Patent Application
20230296760
1. Stephen Harman, “Holographic Radar Development”, Microwave Journal, Aveillant Ltd., Cambridge, UK, February 2021.
2. Armin W. Doerry, “Single-Axis Three-Beam Amplitude. Monopulse Antenna—Signal Processing Issues”, Sandia National Laboratories, Technical Report. May 2015.
3. Stephen E. Lipsky, “Microwave Passive Direction Finder”, SciTech Publishing Inc. Raleigh, NC 27613, 2004.
4. Pavlo A. Molchanov, Ashok Gorwara, “Fly Eye Radar Concept”. IRS2017. International Radar Symposium, Prague, July 2017.
Spatial resolution of imaging radar is determining by the size of the antenna, pulse duration and antenna beamwidth (Prior Art
Non-scanning radar system for detection incoming munition proposed by Stephen A. Harman [1]. Staring antenna array and beam covering presented in Prior Art
Armin W. Doerry shown, that radar resolution can be increased by application of a few (two-three) antennas with overlap antenna patterns [2]. Maximizing the accuracy and precision calculation of direction of arrival related to space tilt between directional antennas with overlap antenna patterns. The optimum space tilt angles can be readily calculated from constituent antenna patterns and the direction of arrival interval.
Lipsky S. E. in U.S. Pat. No 4,257,047 (1981) proposed an antenna array of a plurality of fixed, narrow beamwidth antennas, geographically oriented to provide omnidirectional coverage, as set of antennas is selected. It presents an explanation of the monopulse method for microwave direction finding with two pairs of directional antennas, positioned by azimuth and elevation boresight [3]. Direction finding by way of amplitude comparison methods can provide a root mean square (RMS) accuracy smaller than 2° in 100 ns after a direct wave arrives. High accuracy phase measurements provide high accuracy and fast direction finding. But most important, that monopulse method do not required long time, from millisecond for small amount operations to tens of seconds for FFT (Fast Fourier Transform), computer calculations and can provide critical information about targets position, speed, and identity.
Array of directional antennas with overlap antenna patterns and multi-channel signal's processing provides higher direction finding accuracy, which means better imaging resolution and faster signals processing [4].
Present invention related to radar system adapted for specific applications for mapping or imaging using synthetic aperture techniques, e.g., synthetic aperture radar [SAR] techniques combined with monopulse techniques. More particularly, the present invention relates to synthetic aperture imaging radar applying monopulse method for simultaneous processing of analog signals and multichannel processing of digitized signals. Modular radar with non-scanning non-switching simultaneous lobing, using multiple overlap beams. Image generation based on analog and digital processing of reflected amplitudes, phases and spectrum components associated with scattering and movement parameters of objects.
Ana Martines, Armin W. Doerry in patent U.S. Pat. No. 6,911,932 B1 from Jun. 28, 2005 “Two antenna, two pass interferometric synthetic aperture radar” proposed a multi-antenna, multi-pass IFSAR (interferometric synthetic aperture radar) mode utilizing data driven alignment of multiple independent passes can combine the scaling accuracy of a two-antenna, one-pass IFSAR mode with the height-noise performance of a one-antenna, two-pass IFSAR mode. A two-antenna, two-pass IFSAR mode can accurately estimate the larger antenna baseline from the data itself and reduce height-noise, allowing for more accurate information about target ground position locations and heights. The two-antenna, two-pass IFSAR mode can use coarser IFSAR data to estimate the larger antenna baseline. Multi-pass IFSAR can be extended to more than two (2) passes, thereby allowing true three-dimensional radar imaging from stand-off aircraft and satellite platforms. IFSAR utilizes at least two (2) SAR images of the same scene, formed at slightly different elevation angles relative to each other, to extract information about target heights. Such images can be coherently combined to ascertain the topography of the imaged area and produce three-dimensional maps of the imaged area.
Second antenna allows more accurately estimate the larger antenna baseline from the data itself and reduce height-noise, allowing for more accurate information about target ground position locations and heights. But proposed radar system require two antennas with separate carriers, which not always possible.
Armin W. Doerry in patent “Apodization of spurs in radar receivers using multi-channel processing” from May 14, 2019, proposed various technologies relate to identification and mitigation of spurious energies or signals in radar imaging. Spurious energy in received radar data can be a consequence of non-ideal component and circuit behavior. Such behavior can result from I/Q imbalance, nonlinear component behavior, additive interference (e.g., cross-talk, etc.), etc. The manifestation of the spurious energy in a radar image (e.g., a range-Doppler map) can be influenced by appropriate pulse-to-pulse phase modulation. Comparing multiple images which have been processed using the same data but of different signal paths and modulations enables identification of undesired spurs, with subsequent cropping or apodization of the undesired spurs from a radar image. Spurs can be identified by comparison with a threshold energy. Removal of an undesired spur enables enhanced identification of true targets in a radar image. Proposed radar system is composed of a plurality of components, whereby the components can exhibit some degree of non-ideal behavior, which can influence a purity or quality of a signal being processed by the radar system. Such components often exhibit non-linear behavior and/or other imbalances. In range-Doppler radar data, the effects of the non-linear behavior can be manifested as spurious signals, also known as “spurs”. The spurs can lead to false indication of a target energy, thereby reducing the performance, accuracy, reliability, and/or general utility of the radar system. By applying a received input signal to two signal paths, whereby each signal path utilizes a different modulation, two radar images may be generated. Both radar images can include identical true target responses but have different spurious energy responses.
Multi-channel processing radar system can provide shorter time of processing and more possibilities with reference signals for clutter suppression and spurs apodization but have limitations for detection objects moving with very slow or hypersonic velocity.
Stephen Anthony Harman in his patens from Jul. 13, 2021, U.S. Pat. No 11,061,114 B2 “Radar system for the detection of drones” and patent from December 2020 U.S. Pat. No. 10,877,130 “Drone detection radar” proposed a radar system for the detection of drones, including a transmitter, a receiver and a processor, wherein the processor is arranged to process demodulated return signals in a first process using a Doppler frequency filter, and to store locations of any detections therefrom, and to process the demodulated signals in a second process to look for signal returns indicative of a preliminary target having a rotational element at a location, and should a detection be found in the second process, to then attempt to match a location of the preliminary target with returns from the first process, and to provide a confirmed detection if a location of a detection from the first process matches with the location of a detection from the second process. A drone detection radar can include a plurality of antenna systems, each antenna system being arranged to transmit a signal into an associated sector, and to receive signals reflected from targets in the sector, the sectors collectively forming a monitored volume, and wherein a sub-set of the antenna systems are active at any one time, with the active sub-set of antenna systems being arranged to monitor their respective volumes for a duration sufficient to measure Doppler signals associated with slow moving drones, with the radar being arranged to switch to a different sub-set of antenna systems after each duration, such that the whole volume is monitored within a predetermined period. Combining a staring array from an antenna system with a plurality of switched antenna system allows drones to be both detected and tracked, with appropriate selection of the predetermined period.
But this radar system cannot provide good enough image resolution because long wavelength wide antenna patterns.
An objective of the present invention is development of monopulse synthetic aperture radar for fast, high-resolution imaging of ground and/or airborne objects. Radar consists set of non-scanning transmitting and receiving antennas with overlap antenna patterns wherein antennas positioned in quadrature or multi-axis directions and covering wide space sector. Each of receiving antenna coupled to separate receiver chain and multi-channel processor. Simultaneous application of monopulse and digital multi-axis multi-channel processing of all signals in receiving chains provides fast signal processing from whole space sector. Monopulse method, where amplitudes, phase and frequency components shift of receiving signals processing relative to signals in overlap receiving antennas beams will provide higher imaging resolution and allows to suppress influence of media and clutter.
Monopulse synthetic aperture radar for fast, high-resolution imaging of ground and/or airborne objects comprising transmitting and receiving non-scanning means arranged as monopulse transceiver modules wherein each monopulse transceiver module covering as minimum one said space sector. Each monopulse transceiver module consists monopulse processor and as minimum one transmitter connected to as minimum one transmitting antenna covering said space sector and set of non-scanning receiving antennas with overlap antenna patterns in quadrature or multi-axis directions and covering same space sector, wherein each of receiving antenna coupled to separate receiver chain. Each receiver chain coupled to monopulse processor for simultaneous (monopulse) multi-axis processing of all signals in receiving chains in reference to amplitudes, phase and frequency components shift of signals in receiving antennas with overlap patterns. Outputs of monopulse processor and receiver chains connected to image generator by digital interface. Image generator consists quadrature or multi-axis multi-channel processor for simultaneous processing of all signals in receiving chains including amplitude, phase and frequency components shift of signals in receiving antennas and generating image. All transmitters, receiver chains and processing mean connected with synchronization means. Transceiver modules, image generator and radar output chain connected by microwave and/or fiber optic digital interface.
Transceiver module of proposed monopulse synthetic aperture radar may be positioned on UAS or other airborne or ground carrier or multiple monopulse transceiver modules may be distributed around carrier, or multiple monopulse transceiver modules distributed between the swarm of carriers connected to radar communication network. Transmitting, receiving and processing means may be arranged for simultaneous transmitting, receiving and processing signals on a few different frequencies (multi-frequency signals) and consist separate antennas and filters in each receiving and processing chain or can be arranged for simultaneous transmitting, receiving and processing different modes, such as communication, navigation, control (multi-mode signals) and consist separate antennas, filters and processing means in each transmitter, receiving and processing chain.
PRIOR ART
PRIOR ART
First embodiment of monopulse synthetic aperture radar diagrammatically illustrated in
Accuracy of phase measurement and corresponding increasing image resolution can be made by increasing distance between radar receiving antennas. In
Monopulse transceiver modules can be distributed between swarm of UAS, as shown in
Regular radar systems with one antenna cannot separate reflected object signals from noisy media, eve by using fast gating received signals, as shown in
Fast monopulse high resolution imaging provided by non-scanning quadrature or multidirectional positioned array of directional antennas with overlap antenna patterns. Antennas can be distributed around perimeter of UAS or between swarm of UAS. Monopulse directional antenna arrays can be arranged as non-scanning transceiver modules illuminating imaging space sectors. Each transceiver module covering one said space sector. Reflected signals simultaneously receiving from all objects within imaging space sector by set of directional antennas with overlap antenna patterns distributed in quadrature or multi-axis directions. Processing of received by directional antennas signals simultaneous providing by monopulse processor and multi-channel digital processor, wherein each receiving antenna coupled to separate receiver chain.
The time of signals processing is significantly decreased because signals from all imaging area processing simultaneously, even compare to holographic radar, where receiving signals processing digitally by switching virtual beamforming receiving signals. For example, a scanning radar typically processes only one beam at a time, holographic staring radar processes signals by switching virtual beams and monopulse imaging radar processing all beams simultaneously.
Also, holographic radar transmitting more powerful signals, since a scanning radar contains a high gain antenna on both transmit and receive, and in monopulse radar transmitting power spreading inside the imaging space sector. From another side, high gain antennas in monopulse radar provides better gain and sensitivity than holographic radar, where usually applied array of omnidirectional antennas, which need provide wide area of observation for each antenna array element, and virtual set of receiving signals antennas activated for very short time for one separate point of image (pixel). Practically monopulse imaging radar will provide same gain and sensitivity of antennas, as scanning radar with similar directional antenna.
Monopulse systems can be continuous waves or pulsed [3]. Presented below is an estimation made for a continuous observation pulsed radar with wide field of view, where it was estimated how the distribution of transmitting power will affect radar range. The maximum range equation for a monostatic scanning radar is given by the following equation [3]:
where: R—radar-to-target range; σ—radar target cross section; λ—wavelength; Pr—received-signal power being equal to the receiver minimum detectable signal Smin; Pt—transmitted-signal power (at antenna terminals); Gt—transmitting antenna power gain; Gr—receiving antenna power gain; Ft—pattern propagation factor for transmitting antenna-to-target path; Fr—pattern propagation factor for target-to-receiving antenna path.
The maximum range for a radar when the antenna beam width is, for example, 10 times wider (beam width increased from 3 to 30 degrees). For the same transmitted power, Pt, the energy of the transmitted signal spreads over a 10×10 square and the signal reaching the target (according to the inverse square law) is decreased 100-fold. A radar with a phase antenna array, or holographic radar with virtual switching receiving beams can scan the entire area of observation and receive 1 pixel hit by pulse every second, because one pulse hits the pixel per scan. For imaging radar term pixel will be used instead of term target. For a continuous observation monopulse radar, pulses may be transmitted and reflected from all image pixels continuously. This means that a continuous observation radar can transmit to and receive much more pulses per second for any one pixel direction and all image pixels will be hit and process simultaneously. Integration of these received pulses will dramatically increase information about the all image pixels, or objects.
The maximum range equation for a continuous observation monopulse imaging radar must include the number of integrated pulses:
where: Ie—integrator efficiency; M—number of transmitted/received pulses per period of integration.
In continuous wave monopulse imaging radars with continuous imaging space sector observation of the received signals, integration leading to increasing radar range. Simultaneous correlation and integration of thousands of signals per second from each pixel (or point of observation) provides not only larger radar range but can help increase image resolution and object recognition by using diversity signals, polarization modulation, and intelligent processing. Non-scanning monopulse system allows dramatically decrease in transmitting power and at the same time increase in radar range also by integrating 2-3 orders more signals than regular scanning radar systems. Monopulse radar system can be passive, using ambient RF energy. Monopulse method provides better resolution of 2-3 orders then scanning radars. Synchronizing of signals directly in antennas provide high accuracy amplitude and phase measurement. Non scanning antenna array is phase/frequency independent and can be multi-frequency, multi-function. All receiving chains using ratio of amplitudes, phases and relative frequency components shift of signals for multi-axis signal processing. Multi-axis processed signals from receiving antennas also can be applied for detection and identification of objects in each separate set of receiving antennas and for generating alarm signal and multi-axis signals proportional object range, angle of arrival and velocity. Monopulse means can consist filters in identification circuits for separation clutter signals, target signals form background noise, moving targets, identification of moving targets. Integration time for receiving signals controlling depends on the detected target range and speed, where longer integrating time corresponding to the longer range.
