Showing papers on "Pulse repetition frequency published in 1989"

Two-Dimensional Color Doppler Estimation of the Severity of Atrioventricular Valve Regurgitation: Important Effects of Instrument Gain Setting, Pulse Repetition Frequency, and Carrier Frequency

Abstract: Literature reports of good correlation between two-dimensional color Doppler and ventriculography for estimation of the severity of atrioventricular valve regurgitation have appeared. Local experience has been variable. The purpose of this study was to evaluate three factors for their effect on two-dimensional color Doppler display of the apparent area of regurgitant flow. The factors studied were instrument gain setting, pulse repetition frequency (PRF), and carrier frequency. Sixteen regurgitant valves were examined with high quality commercial instrumentation from two manufacturers. Electrocardiographic-triggered image gating was used for recording and planimetry of maximal imaged flow area (IFA). Examinations were performed at two gain settings with constant carrier frequency and PRF, at three carrier frequencies (2.5, 3.5, and 5 MHz) at constant gain, and at differing PRF (3.0 to 12 kHz). Reduction in gain setting made little difference in asthetic quality of regurgitant jets, but their areas decreased by 29%. Carrier frequency was found to greatly affect IFA, with each higher carrier frequency giving significantly smaller IFAs. PRF also affected IFA consistently, with progressively smaller IFAs at each higher PRF. There was no useful correlation between IFA of a single jet evaluated on the two instruments at generally similar settings. It is concluded that instrument gain setting, PRF, and carrier frequency each significantly affects the size of IFA within a single instrument and between different instruments. Given the variety of instruments and settings employed within the cardiology community, it is unlikely that even moderately accurate grading of severity of regurgitation based on IFA can be done unless instrument settings are specified and standardized. There is a clear need for annotation of instrument settings during color Doppler examinations and a need for concensus regarding "optimal" settings. Finally, the consistency of reduction in IFA with increasing carrier frequency and PRF within each of the instruments tested suggests investigation of "normalization" of IFA displays.

Adaptive waveform radar

Abstract: The present invention allows a radar system to operate in an electromagnetic environment where co-channel narrow band interference is present, without loss of detectability, resolution and ambiguity. The present invention system adapts the radar transmitted signal from a radar transmitter (10) with an adaptive waveform processor (14) so that its spectral energy is significant only in the interference free portions of the radar channel. A Fast Fourier Transform (18) adapts the receiver (8) to detect this transmitted spectrum and then equalizes the signal by means of transversal equalizer coefficients (16) to reduce distortions to the signal sidelobes.

Principles of high-resolution radar based on nonsinusoidal waves. I. Signal representation and pulse compression

Abstract: Carrier-free coded waveforms that promise good clutter rejection and high-resolution capabilities for radar are represented as a sequence of positive and negative Gaussian pulses. On the basis of this signal representation, the matched-filter (or correlated receiver) response to carrier-free coded waveforms is analyzed and plotted for certain Barker and complementary codes, complementary-code pairs being the optimum codes for clutter rejection. The systematic design of a carrier-free, pulse-compression radar system is described. A correlator receiver that is specifically designed for the selective reception of complementary-code pairs is presented. Computer simulation results for the correlator receiver are also presented. >

Frequency agile radar

Abstract: A radar transmitter in which a pseudo-random sequence of pulses are transmitted, each at a different frequency. Prior to the return of the first pulse, a receiver is retuned to the first transmit frequency, then the transmitter is retuned to the second transmit frequency and so on. The return pulses are integrated coherently across the sets at a each frequency to achieve the highest signal level. The pulses are integrated noncoherently within a set over all the frequencies transmitted to improve the signal-to-noise ratio. The technique of the present invention requires accurate knowledge of the range to the target. In a specific multi-line embodiment, the range calculation is provided by the centerline carrier frequency operating at a medium pulse repetition (prf) frequency. The retuning is then performed with the sidebands operating at a low prf. The invention offers the advantages of single frequency operation, pulse-to-pulse switching, and simultaneous processing of two different waveforms (medium and low prf). Thus, the radar system of the present invention offers improved fade elimination, interference insensitivity, and range ambiguity.

High resolution short range radar

Abstract: A short range, high resolution, radar system of particular use in training radar operators. The system transmits very short pulses of a radio-frequency (RF) signal at a very high pulse repetition frequency, PRFo. A signal pre-processor samples a received baseband radar signal and generates a time-expanded signal which is a replica of the real received radar signal, but with a much lower nominal pulse repetition frequency, PRFn. The signal pre-processor includes a sampling circuit which receives the baseband signal, and a timing circuit which controls the sampling circuit. The timing circuit operates by causing samples of the baseband signal to be taken at progressively later portions of subsequent pulses. While the radar system is particularly useful in demonstrating radar principles in a classroom setting, it has other applications.

Wide-swath SAR imaging with multiple azimuth beams

Abstract: Two distinct types of multiple azimuth beam system are described. Both designs attempt to effect a decrease in operating PRF (pulse repetition frequency) whilst maintaining adequate sampling of target returns. The net result is that the unambiguous swath width is increased (roughly by the number of receive beams) whilst maintaining spatial resolution.

Delayed replica radar test set target

Abstract: A radar test set target which is co-located with the radar and produces a faithful replica of the radar signal substantially delayed in time. The radar signal modulates a laser light which is controllable delayed in time by transmitting the modulated laser light through a fiber optic delay line. The output of the delay line is demodulated and the extracted and delayed radar signal is returned to the radar as a test signal.

Radar system and a method for operating a radar system

Abstract: A radar system and a method of operating a radar system. The radar system comprises a transmitter-receiver subsystem to transmit a series of electromagnetic pulses, to scan those pulses across an area of interest, and to receive echoes of the pulses. The radar pulses may be transmitted at a multitude of frequencies, each of which is in one of a multitude of channels. The radar system further includes a channel selection subsystem, first, to measure or sample the electromagnetic interference received by the transmitter-receiver subsystem in each of the channels of the radar system during each of at least selected scans of the radar pulses across the area of interest, and second, on the basis of those interferences measurements, to select automatically a channel of the radar system to transmit the radar pulses and to generate a signal identifying that channel. The transmitter-receiver subsystem includes a channel selector for receiving this signal from the channel selection subsystem, and in response to that signal, for selecting a frequency in the selected channel to transmit the radar pulses the next time those pulses are scanned across the area of interest.

Radar system for determining first time around targets from multiple time around targets

Abstract: A radar system for determining first time around targets from multiple time around targets using two radar pulse trains of different pulse repetition intervals and thresholding means. Echos from pulses at each pulse repetition interval being compared to determine first time around targets from subsequent time around targets. The radar system uses thresholding levels to determine an echo and is less prone to problems of coincidence gate clashing with high duty cycles.

Radar target simulator

Abstract: The present invention provides a radar simulation system with at least two counters. The first counter is a coarse counter 20/30 that counts a transmit pulse interpulse periods to provide a coarse return delay at the beginning of a radar look. The second counter is a fine counter 22/36 which counts, after the coarse counter has counted down to zero, from each subsequent transmit pulse to the time for the return pulse. Once the coarse counter has finished counting, the fine counter, which can be reset and reloaded during the look responsive to each transmit pulse, can provide return targets repeating at the same distance at the pulse repetition frequency. A cyclic counter 24 can also be added that produces the return pulses by cyclically counting the interpulse period one the fine counter has timed out. If the cyclic counter is used the fine counter is not reloaded during the look. If the target moves more than the range resolution of the radar during the radar look, a phase-in counter 70 and secondary return counter 66 adjust the delay of the return pulses to simulate target movement during the radar look.

Removal of aliasing in pulse‐to‐pulse Doppler radar measurements

Abstract: Frequency aliasing is a problem often encountered in pulsed radar studies of some deep targets, such as the radar aurora. It is possible to resolve the aliasing ambiguities by transmitting closely spaced pulse pairs, rather than single pulses. Instead of the usual power spectrum, we compute a cross spectrum of the received signals. The amplitude of this cross spectrum is just the normal, possibly aliased, power spectrum, and the phase determines the aliasing, if any, at each frequency bin. The technique works as long as the spectrum is not seriously wrapped around upon itself, that is, as long as each frequency bin is dominated by signals with a single Doppler shift, no matter what that Doppler shift may be. The price paid for this extra information is the addition of radar clutter, uncorrelated signals from unwanted ranges, but this price may be acceptable.

Method of assessing radar target pulses

Abstract: A method of assessing radar pulses in a passive, i.e. receive-only, radar system to distinguish pulses due to a particular target from pulses due to other targets from noise, interference, etc. Received pulses are assessed, without reference to pulse transmission information, in respect of a number of characteristics by comparing each characteristic of a pulse with the mean value of the respective characteristic for pulses previously received. Using a probability process, confidence values for the pulse can be calculated. By combining the confidence values for each characteristic, an overall confidence value is obtained for the pulse so that it can be determined whether it arises from the particular target.

Weapon training systems

Abstract: A weapon simulator, particularly for simulating small arms, comprises a laser projector for attachment to the weapon. Firing the weapon initiates the production of a narrow, pulsed, beam by the laser, and this beam is scanned vertically downwardly while its pulse repetition frequency (PRF) is varied as a function of scan angle. The weapon/ammunition type can also be encoded in the laser pulses. The beam is received by a spatially diverse pair of detectors on the target, typically comprising a first detector having an annular entry aperture covering about 6 cm in the vertical direction, and a second detector disposed in the center of the annular entry aperture of the first. The central detector effectively determines the width of the beam, thus permitting the range from the weapon to the target to be computed from the beam width and the difference in the prf detected at the start and finish of the illumination of the first detector. The elevation angle of the weapon with respect to the target is computed from the mean prf detected by the central detector. Finally, the accuracy of aim of the weapon (i.e. whether the firing resulted in a hit or a miss) is determined from a combination of the range, the weapon elevation angle, and the weapon/ammunition type.

Digital gate generation for a signal measurement instrument

Abstract: A method of digitally controlling the gate for a timing counter, to open and close the gate based on the occurrence of signal events, rather than on the envelope of the pulse. In a particular embodiment, a digital divider controls the gate, so that, when a pulse burst of RF is encountered, the gate opens on the second signal event. The divider can be programmed to close the gate any number of signal events later. Measurements are taken for an integral number of signal events, while counting time events from a precision clock. A series of measurements can be taken with various integral numbers of signal events for frequency profiling. By incrementing the digital divider from n to n+1 signal events for successive measurements, and subtracting the results, very narrow gates are effectively generated which move through the pulse cycle by cycle for frequency profiling. Because the opening of the gate is triggered by the IF events themselves, the gate is reliably and accurately positioned with respect to the pulse, and is unaffected by jitter in the pulse repetition interval.

Radar target velocity estimator

Abstract: A radar target velocity estimator apparatus and method for computing a radial velocity of radar targets from differences in Doppler frequency shift between pulse-returns of multi-pulse waveforms The velocity estimator uses Doppler frequency shift which is obtained from a finite impulse response (FIR) filter in combination with logarithm tables stored in read-only-memory (ROMs) to calculate the target's velocity The estimation process requires the calculation of each complex FIR filter value twice during a pulse repetition interval; once for returns of a leading set of radar pulses and then for a trailing set The estimated velocity is proportional to the phase difference between each corresponding pair of filter values The estimate is a function of the arctangent of the quotient of the in-phase component of the complex value divided by the quadrature component

New class of pulsed Doppler US ambiguity at short ranges.

Abstract: A form of range ambiguity in Doppler ultrasound has been identified. It occurs when a high pulse repetition frequency (prf) is used with a superficial range gate. "Phantom" range gates are then generated at depths beyond the desired range gate, and these may produce artifactual signals. Extremely high prf values should not normally be used, and duplex machines should be programmed to display phantom range gates wherever they fall within the displayed field of view.

Second time around radar return suppression using PRI modulation

Abstract: A technique for suppressing second-time-around radar returns using pulse-repetition interval (PRI) modulation is presented and analyzed. It is shown that a staggered PRI radar system can offer considerable improvement over a nonstaggered radar system in rejecting second-time-around returns which cause false alarms. This improvement is a function of detector implementation (noncoherent integrator or binary integrator), the number of staggered PRIs, the quiescent false alarm number, the Swerling number of the false return, the transmitted signal power, the second-time-around noise power, and the quiescent noise power of the radar. Small changes in transmitted signal power can be traded off with the quiescent false alarm number to suppress the bogus return significantly. In addition, for a noncoherent integrator, all other parameters being equal, if the second-time-around return is a Swerling case II or IV target, then there is an optimum number of staggered PRIs that can be chosen to minimize the likelihood of its detection. It is also shown that the binary integrator significantly reduces the number of second-time-around return detections when compared with the noncoherent integrator. However, there is an accompanying loss of detection. >

The ETA-II Induction Linac as a High-Average-Power FEL Driver

Abstract: The Experimental Test Accelerator II (ETA-II) is the first induction linac designed specifically to FEL requirements. It is primarily intended to demonstrate induction accelerator technology for high-average-power, high-brightness electron beams, and will be used to drive a 140 and 250 GHz microwave FEL for plasma heating experiments in the Microwave Tokamak Experiment (MTX) at LLNL. Its features include high-vacuum design which allows the use of an intrinsically bright dispenser cathode, induction cells designed to minimize BBU growth rate, and careful attention to magnetic alignment to minimize radial sweep due to beam corkscrew. The use of magnetic switches allows high-average-power operation. At present ETA-II is being used to drive 140 GHz plasma heating experiments. These experiments require nominal beam parameters of 6 MeV energy, 2 kA current, 20 ns pulse width and a brightness of 1x10/sup 8/ A/(m rad)/sup 2/ at the wiggler with a pulse repetition frequency (prf) of 0.5 Hz. Future 250 GHz experiments require beam parameters of 10 MeV energy, 3 kA current, 50 ns pulse width and a brightness of 1x10/sup 8/ A/(m rad)/sup 2/ with a 5 kHz prf for 0.5 s. In this paper we discuss the present status of ETA-II parameters and the phased development program necessary to satisfy these future requirements.

High pulse repetition frequency radar early warning receiver

Abstract: A ten db coupler receives an RF input signal and transmits an attenuated signal through a wideband/narrow band select switch to an RF amplifier. The signal is next processed through an RF amplitude detector. A comparator then receives the signal and generates a pulse in response to signals exceeding a predetermined threshold. A pulse stretcher then eliminates excessive pulses due to noise or other signal oscillations that exceeded the comparator's threshold. A circuit comprising a retriggerable monostable and an OR gate determine if the pulses received exceed a pulse repetition frequency threshold. A ripple counter than eliminates unwanted signals by resetting if a predetermined number is not reached in a specified time. A unique audio signal is then generated if the desired signal is detected. The output signal is audible even though the input signal is of a frequency above the normal hearing threshold. Also a visual signal is provided by a light emitting diode.

Constant amplitude doppler producing radar reflector

Abstract: A constant amplitude Doppler producing radar reflector for simulating a moving target. A plurality of dihedral reflectors having equal radar cross sections are rotated axially at equal rotational speeds. The dihedral reflectors are separated laterally and in the direction towards the illuminating radar by a distance which keeps the radar returns from the two reflectors in phase quadrature. With the radar cross sections of the reflectors being equal, the return signals to a linearly polarized radar maintains constant amplitude as the dihedral reflectors rotate.

Pulse doppler radar system

Abstract: A pulse doppler radar system for measuring the range to a target. The radar system alternately transmits a signal having a constant frequency and a signal modulated by a linearly-changing-frequency and receives an echo signal from a target. When a plurality of doppler frequencies derived from jet engine modulation are detected in the echo signal, the radar system is operable to obtain the number of frequency differences giving the same range value on the basis of the differences between the detected doppler frequencies, or to selectively use methods of range calculation according to the number of detected doppler frequencies.

A vertical beamforming design approach for increased area coverage rate for synthetic aperture sonar

Abstract: The authors present a design approach for increasing the area coverage rate for a bottom mapping synthetic aperture sonar (SAS). The approach uses dual acoustic data channels which are established by vertical beamforming from a single phase-shifted transducer array. An SAR (synthetic aperture radar) image quality criterion known as the ambiguity ratio, which affects image contrasts, is applied to the SAS problem to establish design constraints for beam patterns and pulse repetition frequency. Relationships between transducer array size, spacing of individual elements of the array, sonar Doppler sampling rates, and geometry parameters are presented for an illustrative data collection scenario. Achievement of increased area coverage rate by a factor of 3 is demonstrated within the constraints imposed by the image quality performance criterion. >

Amplifier having suppressed spurious frequency components

Abstract: A circuit for the suppression of spurious frequency components for use in cascaded class C common base transistor amplifiers. The circuit uses a capacitor and shunt diode connected in series between emitter and ground of each stage to slow the pulse falltime and thereby eliminate spurious frequencies resulting from parasitic-caused ringing. The cathode-to-capacitor junction is pulsed through an RF choke at the pulse repetition frequency by a pulse generator. This effectively replaces the fast trailing edge of the RF pulse with the slower turn-on time of the PIN diode circuit. The resulting slower falltime eliminates spurious frequency production that normally occurs at each stage of cascaded amplifiers.

Processing of concatenated radar measurements to re-establish signal phase coherence

Abstract: A method and apparatus for re-establising frequency, phase and amplitude coherence for radar samples generated by mutiple frequency transmitter pulse trains. A plurality of transmitter means 10-12 produce pulses of different frequencies and an antenna 22 and waveguide 24 direct the pulses 26 towards the aircraft 30. The reflected radar signal 32 returns to the antenna 22 l and waveguide 24 and is measured by the receiver 14. The reflected radar signal is then frequency rescaled, phase realigned, and amplitude normalized by the computer processor 18 and output to a processor/display unit 20.

Synthetic aperture radar

Abstract: PURPOSE:To enable normal operation even in an invisible area or regardless of a change in height larger than expected, by detecting a reception timing at each action to generate a transmission pulse repetition frequency (PRF) corresponding to a change in height of a platform CONSTITUTION:An synthetic aperture radar is provided with a reception timing detector 1, a PRF control section 2 and a transmitting section 3 A reception timing signal outputted from the reception timing detecting section 1 is inputted into a reception timing discriminator section 21 of the PRF control section 2 to be compared with a reception timing corresponding to a height at which a PRF must be changed A PRF generating section 22 is controlled by the output thereof to output a transmission trigger corresponding to the height of the platform A transmission pulse is outputted from a transmitter 3 by the transmission trigger

Frequency modulated, phase coded radar

Abstract: A radar ranging system is disclosed which employs a frequency modulated and phase coded transmission signal which can have up to a 100 percent duty cycle and which performs time tracking of the radar target and does not require extreme accuracy in frequency modulation, or extreme receiving antenna to transmit antenna isolation.

Doppler flow rate meter

Abstract: Doppler measuring device for measuring flow velocity of a fluid (11), the particles (12) carries the ultrasonic waves reflect, which device contains the following parts: an ultrasonic transducer (14) for irradiating the fluid with ultrasound wave pulses in response to corresponding transmit pulses that are supplied thereto at a predetermined pulse repetition frequency, for receiving the light reflected from particles in the fluid echo waves, and for delivering a corresponding echo signals, one with the ultrasonic transducer (14) associated transmitter (15) for generating transmit pulses with which the ultrasound transducer for emitting ultrasonic wave pulses is excited, one with the ultrasonic transducer (14) receiver (16) connected for receiving and processing of echo signals corresponding to at least two groups of echo waves, which are reflected by particles in the fluid m response to a first and a second wave pulse transmitted, but adjacent each other, separate frequency bands of the echo signals in each a separate signal processing path (21, 22) are processed, and one with the output of the receiver (16) verbunjenen evaluation unit (18) with which an output signal is derived from the recovered with the receiver Doppler information corresponding to the flow velocity, To increase the sensitivity of the device the transmitter (15) is arranged so that the frequency spectrum of the periodic sequence of transmit pulses consists of two adjacent but separate frequency bands-is.

Low-threshold erbium glass minilaser

Abstract: A new chromium ytterbium erbium laser glass LGS-Kh was used for the first time in minilasers emitting at 1.54 μm and characterized by a very low (less than 5 J) threshold and a relatively high (up to 1.7%) efficiency. An average output power of 0.7 W was obtained at a pulse repetition frequency of 7 Hz.