CA1121035A - Radar target reflector - Google Patents

Abstract

RADAR REFLECTOR

Abstract of the Disclosure A radar reflector has at least six corner reflectors directed outwardly of a major axis. The reflectors are disposed along two successive helical paths one of which paths is sinistrorse and the other of which paths is dex-trorse. In a preferred embodiment, ten corner reflectors are employed which are directed evenly about an angle of 360°.

Description

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The invention relates to radar re1ectors and more particularly but not solely to such reflectors for use on sea vessels.
Radar reflectors are.employed to improve the radar echoing properties of objects or land formations with a view to improving the detection of such objects or formation by radar scanning equipment. Radar reflectors of this type to be fully efficient should reflect radar waves back parallel to their initial direction.
In many applications it is advantageous if the reflector is capable of providing reflection of radar signals in any direction and in applications such as in sea vessels it is advantageous if this capability is not badly affected upon heeling of the vessel, Corner reflectors, constructed o three sheets of reflective material which are mutually perpendicular, i.e. orthogonal re-entrant trihedrals, are known to provi.de effective reflection over a,range of angles of incidence, with the signal strength decreasing as the obliquity increases, forming a lobe.

This invention ha~ heen arrived at by consideration of the above mentloned requirements and seeks to provide a radar reflector which provides effective reflection of signals received from any direction in a horizontal plane.
According to the invention there is provided a radar reflector comprising at least six corner ref'lectors directed outwardly of and disposed 'helically about a major axis of the reflector along two successive hel~cal paths one of which paths is sinistrorse and the other o which paths is dextro~se.
The corner reflectors are preferably evenly distributed to cover the full 360 of horizon, In one advantageous :form of the invention ten corner reflec tors are employed.
A reflector in accordance T.~ith the invent.ion mav be formetl from a strip of radar reflective sheet material folded i.n alternate directions along fold axes spaced apart on the strip

2 ~

~ V35 and extending tr nsversol~ across tne str;p with t-~o consec~ltl~te ones of the '-OlCI axes d 1 sposed intermediately being substantially parallel and the remaining folds being alternately convergent and divergent in a direction from one edge to the opposite edge of the strip the folds dividing the strip into sections adjacent sections being disposed at right angles and a separator plate being provided between and at right angles to each pair of adja cent sections to form therewith two corner reflectors. The separator plates may be rectangular but rectangular plates naving one point cut off are to be preferred, the plate being positioned such that the edge where the point ha~ been removed is remote from the adjacent sections. This cut away avoids interaction with reflections from other ones of the corner reflectors.
The edge of the strip and/or the cut away point of the separator plates can be profiled such that they have an edge profile conforming to part of the internal surface of a cylind-rical housing to permit slidable and secure location of the reflector within the housing.

.
In order that the invention and its various other preferred features may be understood more easily, an embodiment thereof will now be described, by way of example only9 with reerence to the drawings, in which:-Figure 1 is an elevational view of a radar reflector constructed in accordance with the invention, Figure 2 shows a blank strip for bending to form the reflec-tor of Figure 1 illustrating the bending axes, Figure 3 shows horizontal projections of two adjacent sections of the target radar reflector of Figure 1 illustrating angle of twist, Figures 4a and 4b are circular and elliptical sections of a stepped helix, Figures Sa and 5b are schematic elevational views of oppo-site sides of a stepped helix, Figure 6a is a schematic elevational view of a corner reflector 9 Figure 6b is a schematic plan view of the corner reflector ~ 35 of Flgure 6a, Figures 7~. and 7b are schematir tilted corner views in plan and perspective respectively, Figure 8 is a polar diagram showing schematically the con-struction viewed from above, Figure 9 is a predicted polar diagram showing the response of the radar reflector, and Figure 10 is a side view of a demountable reflector con-structed in accordance with the invention and folded into a flat condition.

In the drawings Figure 1 shows a particularly advantageous form of the invention hauled up to the cross tree of a mast. The radar reflector indicated generally at lO is formed of a strip of radar reflective material e.g. 18 s.w.g. sheet duralum-inium or stainless steel. The strip is folded along axes which extend transversely across the strip in concertina fashion. The folds divide the strip into a series of sections 11, 12 and 13 adj~cent ones of which are disposed at right angles.
A flat strip suitable for folding to ~orm in this case triangular divislons is shown in Figure 2. The chain lines indicate axes at which the fold is to be forwards and the dot and chain lines indicate axes at which the fold is to be back-wards. It will be apparent from the drawing that the fold axes in this case are all of the same length.
The folds defining the centre ~ection 12 of the strip are parallel, the centre section being of parallelogram form. The other folds are alternatPly convergent and divergent in a direc-tion from one edge to the opposite edge of the strip and divide the strip into triangular sections 11 and end sections 13 of basically trapezium form which end sections are cut away to one side of an axis extending at right angles to their adjacent fold axis to leave only the portion with the shorter side at the edge of the strip.
The folded strip forms a spine having seven sec~ions adjac-ent ones of which are disposed at right angles. Each pair of adjacent surfaces of the sections is provided with a sheet metal ~ 35 divider 14 which is afixed thereto 'oy ~or e~amp'le rive~Lting Ot welding at righr, ang~es to both surfaces to ~orm a pair of cor~
ner reflectors ln the form of ortllogonal re;-entrant trihedrals which are capable of acting as elementary reflectors.
The radar reflector can be hung from one end from a point adjacent the axis at which the end section is cut away or can be hoisted by a similar connection at each end as shown in Figure 1. The reflector hangs normally by its own weight with the surfaces of the sections inclined alternately at 45 above and below the horizontal.
, The maximum reflecting capability of a corner reflector occurs along an axis extending equiangularly between the faces of the corner and this axis may be,termed the directional axis of the reflector. When the reflector is hung as previously des-cribed the directional axes are inclined above or below the horizontal at a constant angle.
The folding of the strip to form the spine results in an effective twist or change in azimuth of each fold relative to its adjacent one. Figure 3 shows only two adjacent sections to facilitate illustration of the twist which occurs.
It will be seen that bisectors of the two sections are disposed at horizontal angles 2~o to each other. It has been discovered that if the twist is arranged such that the reflectors on adjacent olds are directed with an azimuthal displacement of about 36 then a most efficient "all round" reflection coverage results. The reflected signal strength at a lobe width or 36, i.e. ~ 18 from the directional axis, is sufficiently low that overlap of the lobes of diferent ones of the reflective corners at this level have been found to introduce an acceptably narrow deterioration of the polar response of the radar target reflec-tor due to phase cancellation. Accordingly ten elemental reflectors evenly disposed ar~und a polar axis have been found to give a particularly good polar responseO To provide this displacement the angle "~ o" should be about 1 8o It will be , appreciated that in view of the twist the solid ang~es of the elemental reflectors all diver~e radially from 'cwo helical axes one of which is sinistrorse and the ot'her of which is dextrorse.

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The sections 11 need nct be trl~ngular but can be oL trun-cated trian~ul-~r iorm t~at is of trapezium shapeO
There now ~ollows a mathematical analysis of the const~lc-tion.

The circle in Figure4a represents a right section of a cylinder in which are contained the stepped helices of a reflector. The trapezium shown is the pro~ection of an actual trapezium of construction on to the circular plane which is normally horizontal. All intersections, dimensions and angles in this plane will bear a zero suffix. The actual trapezium of construction is at 45 deg to the circular plane. Its plane will be an ellipse. O, W and W' are in both planes because ~hey are on the axis of rotation.
Note QOPo is para~lel to SoNo (and parallel to OVO) , ~ ~
OV T , OU S , Q T S are constructed right angles Let Q S = P N = P
-o o o o o QoPo qO
S N = s - o o o ~0 OU = x Q T = t o o o Q O = OS = r o o o SoQoTo = ~o, the half-twist angle O Q T = ~
o o o o Problem : Given r , ~ and x . o o o (i) Calculate pO, qO~ sO, to etc 9 then (ii) Calculate p9 q9 ~, t etc In t'ne tilted plane formed by a 4' deg rotation abou~ axis T,~W~.

Because 0U bi.sects Q S
o o o P = 2 ¦ r 2 _ x 2 .,...... ,.~... ~,... (1) In ~ 0Q V
o o qO 2 rO sin ~ O~O~ (2 In ~0Q U
o o tan ( ~O + ~ O) = 2 xO .. O... ..~... ,.... (3) Po Combining (2) and (3) q = 2 r sin (tan 2 xO ~ ~ ) ..................... (4) (the brackets con~
pO tain ~ O) In ~ S Q T
o o o sin ~ - s - q o o o ~ Po ie sO - 2 pO sin ~ O ~ qO ~I~DO~ (5) Now, in the tilted plane, q = q ~2 (see Figure Sa) s sO ~2 : Therefore, from (4) and (5) q = 2 ~2 rOsin (tan 2 xO - ~ O~ o - - ~ (6) pO
and s = 2 ~2 p sin ~ O ~ J2q ~O~ (7) In ~ S Q T
o o o tan ~ O (sO qO~ r ~ ~ ~ e ~ ~ o ~ 8 ) 2t ~ 35 And in ~ SQT
tan ~ ~ ........... (9) 2t ( O qO~ ~2 2t = J2 tan ~ O

Therefore ~ = tan (J2. tan ~ .O.~..... (10) Because planes QQOPOP and SSoNoN are parallel QoTo = t c QT
Examining the plane SS N N (Figure 5b), Q will be directly above T, distance t SST = ST - TST = ~ - q2 ~ 2 QST PO S QOSO

Consider ~ SQST

`SST = tan SQST
.' T
ie SQST - tan ~ ........... (11~ (call 2 ~2 pO , see later) also QST = COS S~ST = PO

.: p= P
O
cos tan 1~ s - q __ 2~po ~ O~ 12) cos tan 1 sin ~

,:

Finally note in ~ SoQoTo ~Figure 4a~
Po Y o and in ~ OQWQ

= tan~ tan ~ _9L___ \ ......... ,... (14 OWQ ~rO cos ~

Definition of the unit trapezium is now complete.
The position of the separator plates must now be definedO In the circular plane of Figure 4a each is defined by the line U O Y . U is at the apex of the two reflecting corners. (Note however U - UO, because both are in the circular and tilted planes~. O is on the cylinder axis (midway) between the inter-sections of the axis with adjacent trapezia. YO is located arbitrarily on the U O axis at some point within the cylinder envelope.
Because QS is tilted at angle from the horizontal, so the plane of the separator plate will be tilted at angle from the vertical. Thus the separator plate will be situated on the.
tilted.plane QSNP a~ UX where X is on PN (see Figure 4b~, On the next PN ~old above XYZ9 ~N~ say, there will be another point X9 where the plane of the separator i.ntersects P/N'~ How ever, P9N~ will not be in the vertical plane of PN~ but another, also vertical. but rotated through the tWlSt angle, In fact UX
= U~' by symmetry.
Also SUX = QUX = SUX' = QUX' = 90 deg.
Now calculate the dimensions of the individual reflectors. They are QXX' which has edges UQ9 UX9 UX' and SXX9 which has edges US, UX, UX~
: Of these edges UQ = US ~bisected chord Qk an ellipse~ and so an~ UX = UX' (see above) constructed) UO~ US ........................... ~. J A ,.. 0 (15) Consider ~ XJP in Figure 4b q + -~ . ux ~ p 2 sin`~ - 2 tan ~ 16) sin (90-2~) sin (90 .~. UX = ~ (q ~ ......... (17 sin 90 + 2Y 2 sin ~ 2 tan ~

= cos ~ (q + ~ ......... (18) cos 2 ~ 2 sin ~ ~ 2 tan ~
A hypotenuse length can now be calculated using -the smallest of the edges (15) or (18) and multiplying by ~2.
10 ~Z~:
It has been assumed this far that the stepped helix has been constructed of trapezia with sides QP and SN straight and para-llel. In fact they could be extended to the wall of the enclosing cylinder when they would assume an elliptical curva-ture.
It can be simply shown that the smaller semi-diameter is on the axis WW' and is rO~ the radius of cyl:inder~ The major semi diameter is then J~ rO.
~C~
Let be the angle of tilt of the fold to the horizontal. This is angle ~ST described in association with Figure 5b.

From (11) ~ = tan (s - q) = sin (s - q) 2 ~po 2v~ p sin /sin ~ o~(l9) ~ J .
Thus, in Figure 4a direction US is inclined upwards at deg " UQ " " downwards at deg UOO " Horizontally Each lobe wlll therefore be inclined at a characteristic eleva-tion, between 0 and deg, up or down as appropriate, as determined by its azimuth between the face and edge of the corner (see Figure 6a).

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Reca]l tha~ the lo~e a~imuth is at tan y~ )from the face of the corner, Recall that the lobe ~zimuth ls at tan ~ from the ~ of the corner, the plane of edge-to-face-centre is in the plane of the incident radiation (ss Figure 6b). But it is not, S is ~ilted upwards deg about axis FU (and Q is tilted down), see Figure 7a.
If S is the projection of S in the horizontal plane, note (i) FUS being 90 deg, FUS C 90 deg, (ii) the angle between the lobe peak and the fold US (LUS in Figure 7b)~ which was formerly tan 1 J 2 must now be less. Call this angle ~ (= L US in Figure 7b).
First calculate the lobe elevation. As it is a concomitant of heel (~ ) it can usefully be calle~ ~ (= LUL in Figure 7b).
~ "~ ~ o ~ ~_~
Note in Figure 7b th~t SUF, SLU, SSoF, SS U, LL U and LL F are all 90 deg.
Thus in ~ s LL F and SS F
o o sin LFL ~ LL = SS
o _ _ LL - SS LF
o o FS

Because sin ~O = LLo UL
sin ~ = SS I.F
o o .
FS UL .O~ (20) But SS = US sin ~

LF = US sin tan ( ~ ) ~ 2 30FS = US
cos tan ~2) ~1 ~ 33~

and UL = rJs eo5 t~n 1(~2~

Therefore sin~ = sin sin tan 1 ( - ) v~ o~(21) Now find ~ - 1, USO, the angle between the azimuths of the direc-tional axis of the lobe and the fold.
cos ~ = rJL
USO
=~
- US cos ~

~ ~ (22) - -- - O .. , . O ~ ~
l 2 co~ ,~

Considerlng the construction of Figures 1 and 2, which I
call an ambiorse construction, with the sinistrorse folds Nos:
1, 2 and 3 on top, and No: 1 topmost, The spine before folding i9 shown in Figure 2, Let U9 start at fold No: 3 for (ultimate) simplicity. Fold No: 3 define~ the azimuth datum) 09 in the horizontal projection ~hown in Figura ~9 where the construction i9 viewed from above. Each foid i9 tangential to the circle, radius x which is the locus of the corners U, The face of the o plate shown in Figure 2 is defined as its 'front' face, and the odd-n~mbered folds (which are shown as chain lines in Figure 2 and dotted in Figure 8~ and which have reference numerals en- -2~ circled in Figures 2 and 8) are produced by folding the plate forwards for example see fold No: 3, i.e. the front is the face on whlch the corners 3L ~nd 3R will be situated. The other face is the 'back', and the (even~numbered) backwards folds are ; shown as do~ and chain lines in Figure 2 and as solid lines in Figure 8 and with reference numerals not circled in Figures 2 and 8.
Adjacent fol~s are folded in opposite senses (Figure 2), i.e, the plate is folded rom top t~ bottom f~ rn~tely for~ff~rGs and backwards9 wif~h odd~numbered folds forwards (encircled) and even-numbered folds backwards.
Going from ("start" in F~gure 8) Fold No: 3 to Fold No: 2 up thefsinistrorse he~ix causes a ~ hand turn throu~h the ~2 twist angle ~2~ 35.8 in ChiS examp1e). Similarly going from Fold No: 2 to Fold No: 1 causes the same 35.8 right handed turn.
These are shown in Figure 8~
Fold No: 4 is parallel to Fold No: 3, and is of opposite sense. It is the uppermost of the three ~Nos 4, 5 and 6) dex-trorse folds forming the bottom half of the whole construction.
Going from Fold No: 4 to Fold No: 5 down the dextrorse helix causes a ~ -hand turn through the twist angle, and similarly again from Fold No: 5 to Fold No: 6 ("Finish")O
The horizontal projection of each pair of corners for each fold is shown in Figure 8 following the construction described above. In the following Table 1 are shown the fold azimuths (left and right, when viewing from behind the reflector, i.eO
towards the central axis). Hence the lobe azimuths (left and right) for each fold are given9 being ~ degrees (see Eqn. 22) into each corner from each fold azirnuth. The lobe azimuths for the de~xtrorse helix are exactly at 180 to those for the enantiomorphic sinistrorse helix. The lobe azimuths are shown around Figure 8.

. . ~.
Fold . Lobe Azimuths Lobe Fold Azlmuths9 deg ~. .
No: deg. ~levatlon*
. . , _ . ~ . ~ ~ .. _.
1 (L) 71.69 251.6 (R) (L 125.1) (-) R 198.1 2 (R) 35.8, 215.8 (L) R 342 3

3 (L~ 0 , 180 (R) L 53.5 _ R 126.5

4 (R) 0 , 180 (L) L 233`.5 +
'~ R 306.5 _ , 5 (L) 35.8, 215.8 (R) L 89.3 +
R ]62.3 6 (R) 71.6, 251.6 ~L) (L 305~1) (~) R 18.1 , ...... . _ ~_ _.,, .. _ . __ . . ~_ ~ 9.77 deg above (~ or below ~) the horiæon.
Thus the whole 360 degrees of azimuth are covered by 12 corners with two overlapping pairs 7 o~e corner of each of which can be eliminated as they are at GppOslte ends of ~he construction (lL

~3 3~;

and 6L, bracketted in the Table), leaving 1.0 lobes.
So the azim~thal sequence of the remain.ing lobes is as in Table 2.

__ .
Lobe No: 6R 3L 5L 3R
Elevation - - + +
Azimuth, deg. 18.1 53.5 89.3 126.5 Spacing, deg. 35.4 35~8 37.2 Deviation from 36.0 -0.6 -0.2 +1.2 Lobe No: 3R 5R lR
Elevation + - ~
Azimuth, deg. 126.5 162.3 198~1 Spacing, deg~ 37.2 35.8 35.8 Deviation from 36.0 +1.2 -0.2 -0.2 Lobe No: lR 4L 2L
Elevation ~ ~ -Azimuth, deg. 198.1 233.S 269.3 Spacing, deg. 35.8 35.4 35.8 Deviation from 36.0 -0.2 -0.6 -0.2 Lobe No: 2L 4R 2R
Elevation ~ ~ +
Azimuth, deg. 269.3 306.5 342.3 Spacing, deg. 35O8 37.2 35.8 Deviation from 36.0 -0.2 ~1.2 -0.2 ; Lobe No: 2R 6R etc Elevation ~ -Azimuth, deg. 342.3 18.1 Spacing, cleg. 35~8 35.8 Deviation from 36.0 -0.2 -0.2 That is to say, the 10 corners are disposed substantially evenly around the azimuth, as indicated in Figure 9.

~ 35 Arl ait2;rat~;~ oollaps-ole ~ers~on of a refiector in accordance ~i.C~ e Inventlorl is shown in Figure 10. In this embodiment sections 21 and 22 of radar reflective sheet material are hingedly interconnected in edge to edge relationship to form a strip by means of hinges 23. The portions 21 are of similar shaping to the portions 11 and the portion 22 is of similar shaping to the portion 12 of Figure 2. The hinges permit the strip to be folded back~ards and forwards in concerting fashion into a small space. The opposite edges of the portion 22 which are hingedly connected to adjacent portions 21 are substantially parallel~ The hingedly connected edges of the other portions 21 are alternately divergent and convergent in a direction from one edge to the other edge of the sectional strip.
Each of the portions 21 and 22 except the top portion is provided with a separator plate 24 which are hingedly connected to their respective por~ion ~l~ernately to opposite faces of the plate. The separator plates are shaped and positioned so as to be movable into a position at right angles to their respective portion and to permit the adiacent portion to be hinged into contact therewith at which ~osition the adjacent portions are mutually ac right anglesO A cl~p 25 is provided w~ich engages the edge of the separator pla~e and secures the plate in posi-tion. The two adjacent portions and the separator plate form a ? pair of orthogonal re-entrant trihedrals in the same form as Figure 1.
It will be appreciated that this version of the reflector can be folded down for storage in a confined space yet is quickly reassembled for use.
It is believed that the constructions described fully meet the stringent performance requirements of the Department of Trade ~larine Radar Reflector Performance Specification 1977. In particular, since the response for the vertical plane is also extremely good the vertical angle response~ so imporcant to main~ain reflectio-n d~.lr,ng hee~ing -;n ro~gh seas9 meets the requirement thaL the vert c~i coverage 7 t l5 to t'ne norizontal, shali not remaîn ~e~ow ~6dB relat-Lve to the LOm vaLue over any single angle of more than ïO~;CO

3~6~35 It will be appreciated that more o~ less reflective corners could be employed and that provided at least six are distributed around a 360 arc, a useful construction may be obtained.
Refl~ctors employing more than 10 reflective corners in which overlapping of lobes at higher signal strengths occurs may well provide useful constructions and such constructions are at pres-ent being analysed as their usefulness is influenced by their response at diferent heeling angles as well as by several other complex factors.
Although the spine and dividers of the described reflector are formed from a single sheet of material the invention is not restricted to such a construction and any other radar reflective material can be employed~ For example, the whole could be moulded in plastics e.g. by injection moulding. Such a moulding could be effected with a moulding material containing particles of radar reflective material so that these particles are embedde~
in the moulded reflector. Another possibility is the provision of acings of radar reflective material on a plastics moulded construction e.g. by metal plating or metalization.
A radar reflector as previously described may be encapsul-ated or hermetically sealed in a container of for example glass reinforced plastics material~
It will be understood that the above description of the ! present invention is susceptible to various modification changes and adaptations,

Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A radar reflector having a major axis and comprising at least six corner reflectors directed outwardly of said major axis and disposed along two successive helical paths one of which paths is sinistrorse and the other of which paths is dextrorse.

2. A radar reflector according to Claim 1, wherein the reflectors are evenly distributed within an angle of 360°.

3, A radar reflector according to Claim 1, wherein the corner reflectors are orthogonal re-entrant trihedrals.

4. A radar reflector according to Claim 3, comprising a strip of radar reflective sheet material folded in alternate directions along fold axes spaced apart on the strip and extending transversely across the strip with two consecutive ones of the fold axes disposed intermediately being substanti-ally parallel and the remaining folds being alternately conver-gent and divergent in a direction from one edge to the opposite edge of the strip the folds dividing the strip into sections adjacent sections being disposed at right angles and a separator plate being provided between and at right angles to each pair of adjacent sections to form therewith two corner reflectors.

5, A radar reflector according to Claim 4, wherein the separator plates are rectangular.

6. A radar reflector according to Claim 4, wherein the separator plates are rectangular with one point cut off to pro-vide an edge and are each positioned such that said edge is remote from adjacent sections.

7, A radar reflector according to Claim 4 wherein the strip is profiled to provide an edge profile conforming to part of the internal surface of a cylinder.

8. A radar reflector according to Claim 4 wherein the separator plates are profiled to provide an edge profile con-forming to part of the internal profile of said cylinder.

9. A radar reflector as claimed in Claim 7 including a cylindrical housing containing the profiled strip with separa-tor plates.

10. A radar reflector as claimed in Claim 2 comprising ten corner reflectors.

11. A radar reflector according to Claim 3, 4 or 6 comprising a strip of radar reflective sheet material formed by a multiplicity of sheet sections having edges in edge to edge relationship extending across the strip, said edges of an intermediate one of the sections being substantially parallel and the remaining ones of said edges being alternately conver-gent and divergent in a direction from one edge to the opposite edge of the strip, and for each pair of adjacent sections hinge means coupled between said sections and adapted to permit hinged movement of said sections into a position where they are mutually at right angles and a separator plate hingedly connected to one of said sections adapted to permit hinged movement into a position at right angles to each of said pair of adjacent sections to form therewith two corner reflectors.