FFT-based filtering for low-quality signal direction finding

• US 6,469,657 B1
• Filed: 10/10/2001
• Issued: 10/22/2002
• Est. Priority Date: 10/17/2000
• Status: Expired due to Fees

• Current Assignee: Harris Corporation (L3Harris Technologies, Inc.)
• Original Assignee: ITT Manufacturing Enterprises, Inc. (ITT, Inc.)
• Inventors: Fox, Neil David
• Primary Examiner(s): Sotomayor, John B.
• Application Number:
  US09/972,874
• Time in Patent Office: 377 Days
• Field of Search: 342/21, 342/98, 342/99, 342/147, 342/195, 342/196, 342/417, 342/418, 342/443
• US Class Current: 342/147
• CPC Class Codes: G01S 3/74 Multi-channel systems specially adapted for direction-finding, i.e. having a single antenna system capable of giving simultaneous indications of the directions of different signals systems in which the directions of different signals are determined sequentially and displayed simultaneously G01S3/04, G01S3/14

Abstract

A radio direction finding system for a low signal quality transmission signal performs a Fast Fourier Transform (FFT) on an entire received RF band into a number of FFT bins, each serving as an IF filter. The contents of each FFT bin are demodulated to decode a frequency signature from the encoded RF signal. A signal-to-noise ratio comparator receives the FFT baseband samples and determines the signal level from the known frequency signature. The FFT baseband buffer with the greatest SNR corresponding to a particular FFT frequency bin is outputted as a direction finding metric.

Claims

1. A radio direction finding system for determining a direction of origin of a source of a transmitted radio frequency (RF) signal, comprising:

an antenna capable of receiving said RF signal;

an RF receiver coupled to said antenna system, said RF receiver having a bandwidth for receiving an entire RF band of interest in which said transmitted RF signal may lie, and outputting a detected signal;

an analog-to-digital (A/D) converter coupled to said RF receiver and digitizing said detected signal to provide a series of RF samples;

a Fast Fourier Transform (FFT) processor coupled to said A/D converter and performing a Fourier transform on said series of RF samples, said FFT processor producing a plurality of component frequency approximations representing frequency components of said detected signal into a plurality of FFT bins, each having a predetermined frequency size;

a plurality of demodulators, each receiving the contents of a corresponding FFT bin, and subjecting said contents to a demodulation process whereby an FFT baseband sample is produced for each of said plurality of FFT bins; and

a signal-to-noise ratio (SNR) processor that determines a maximum SNR from among said plurality of FFT baseband samples, and outputs an FFT baseband signal whose SNR is determined to be greatest;

wherein said outputted FFT baseband signal having a greatest SNR is used as a direction finding metric for determining a direction of arrival (DOA) angle of said RF signal.

2. The radio direction finding system of claim 1, wherein said transmitted RF signal includes a predetermined frequency signature.

3. The radio direction finding system of claim 2, wherein said plurality of demodulators comprise squaring process demodulators for demodulating an encoded audio frequency from each of said FFT bins.

4. The radio direction finding system of claim 3, wherein said SNR processor determines a signal level according an averaged value from an FFT bin corresponding to said predetermined frequency signature.

5. The radio direction finding system of claim 2, wherein said predetermined frequency signature is an audio frequency.

6. The radio direction finding system of claim 1, further including a plurality of decimators for reducing the number of FFT baseband samples provided to said SNR processor from said plurality of demodulators to a number required by a Nyquist sampling criterion.

7. A method of determining a direction of origin of a source of a transmitted radio frequency (RF) signal, comprising the steps of:

receiving an entire RF band of interest in which said transmitted RF signal may lie, and outputting a detected signal;

digitizing said detected signal to provide a series of RF samples;

performing a Fast Fourier Transform (FFT) on said series of RF samples, said FFT producing a plurality of component frequency approximations representing frequency components of said detected signal into a plurality of FFT bins, each having a predetermined frequency size;

subjecting the contents of a corresponding FFT bin to a demodulation process whereby an FFT baseband sample is produced for each of said plurality of FFT bins; and

determining a maximum SNR from among said plurality of FFT baseband samples, and outputting an FFT baseband signal whose SNR is determined to be greatest;

wherein said outputted FFT baseband signal having a greatest SNR is used as a direction finding metric for determining a direction of arrival (DOA) angle of said RF signal.

8. The method of claim 7, wherein said transmitted RF signal includes a predetermined frequency signature.

9. The method of claim 8, wherein said demodulation process comprises a squaring process for demodulating an encoded audio frequency from each of said FFT bins.

10. The method of claim 9, wherein said SNR processor determines a signal level according an averaged value from an FFT bin corresponding to said predetermined frequency signature.

11. The method of claim 8, wherein said predetermined frequency signature is an audio frequency.

12. The method of claim 7, further including the step of reducing the number of FFT baseband samples used in said SNR process from said demodulation process to a number required by a Nyquist sampling criterion.