 18, further including: a plenum chamber for each of said intakes, to receive air from said intakes;   a plurality of orifices on one end of said rotatable member which can, by rotation of said member, be placed in fluid communication with said cavities; and   obturating means disposed between said rotatable means and said cavities, and having conduits for fluid communication with each of said cavities,   said flow passages are disposed within said rotatable member and interconnect said plenum chambers with said orifices, which can, by rotation of said member, be aligned with the conduits of said obturating means, to alternately and intermittently conduct air samples from each of said plenum chambers to each of said cavities.   
     
     
       21. An apparatus according to claim 12, wherein each of said cavities is made of material having a low thermal expansion coefficient and the interior of each of said cavities is coated with a material having high conductivity to provide said cavity with high Q-factors. 
     
     
       22. An apparatus for measuring the difference of the electromagnetic refractivity of two gases, comprising: a first source and a second source for providing samples of the gas, said sources being vertically separated a known distance;   two resonant cavities for receiving samples of said gas, said cavities being tuned to different frequencies;   means for directing samples of gas from each of said sources to each of said cavities;   means for determining the difference frequency for each of said cavities, with gas samples from each of said sources; and   processing means for determining the gradient of the refractive index for said gas for the separation distance of said sources.