Note: Descriptions are shown in the official language in which they were submitted.
~ 5~5~4
SYSTEM FOR DETECTION OF OBJECTS WITH GIVEN, KNOWN
CHARACTERISTICS AGAINST A ~ACKGROUND.
This invention relates to the detection of objects
with given, known characteristics against a background.
The problems associated with the detection and identi-
fication of for example vessels against the sea surface
has interest in this connection. Primarily the invention
has been developed for radar systems and the employment
of electromagnetic waves, but obviously it may also be
applied in sonar and the like which is based upon
acoustic waves.
In radar systems which include powerful data
processing equipment the invention can give a monitoring
capability which has beenquite impossible hitherto, by
employing corresponding optimal radarstructures and
signal processing algorithms.
It is known in radar systems to transmit coherent
electromagnetic waves having characteristics adapted
to the object or objects, backgroundwavepattern or
other wavepatternsto be brought forth, or possibly
suppressed in the detection. Such adaption may be de-
signatedtransmittergenerated filter functions. The use
of filter functions at the transmitter side involves
that after an adapted detection at the receiver
side there is obtained a decision as to whether an
object corresponding to the filter function concerned,
is present. The shape of the object in the space
domain as well as its movements in the time domain are
independent parameter sets which may be analysed. A
hologram-like interference patternOf a wave or vessel
structure or both is put on thesea surface by an out-
going wave and a reflective wave interfering and
forming standing waves. Then the hologram or inter-
1;~5~5~L
ference pattern can be swept so as to investigate whichspecific propagation directions or orientations in space
are present. Because there exist relationships between
for example the movement of vessels and associated water
waves induced on and in the water, a wave hologram and
a vessel hologram can be established on the sea surface
simultaneously in order to enhance the detection
process.
It is clear that what is an "object" and what is
'~ackground", is purely a question of definition. Thus,
for example the object of interest may possibly be a
"naturally" occuring water wave pattern, whereas
vessels and accompanying wave patterns on the water
surface may be regarded as an interfering background
or noise, which the detection process attempts to sup~
press.
For a closer description of the more advanced radar
systems of the recent time, being of interest in con-
nection with the above and forming the background of
this invention, reference may be made to the following:
1. Gjessing, D T, 1977, "A Generalized Method for Environ-
mental Surveillance by Remote Sensing", Radio Science,
Vol 13, No 2. .
2. Gjessing, D T, 1978, "Re~ote Surveillance by Electromag-
netic Waves for Air-'.~ater-Land", Ann Arbor Pu~lishers Inc,
Ann Arbor, USA.
3. Gjessing, D T, 1981a, "Adaptive Radar in Remote Sensing",
Ann Arbor Publishers Inc, Ann Arbor, USA.
4. Gjessing, D T, 1981b, "Adaptive Techniques for Radar Detec-
tion and Identification of Objects in an Ocean ~nviron-
ment", IEEE Journal of Ocean Engineering, OE-6.1, 5-17.
5. Gjessing, D T, 1979, "Environmental Remote Sensing. Part
I: Methods Based on Scattering and Diffraction of Radio
Waves", Phy~ Technol, Vol 10.
6. Gjessing, D T, }~jelmstad~ J, Lund, T, 1982, 'A Multifre-
quency Adaptive Radar for Detection and Identlfic~ion of
Objects. Results on Preliminary Experiments on Aircraft
against a Sea Clutter Background", IEEE Transactions on
Antennas and Propagation, AP-30, 3, 351-365
~L25~5~
- 7. Gjessing, D T, Hjelmstad, J, 19B2, "Adaptive Radar in
Remote Sensing space, Frequency and P~larization Pro-
ce~ses", Proc IEE Radar 82, London, ~ctober 1982.
8. Gjessing, D T, Hjelmstad, J, Lund, T, 1983, "Directional
Ocean Spectra as Observed with a Multi-Frequency CW
Doppler Radar System", Int J Remote Sensing 1984, 5, 2.
9. Bass, F B, Fuks, I M, 1979, "Wave Scattering from Statis-
tically Rough Surfaces", Pergamon, New York,
l0. Dysthe, K, 1980, "Havb01ger og fysikk", Fra Fysikkens
Verden.
In the typical case of anairborne radar system the
invention - shortly expressed - is based upon the em-
ployment of antennaes being separated in space in order
to structure the hologram (the interference pattern)
in the flight direction of the aircraft. This presumes
that the main beam direction of the antennaes is trans-
versally to the flight direction and that the anten-
naes have independent modulation. In a corresponding
manner acoustic transducers will be provided in
acoustic systems, for example sonar systems. Also
systems operating within the optical part of the
electromagnetic frequency range are possible within
the scope of this invention.
Systems of the kind contemplated here comprise
receiver means for receiving backscattered electro-
magnetic or acoustic waves respectively, and for co-
herent demodulation employing signal adapted filters,
as indicated above. This signal processing at the
receiver side may be designated receiver generated
filter functions which on the basis of an assumed
or given transmitter function, performs corresponding
signal operations so as to obtain resolution in space
and time resolution in the processing domain in which
a maximum of contrast between object (target) and
background is obtained.
~:~5~LS~
--4--
On the above basis this invention provides a fundamen-
tal solution which opens up for many new and different possibili-
ties and applications, and which is primarily characterized
therein that the transmitter means and/or the receiver means
comprises two or more apertures for coherent transmission and/or
reception of the waves, and in that the mutual distance between
the apertures is ].arger than one wavelength of the waves being
transmitted.
More particularly, the present invention provides a
system for the detection of objects with given, known character-
istics against a background, comprising a transmitter means for
transmitting one of either electromagnetic or acoustic waves
having characteristics adapted for the objects to be detected or
correspondingly adapted to suppress the background prior to detec-
tion, said waves being composed of a number of coherent frequency
components in general being a function of time, and a receiver
means for receiving backscattered electromagnetic or acoustic
waves and for coherent demodulation using signal adaptive filters
and comprising homodyne detectors, wherein at least one of said
transmitter means and said receiver means comprise at least two
apertures for coherent transmission or reception of the waves
and wherein a distance between said at least two apertures is
larger than one wavelength of the transmitted waves, and wherein
signals derived from the received waves by said demodulation and
filtering, and related to at least two simultaneously transmitted
frequencies and at least two apertures respectively, are multi-
~:~5~5~
-4a-
plied by complex multiplication, wherein a signal detected by
one homodyne detector at one frequency and at one aperture, is
subjected to said complex multiplication with another signal
detected by another homodyne detector at another simultaneously
transmitted frequency at another aperture separated from said one
aperture by at least one wavelength of said waves, said complex
multiplication providing a product which constitutes a matched
filter for a given object or background.
Aircraft radar systems have been mentioned as an example
above, but it will be realized that a system according to the
invention may be employed on various types of carrying platforms,
and not only on aircraft. Here both stationary as well as movable
platforms may be contemplated. The two or more apertures may be
located at one and the same platform or they may be arranged
each on a separate platform which is mechanically independent of
the one or more other platforms.
As will appear from the following there is also in other
ways possibilities of various modifications of this new system,
including the feeding of the apertures with the same or different
frequencies, possibly time varying frequencies, and variation in
the mutual positions and orientations respectively, of the
apertures.
Particularly preferred embodiments of this invention,
inter alia at the points just mentioned, appear from the patent
claims.
In the following description the invention is to be
S~L~
-4b-
explained more closely with reference to the drawings, in which
FigO 1 shows a simplified block diagram of a radar sys-
tem utilizing the principles according to this invention,
Fig. 2 illustrates receiver generated filter functions
~;~5~ 4
which may be involved in the system of fig. 1,
Fig. 3A shows a particular embodiment according to the
invention,
Fig. 3B shows a diagram related to the embodiment of
fig. 3A,
Fig. 4 illustrates the geometry of the scattering pro-
cess in wave number filtering along the aircraft axis
(direction of movement of the platform),
Fig. 5 shows an interferogram (interference pattern)
when employing multifrequency waves, for illustrating
specific functions,
Fig. 6 shows in a diagram the socalled K-space-signature
of a seasurface and a ship respectively,
Fig. 7 is a diagram showing Doppler shift as a function
of the frequency separation between multifrequency
components used in a radar system,
Fig. 8 illustrates schematically an advantageous
Doppler signal treatment in the system according to
the invention,
Fig. 9 shows a further particular embodiment according
to the invention.
Fig. 1 is a simplified block diagram for illustrating
a complete radar system in which the principles according
to the present invention are employed. The radar of
fig. 1 is structured in blocks which are all controlled
by a computer (not shown). For the various tasks or
functions to be attended to by the radar system during
operation, for example searching, tracking, identifica-
tion), the filters or signal processing blocks of the
radar will be configurated correspondingly and for the
purpose of obtaining an optimal result. The filter or
signal processing functions involved may be implemented
to a substantial degree in the form of software for the
computer. Algorithms for reconfigurating the radar
system for the different tasks may also suitably be
~:~5~
a part of this software. Line C-C in the figure indicates
at which level it may be normally assumed that the soft-
ware part (below the line) starts in relation to more or
less conventional electronic circuits (above line C-C)
being included in the system. It is obvious however,
that this dividing line is not fixed and may be dis-
placed towards more hardwired electronics or towards a
greater proportion of software and computer processing,
depending upon available technology. In view of pre-
sent technology computer processing may as a maximum
take place starting with the band pass filtering and
proceeding downwards in fig. 1.
The transmitter part of the system in fig. 1 com-
prises in the first instance a group of frequency
synthesi~ers lFl-lF6 operating in the frequency band
50 to 90 MHz for the purpose of illuminating the object
or area being of interest, with a suitable set of fre-
quencies (waves). Thus in the example shown there are
provided six frequency synthesizers lFl-lF6 having
selected mutual frequency spacings.
Among relevant transmitter generated filter functions
in this system multi frequency illumination is mentioned
at the first hand, i.e. the simultaneous transmission
of several coherent waves or frequency components. For
example by illuminating a seasurface from one or more
transmitter apertures with a signal being composed of
a number of frequency components, the combining of
these at the receiver side will result in the genera-
tion of anumber of standing waves against the sea-
surface. The period of these waves is inversely
proportional to the frequency spacing between the fre
quency components. Several of the references mentioned
above discuss more closely such transmitter generated
filter functions, for example reference (6), page 356
and fig. 4.
S ~L 5 L~ ~
The transmitterfrequencies mentioned consitute a
coherent line spectrum of transmitted waves, which after
having been comhined in a combiner lB, possibly may be
given a polarization in a polarization coder 2 and a
polarization basis processor 3. Then follows a Doppler
pxocessor 4 and a range-gating unit 5 before the signals
proceed to a SHF-unit 6 which transposes the signals to
a suitable microwave frequency. The purpose of the po-
larization processor 3 is to establish a plurality of
channels for independent measurements of all elements
in the backscattering matrix. The Doppler processor
4 performs a precompensation for the velocity of the
object or target. The units or function blocks 2, 3
and 4 as mentioned here are not necessary for the so-
lution principle according to the invention, but may
make possible particularandadvantageous functions in
association therewith. It will be understood that cor-
responding functions may also be included at the re-
ceiver side, as shown in fig. 1.
The example of fig. 1 thus shows a structure having
twotransmitter channels which from the combiner lB leads
to two separate antennaes or apertures 7A and 7B, the
feeding of which gives a coherent transmission of elec-
tromagnetic waves. The mutual distance between the an-
tennaes 7A and 7B must be larger than one wave length
of the waves transmitted. This shall be explained more
closely below with reference to figures 3A and 3B.
Although fig. 1 is a schematic and simplified
drawing it will be realized that the antennaes 7A and
7B emit directional radiation with parallel main
beam directions for both antenneaes. In an advantageous
embodiment these are moreover located in one and the
same plane being normal to the main beam direction.
At the receiver side there are provided two an-
tennaes 8A and 8B with a mutual arrangement which may
~51~
be quite similar to what is described with respect to
the transmitter antennaes. In two corresponding and se-
parate receiver channels there may possibly be included
functional blocks having functions being complementary
to the blocks 2, 3 and 4 at the transmitter side. The
polarization coder in the receiver part in addition to
the through-going two receiver channels from the respec-
tive antennaes 8A and 8s,comprises a specific unit 10
which is necessary in order that the detection shall
provide a complete measurement of polarization scatter
values, which has no direct interest to the main idea
according to the invention. The receiver part further
comprises a number of homodyne detectors ll which in a
way known per se provides for the necessary detection.
From the detectors there is delivered to a band pass
filter bank 12 a number of signals corresponding to
the six transmitted frequency components. Since the de-
tection involves both amplitude and phase, the number of
signals delivered by the band pass filters wi~ be
twelve.
The band pass filter functionrepresented by block
12 in fig. 1 as known and conventional is adapted to
provide a narrow-band sensitivity within the respective
frequency bands, soas to eliminate interference and
noise. The following blocks 13 and 14 comprise more spe-
cific functions, namely a multiplication in 13 and a
coherency filter function in 14. These shall be explained
more closely below.
As the last (lowermost) functional block in fig. 1
there is shown a display block 15. This serves to pre-
sent the final product of the processing to the out-
side world, usually represented by an operator. In ad-
dition to or as an alternative to representation to an
operator, the block 15 may of course comprise automatic
alarm functions or control functions activated for
~25~5~14
example upon the detection and identification of a
certain object.
Picture (frames)A-G in fig. 2 illustrate functions
which are included in or may be incorporated into the
system of fig. 1. Thus fig. 2A shows a practical example
of an application of a radar system, namely towards a
seasurface having water waves 21 as indicated and with
a vessel 22 in movement. The oval picture section shown
may be regarded as the "scene" which is illuminated by
the directional transmission of electromagnetic waves
from a platform, for example an aircraft.
In figures 2~ and 2C there are illustrated an
azimuthalcompression and range gating or focusinq func-
tions respectively, not being directly related to the
present invention, but indicating that the principles
according to the invention may also be used in connec-
tion with conventional SAR-processing (SAR: synthetic
aperture radar) and for special additional functions,
such as range focusing. There is here the question of
receiver generated filter functions being mainly based
upon wave number filtering in order to obtain an en-
hanced spacial resolution.
Functions as illustrated by frame pictures 2D, 2E
and 2F in the figure are related to wave number Doppler-
filtering for establishing wave number spectra and
associated Doppler-spectra. More specifically fig. 2D
shows in the principle a diagram of the socalled K-space
signature for sea 23 and ship 24, which shall be explained
more closely below with reference to fig. 6. This form
of presentation (in the K-space) is described in re-
ferences (6), (7) and (8).
The multiplier block 13 in fig. 1 in analogue circuit
technology and/or in a programmed computer attends to,
inter alia,the wave number filtering which gives a pre-
sentation as in fig. 2D.
~5~
By illuminating thesea surface from a transmitter
aperture with a signal being composed of a number of
frequency components, and by combining these at the
receiver side there will be generated a number of stan-
ding waves against the seasurface with a period being
inversely proportional to the frequency separation
(distance) between the frequency components.
When the frequency separation between two frequency
components is ~ F there will be a modulation in space
with period c/2~ F. By transmitting a number of fre-
quency components equal to n having differing mutual
frequency separations, there is obtained by combination
a number of n(n-l)/2 different modulation periods
(wave number).
The transmitted signal s(t) will then be written as
a sum of frequency components
fn
s(t) = exp(j2~fit~ w(t)
fi
in which
w(t~ = a suitable window function in the time domain
in which space selective analysis of wave numbers is
desired.
The above number n(n-1)/2 appears at the receiver
side as a number of signals from block 13. Employing
for example six transmitted frequency components this
number of signals being processed is equal to 15.
Fig. 2E, 2F and 2G relate to possible and desirable
additional operations after the actual detection, namely
Doppler-filtering (fig. 2E) in which the ship is brought
forth at 26 in relation to the sea 25, and beatfre-
quency-Doppler-filtering (fig. 2F) in which the curve
27 (approximately rectilinear) is associated with the
~51$~
ship, whereas curve 28 relatesto the sea. This latter
form of diagram shall be explained more closely below
with reference to fig. 7. Finally fig. 2G is concerned
with the formation of mutual space/time coherency spectra
by means of specific filter functions being represented
by block 14 in fig. l.See also explanations below with
reference to fig. 8.
When applying filter functions which in a simplified
and fundamental manner are illustrated in fig. 2, the
various characteristics of objects such as ships, in-
duced waves and background waves, will be utilized for
enhancing the detection capability of a radar system,
possibly a sonar system or the like.
For the fundamental solution according to the inven-
tion there is a concept of substantial significance,
which may be designated transverse space variable
multi frequency illumination, i.e. the transmission of
electromagnetic or acoustic waves towards a scene com-
prising an object and/or a background. In the example of
airborne radar and by employing several antenna apertures
along the flight direction of the aircraft, wave
numbers may be assigned a component in the flight
direction which may be utilized for generating peri-
odities random orientation in relation to the flight
direction of the aircraft and to the pointing direction
of the antennaes.
By using a signal form
s ~fn
l(t) = f exp (j2~fit) ~(t)
i
in aperture 1 and a signal form
fn
s2(t) = ~ exp (j2~(fi+ ~ f(t)t) ~(t)
fi
~:~515'~
12
in aperture 2 separated by a distance x there is ob-
tained a sweep of the wave number vector as illustrated
in fig. 3A. The above apertures 1 and 2 are represented
in this figure by the two antennaes 31 and 32 schema-
tically indicated as belonging to an aircraft 30. The
antennaes illuminate an area of a seasurface 33 with
an interference pattern which can vary with time, as
will appear from the above mathematical expressions
regarding signal forms sl(t) and s2tt).
The resulting spacial wave number vector KreS may
be given a rotation (~) in the horisontal plane by
letting the transmitted signal at one of the antenna
apertures 31, 32 have a differential frequency dis-
placement (see the term ~f(t) in the expression for
s2(t)). This may vary with time. Moreover it will be
understood that a randomly chosen phase difference
between the apertures may be employed.
Considering that the resulting hologram or inter-
ference pattern which the antennaes 31 and 32 esta-
blish in common, shall have desired or predetermined
particular configurations which may deviate very sig-
nificantly from what is established by the antennaes
individually, it will be necessary to locate the an-
tennaes at a mutual distance (x) which is larger than
one wavelength of the frequencies employed.
Similar to fig. 1 there is shown in fig. 3A a lo-
cation of the antennaes 31 and 32 side by side in the
same plane with parallel main beam directions in the
same sense.
For acloser explanation of the operation according
to fig. 3A, there is shown in fig. 3B how the frequen-
cies being applied to the respective antennaes 31 and
32 vary with time. While antennae 31 has a constant
frequency, the antenna 32 has a periodic time varia-
tion as a sinus curve. The period is -~ , wherein ~ is
13
the rotational velocity of the vector Kres in fig. 3A.
From the preceeding discussion of figures 3A and 3B
it appears that the system may operate with the same
frequencies or with different frequencies and possibly
with time varying frequencies at the two apertures. The
choice of frequency variation and combination will de-
pend upon which objects and/or bakgrounds are of sig-
nificance in a given practical use.
It will also be understood that the apertures must
not necessarily lie in one and the same plane normally
to the main beam directionor directions. The apertures
may be arranged with a mutual displacement in the beam
direction and in that way form interference patterns
corresponding to that object or those objects which are
to be detected, possibly to a background which shall be
suppressed or emphasized. See fig. 9.
The geometry of the scattering process in wave
number filtering along the aircraft axis is illus-
trated in fig. 4. This shows a platform 40 provided with
a number of transversally directed apertures 41-48
which illuminate an object 50 the reflection co-efficient
of whichvariesaccording to curve 51 for the waves em-
ployed. Upon back_scattering from the object there will
appear at the receiver side a resulting field strength
distribution as shown with curve 49. The apertures 41-49
or corresponding receiver apertures will then sense
respective field strength values asshown at 41A-48A.
Theangular spectrum ES(K) of backscattered field
strength 49 from the structure represented by the object
50, is the Fourier-transform of spacial field strength
E(K) as seen in fig. 4.
E(K) ^~ ¦ E(x~ e j dx
in the same way the angular power sp~ctrum P(~) will be
P(~) = E(K3 E*(K)
~ ¦ RE(r) e jKr dr
~ :~51~
14
in which
RE(r) = the autocorrelation function
of the field strength interferogram E(x) and r is an
increment of x. Since
IKI = ~ - sin ~/2
and
r = V
whereby
A = the microwave wavelength
= the aircraft velocity (when the platform is an
aircraft)
~ = time increment
K = ~/C
one has
P (~) = J RE (V~) e i ~c d(V~)
which means
p(~) ~ W(f) ~ a(R)
This means that by taking the power spectrum of the
interferogram (interference pattern, "hologram")
through which the platform (aircraft) 40 moves, there
is obtained a direct expression for the transverse
distribution of scattering elements in the object
structure 50.
The above references (1) and (2) treat the basic
principles for what is discussed immediately above in
connection with fig. 4.
Another and equivalent presentation which explains
the same as fig. 4, but from a different point of view
is shown in figures 5A and 5B. The situation according
to fig. 5A corresponds in the principle to the one in
fig. 4 in that a platform 55 with antenna 55A is
moving at a velocity v, whereas an object represented by
two points at a mutual distance ~x locatedat a dis-
tance R from the platform 55, is illuminated by the
antenna 55A thereof. Between the antenna and the object
there will be standing an interferogram as shown in
fig. 5B, wherein curve 56 indicates the antenna
pattern (interferogram) as a whole, whereas for
example sections 56A and 56B show the position and the
size respectively of the object, obtained as a result
of the previously described multi frequency illumination.
By putting together wave number information provided
according to figures 4 and 5 it will be possible to im-
prove the detection in a radar system, for example for
discriminating between vessels and waves. The presen-
tation then takes place by means of a diagram as shown
in the principle in fig. 2D and generally discussed
above. More detailed an example of such adiagram is
shown fig. 6. Along both coordinates in the diagram
there is a frequency separation scale and there is plot-
ted a K-space signature for a ship at 61 and for the
sea at 62 respectively. The signature 61 as shown has
been calculated theoretically, whereas the sea sig-
nature 62 has been measured.
Wave number plotting in a diagram as in fig. 6 thus
generally can be used for representing a scene including
objects/background by establishing a harmonic hologram
(interferogram, interference pattern) as explained pre-
viously.Quitegenerally by using complicated hologram
patterns with respect to time variation, frequency
components and aperture arrangement as mentioned above,
it is moreover possible to adapt the system for scenes
(objects and background~ of the most differing types,
but still describe the response in a diagram of the
kind shown in fig. 6 (and fig. 2D). All the previously
discussed filter functions may be of interest in this
~:Z SlS~
16
connection. On the basis of what is described here with
reference to figures 4, 5 and 6, the signal processing
at the receiver side by wave number filtering may in
other wordsgive a registration of the response of a
number of simultaneous interference patterns formed by
transmitting waves towards an object, and determination
of the degree of correspondence between the interference
patterns and the object to be detected.
Fig. 7 is a diagram obtained by subjecting a point
sample from fig. 6 to a time analysis by Doppler-
processing (see fig. 1 and 2). Such processing and pre-
sentation is explained in the above references (6), (7)
and (8). The diagram of fig. 7 shows Doppler-shift as
a function of frequency separation and two curves re-
lating respectively to a rigid object in movement (curve
71) and to gravitational waves on water (curve 72). I'he
latter curve is based upon the theoretical dispersion
relation for gravitational waves on deep water.
As it appears from curves 71 and 72 in fig. 7 the
wave number Doppler-spectra for a vessel (rigid object)
and a seasurface with waves, can only have a co-incident
maximum spectral maximum for one value of the frequency
separation ~F. For other values of ~F therefore this
form of filter function will result in a
further enhanced contrast between sea and vessel, over
and beyond that which is obtained with the remaining
filter functions described.
This Doppler-filtering of the wave number spectrum
with a calculation of the Doppler-spectrum of the non-
averaged autocorrelation function R(~F) gives informa-
tion as to how different structures which resonate with
the frequency separation concerned, are moving. The
Doppler-spectrum is calculated as
P~F~f) = ¦ Efi(t) Efj*(t) expij2~ft), f~-fj = ~F
T
15~
17
Accordingly such Doppler-processing makes possible
the determination of the movements of the object with
respect to the interference pattern (hologram, inter-
ferogram) established.
More specifically the Doppler-curve 72 for the sea
has a square law shape, whereas the vessel curve 71 is
linear with an angular coe fficient which is propor-
tional to theradial velocity of the vessel (in re-
lation to the observation platform). The one possible
coinciding value mentioned, is given by an intersection
point between curves 71 and 72, lying at the right out-
side the diagram of fig. 7.
As the last functional block in front of the display
block 15 in fig. 1 there is shown a particular filter
function designated mutual coherency, i.e. by filtering
wave number series. This is implemented for example as
illustrated in fig. 8. Also here there is taken as an
interesting use a rigid object,such as a vessel against
a background in the form of the sea surface.
After deconvolution and normali~ation processes the
time axis of the autocorrelation functions R(QF,t) is
stretched (at 81 and 82 respectively) sothat the dif-
ferencies in QF are eliminated. This is effected by
stretching the time axis with a factor QFr/QFi, where~y
~Fi is the requency difference of the R-function and
QFn is the reference frequency difference.
Then each of the frequency components in the nor-
malized functions R(QFi,t) and R~QFn,t) are correlated
(at 83). Because the seasurface is dispersive and com-
pressible it will not give any correlation, whereas in
contrast rigid objects such as vessels will give a
significant correlation.
Instead of being with respect to the time access
the stretching described can possibly be applied to the
frequency axis, which is in full analogy. At this point
the invention generally implies that the received and
processed, complex Doppler-signal components resulting
from the multi frequency waves transmitted, are treated
in pairs by stretching of the time axis or the frequency
axis of the highest frequency component in each pair
and/or compressing of the time axis or the frequency
axis for the lowest frequency component in the pair,
in order to bring these components to the same frequency.
This stretching and/or compressing is carried out with
a factor being dependent upon what is to be detected.
After this pair processing the two signal components
are correlated so that there is obtalned an improved
detection of objects.
In the case of detection of underwater objects in
movement generating internal waves in a water mass with
consequent modification of a surface wave pattern on
the water, the above stretching and/or compressing of
the time axis or the frequency axis is carried out with
a factor given by the dispersion relation for internal
waves at the density profile concerned in the water mass.
The presently discussed mutual-coherency-function
obtained by correlating normalized Doppler-spectra from
one difference-frequency-pair with another, makes it
possible to obtain an indication of the rigidity of the
object or the structure being of interest.
When installing a system according to the invention
on a platform intended for movement, for example an air-
craft, a satelite or a ship, this platform can advantage-
ously and in a manner known per se, be provided with an
inertial reference device for stabilizing the antenna or
the antennaes (apertures) being included in the system.
It will be realized that in the case of movable platforms,
in particular when the two or more co-operating apertures
which shall have a determined spacing, are divided be-
tween several such platforms, special precautions must
be taken in order to compensate for possible mutual
~s~s~
19
movements. Advantageously this can be obtained by means
of a compensation device based upon phase shift of signals
involved, for establishing a local and time limited sta-
tionary interference pattern of the waves. Such a solu-
tion will make possible,inter alia,particular arrange-
ments in which at least two apertures are located each
on a separate platform which is mechanically independent
of each other, for example two airplanes.
For a closer explanation of the possibilities just
mentioned, reference is made to fig. 9. This shows an
aircraft 90 provided with a number of antennaes 91-99
the main beam direction 90A of which may be parallel to
the longitudional axis of the aircraft. With the location
schematically shown the antennaes have a mutual displace-
ment in the main beam direction. Normally only a smaller
number of these antennaes 91-99, in particular the pairs
thereof, will be used for simultaneous, coherent trans-
mission and reception respectively. By means of a com-
mutator 100 there may be effected an electronic switching
between the antennaes, so that among other things there
can be obtained a time variation of the mutual distance
between those antennaes being activated at any given
time. Moreover in fig. 9 there is shown schematically
an inertial reference device 101 which through a com-
pensation device 102 based upon phase shift of the
antenna signals,makes it possible to establish a desired
interference pattern, in particular alocal and time
limited stationary interference pattern.
Finally experts in the field will understand that
in such systems there is in the principle a complete
reciprocity between the transmitter side and the re-
ceiver side. In substance therefore, features which
have been discussed in the above description with
reference to either the transmitter side or to the
receiver side, will apply correspondingly to the
~Sl~fl,~.
other or complementary side, i.e. the receiver side and
the transmitter side respectively.
