Digital array stretch processor employing two delays

• US 6,624,783 B1
• Filed: 02/28/2001
• Issued: 09/23/2003
• Est. Priority Date: 02/28/2001
• Status: Expired due to Fees

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Rabideau, Daniel
• Primary Examiner(s): Gregory, Bernarr E.
• Application Number:
  US09/796,049
• Time in Patent Office: 937 Days
• Field of Search: 342/74-81, 342/89, 342/98-103, 342/122, 342/128-133, 342/147, 342/158, 342/175, 342/195, 342/196, 342/197, 342/200, 342/201, 342/368-377, 342/118-121
• US Class Current: 342/195
• CPC Class Codes: G01S 13/286 frequency shift keyed
• CPC Class Codes: G01S 2013/0245 Radar with phased array antenna
• CPC Class Codes: G01S 7/021 Auxiliary means for detecting or identifying radar signals or the like, e.g. radar jamming signals
• CPC Class Codes: G01S 7/2883 using FFT processing
• CPC Class Codes: G01S 7/292 Extracting wanted echo-signals

Abstract

A pulse-type beamforming apparatus, such as a radar array system, is used for receiving, detecting, localizing, and/or imaging desired signals. The apparatus is used to receive wideband chirp signals. The apparatus contains a receive aperture that is partitioned into multiple channels. The received signal at each channel is mixed with a replica chirp. The replica chirp is effectively delayed in a way that partially removes range-dependent distortion of desired signals. The mixer outputs are then sampled and filtered. The filters on each channel incorporate a time delay that completely removes the remaining range-dependent distortion for all signals in a desired direction. Signals are also compressed and integrated by a digital beamformer.

Claims

1. A system for beamforming in a pulse-type sensor array, comprising:

a source of pulsed wideband linear frequency modulated (LFM) signals;

an array of sensors through which pulsed wideband signals are received, said array generating N channels of data;

means for processing the N channels of data, wherein each channel is sampled at a rate below the Nyquist frequency associated with the signal bandwidth, and each channel is processed to completely align, from channel to channel, the frequency and phase of all pulsed wideband signals received from a desired direction, said processing being performed without the aid of analog time-delays; and

means for further processing the N processed channels such that channels are combined coherently, allowing the full capability of said array to be steered equally to all signals in a desired direction, and signals are detected, localized, imaged and otherwise resolved to a degree that is commensurate with the wide bandwidth of the source pulses.

2. The system of claim 1 further comprising:

means for producing LFM chirp replica signals;

mixers supplied with said received wideband signals and said LFM chirp replica signals for producing resultant pulses that, after filtering, have frequency-time slopes less than that of the received pulses;

sampling devices; and

means for completely aligning from channel to channel the frequency and phase of all pulsed wideband signals received from a desired direction.

3. The system of claim 2 further comprising one or more digital compression filter supplied with sampled pulse-type signals and producing resultant non-dispersed pulses.

4. The system of claim 2, wherein the LFM chirp replica signals are time delayed, with delay values that vary from channel to channel.

5. The system of claim 4, wherein the delay value used for the LFM chirp replica signal at channel i is Γ

+β

_i, where Γ

is common to all channels, and β

_i represents the time needed for desired signals to travel from a common reference point to channel i.

6. The system of claim 4, wherein delays of the LFM replica chirp signals is achieved by either generating chirps with the desired delays;

applying digital time-delay filters to undelayed digitized LFM chirp replicas, then converting to analog;

generating undelayed chirps at an intermediate frequency, then processing to effect time-delays and phase shifts, then upconverting to the desired frequency;

or generating undelayed chirps, then applying frequency shifts and phase shifts before or after mixing.

7. The system of claim 2, wherein the outputs of said sampling devices are time-delayed, with delay values that vary from channel to channel.

8. The system of claim 7, wherein the time-delay value used at channel i is −

β

_i.

9. The system of claim 7, wherein time-delaying of sampled signals is implemented through Finite Impulse Response (FIR) filtering in the time domain or frequency domain.

10. The system of claim 9, wherein time-delay filtering is mathematically combined with one or more other digital filters.

11. The system of claim 10, wherein time-delay FIR filtering is implemented in the frequency domain, and one other filter is a digital compression filter, the combination of which results in the cancellation of Fourier transformation operations.

12. The system of claim 2, wherein said means for completely aligning desired signals at channel i comprises time-delaying the chirp replica by Γ

+β

_i, then time delaying the sampled signal by −

β

_i.

13. The system of claim 1, wherein the coherent channel combining is performed by a digital beamformer.

14. A method of beamforming in a pulse-type sensor array, comprising:

providing a source of pulsed wideband LFM signals;

receiving pulsed wideband signals with an array of sensors, said array generating N channels of data;

processing the N channels of data, wherein each channel is sampled at a rate below the Nyquist frequency associated with the signal bandwidth, and each channel is processed to completely align, from channel to channel, the frequency and phase of all pulsed wideband signals received from a desired direction, said processing being performed without the aid of analog time-delays; and

further processing the N processed channels such that channels are combined coherently, allowing the full capability of said array to be steered in a desired signal direction, and signals are detected, localized, imaged and otherwise resolved to a degree that is commensurate with the wide bandwidth of the source pulses.

15. The method of claim 14, wherein the processing at each receiver further comprises:

generating LFM chirp replica signals;

generating resultant pulses which, after filtering, have a frequency-time slope less than that of the received pulses;

sampling; and

completely aligning from channel to channel the frequency and phase of all pulsed wideband signals received from a desired direction.

16. The method of claim 15 further comprising digital compression filtering and digital beamforming.

17. The method of claim 15, wherein the LFM chirp replica signals are time delayed, with delay values that vary from channel to channel.

18. The method of claim 17, wherein the delay value used for the LFM chirp replica signal at channel i is Γ

+β

_i, where Γ

is common to all channels, and β

_i represents the time needed for desired signals to travel from a common reference point to channel i.

19. The method of claim 15, wherein the output of said sampling device is time-delayed, with a delay value that varies from channel to channel.

20. The method of claim 19, wherein the time-delay value used at channel i is −

β

_i.