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Old 08-28-2018, 11:50 AM   #1
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Interesting radar comparison, magnetron vs doppler

On Mahalo I have two Garmin radars, a GMR626 XHD2 traditional magnetron radar, and a new Fantom 24 digital radar.

I was tooling along in the Strait of Georgia last week, in intense forest fire smoke. The sky and water were just shades of gray.

I generally just show one radar, typically the Fantom, because the images are a bit more crisp than the other.

I came across what appears to be hundreds of small fishing boats, yet there are just a few blips on the radar. I range out and back in, almost nothing shows?

I fire up the 626 and there are the targets. See the picture for yourself. One thing I didn't think to review is the gain setting on the Fantom. It's probably just at the factory default.

In the second picture you can see what it looked like outside.

I did a little googling and the fire community is excited about doppler radar because it identifies smoke well. Leaves me to wonder if it can see through the smoke or not?

The HYG service guy is going to inquire with Garmin...
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Old 08-28-2018, 12:09 PM   #2
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Originally Posted by JustBob View Post
On Mahalo I have two Garmin radars, a GMR626 XHD2 traditional magnetron radar, and a new Fantom 24 digital radar.

I was tooling along in the Strait of Georgia last week, in intense forest fire smoke. The sky and water were just shades of gray.

I generally just show one radar, typically the Fantom, because the images are a bit more crisp than the other.

I came across what appears to be hundreds of small fishing boats, yet there are just a few blips on the radar. I range out and back in, almost nothing shows?

I fire up the 626 and there are the targets. See the picture for yourself. One thing I didn't think to review is the gain setting on the Fantom. It's probably just at the factory default.

In the second picture you can see what it looked like outside.

I did a little googling and the fire community is excited about doppler radar because it identifies smoke well. Leaves me to wonder if it can see through the smoke or not?

The HYG service guy is going to inquire with Garmin...
Well, to begin with, it is unlikely that your Fantom is doppler radar, which is used for mapping tiny particles such as individual raindrops and would, as you suspect, cover any targets you are interested in.

Without knowing the gain on either transmitter, no comparison is possible, but digital displays are popular precisely because the signal processing rejects all that annoying "clutter"...just as you discovered.

Seems like running both units and continually comparing performance seems like a good solution.

In my first career, as an air traffic controller, my employer transitioned from analog to digitized radar, but I never really liked it, still prefer "real" radar, which is what I have on Seeadler, and used with confidence as recently as yesterday in 1/4 mile vis. BTW, I always use the manual "gain" and frequently tweak it; can't imagine why one would not.
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Old 08-28-2018, 01:05 PM   #3
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Well, to begin with, it is unlikely that your Fantom is doppler radar, which is used for mapping tiny particles such as individual raindrops and would, as you suspect, cover any targets you are interested in.
Ah, my mistake. Here's what is on Garmin's product page:

MotionScope™ technology uses the Doppler effect to detect and highlight targets moving toward you or away in different colors to help avoid collisions, find flocks of birds, and track weather.
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Old 08-28-2018, 01:16 PM   #4
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Furuno is advertising their radar as having doppler technology....whatever marketing ploy that is....

From their website...

"The*NXT*step with doppler Radar technology

The Furuno*NXT*Radar*is the next step in Radar evolution; a Solid-State Radar with pulse compression, Target Analyzer™ and Fast Target Tracking™ utilizing Doppler technology. Combined with FURUNO exclusive RezBoost™ technology for beam sharpening, NXT packs the performance of a larger Radar in a more compact package."
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Old 08-28-2018, 02:30 PM   #5
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I will comment because the thread topic changed to what I have, and I like facts. PS, how many hours do you have with this radar? Did my third, 10 hr. overnight run last night with the new to me Furono NXT with Doppler. I have Had a traditional magnetron Ray, and a solid state Simrad prior, so I do have something to compare it to. Absolutely love it, and it does what it says it does...very well.
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Old 08-28-2018, 02:43 PM   #6
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I have zero time with the "newgen radars".

I am not commenting on radar performance just how sometimes companies use words like "Doppler technology" and the only tech used is the paint on the array.

I have no idea whether it is true doppler or not, just putting up a lead if anyone wants to believe doppler tech is in a small boat operators grasp or not.
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Old 08-28-2018, 03:12 PM   #7
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On 3 miles you would think the Fantom would be painting targets better than that. It must be a fault with the settings even though you're comparing a 6' 6kW open array antenna to a much smaller 2' enclosed dome antenna where the 6' is going to have a far greater advantage. I'm assuming the antennas are both parallel to the horizon and both close to the same height.
I've had sales people at dealer meetings insist that their broadband radars will pick up targets like birds just fine but they'll never convince me. There's currently nothing better for long range target detection that good old raw microwave power.
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Old 08-28-2018, 03:23 PM   #8
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I still have an old Furuno magnetron radar that I keep putting off replacing. And no, I have not studied the newer stuff.

For my application though, I plan to move away from pulsed systems to a new CW low power unit maybe next year. Hard to chuck something that works. But back to the newer CW stuff. It should be a cake walk for manufacturers to add doppler to a CW system. Digital hardware today is very inexpensive once the software is complete.

One thing to remember, higher frequencies used with old maggies are in the X band with wavelengths of only a few centimeters. Great for clear weather but not so great when it rains and for most, a radar is most valuable at night, in the fog, rain and just sucky weather. But the short wavelengths of X band are absorbed by the water droplets that compromise the system. KU band is worse for water absorbing signals.
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Old 08-28-2018, 03:36 PM   #9
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It's hard to come to any conclusion without equal hand tuning on each radar. But in my experience there is no comparison between a pulse (conventional) radar and a "broadband" or other continuous wave radar. The pulse radar wins, hands down.
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Old 08-28-2018, 04:54 PM   #10
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Ah, my mistake. Here's what is on Garmin's product page:

MotionScope™ technology uses the Doppler effect to detect and highlight...
No error, if that's what it says, I believe that's what you bought, but im not sure I see the utility of it. How much bird finding and weather tracking do you do?

I make no claim to engineering expertise, but I regard myself as a sophisticated user. The one radar innovation which I really like is the phased-array antennas, whichare swept electronically without actually rotating.

'prof in Lagoon Cove BC
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Old 08-28-2018, 05:11 PM   #11
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The one radar innovation which I really like is the phased-array antennas, whichare swept electronically without actually rotating.

'prof in Lagoon Cove BC


For small boats???
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Old 08-28-2018, 05:27 PM   #12
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For small boats???

Don't know. Certainly feasible.
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Old 08-28-2018, 05:29 PM   #13
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I wish I knew more about the new radar. 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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Old 08-28-2018, 05:37 PM   #14
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I wish I knew more about the new radar. 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?
Doppler senses the frequency shift caused by motion of the individual rain drop...many many rain drops, most of which are not moving at the "blind speed".
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Old 08-28-2018, 07:42 PM   #15
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I'm not so good with radar, so take my experience for what it is worth (not much). But I recently switched from an old conventional raymarine 24" dome to a Garmin GMR digital 18". I also updated MFD's etc.


I wasn't real happy with the new radar at first, it wasn't finding targets like I thought it should, though it was much better than my old radar with stuff that is close to the boat, and the colors sure are pretty.


It really kind of burned me when we were crossing the stream on a clear day and it wasn't picking up a cargo ship 8 miles away that I could see clearly with the naked eye.


Eventually though I started messing with the gain and other settings and it got way better. It does a good job showing rain squalls, which is the main thing I use radar for.


Picking up birds, like for tuna fishing? Not my set. Maybe a bigger more powerful dome.
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Old 08-28-2018, 08:17 PM   #16
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Doppler senses the frequency shift caused by motion of the individual rain drop...many many rain drops, most of which are not moving at the "blind speed".
The frequency referred to by Prof above is in the returned echo. The faster the target closes, the higher the returned echo frequency. The reverse is true for targets moving away. The faster moving away, the lower the returned echo frequency. And if echo returns at the same frequency as transmitted indicates target is stationary.

A good example of doppler is to listen to a train’s whistle as the train approaches and then passes by
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Old 08-28-2018, 08:23 PM   #17
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Doppler radar just means that the unit can detect a (very slight) change in echo frequency compared to transmit frequency due to relative motion between target and ship. It means nothing else. The "Doppler shift" thingy. It can calc relative speeds, that's it. Has nothing to do with target resolution. That could be better, or worse.
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Old 08-28-2018, 08:31 PM   #18
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...And if echo returns at the same frequency as transmitted indicates target is stationary....
Assuming that I am not moving. If I'm moving, then we're only looking at RELATIVE motion.

As was stated above, knowing our own course and speed, the unit could do computational vector analysis to give the target's course and speed.
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Old 08-28-2018, 10:09 PM   #19
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Don't know. Certainly feasible.
Garmin, for one builds such units for aircraft, pulsed weather radar.
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Old 08-28-2018, 11:46 PM   #20
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Assuming that I am not moving. If I'm moving, then we're only looking at RELATIVE motion.



As was stated above, knowing our own course and speed, the unit could do computational vector analysis to give the target's course and speed.


I believe Doppler measures the frequency shift relative to the signal itself. I could be wrong, but that is my understanding. Thus your relative speed is essentially meaningless in comparison. The Doppler shift is simply measuring the delta shift and calculating a resultant velocity. This is opposed to “regular” radar where distance is calculated based on the reflected return time of the signal as compared to when it was sent and no measurement of the frequency shift is measured. Though as you note, if you know course and speed, you could calc a relative velocity.

At least, this is what I think I understand.
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