Unusual RADAR interference patterns

[DirtSailor]

To my fellow RADAR operators out there, I’ve got a conundrum that maybe one of you can help me solve.

We’ve all studied the basic forms of false RADAR echoes, such as side lobe echoes, second trace echoes, radar-radar interference, stack/crane reflections, etc. I regularly see something else, though, in a few specific geographical regions. These screenshots, in particular, are south of the Channel Islands off Santa Barbara, CA, and I regularly see these sorts of odd patterns around there.

None of those are real targets, points of land, or anything. The patterns change when I change pulse length, and occasionally go away. Eventually the patterns fade out.

In this instance, I’ve got solid ARPA locks on two moving targets - neither of which exist. In these instances, if i change pulse length the contact vanishes, and it also vanishes if it gets within a couple miles of me.

What on earth (or elsewhere) am I seeing? And, are there any solutions to get rid of this bizarre clutter?

[LupusProximo]

May be this will help for further study of this phenomenon:

You’re dealing with indirect echoes (also known as “false reflections” or “mirror images”).

Essentially, your radar energy isn’t bouncing off a real ship and coming straight back. Instead, the beam is hitting a fixed part of your own vessel’s structure first—likely a mast, crane, or stack—which then deflects the energy toward a real target or a large shore object.

Here is why your specific symptoms are happening:

• Vanishing at close range: As you get closer to the object causing the reflection, the geometry changes. The “mirror” on your boat no longer aligns with the target, or the target moves into a “blind sector” (shadow) created by your own superstructure.

• Pulse length sensitivity: Changing the pulse length changes the range resolution and the energy density of the return. Shortening the pulse often drops these “weak” reflected signals below the detection threshold, making them disappear.

• Moving targets: Even if the reflecting object is stationary, your own ship’s movement makes the “mirror image” appear to have its own course and speed on the ARPA.

How to fix it:

1. Identify the “Shadow Sectors”: Check your radar’s manual or shipyard diagrams for “blind” or “shadow” sectors. These are areas where your mast or funnel blocks the beam. If the ghosts always appear at the same relative bearing to your bow, that’s your culprit.

2. Adjust the Gain/Sea Clutter: Turn down the Gain slightly. Indirect echoes are usually weaker than real ones.

3. Check for “Secondary Trace” Echoes: If you are using a high Pulse Repetition Frequency (PRF), you might be seeing targets from a previous pulse. Switching to a longer range scale (which lowers the PRF) can sometimes clear this.

4. Physical Shimming: In some cases, technicians install “blind sector” software settings or physical radar absorbent material (RAM) on the obstructing mast to stop the reflection at the source.

Quick tip: If the “ghost” is always on the same bearing relative to your own ship, even as you turn, it is definitely an internal reflection from your own gear

[AKBJR]

LP may be correct. I always attributed such phenomena to electronic emissions from military installations in whatever area my ship happened to be.

[DirtSailor]

Thanks, but it’s something a little more complicated than that I think. For instance:

These are false echoes from my stack reflecting off the ships on my port bow. They’re pretty easily identifiable as such. Same with double echoes from nearby ships where the pulse makes it out and back then bounces again before hitting the antenna. The stuff that’s baffling me occurs when there’s nothing but ocean for 20+nm around, sometimes much more, and occurs even with the longest pulse lengths my unit can put out.

I considered if it could be distant terrain and mountains, getting returns off a second full sweep, but there are a lot of mountain ranges I pass by where I don’t see these effects and I’ll get them even with the gain turned down.

I’ve also wondered if it was military EMF emissions, but I’ve also seen this in areas that I’m not aware of any big emitters nearby. Could be, though, I suppose.

[LupusProximo]

Sure think it is. Can You try on same S band but reduce range to 6Nm and shift off center like indicated in the pic below.

repeat same with X band.

Extend your course line with VBL +/- 180 , check the range of both items as accurately as possible and examine if both items look like mirror immages in relation to yellow line as per below. Interesting stuff indeed need to hit some books for some clues.

[dbeierl]

On my little radar attached to the front of an oval aluminum mast I had to make a rectangular shield out of flashing stock to prevent false echoes showing up astern from strong targets near abeam.

[fogmachine]

I’ve seen similar when transiting between LA and San Diego and assumed it was due to military operations, usually when the Marines were practicing night assaults on San Onofre.

[DirtSailor]

Yeah, my ship has two stacks which is why there are mirrored reflections on either side of my reciprocal heading. One pair of echoes there almost precisely matches the range of a large freighter in ballast and the second pair is at the same range as a large loaded container ship. Those four returns are, for sure, reflections from them bouncing off the twin funnels toward my antenna. I’m not worried about them. (Just be sure to check visually anyway to make sure you’re not also being overtaken by a destroyer with its AIS off! Ask a shipmate how he knows…)

It’s the huge stripes and the unknown moving point contacts shown in my first post that are confusing to me, and that aren’t explained by my stacks or other deck gear.

[LupusProximo]

@DirtSailor

destroyer with its AIS off! Ask a shipmate how he knows…)

Have completely forgotten about AIS . Well as a 3rd, 2nd , chief mate had never used it to help identify false immages as it did not exist then as a tool . And old age kicks in too.

When you observe this strange echoes as described in first case ,what does happen when you change course by some 10- 20 deg to stb or port. Do they remain still? or vanish.

Have experienced many strange phenomena on radar screens in my life but the problem with some was these appeared only once and never again .

Something what occurs regularly is a lot easier to study in order to find/guess it’s cause.

Leafing through several books abt radars searching for something similar but untill now have found nothing.

[yacht_sailor]

I have seen something similar southbound passing over Andros Island on the airplane radar and always assumed it was the Navy playing games.

[DirtSailor]

Have a possible solution to one kind of odd interference encountered. We were about 100nm offshore and getting some big phantom blobs on the S-Band. I maneuvered the ship about 20º off our heading in each direction, and the blobs kept their same true bearing, pretty conclusively proving it wasn’t related to the ship’s structure I believe.

I put a pair of EBLs through the blobs to track bearing change, if any. Here’s what it looked like with “Long Pulse 1”

“Long Pulse 2”

And “Medium Pulse 3”

Each pulse length produces a different pattern and different ranges but very similar in azimuth. I extended the EBLs out to the coast to see if there was anything on those bearings, and while there are mountains in the area, there aren’t any particular peaks noted on those bearings - but, with the pulses going up to 130 miles out and back, the antenna would have had time to rotate a bit, which could throw off the return angle.

The blobs seemed to move almost tangentially to their bearing from the ship, but slowly moving closer at roughly the speed the ship was going. Observe the difference between true and relative trails. I also noticed that returns were directly proportional to signal gain:

The X-band showed a little bit of interference in the same area, but only a tiny bit at max gain on one pulse length:

So, for this set of echoes, my best current guess is atmospheric ducting giving me returns off distant mountains that the RADAR doesn’t know how to interpret. I regularly encounter this sort of thing in this region, but don’t see it in other mountainous areas we frequently pass. There’s probably something in the prevailing weather that makes the difference, I’d guess?

EDIT: I missed this old Gcaptain thread at first, discussing “second trace” echoes that would seem to explain this particular scenario. My S-band unit uses variable PRFs; MP3 at 1400hz, LP1 at 750hz and LP2 at 650hz. For the ranges I’m seeing the blobs at in this case, this would make perfect sense and not even need favorable weather and ducting to show up. Section 3.9.6 of this article describes it really well, now that I know what to Google.

Though that still doesn’t explain the false echoes that look like seaborne contacts while 200+ miles offshore moving contrary to wind, ship course and speed (unless I’m just seeing North Korean submarines tracking me with their big reflective periscopes and snorkels up, I suppose)

[DirtSailor]

Another obvious candidate to explain these echoes is gravitational lensing from the relativistic interstellar drives of extraterrestrial visitors.

… Leaving aside the use of aliens as an explanation though, the radar returns these guys show are very similar to what I’m used to seeing in certain areas, like off southern California, especially figures 17, 19 and 23.

Figure 19. A correlation between two independent radar systems in two scan geometries. The image on the left is scanning on a Y-axis, 180 degrees skyward front to back. The image on the right is an X-axis, 360-degree circular scan of the horizon. The patterns are the same, with echo pitch and count. This indicates that the timing/distance relationship is the same for both independent signals—an omnidirectional effect with no directionality other than the source.

But if it’s not aliens, then what the heck is it?

[LupusProximo]

This thread of yours can be a good chapter and update to some of the books on radar.

Greatly admire your zeal and passion in experimental science and truth seeking . You are a perfect example of good and dedicated reasearcher . Surely a great asset for your company as generally never mind attitudes prevail. Really appreciate your metodology and road to discovery . My hat off man ..

[DirtSailor]

Actually…

@DirtSailor

In this instance, I’ve got solid ARPA locks on two moving targets - neither of which exist. In these instances, if I change pulse length the contact vanishes, and it also vanishes if it gets within a couple miles of me.

… “Second trace” echoes combined with super-refraction would explain this. Those are probably real returns off real ships, that are just a hundred miles or more away. The fact that changing the pulse length changes the PRF explains why they only appear on certain settings - the contacts are probably only within the correct range band of one PRF’s second trace at a time. I’ll have to analyze the weather next time I see this.

Compare that to a huge mountain range and you get returns on any PRF just because there’s so much terrain to reflect signals from.

Aha! There’s two mysteries potentially solved (unless someone can get me a better explanation, I’m all ears), but the bizarre interference off Southern California is still making me scratch my head. I’ll have to do some more experimenting when I’m next down there.