Interesting radar comparison, magnetron vs doppler

Doppler helps with motion detection, but is only an aid. The most capable motion detection is ARPA which will show you the course and speed of another vessel. It does it by tracking successive positions of the target over time, and calculates the direction and speed of travel.


Doppler is another tool to help, providing a much faster way to measure the approaching speed of a target. Rather than needed to track a bunch of positions over time to assess speed, doppler can measure it in a single sweep. That's really useful, but it only measured the closing speed, i.e. how fast the target is getting closer to you. If the target is moving, but not in a way that is getting closer, doppler won't know it's moving, and you have to revert back to tracking successive positions. So dopper is an aid to target tracking, but not a replacement for ARPA-style tracking. But collision risks are from boats that are getting closer, so Doppler quickly identifies the ones you case about most, and that is really good.


It's also worth noting that there are other ways to quickly identify trouble targets. Furuno's fast tracking (or whatever they call it) basically tracks all targets all the time. Then, when you say you want to "acquire" a target, it starts displaying right away because it's been getting tracked all along. It's pretty cool, and a good way to make use of lots of processing power in current computers.


As for needed to know the boat's current speed and direction, that's actually vital to all of these tracking technologies. Doppler measures the closing speed between you and a target. But to figure out the speed over ground of the target you need to subtract out your own speed and direction using vector math. ARPA won't even work without speed and heading inputs.

Ultimately, those good with radar only need the basic function of radar....target distance and direction.

Targets closing in on the center of the screen are potential collisions, those tracking elsewhere...are not.

All other functions help make some of your navigation decisions faster or more accurate which is good.

Per another thread, pick up a copy of The Radar Book if you don't know how to use yours; in tandem with the operating manual for your model, you will discover the wide number of uses for this tool.

Some folks use it for weather avoidance, but I could argue it has a lot of shortcomings for that. And ya really can't outrun the weather in a boat anyway.

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I really disagree with this, having used radar to both avoid storms and to prepare for them. In a slow boat you avoid storms b.y getting out of their path. Storm tracking is why I like to have a longer range open array radar, mounted as high as possible. I am curious what those "lot of shortcomings" are. It is superior to weather radar obtained on the internet, because it shows the storm relative to your specific position. Used in combination with internet or better yet satellite weather, it is a very powerful weather tool

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.

Being able to pick a path between ts cells in real time is useful but then so is seeing no path. Internet radar is 5 or 10 minute snapshots. Better than nothing.

AlaskaProf said:

... The one radar innovation which I really like is the phased-array antennas, whichare swept electronically without actually rotating.
...

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The problem with phased array radars is that they have limited sweep angles.
For aircraft that are more concerned with what is in front of them, this type of radar can work. To cover 360 deg like most boat radars would require multiple arrays and a fused display system. Aegis cruisers have large phased array radars, but they require an antenna for each quadrant around the ship.

It is unlikely to be practical for a smallish boat at a reasonable cost for not a lot of added benefits.

Loved my 40yr old Furuno magnetron,CRT with the gain, tune, ftc, stc and range with individual knobs.
Great for entering a strange harbour in high winds, rain and in the dark. Could manipulate each without ever menus trying to quickly find the appropriate items.

I have a newer Garmin HD and it's ok but when you need it and you need it fast the auto settings are inadequate and 10 seconds in a strange dark harbour mouth is a long time.

However, finding the newer Garmin controls
is breeze compared to the much buried menus in Raymarine equipment.

I have a beef with all the manufacturers pushing the auto features and suggesting that anyone can be proficient with these machines after reading their manuals. 25yrs ago it took me an eleven week course to get Radar/ARPA/MARPA certified and another year of almost daily use to get good at it..

For what it's worth, as I have nothing to compare to; crossing Albemarle Sound I noted a target 3 mi off our port beam. First mate was tasked with IDing it as it was paralleling us. This is a woman who can spot a crab pot at a half mile with the naked eye.
Target now steering an intercept, still no ID. Target now one mile out on collision course. No ID. We slowed not sure what was going on. Mid day, no wave chop. As the target crossed in front we had to laff. Three pelicans.
Our Garmin HDx seems quite capable, has even picked up boats on the back of thunder cells.

diver dave said:

BUT, how does doppler work when the radar itself is moving at about the same rate as the rain target, for instance? Does the radar know the boat speed and direction and subtract that from the relative target speed?

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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)

NightCrawler said:

For what it's worth, as I have nothing to compare to; crossing Albemarle Sound I noted a target 3 mi off our port beam. First mate was tasked with IDing it as it was paralleling us. This is a woman who can spot a crab pot at a half mile with the naked eye.
Target now steering an intercept, still no ID. Target now one mile out on collision course. No ID. We slowed not sure what was going on. Mid day, no wave chop. As the target crossed in front we had to laff. Three pelicans.
Our Garmin HDx seems quite capable, has even picked up boats on the back of thunder cells.

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Yup, but throw in some weather and you can lose a 50 footer if you can't adjust fast enough.

SoWhat said:

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)

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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).

MYTraveler said:

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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The simplest answer is that the radar return comes from each individual raindrop, which are moving in many directions at various velocities.

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.

Sabre602 said:

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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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.

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).

Learning something every day!

psneeld said:

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.

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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.

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.

Sabre602 said:

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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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.

Wxx3 said:

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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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.

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.

Wxx3 said:

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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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"......

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.

Wxx3 said:

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. ....

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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.

Wxx3 said:

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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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.

ssobol said:

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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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.

Seevee said:

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.
....

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Collins calls it PAC alert. Been around for at least 12 years.

Honeywell calls it REACT.

Fletcher500 said:

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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