Dual frequency radiometer

• US 3,911,435 A
• Filed: 06/01/1971
• Issued: 10/07/1975
• Est. Priority Date: 06/01/1970
• Status: Expired due to Term

• Current Assignee: Austin Mardon, Franklin H. Prestwood
• Original Assignee: Austin Mardon, Franklin H. Prestwood
• Inventors: Mardon, Austin, Prestwood, Franklin H.
• Primary Examiner(s): Wilbur, Maynard R.
• Assistant Examiner(s): Montone, G. E.
• Application Number:
  US04/150,864
• Time in Patent Office: 1,589 Days
• Field of Search: 343/1ME 324/4,7 73/355R,355EM
• US Class Current: 342/351
• CPC Class Codes: G01K 11/006 using measurement of the effect of a material on microwaves or longer electromagnetic waves, e.g. measuring temperature via microwaves emitted by the object G01K17/003, G01J5/00 take precedence; measuring the effect of a material on X-, gamma- or particle radiation G01K11/30
• CPC Class Codes: G01V 8/005 operating with millimetre waves, e.g. measuring the black losey radiation

Abstract

Disclosed is a dual frequency radiometer concept whereby selection of operating frequencies allows the detection and discrimination of metal, water, tarmac, concrete and wet soil from natural terrain backgrounds. Disclosed is one radiometry system designed to detect metal in the presence of background of normal terrain features, including vegetation, ground soil and water. Embodiments of the system are disclosed, employing two radiometer sensing heads operating at different preselected frequencies. The output of each sensing head is mixed with a respective locally generated signal f1 and f2 to produce a common IF frequency and alternately applied to an IF channel. The output of the IF channel is synchronously detected and the difference in level of radiometric energy at the two frequencies is synchronously detected and integrated to produce a difference signal indicative of the presence or absence of metal in the field of view. In one embodiment, the local frequency sources are klystron oscillators and in the second embodiment, solid state Gunn diodes are used. The physical embodiment of one preferred form of sensing head is disclosed. One third embodiment employs a single oscillator and multiplier to produce a second harmonic frequency. The fundamental and the second harmonic are used to serve as reference for the two superhet signal channels. Gain drift, frequency drift and oscillator noise cancellation are all achieved in this embodiment.

Claims

1. A system for the detection of metal objects in the presence of interfering background including water comprising:

a pair of radiometric receivers including energy receiving ports positioned to receive energy from substantially the same field of view, one of said receivers tuned to a frequency f1 and the second of said receivers tuned to a frequency f2, means for converting incoming energy at said ports to a common IF frequency, means for sensing the difference in level of energy received at said two frequencies indicative of the presence or absence of metal in the field of view, said frequencies f1 and f2 being characterized as having detectable different apparent sky temperature and approximately identical apparent water temperature.

2. The combination in accordance with claim 1 wherein said energy receiving ports are polarized to receive energy in two orthogonal axes.

3. The combination in accordance with claim 1 wherein energy received by one energy receiving port may be represented as Tf epsilon f To + Rho f Ts and the energy received by the second receiver may be represented by Tf epsilon f To + Rho f Ts where Tf is the absolute temperature in degrees Kelvin of water at the frequency f1, Tf is the absolute temperature in degrees Kelvin of water at the frequency f2. epsilon f is the emissivity of water at frequency f1, epsilon f is the emissivity of water at frequency f2, Rho f is the reflectivity of water at frequency f1, Rho f is the reflectivity of water at frequency f2, Ts is the apparent sky temperature at frequency f1, and Ts is the apparent sky temperature at frequency f2 To is the ambient thermametric temperature and the frequency f1 and f2 are selected whereby Tf about Tf .

4. The combination in accordance with claim 1 wherein said pair of radiometric receivers include local oscillator means for Producing said frequencies f1 and f2, said local oscillator means comprising a single oscillator operating at the frequency f1 and including multiplier means for producing the frequency f2 as a harmonic of the frequency f1.

5. The combination in accordance with claim 1 wherein said frequencies are in the order of f1 about 20 GHz and f2 about 34 GHz

6. The combination in accordance with claim 5 wherein the energy receiving port of the radiometric receiver tuned to frequency f1 is polarized to receive energy in the horizontal plane.

7. The combination in accordance with claim 5 wherein the energy receiving port of the radiometric receiver tuned to frequency f2 is polarized to receive energy in the vertical plane.

8. A dual frequency radiometer comprising a first antenna tuned to receive energy at a selected frequency f1 a second antenna tuned to receive energy at a selected frequency f2, a first local oscillator, a first mixer coupled to said first antenna and first local oscillator to produce an intermediate frequency IF, a second local oscillator, a second mixer coupled to said second antenna and second local oscillator, said second local oscillator tuned to provide the same intermediate frequency IF from said second mixer when combined with energy at said frequency f2, Switch means for alternately applying an IF signal from the first and second mixers to a common output signal channel, a synchronous detector for energy from said switch means, and output means for producing a signal representative of the difference in level of received energy at said two antennae.

9. The combination in accordance with claim 8 wherein said first and second antennae are polarized in two orthogonal planes.

10. A radiometric system for detecting one material in a field of view containing different background material comprising:

a first polarized antenna directed toward a field of interest, a second antenna directed toward substantially the same field of interest orthaganally polarized with respect to said first antenna, a local oscillator for producing a frequency fo, first mixing means connected to said first antenna and said local oscillator to produce an intermediate frequency from the product of incoming energy at said first antenna and the output of said local oscillator, a multiplier connected to said local oscillator for producing a frequency nf , second mixing means connected to said second antenna and said multiplier to produce an intermediate frequency from the product of incoming energy at said second antenna and the outupt of said multiplier, first and second detector means connected to respective first and second mixing means for producing video signals representative of incoming energy at two different frequencies f1 and f2, means for differentially combining said video signals to produce a difference signal, threshold means for passing signals from said combining means having a level above a preselected threshold, a means for displaying signals above said threshold.

11. The combination in accordance with claim 10 wherein said multiplier is a frequency doubler.

12. The combination in accordance with claim 10 including means connected to one of said detector means for displaying the signal output of said detector means as a representation of the radiometric temperature of the field of view of the antenna associated with said detector means.

13. The combination in accordance with claim 12 including means for introducing the output of said threshold means into said display device to provide a signal indicative of a detected event superimposed upon the display of the output of said detector means.

14. The combination in accordance with claim 10 wherein Said system is operative to detect metal in a background of terrain features and said frequency f1 is in the range of 20.5 + or -0.5 GHz and said frequency f2 is in the range of 41 + or - 1 GHz.

15. The combination in accordance with claim 10 wherein said system is operative to detect water in a background of natural terrain features and said frequency f1 is in the range of 10.3 + or - 0.25 GHz. and said frequency f2 is in the range of 20.6 + or - 0.5 GHz.

16. The combination in accordance with claim 10 wherein said system is operative to detect tarmac in a background of natural terrain features and said frequency f1 is in the range of 8.5 + or - 0.5 GHz and said frequency f2 is in the range of 17 + or -1.0 GHz.

17. The combination in accordance with claim 10 wherein said system is operative to detect fresh water in a background of sea water and said frequency f1 is in the range of 800 + or - 50 MHz and said frequency f2 is in the range of 1600 + or - 100 MHz.