Abstract: The present disclosure relates to a array radar system. More particularly, to a technique for determining a ghost target in a radar equipped with a nonlinear array antenna. There is provided a radar apparatus including a receiver configured to receive real non-linear array signals of N channels through N channel non-linear array antennas arranged at mutually different intervals, a signal generator configured to generate virtual non-linear array signals of M channels capable of spacing equal intervals between the adjacent channels together with the N channels based on N real channel non-linear array signals, and configured to generate virtual linear array signals of M+N channels including the real non-linear array signals of N channels and the virtual non-linear array signals of M channels M channels, and a determiner configured to determine a ghost target based on the virtual linear array signals of M+N channels.

Abstract: According to one embodiment, an electronic device comprises a first antenna includes first antenna elements on a first planar substrate, a second antenna includes second antenna elements on a second planar substrate, an orientation of the second planar substrate being different from an orientation of the first planar substrate, and a circuit that forms a first beam pattern using the first antenna elements, forms a second beam pattern using the second antenna elements, and forms a first combined beam pattern using some of the first antenna elements and some of the second antenna elements.

Abstract: A radar apparatus includes: a transmitting portion including transmitter elements that emits a transmitting signal as a radar wave toward an object; a receiving portion including receiver elements receiving a reflection signal; and an estimation portion. The estimation portion generates a virtual receiving array including virtual receiver elements. The estimation portion divides the virtual receiving array into identical sub-arrays. The estimation portion generates covariance matrix data for each of the sub-arrays. The estimation portion averages the covariance matrix data for each of the sub-arrays to decorrelate a direct reflection signal from the object and multipath reflection signal among the reflection signal and spatially smooths the reflection signal. The estimation portion resolves the direct reflection signal from the object and the multipath reflection signal. The estimation portion determines the direction of the object.

Abstract: A method and system is described for using radar to provide non-contact detection of human breathing at a distance. Radar data is processed to remove clutter and enable detection of a breathing signal. The breathing signal can be used to determine breathing rate or posture of the target. Static obstructions can be removed from the radar data to identify target breathing signal. The system and method can be applicable to emergency response, home or institutional monitoring applications and for detection of medical conditions.

Abstract: Disclosed herein are embodiments that relate to phase coded linear frequency modulation for a radar system. Embodiments include transmitting at least one signal pulse with a binary phase shift keying (BPSK) encoding. The method also includes receiving a signal associated with reflection of the at least one transmitted signal pulse. The received signal may include at least two channels. Further, the method may also include processing the received signal to determine target information. The processing may include performing a despread operation that provides a phase offset based on a filtering range. Additionally, the processing may include performing a reconstruction operation that comprises creating a virtual spatial channel based on combining the two received channels. Yet further, the processing may include determining the target information based on the virtual spatial channel. An autonomous vehicle may be controlled based on the determined target information.

Abstract: Methods and systems of determining the altitude of an aircraft are provided. The method includes receiving data associated with aircraft position, a position of a first point and a second point on the runway, and an altitude of the first point and the second point, radar returns from the runway. The method includes determining a first range and second range between the aircraft, and the first point and the second point. The method includes determining a first angle and a second angle between the first point and second point, and the aircraft. The method includes determining a corrected angle. The method includes determining the altitude of the aircraft based on the corrected angle, the runway altitude of at least one of the first point and the second point, and at least one of the first range and the second range.

Abstract: A weather radar system is coupled to a weather radar antenna. The weather radar system includes a processor for combining or summing portions of the radar return data to obtain a null. The processor associates the null with the ground (e.g., steers the null toward ground or processes data so that the null corresponds to ground) to obtain a null to alleviate ground clutter when sensing weather. The null can be a single null or multiple nulls associated with different ranges.

Abstract: Aspects of the present invention relate to a system (10) and a method for tracking one or more targets by a radar using a multiple hypothesis tracking (MHT) algorithm, the method including operating the radar to transmit a radar beam from a first location toward the one or more targets, operating the radar to receive a plurality of return signals at the first location from the one or more targets, and to generate a plurality of observations for a single radar dwell respectively corresponding to the plurality of return signals, and processing the plurality of observations in accordance with the MHT algorithm for at least two passes such that more than one of the plurality of observations are associated with a single track of the one or more targets.

Abstract: A Luneburg lens is used in conjunction with a patch antenna array. The patch antenna array is conformed or adapted to cover a portion or backside of the Luneburg len's surface with the backplane of the conformed antenna array defining a field of regard (FOR) in which objects are detected and tracked. A processor is connected to a receiver/exciter module which connects to transmit/receive modules which are connected to the individual patch antennas through a network of MEMS switches. In a receive mode, selected subarrays of the conformed patch antenna array are scanned during selected time intervals with the sum and delta beams being formed coherently in amplitude and phase to realize amplitude monopulse sensing and angle tracking of an object.

Abstract: Embodiments of a target classifier and method for target classification using measured target epsilons and target glint information are generally described herein. The target classifier is configured to compare a total epsilon measurement with target glint information to determine whether to the target being tracked corresponds to an intended target type. Based on the comparison, the target classifier may cause target tracking circuitry of a target-tracking radar to either continue tracking the target or break-off from tracking the target. Glint of different target types may be characterized at different ranges and the target's glint characteristics may be used to distinguish intended from non-intended targets. Accordingly, intended targets such as incoming artillery may be distinguished from non-intended targets such as aircraft to help prevent countermeasures from being launched against non-intended targets.

Abstract: The present disclosure relates to an active antenna array for a mobile communications network. The active antenna comprises a base band unit coupled to a base station, a plurality of transceiver units and at least one link. The plurality of transceiver units is terminated by at least one antenna element. The at least one link couples individual ones of the plurality of transceiver units to the base band unit 10. The at least one link is a digital link and is adapted to relay an individual transmit signal concurrently and in synchronisation with a transmit clock signal. The present disclosure further teaches a method for relaying radio signals in a mobile communications network. The present disclosure further relates to a computer program enabling a computer to manufacture the active antenna array of the present disclosure and to execute the method of relaying radio signal in a mobile communications system.

Abstract: A multi-band low-profile, low-volume two-way mobile panel array antenna system is described. Operation of the antenna may automatically switch between bands based on various user-entered parameters.

Abstract: A system and method for increasing the effective angular resolution of a multi-function phased array radar system is provided. The system is operative to simultaneously transmit a plurality of overlapping sub-beams covering a representative central beam. A de-convolution process is applied to received return signals. The process includes determining the reflectivity within sub-beamwidth resolution cells defined by the overlapping sub-beams. Generated sub-beamwidth data provides the radar system with an effective angular resolution beyond that of any single transmitted beam.

Abstract: System and methods provide tracking capabilities by one or a plurality of satellites for a mobile terrestrial terminal. A user requests that a satellite track a particular mobile terrestrial terminal. If the user privilege level allows for this level of control, the satellite adjusts to track the identified terminal. One method for tracking involves the use of a steerable antenna in which the antenna steers to maintain a footprint over the identified mobile terminal. Another method for tracking involves moving the satellite itself to maintain a footprint over the identified mobile terminal. The tracking functionality may utilize a closed loop tracking method.

Abstract: A radar sensor for motor vehicles, having an antenna system that can be controlled by a control device so that it has a temporally varying directional characteristic, and having an evaluation device for evaluating the radar echoes received by the antenna system and for the location of objects using angular resolution, wherein the antenna system has at least two groups of antenna elements that differ in elevation in their effective direction, and the control device is fashioned to activate and deactivate the two groups in periodically alternating fashion, and the evaluation device is configured to estimate the elevation angle of the objects on the basis of a contrast between the radar echoes received by the various groups.

Abstract: The present invention includes a method for identifying a facility on the ground or at sea, the method being implemented on an airborne responder linked to at least two antennas, the method including a step of choosing a first transmission antenna and a step of transmitting an interrogation message from the chosen antenna. The method further includes testing whether a response has been received by the responder, and, if at least one response signal is received by at least one of the antennas, choosing a transmission antenna as a function of the response signal or signals received. If no response message is received, a different transmission antenna is chosen from the antenna that transmitted the last interrogation message. The method is repeated from the step of transmitting the interrogation message.

Abstract: Embodiments of a target-tracking radar and methods for responding to fluctuations in target SNR are generally described herein. In some embodiments, the target-tracking radar may be configured to determine whether a target can be considered a point target based on the SNR of received signals and off-boresight error estimates. Measurement-variance estimates generated from the SNR are provided to a target-state estimator when the target is determined to be a point target. When the target is determined not to be a point target, the measurement-variance estimates generated from SNR are not used by the target-state estimator. This may allow targets to be engaged at increased range.

Abstract: The present disclosure teaches an active antenna array for a mobile communications network. The active antenna array comprises a base band unit, a plurality of transceiver units terminated by at least one antenna element; and at least one link. The link couples the individual ones of the plurality of transceiver units to the base band unit. The link is a digital link and is adapted to relay a payload signal at a selectable payload rate. The digital link is further adapted to relay a timing signal at a fixed timing rate, when the timing signal is embedded in the payload at a selectable payload rate. The present disclosure further teaches a method for relaying radio signals and a computer program for manufacturing the active antenna array and for executing the method.

Abstract: A radar system includes at least one transmit array comprising a plurality of metamaterial elements. The radar system further includes at least one near-field stimulator for inputting electromagnetic signal to the transmit array so that a sub-wavelength target is illuminated with an electromagnetic wave.

Abstract: An antenna control system enabling the remote variation of antenna beam tilt. A drive means continuously adjusts phase shifters of a feed distribution network to radiating elements to continuously vary antenna beam tilt. A controller enables the beam tilt of a number of antenna at a site to be remotely varied.

Abstract: A method of operating a multi-function phased array radar system having an antenna array populated with dual-pol antenna elements is described. The method includes selectively controlling individual dipoles of each dual-pol antenna element in order to operate the array in either a polarimetric mode with improved cross-pol isolation, or in a multi-mission mode, wherein a plurality of singularly-polarized signals are produced.

Abstract: The present invention relates to a communication transmission field, and particularly provides a device, a method, and a system for aligning an antenna. The method includes steps as follows. An input signal is received, and a receiver signal strength indication (RSSI) value is output according to a strength of the input signal. The RSSI value is received, and a practically received power value corresponding to the input signal is acquired by looking up a corresponding table between the RSSI and power. An antenna rotation signal and an alignment indication signal are output. The antenna is adjusted according to the antenna rotation signal so as to change a receiving angle of the antenna. Then, it is displayed that the antenna is aligned according to the alignment indication signal. In the present invention, the man-made operation errors are reduced, thus the network quality is effectively ensured.

Abstract: According to one embodiment, a weather radar apparatus includes an antenna unit, a drive unit, an adjustment unit, and a control unit. The antenna unit is configured to transmit radio waves from a plurality of antenna elements, perform beam scan in a direction of elevation angle by phase control, and receive waves reflected by a weather target. The drive unit is configured to drive an elevation angle and an azimuth angle of an aperture plane of the antenna unit. The adjustment unit is configured to adjust an observation range and an observation elevation angle by the antenna unit and the drive unit in accordance with a plurality of observation modes. The control unit is configured to set the observation mode based on a received signal of the reflected waves.

Abstract: A radar system is disclosed for forming a scanning receive beam from signals received by a phased array having a plurality of sub arrays. An exemplary radar system includes a plurality of phase units each configured to receive a signal from one or more sub arrays. Each phase unit includes a waveform generator configured to generate an analog waveform having a frequency corresponding to a time-varying phase shift. Each waveform generator is arranged to digitally generate the analog waveform, and output a comparison of the received signal with the waveform, incorporating the time-varying phase shift. The system further includes a combining unit configured to combine the outputs from the plurality of phase units to form a scanning receive beam.

Abstract: An apparatus or a method for preventing radio-frequency (RF) interferences between rotating antennas. For example, the invention is particularly applicable to radar and communication systems on board naval ships.

Abstract: Provided are a sensor capable of adjusting vertical alignment and a sensor vertical alignment adjusting apparatus using the same. The sensor has a structure with a plurality of switchable transmitting and receiving antennas so as to be able to adjust the vertical alignment, or a structure with a tilting motor for adjusting a radiating or receiving angle. The sensor vertical alignment adjusting apparatus using such a sensor corrects vertical misalignment of the sensor by determining whether or not the vertical misalignment of the sensor occurs, variably switching one from among the plurality of transmitting or receiving antennas of the sensor or controlling the tilting motor, and adjusting the radiating angle of the sensor signal or the receiving angle of a reflected wave of the sensor signal.

Abstract: In one embodiment, a system includes a first cold noise source, a first radiometer receiver, and a first detector. The first cold noise source generates a first thermal radiation signal having a first carrier frequency band. The first thermal radiation signal carries a first information signal. The first cold noise source also transmits the first thermal radiation signal through a first antenna. The first radiometer receiver receives the first thermal radiation signal through a second antenna, and the first detector extracts the first information signal from the first thermal radiation signal.

Abstract: A device and method for wide area surveillance of a geographic region includes identifying a search space associated with a geographic region having a plurality of sub-regions. The search space is divided into a plurality of search cells and observation data is collected for each of the plurality of search cells based on a scan of the geographic region. An event generation rate is calculated for each of the search cells based on the collected observation data. One or more of the sub-regions are identified as a region(s) of interest based on the calculated event generation rates. A coverage pattern is estimated for each of the sub-regions and a desired revisit rate is calculated for the region of interest based on the estimated coverage pattern. The desired revisit rate is then output for scanning the region of interest.

Abstract: A radar device including: a reception antenna that receives radio waves, includes at least three antennas and is arranged so that the phase center points thereof form an isosceles triangle; an arrival direction detection unit that detects an arrival direction of the radio waves by a phase monopulse method; and a phase correction unit that corrects a phase difference between phases of radio waves respectively received by two adjacent antennas among the three antennas based on the relationship of the phases of the radio waves respectively received by the three antennas.

Abstract: In a radar device including: a transmission antenna group having a plurality of transmission antennas; a reception antenna group having a plurality of reception antennas; and transmission switchover means to transmit transmission waves in a way that sequentially switches over the transmission antennas of the transmission antenna group, the reception antenna group receiving reflected waves, reflected from a target, of transmission waves of the transmission antennas when sequentially transmitting the transmission waves, is made to function as a virtual sub-array corresponding to the transmission antennas. The target is thus detected by executing a space averaging process based on a relative position, to the reception antenna group, of each of the transmission antennas undergoing the transmission switchover with respect to the virtual correlation values corresponding to the plurality of virtual sub-arrays.

Abstract: A method and system of determining spatial identification of an object, such as orientation, size, location, range, and/or movement, using an RFID system is disclosed. An RFID system can comprise one or more RFID reader receiving antennas that query one or more RFID tags coupled to the object. The measurement of the phase of the tag responses at the reader antennas and phase differentials as a function of distance, frequency, and time are the basis of spatial identification. The system can work with conventional Gen 2 tags and readers without modification of the tags or protocol.

Abstract: A radar, mounted on a vehicle, emits electromagnetic waves to a side area of a vehicle and receives reflected electromagnetic waves to detect a distance and a bearing of a physical object. A speed of the vehicle is detected. When the vehicle speed is greater than a predetermined value, it is determined whether multiple reflection occurs based on at least distances detected by the radar. If it is determined that the multiple reflection occurs, an axis deviation of the radar is determined based on a difference between the detected bearing of the multiple reflection waves and a reference bearing.

Abstract: Embodiments of a target tracking system and method with jitter reduction suitable for directed energy systems are generally described herein. In some embodiments, the directed energy system includes a target tracking system to track one or more track points on a moving target, and a beam transmission unit to maintain a directed energy beam on a selected one of the track points in response to tracking control signals provided by the target tracking system. The track points may be smaller than a spot size of the directed energy beam maintained on the target.

Abstract: Radar beams for searching a volume are selected by determining the central angle and azimuth and elevation extents to define an acquisition face. The number of beams NMBA required to cover the acquisition face is determined by N MBA = ( 2 ? n + 1 ) ? ( m + 1 2 ) + ( - 1 ) n + m 2 ( 2 ) The number of beams NMBA is multiplied by the dwell per beam to determine the total search time, which is compared with a maximum time; (a) if the total search time is greater than the permissible time, the acquisition face is partitioned, and (b) if the total search time is less, the acquisition face information is applied to a radar processor for filling the unextended acquisition face with the number NMBA of beams in a particular pattern.

Abstract: According to one embodiment, bias estimation and orbit determination include receiving measurements in real time. The measurements include radar measurements and radar array orientation measurements. The radar measurements are generated by a radar system and indicate the location of a target. The radar array orientation measurements are generated by a navigation system and indicate the orientation of a radar array of the radar system. A state variable set is used. The state variable set includes measurement variables and dynamic bias variables. For example, a state variable set may include orbit position, orbit velocity, radar orientation, and radar measurement variables, which in turn may include dynamic bias variables such as orientation bias variables and measurement bias variables. A measurement variable is associated with a measurement, and a dynamic bias variable is associated with bias of a measurement.

Abstract: To suppress cross-ambiguities in the examination of an ice region or dry region by means of aircraft- or aerospace-supported radar echo sounding, the region to be examined is overflown by a radar sensor (6) by multiple compatible radar sensors of the same operating wavelength on multiple spatially separated, substantially parallel paths, wherein the radar signal data received on each path are recorded. The radar signal data recorded for each of the different paths are summed coherently and using a weighting to form a radargram, wherein an adaptive complex-valued weighting for each of the individual paths is performed using a geometrical model which takes into account the topography of the environment of the region to be examined. The weighting for every depth of the examined region is determined by solving a system of linear equations from which is calculated a synthetic antenna pattern which has zeros in the direction of the ambiguities.

Abstract: A motion compensation method and system is included in a radar antenna system mounted on a moving platform which is subject to pitch, yaw and roll. The radar antenna system includes a main array antenna, and an auxiliary antenna. The auxiliary channel associated with the auxiliary antenna utilizes roll, pitch and yaw angle motion compensations as its auxiliary antenna always steers a horizontal fan shape beam at the horizon to blank any surface (land or sea) based EM interferences. Such motion compensations are provided by a ship motion compensator component and process included within the antenna system. The ship motion compensator component in response to platform motion signals indicative of changes in platform motion angles generates new sets of values using an initial set of weighting coefficient values as a function of such angle motion changes.

Abstract: A method is provided for controlling an aircraft-mounted radar system configured to project radar beams and to receive radar returns relating to the projected radar beams. The method includes providing at least one output to the radar system that causes the radar system to sweep the radar beam horizontally to create a horizontal sweep set. The method further includes providing at least one output to the radar system that causes the radar system to conduct two vertical sweeps during the horizontal sweep set, the two vertical sweeps offset from a center horizontal location by at least one beam width, and wherein each of the two vertical sweeps are on opposite sides of the center horizontal location.

Abstract: A method and a system for sensing a boosting target missile, estimate position and velocity and boost acceleration parameters of the target missile, and control an interceptor missile to the target missile. A boost-phase missile target state estimator estimates at least acceleration, velocity, and position using an acceleration template for the target vehicle. The nominal template is incorporated into an extended Kalman filter which corrects the nominal template acceleration with the filter states to predict future thrust acceleration, velocity and position. The correction can compensate for motor burn variations and missile energy management (lofted/depressed trajectory).

Abstract: A terrain awareness system includes a processor for receiving radar returns and providing terrain and/or obstacle alerts or warnings in response to the radar returns. The processor receives information from a database and the information is used to select the radar transmit function and/or the radar reception function to optimize the performance of the system.

Abstract: An exemplary system and method are for tracking a target in a decentralised network having a plurality of sensing nodes. Each node makes observations of a target, performs a multiple models tracking algorithm based on the observations, and updates tracking information stored therein. Each node communicates the updated track information to selected other nodes in the network. In response to receiving track information from another node, each node fuses the receiving track information with local track information.

Abstract: An adaptive antenna control method is used for a radio communication system built by a plurality of radio base stations and a plurality of terminal stations capable of communicating with the radio base stations. Each radio base station includes an adaptive antenna having a plurality of antenna elements, a distributor for generating signals to be input to the plurality of antenna elements by branching a signal of one system to be transmitted, and weighting circuits for respectively weighting transmission signals to the plurality of antenna elements. For reception by each terminal station, an interference wave power given by the transmission signal from each of the plurality of radio base stations is estimated. A weight in the adaptive antenna of each radio base station is determined to minimize a sum of square errors between reception signals and desired signals for all the radio base stations which simultaneously use the same communication channel.

Abstract: A system and method provide the ability to collect, store and transmit information statements concerning satellite communication system use. The satellite system may collect billing information, such as roaming data, network use in terms of both total time and total bandwidth and functionality utilized on a user by user basis. Further, the satellite system may collect network usage information from the totality of users of a satellite system. The billing information and network usage information may be collected and stored on-board satellite(s) in a network in a data structure including a database. This information may then be collected, organized and transmitted to a variety of users in the earth segment of the system.

Abstract: An Advanced Focal Plane Array (“AFPA”) for parabolic dish antennas that exploits spatial diversity to achieve better channel equalization performance in the presence of multipath (better than temporal equalization alone), and which is capable of receiving from two or more sources within a field-of-view in the presence of multipath. The AFPA uses a focal plane array of receiving elements plus a spatio-temporal filter that keeps information on the adaptive FIR filter weights, relative amplitudes and phases of the incoming signals, and which employs an Interference Cancelling Constant Modulus Algorithm (IC-CMA) that resolves multiple telemetry streams simultaneously from the respective aero-nautical platforms. This data is sent to an angle estimator to calculate the target's angular position, and then on to Kalman filters FOR smoothing and time series prediction. The resulting velocity and acceleration estimates from the time series data are sent to an antenna control unit (ACU) to be used for pointing control.

Abstract: A system for estimating an antenna boresight direction. The novel system includes a first circuit for receiving a Doppler measurement and a line-of-sight direction measurement corresponding with the Doppler measurement, and a processor adapted to search for an estimated boresight direction that minimizes a Doppler error between the Doppler measurement and a calculated Doppler calculated from the estimated boresight direction and the line-of-sight direction measurement. The line-of-sight direction measurement is measured relative to the true antenna boresight, and the calculated Doppler is the Doppler calculated for a direction found by applying the line-of-sight direction measurement to the estimated boresight direction. In a preferred embodiment, the first circuit receives a Doppler measurement and a line-of-sight direction measurement from each of a plurality of pixels, and the processor searches for an estimated boresight direction that minimizes a sum of squares of Doppler errors for each of the pixels.

Abstract: This scanning array scans an area around the array for nearby objects, collision obstructions, and terrain topography. The scanning array can scan for sounds emitted by objects in the vicinity of the scanning array, passive energy receipt sources, or it can also send out an energy beam and scan for reflections from objects within the energy beam. The energy beam can be optical, laser, radar or other energy emitting sources. The scanning array of the invention can be used for helicopter detection and avoidance of collision risk and can be used for other scanning purposes. Scanning of an entire hemisphere or greater is accomplished by manipulating the scanner platform through the coordination of either linear actuators or gimbals so as to produce nutation without rotation. This motion allows transceivers to be directly coupled to transmitting and sensing modules without the losses associated with slip rings and other coupling devices.

Abstract: A detector apparatus, detection system, and method are provided for determining optimum operational angles based on the statistical correlation of wavelength-specific electromagnetic propagation and surface interaction. These techniques can be used within the radar community in both military and commercial radar applications for airborne radar system users to determine optimum operational depression angles based on the purpose of the effort, the operational frequency, and the terrain-type to be encountered. The method requires the user to interface with a standard computer equipped with the commercially available MATLAB® software package where the operation is presented as a graphic user interface (GUI) that once invoked allows the user to set specific parameters corresponding to the desired terrain type. Upon doing so, the algorithms are exercised and the results are displayed in a series of figures identifying the optimum operational angles.

Abstract: This disclosure provides an antenna device that includes an electromagnetic wave radiation source for radiating an electromagnetic wave, and an electromagnetic wave shaping module, arranged forward of the electromagnetic wave radiation source, where a plurality of slot array rows each including a plurality of slots arranged in the horizontal direction are arranged in the vertical direction.

Abstract: In a radar device including: a transmission antenna group having a plurality of transmission antennas; a reception antenna group having a plurality of reception antennas; and transmission switchover means to transmit transmission waves in a way that sequentially switches over the transmission antennas of the transmission antenna group, the reception antenna group receiving reflected waves, reflected from a target, of transmission waves of the transmission antennas when sequentially transmitting the transmission waves, is made to function as a virtual sub-array corresponding to the transmission antennas. The target is thus detected by executing a space averaging process based on a relative position, to the reception antenna group, of each of the transmission antennas undergoing the transmission switchover with respect to the virtual correlation values corresponding to the plurality of virtual sub-arrays.

Abstract: A method for main beam alignment verification includes providing data pertaining to one or more patterns associated with an antenna, measuring power levels of a signal acquired by the antenna, and comparing the measured power levels with the data to determine whether a direction of the signal is incident upon a main beam of the antenna.