20:
321:
With this calculation, the horizon for a radar at a 1-mile (1.6 km) altitude is 89-mile (143 km). The radar horizon with an antenna height of 75 feet (23 m) over the ocean is 10-mile (16 km). However, since the pressure and water vapor content of the atmosphere varies with height,
805:
as a reflector and beams the signal skyward and then listens for the tiny signals that are returned from the sky, others use a bistatic arrangement with distant antennas looking for objects that pass between them, and a small number of systems use "creeping waves" that travel into the shadow zone.
490:
Ducting influence becomes stronger as frequency drops. Below 3 MHz, the whole volume of the air acts as a waveguide to fill in the radar shadow and also reduces radar sensitivity above the duct zone. Ducting fills in the shadow zone, extends the distance of the clutter zone, and can create
621:
741:
wide will illuminate millions of square feet of surface by the time the radar pulse reaches 10 miles (16 km). Targets are generally much smaller, so will be masked by clutter. Clutter reflections can create unwanted false targets.
695:
There are a large number of reflectors on the ground at these elevation angles. Prevailing winds of about 15 mile/hour cause these reflectors to move, and this wind stirs up smaller objects into the air. This interference is called
464:
225:
751:(MTI) can reduce clutter by about 35 dB. This allows objects as small as 1,000 square feet (93 m) to be detected. Prevailing wind and weather can degrade MTI performance, and MTI introduces
469:
And for the same examples : the radar horizon for the radar at a 1-mile (1.6 km) altitude will be 102-mile (164 km) and the one at 75 feet (23 m) will be 12-mile (19 km).
314:
761:
can reduce clutter by over 60 dB, which can allow objects smaller than 1-square-foot (0.093 m) to be detected without overloading computers and users. Systems using
518:
739:
678:
651:
382:
355:
255:
153:
106:
126:
745:
The antenna for radar with no signal processing clutter-reduction improvement is not normally aimed near the ground to avoid overwhelming computers and users.
393:
476:, which bends the beam downward or even traps radio waves so that they do not spread out vertically. This phenomenon occurs in two circumstances:
54:
region of performance, and its geometry depends on terrain, radar height, and signal processing. This concept is associated with the notions of
797:
A number of radar systems have been developed that allow detection of targets in the shadow zone. These systems are collectively known as
765:
with speed rejection set above the wind speed have no clutter zone. This means that the clear region extends all the way to the ground.
326:
by the change in density. With a standard atmosphere, electromagnetic waves are generally bent or refracted downward. This reduces the
692:
is where radar energy is in the lowest several thousand feet of air. This extends to a distance of about 120% of the radar horizon.
903:
161:
762:
783:
There is no clear region in areas with weather and heavy biological activity (rain, snow, hail, high winds, and migration).
263:
69:
Airborne objects can exploit the radar shadow zone and clutter zone to avoid radar detection by using a technique called
81:
Without taking into account the refraction through the atmosphere, the radar horizon would be the geometrical distance
840:
492:
815:
748:
484:
798:
792:
908:
616:{\displaystyle H_{T}>{\frac {\left(R_{T}-{\sqrt {2\times H\times R_{e}}}\right)^{2}}{2\times R_{e}}}}
50:'s surface to make detection of a target at the lowest level possible. It is associated with the low
758:
777:
is the zone that begins several kilometers beyond the radar horizon at low elevation angles.
752:
496:
473:
70:
717:
656:
629:
360:
333:
233:
131:
84:
697:
512:
Objects beyond Dh will be visible only if the height satisfies the following requirement:
111:
897:
704:
28:
19:
857:
459:{\displaystyle D_{h}={\sqrt {2\times H\times \left({\frac {4R_{e}}{3}}\right)}}}
39:
780:
The clear region is also the zone above low elevation angles with clear skies.
357:, one must be using a value of 8.5·10 km for the effective Earth's radius
802:
323:
820:
51:
472:
Furthermore, layers with an inverse trend of temperature or humidity cause
330:, but causes errors in distance and height measuring. In practice, to find
680:
is the target range. Objects below this height are in the radar shadow.
708:
108:
from the radar to the horizon only taking into account the height
47:
43:
18:
801:. Three systems are generally used; the most common uses the
220:{\displaystyle D_{h}={\sqrt {2\times H\times R_{e}+H^{2}}}}
128:
of the radar above sea-level, and the radius of the earth
720:
659:
632:
521:
396:
363:
336:
309:{\displaystyle D_{h}={\sqrt {2\times H\times R_{e}}}}
266:
236:
164:
134:
114:
87:
733:
672:
645:
615:
458:
376:
349:
308:
249:
219:
147:
120:
100:
42:systems, defined by the distance at which the
8:
16:Distance at which ground targets are hidden
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113:
92:
86:
480:A thin stable layer of elevated humidity
27:For broader coverage of this topic, see
832:
384:(4/3 of it), instead of the real one.
38:is a critical area of performance for
7:
322:the path used by the radar beam is
14:
711:when operating on or near land.
155:(approximately 6.4·10 km):
763:pulse-Doppler signal processing
257:, this can be approximated by:
703:The clutter zone includes the
1:
230:When H is small compared to
46:beam rises enough above the
925:
790:
495:radar that are beyond the
26:
846:. University of Illinois.
816:Line-of-sight propagation
653:is the target height and
387:So the equation becomes:
749:Moving Target Indication
904:Radar signal processing
799:over-the-horizon radars
793:Over-the-horizon radar
735:
674:
647:
617:
460:
378:
351:
310:
251:
221:
149:
122:
102:
24:
858:"Radar Line of Sight"
736:
734:{\displaystyle 1^{o}}
675:
673:{\displaystyle R_{T}}
648:
646:{\displaystyle H_{T}}
618:
485:temperature inversion
461:
379:
377:{\displaystyle R_{e}}
352:
350:{\displaystyle D_{h}}
311:
252:
250:{\displaystyle R_{e}}
222:
150:
148:{\displaystyle R_{e}}
123:
103:
101:{\displaystyle D_{h}}
22:
718:
657:
630:
519:
394:
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112:
85:
759:Pulse-Doppler radar
474:atmospheric ducting
882:Merill I Skolnik.
841:"Radar Principles"
731:
670:
643:
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497:instrumented range
456:
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40:aircraft detection
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121:{\displaystyle H}
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787:Over-the-horizon
753:blind velocities
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503:Limiting factors
491:reflections for
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71:nap-of-the-earth
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911:
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886:. McGraw-Hill.
884:Radar Handbook
874:
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791:Main article:
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23:Radar horizon.
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862:Radartutorial
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705:littoral zone
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36:radar horizon
30:
29:Radio horizon
21:
909:Radar theory
883:
877:
867:November 27,
865:. Retrieved
861:
852:
835:
796:
782:
779:
775:Clear Region
774:
772:
769:Clear Region
757:
747:
744:
713:
702:
694:
690:Clutter Zone
689:
687:
684:Clutter Zone
625:
511:
489:
471:
468:
386:
327:
320:
318:
229:
80:
73:navigation.
68:
63:
60:clutter zone
59:
56:radar shadow
55:
35:
33:
508:Shadow Zone
328:shadow zone
898:Categories
827:References
803:ionosphere
77:Definition
64:clear zone
62:, and the
821:Hull-down
598:×
568:×
562:×
554:−
422:×
416:×
324:refracted
292:×
286:×
190:×
184:×
52:elevation
810:See also
714:A beam
709:terrain
698:clutter
493:low PRF
483:Stable
626:where
58:, the
844:(PDF)
48:Earth
44:radar
869:2011
773:The
707:and
688:The
533:>
34:The
900::
860:.
755:.
700:.
499:.
66:.
871:.
727:o
723:1
666:T
662:R
639:T
635:H
606:e
602:R
595:2
589:2
584:)
576:e
572:R
565:H
559:2
549:T
545:R
540:(
528:T
524:H
451:)
446:3
440:e
436:R
432:4
426:(
419:H
413:2
408:=
403:h
399:D
370:e
366:R
343:h
339:D
300:e
296:R
289:H
283:2
278:=
273:h
269:D
243:e
239:R
211:2
207:H
203:+
198:e
194:R
187:H
181:2
176:=
171:h
167:D
141:e
137:R
116:H
94:h
90:D
31:.
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