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Sky lapse in two tone

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Re: Sky lapse in two tone

#21

Post by mysterypie »

hardlianotion wrote: post #3

I had no idea shipping lanes were so densely populated. EDIT - thanks all. I'll read it next time ...

Here's a site that does live tracking of ships:

https://www.marinetraffic.com/en/ais/home/centerx:1.7/center...

"Since December 2004, the International Maritime Organisation (IMO) requires all passenger and commercial vessels over 299 Gross Tonnage that travel internationally to carry an AIS transponder that include a GPS receiver which collects the vessel's position and movement details."

Counting across the longest segment of ships at the narrowest point of the English Channel in the feature article I see about 18 ships. In the live image from marinetraffic I see about 8. So even though the feature article is a composite image, the English Channel is indeed pretty crowded.

Re: Sky lapse in two tone

#23

Post by louthy »

hardlianotion wrote: post #3

I had no idea shipping lanes were so densely populated. EDIT - thanks all. I'll read it next time ...

The English channel is apparently busiest shipping lane in the world, so it’s not surprising that it looks so busy on these images.

Re: Sky lapse in two tone

#24

Post by hardlianotion »

mysterypie wrote: post #21
hardlianotion wrote: post #3

I had no idea shipping lanes were so densely populated. EDIT - thanks all. I'll read it next time ...

Here's a site that does live tracking of ships: https://www.marinetraffic.com/en/ais/home/centerx:1.7/center... "Since December 2004, the International Maritime Organisation (IMO) requires all passenger and commercial vessels over 299 Gross Tonnage that travel internationally to carry an AIS transponder that include a GPS receiver which collects the vessel's position and movement details." Counting across the longest…

Yes, you can still see the shipping lanes very clearly.

Re: Sky lapse in two tone

#25

Post by Cthulhu_ »

algo_trader wrote: post #22

Off topic: are there datasets for solar incidence over the oceans ? There are plenty of accurate maps/widgets for ground based PV, but ocean data seems much rarer

That's because ocean PV is... unrealistic; the wear and tear on these things is immense. I am guessing windmills are affected less, with the blades being made out of fiberglass and most of the construction being above the waves.

Source: I made it up, I'm a software engineer not a maritime power expert.

Re: Sky lapse in two tone

#26

Post by 3dsnano »

cool way to see the formation of wind farms from above... hornsea seems to follow a pattern then it gets all organic looking on the left hand side. i wonder why?

my absolute favorite of the wind farms shown is sheringham, a true ocean rhomboid. i got curious and found the website for the wind farm [1], and here's an aerial view [2]

[1] https://sheringhamshoal.co.uk/about/overview.php

[2] https://sheringhamshoal.co.uk/about/benefits-of-offshore-win...

Re: Sky lapse in two tone

#27

Post by enriquto »

m2fkxy wrote: post #15
enriquto wrote: post #12

Earlier quoted context omitted.

The liquid water absorbs most of the radar wave (at least for the C band, as is the case of sentinel-1). Thus, in the images it appears natively "black". When there are a lot of waves, the surface of the water forms some spurious reflectors that appear as a light texture in the surface, but the signal is definitely less powerful than metal/concrete reflectors of buildings and ships. Notice that in the "Humber" image…

It actually does not absorb radar energy, rather it reflects it away (specular reflection). That's why rougher sea surface appears brighter on SAR as it turns from a specular to a diffuse scatterer. Furthermore, in some specific conditions (low incidence angle, closer to nadir), calm water can appear much brighter than other land surfaces.

thanks for the clarification! I'm just used to looking to the images without thinking too much about the water.

I've never seen "close to nadir" radar images... wouldn't the ground fold over itself?

Re: Sky lapse in two tone

#28

Post by fithisux »

m2fkxy wrote: post #16
jvanderbot wrote: post #4

Technical SAR question: Is the sea surface removed post-hoc, or does the sea not reflect these radar waves in a sufficient amount to show up? I seem to remember NISAR [1] will map land and ice - so presumably water ice shows up. I ask b/c if they are synthetically removing the sea surface at 1-5m resolution that seems really hard given tides, waves, etc. 1. https://en.wikipedia.org/wiki/NISAR_(satellite)

Calm water surface is a specular scatterer, meaning radar energy will be reflected away, and proportionally so as the incidence angle increases. Images collected at lower incidence (closer to nadir) might feature very bright surface water if it is calm, as more energy is reflected towards the radar. Rough water surface is a diffuse scatterer, and will generally appear brighter than calm waters.

Any good source for SAR image formation? From antennas to image processing?

Re: Sky lapse in two tone

#29

Post by m2fkxy »

enriquto wrote: post #27
m2fkxy wrote: post #15

Earlier quoted context omitted.

It actually does not absorb radar energy, rather it reflects it away (specular reflection). That's why rougher sea surface appears brighter on SAR as it turns from a specular to a diffuse scatterer. Furthermore, in some specific conditions (low incidence angle, closer to nadir), calm water can appear much brighter than other land surfaces.

thanks for the clarification! I'm just used to looking to the images without thinking too much about the water. I've never seen "close to nadir" radar images... wouldn't the ground fold over itself?

Nadir-SAR would look very confusing, full of ambiguities, and probably unexploitable since the ranging part of RADAR would not be able to distinguish the left returns from the right returns -- that's why SAR is side-looking.

even with a strictly side-looking geometry, images collected closer to nadir (steep/low incidence) start exhibiting some artifacts such as nadir returns [1].

[1] https://www.researchgate.net/figure/Example-of-nadir-echo-in...

Re: Sky lapse in two tone

#30

Post by m2fkxy »

fithisux wrote: post #28
m2fkxy wrote: post #16

Earlier quoted context omitted.

Calm water surface is a specular scatterer, meaning radar energy will be reflected away, and proportionally so as the incidence angle increases. Images collected at lower incidence (closer to nadir) might feature very bright surface water if it is calm, as more energy is reflected towards the radar. Rough water surface is a diffuse scatterer, and will generally appear brighter than calm waters.

Any good source for SAR image formation? From antennas to image processing?

I am just a half-educated layman when it comes to SAR, which has a very heavy electrical engineering heritage. I hear that one very good technical resource is this book written by Iain Woodhouse [1], but I would be lucky to understand a tenth of it myself.

There is also a very good SAR vulgarisation book written by Tom Ager [2].

[1] https://www.routledge.com/Introduction-to-Microwave-Remote-S...

[2] https://www.amazon.com/Essentials-SAR-Conceptual-Remarkable-...

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