N093-204 TITLE: Synthetic Elements for Moving Line Arrays

TECHNOLOGY AREAS: Sensors

ACQUISITION PROGRAM: PMS401 (TB-16 submarine towed array)

The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), which controls the export and import of defense-related material and services. Offerors must disclose any proposed use of foreign nationals, their country of origin, and what tasks each would accomplish in the statement of work in accordance with section 3.5.b.(7) of the solicitation.

OBJECTIVE: To develop a methodology for exploiting towed array motion to process higher frequency regions at all steered directions in order to use the full frequency capability of a towed array.

DESCRIPTION: Beamformed line arrays are limited in frequency coverage by both the Nyquist rate imposed by temporal sampling and by the spatial separation of the individual elements of the array. The spatial limitation, analogous to the Nyquist rate, is imposed by the necessity of adequately sampling the waveform in space in order to distinguish one arrival angle from another. A moving line array effectively samples the space more densely than a stationary array, and, thus, with an adequate temporal sampling rate, meets the sampling criterion for higher frequency coverage than does the stationary array.

Existing work in the closely related area of passive synthetic aperture sonar (SAS) seeks to use array motion to extend the effective aperture, subject to the constraint of signal coherence in time and space, to provide improved estimation of arrival angle. This topic focuses on providing additional frequency coverage by increasing the effective array population with "synthetic elements".

Signal processing�s most common constraint is, perhaps, the Nyquist criterion, limiting frequency coverage based on the time sampling rate. The cost of violating this temporal constraint is aliasing � signals of one frequency appearing at another, incorrect frequency. Arrays sample spatially by using an arrangement of discrete elements. In array beamforming, the analogous frequency constraint due to spatial sampling depends on the separation of the elements versus the wavelength of the signal in the medium. Processing above the spatial design frequency incurs the penalty of grating lobes � aliasing of a signal on one beam to other beams, possibly obscuring other signals. The grating lobes that result depend on the location in azimuth of the source, as well as the frequency of the signal and are completely predictable.

In a stationary array, there is no remedy for the grating lobes because there is no additional information to be exploited. In the case of moving arrays, however, additional spatial sampling results from the array�s motion. In particular, synthetic elements can be created using a combination of time delay and phase shifting.

The phase shifts depend on the source frequency, not the bin frequency or the observed frequency. The observed frequency will be a function of the relative motion between the array element and the source, but the phase shift required to remove the effect of the imposed time delay is a function of the change in phase of the field at a static location.

PHASE I: Perform the R&D to develop an approach to increase the useful frequency coverage of beamformed line array data and mitigate the effects of missing array elements to improve beamformer performance in a degraded array.

PHASE II: Expand the development to an actual real data set. Demonstrate the implementation of these concepts with test data involving a moving line array relative to far-field static sources from a range of azimuths and demonstrates the mitigation of grating lobes. Report the results with needed algorithm refinement and repeat demonstrations.

PHASE III: Transition algorithms developed in Phase II to the fleet.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The work developed here should be applicable to all moving array receivers in all fields.

REFERENCES:
1. M. H. Johnson, "Synthetic elements for the towed array" (IAR project). https://donst.nrl.navy.mil/cgi-bin/login-form.cgi

KEYWORDS: towed array; synthetic aperture; beamforming; grating lobes; sampling; Nyquist

** TOPIC AUTHOR (TPOC) **

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