Radar brain teaser

[twistedtree]

https://www.navcen.uscg.gov/?pageName=enavRadarBeacons

The displayed response has a length on the radar display corresponding to a few nautical miles, encoded as a Morse character beginning with a dash for identification.� The inherent delay in the racon causes the displayed response to appear behind the echo from the structure on which the racon is mounted.�

Got it, but also directly in line with the RACON, not off to the side as in this case. I think that's one of the tell tales in this case that says it's not a RACON.

 

[psneeld]

Plus it would probably be in yellow as a primary target not a blue "shadow".

I have tracked many an aircraft on ships radar but those were not set up with "tails"... thus the 12 pings of nearly uniform size and shape would be an almost dead guveaway for a SART...but not being in line perplexed me and with the "blue tail" shadows though it might just be a SART return while the boat was in a turn.

 

[FoxtrotCharlie]

737 my foot, that Santa's sleigh and reindeer

 

[twistedtree]

Plus it would probably be in yellow as a primary target not a blue "shadow".

I have tracked many an aircraft on ships radar but those were not set up with "tails"... thus the 12 pings of nearly uniform size and shape would be an almost dead guveaway for a SART...but not being in line perplexed me and with the "blue tail" shadows though it might just be a SART return while the boat was in a turn.

The fact that echo trails are on is key.


If it were SART, I it would be yellow (active echo), not blue (historic echo), and of course in line with the target. And if trails were turned off, you wouldn't see a series of returns like for SART, but rather a single yellow dot for the plane's current location.


It's a tricky one, and a really good exercise.

 

[tpbrady]

Twisted Tree

You were right first. I calculated the approach speed at around 150 mph or slightly faster depending on interpretation of how many returns were between the half mile rings. That puts it into the approximate approach speed of a 737. Since it was a fairly constant velocity, that made it approach and not departure as on departure the velocity should have been increasing. The turn on approach seems to indicate the pilot was hand flying which they do in good visibility. Normally the approach would be straight in. The difference in the target size of the return is probably due to the changing radar cross section of the airplane. Initially the radar was seeing the underside of the wings and fuselage, and as it descended, the wing became edge on reducing the cross section. This would make a good physics exam question.

Tom

 

[twistedtree]

The difference in the target size of the return is probably due to the changing radar cross section of the airplane. Initially the radar was seeing the underside of the wings and fuselage, and as it descended, the wing became edge on reducing the cross section. This would make a good physics exam question.

Tom

Are you referring to the length of the blue streaks which are longer further out, and progressively shorter as it gets closer?


If so, those are strictly the echo trails which on a modern radar are artificially created. It's a re-creation of the persistent phosphorescent afterglow on old CRT radar displays. The yellow in the image is the current return, and the blue is a historical recording of where the return has been. So anything that moves over time with respect to the boat leaves a blue trail. It's one of several radar tools helpful in picking out things that are moving towards you.


So each of those streaks started out as a yellow blip. Then the next sweep the blip had moved ahead, and the last blip was recorded in blue. Then the next scan the yellow blip moves further, and now the past two blips are blue. It looks like Richard was turning during this, which causes the sideways streak recording the past locations of the plane. The furthest out blip happened the longest time ago, so has had the most time to streak, and each progressively close blip has had successively less time, so are progressively shorter.

 

[Plane Nauti]

Wxx3
I waited to ask this question until thread was resolved.
You were 1-2 NM from the series of blips. My question is I thought marine radar was focused on the horizon, how low was that plane flying, OR is your radar pointing skyward, thus the reason the float was not picked up?

Most glideslopes for Aircraft are roughly 3 to 1 so that makes the plane approx 900 ft AGL (above ground level) 3 miles out. Not sure what the beam width is on a marine radar but could paint the plane i'm sure. 1 mile out 300 ft, 2 miles out 600 feet AGL. etc.

 

[OldDan1943]

A 'raft' of migrating ducks or geese?
Yes, the do show up on RADAR if you aim low.

 

[twistedtree]

Most glideslopes for Aircraft are roughly 3 to 1 so that makes the plane approx 900 ft AGL (above ground level) 3 miles out. Not sure what the beam width is on a marine radar but could paint the plane i'm sure. 1 mile out 300 ft, 2 miles out 600 feet AGL. etc.

Vertical beam width is nominally 12 deg above the horizon (and 12 below). So does that math work out? My assumption is that the plane's first appearance is when it descended low enough to enter the 12 deg radar beam.

 

[Plane Nauti]

Vertical beam width is nominally 12 deg above the horizon (and 12 below). So does that math work out? My assumption is that the plane's first appearance is when it descended low enough to enter the 12 deg radar beam.

Richard was approx 2 miles from the target on the first hit. Guessing about where the airplane would land puts the plane at most approx 1 1/2 miles from landing so the plane would be 450ft ARB (Above Richards Boat ) 2 miles from Richard on the first hit.


A +12 degree beam should easily pick this up.


I'm avoiding public math. 60 to 1 at 35K feet is easier.

 

[Portage_Bay]

Vertical beam width is nominally 12 deg above the horizon (and 12 below). So does that math work out? My assumption is that the plane's first appearance is when it descended low enough to enter the 12 deg radar beam.

12 degrees at 3 nautical miles out is about 1260 ft. I've tracked planes departing and arriving Sidney BC when on Saanich Inlet. Many times float planes over Puget Sound.

 

[SteveK]

Well, I am surprised. The radar beam is 50% in the 12.5* above/below level AND 50% of the power is 25* above/below. I thought it was more focused directly ahead, it is not. Using a triangle calculator HERE @ 12.5* just one nautical mile the beam is already at 1347 feet above the water.
Did not expect a marine radar to be scanning the sky wasting that power.

 

[Portage_Bay]

Well, I am surprised. The radar beam is 50% in the 12.5* above/below level AND 50% of the power is 25* above/below. I thought it was more focused directly ahead, it is not. Using a triangle calculator HERE @ 12.5* just one nautical mile the beam is already at 1347 feet above the water.
Did not expect a marine radar to be scanning the sky wasting that power.

Apparently wasteful beam width was explained to me by a tech as the need to compensate for pitch and roll. If the beam is too narrow in an attempt to gain range when you pitch and roll the beam misses the target. Current beam width is a trade off but one that seems to work well most of the time.

 

[psneeld]

Many radars on power boats are mounted tilted down a bit to compensate for bow rise underway and still see things right in front of the boat.

The outfits I worked for always did.

 

[twistedtree]

Apparently wasteful beam width was explained to me by a tech as the need to compensate for pitch and roll. If the beam is too narrow in an attempt to gain range when you pitch and roll the beam misses the target. Current beam width is a trade off but one that seems to work well most of the time.

Exactly.


And the cutoff isn't abrupt. Just that a large portion of the beam is concentrated in that 25 deg spread.

 

[twistedtree]

Many radars on power boats are mounted tilted down a bit to compensate for bow rise underway and still see things right in front of the boat.

The outfits I worked for always did.

Yes, very important for boats with bow rise underway, i.e. planing boats. Less important for a displacement boat that stays level (excluding wave action) across its speed range.

 

[psneeld]

Even displacement boats can benefit....as locating things close aboard and still out to the horizon is important...so a simple calculation solves the issue of desired tilt. Usually well explained in install manuals.

Semi displacement can have the biggest benifit.

 

[SteveK]

One wonders why a narrower beam cannot be kept level electronically.
Perhaps too expensive to include.
Heel leveling is available for sailboats.

 

[psneeld]

Gimble mounting or phased array would be pricey.

 

[rgano]

Please see the attached picture. It is of my Raymarine E80 radar screen.

The question is who can identify what caused thst return if about a dozen blips and tell tails, that start about a mile ahead of the boat and go off at a 45° angle?

A hint. Ihad to adjust the gain a bit to see it.
Yes, I know what it is.

OK, I am not gonna read below the initial post and say that it is birds. That's what a line of brown pelicans (aka a squadron) skimming the water looks like on my Furuno.

Now to see what the rest of the story...

 

[rgano]

Oh well, at least it was flying.

 

[JDCAVE]

I’m replying before I read too far into the thread. Could be a vessel wake. However, you haven’t given us all the details we need to know to understand the screen capture. A real-time observer would have the opportunity to make additional observations to determine the origin of the return.

Jim

 

[JDCAVE]

So I’ve read the rest of the thread and I stand corrected. However, I will say that I often observe aircraft as I pass by YVR and have never seen a similar radar return on my older Furuno 1942 4’ open array. Typically they are simply fast moving targets. That said I don’t have ARPA on my unit.

 

[twistedtree]

So I’ve read the rest of the thread and I stand corrected. However, I will say that I often observe aircraft as I pass by YVR and have never seen a similar radar return on my older Furuno 1942 4’ open array. Typically they are simply fast moving targets. That said I don’t have ARPA on my unit.

I think that's because you don't have echo trails turned on. The airplane return would be nothing more than a dot with echo trails turned off. That's why the teaser is such a teaser.

 

[Fish53]

RACON usually has varied pulses but looking at a still picture without the ability to watch the target develop or tune the radar yourself it's pretty hard to answer intelligently.

 

[skipgundlach]

Nine, with Rudolph.

10 ,actually - her name is featured in the song whilst referring to her nasty and excluding nature...

 

[Reelalure]

Super cool thread, thanks for sharing!!!!!!!!

 

[firstbase]

My head hurts. I m going to go lay down.

 

[Bigsfish]

I seen that many times and it is always one on my ex wives coming in on her broom for more money.

 

[JDCAVE]

I think that's because you don't have echo trails turned on. The airplane return would be nothing more than a dot with echo trails turned off. That's why the teaser is such a teaser.

Ahhhh! I see said the blind man to his deaf son! I read about that in the manual, but felt that I didn’t need addition clutter on the screen so left it off and since forgot about it. Anyways, I know what airplane targets are on the screen by the speed they move.

Jim