Diffraction Spike in Satellite Images

Giddierone

Senior Member.
The Sentinel 1(a) satellite captures Synthetic Apature Radar images (SAR).
It scans a 250km wide swath of earth surface (interferometric wide swath mode, or IW), traveling on a sun-synchronous, polar-orbiting plane.
It's images frequently feature what appear to be diffraction spikes, with their four points are orientated along the ground track of the satellite.
1733531380202.png

Source: https://sentiwiki.copernicus.eu/web/s1-mission#S1-Mission-SAR-Instrument

It doesn't look directly down but at an angle between 29-46 degrees.
Source: https://sentiwiki.copernicus.eu/web/s1-mission

Vessels at sea (and wind turbines) tend to produce a consistent spike— often very large. For example this image of Shanghai Port is reminiscent of a Hubble Space telescope image of stars.
Screenshot 2024-12-06 at 22.31.23.png


Spikes on land seem less common.
I noticed this one in Cologne Germany.
1733531899213.png

Source: https://browser.dataspace.copernicu...e3D="MAPZEN"&cloudCoverage=30&dateMode=SINGLE

It appears to come from a particualr building.
RWE Energy company. https://www.rwe.com/der-konzern/rwe-power/
1733532063013.png

The spike appears over numerous dates, and changes orientation (presumably when captured by the satellite on a different path—perhaps coming the oposite way) so doesn't appear to be a chance glinting reflection.
E.g. this image shows a much smaller spike with the points rotated from the previous image.
1733532009469.png


I'm curious to know what might cause the spike, and why it seems to be consistently associated with that particular building.
There are other spikes in the same scan swath on that same date, but this one seems quite distinct.
Looking at other sites like solar farms, radio telescopes, nuclear power stations, green houses etc — which I'd assumed might all have a high radar reflectance don't seem to produce such a distinct cross-shaped spike.
But, why does this building produce a spike like it's a ship at sea?
 
Vessels at sea (and wind turbines) tend to produce a consistent spike— often very large.
Do they carry radar reflectors?
73_RadarReflector.jpg

(Source: https://www.itonavaids.com/products/wind-farms/permanent-marking/wind-farms-radar-reflector-/irr )
It appears to come from a particualr building.
RWE Energy company. https://www.rwe.com/der-konzern/rwe-power/
By browsing different dates, I notice the spikes are not present on every pass.

This is the building:
aussen5_AB.jpg


This is the inner courtyard:
aussenInnehof.jpg


It looks like it would act like a radar reflector with its perpendicular sides and reflective floor, at least from the directions where it is visible.

Diffraction is an optical phenomenon, but a synthetic aperture radar is not optical. The spikes in the SAR image are likely a SAR artifact. My hypothesis: Basically, SAR correlates the phases of multiple antennas to receive a strongly directional signal; but if a signal aligns not on every antenna, but on every second (3rd, 4th, ...) antenna, it would still be boosted somewhat, but the angle would be offset from the primary angle. The direction of the spikes would then reflect the alignment of the antenna grid. This also explains why these spikes are not continuous.
 
The right angles of the sides of the courtyards did say "trying to be a retroreflector" to me. Finding some other examples would be useful. A retroreflector the size of a football can have a cross section larger than a plane, it's a huge spike.
 
Further down in the same image (2022-03-07) off the coast of Marseille there's a much larger spike (around 10km across) with additonal bright spots along the arms. I wonder if the vessle that produced it can be identified?
Screenshot 2024-12-07 at 13.31.08.png

— By the way I put this in practical debunking because, like with ISS images of cities at night, and squid boats these images would no doubt generate their share of UFO sightings, so it's useful to know what's going on with them. For example this apparent "fleet" of UFOs in formation off the English coast...(actually the Kentish offshore wind farm).
Screenshot 2024-12-07 at 13.40.07.png
 
Further down in the same image (2022-03-07) off the coast of Marseille there's a much larger spike (around 10km across) with additonal bright spots along the arms. I wonder if the vessle that produced it can be identified?
I would bet on a retro reflector, as its signature seems magnitudes higher than everything in the scene. Certainly overpowers the (radar) signature from the boat underneath it.
 
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