Abstract: A process and apparatus to automatically determine whether a police radar is installed in a vehicle which has a coupling between a vehicle speed sensor and the police radar. The process automatically, without any operator input, determines the correct ratio between the true ground speed and the frequency output by vehicle speed sensor at that ground speed. The process and apparatus then use that ratio and the frequency from the vehicle speed sensor to establish a software search window that limits the speed range of the search by the police traffic radar for the true ground speed.

Abstract: A transceiver assembly is provided for use in a Doppler-based traffic radar system for determining the speed of at least one target. The transceiver assembly includes a cover for receiving therein at least some of the component parts of the transceiver assembly. In other words, the cover is positioned over the component parts of the transceiver assembly which are enclosed by a cap removably attached to the cover. A conical antenna horn and turnstile waveguide cavities are formed from a unitary material. The circuitry associated with the transceiver assembly is modularized such that the circuitry may be removed from the transceiver assembly, or more specifically separated from the microwave components of the transceiver assembly, for testing.

Abstract: The present invention is a method of deriving a ground speed of an aircraft on a descent along a flight path. A vertical speed signal is produced as a function of an altitude signal and a vertical acceleration signal. The vertical speed signal is transformed to a nominal ground speed signal based upon a glide slope defined by a glide slope beam. A correction is produced based on a glide slope deviation rate representative of deviation of the aircraft from the glide slope. The nominal ground speed signal is corrected with the correction to produce a corrected ground speed signal. The corrected ground speed signal is filtered with a horizontal acceleration signal and a runway heading signal to produce a smoothed ground speed signal.

Abstract: A method and apparatus for measuring the parameters of atmospheric turbulent flows utilizes the Doppler shifted frequencies of received radar signals backscattered from sound generated aerodynamically by atmospheric turbulent flows. Doppler frequency bandwidths of the received backscattered signals are used to estimate the atmospheric flow turbulence and the mean frequency within a bandwidth is processed to estimate its radial flow velocity. Total flow velocity and the flow velocity angle with respect to the antenna boresight of the atmospheric turbulent flow may be estimated by estimating the radial flow velocity at two radial positions and processing these radial velocities. Processing of the Doppler data is initiated when the total signal power within the Doppler frequency band exceeds a predetermined power level.

Abstract: A method and a radar sensor for determining an elevation angle error of a multibeam radar sensor are described. In order to detect an elevation angle of the multibeam radar system with respect to a predefined target, a plurality of laterally arranged cutting planes at a predefined distance are formed. The values of the corresponding antenna diagrams in each plane are stored in a suitable form, for example, normalized and in a parametric form taking into account the elevation angle &agr;. In order to reduce the size of the memory, it is sufficient to store one symmetry half for reasons of symmetry if additional information, for example, road clutter values are added, so that the upward or downward direction of the angle can be recognized. By comparing the measured echo values obtained by normalization and application of a quality factor, a corresponding elevation angle &agr; is obtained for each cutting plane.

Abstract: In a mobile communication system, signals which are transmitted from mobile stations moving relative to a base station are subject to a Doppler effect. A technique is described for compensating for that Doppler effect in the received signal, depending on the channel conditions for the received signal. Thus, the Doppler shift compensation is implemented only or mainly in good enough channel conditions.

Abstract: A method is described for detecting moving and/or stationary objects in the path of a vehicle, where the distance and velocity of the reflecting object are determined by using a radar sensor, where the frequency of the signal transmitted is modulated in the shape of a ramp according to a multi-ramp method, and some ramps have a lower slope in the frequency curve. The higher spectral components of the mixed signals from the ramps having the lower slope, which are to be assigned to a distance range of the reflecting object which is greater than the distance range belonging to the ramps having the greater slope, are used to determine the distance, speed and angle of objects.

Abstract: A Doppler-based radar system and related method are provided for determining the direction and speed of at least one selected target traveling in the same lane as a moving patrol vehicle supporting the radar system independent of the direction of the target relative to the platform. The radar system includes an oscillator to generate a signal, an antenna to transmit the signal toward the at least one target and to receive a return signal reflected from the at least one target, a turnstile in communication with the antenna for receiving the return signal and forming processing signals which are different in phase, and circuitry for determining the direction of the at least one target relative to the platform and the speed of the at least one target dependent upon a mode of operation of the radar system.

Abstract: A police radar and laser detector with networking capability is disclosed. By including the ability to wirelessly transmit the existence or even location of a detected radar or laser source, overall performance can be improved. Transmission of an actual radar or laser location gives other detectors advanced warning. Additionally, transmission of false alarm locations will allow other detectors to reduce false alarms.

Abstract: A system is disclosed that can determine the speed of a golf club or golf ball, and report that speed in a format suitable for use on a television broadcast, a radio broadcast, the Internet or another medium. In one embodiment, the system includes a set of radars pointed toward the golf ball. Data from the radars is collected and sent to a computer which can determine the speed of the club during a swing, the speed of the ball after being hit, the angle of the path of the ball and/or other related statistics.

Abstract: A communication control apparatus installed in a base station or a mobile station in mobile communications system estimates the relative moving speed of an opposing station, and adjusts the values of communication parameters for a searcher, a transmission power control unit, an absolute synchronous detector unit, etc. according to an obtained moving speed.

Abstract: A radar device in which a much more precise demodulation can be made with an inexpensive circuit, free of the output variation of the frequency-modulated transmission signal. The radar device includes an oscillator which produces and outputs a frequency-modulated electromagnetic wave to a switching circuit by way of a directional coupler. The switching circuit uses switching modulation to radiate the electromagnetic wave from a transmitting antenna. The radiated electromagnetic wave after being reflected from an obstacle is received at a reception antenna. A mixer mixes this reception signal and a local signal fed from the directional coupler to produce a mixing signal which is to be fed to a high pass filter. The high pass filter deletes a low frequency output variation noise which is contained in the mixing signal and which occurs in the frequency modulation in the oscillator. The resulting mixing signal is, with the noise deleted, fed by way of an AC amplifier, to a switching demodulating circuit.

Abstract: A detecting apparatus of vehicle speed including detecting means for emitting an electromagnetic wave or an ultrasonic wave to a road surface and detecting traveling speed of a vehicle from the Doppler effect produced in a reflected wave thereof, a transmitting circuit for sending information about the traveling speed detected by the detecting means by radio, a power generation mechanism for generating power by vibration produced in the vehicle, and a charging circuit for charging a charging type battery by an output voltage of the power generation mechanism.

Abstract: The invention concerns a method and a device to measure the speed (v) of a vehicle relative to a road surface. To increase the accuracy of the speed measurement and in order to be able to determine the speed (v) of the vehicle independently of the diameter and the adhesion of a vehicle wheel on the road surface, the invention proposes that the speed (v) of the vehicle is measured directly on the road surface by utilizing the Doppler effect.

Abstract: A police activity transponder utilizes data available through a vehicle on-board diagnostic system or a intelligent vehicle data bus to adjust the sensitivity of the police activity transponder to law enforcement signals based on speed data. A police activity transponder is configured to read speed data available through an on-board diagnostic system or an intelligent vehicle data bus and using an internal clock, calculate a 0 to 60 mile per hour or a quarter mile time for display and function as a speedometer. A police activity transponder is configured to read engine revolutions per minute (rpm) data and function as a tachometer. A police activity transponder is configured to allow a user to input shift points, read engine rpm data, and provide a user indication.

Abstract: A road antenna apparatus includes a road antenna 104 which is mounted on a post 103 and at an elevated position on a road R and establishes radio communication with an on-vehicle radio device 102 mounted in a vehicle 101 which is traveling over the road; and a laser-beam emitting device 111 which is mounted on the road antenna and radiates a laser beam onto a predetermined position 113 on the surface of the road. An offset in the angle at which a road antenna is mounted can be readily ascertained on the basis of a distance between a predetermined position on the surface of the road and a position 114 on the road surface onto which a laser beam is actually radiated.

Abstract: System and methods are disclosed for fluid measurements which may be utilized to determine mass flow rates such as instantaneous mass flow of a fluid stream. In a preferred embodiment, the present invention may be utilized to compare an input mass flow to an output mass flow of a drilling fluid circulation stream. In one embodiment, a fluid flow rate is determined by utilizing a microwave detector in combination with an acoustic sensor. The acoustic signal is utilized to eliminate 2 &pgr; phase ambiguities in a reflected microwave signal. In another embodiment, a fluid flow rate may be determined by detecting a phase shift of an acoustic signal across two different predetermined transmission paths. A fluid density may be determined by detecting a calibrated phase shift of an acoustic signal through the fluid. In another embodiment, a second acoustic signal may be transmitted through the fluid to define a particular 2&pgr; phase range which defines the phase shift.

Abstract: A Doppler-based traffic radar system determines a speed of a moving target from a platform while substantially eliminating the possibility of detection by an operator of the moving target utilizing a traffic radar detector. The radar system includes a controller or digital signal processor for limiting the period of time the system is initially transmitting. The duration of the initial transmission period is limited such that presently available commercial radar detectors are substantially unable to detect and/or elect not to report the presence of the radar signal. Specifically, the radar system is adapted to monitor the number of accumulated digital samples from a reflected portion of the radar signal. Upon receipt of a number of samples sufficient to determine the speed of the target, the radar system ends the initial transmission. Once the samples are obtained, the radar system determines the speed of the target and displays the speed.

Abstract: A Doppler-based radar system and related method are provided for determining the direction and speed of at least one selected target traveling in the same lane as a moving patrol vehicle supporting the radar system independent of the direction of the target relative to the platform. The radar system includes an oscillator to generate a signal, an antenna to transmit the signal toward the at least one target and to receive a return signal reflected from the at least one target, a turnstile in communication with the antenna for receiving the return signal and forming processing signals which are different in phase, and circuitry for determining the direction of the at least one target relative to the platform. In particular, samples of the processing signals are transformed into the frequency domain and cross-correlated forming cross-correlation components.

Abstract: A radar apparatus mounted on a vehicle includes a detecting section and a fault determining section. The detecting section includes a radar unit and detects a detection object using radar wave radiated from a radar unit toward the detection object and reflected radar wave from the detection target to the radar unit. The fault determining section determines whether any fault has occurred in the radar unit, based on the detecting result of the detection object and a movement distance of the vehicle, and generates a fault detection signal, when it is determined that any fault has occurred in the radar unit.

Abstract: A series of police doppler single mode radars and a multimode police doppler radar, all with direction sensing capability are disclosed. A quadrature front end which mixes received RF with a local oscillator to generate two channels of doppler signals, one channel being shifted by an integer multiple of 90 degrees in phase relative to the other by shifting either the RF or the local oscillator signal being fed to one mixer but not the other. The two doppler signals are digitized and the samples are processed by a digital signal processor programmed to find one or more selected target speeds. Single modes disclosed are: stationary strongest target; stationary, fastest target; stationary, strongest and fastest targets; moving, strongest, opposite lane; moving, strongest, same lane; moving, fastest, opposite lane; moving, fastest and strongest, opposite lane; moving, fastest, same lane; moving fastest and strongest, same lane.

Abstract: A true predicted area of danger providing a preselected miss distance between an intruding vehicle and own ship is provided. A miss distance circle, having a radius equal to the desired miss distance, is generated about the initial position of the intruding vehicle and the relative coordinates and distances from own vehicle of selected points on this miss distance circle are determined. Points of own vehicle intercept of these selected points are established and a smooth curve is drawn through these points to provide the boundary of the predicted area of danger.

Abstract: A Doppler-based traffic radar system determines a speed of a moving target from a platform while substantially eliminating the possibility of detection by an operator of the moving target utilizing a traffic radar detector. The radar system limits a period of time the system is initially transmitting. The radar system is adapted to determine the speed of the target and display the speed. Dependent upon the displayed rate of speed, the radar system operator further has the ability to halt operation of the radar system with respect to the particular target or to reinitiate transmission of the transmit signal for subsequently tracking the speed of the moving target.

Abstract: The combination of a global positioning satellite system (GPS) and a radar detection unit, in wireless communication with the GPU, for tracking and determining the speed of a vehicle. The system may be manually activated, or more preferably activated by an external source of radar signals, such as may be emitted by a police “speed trap”. The unit includes means for recording and storing speed data of the vehicle, and to alerting the operator of the vehicle to a “speed trap” situation.

Abstract: A high resolution spectral estimator (HREE) filter coupled to a spectral plotter processes either Doppler frequencies provided from the output of a pulse-Doppler radar or a frequency based output provided by a Fourier transformer coupled to a sensing device to allow the spectral plotter to determine the power frequency spectrum of either the pulse-Doppler radar output or sensing device output. The HREE filter preferably comprises a bank of first order filters tuned to a pre-selected frequency, a covariance estimator coupled to the filter bank for estimating filter covariances, and a decoder coupled to the covariance estimator for producing a plurality of filter parameters. Further, it is preferable that the filters comprising the filter bank be adjustable to permit their being tuned to a desired frequency based on a priori information.

Abstract: The present invention relates to a CW radar method for measuring distances between and relative speeds of a vehicle and one or more obstacles. The present invention further provides that the transmission (s(t)) can be composed of at least four consecutive chirps (A, B, C, D), each having different slopes. The intersection points of all lines in the distance-relative speed diagram from two chirps (A, B) can be calculated from all the ascertained frequency positions K1,n and K2,p. To validate those intersection points, one may observe whether a peak exists in the Fourier spectrum of a third chirp C at a frequency position K3,q, whose assigned line intersects a surrounding area of the intersection point in the distance-relative speed diagram.

Abstract: A microstrip antenna radiates and receives six beams in sequence out of a common aperture and processes return signals with common electronics in order to measure the velocity components relating to a backscattering surface. Sequentially, the first four beams are processed by measuring Doppler shift in a return signal while the last two beams are processed for time correlation. The result of the present invention is a combined time correlation and Doppler measurement velocity sensor available for operation over virtually all terrains and the vehicle speed range.

Abstract: A Doppler radar transceiver in a traffic monitoring system includes a pyramidal horn antenna having a rectangular aperture and a dielectric lens mounted at the rectangular aperture. Preferably the pyramidal horn antenna is corrugated and the rectangular aperture has a breadth at least twice its width in order to provide a fan-shaped radar beam most effective for detecting moving vehicles in a stream of traffic. To reduce standing waves in the horn antenna that would otherwise cause nulls that would intermittently interfere with detection of a Doppler signal, the dielectric lens has impedance matching means for matching impedance of the dielectric lens to impedance of free space to reduce standing waves in said horn antenna. Suitable impedance matching means include a quarter-wave transformer or a tapered transformer structure at the interfaces between the dielectric body of the lens and the free-space region inside the horn and outside of the horn.

Abstract: AN FM pulse Doppler radar apparatus performs pulse modulation of modulating waves having repeatedly increasing and decreasing frequency, transmits thus modulated waves, receives at each range gate having an interval equivalent to a pulse width, reflected waves reflected from an object, determines a distance according to the range gate, and calculates the distance to the object and the relative velocity of the object based on the difference between frequencies of the transmission waves and the received waves. The apparatus includes a velocity determining unit for determining velocity of the radar-mounted vehicle and a comparison-and-detection unit for comparing the detected distance according to the range gate and the distance calculated based on the difference between the frequencies of the transmission waves and the received waves.

Abstract: The present invention discriminates between moving targets and stationary targets in order to measure distances and speeds of the targets without producing a false target. The invention transmits signals whose modulating frequencies ascend or descend to a target, and receives signals reflected from this target. A frequency spectrum of a beat signal of each target is detected during a modulating frequency ascent period and a modulating frequency descent period. The frequency spectrum of the modulating frequency ascent period and the frequency spectrum of the modulating frequency descent period are used to set a reference spectrum. The reference beat frequencies of either the moving targets or the stationary targets are detected based upon the reference spectrum.

Abstract: A moving speed detecting method of a mobile radio terminal which makes communication with a base station outputting a pilot signal includes a step of predicting phases and amplitudes of the pilot signal from the base station by using a plurality of prediction methods having different characteristics. A calculating step calculates errors for the prediction results, which are obtained for respective prediction methods in the prediction step, and a prediction error comparing step compares the prediction errors with respect to each of the prediction methods, which are calculated by the calculating step, to detect a moving speed of the mobile radio terminal.

Abstract: A radar system mounted on a vehicle is capable of properly measuring an angle even when a linearity is impaired without increasing a scale of hardware and a load on signal processing, and exhibiting a high performance at a low cost.

Abstract: A Pulse-Doppler radar apparatus is constructed such that an output of an oscillator is divided by a distributor and an on/off switch for generating pulses is coupled to an IF input of a first harmonic mixer, in order to improve precision in measurement of a distance and speed.

Abstract: A detecting apparatus of vehicle speed including detecting means for emitting an electromagnetic wave or an ultrasonic wave to a road surface and detecting traveling speed of a vehicle from the Doppler effect produced in a reflected wave thereof, a transmitting circuit for sending information about the traveling speed detected by the detecting means by radio, a power generation mechanism for generating power by vibration produced in the vehicle, and a charging circuit for charging a charging type battery by an output voltage of the power generation mechanism.

Abstract: A stop determination for a vehicle is provided by requiring simultaneous stop determinations by at least one wheel speed sensor providing output pulses at a rate inversely proportional to vehicle speed and a forward looking radar unit for sensing an object in the vehicle's path and providing a range rate signal with respect to that object. The determination is particularly useful when the time between consecutive pulses at a predetermined minimum speed indicative of an essentially stopped vehicle is greater than a predetermined maximum time period allowed for the determination. Each of the signals complements the other to provide a vehicle stopped determination that is more reliable than either can provide by itself and is workable for most purposes, including stop and go vehicle speed control.

Abstract: A method and a device for non-invasively and remotely detecting or monitoring visible or invisible concealed moving bodies and objects (9) located in scrutinized media (8), by means of tracking the RF impedance match variations between the inspected media which contains the targeted moving object and a stable RF transmitter (1), through surrounding the inspected media with a power field of HF EM-waves, by one or more transmitting antennas (6) made from metallic sheets or wires.

Abstract: A control arrangement for an adjustable housing of a radar sensor has a holder on which a position of the housing is changeable by at least one adjusting screw which is turnable by a regulatable adjusting drive, and a control electronic unit provided for adjustment, via which said signals produced by the radar signal are evaluated or adjusting data stored in a storage are read, supplied to the corresponding adjusting drive and the position response command signals are transmitted to the corresponding adjusting drives.

Abstract: A system for rapidly acquiring a time reference for the location determination of a wireless communication device. The system includes a wireless communication device (200), a GPS satellite (202) and a communication satellite (208). The method comprises the steps of acquiring a communication satellite signal (210) and using the finite frame time to establish a course time reference. Once the course time reference is established, an absolute device time is determined by the wireless communication device, which as a result is synchronized to the absolute time of the communication satellite (208) to within ten milliseconds. The absolute device time is then used to synchronize the GPS portion of the wireless communication device with the GPS satellite system.

Abstract: Method and device for calculating a velocity of a moving transmitter using Rayleigh-fading of a received radio signal are presented. The method includes the step of making at least two parallel calculations of at least two respective values using the received radio signal. The at least two respective values resulting from the calculations are then used as the basis for making a decision. The decision results in an optimal value for calculating the velocity of the moving transmitter being chosen from among the at least two respective values. Each of the at least two parallel calculations is carried out by respective devices each having at least one filter and at least one level crossing counter.

Abstract: The present invention relates to a method for correction of a signal which is derived from wheel rotation speed signals and represents the vehicle speed, wherein the vehicle has a device by means of which the environment of the vehicle can be identified, and wherein the correction of the signal which is derived from the wheel rotation speeds and represents the vehicle speed is carried out on the basis of the speed at which objects in the environment which are identified as being stationary move relative to the vehicle.

Abstract: An electronic counter-measure receiver can be colocated with a threat simulator radar, which transmits a radar signal. As a pulsed-Doppler signal, this transmitted radar signal is sampled as a reference signal by the receiver to determine quadrature, coherent matched filter coefficients. A return signal is received and processed with the reference signal as quadrature signal components with the quadrature, coherent matched filter coefficients within a matched filter. The return signal is processed as a first intermediate frequency via a mixer and local oscillator and a second quadrature intermediate frequency via a digital local oscillator and mixer. A fast Fourier transform is performed to determine Doppler information. In a continuous wave transmission for sampling arbitrary but unevenly spaced in time pulses are generated from a pulse generator source internal to the receiver.

Abstract: A system is disclosed that can determine the speed of an object and report that speed in a format suitable for use on a television broadcast, a radio broadcast, the Internet or another medium. One example of a suitable use for the present invention includes determining the speed that a baseball player swings a bat. Another use of the present invention is to measure the speed of a moving ball. In one embodiment, the system includes a set of radars positioned behind and pointed toward the batter. Data from all of the radars is collected and sent to a computer which can determine the start of a pitch, when a ball was hit, the speed of the ball and the speed of the bat.

Abstract: A timing and control method and apparatus (111) for performing precise range rate aiding includes a range gate delay means (114) for generating an estimate of the range gate delay (135) each pulse repetition interval as a function of the initial range (134) and velocity (133) provided by a processor (104). The range gate delay (135) is converted into a coarse delay (138) defining the integral number of clock cycles preceding the range gate, and a fine delay (139) for positioning a range gate to within a fraction of a clock cycle. Fine temporal control is achieved using programmable delay lines (117) and (118), which retard various control signals, including the system clock signal (131), in accordance with the fine delay (139). A modified signal (126) then drives a counter means (119) which outputs a signal (128) that defines an analog-to-digital sampling window beginning at the elapse of the range gate delay (135).

Abstract: The invention relates to a radar method for an automatic intelligent traffic control (AICC) in a motor vehicle. The use of a frequency modulation continuous wave method (FM-CM) is suggested in order to securely detect the distance to, relative speed and angle of a preceding motor vehicle. It is furthermore suggested according to the invention that when using an A/D converter 5 with 8-bit resolution, the necessary dynamics are generated by means of a level switchover, that the R, V information is generated in FFTs [Fast Fourier Transformations] 6 with blocked R and V-FFTs, that the useful signals are separated from the noise in a detection device 7 by means of a R-dependent adaptive CFAR threshold, that in a track formation 8, the detection is directly assigned to the tracks and that the association of a detection i to a track j is in the process computed as probability r (i, j).

Abstract: A method and apparatus for generating microwave signal frequencies. An incident reference signal is provided. A first stimulus signal is also provided, the first stimulus signal having a first polarization and having a first predetermined relationship with the incident reference signal. A second stimulus signal is also provided, the second stimulus signal having a second polarization and having a second predetermined relationship with the incident reference signal. The incident reference signal is split into a first polarization reference signal and into a second polarization reference signal. The first stimulus signal is coupled with the first polarization reference signal to provide first polarization mixed signals. The second stimulus signal is coupled with the second polarization reference signal to provide second polarization mixed signals.

Abstract: A radar system is described for use in vehicular applications. The radar system is particularly suited to backup warning systems and lane-change warning systems. The radar minimizes many of the problems found in the prior art by providing programmable delays and programmable gain. The radar uses a range search algorithm to detect and sort targets at various ranges within the field of view of the radar. Each target range corresponds to a particular delay and gain setting. The radar searches for targets at the various ranges by running a target search algorithm. For each target range, the search algorithm causes the proper time delay and gain setting. Targets within the selected range are detected and catalogued. Speed of the targets is obtained through Doppler processing. A display is used to warn the driver of the vehicle of the presence of targets at the various ranges. The warning may be visual and/or audible.

Abstract: Characteristics of a target are measured by a radar or sonar. Pulses (101, 102, 103) are transmitted and in between (X) the transmissions of pulses a signal is received which depends on the transmitted pulses and on the distribution of the characteristics measured at different ranges. The distribution at different ranges of the characteristics measured is determined by representing it by means of a substantially linear system of equations in which the variables are the values of the characteristics measured at desired ranges, and by solving the system of equations for the variables. The transmitted pulses form a cyclically repeated pulse code or a continuously changing pulse train.

Abstract: The invention particularly relates to the detection of fast-flying targets by means of an HPRF radar system that operates with a plurality of switchable pulse-repetition frequencies (PRFs). In the method, a high velocity resolution is attained, which permits a reliable detection of a multiple-target situation. At the same time, a precise range determination is attained with a high range resolution by means of a pure transit-time measurement of the pulses. The length of the used range gates is selected to correspond to the anticipated target length.

Abstract: Correction for compensating for errors caused by the pitching movement of a vehicle is executed with high accuracy at low cost. The ground speed of the vehicle sensed by a Doppler sensor is corrected with angular velocities with respect to the center of gravity of the vehicle. In this correction, the center of rotation of the vehicle and the rotation of the Doppler sensor itself during the pitching movement of the vehicle are taken into account.

Abstract: A radar system adapted to be installed on a vehicle (1) is operable to detect objects in different fields of view of the system. The system has transceiver means (2, 3; 44, 46) for transmitting a radar signal and receiving the signal after the latter has been reflected from an object to be detected. A reflected signal is sampled by sampling means (12; 66) during a succession of sampling periods each of which is delayed with respect to a corresponding portion of the transmitted signal. The gating of the reflected signal effectively generates range shells, each of which corresponds to the time taken for a transmitted signal to reach an object at the shell, and to be reflected back to the system and is hence related to the delay between the transmission of the signal and the corresponding sampling period. The range shells define the field of view of the system, which can be changed by altering said delay.