Airbourne weather radar (Beams)
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Airbourne weather radar (Beams)
Abstract on weather radars:
"The radar can has two modes, weather and mapping. The weather beam is used for detecting clouds and is a conical pencil beam with a width of 5 degrees and the mapping is for ground features and uses a fan shaped or cosecant squared beam.
The directional properties of the radar produce side lobes. One side lobe goes vertically down to the ground and is received back by the weather radar receiver. This received signal produces a height ring on the display. The ring indicates that the radar is working and appears at the approximate height of the aeroplane above the gound. An aeroplane flying at 12000ft will have a permanent return at approximately 2nm."
I would like to know why the weather radar uses a conical pencil beam for detecting clouds and the different fan shaped consecent squared? beam for ground features?
From what i have read i thought that side lobes are useless, that they give false responses outsde the main beam and produces unwanted clutter. Although side lobe is used to determine height on the AWR why is this?
Cheers
"The radar can has two modes, weather and mapping. The weather beam is used for detecting clouds and is a conical pencil beam with a width of 5 degrees and the mapping is for ground features and uses a fan shaped or cosecant squared beam.
The directional properties of the radar produce side lobes. One side lobe goes vertically down to the ground and is received back by the weather radar receiver. This received signal produces a height ring on the display. The ring indicates that the radar is working and appears at the approximate height of the aeroplane above the gound. An aeroplane flying at 12000ft will have a permanent return at approximately 2nm."
I would like to know why the weather radar uses a conical pencil beam for detecting clouds and the different fan shaped consecent squared? beam for ground features?
From what i have read i thought that side lobes are useless, that they give false responses outsde the main beam and produces unwanted clutter. Although side lobe is used to determine height on the AWR why is this?
Cheers
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When looking at the ground for mapping purposes at fairly close range, the beam must be widened vertically as well as having its energy distribution controlled so that returns from longer range (the top of the beam) are of similar strength to those from shorter range (the bottom of the beam). The strength of the signal vertically within the beam depends upon the square of the cosecant of the angle of depression, so more energy is radiated in the upper part of the beam than the lower part.
The sidelobe mentioned is a side effect and coincidentally can be used to check if the system is working. It is not accurate enough to determine height with. The statement is probably to do with a question in the exams
The sidelobe mentioned is a side effect and coincidentally can be used to check if the system is working. It is not accurate enough to determine height with. The statement is probably to do with a question in the exams
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The strength of the signal vertically within the beam depends upon the square of the cosecant of the angle of depression, so more energy is radiated in the upper part of the beam than the lower part.
Hi phil, sorry but i dont understand that, i know that the angle of depression is the angle that an observer can not see, could you explain "the strength of the signal vertically within the beam depends upon the square of the cosecant of the angle of depression"
Cheers
Hi phil, sorry but i dont understand that, i know that the angle of depression is the angle that an observer can not see, could you explain "the strength of the signal vertically within the beam depends upon the square of the cosecant of the angle of depression"
Cheers
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As the beam leaves the aircraft, the upper part of it is more horizontal than the lower part, which is probably around 45 degrees to the surface of the earth. The angle of depression when leaving the aircraft is therefore different for them (imagine looking at it from the side). Numbercrunching the angles with the square of the cosecant ensures that more of the signal is radiated in the top part of the beam to cover for the fact that when it reaches the surface it is attenuated over a wider area (because of the lower angle of depression). Thus the signal strength is more uniform.
The idea is not to waste energy when a normal fan beam radiates into space while you're looking on the ground - the cosecant beam has a more defined upper boundary, and the upside down version is used for ground mapping.
But this is old hat anyway - modern radars appear to have no difference between the shape of the beam in WX or MAP modes. We had an old Bendix on one of our 212s which mentioned it in the manual, but I have never seen it otherwise.
But then, many questions are way out of date, as witnessed by the symbols used for logic gates
The idea is not to waste energy when a normal fan beam radiates into space while you're looking on the ground - the cosecant beam has a more defined upper boundary, and the upside down version is used for ground mapping.
But this is old hat anyway - modern radars appear to have no difference between the shape of the beam in WX or MAP modes. We had an old Bendix on one of our 212s which mentioned it in the manual, but I have never seen it otherwise.
But then, many questions are way out of date, as witnessed by the symbols used for logic gates
Last edited by paco; 24th Sep 2012 at 03:11.
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Shadow question (Airborne Weather Radar)
Can anyone explain shadow
From my ATPL book i have this abstract;
"Water surfaces at any reasonable range reflect the radar energy away from the aeroplane and give little or no return, causing the screen to remain quite clear. Beware, shadow areas appear behind hills and could easily be mistaken for water"
Firstly i would of thought the water surface would be reflected back to the aeroplane and not away why does the range do this? and beware of shadow areas which appear behind hills that could easily be mistaken for water? so i presume what it is saying is that there is more land behind the hill?
Thanks
From my ATPL book i have this abstract;
"Water surfaces at any reasonable range reflect the radar energy away from the aeroplane and give little or no return, causing the screen to remain quite clear. Beware, shadow areas appear behind hills and could easily be mistaken for water"
Firstly i would of thought the water surface would be reflected back to the aeroplane and not away why does the range do this? and beware of shadow areas which appear behind hills that could easily be mistaken for water? so i presume what it is saying is that there is more land behind the hill?
Thanks
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radar energy hitting a smooth surface at a shallow angle will skip straight on and not reflect. On the return scope it will look like a blank area. But don't think all blank areas are harmless water. Some could be mountains in a shadow area
long range equals shallow angle. Think about it
long range equals shallow angle. Think about it
Last edited by Dick Whittingham; 25th Sep 2012 at 18:28.
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aw i see, thanks for that Dick, but i still can't see why you should be aware of shallow areas that are behind hills as that would be of no concern unless the terrain behind the hill rise above it, but then the radar would pick it up?
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The shadow is due to attenuation. If you get plenty of ground returns on the screen, look for a shadow (V shaped), then look forward to see if there is a thunderstorm in front, or a hill. Cities do not produce shadows.
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The observation comes from the days when radar was used for ground mapping and fixing. In this case the radar operator would look at the radar picture and try and match it to a map. At low level ranges of hills would produce a shadow as described above and the coastline and features behind them would not be visible to the radar, and hence the radar operator would not get a picture which exactly matched the map.
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It seems that when on Auto mode, the side lobe that produces the false echo at a pseudo distance equal to the height of the aircraft is removed, whereas in Manual mode, it is not.
Could someone explain if this makes sense and if yes why ? I.e. what would be the difference in Auto mode regarding de cluttering of the return of the side lobe as opposed to Man mode?
Also, the false echo seems to get worse if the radome is wet, or in bad shape, or covered with ice, or not fully adjusted correct?
What could be done to remove or diminish this phenomenon, even on a brand new aircraft ?
Could someone explain if this makes sense and if yes why ? I.e. what would be the difference in Auto mode regarding de cluttering of the return of the side lobe as opposed to Man mode?
Also, the false echo seems to get worse if the radome is wet, or in bad shape, or covered with ice, or not fully adjusted correct?
What could be done to remove or diminish this phenomenon, even on a brand new aircraft ?
