Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Analysis of complex waves
Reexamination Certificate
1976-11-15
2002-07-23
Blum, Theodore M. (Department: 3662)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Analysis of complex waves
C324S076330, C342S192000
Reexamination Certificate
active
06424138
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to spectral analysis and more particularly to a high resolution spectrum analyzer for use in anti-submarine warfare applications.
BACKGROUND OF THE INVENTION
One of the most pressing problems in anti-submarine warfare (ASW) applications is the detection of the spectral signature, direction and velocity of both surface and subsurface vessels by either passive or active listening devices located at sonobuoys which are air dropped across a given area suspected of having enemy vessels.
The purpose of the passive sonobuoys is to listen for subsurface or surface activity and transmit detected signals, usually in the acoustic range, to overflying aircraft. The system to be described is useable with both LOFAR and DIFAR buoys in which the LOFAR buoy is an omnidirectional listening type buoy and the DIFAR buoy includes direction determining apparatus so as to be able to locate the direction of the incoming signals.
The signals from the sonobuoys are, in general, transmitted to the overflying aircraft which identifies the particular sonobuoy from which a signal is coming, its position, and the information carried on the signal. The incoming signals from the sonobuoys are usually analyzed in detail as to the particular signature of the sound source. This includes analysis as to the particular frequencies which are present in the incoming signal, usually in a range from 2.5 Hz to 3,000 Hz; any doppler shift of the spectral lines of the signal; the line width of the spectra so as to identify whether the particular vessel is driven by an internal combustion engine (broad spectral line) or by turbine or electric power (narrow spectral line); the amplitude of each of the spectra of the signal; and the direction of the particular incoming signal, so as to identify the source of either continuous signals or periodic signals indicative of known types of vessels.
In a typical passive listening system, the signals from the sonobuoy are sampled and digitized so as to produce a digital representation of the incoming signal. In order to perform a spectrum, analysis, the digital representation of the incoming signal is multiplied with a digital representation of a reference signal of a predetermined frequency, with the product thereof being integrated to obtain the degree of correlation between the reference signal and the incoming signal. The reference signal is periodically changed or stepped in a known and rapid fashion so that the incoming sampled signal is compared with a large number of reference signals, each at a different frequency. When correlation occurs, the reference frequency is noted as well as the amplitude of the signal. The notation is usually in the form of a “gram” type display which gives a time history of the frequency and amplitude of the incoming signal. From this type display, spectral signature information as well as any doppler frequency shift indicating a relative velocity with respect to the particular sonobuoy can be readily observed. The same type information can be obtained from a so-called “waterfall” display or even from the traditional display of the spectra of the incoming in terms of frequency versus amplitude.
The system described utilizes a narrow band digital filter, the center frequency of which is rapidly stepped so that a spectral analysis over a range of frequencies takes place in real time. The system differs generically from Fast Fourier Transform (FFT) analysis as follows: FFT analysis reduces the number of operations to be carried out in performing a spectral analysis while at the same time accepting whatever complexity is involved for the individual operations. The narrow band digital filter reduces the complexity of the individual operations while accepting whatever number of operations naturally arises.
This process is equivalent to passing the signal through a narrow band filter whose band width is equal to the reciprocal of the time epoch over which the cross correlation is performed. By storing a sequence of digitized signal samples (which we call a “record”) in an electronic memory, the samples may be sequenced through the cross correlator at many times the real-time rate. Each time the sequence is recirculated, the reference frequency is stepped to a different frequency thereby accomplishing real time spectrum analysis.
One of the problems with prior ASW spectrum analyzers has been the sensitivity of the spectrum analysis, especially at the relatively low acoustic frequencies which typify the types of radiation from a moving target vessel such as a submarine. The sensitivity of the spectrum analyzer in general is directly proportional to the length of the record of the incoming signals which is processed. The longer the record the better able will be the apparatus to resolve stable signals by virtue of long integration times. Moreover, in terms of bandwidth (BW), bandwidth is inversely proportional to the length of a record processed in a coherent fashion. This means that the longer the coherent record processed the narrower the bandwidth and the greater, the resolution.
The criticality of the resolution of a given spectrum analyzer for use in ASW applications can be seen in a comparison between prior art systems and the present system. It is a feature of the prior art systems that prior art apparatus do not detect doppler shifts of under 2.5 Hz at 1,000 Hz. This doppler shift corresponds to a velocity of the source relative to the sonobuoy of 7.4 knots. Thus, present day apparatus does not detect movement of targets which are proceeding at a speed of under 7.4 knots. This is because in general, present spectrum analyzers utilized in ASW applications have a resolution of ±¼% of the reference frequency. On the other hand, the subject system to be described permits resolution of ±{fraction (1/64)}% of the reference frequency. This results in a resolution at 1,000 Hz. of 0.156 Hz which is equivalent to 0.46 knots. Thus, it is virtually impossible for a target vessel to escape a doppler measurement by virtue of moving slowly through the water.
The subject system is virtually unlimited in resolution by virtue of performing the spectrum analysis with a number of short, albeit discontinuous, records of the sampled signal which can be built up to any desired length and which can be made to simulate continuous records. The ability to use discontinuous records permits batch processing such that while one short record is being correlated with a large number of reference signals each having a different frequency, a second record is being built up in the memory of the system to form another short record. With the subject invention it is possible to process data blocks separately and add them up so that the effective length of the record may be increased, for instance, by an order of magnitude without a corresponding increase either in the physical size of the memory or a corresponding increase in the amount of processing equipment.
Virtually unlimited resolution is accomplished since if T= record length, the resolution of the system is 1/T For example, if a record length at 100 Hz=2000 samples, then T=4 seconds and the resolution or bandwidth BW=¼ Hz. To increase the resolution, the record length is increased. For a resolution of {fraction (1/32)} Hz the record length is multiplied by 8 so as to increase the number of samples from 2,000 samples to 16,000 samples. To accumulate 16,000 samples in one register and to process them would require a large amount of additional storage and processing.
In the subject invention, to solve the problem of the amount of storage and processing normally necessary, spectrum analysis is performed by breaking up the record into subrecords or segments, by processing the information in the subrecords or segments so as to eliminate the effects of the discontinuities and then by accumulating the results in accordance with the following equation for the above example:
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BAE SYSTEMS Information and Electronic Systems Integrations, Inc
Blum Theodore M.
Long Daniel J.
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