Wow! Where's a helmet when you need one?
OK I'm gonna keep it friendly and answer all you critics.
Firstly I want everyone to know where I am comming from. I'm not trolling. I'm not trying to wind anyone up. It was a bona fide attempt to help others avoid the ultimate bad day at the office!
ITCZ Firstly thankyou for your thoughtful reply.
First problem: Wx radar unit design v primary surveillance radar.
Wx radar is 'tuned' to a particular band of the microwave range of the e/m spectrum. Unsurprisingly, the engineers that designed it used a band that gives best reflections/scatter/returns when striking precipitation. Not moisture as cloud, which is orders of magnitude smaller than precip, nor light aircraft, which are several orders of magnitude larger than precip, with different reflectivity qualities.
You will notice that primary surveillance radar receiver/transmitters on major airports are not also used as wx radar rx/tx for RAPICs. They do entirely different jobs. The PSR head is rotating happily on a gantry on one part of the airfield, whilst the BoM Wx radar head is in a golfball on a gantry on another part of the airfield.
So for a start we are using the wrong sort of radar energy to spot unannounced aircraft.
I agree with all you say. Even the last sentence to some degree. I never said the wx radar was ideal for the purpose of picking up traffic. But because of it's flaws, it CAN be used to pick up things other than water droplets.
Secondly, the radar display in your cheiftain/B737 has some software in it. This software is designed to present you with the best wx information. It might be designed to cut out unnecessary clutter. It certainly will not be optimised to show a rogue C402 or smaller.
Yes we have a button labelled "Ground Clutter Suppressor". It tries to filter any returns that are moving horizontally at less than about 1 metre per second. Can be a bit of a trap if the weather is stationary.
Thirdly, resolution in azimuth. Say you wind down your radar to the lowest range, perhaps 10nm. Half way along is 5nm. Maybe a blip at 5nm ahead is warning you of possible conflict less than 30 seconds from closest point of approach (impact!) if you are doing 200 knots and he/she is a very slow 100 knots.
So how big would the blip be if it were a C402, five miles ahead? Say the radar display displays 120 degrees arc of azimuth. One degree of azimuth at one mile is a target 100 feet in width. One degree of azimuth at five miles is a target 500 feet, or roughly 180 metres, in width. What was the wingspan of that C402? Or a better question -- what was the frontal area of that C402? The width of the smallest "blip" able to be displayed by wx radar (and visible to an operator that is intently looking at only the wx radar display) might be 3 degrees of arc, so we are only going to see objects that are greater than 1500 feet in width?
This part I think you have got it around the wrong way. It's not that it can't pick up a target that is less than the width of the beam, it's that it won't DISPLAY a target on the screen any less than the width of the beam.
This is going to be difficult to describe without a few pictures but I will try.
Imagine the radar beam is a hand held torch (flashlight) that has a beam 6 degrees wide (3 degrees either side of centre).
Now although the beam is 6 degrees wide, you know which direction the CENTRE of the beam is pointing to a small fraction (0.1) of a degree.
Standing in dark padock directly south of of someone holding a shiny tin can 20 metres away, you slowly sweep the beam of light from west, through north, to east, and back again. You note what direction the CENTRE of the beam is pointing to the accuracy of 0.1 degrees and make a note on a piece of paper whenever you see the beam of light reflecting off the can also noting the direction the centre of the light beam is pointing.
What do you think the result will be? My guess is this.
From 270 (west) to 356 there is no reflection seen.
Approaching 357 you just start to pick up a faint reflection.
Now this is an important point!
Although the reflection is comming from the very edge of the beam which you know is 3 degress to the right of the beam centre, you are noting where the CENTRE of the beam is pointing. So that means your results indicate that a reflection is being picked up from 357!
You continue to sweep to the right. At 358 the reflection is even stronger now. That's because the can is closer to the centre of the light beam. The can has not moved. It is still directly north of you, but because the centre of the beam is pointing at 358, that is where the reflection is considered to have come from.
Through 359, same principle. The reflection is noted as coming from 359 when it's acually coming from 1 degree right of the centre of the beam.
Now the light beam is pointing at 000. No doubt about it. There is a reflection comming off that can. The reflection is at a maximum, and is noted as comming from 000.
Continuing the sweep across to
the east, at 001 there is still a reflection.
At 002 still a reflection, only it's getting a little weaker now. Still there though.
At 003 the reflection is still there but it is dropping off now. Not nearly as bright as what is was when pointing at 360.
At 004, there is no longer a reflection.
005 to 090 (east), no reflection.
So although the can is only 10 cm wide in real life, it produced a reflection from 357 to 003. (The width of the radar beam!) At 20 meters, 6 degrees of arc equals 2.1 metres.
So if that pattern of return was painted on a radar display, you would have a round blob, with a red centre from 359 to 001, with an amber and green ring from 357 to 359 and 001 to 003. The whole blob would appear to have a diameter of 2.1 metres.
Now for a practical example of this.
Have you not been in the situation where another airliner has departed in front of you, bound for the same destination, say from BNE to MEL? You might depart 3 minutes after them due to a landing aircraft in between the two of you. Once in the cruise at say FL320 you wonder how far in front of them they are and whether it might be worth going faster and\or lower than planned to make up time and beat them to the destination.
They may have climbed straight up to FL360 so TCAS won't show them on your map, or if your TCAS uses the VSI, it can only display about 6 nm ahead anyway.
You estimate that at 8 nm per minute, they should be about 3x8=24nm so you reduce the map range to 40 nm and tilt the radar up and let it sweep a few times. If nothing is seen on the screen, you tilt it up another degree or so and let it sweep a few times. Eventually you pick a tilt, say 3 degrees UP and there it is, a green blip on the screen about 20 nm ahead. Tilt it up another degree and it becomes a red blip with green edges. Five minutes later you note that the blip is 18 miles ahead and need to occasionally increase the tilt to keep it on the screen. Then you notice that a vapour trail is starting to appear and so now you have him visual and you know you are gaining on him.
If you haven't tried this on a day when enroute weather is not a issue. I suggest you try it. It's actually quite fun to do.
Fourth, coverage in elevation. You tilt your wx radar down 3 degrees. In many turboprops like a metro, your beam width in elevation is 8 degrees, and in aircraft with larger wx radar antennae, probably 6 degrees top to bottom.
Best case, 8 degrees of coverage in elevation for a smaller wx equipped aircraft top to bottom of beam. Tilt the centre of the beam down 3 degrees. So your radar is now scanning an elevation 7 degrees below level to one degree above level.
You then descend on a 3x profile, which is a very rough 3 degree slope. You then have protection against an aircraft that is level with you, or climbing at less than 400'/nm.
A C210 climbing at 90knots showing 1000 fpm climb is climbing steeper than 400'/nm. Your radar would not show him, even if the other problems above were solved. He/she is 'underneath your radar!'
Sorry you lost me when you said "you then have protection against an aircraft that is level with you, or climbing at less than 400'/min"
If I'm descending through FL180 at say 250 kts IAS, that's 330 kts TAS or 5.5nm per minute. At a descent gradient of 20:1 that's a descent rate of around 1700 ft/min.
At 5.5nm, a 6 degree radar beam will be 3500 ft wide. That is 1750ft above and 1750 ft below the descent profile if the tilt is the same as the actual descent path. At 3 degrees down tilt, it's gonna be close enough.
So your Cessna is climbing at 1000ft / min. Seems to me that if it was outside the radar beam and one minute ahead, it will still be 700 odd feet below me as I pass that point.
Anyway there are so many variations. Who's going which way, bla bla bla. I'll make my point on this later.
And have you done a quick rule of thumb check to ensure that your radar is REALLY tilted 3 degrees down, rather than just what the display says? What if it is really only tilted 2 degrees down?
Yes I know there can be a discrepancy here. But if you remeber the beam is about 6 degrees wide, it's not really a big deal.
Fifth. even if your radar had the right bandwith to detect metal/ragwing/composite construction objects the size and dimensions of Jabirus, Luscombes and C402's, had a high enough resolution in azimuth to actually paint it on the screen, and was within your tilt in elevation range without being obscured by ground returns....... so it would actually be displayed 30 seconds or so before you hit it..... were you watching the screen as you descended into the MBZ, called company, centre and other traffic and did all the other things you had to do? There is a reason why TCAS units issue audio commands and warnings!
I'd be including the map display in my scan as much as I would any other time. It's not like I suddenly get tunnel vision and focus all my attention on the map. Just because you're looking outside more does not mean you neglect what's inside either. I'm simply including it in my instrument scan and look out.
I'm simply using it as a tool to back up my eyes limitations. If I see that blip on the screen 20 degrees off the nose, I know were I'm going concentrating my visual search outside.
Nothing amazing about that is there?
Sixth, if you are doing what you recommend when you are inbound to a class C 'drome, then what is the problem?
The problem is the Virgin scenario north of Melbourne where others don't turn on their transponder.
The problem is GA transponders sending out altitude misinformation and making my TCAS system useless or worse.
The problem is Class E outside radar coverage.
These are all topics well discussed elsewhere around PPRuNe.
I think I've answered the other's concerns about me neglecting (not) my resposibilities around the flightdeck, both inside and out.
So in short, yes wx radar is designed for detecting water droplets and hence precipitation and hopefully the bumps that go with them. But you can't deny that it picks up on other things as well. Try the ground for example. Cities reflect better than flat forests. Mountain too paint much stronger than flat earth. Land reflects better than the ocean. And yes aeroplanes reflect too.
I never said it was a perfect solution. Of course there are limitiations on the concept. But for goodness sake! I have seen aircraft paint on the radar screen. I know that I am descending at very close to 3 degrees. I know that "See and Avoid" is a joke. If I think the radar can help me out with my visual scan (which I think it can) I will use it.
Like I said, I hope this idea helps.
