TRACKING APERTURE RADAR

RADAR DE POURSUITE

English 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.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
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.
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
<IMG>
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
11

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
12

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.
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.
13

Description

~ ~5~39
01
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~
01 3
02 PAV = a7J~ v~ (a)
04
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 (~
01
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
36
37 W = ~ ~ (c)
38 DV~5;A
~39
,

.1. ,~S83g
01 5
02 and the range extent or length Wr o* the recelve antenna
03 illuminated area i5
04
05
06 Wr = ~~ - (d)
07 DV~s~ 0
08
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
16
17
~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~
01
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~
01 7
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 ~ ,
01 8
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
01
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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