CA1165839A - Tracking aperture radar - Google Patents
Tracking aperture radarInfo
- Publication number
- CA1165839A CA1165839A CA000372431A CA372431A CA1165839A CA 1165839 A CA1165839 A CA 1165839A CA 000372431 A CA000372431 A CA 000372431A CA 372431 A CA372431 A CA 372431A CA 1165839 A CA1165839 A CA 1165839A
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- Prior art keywords
- patch
- signal
- array
- mixers
- radar
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
- G01S13/88—Radar or analogous systems specially adapted for specific applications
- G01S13/89—Radar or analogous systems specially adapted for specific applications for mapping or imaging
- G01S13/90—Radar or analogous systems specially adapted for specific applications for mapping or imaging using synthetic aperture techniques, e.g. synthetic aperture radar [SAR] techniques
- G01S13/904—SAR modes
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- Engineering & Computer Science (AREA)
- Remote Sensing (AREA)
- Radar, Positioning & Navigation (AREA)
- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Computer Networks & Wireless Communication (AREA)
- General Physics & Mathematics (AREA)
- Radar Systems Or Details Thereof (AREA)
Abstract
TRACKING APERTURE RADAR
ABSTRACT OF THE DISCLOSURE
The transmitting antenna of a side looking radar system for high flying aircraft and space vehicles emits radar pulses which illuminate a narrow patch of terrain which is elongated in a direction orthogonal to the direction of progression of the vehicle carrying the radar system. Reflected pulses are received in a receiving antenna which illuminates an area contained within the patch and has the width of the patch but a small fraction of its range extent. The patch is scanned with the receive area.
By the use of this system, for a given antenna size the previous transmitter power dependence on frequency is avoided, thus allowing much higher R.F. frequencies to be used with available transmitter power.
ABSTRACT OF THE DISCLOSURE
The transmitting antenna of a side looking radar system for high flying aircraft and space vehicles emits radar pulses which illuminate a narrow patch of terrain which is elongated in a direction orthogonal to the direction of progression of the vehicle carrying the radar system. Reflected pulses are received in a receiving antenna which illuminates an area contained within the patch and has the width of the patch but a small fraction of its range extent. The patch is scanned with the receive area.
By the use of this system, for a given antenna size the previous transmitter power dependence on frequency is avoided, thus allowing much higher R.F. frequencies to be used with available transmitter power.
Description
~ ~5~39 02 This invention relates to s~n-thetic aperture (side 03 looking) radar which is particularly use~ul in conjunction with 04 high flying aircraft or space vehicles.
05 Antennas of side looking radar systems carried aboard 06 aircraft or space vehicles typically illumina-te terrain to be 07 mapped in the shape of an elongated patch having a narrow width.
08 Reflected radar pulse signals are received by the same antenna as 09 is used for transmission of the signals; use of the single antenna aperture ensures that all elements of the terrain to be 11 mapped are received. The length of the patch defines one 12 dimension to be mapped, and the distance travelled due -to the 13 forward motion o~ the vehicle carrying the radar system defines 14 the other dimension.
In order to achieve high range resolution, short pulses 16 are transmitted, and in order to achieve good azimuth resolution 17 (in the direction of movement by the vehicle) synthetic aperture 18 radar techniques are utilized.
19 Side looking radar and certain problems associated with high pulse rates are described in U.S. Patent 4,064,510 issued 21 December 20, 1977, invented by Maurice CHABAH, Paris, France.
22 It is clear that increasingly high transmitted pulse rates, and 23 increasingly high R.F. carrier signal frequencies are desirable.
24 However ~or very high flying vehicles carrying the radar system, such as space vehicles, a high pulse rate results in the 26 reception of possibly ambiguous signals and blind areas, the 27 noted patent proposes a solution to these problems.
28 In a solution as described in the noted patent the 29 antenna vertical aperture which defines the swath width must be made smaller, with increasing frequency. The well known radar 31 equation clearly demonstrates however that as the frequency is 32 increased, the required transmitter power must be increased to 33 obtain a given average power. Yet as the frequency is increased, 34 the generation o~ such increased power becomes more and more difEicult.
36 The presen-t invention provides means for eliminating 37 dependence o~ required power on ~requency. Xn addition, the 38 present invention allows the ver-tical aperture of the antenna to 39 be designed ~or most efficient operation.
. "~
~' 1. ~L f;~t~39 ol 2 02 Accorcling to the pre~ent i.nven-tion, the transmi-tted 03 beam illuminates a narrow, elongated patch with a pulse signal, 04 and a receiving antenna illuminates an area within -the patch of 05 similar width as the patch but with a substantiall-y smaller 06 range, the illuminated receive area scanning the transmit patch.
07 It has been found that using this mode of operation, the average 08 required transmitting power can be made to be independent of the ~09 frequency of the radio frequency signal.
In general, the present invention is a radar system 11 comprising a transmitting antenna for illumina-ting an elongated 12 patch with a pulse signal, a receiving antenna for illuminating 13 an area within the patch having a similar width but a raction of ~14 the range extent of the patch whereby reflected pulse signals from said area can be received, and apparatus for scanning the ~16 swath with the receiving antenna within the area illuminated ~17 by the transmit-ting antenna.
~18 The scanning apparatus can be adapted to illuminate the ~19 swath either progressively from one end to the o-ther, or in steps of adjacent areas.
21 A better understanding of the invention wlll be 22 obtained by reference to the detailed description below, and to 23 the following drawings, in which:
24 Figure 1 is a perspective pictorial view illustrating the operation of side looking radar, and 26 Figure 2 is a block schematic of an operation system ~27 for the radar in accordance with the preferred embodiment of the 28 invention.
;29 Turning now to Figure 1, a vehicle at posi-tion 1 ~30 carries the side looking radar system. In a conventional system, ~31 electromagnetic radar pulses illuminate a patch 2 having a range 32 extent W and a narrow width. The vehicle 1 travels in the 33 direction V, the patch thus effectively providing a line scan of ~34 a swatch o terrain to be mapped.
It is well known that the average power requirement for 36 the transmitted signal of a synthetic aperture side looking radar ~37 is as follows:
~38 ... ~
1 16~33~
02 PAV = a7J~ v~ (a) 05 where H is the altitude of the transmitter 06 K is Boltzman's constant 07 To is the effective temperature of the receiver, 08 Fn is the receiver noise frequency, 09 V îs the velocity of the transmi.tter and receiver, W is the extent range of the swath (i.e. swatch width) 11 NL is the number oE statistically independent images 12 that are summed incoherently to form the synthetic aperture 13 image, 14 ~T is the aperture efficiency, i.e. the main to sidelobes ratio of the transmittin~ antenna, 16 DvR is the receiving antenna aperture, i.e. the 17 vertical dimension of the receiver antenna, 18 ~r is the range resolution, 19 ~a is the azimuth or width resolution, ~O is the radar crossection of a target within the 21 area, 22 ~ is the elevation angle of the centre of the swath 23 (W) to the radar, 24 S/~ is the signal to noise ratio and ~R is the aperture efEiciency of the receiving 26 antenna.
27 It is clear that as the transmitted frequency 28 increases, the wavelength becomes smaller, and the average 29 required power increases.
It is well known that with increase in pulse repetition 31 ~rsuqency, an ambiguity can occur if the width of the patch is 32 too great, since pulses can be received from overlapping portions 33 o~ the patch, thus initiating ambiguity as to the source of the 34 pulse reflections. It is therefore important to reduce the width of the patch (i.e. beam width) as much as possible. However, in ~3~ order to produce a narrow beam width, a larc3e array i9 re~llired, 37 which gives ri~e to tolerance and antenna mechanical ~abrication 38 difficulties. ~hese are reduced somewhat i~ the operating 3~ frequency is raised, but according to the radar equation noted .. ~. .
.
' '3 (~
02 above, gives rise to ~he requirement or increased power.
03 Clearly the operating frequency o~ such radars has been kept 04 sufficiently low so that the necessary transmitter power is 05 within ~easible limits. In satellite-borne ra~ar systems, this 06 power limit is lower than is desirable for mini.mizing deleterious 07 ionospheric effects, simplifying the synthetic aperture signal 08 processing requirements, and reducing the sensitivity to spurious 09 target motion.
The present invention provides means for removing ~he 11 dependence of transmitter power on operating frequency. In 12 transmitting the radar pulse signal, the full swath 2 (Figure 1) 13 to be mapped is illuminated. However, for receiving the 14 reflected signal, only a very narrow antenna beam which lS illuminates a small area 3 is used, which area is scanned across 16 the swath in coincidence with the arrival of the reflections.
17 The receiving antenna beam can be a single beam which scans 18 continuously~ or a plurality of adjacent beams from which the 19 appropriate beam is selected so as to produce a stepped type of scan.
21 Accordingly two antennas are used, a transmitting 22 antenna having a vertical aperture DVT which size is chosen to ~23 just illuminate the patch, and a receiving antenna which has a 24 vertical aperture DVR which size is made as large as possible, in order to produce a narrow beam which is contained in the 26 transmitter beam. The transmitting antenna can form part of the 27 receiving antenna.
28 A time dependent linear phase shift is introduced 29 across the vertical receiving antenna aperture, causing the receiver beam to scan across the patch, in coincidence with 31 signal returns arriving from increasing ranges. The kwo antennas 32 preferably have the same aperture length in the azimuth 33 direction, such that they should illuminate the same width.
34 The range W of the swath is 37 W = ~ ~ (c) 38 DV~5;A
~39 , .1. ,~S83g 02 and the range extent or length Wr o* the recelve antenna 03 illuminated area i5 06 Wr = ~~ - (d) 07 DV~s~ 0 09 where Ro i8 the distance between the vehicle carrying the radar system and the area being mapped (the radar range), ~11 ?~ is the wavelength, and 12 ~ is the elevation angle between the area being mapped 13 and the vehicle carrying the radar system.
14 With the use of the present system, the radar equa-tion is ~18PAV = ~ 7~ F~ v~ b) ` 1 9 ~ ,2 ~ Sj~ J
21 where H is t~e altitude of the transmitter, 22 K iæ Boltzman's constant ~23 To is the effective ternperature of the receiver, 24 Fn is the receiver noise figure, V i5 the velocity of the transmitter and receiver, 26 W is the range of swath ~27 NL iæ the number of statistically independent 28 images that are summed incoherently to form the synthetic ~29 aperture image, ~ T is the aperture efficiency, i.e. the main to ~31 sidelobes ratio of the transmitting antenna, - 32 DVR is the receiving antenna aperture defining the 33 length of the area, i.e. the vertical dimension of the receiver 34 antenna, ~35 ~ r is the range reæolution, `36 ~ a i8 the azimuth or width reæolution, 37 ~ O is the normalized radar crossection oE terrain ~38 within the area, ~39 S/N is -the æignal to noise ratio and :
~. lS583~
02 ~ R is the aperture efEiciency of the receiving 03 antenna.
04 It should be noted that the frequency or wavelength of 05 the radar signal forms no part o~ this equation, and -thus it is 06 clear that the required power is not related to -the radar 07 frequency.
08 In prior art systems, in which the receive antenna beam 09 illuminates the complete swath, the antenna vertical aperture must be decreased with increasing ~requency. However in the 11 present inven-tion, the vertical aperture can be made much larger 12 and does not have to be decreased with increasing ~requency.
13 Furthermore, although the swath to be mapped may 14 contain an ambiguous range, with the pr~sent invention the receiving beam does not see the ambiguous returns. The pulse 16 repetition frequency may thus be elevated to alleviate azimuth 17 ambiguities that may be troublesome when wide swath coverage is 18 attempted. The limiting factor in the present invention then i5 19 the technical tolerance of the particular antenna design which is used.
21 In a typical example, the values of the elements given 22 in the radar equation would be as follows:
23 R = 800 kms; V - 7.5 km/sec.;~O = -15db; S/~ = 25db;
24 Fn = 4.5db; W = 200 km, ~ = 66 degrees; ~a ~ ~r = 20 meters;
~ = 80~, ~ = 0.03 meters.
26 For DVR approximately ~.5 me-ters, the required 27 average power is about 200 watts. Thi~ value is approximately 28 16db lower than the corresponding power which w~uld be required 29 in a conventional synthetic aperture radar. Clearly the present invention significantly reduces the necessary transmitted power.
31 Preferably the transmitting and receiving antennas ;32 should be combined. The transmitter feeds a small number of 33 centre elements to form a wide aperture. All o~ the elements are 34 scanned for receiving, forming a narrow receiving aperture, using phased array techniques.
36 The transmit ~eed system for the cen-ter elements o~ the 37 antenna derives its R.F. power rom a stable local osc:illator.
38 The feed system i9 coupled to the antenna elements through one or 39 more circulators whose purpose is to direct -transmitter energy to ' ~ ~65~3~
02 the antenna and received energy Erom ~he antenna elernents -to the 03 receiver channels.
04 Turning to Figure 2, a block diagram of -the system 05 according to a preferred embodiment of the invention is shown.
06 A physical antenna which can be used with this invention is of 07 the kind described by C.F. Winter in the article "Phase Scanning 08 Experiments with Two Reflector Antenna Systems", Proceedings of 09 the I.E.E.E., Vol. 56, No. 11, November 1968, pp. 1984-1998.
Center elements 7 of the antenna system are fed via circulators 11 8, which receive signal~ via a transmit channel feed system 9.
12 Typical output pulse widths would be in the range of 1 to 100 13 microseconds. The transmit channel feed system receives power 14 from a power amplifier 10, which itself receives signals from a stable local oscillator ll which feeds its signals via bandpass 16 filter 12 and waveform generator 13 to the input of power 17 amplifier lO. The local oscillator signals are applied to the ~18 input of bandpass filter 12 after being split in po~er splitter l9 14, one portion of which is divided down in divider 15 and ~0 applied via a further splitter 16 to one input of up converter ~1 17, to which a second portion o~ the local oscillator signal is ~22 also applied. The output of the up converter is applied to ~23 bandpass filter 12.
~24 The reflected radar pulse signals received from all of the antenna elements 17 and 7 which constitute the entire array, 26 are fed to limiters 18, the signals from the center elements 7 o ~27 the antenna passing through circulators 8. The output signals 28 from limiters 18 are passed through low noise amplifiers l9 to 29 one input of down converters 20. A signal having successively ~30 increased time delay (i.e. delayed phase) is fed to the second 31 inputs of successive ones of down converters 20, as ~7ill be 32 described below.
33 For each radar transmit pulse, it is required that the ~34 receive antenna vertical pattern should be scanned such that it points in the direction of radar reflections as they arrive at 36 the antenna from successively longer ranges. The relative phase ~37 of each element is determined b~ the relative phase Oe the 38 phasing signal which is rnixed in down converters 20 with the 39 receive signal in each channel. This signal is yenerated in a t; 8 3 ~ , 02 voltage controlled oscillator 21, the output signal of which is 03 passed through power splitter 22 to a tappe~ delay line 23. ~ach 04 tap 24 is connected to the second input of down converter 20.
05 The voltage controlled oscillator 21 is of course enabled by scan 06 initiate pulses received from conven-tional and well known 07 circuitry at terminal 25.
08 The outputs of each of the down converters 20 are 09 applied to corresponding inputs of a combiner 25 which sums the respective input signals.
11 The electrical length between ~he taps o~ the delay 12 line, corresponding to an unique voltage controlled osciLlator 13 frequency, is such that the down converted signals in all element 14 channels interfere constructively only for those signals incident on the array at an unique vertical angle of incidence. Therefore 16 varying the voltage control oscillator frequency alters the 17 electrical length between the taps, the relative phases of the 18 down converted signals in the element channels, and therefore the ~19 vertical angle of incidence on thè array at which the element signals constructively interfere in the combiner. Therefore by ~`21 changing the fre~uency of the voltage controlled oscillator, the 22 receive illuminated area can be caused to scan within the ~`23 transmit swath. If the frequency is caused to vary linearly, a 24 linear scan will be produced; if the frequency varies stepwise, adjacent areas will be illuminated for the periods of each step.
26 The output signal from combiner 25 is applied through 27 wideband amplifier 26, through a bandpass ilter 27 to one input 28 of up converter 28. The second input of up converter 28 is 29 connected to the second output of power splitter 22, to which the original voltage controlled oscilIator signal is applied. The 31 frequency offset caused by the voltage controlled oscillator is 32 thus removed from the output signal from up converter 28.
33 The remainder of the radar system is similar to that of 34 conventional coherent radars. The output signal of up converter 28 is applied through bandpass filter 29 to one input of down ~36 converter 30; the second input o~ down converter 30 is connec-ted ~37 to the output of frequency divider 15 -through power splitter 16.
38 ~ccordingly the output signal of up converter 28 is mixed wi-th 39 the local oecillator signal which is coherent with ~he ~ 16~39 02 transmi-tted siynal. The resulting output of down converter 30 is 03 an I.F. signal, which is applied through I.~. amplifier 31 to 04 coherent demodulator 32 (to which the oscillator 11 signal is 05 applied through the third branch of power sp].itter 1~, to 06 provide in phase and quadrature si~nals at terminals I and Q.
07 These signals are processed using well known synthetic aperature 08 techniques to produce a radar image.
09 A person skilled in the art understanding this invention may now conceive of variations or other embodiments.
11 For example, more than one scanning beam can be used. Each 12 element of the li.near antenna array can be comprised oE a 13 plurality in-phase fed elements. All such variations or other 14 embodiments are believed to be within the scope of the present invention, as defined in the claims appended hereto.
:
..
,
05 Antennas of side looking radar systems carried aboard 06 aircraft or space vehicles typically illumina-te terrain to be 07 mapped in the shape of an elongated patch having a narrow width.
08 Reflected radar pulse signals are received by the same antenna as 09 is used for transmission of the signals; use of the single antenna aperture ensures that all elements of the terrain to be 11 mapped are received. The length of the patch defines one 12 dimension to be mapped, and the distance travelled due -to the 13 forward motion o~ the vehicle carrying the radar system defines 14 the other dimension.
In order to achieve high range resolution, short pulses 16 are transmitted, and in order to achieve good azimuth resolution 17 (in the direction of movement by the vehicle) synthetic aperture 18 radar techniques are utilized.
19 Side looking radar and certain problems associated with high pulse rates are described in U.S. Patent 4,064,510 issued 21 December 20, 1977, invented by Maurice CHABAH, Paris, France.
22 It is clear that increasingly high transmitted pulse rates, and 23 increasingly high R.F. carrier signal frequencies are desirable.
24 However ~or very high flying vehicles carrying the radar system, such as space vehicles, a high pulse rate results in the 26 reception of possibly ambiguous signals and blind areas, the 27 noted patent proposes a solution to these problems.
28 In a solution as described in the noted patent the 29 antenna vertical aperture which defines the swath width must be made smaller, with increasing frequency. The well known radar 31 equation clearly demonstrates however that as the frequency is 32 increased, the required transmitter power must be increased to 33 obtain a given average power. Yet as the frequency is increased, 34 the generation o~ such increased power becomes more and more difEicult.
36 The presen-t invention provides means for eliminating 37 dependence o~ required power on ~requency. Xn addition, the 38 present invention allows the ver-tical aperture of the antenna to 39 be designed ~or most efficient operation.
. "~
~' 1. ~L f;~t~39 ol 2 02 Accorcling to the pre~ent i.nven-tion, the transmi-tted 03 beam illuminates a narrow, elongated patch with a pulse signal, 04 and a receiving antenna illuminates an area within -the patch of 05 similar width as the patch but with a substantiall-y smaller 06 range, the illuminated receive area scanning the transmit patch.
07 It has been found that using this mode of operation, the average 08 required transmitting power can be made to be independent of the ~09 frequency of the radio frequency signal.
In general, the present invention is a radar system 11 comprising a transmitting antenna for illumina-ting an elongated 12 patch with a pulse signal, a receiving antenna for illuminating 13 an area within the patch having a similar width but a raction of ~14 the range extent of the patch whereby reflected pulse signals from said area can be received, and apparatus for scanning the ~16 swath with the receiving antenna within the area illuminated ~17 by the transmit-ting antenna.
~18 The scanning apparatus can be adapted to illuminate the ~19 swath either progressively from one end to the o-ther, or in steps of adjacent areas.
21 A better understanding of the invention wlll be 22 obtained by reference to the detailed description below, and to 23 the following drawings, in which:
24 Figure 1 is a perspective pictorial view illustrating the operation of side looking radar, and 26 Figure 2 is a block schematic of an operation system ~27 for the radar in accordance with the preferred embodiment of the 28 invention.
;29 Turning now to Figure 1, a vehicle at posi-tion 1 ~30 carries the side looking radar system. In a conventional system, ~31 electromagnetic radar pulses illuminate a patch 2 having a range 32 extent W and a narrow width. The vehicle 1 travels in the 33 direction V, the patch thus effectively providing a line scan of ~34 a swatch o terrain to be mapped.
It is well known that the average power requirement for 36 the transmitted signal of a synthetic aperture side looking radar ~37 is as follows:
~38 ... ~
1 16~33~
02 PAV = a7J~ v~ (a) 05 where H is the altitude of the transmitter 06 K is Boltzman's constant 07 To is the effective temperature of the receiver, 08 Fn is the receiver noise frequency, 09 V îs the velocity of the transmi.tter and receiver, W is the extent range of the swath (i.e. swatch width) 11 NL is the number oE statistically independent images 12 that are summed incoherently to form the synthetic aperture 13 image, 14 ~T is the aperture efficiency, i.e. the main to sidelobes ratio of the transmittin~ antenna, 16 DvR is the receiving antenna aperture, i.e. the 17 vertical dimension of the receiver antenna, 18 ~r is the range resolution, 19 ~a is the azimuth or width resolution, ~O is the radar crossection of a target within the 21 area, 22 ~ is the elevation angle of the centre of the swath 23 (W) to the radar, 24 S/~ is the signal to noise ratio and ~R is the aperture efEiciency of the receiving 26 antenna.
27 It is clear that as the transmitted frequency 28 increases, the wavelength becomes smaller, and the average 29 required power increases.
It is well known that with increase in pulse repetition 31 ~rsuqency, an ambiguity can occur if the width of the patch is 32 too great, since pulses can be received from overlapping portions 33 o~ the patch, thus initiating ambiguity as to the source of the 34 pulse reflections. It is therefore important to reduce the width of the patch (i.e. beam width) as much as possible. However, in ~3~ order to produce a narrow beam width, a larc3e array i9 re~llired, 37 which gives ri~e to tolerance and antenna mechanical ~abrication 38 difficulties. ~hese are reduced somewhat i~ the operating 3~ frequency is raised, but according to the radar equation noted .. ~. .
.
' '3 (~
02 above, gives rise to ~he requirement or increased power.
03 Clearly the operating frequency o~ such radars has been kept 04 sufficiently low so that the necessary transmitter power is 05 within ~easible limits. In satellite-borne ra~ar systems, this 06 power limit is lower than is desirable for mini.mizing deleterious 07 ionospheric effects, simplifying the synthetic aperture signal 08 processing requirements, and reducing the sensitivity to spurious 09 target motion.
The present invention provides means for removing ~he 11 dependence of transmitter power on operating frequency. In 12 transmitting the radar pulse signal, the full swath 2 (Figure 1) 13 to be mapped is illuminated. However, for receiving the 14 reflected signal, only a very narrow antenna beam which lS illuminates a small area 3 is used, which area is scanned across 16 the swath in coincidence with the arrival of the reflections.
17 The receiving antenna beam can be a single beam which scans 18 continuously~ or a plurality of adjacent beams from which the 19 appropriate beam is selected so as to produce a stepped type of scan.
21 Accordingly two antennas are used, a transmitting 22 antenna having a vertical aperture DVT which size is chosen to ~23 just illuminate the patch, and a receiving antenna which has a 24 vertical aperture DVR which size is made as large as possible, in order to produce a narrow beam which is contained in the 26 transmitter beam. The transmitting antenna can form part of the 27 receiving antenna.
28 A time dependent linear phase shift is introduced 29 across the vertical receiving antenna aperture, causing the receiver beam to scan across the patch, in coincidence with 31 signal returns arriving from increasing ranges. The kwo antennas 32 preferably have the same aperture length in the azimuth 33 direction, such that they should illuminate the same width.
34 The range W of the swath is 37 W = ~ ~ (c) 38 DV~5;A
~39 , .1. ,~S83g 02 and the range extent or length Wr o* the recelve antenna 03 illuminated area i5 06 Wr = ~~ - (d) 07 DV~s~ 0 09 where Ro i8 the distance between the vehicle carrying the radar system and the area being mapped (the radar range), ~11 ?~ is the wavelength, and 12 ~ is the elevation angle between the area being mapped 13 and the vehicle carrying the radar system.
14 With the use of the present system, the radar equa-tion is ~18PAV = ~ 7~ F~ v~ b) ` 1 9 ~ ,2 ~ Sj~ J
21 where H is t~e altitude of the transmitter, 22 K iæ Boltzman's constant ~23 To is the effective ternperature of the receiver, 24 Fn is the receiver noise figure, V i5 the velocity of the transmitter and receiver, 26 W is the range of swath ~27 NL iæ the number of statistically independent 28 images that are summed incoherently to form the synthetic ~29 aperture image, ~ T is the aperture efficiency, i.e. the main to ~31 sidelobes ratio of the transmitting antenna, - 32 DVR is the receiving antenna aperture defining the 33 length of the area, i.e. the vertical dimension of the receiver 34 antenna, ~35 ~ r is the range reæolution, `36 ~ a i8 the azimuth or width reæolution, 37 ~ O is the normalized radar crossection oE terrain ~38 within the area, ~39 S/N is -the æignal to noise ratio and :
~. lS583~
02 ~ R is the aperture efEiciency of the receiving 03 antenna.
04 It should be noted that the frequency or wavelength of 05 the radar signal forms no part o~ this equation, and -thus it is 06 clear that the required power is not related to -the radar 07 frequency.
08 In prior art systems, in which the receive antenna beam 09 illuminates the complete swath, the antenna vertical aperture must be decreased with increasing ~requency. However in the 11 present inven-tion, the vertical aperture can be made much larger 12 and does not have to be decreased with increasing ~requency.
13 Furthermore, although the swath to be mapped may 14 contain an ambiguous range, with the pr~sent invention the receiving beam does not see the ambiguous returns. The pulse 16 repetition frequency may thus be elevated to alleviate azimuth 17 ambiguities that may be troublesome when wide swath coverage is 18 attempted. The limiting factor in the present invention then i5 19 the technical tolerance of the particular antenna design which is used.
21 In a typical example, the values of the elements given 22 in the radar equation would be as follows:
23 R = 800 kms; V - 7.5 km/sec.;~O = -15db; S/~ = 25db;
24 Fn = 4.5db; W = 200 km, ~ = 66 degrees; ~a ~ ~r = 20 meters;
~ = 80~, ~ = 0.03 meters.
26 For DVR approximately ~.5 me-ters, the required 27 average power is about 200 watts. Thi~ value is approximately 28 16db lower than the corresponding power which w~uld be required 29 in a conventional synthetic aperture radar. Clearly the present invention significantly reduces the necessary transmitted power.
31 Preferably the transmitting and receiving antennas ;32 should be combined. The transmitter feeds a small number of 33 centre elements to form a wide aperture. All o~ the elements are 34 scanned for receiving, forming a narrow receiving aperture, using phased array techniques.
36 The transmit ~eed system for the cen-ter elements o~ the 37 antenna derives its R.F. power rom a stable local osc:illator.
38 The feed system i9 coupled to the antenna elements through one or 39 more circulators whose purpose is to direct -transmitter energy to ' ~ ~65~3~
02 the antenna and received energy Erom ~he antenna elernents -to the 03 receiver channels.
04 Turning to Figure 2, a block diagram of -the system 05 according to a preferred embodiment of the invention is shown.
06 A physical antenna which can be used with this invention is of 07 the kind described by C.F. Winter in the article "Phase Scanning 08 Experiments with Two Reflector Antenna Systems", Proceedings of 09 the I.E.E.E., Vol. 56, No. 11, November 1968, pp. 1984-1998.
Center elements 7 of the antenna system are fed via circulators 11 8, which receive signal~ via a transmit channel feed system 9.
12 Typical output pulse widths would be in the range of 1 to 100 13 microseconds. The transmit channel feed system receives power 14 from a power amplifier 10, which itself receives signals from a stable local oscillator ll which feeds its signals via bandpass 16 filter 12 and waveform generator 13 to the input of power 17 amplifier lO. The local oscillator signals are applied to the ~18 input of bandpass filter 12 after being split in po~er splitter l9 14, one portion of which is divided down in divider 15 and ~0 applied via a further splitter 16 to one input of up converter ~1 17, to which a second portion o~ the local oscillator signal is ~22 also applied. The output of the up converter is applied to ~23 bandpass filter 12.
~24 The reflected radar pulse signals received from all of the antenna elements 17 and 7 which constitute the entire array, 26 are fed to limiters 18, the signals from the center elements 7 o ~27 the antenna passing through circulators 8. The output signals 28 from limiters 18 are passed through low noise amplifiers l9 to 29 one input of down converters 20. A signal having successively ~30 increased time delay (i.e. delayed phase) is fed to the second 31 inputs of successive ones of down converters 20, as ~7ill be 32 described below.
33 For each radar transmit pulse, it is required that the ~34 receive antenna vertical pattern should be scanned such that it points in the direction of radar reflections as they arrive at 36 the antenna from successively longer ranges. The relative phase ~37 of each element is determined b~ the relative phase Oe the 38 phasing signal which is rnixed in down converters 20 with the 39 receive signal in each channel. This signal is yenerated in a t; 8 3 ~ , 02 voltage controlled oscillator 21, the output signal of which is 03 passed through power splitter 22 to a tappe~ delay line 23. ~ach 04 tap 24 is connected to the second input of down converter 20.
05 The voltage controlled oscillator 21 is of course enabled by scan 06 initiate pulses received from conven-tional and well known 07 circuitry at terminal 25.
08 The outputs of each of the down converters 20 are 09 applied to corresponding inputs of a combiner 25 which sums the respective input signals.
11 The electrical length between ~he taps o~ the delay 12 line, corresponding to an unique voltage controlled osciLlator 13 frequency, is such that the down converted signals in all element 14 channels interfere constructively only for those signals incident on the array at an unique vertical angle of incidence. Therefore 16 varying the voltage control oscillator frequency alters the 17 electrical length between the taps, the relative phases of the 18 down converted signals in the element channels, and therefore the ~19 vertical angle of incidence on thè array at which the element signals constructively interfere in the combiner. Therefore by ~`21 changing the fre~uency of the voltage controlled oscillator, the 22 receive illuminated area can be caused to scan within the ~`23 transmit swath. If the frequency is caused to vary linearly, a 24 linear scan will be produced; if the frequency varies stepwise, adjacent areas will be illuminated for the periods of each step.
26 The output signal from combiner 25 is applied through 27 wideband amplifier 26, through a bandpass ilter 27 to one input 28 of up converter 28. The second input of up converter 28 is 29 connected to the second output of power splitter 22, to which the original voltage controlled oscilIator signal is applied. The 31 frequency offset caused by the voltage controlled oscillator is 32 thus removed from the output signal from up converter 28.
33 The remainder of the radar system is similar to that of 34 conventional coherent radars. The output signal of up converter 28 is applied through bandpass filter 29 to one input of down ~36 converter 30; the second input o~ down converter 30 is connec-ted ~37 to the output of frequency divider 15 -through power splitter 16.
38 ~ccordingly the output signal of up converter 28 is mixed wi-th 39 the local oecillator signal which is coherent with ~he ~ 16~39 02 transmi-tted siynal. The resulting output of down converter 30 is 03 an I.F. signal, which is applied through I.~. amplifier 31 to 04 coherent demodulator 32 (to which the oscillator 11 signal is 05 applied through the third branch of power sp].itter 1~, to 06 provide in phase and quadrature si~nals at terminals I and Q.
07 These signals are processed using well known synthetic aperature 08 techniques to produce a radar image.
09 A person skilled in the art understanding this invention may now conceive of variations or other embodiments.
11 For example, more than one scanning beam can be used. Each 12 element of the li.near antenna array can be comprised oE a 13 plurality in-phase fed elements. All such variations or other 14 embodiments are believed to be within the scope of the present invention, as defined in the claims appended hereto.
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Claims (11)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A radar system comprising:
(a) transmitting means for illuminating an elongated patch with a pulse signal, (b) receiving means for illuminating an area within the patch having a similar width as the patch, and a fraction of the range extent of the patch whereby reflected pulse signals from said area can be received, and, (c) means for scanning the patch with said illuminated area.
(a) transmitting means for illuminating an elongated patch with a pulse signal, (b) receiving means for illuminating an area within the patch having a similar width as the patch, and a fraction of the range extent of the patch whereby reflected pulse signals from said area can be received, and, (c) means for scanning the patch with said illuminated area.
2. A radar system as defined in claim 1, in which said fraction is small relative to the range extent of the patch.
3. A radar system as defined in claim 2, in which said scanning means is adapted to illuminate said patch progressively from end to the other.
4. A radar system as defined in claim 2, in which said scanning means is adapted to illuminate said patch in steps of adjacent areas.
5. A radar system as defined in claim 1, 3 or 4, in which the transmitted signal average Power is where H is the altitude of the transmitter K is Boltzman's constant To is the effective temperature of the receiver Fn is the receiver noise figure V is the velocity of the transmitter and receiver W is the range of the swath NL is the number of statistically independent images that are summed incoherently to form the synthetic aperture image, ?T is the aperture efficiency, i.e. the main to sidelobes ratio of the transmitting antenna DvR is the receiving antenna aperture, i.e. the vertical dimension of the receiver antenna ?r is the range resolution ?a is the azimuth or width resolution ?o is the radar crossection of a target within the area ? is the depression angle of the transmitter to the swath S/N is the signal to noise ratio ? R is the aperture efficiency of the receiving antenna.
6. A radar system as defined in claim 1 or 3, in which the transmitting means is comprised of a first array of stacked elements, a local oscillator for generating an R.F. signal, and a circulator connecting the local oscillator to said elements, and the rsceiving means is comprised of a second array of stacked elements incorporating the first array, means connecting each of the circulators to first inputs of corresponding mixers, means connecting each element of the second array of said elements other than said first array to first inputs of corresponding further mixers, means for applying a single frequency signal to second inputs of successive ones of said mixers with a progressively increasing phase delay, means for summing the outputs of each of the mixers and for receiving a signal from the output thereof corresponding to the scanned swath.
7. A radar system as defined in claim 1 or 3, in which the transmitting means is comprised of a first array of stacked elements, a local oscillator for generating an R.F. signal, and a circulator connecting the local oscillator to said elements, and the receiving means is comprised of a second array of stacked elements incorporating the first array, means connecting each of the circulators to first inputs of corresponding mixers, means connecting each element of the second array of said elements other than said first array to first inputs of corresponding further mixers, means for applying a controllable frequency signal to said successive ones of mixers with progressively increasing phase delay, means for summing the outputs of each of the mixers and for receiving a signal from the output therefore corresponding to the scanned swath.
8. A radar system as defined in claim 1 or 3, in which the transmitting means is comprised of a first array of stacked elements, a local oscillator for generating an R.F. signal, and a circulator connecting the local oscillator to said elements, and the receiving means is comprised of a second array of stacked elements incorporating the first array, means connecting each of the circulators to first inputs of corresponding mixers, means connecting each element of the second array of said elements other than said first array to first inputs of corresponding further mixers, means for applying a single frequency signal to second inputs of successive ones of said mixers with a progressively increasing phase delay, means for summing the outputs of each of the mixers and for receiving an output signal from the output thereof corresponding to the scanned swath, and means for mixing the output signal with said single frequency signal to form an up-converted signal, and for mixing the up-converted signal with a sample of said R.F. signal to form an I.F. signal for demodulation and conversion into in-phase and quadrature signals for producing a radar image.
9. A radar system as defined in claim 1 or 3, in which the transmitting means is comprised of a first array of stacked elements, a local oscillator for generating an R.F. signal, and a circulator connecting the local oscillator to said elements, and the receiving means is comprised of a second array of stacked elements incorporating the first array, means connecting each of the circulators to first inputs of corresponding mixers, means connecting each element of the second array of said elements other than said first array to first inputs of corresponding further mixers, means for applying a controllable frequency signal to successive ones of said mixers with progressively increasing phase delay, means for summing the outputs of each of the mixers and for receiving an output signal from the output thereof corresponding to the scanned swath and means for mixing the output signal with said controllable frequency signal to form an up-converted signal, and for mixing the up converted signal with a sample of said R.F. signal to form an I.F. signal for demodulating and conversion into in-phase and quadrature signals for producing a radar image.
10. A method of detecting radar reflective objects comprising:
(a) transmitting a succession of radar pulses, illuminating a narrow elongated patch, (b) receiving reflected radar pulses from an area within the patch having the width of the patch and a range extent which is a small fraction of the range extent of the swath, (c) scanning the patch with the illuminated area.
(a) transmitting a succession of radar pulses, illuminating a narrow elongated patch, (b) receiving reflected radar pulses from an area within the patch having the width of the patch and a range extent which is a small fraction of the range extent of the swath, (c) scanning the patch with the illuminated area.
11. A method of radar mapping of terrain from a moving vehicle comprising:
(a) transmitting a succession of radar pulses toward the terrain, illuminating a narrow patch elongated to the side of the vehicle as the vehicle progresses, (b) receiving reflected radar pulses from an area within the patch having the width of the patch and a range extent which is a small fraction of the range extent of the patch, (c) scanning the patch with the illuminated area.
(a) transmitting a succession of radar pulses toward the terrain, illuminating a narrow patch elongated to the side of the vehicle as the vehicle progresses, (b) receiving reflected radar pulses from an area within the patch having the width of the patch and a range extent which is a small fraction of the range extent of the patch, (c) scanning the patch with the illuminated area.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000372431A CA1165839A (en) | 1981-03-05 | 1981-03-05 | Tracking aperture radar |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000372431A CA1165839A (en) | 1981-03-05 | 1981-03-05 | Tracking aperture radar |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1165839A true CA1165839A (en) | 1984-04-17 |
Family
ID=4119386
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000372431A Expired CA1165839A (en) | 1981-03-05 | 1981-03-05 | Tracking aperture radar |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1165839A (en) |
-
1981
- 1981-03-05 CA CA000372431A patent/CA1165839A/en not_active Expired
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