Method and apparatus for enhancing radiometric imaging

• US 4,521,861 A
• Filed: 04/30/1982
• Issued: 06/04/1985
• Est. Priority Date: 04/30/1982
• Status: Expired due to Term

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Texas Instruments, Inc.
• Inventors: Poradish, Frank J., Logan, Russell H.
• Primary Examiner(s): Zache, Raulfe B.
• Assistant Examiner(s): Shaw, Dale M.
• Application Number:
  US06/373,583
• Time in Patent Office: 1,131 Days
• Field of Search: 364/423, 364/514, 364/516, 364/517, 364/414, 250/250, 382/6, 382/54, 343/351, 343/5 MM, 343/5 PD, 343/5 CE
• US Class Current: 702/57
• CPC Class Codes: G01S 13/86 Combinations of radar systems with non-radar systems, e.g. sonar, direction finder
• CPC Class Codes: G01S 7/4017 of HF systems
• CPC Class Codes: G01S 7/4021 of receivers
• CPC Class Codes: G01V 8/005 operating with millimetre waves, e.g. measuring the black losey radiation

Abstract

Disclosed is a method and apparatus for enhancing target detection, particularly in the millimeter wave frequency range, through the utilization of an imaging radiometer. The radiometer, which is a passive thermal receiver, detects the reflected and emitted thermal radiation of targets within a predetermined antenna/receiver beamwidth. By scanning the radiometer over a target area, a thermal image is created. At millimeter wave frequencies, the received emissions from the target area are highly dependent on the emissivity of the target of interest. Foliage will appear "hot" due to its high emissivity and metals will appear cold due to their low emissivities. A noise power illuminator is periodically actuated to illuminate the target of interest. When the illuminator is actuated, the role of emissivity is reversed, namely poorly emissive targets will generally be good reflectors which in the presence of an illuminator will appear "hot". The highly emissive targets (such as foliage and dirt) which absorb most of the transmitted energy will appear almost the same as in a non-illuminated, passive image. Using a data processor, the intensity of the passive image is subtracted from the intensity of the illuminated, active image which thereby cancels the background foliage, dirt, etc. and the reflective metallic targets are enhanced.

Claims

1. A radiometric imaging system for sensing electromagnetic energy from a target area, comprising:

an illuminator means comprising a transmit antenna and transmitter means for producing random electromagnetic energy for illuminating said target area;

a radiometer comprising a receive antenna and receiver means, said radiometer for receiving and measuring the intensities of both the illuminated thermal image as well as the non-illuminated passive image from said target area in a predetermined frequency band, and

means coupled to said radiometer for inverse filtering and subtracting the intensities of the non-illuminated target area from the intensities of the illuminated target area to provide enhanced target detection.

2. A radiometric imaging system according to claim 1 wherein the illuminator means generates electromagnetic energy having a preselected polarization and said radiometer further includes a polarization splitter operatively connected to the receive antenna for receiving the preselected polarized electromagnetic energy from the illuminator means and horizontally and vertically polarized natural emissions from the target area.

3. A radiometric system comprising:

(a) receiver means for receiving energy representative of images from an area, the receiver means including a receive antenna tuned to operate in the receiver band, a reference load and a calibration means for calibrating the first means for producing voltages proportional to the difference between the power received by the antenna and the power emitted by the reference load and an A/D converter means for digitizing the voltages;

(b) a display means including an image processing computer and a display means, the image processing computer operatively connected to the A/D converter for receiving the digitized outputs thereof and producing signals representative of the difference between preselected images of the area for display by the display means;

(c) illuminator means inlcuding a noise generating means and a transmitter means having a transmit antenna for transmitting the noise; and

(d) control means including servo mechanism and control computer, the servo mechanism operatively connected to the receiver and transmitter and antennas and control computer for coordinating movement of the receive and transmit antennas over a target area and the control computer operatively connected to the calibration means of the receiver means, image processing means and illuminator means for operating;

the illuminator means to illuminate selectively the target area;

the receiver means to receive and digitize emissive energy and reflective energy from the target area; and

the processing computer to store the digitized signals for passive and active images and subtract the digitized intensities of the passive image from the intensities of the illuminated image whereby highly reflective targets are differentiated from the emissive background terrain for enhanced target detection.

4. A radiometric imaging method for enhancing target detection and image contrast consisting of the steps of:

(a) generating a passive thermal image of the intensities from a target area;

(b) generating an active thermal image of the intensities from the target area;

(c) subtracting the intensities of the passive thermal image from the intensities of the active thermal image for forming a subtracted image; and

(d) inverse filtering the subtracted image for providing an enhanced target detection.

5. A radiometric imaging method for enhancing target detection and image contrast consisting of the steps of:

(a) generating a passive thermal image of the intensities from a target area;

(b) generating an active thermal image of the intensities from the target area;

(c) inverse filtering the passive and active thermal images for improving their resolution; and

(d) subtracting the inverse filtered passive and active thermal images for enhancing target detection in the target area.

6. A radiometric imaging method for enhancing target detection and image contrast consisting of the steps of:

(a) generating a passive thermal image of the intensities from a target area;

(b) generating an active thermal image of the intensities from the target area;

(c) inverse filtering selectively the passive and active thermal images for improving their resolution; and

(d) subtracting the passive thermal image from the active thermal image for enhancing target detection in the target area.

7. A radiometric imaging system comprising:

a control computer system, a radiometer, an illuminator and a display means;

the control computer system including a servo mechanism operatively connected to the radiometer and illuminator for selecting a target area, and a control computer for selectively controlling the radiometer for measuring the thermal intensity of a non-illuminated target area for activating the illuminator for illuminating the target area and for controlling the radiometer for measuring the intensity of the illuminated target area for the display means;

the display means including an image processing computer operatively connected to the radiometer and control computer for subtracting the intensities of the non-illuminated and illuminated target area for providing enhanced target detection and a readout means for producing an enhanced image of any target in the target area;

the radiometer including a receive antenna operatively connected to the servo mechanism for motion control, a reference load, a switch operatively connected to the receive antenna and reference load, a local oscillator, a double sideband mixer having first and second inputs operatively connected, respectively, to the local oscillator and selectively to the reference load and receive antenna, an amplifier operatively connected to the mixer, a diode detector connected to the amplifier, a low pass filter operatively connected to the diode detector for eliminating ripple and producing a dc signal at different levels proportional to the thermal energy sensed, a difference amplifier having a positive input terminal operably connected to the low pass filter, calibration circuitry operatively connected in two-way communication with the control computer of the control system and to the switch, a D/A converter operably connected to the calibration circuitry and negative terminal of the difference amplifier whereby when the switch is connected to the load reference the calibration circuitry applies an offset voltage to the negative terminal of the difference amplifier for ensuring that its output is zero for ensuring that when the switch is connected to the receive antenna the difference amplifier outputs a voltage proportional to the difference between the power received by the receive antenna and the power emitted by the reference load, an A/D converter operatively connected to the difference amplifier, the image processing computer of the display means and the calibration circuitry operatively connected to the A/D converter for receiving the digitized output of the difference amplifier;

the illuminator having a noise diode for generating wideband random noise, at least one amplifier operatively connected to the noise diode for amplifying the noise, a bandpass filter for bandlimiting the amplified noise, a local oscillator, a double sideband mixer operatively connected to the local oscillator and bandpass filter for up-converting the noise signal to a preselected transmit frequency, a bandreject filter operatively connected to the mixer for removing the local oscillator frequency, a transmit amplifier operatively connected to the bandreject filter and the control computer of the control system, and a transmit antenna operatively connected to the transmit amplifier and servo mechnaism of the control system, respectively, for transmitting energy at a power level for a preselected time and for motion control.