MONOPULSE SYNTHETIC APERTURE RADAR

Abstract

Monopulse synthetic aperture radar for fast, high-resolution imaging of ground and/or airborne objects consists set of non-scanning transmitting and receiving antennas with overlap antenna patterns positioned in quadrature or multi-axis directions and covering wide space sector, wherein each of receiving antenna coupled to monopulse processor and separate receiver chain coupled with digital multi-channel processor. Application of monopulse and digital multi-axis multi-channel processing of all signals in receiving chains provides simultaneous fast signal processing from all space sector. Monopulse method in combine with multi-channel digital processing, where amplitudes, phase and frequency components shift of receiving signals processing relative to signals in overlap receiving antennas beams provides 3-5 times higher imaging resolution and allows to suppress influence of media and clutter. Array of directional antennas may be arranged for multi-frequency, multi-mode regimes.

Claims

1. Monopulse synthetic aperture radar for fast, high-resolution imaging of ground and/or airborne objects comprising illuminating transmitting means, receiving means covering entire space sector by set of non-scanning antenna beams and processing means for processing reflected signals wherein: transmitting and receiving 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 multi-axis processing of all signals in receiving chains in reference to amplitudes, phase and frequency components shift of signals in overlap receiving antennas; 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 including information received from monopulse processor; all transmitters, receiver chains and processing means connected with synchronization means; transceiver modules, image generator and radar output chain connected by microwave and/or fiber optic digital interface.

2. Monopulse synthetic aperture radar of claim 1, wherein each monopulse transceiver module consists set of non-scanning transmitting and receiving antennas covering space sector and each of transmitting antennas coupled with receiving antenna and separate receiver chain.

3. Monopulse synthetic aperture radar of claim 1, wherein each monopulse transceiver module consists set of non-scanning transmitting and receiving antennas covering space sector and each of receiving antenna arranged for transmitting and receiving signals.

4. Monopulse synthetic aperture radar of claim 1, wherein each monopulse transceiver module transmitting continuous waves, or pulses or pulses forming single sinusoidal wave without or with multi-mode different kinds of modulation.

5. Monopulse synthetic aperture radar of claim 1, wherein monopulse transceiver module positioned on UAS or other airborne or ground carrier or multiple monopulse transceiver modules distributed around carrier, or multiple monopulse transceiver modules distributed between the swarm of carriers connected to radar communication network.

6. Monopulse synthetic aperture radar of claim 1, wherein said transmitting, receiving and processing means are 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.

7. Monopulse synthetic aperture radar of claim 1, wherein said transmitting, receiving and processing means are 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.

8. Monopulse synthetic aperture radar of claim 1, wherein one antenna of said set of non-scanning receiving antennas with overlap antenna patterns arranged as reference for monopulse (one iteration) or/and adaptive (multiple iterations) signals processing for noise suppression or/and time delay adjustment to increase image quality.

9. Monopulse synthetic aperture radar for fast, high-resolution imaging of ground and/or airborne objects comprising illuminating transmitting means, receiving means covering entire space sector by set of non-scanning antenna beams and processing means for processing reflected signals wherein: as minimum one ambient cooperative or non-cooperative source of radio-frequency or/and microwave signals is using as transmitting means for illuminating said entire space sector; receiving means consists set of non-scanning receiving antennas with overlap antenna patterns in quadrature or multi-axis directions and covering entire space sector, wherein each of receiving antenna coupled to monopulse processor for simultaneous multi-axis processing of all signals in receiving chains in reference to amplitudes, phase and frequency components shift of signals in overlap receiving antennas and each of receiving antenna coupled to separate receiver chain; each receiver chain consists analog-to-digital converter and coupled 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 including information received from monopulse processor; all receiver chains and processing means connected with synchronization means; receiving means, image generator and radar output chain connected by microwave and/or fiber optic digital interface.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0021] PRIOR ART FIG. 1 diagrammatically illustrates of the real aperture radar principle.

[0022] PRIOR ART FIG. 2 shows known radar antenna system (a) and antenna patterns distribution (b) for continuous tracking of multiple objects covering 360-degree.

[0023] FIG. 1 diagrammatically illustrates first embodiment of monopulse synthetic aperture radar (a), where non-scanning quadrature antenna array with overlap antenna patterns covering imaging space sector (b).

[0024] FIG. 2 diagrammatically illustrates of two space tilted antenna patterns of directional antennas in Cartesian coordinates (a), and antenna pattern in polar coordinates (b) with two positions of object relative to center of antenna pattern.

[0025] FIG. 3 shows two-axis distribution of directional antennas within transceiver module.

[0026] FIG. 4 shows three-axis distribution of directional antennas within transceiver module.

[0027] FIG. 5 shows distribution of directional antennas around UAS perimeter.

[0028] FIG. 6 shows distribution of directional antenna arrays around UAS perimeter.

[0029] FIG. 7 shows distribution of directional antennas between UAS swarm.

[0030] FIG. 8 representing first embodiment of block-diagram of transceiver module with two-axis distribution of receiving antennas.

[0031] FIG. 9 shows set of non-scanning receiving antennas with overlap antenna patterns wherein one antenna arranged as reference for monopulse (one iteration) or/and adaptive (multiple iterations) signals processing.

[0032] FIG. 10 shows two models of monopulse set of receiving directional antenna array for micro-UAS.

DETAILED DESCRIPTION OF THE INVENTION

[0033] First embodiment of monopulse synthetic aperture radar diagrammatically illustrated in FIG. 1 (a). Non-scanning array of directional antennas positioned in two directions X, Y (quadrature directions) on UAS 101. Main beam 102 covering central, imaging space sector (main beam). Antenna patterns 103, 104 partially overlap main beam 102 and partially covering imaging space sector in X direction as shown in FIG. 1 (a). Overlap antenna patterns in Y direction are not shown in picture. In FIG. 1 (b) shown on-ground overlap beams footprints. Main beam's footprint 105 in partially overlapping by footprints 106, 107 in X direction, which corresponding to antenna patterns 103, 104. Footprints 108, 109 partially overlapping main beam's footprint in Y direction.

[0034] FIG. 2 (a) diagrammatically illustrates of two space tilted directional antenna patterns in Cartesian X, Y coordinates, where main beam 202 partially overlapping antenna pattern 203, which space tilted relative to center of antenna pattern 202. In FIG. 2 (b) diagrammatically illustrated antenna pattern of receiving directional antenna 204 in polar coordinates with two positions of detecting object relative to center of antenna pattern 205 and 206. If object positioned close to center of antenna pattern (position 205), some receiving signal phase shift 207, which can be for example result of change distance to object or change another object parameters, lead to corresponding change of signal amplitude 208. If antenna tilted in space relative of object, as shown in FIG. 2 (a), object position will be change to position 206, as shown in FIG. 2 (b). Same phase shift 207 (for example resulting of change distance to object) will lead to change of signal amplitude 209. As follow from picture, change of receiving signal amplitude will be 3-5 times larger for same phase shift, if antenna tilted to some angle (for example15-75 degrees, approx. 30 degrees optimal) relative to imaging object because antenna pattern slope. It means, array of directional antennas with overlap antenna patterns by antennas space tilt relative to main antenna center can provide 3-5 times better image resolution if monopulse phase measurement is using. Radar resolution can be increased by application of a few (two-three) antennas with overlap antenna patterns. 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.

[0035] FIG. 3 shows two-axis distribution of directional antennas 301 within monopulse transceiver module. Such two-axis distribution 302 can create overlap antenna patterns presented in FIG. 1 (a), (b). FIG. 4 shows three axis distribution of directional antennas 401 within monopulse transceiver module.

[0036] Accuracy of phase measurement and corresponding increasing image resolution can be made by increasing distance between radar receiving antennas. In FIG. 5 shown first embodiment of distribution of directional antennas around UAS perimeter. Second embodiment with distribution of monopulse transceiver modules with directional antenna arrays around UAS perimeter presented in FIG. 6. Three receiving antennas positioned on different distances in one direction axis also allows to solve phase ambiguity problem and provide high accuracy range measurement, which corresponding to better image range resolution.

[0037] Monopulse transceiver modules can be distributed between swarm of UAS, as shown in FIG. 7. Synchronization of all transmitting and receiving chains, digitizing of reflected from objects signals directly on antennas and connection of radar modules to network allow distribution of modules even when distance between UAS with modules is variable.

[0038] FIG. 8 representing first embodiment of block-diagram of two-axis distribution of receiving antennas within separate transceiver module. Monopulse transceiver module 801 consists monopulse processor 802 and transmitter 803 with power amplifier 804 connected to one transmitting antenna 805 covering said space sector 806. Set of non-scanning receiving antennas with overlap antenna patterns 807 in X direction 808 and Y direction 809 covering same space sector 806, wherein each of receiving antenna coupled to separate receiver chain consists low noise amplifier 810, direction coupler 811 and analog-to-digital converter 812. Each receiver chain coupled to monopulse processor 802 by directional coupler 811 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 connected to analog-to digital converter 813 and receiver chains 808, 809 connected to image generator 814 by digital interface 818. Image generator 814 consists quadrature or multi-axis multi-channel processor 815 for simultaneous processing of all signals in receiving chains including amplitude, phase and frequency components shift of signals in receiving antennas and generating image. Transmitter 803, 804, receiver chains 808, 809 and processing means 813, 814 connected with synchronization means 816. Transceiver module 801, image generator 814 and radar output chain 817 connected by microwave and/or fiber optic digital interfaces 818.

[0039] Regular radar systems with one antenna cannot separate reflected object signals from noisy media, eve by using fast gating received signals, as shown in FIG. 9 (a). For noisy media One directional antenna of set of non-scanning receiving antennas with overlap antenna patterns can be arranged as reference for monopulse (one iteration) or/and adaptive (multiple iterations) signals processing as shown in FIG. 9 (b). Reference antenna provides possibility to separate signals reflected from imaging object and signals reflected from media between object and receiving antenna. Reference antenna also allows to adjust time delay of reflected from object signals for example by adaptation algorithm. It allows to enhance SNR (signal-noise ratio) and suppress clutter.

[0040] FIG. 10 shows models of monopulse set of receiving directional antenna array for micro UAS.

REFERENCE NUMBERS

[0041] 101—UAS [0042] 102—space imaging sector, antenna pattern, main beam [0043] 103—first overlap antenna pattern in X direction [0044] 104—second overlap antenna pattern in X direction [0045] 105—footprint of main beam, space imaging sector [0046] 106—footprint of first overlap antenna pattern in X direction [0047] 107—footprint of second overlap antenna pattern in X direction [0048] 108—footprint of first overlap antenna pattern in Y direction [0049] 109—footprint of second overlap antenna pattern in Y direction [0050] 202—antenna pattern, main beam [0051] 203—space tilted antenna pattern [0052] 204—antenna pattern in polar coordinates [0053] 205—object position close to center of antenna pattern [0054] 206—object position tilted relative to center of antenna pattern [0055] 207—phase shift [0056] 208—amplitude change corresponding to phase shift [0057] 209—amplitude change corresponding to phase shift for tilted relative to center antenna pattern [0058] 301—two axis directional antenna array [0059] 401—three axis directional antenna array [0060] 501—directional antennas with overlap antenna patterns distributed around UAS [0061] 601—monopulse transceiver modules distributed around UAS [0062] 701—directional antennas with overlap antenna patterns distributed between swarm of UAS [0063] 801—monopulse transceiver module [0064] 802—monopulse processor [0065] 803—transmitter [0066] 804—power amplifier [0067] 805—transmitting antenna [0068] 806—covered imaging space sector [0069] 807—overlap antenna patterns [0070] 808—X axis receiver chains [0071] 809—Y axis receiver chains [0072] 810—low noise amplifier [0073] 811—directional coupler [0074] 812—analog-to-digital converter [0075] 813—analog-to-digital converter [0076] 814—image generator [0077] 815—multi-channel processor [0078] 816—synchronization means [0079] 817—output chain [0080] 818—digital interface

OPERATION

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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]:

[00001]$\begin{array}{cc}R={\left[\frac{P_t{G}_t{G}_r\sigma {\lambda }^2{F}_t^2{F}_r^2}{{\left(4\pi \right)}^3{P}_r}\right]}^{\frac{1}{4}},& \left(1\right)\end{array}$

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.

[0085] The maximum range equation for a continuous observation monopulse imaging radar must include the number of integrated pulses:

[00002]$\begin{array}{cc}R={\left[\frac{P_t{I}_{e}M{G}_t{G}_r\sigma {\lambda }^2{F}_t^2{F}_r^2}{{\left(4\pi \right)}^3{P}_r}\right]}^{\frac{1}{4}},& \left(2\right)\end{array}$

[0086] where: I.sub.e—integrator efficiency; M—number of transmitted/received pulses per period of integration.

[0087] 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.