WO1994016339A1

WO1994016339A1 - A method for scanning with a coherent radar and a radar for carrying out the method

Info

Publication number: WO1994016339A1
Application number: PCT/SE1994/000014
Authority: WO
Inventors: Jan Kjellgren; Gunnar STENSTRÖM
Original assignee: Försvarets Forskningsanstalt
Priority date: 1993-01-12
Filing date: 1994-01-12
Publication date: 1994-07-21
Also published as: SE9300063L; SE500809C2; SE9300063D0

Abstract

The present invention relates to a method for scanning with a coherent radar and a radar for carrying out the method. The basic object of the invention is to get small mechanical mass movements and thereby in a simple way facilitate fast scanning cycles. This is solved according to the invention by a radar comprising a transmitter, an antenna system, a receiver and a signal processing equipment. Further, the radar is caracterized in that the antenna system comprises a physical antenna having a reflector surface (A) or antenna lens and a feed system arranged to successively direct radar signals towards and/or receive radar signals from different part surfaces (ΔA) of the antenna or lens and that the signal processing equipment is arranged to use coherent integration of received signals.

Description

A method for scanning with a coherent radar and a radar for carrying out the method

The present invention relates to a method for scanning with a coherent radar and a radar for carrying out the method. With a radar scanning met is here to be understood a method which makes it possible to observe sev al objects or a distributed object within a scanning area.

Both in civil and military applications of radar methods in the fields o reconnaissance, surveillance and navigation, it is usually desirable to recurrently scan a space or an area. Also in applications for tracking, scanning methods as defined above can be of interest.

It is previously known to scan an area by moving a directional antenna. Such antennas do, however, become large, if you wish to have a narrow ra beam, and due to the size of the antenna, difficult to move fast and pre cisely enough. These drawbacks are overcome by the invention by designin it the way that is evident from the following independent claims.

In the following the invention will be described in more detail with reference to the attached drawings, where

fig 1 shows in principle the design of an embodiment of the invention in perspective view and

fig 2 shows in principle the design of an embodiment of the invention in cross-sectional view.

The basic object of the invention is to get small mechanical mass moveme and thereby in a simple way fascilitate fast scanning cycles. The signal of a large and quickly scanning antenna area are generated synthetically making a large number of successive radar measurements that each only us a small part of the antenna. The size and illumination of the part areas are adapted in order to give an elementary antenna, the antenna beam of which includes all the desired field of view. The desired high spatial resolution in the angular dimension is then obtained by coherent integra tion of the signals via all elementary antennas, that are distributed ov the antenna. By varying the coherent integration, different directions o the antenna can be synthesized within the field of view, cf. figure 1. Coherent means that the transmitted waveform is so stable that the phase relation between several consecutive radar pulses (waveform periods) can used in order to extract information.

From a mechanical point of view, the radar method is chiefly characteriz by the antenna system comprising a movable sub-reflector or lens control ling the illumination of the feed. The aperture of the antenna is dimen¬ sioned to give the desired final resolution in the angular dimension and the sub-reflector and feed are di entioned to give the desired field of view. If the illumination is focused on the reflector surface, the field view corresponds to the angle occupied by the sub-reflector as seen from the part area in question of the main reflector. The feed can consist of horn and either a revolving sub-reflector, a lens and a revolving flat mirror or a lens and a revolving prism for transmission and reception or possibly only one way. The sub-reflector or the equivalent, connecting t feed and the reflector can be an ellipsoid reflector having the feed and the respective part of the antenna surface in the foci of the ellipsoid. When there is a motion pattern giving varying distances to the different parts of the antenna surface, one focus is related to the surface in a suitable way.

The angular scanning is accomplished by rotating the sub-reflector or th equivalent around the symmetry axis of the feed or the boresight. With a fixed circular symmetric and circular polarized feed, rotation around th boresight of the feed gives an illumination of the sub-reflector that is independent or relatively independent of the angle of rotation.

In fig 2 the boresight of the antenna system coincides with the axis of rotation (in the plane of the paper) and the illuminated part aperture will describe, when rotated, a circular movement creating an anular ante for e.g. scanning within a cone. If the axis of rotation is perpendicula to the boresight, a long and narrow antenna for e.g. sector search is created.

The signal processing of the method is characterized by comprising sever part measurements, each part measurement only using a small part of the surface of the main antenna. Between the part measurements the illuminat part of the antenna surface is changed, so that the entire surface becom illuminated when all part measurements is carried out. The desired high resolution within the field of view is obtained in the signal processing coherently integrate the signals via all small part surfaces of the enti antenna. This signal processing comprises in the first place an integrat of signals from different directions (a synthetic rotation of the antenn If the measurement distance is within the near-field of the antenna, an integration combining direction and distance (synthetic rotation and focusing of the antenna is required).

The signal processing can be carried out with methods that are known in connection with the use of radar with a synthetic aperture. The applicat of these does not give the person skilled in the art any specific proble and therefore the signal processing is not here treated in full. The difference between the known synthetic aperture technique, SAR, and the present invention will, however, be underlined, as the similarities in t signal processing has been mentioned. In SAR an antenna transmits radar signals and receives them while the antenna and the vehicle carrying it per-forming an usually linear movement. The antenna is directed to the s as seen in the direction of movement. The signal processing then synthes a - possibly kilometer-long - radar aperture. In the present case an ordinary physical antenna is used, which is caused to transmit and recei radar signals by means of part surfaces of the antenna. The signal processing then synthesize an antenna based on the entire surface of the physical antenna.

The resulting angular resolution of the method is given by the aperture dimensions d, x d₂ or the aperture area A of the antenna. If both the transmission and the reception are carried out via successivly chosen pa surfaces, a resolution Λ L that is four times as good than for a conven tional antenna is obtained. This is due to the fact that the transmissio via a certain part surface can be coupled to the reception by the same p surface.

The angular resolution Δ-Ω-for the aperture area A is given by

The desired resolution must be balanced primarily against the disadvanta of having a physically large antenna. A fixed or possibly slowly revolvi antenna should, however, allow a larger antenna to be used as compared t conventionally mechanically scanning antenna.

The field of view - - is given by the size of the illuminated antenna surface with the wavelength as measure. A large field of view will lead smaller and more elementary antennas in a given antenna area. The beam angle of the field of view for an illuminated area A h is given as

- ²

-^{L ~} 2x ^•

The number of picture elements N_Δ in angular direction is given by th relation between the field of view and the resolution according to the below.

"^ ⁼ 15. ^•

The size of the picture or the number of picture elements in angular direction is primarily balanced against the scanning period Tc_l,V,S_ or the picture frequency fα..V.S... Further, the picture frequency depends on the repitition frequency of the waveform f or the maximum unambiguous measuring distance. A simultaneous observation with several elementary antennas na,,r,.r-a,y.. increases the performance but makes the system more complicated. In the extreme case a complete group antenna is obtained including elementary receivers, that is the basis of a so called digital antenna. For each elementary antenna a number of measurements can be carried out n ,.,,, which improves the integration factor and facilitates doppler signal processing, but at the same time leads to an increased observation time T ^ . The observation time depends on the factors n distance R and the number according to

^Tobs ⁼ Tprf "array c ^{where f}prf = ?Εmax Normally during scanning the time can not be completely used for observa tion, to this a certain so called dead time T. must be added, as imple¬ mented methods difter from the ideal ones so that

^Tavs ^{= T}obs ^{+ T}d^*

The relation btween the observation time and scanning time for a scannin period is given by the quotient k, k < 1, according to

where the potential picture frequency fu^-ι_d of the radar method is obtai as

^{fb11d "} -TT_* ^'

The measuring dimensions of the radar method can in addition to two angu dimensions and range, that is a spatical 3-dimensional observation, also consist of combinations of only one angle and range.

In order to be able to carry out the integration of all signals belongin to a scan, changes in the measuring geometry must be more or less known during the scanning interval. In the most simple case an unchanged, fixe measuring geometry or a linear movement in relation to the antenna apert generated during the scan is assumed. The observation time T , and wave length A of the radar method determine the velocity limit V , between fixed and a moving object and the acceleration limit a -■ between linear and not linear movement. With a certain degree of arbitrariness, expres¬ sions for these limits can be given according to the following.

Condition for a fixed object V , < -y — obs

Condition for a linear movement a -. <

4T'o^fcbs

Especially the first but also the second condition above is an example o assumptions to simplify the inversion of measuring data in SAR-methods. In the following a mechanization of the scan is outlined more concretely two cases, one dealing with a volumetric scan of a conical solid angle a one dealing with scanning an area within a limited angular sector. The s of a conical solid angle can be performed by making a small part antenna having a conical wide antenna beam rotate in a circular orbit with a lar diameter. The boresight of the part antenna is arranged to be parallel wi the symmetry axis of the circular orbit, which then gives the direction observation. The rotation of the antenna in the circular orbit is perfor by deflecting and focusing the radiation to illuminate a circular reflec with the help of a rotating mirror or a lens arrangement. The circular reflector can be cut from a parbolic or conical surface.

Sector scan of an area within a limited angle can be performed by making small part antenna having a conical wide antenna beam run along an appro mately straight aperture path of great length. The movement is produced with the help of an ellipsoidical sub-reflector that is rotated while it illuminating a parabolical main reflector. The axis of rotation is arran to be perpendicular to the direction of observation and to go through th focus of the main reflector. The feed is located with the axis of rotati as a symmetry axis in order to give a homogeneous illumination of the su reflector in different angles of rotation. The field of view corresponds approximately to the angle occupied by the sub-reflector as seen from th reflector. A fixed distance from the axis of rotation to the focus of th radiation means that the focus can be located to the surface of the main reflector in two points at most, which in such cases are located symmetri cally around the plane that is defined by the parabolic axis and the axi of rotation for the sub-reflector. Otherwise, the sub-reflector will foc the radiation a short distance from the reflector surface. The field of view is achieved by the reflection of a conical beam, but in this case t reflection does usually not take place in the point of intersection for beam, but different directions are reflected in different points of the reflector surface. As the reflector surface is curved, the field of view will be affected and reduced when the illuminating beam is turned from t axis of the parabola. This unwanted variation of the field of view can b seen as a reduction of the efficiency of the antenna. The antenna is estimated to manage turns of about + 60°.

Claims

Claims :

1. A method for scanning an area by means of a radar, characterized in t radar signals are successively transmitted and/or received via differnt part surfaces (ΛA) of an antenna surface (A) or antenna lens of a physi antenna and that the angular resolution within the field of view of the antenna (-1) is obtained by coherent integration of received signals.

2. A radar for scanning an area comprising a transmitter, an antenna system, a receiver and a signal processing equipment, characterized in t the antenna system comprises a physical antenna having a reflector surfa (A) or antenna lens and a feed system arranged to successively direct ra signals towards and/or receive radar signals from different part surface (Δl ) of the antenna or lens and that the signal processing equipment is arranged to use coherent integration of received signals.

3. A radar according to claim 2, characterized in that the feed system consists of a fixed feed (1) and a revolving sub-reflector (2).

4. A radar according to claim 2, characterized in that the feed system consists of a fixed feed (1), a feed lens and a revolving flat mirror.

5. A radar according to claim 2, characterized in that the feed system consists of a fixed feed (1), a feed lens and a revolving prism.

6. A radar according to claim 3, characterized in that the sub-reflector (2) is ellipsoidical and arranged to have the feed (1) and the part in question (-ΔA) of the reflector surface (A) of the main antenna in the respective focus.

7. A radar according one of the claims 2-6, characterized in that the ax of rotation (3) for the feed system coincides with the symmetry axis of antenna system.

8. A radar according one of the claims 2-6, characterized in that the ax of rotation (3) for the feed system is perpendicular to the symmetry axi of the antenna system.

9. A radar according one of the claims 2-8, characterized in that the fee system is arranged to instantaneously direct radar signals towards and/or receive radar signals from two or more part surfaces (ΛA) of the antenna