Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to radar antenna systems for use
in Doppler navigation equipment with automatic land-sea error correction, the
antenna system being designed to produce two slightly differently inclined
groups of lobes (beam lobing), using a flat radiator array consisting of individ-
ual radiators in parallel rows each end-fed at both ends, using feeder lines
extending transversely to said rows.
In Doppler navigational systems, an aircraft or the like directs
radar beams onto the earth's surface and measures the Doppler shift of the
reflected waYes. In determining the well known frequency shift phenomena only
those velocity components which are vectored back in the direction of the
particular radar beam are effective. In order to obtain the velocity compon-
ents necessary to determine movement in space, at least three measurements in
different directions and more than one plane have to bc carried out. In systems
using continuous measurement, this means that at least three beams are required,
and a fourth beam is generally provided as a standby.
The known systems enable a relatively high degree of accuracy
to be achieved in overland navigation, whereas over water an error occurs due
to the dependence of the reflected energy upon the angle of incidence of the
beam on the water. This error can be explained by the fact that the energy of
the ground echo over l~md is virtually independent o the angle of incidence
whilst the reflectcd energy over water varies substantially, both as a function
of the angle of incidence of the beam on the water, and also as a function of
the condition of the water surface tstate of the sea). Consequently the
frequency spectrum of the echo signals is distorted relative to the frequency
spectrum obtained over land ant this leads to an error shift in the measured
centre frequency. Consequently, when flying over water a correction must be
introduced.
In most of the systems thus far known, this correction is intro-
ducod by operation of a so-called "land-sea" switch. However, this is only a
compromise solution, and simply eliminates an average error over water, taking
no account of the sea condition. Furthenmore, it imposes an additional work-
load on the pilot, and requires that the pilot must be able to see the ground
or have first-hand knowledge of the terrain over which he is flying.
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The reflected energy is dependant upon the angle of incidence~
which differs for land and sea, so that an automat;c land-sea error correction
can be derived. One known system employs Doppler navigational antenna to
radiate one group of three or four variously inclined lobes onto the ground
and a further set of three or four iobes, generally radiated with a time-
staggered relationship to the first group, the lobes of this further group
having inclinations slightly modified relative to the first group (beam lobing).
The energy difference between neighbouring beams will depend upon the sea
condition, so that a correction which is actually dependent on this sea con-
dition can be automatically derived.
In the known arrangements of this type, the two lobe groups haveeither been produced by two separate antenna systems side by side, or by two
interleaved antenna systems. There is usually only a specific area available
for the entire antenna arrangement, so that if separate antenna systems are
used, then they can each be only half the size of a system used without beam
lobing, and this leads to double the beam width in a plane, and is therefore
less favourable. With two interleaved antenna systems, then a radiator of one
system must be located between each two radiators of the other, and slotted
waveguides may be used, for examplc. ~lthough thc overall arrangement is more
compact, the resulting spacing between the individual radiators of either
antenna system becomes so large that several main lobes are produced. Where
two main lobes occur at an antenna input, then the signals from these two
lobes are distinguished only by their different Doppler frequencies, and in
the case of slow-flying aircraft such as helicopters for example, they can
no longer be effectively separated from one another.
One object of the invention is ~o provide an antenna system for
Doppler navigation equipment which avoids these drawbac~s.
In accordance with the present invention there is provided a
doppler navigational radar antenna with automatic land sea correction
utilizing the production of two somewhat differently inclined mutually over-
lapping lobe groups each having four lobes, comprising a plane radiation
array group having individual radiators disposed in parallel rows each
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radiator having two ends having respective feed points, each radiator being
actually fed or fed by wa~ of radiation at said feed points by a first pair
of feed lines extending transverse to said rows of radiators and coupled
thereto at the respective feed points at one end thereof and a second pair of
feed lines extending transverse to said rows of radiators and coupled thereto
at respective feed points at the other end thereof, one feed line of each
pair having a different phase delay than the other feed line of the same
pair, the different phase delay being set correspondingly small so as to
obtain the overlapping of the lobe groups.
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Preferably, each radiator is fed at each end by a respective
pair of feeder linesi one for each lobe group. One line of each feeder line
pair is fed with the same phase delay, and a differing lobe group inclination
in the longitudinal direction of the feeder lines is obtained in relation to
that produced by the other two feeder lines. This enables a common antenna
system to be used for producing the two lobe groups the two feeder lines of
each feeder line pair being connected to one supply source or, in the case of
a receiving station, to the receiver input.
The invention will now be described with reference to the
drawings, in which:
Figure 1 schematically illustrates an aircraft provided with
means for directing two lobe groups to the ground;
Figure 2 is a perspective view of one exemplary embodiment of
~n antenna system in accordance with the invention;
Figure 3 is a plan view illustrating the ground position of the
eight main lobes ratiated by the antenna system shown in Figure 2; and
Figures 4a to 4d are fragmentary details of four alternative
coupling arrangements for feeding each slotted waveguide radiator of the embodi-
mentS of the type shown in Figure 2, each view being a transverse cross-section
at ono end of a radiator.
; Pigure 1 illustrates an aircraft 1 vertically above a ground loca-
tion 2, and provided with a Doppler navigational equipment whose antenna system
radiates a first lobe group 3 and a second lobe group 4, the respective inclina-
tions of the lobes of the two groups 3 and 4 being slightly diferent in relation
to the perpenticular between the aircraft and the ground. Fundamentally, only
three measurements, i.e. three beams in different directions and more than one
plane, are required, although it is advantageous to have a standby beam available.
Using the two lobe groups 3 and 4, it is possible to establish the dependence of
the reflected energy upon the angle of incidence, a factor which differs in the
caso of the sea from the land, so that the energy ratios of the received signals
onable sn automatic l~nt-sea error correcticn to be derived.
The exemplary embodiment of a Doppler navigation antenna system ;
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in accordance with the invention shown in Figure 2 consists of an array of
eleven rectangular waveguides 5, in whose narrow sides a plurality of slots 6
have been milled, which are slightly inclined in alterna~e directions. A
constant centre-to-centre spacing applies for the slots, and this is slightly
greater than half a wavelength. If a wave is propagated through a slotted
waveguide 5 of this type, then the output radiation is slightly inclined in
relation to ~he waveguide input. Therefore, with a double-ended feed two
oppositely inclined beams are produced. The beam direction and the radiation
pattern are particularly dependent upon the frequency, the waveguide size, and
upon the number, spacing and inclination of the slots. The output energy rises
with increasing slot inclination, and it is advantageous to arrange for the
inclination to increase towards the waveguide centre, to promote secondary
lobe attenuation. One end of each of tha eleven waveguides 5 is connected
and coupled to two feeder waveguides 7 and 8, the other end of each waveguide
is connected to two guides 9 and 10. By suitable dimensioning, propagating
waves develop in the feeder waveguides 7, 8, 9 and 10. The feeder waveguides
of each pair have the same phase delay, that is, the delay is similar for the
waveguides 7 and 9, and for the feeder waveguides 8 and 1OJ but the two wave-
guides at the same end of the radiators~ one from each pair, have slightly
different delays, i.e. that of the guide 7 differs from that of 8, and thus
that of the guide 9 diffors from that of 10. This differenco is achieved
eithor by giving them different cross-sectional dimensions or by introducing
a dielectric. Feeder points 11, 12, 15 and 16 are located at the ends of the
pair of feeder waveguides 7 and 9 for producing one group of lobes, whilst
feeder points 13, 14, 17 and 18 are located at the ends of the two feeder wave-
guides 8 and 10 for producing the second, differently inclined lobe group.
In Figure 3 shows the respective ground positions of the eight
main lobes radiated by this antenna system when fed at the fe~der points
correspondingly marked in Figure 2. The samo relationship applies to the case
of a receiving ant~nna. A central point 2 represents the incidence of the
perpendicular from the aircraft or the like to the ground. The two groups of
lobes are differently inclined with respect to the longitudinal direction of
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the feeder waveguidesO In the example illustrated, this direction corresponds
with the direction of flight.
Figures 4a to 4d illustrate various possible ways of coupling
each slotted waveguide radiator, these coupling arrangements serving simultane-
ously to opti~ise the mutual decoupling of the two feeder waveguides where a
pair of feeders is involved. In Figure 4a, two feeder waveguides 20 and 21 of
a pair are arranged on opposite side walls 22 and 23 of a slotted radiator 24
which takes the form of a rectangular waveguide. Part of each of the waveguides
20 snd 21 projects into the slotted radiator 24, where it is coupled through
slots 25 and 26. In the alternative arrangement shown in FiguTe 4b, a slotted
radiator 27 merges at its end into a chamber 28, into which there partially
penetrate two waveguidcs 29 and 30 of a feeder line pair. Figure 4c illustrates
a slotted radiator 31 whose end forks into two arms 32 and 33, in each of which
there penetrates a respective waveguide, 34 or 35, of a feeder line pair. In
Figure 4d, instead of two waveguide feeders at each end, one feeder pair is
used, a single waveguide 36 being provided for each end of esch radiatorO This
waveguide 36 has sn a~nost square section, and carries two Hlo waves which are
polarised in mutually perpendicular directions, as indicated by arrows 37 and
38. The respective delays or transit times are determined by the dimension in
the particular H-plane, and differ from one another in accordance with the sido
ratio of the cross-section of the waveguide 36. Coupling to a waveguide radiator
39 is effectod via two coaxial lines 40 and 41. The inner conductor 42 of the
coaxial line 40 projects through a side wall 43 and ths inner conductor 44 of
tho line 41 projects through a side wall 45, into the interior of the waveguide
36. At the other end of the coaxial line 40 and 41, the inner conductors 42 snd
44 penetrate into the interior of the slotted radiator 39.
It is also possible to employ a single feeder waveguide at each
end, with a standard square, rectsngular or circular cross-section9 in which
an electrically controlled phase-shift device is successively switched in or
out to slter the lobe inclination.
Instead of rows of slotted radiators, other radiators, for example
periodically curved conductors forming strip line radiators, dipole radiators,
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or slotted radiators in metal plates, Furthermore, the feeder lines need not
take the form of waveguides, but can be coaxial lines, for example.
