Radar signal processing system

• US 4,665,402 A
• Filed: 05/06/1983
• Issued: 05/12/1987
• Est. Priority Date: 05/18/1982
• Status: Expired due to Term

• Current Assignee: L3 Technologies ESS, Inc. (L3Harris Technologies, Inc.)
• Original Assignee: L3 Technologies ESS, Inc. (L3Harris Technologies, Inc.)
• Inventors: Young, Willard A.
• Primary Examiner(s): Tarcza, Thomas H.
• Assistant Examiner(s): Gregory, Bernarr Earl
• Application Number:
  US06/492,206
• Time in Patent Office: 1,467 Days
• Field of Search: 343/17.2 PC, 343/5 CE, 343/5 CF, 343/7 ED, 343/5 DP, 343/5 CD, 343/5 PN, 343/5 SA, 343/5 SM, 343/5 VQ, 343/7 PG, 358/168, 358/169, 358/219, 358/160, 358/166, 455/234, 330/75, 330/129, 330/144, 328/53, 328/146, 328/175, 364/734, 364/811, 342/91, 342/92, 342/176
• US Class Current: 342/91
• CPC Class Codes: G01S 7/064 using a display memory for image processing G01S7/298 takes precedence
• CPC Class Codes: G01S 7/285 Receivers

Abstract

The application describes digital signal processing of radar video signals to provide an optimum display. Three interacting components of the system are further described in detail: a global mean estimating circuit, a statistical analyzer circuit, and a dynamic range compression circuit. The global mean estimating circuit determines the mean of the amplitude distribution of video signals over a wide section of the display as opposed to a local mean of signals. A local mean of signals is the mean of the amplitude distribution of video signals occurring in the near vicinity of a point on the display. The statistical analyzer circuit evaluates the amplitude distribution of video signal samples and controls the gain of the signal path so that the variance of the sampled amplitude distribution follows the variance of a model distribution. The dynamic range compression circuit functions to compress the range of amplitude distribution in a manner to provide maximum information from the display.

Claims

1. A signal processing system to adapt a received signal for use with a display having a limited range of brightness comprising:

means estimating a mean value of said signal over a significant portion of the area of the display and subtracting the estimated mean from the signal to form a modified signal having amplitude values distributed about a zero mean,

means supplying the modified signal to a variable gain amplifier having its output connected to an analog to digital converter to provide a sequence of signal samples,

a statistical analyzer responsive to said sequence of signal samples to determine the spread of the amplitude distribution of said signal samples.

means controlling the gain of said amplifier to maintain the spread matched to that of a model distribution, and

digital signal compression means responsive to the output of said analog to digital converter to produce an output digital signal of limited range for use with said display.

2. A system as set out in claim 1 wherein said estimating means operates on samples of the signal selected by a pseudo-random sequence.

3. A system as set out in claim 1 wherein said estimating means operates on samples of the signal selected by a random sequence.

4. A method of estimating the mean of a sequence of digital signals representing a radar return signal comprising the steps of:

generating a pseudo-random sequences of pulses synchronized to the digital signal sequence to obtain a sequence of sampling signals from one characteristic of the pseudo-random sequence,

selecting and storing such digital signals as occur in coincidence with the sampling signals;

accumulating the values of said selected and stored digital signals up to a predetermined number thereof, therafter increasing the accumulated amount by the value of each next selected digital signal and decreasing the accumulated amount by the value of the digital signal occurring earlier by said predetermined number in the sequence selected, and

using the accumulated amount as a measure of said mean.

5. A method of estimating the mean of a sequence of digital signals representing a radar signal comprising the steps of:

generating a random sequence of pulses synchronized to the digital signal sequence to obtain a sequence of sampling signals from one characteristic of the random sequence,

selecting the storing such digital signals as occur in coincidence with the sampling signals,

accumulating the values of said selected and stored digital signals up to a predetermined number thereof, thereafter increasing the accumulated amount by each next selected digital signal and decreasing the accumulated amount by the value of the digital signal occurring earlier by said predetermined number in the sequence selected, and

using the accumulated amount as a measure of said mean.

6. Apparatus for estimating the mean of a sequence of digital signals representing a radar signal comprising:

a random clock generator responsive to said digital signal sequence to produce sampling signals in synchronism with some of said digital signals,

a memory with a corresponding address counter,

said address counter being responsive to said trigger signals to increment the address by one and store the corresponding digital signal at that address,

an accumulator receiving the sequence of stored digital signals,

means reading out previously stored signals at an address just prior to the new signal being stored at that address, and

7. means subtracting each read out previously stored signal from the total in the accumulator, whereby the total in the accumulator is a measure of the mean.

8. Apparatus for estimating the mean of a sequence of digital signals representing a radar return signal comprising:

a pseudo-random clock generator responsive to said digital signal sequence to produce sampling signals in synchronism with some of said digital signals,

a memory with a corresponding address counter,

said address counter being responsive to said trigger signals to increment the address by one and store the corresponding digital signal at that address,

an accumulator receiving the sequence of stored digital signals,

means reading out previously stored signals at an address just prior to the new signal being stored at that address, and

means subtracting each read out previously stored signal from the total in the accumulator, whereby the total in the accumulator is a measure of the mean.

10. A circuit as set out in claim 8 wherein the gain of the amplifier is reduced if the identified count in said first counter exceeds a predetermined number and vice versa.

11. A circuit as set out in claim 8 wherein the count in said first counter represents the upper tail breakpoint of the distribution.

13. Apparatus as in claim 11, further including a compression table memory, means for entering a first level signal in the first address location, a second level signal in addresses between the second address location and the first break-point, a third level signal in addresses between the first and second break-points and so on.

14. Apparatus as in claim 11 further including weighting means modifying the number of samples in each amplitude level of the compressed distribution so as to be unequal.

9. A circuit for maintaining the spread of the amplitude distribution of a signal matched to the spread of a model distribution, comprising:

a variable gain amplifier having a gain control input receiving said signal,

sampling means responsive to the output of said amplifier to provide a series of discrete sample values,

amplitude discriminating means responsive to said series of discrete sample values to transmit those exceeding a preset value,

a first counter coupled to said amplitude discriminating means to accumulate the number of sample values exceeding said preset value,

a second counter coupled to said sampling means to accumulate the total number of sample values,

means responsive to the accumulation of a predetermined count in said second counter to stop sampling and to supply a signal based on the count in said first counter to the gain control input of said amplifier.

12. Apparatus for transforming input signal samples of a certain amplitude distribution to a compressed amplitude distribution, including a system for establishing the break-points of the compressed distribution, comprising:

storage means having addresses corresponding to each possible input sample amplitude,

means incrementing by one the number stored at an address on detection of an input sample of corresponding amplitude,

a first counter counting the total number of signal samples and dividing the total by a factor representing the number of possible amplitudes in the compressed distribution to obtain the number of samples occurring at each amplitude level of the compressed distribution,

a second counter responsive to said storage means to accumulate the stored numbers when read out in sequence including means to indicate when the accumulated count exceeds said last-mentioned number thereby establishing the address last read as the first break-point and further indicating when the accumulated count exceeds twice the last-mentioned number thereby establishing the second break-point and so on.

15. A method of compressing the amplitude distribution of a series of signal samples falling within a finite amplitude range, comprising:

supplying the series of samples in parallel to a plurality of comparators having monotonically increasing level settings,

detecting the comparator having a level setting immediately above the amplitude of the applied signal sample and generating an output signal indicative of that comparator,

counting the number output signals supplied by each comparator in a complete series of signal samples, and

adjusting the level settings of the comparators so that the counts attributable to each comparator remain substantially equal.

16. Apparatus for transforming input signal samples of a certain amplitude distribution to a compressed amplitude distribution comprising:

a plurality of comparators having monotonically increasing trip-level settings,

means supplying said input signal in parallel to said comparators,

a selector circuit connected to said comparators and responsive to the highest tripped comparator to generate an output signal indicative of that comparator,

means counting the number of output signals relating to each comparator in a complete series of signal samples, and

control means responsive to the relative numbers to adjust the trip-level settings so that each comparator has a substantially equal count.

17. A signal processing system to adapt a received signal for use with a display having a limited range of brightness comprising:

means estimating a mean value of said signal over a significant portion of the area of the display,

a variable gain amplifier in the path of said signal followed by an analog to digital converter to supply a sequence of signal samples,

a statistical analyzer responsive to said signal samples to determine the spread of the amplitude distribution of said signal samples, by determining the number of samples having an amplitude exceeding an upper tail break-point value,

means controlling the gain of said amplifier to maintain the spread matched to that of a model distribution, and

signal compression means responsive to the output of said amplifier to produce an output signal of limited range for use with said display.

19. A system as set out in claim 17 wherein the stored data is derived from a stored distribution of the signal samples.

18. A signal processing system to adapt a received signal for use with a display having a limited range of brightness comprising:

means estimating a mean value of said signal over a significant portion of the area of the display,

a variable gain amplifier in the path of said signal followed by an analog to digital converter to supply a sequence of signal samples,

a statistical analyzer responsive to said signal samples to determine the spread of the amplitude distribution of said signal samples,

means controlling the gain of said amplifier to maintain the spread matched to that of a model distribution, and

signal compression means utilizing data stored in a random access memory and responsive to the output of said amplifier to produce an output signal of limited range for use with said display.