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)