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Old 09-03-2018, 04:23 PM   #41
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Microwaves move at the speed of light. Pretty sure rain targets are moving slower than that. (I'm ignoring the pokey processing speed of the electronics)
I think you misunderstood the point of his question. He was asking how doppler works if there is no speed differential between the target and the boat's radar (not the radar wave).
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Old 09-03-2018, 04:33 PM   #42
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I think you misunderstood the point of his question. He was asking how doppler works if there is no speed differential between the target and the boat's radar (not the radar wave).
The simplest answer is that the radar return comes from each individual raindrop, which are moving in many directions at various velocities.
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Old 09-04-2018, 02:35 PM   #43
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I'm no expert, but my impression is that a true Doppler radar is distinctly different from modern marine sets for the general public which tout "Doppler technology." In essence, such units change the color of targets based upon whether the target is moving toward or away from the observer. This in and of itself has nothing to do with the actual Doppler shift that electromagnetic waves demonstrate.
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Old 09-04-2018, 03:19 PM   #44
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I'm no expert, but my impression is that a true Doppler radar is distinctly different from modern marine sets for the general public which tout "Doppler technology." In essence, such units change the color of targets based upon whether the target is moving toward or away from the observer. This in and of itself has nothing to do with the actual Doppler shift that electromagnetic waves demonstrate.

I'm pretty sure they DO detect the doppler shift, so it's not total BS. Now I have no idea how it compares to a TV stations Doppler Weather Radar, but I'd frankly expect more BS from the TV weather people than the marine guys.
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Old 09-04-2018, 10:35 PM   #45
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I think I resolved what was going on. Two days ago I was motoring along, with the Fantom set at 2 miles. Dead ahead was a saliboat (without sails and coming at me) and another smaller boat. No target on the radar. I thought they must be further out, and ranged to four miles. Still nothing.

THIS time I remembered to play with the settings (amazing when you don't have physical dials you forget these things?). The first thing I saw was that my sea clutter was set to "auto high." I set this to "auto low" and boom there they were.

Going back to my original post... The day before we were about an hour late getting out of the anchorage, and a Small Craft Warning had the ignominy to show up a couple of hours early. I had set the Fantom to auto high that day, but forgot about it the next day (i.e. when the photo was taken in the smoke haze).

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Old 09-05-2018, 07:25 AM   #46
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Sure you can't out run weather sometimes, but if you are out of internet/cell range and you don't have satellite weather...radar at least gives you the info for an informed decision.

You may not be able to out running it, but you may be able to pick your way though the weaker storms.
PS I surprised at you.
In the '70's and 80's we had a number of aircraft crashes because pilots were picking their way thru what they thought were "weaker" returns. The "weaker" returns were a manifestation of the attenuation of the return. The radar could not penetrate further into the storm, thus is looked better, but was actually worse.

The problem with using a boar radar for storm avoidance is that it works until it doesn't, and when it doesn't you will be in for a hell of a ride.

Boat radars must compromise many things, you want to be able to see the kayak right in front of you and the fibreglass sailboat close to the horizon which at an average antenna height may be 6 to 10 miles??

They know metal boats and larger returns will show up.

What you perceive as weather is a whole other ballgame. You don't have to worry about 10 mile long tankers, but a line of thunderstorms?
I doubt any boat radar can penetrate more than a couple miles thru a thunderstorm with large raindrops.

Attenuation is the issue.

Best way to think of it is that just like your radar is not telling you what's on the other side of the tanker, when it comes to storms, the more powerful the storm, the bigger the raindrops, so all your radar is REALLY telling you is the face of the storm by not really anything behind it.
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Old 09-05-2018, 07:32 AM   #47
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Originally Posted by Sabre602 View Post
I'm no expert, but my impression is that a true Doppler radar is distinctly different from modern marine sets for the general public which tout "Doppler technology." In essence, such units change the color of targets based upon whether the target is moving toward or away from the observer. This in and of itself has nothing to do with the actual Doppler shift that electromagnetic waves demonstrate.
I think this is correct. Twisted tree may also be right, but the Doppler in marine radars is a marketing tool to get you to spend your money.
The displays have more color coding.

But there is no way Doppler can replace ARPA. Why? Because ARPA takes into account an acceleration of the target (change of speed or direction) over time, while Doppler can not sense direction, all it senses is the component of speed in relation to the radar.
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Old 09-05-2018, 08:39 AM   #48
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Originally Posted by Wxx3 View Post
PS I surprised at you.
In the '70's and 80's we had a number of aircraft crashes because pilots were picking their way thru what they thought were "weaker" returns. The "weaker" returns were a manifestation of the attenuation of the return. The radar could not penetrate further into the storm, thus is looked better, but was actually worse.

The problem with using a boar radar for storm avoidance is that it works until it doesn't, and when it doesn't you will be in for a hell of a ride.

Boat radars must compromise many things, you want to be able to see the kayak right in front of you and the fibreglass sailboat close to the horizon which at an average antenna height may be 6 to 10 miles??

They know metal boats and larger returns will show up.

What you perceive as weather is a whole other ballgame. You don't have to worry about 10 mile long tankers, but a line of thunderstorms?
I doubt any boat radar can penetrate more than a couple miles thru a thunderstorm with large raindrops.

Attenuation is the issue.

Best way to think of it is that just like your radar is not telling you what's on the other side of the tanker, when it comes to storms, the more powerful the storm, the bigger the raindrops, so all your radar is REALLY telling you is the face of the storm by not really anything behind it.
Good post Richard, but truth be told I am not that concerned about the details of what's behind the storm, I am most interested in the edge closest to me, how wide it is and how fast and what direction it is moving.
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Old 09-05-2018, 08:50 AM   #49
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This is the best boat related forum on the web, and I have learned a lot from other experienced mariners who post here. However, it is not a good place to learn about current electronics technology and radars. I realized that when researching systems last year to decide what I was going to use on our new boat. I therefore read through other technical sources, and a couple other forums where people where more up to date.
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Old 09-05-2018, 09:11 AM   #50
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Quote:
Originally Posted by Wxx3 View Post
PS I surprised at you.
In the '70's and 80's we had a number of aircraft crashes because pilots were picking their way thru what they thought were "weaker" returns. The "weaker" returns were a manifestation of the attenuation of the return. The radar could not penetrate further into the storm, thus is looked better, but was actually worse.

The problem with using a boar radar for storm avoidance is that it works until it doesn't, and when it doesn't you will be in for a hell of a ride.

Boat radars must compromise many things, you want to be able to see the kayak right in front of you and the fibreglass sailboat close to the horizon which at an average antenna height may be 6 to 10 miles??

They know metal boats and larger returns will show up.

What you perceive as weather is a whole other ballgame. You don't have to worry about 10 mile long tankers, but a line of thunderstorms?
I doubt any boat radar can penetrate more than a couple miles thru a thunderstorm with large raindrops.

Attenuation is the issue.

Best way to think of it is that just like your radar is not telling you what's on the other side of the tanker, when it comes to storms, the more powerful the storm, the bigger the raindrops, so all your radar is REALLY telling you is the face of the storm by not really anything behind it.
I flew a lot of years without radar or gps....heck, not even remotely accurate charts sometimes.

From the Arctic to the Antarctic, I also flew without traffic control helping most of the time.

I will keep both and enjoy.

You are absolutely correct about misinterpreting radar.....but taking bad info from any source can be a fatal flaw. Imagine flying through those raindrops into a 500 foot rock pinnacle behind them....

One of my favorite expresions.... " ya gotta be smarter than the equipment you are using"......
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Old 09-10-2018, 08:47 PM   #51
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I could argue that marine radar as a weather avoidance tool, is a poor tool. It has one MAJOR disadvantage over aircraft radar.... it has no tilt, which is vital for weather avoidance.

Sure you can see some rain with marine radar, but hard to distinguish it from land at times. Perhaps if your out in the ocean there's "some" value. And because there's no tilt, it's hard to see radar shadows (where there's no return after the heavy rain area and you just don't know what's back there.

Again, hard to outrun, but perhaps one could maneuver a bit to hit the lesser spots. Xm may provide better details, but with it's delay, it has issues, too.

The airline crashes because the guy couldn't determine a shadow was from the guy sleeping thru radar school. I believe there was only one crash because of this issue a Southern Air DC9 with a double flame out. But there has sure been a few that ended up with a rough ride.
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Old 09-10-2018, 09:06 PM   #52
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PS I surprised at you.
In the '70's and 80's we had a number of aircraft crashes because pilots were picking their way thru what they thought were "weaker" returns. The "weaker" returns were a manifestation of the attenuation of the return. The radar could not penetrate further into the storm, thus is looked better, but was actually worse. ....
Modern aviation radars flag the scan in areas where there may be path attenuation (poor penetration of the radar beam). This alerts the pilots that the deeper returns in that part of the scan may not show properly.
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Old 09-10-2018, 09:09 PM   #53
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I think this is correct. Twisted tree may also be right, but the Doppler in marine radars is a marketing tool to get you to spend your money.
The displays have more color coding.

But there is no way Doppler can replace ARPA. Why? Because ARPA takes into account an acceleration of the target (change of speed or direction) over time, while Doppler can not sense direction, all it senses is the component of speed in relation to the radar.

Bingo. Doppler can enhance ARPA via quicker detection of movement, but only along a path radial to yourself. But it's no replacement for ARPA. And Furuno's "fast tracking" gives you instant display of movement just like doppler, but with the full functionality of ARPA. So arguably the best of both worlds.
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Old 09-11-2018, 09:48 PM   #54
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Modern aviation radars flag the scan in areas where there may be path attenuation (poor penetration of the radar beam). This alerts the pilots that the deeper returns in that part of the scan may not show properly.
Sobol,

Not familiar with that "flag" function and I've been flying some pretty nice stuff until I retired from the airlines a few years ago.

However, even with my own old RDR 130 (that's 40 yr old technology) with a newer color display I can clearly determine attenuations and see the radar shadows to avoid them.

Unfortunately we don't have that capability with our boats. And even if we did, it's still pretty hard to avoid the weather. Sometimes the best we can do is to position ourselves for the lightest returns (which aren't always the best). And even that can change by the time we get there. But fortunately, if it's a nasty squall line, it's not gonna last long.

If it's embedded thunderstorms in a warm or occluded front, the likely hood of really severe stuff is pretty low, but one can get a ton of attenuation with that kind of weather... depending on the quality of radar.
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Old 09-11-2018, 11:34 PM   #55
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Sobol,

Not familiar with that "flag" function and I've been flying some pretty nice stuff until I retired from the airlines a few years ago.
....
Collins calls it PAC alert. Been around for at least 12 years.

Honeywell calls it REACT.
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Old 09-13-2018, 03:35 PM   #56
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This is the best boat related forum on the web, and I have learned a lot from other experienced mariners who post here. However, it is not a good place to learn about current electronics technology and radars. I realized that when researching systems last year to decide what I was going to use on our new boat. I therefore read through other technical sources, and a couple other forums where people where more up to date.


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