[HN Gopher] Satellites Spotting Aircraft
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Satellites Spotting Aircraft
Author : marklit
Score : 117 points
Date : 2024-09-10 05:15 UTC (17 hours ago)
(HTM) web link (tech.marksblogg.com)
(TXT) w3m dump (tech.marksblogg.com)
| ck2 wrote:
| Is bouncing radar from above on an active commercial airport an
| okay idea?
| snakeyjake wrote:
| The vast majority of SAR satellites are below 500W and have a
| beam "footprint" of about 5km by 5km at minimum.
|
| Imagine 5 100W lightbulbs reflected and focused to channel as
| much light as possible onto a 25 square kilometer field from
| hundreds of kilometers away.
|
| The planes are fine.
| m2fkxy wrote:
| yes, received power is inversely proportional to the distance
| squared. it's hard to overstate how ridiculously small the
| amounts of power are at play with spaceborne SAR systems.
| Sanzig wrote:
| Sure, happens all the time. Synthetic aperture radar satellites
| operate in different bands than air traffic control radar, so
| there is no risk. Your house is scanned at least once every few
| days from wide swath radar satellites like Sentinel-1.
| jprd wrote:
| Regardless of topic, Mark's blog has been consistently making me
| smarter.
|
| ClickHouse, DuckDB, literally anything GIS related.
|
| Thank you dude.
| jauntywundrkind wrote:
| Agreed, great blog.
|
| "satellites spotting ships" was only 82d ago.
| https://tech.marksblogg.com/yolo-umbra-sar-satellites-ship-d...
| https://news.ycombinator.com/item?id=40716032
| tomcam wrote:
| Yeah this blog is a find. Welcome to the 21st.
| 0cf8612b2e1e wrote:
| It used to be a joke that satellites could read your license
| plate from space. Then I see commercial images like this and I am
| less sure.
| actionfromafar wrote:
| Well the Hubble is 70s tech, right?
| mandevil wrote:
| Because Hubble was designed for in-orbit servicing by the
| STS, it had it's optics, CCD systems, and even the flight
| computers (not originally designed for in-orbit servicing!)
| replaced over the various servicing missions. The differences
| between WF/PC 1 and WFC 3 is the difference between going
| into space in 1990 with mid-80's hardware and going into
| space in 2009.
|
| Since the 2009 Service Mission 4, Hubble tech has held
| steady, but until then it was regularly being upgraded to
| state-of-the-art.
| ceejayoz wrote:
| We got a hint of how much better spy satellites are during
| Trump's presidency.
|
| https://www.npr.org/2022/11/18/1137474748/trump-tweeted-an-i...
| / https://www.npr.org/2019/08/30/755994591/president-trump-
| twe...
|
| Folks figured out the satellite that took it, too; a 2011
| KH-11: https://www.npr.org/2019/09/02/756673481/amateurs-
| identify-u...
| 1024core wrote:
| Just look at the image: https://x.com/realDonaldTrump/status/
| 1167493371973255170/pho...
|
| You can see lamp posts and fenceposts. Fenceposts are
| literally < 2" thick! So the resolution must be on the order
| of 2cm.
| perihelions wrote:
| - _" Fenceposts are literally < 2" thick! So the
| resolution must be on the order of 2cm."_
|
| That's not quite how it works. If you apply a Gaussian blur
| on the scale of 20 cm to a 2 cm imaged object, it will
| persist in some form, if the contrast ratio is very high.
| That doesn't mean you have 2 cm resolution. Spatial
| resolution is rather different: it asks something more
| like, can you distinguish *two* objects at a 2 cm
| separation distance? Can you distinguish the case there two
| such separated objects, from that where there is only one?
|
| That's closer to what you need to answer the question "how
| small text can be read?"
|
| (Some of the visible stars in the sky, by the way, are
| _ridiculously_ small (angular size) compared to the human
| eye resolution--there 's no contradiction there either! The
| angular diameter of (for instance) Rigel is smaller than
| 1/10,000th the resolution of a human eye!)
| jofer wrote:
| That's a point that far too few people in the remote
| sensing industry understand!!
|
| Put another way, try measuring the width of small objects
| in the scene. You'll find there's a minimum width things
| will appear regardless of how small they actually are.
| Small, high-contrast objects will be visible, but will be
| wider than they actually are. Measuring the width of
| small bright objects is one way of estimating the spatial
| resolution (i.e. FWHM of the PSF) of optical imagery.
|
| And with that said, those fenceposts are not that thin.
| They're likely on the order of 10cm. It's widely assumed
| from regulations/etc that "spy" sats can get below 10cm
| spatial resolution. In the US, commercial sats are not
| allowed to collect imagery better than 10cm spatial
| resolution. At the 10cm resolution point, things like
| atmospheric lensing due to temperature variations become
| major issues and need to be corrected for (e.g. that
| "shimmer" you see above the pavement on a hot day). That
| type of tech gets tightly regulated very quickly even in
| the US's current "let private industry image how they
| want" regulatory environment (used to be more restrictive
| not long ago).
|
| So that imagery is likely somewhere on the order of 5cm
| to 10cm resolution in its native form. Which is pretty
| nuts. It's crazy what the NGA can do!
| dylan604 wrote:
| What is released commercially is not the same thing the NRO
| uses.
| jp42 wrote:
| "Those satellite photographs -- the landsat photographs -- are
| so darn good that when they re fully enhanced by computer, we
| can actually tell how high the waves are out in the middle of
| the Pacific; we can tell what the temperature of the ocean is
| 20 feet below.." - Grace Hopper in 1982 NSA lecture. YT link:
| https://www.youtube.com/watch?v=si9iqF5uTFk&t=1612s
|
| I bet current spy satellite will be doing a lot things we can
| hardly imagine, may be we will know in 40-50 years down the
| line.
| ianburrell wrote:
| This is a SAR satellite which uses radar to produce altitude
| map. I thought the resolution would be bad but it is really
| good at up to 16cm. This isn't going to read anything since it
| isn't a camera.
|
| The other thing is that commercial satellites are lower
| resolution than military ones. The giant ones are super
| expensive, and commercial users don't need to read license
| plates. They would much rather have multiple satellites that
| visit same spot every day.
| 1f60c wrote:
| > Umbra has an open data programme where they share SAR imagery
| from 500+ locations around the world
|
| I think they're referring to this: https://umbra.space/open-data/
| (warning: most files are absolutely ginormous)
| 1024core wrote:
| Is there a mapping from, say, a geo location to a list of Umbra
| files which cover that location? I'd hate to download 42TB of
| data and then have to grep through it :-D
| m2fkxy wrote:
| A little nitpick about: The number of looks
| correlates with a higher resolution.
|
| Yes and no. When you task an image, you usually (as is the case
| with Umbra) specify your desired ground resolution, eg. 25, 50,
| 100 cm, etc. There are two dimensions in a SAR image: range and
| azimuth. Range resolution is determined by the SAR system
| bandwidth. Azimuth resolution is determined by the integration
| angle (the angle formed between your target and your satellite
| from start to end of the collection).
|
| Let's assume you want a 50 cm image. Your range resolution will
| be equal to that and, in a 1-look image, your native azimuth
| resolution will also be 50 cm. What happens when you request a
| multi-looked image, is that the satellite will collect data over
| your target for a longer amount of time (and thus over a greater
| angle diversity). Range resolution will not change; however, in
| the natural ("native") image, you get asymmetrical pixels: taking
| the same target resolution of 50 cm, a 2-looks image will have a
| 25 cm azimuth resolution. For 3-looks, ~16 cm. And so on.
|
| What then happens during the processing of derived products (eg.
| GEC) is that the pixels are squared: to do that, you have to
| average out the pixels in the azimuth dimension. This greatly
| improves what is called the radiometric resolution (ie. how much
| information a pixel contains), by cancelling out the speckle and
| averaging the noise. But for all intents and purposes, on a
| multi-looked image ( _which is what the GEC products that you use
| are_ ), spatial resolution remains the same, square pixel.
|
| [SAR nerds here: I am not mentioning the slant-range-to-ground-
| range process, and I am also ignoring the resolution vs. sampling
| distinction for simplicity]
| tomcam wrote:
| Man there's everything to love about this. A novel free dataset,
| complete example code, clear description of how to get it up and
| running--the ideal blog post IMHO. Delighted to learn about it.
| Luc wrote:
| > Below is Umbra's image of the same location. Though it was
| taken on a different day and some aircraft might have been moved
| around, you can see that a lot of the aircraft in the bottom left
| are barely visible unless you zoom in very closely and pay
| attention to artefacts that give away a large man-made object is
| present.
|
| Bad example, because the radar image simply shows a different
| situation with all but two of the aircraft not present. The two
| that are present are easy enough to spot.
|
| Here's another image with 5 aircraft present (including the two
| from the radar image). It's rotated, the aircraft are in the top
| left:
| https://x.com/___Harald___/status/1825362047061971309/photo/...
| greggsy wrote:
| I think it successfully got the point across that visual vs SAR
| have different benefits.
| shitlord wrote:
| Would the F-35 actually show up on SAR or does the radar
| absorbent material distort the image somehow?
| chris_va wrote:
| Related, they are also visible while flying:
| https://medium.com/google-earth/planespotting-465ee081c168
|
| (not SAR in this case)
| walrus01 wrote:
| If some guy on the internet can do this in his home office,
| imagine the SAR and analysis capabilities the NRO and NSA have
| built in-house.
| sangnoir wrote:
| "Some guy" who happens to be a consultant with a long list of
| household-name clients, and seems like a GIS pro
| jofer wrote:
| Quick geospatial note that's important for accurately geolocating
| these images:
|
| You need a DEM to use RPCs for geolocation. Running things
| through gdalwarp as was done here will assume no terrain and 0
| elevation. That will lead to significant mislocations anytime
| it's not flat and at sea level, especially given the off-nadir
| view angle of the data used here.
|
| In other words: gdalwarp \ -t_srs
| "EPSG:4326" \ 2024-05-25-15-37-54_UMBRA-06_GEC.tif \
| warped.tif
|
| Should be: gdalwarp \ -to
| RPC_DEM=some_dem.tif \ -rpc \ -t_srs
| "EPSG:4326" \ 2024-05-25-15-37-54_UMBRA-06_GEC.tif \
| warped.tif
|
| If you don't want to use a DEM for orthographic corrections, then
| you should at least include a constant elevation in meters of the
| scene with RPC_HEIGHT. Otherwise things can be shifted kilometers
| from where the image actually is.
| wslh wrote:
| Great hands-on example! I recently read a news article [1] that
| discussed the monitoring of Russian "Mainstay" AEW&C Beriev A-50
| aircraft [1] but unfortunately, it didn't include the actual
| code.
|
| [1] https://www.linkedin.com/posts/spaceknow_spaceknow-inc-in-
| co...
| seoulmetro wrote:
| In the blog there's a video of what looks like cars moving
| around, but the lighting is constantly changing. How does that
| work?
|
| It looks like a timelapse but then there are cars doing normal
| car things.
|
| https://www.youtube.com/watch?v=cwDjJqtx_og
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(page generated 2024-09-10 23:00 UTC)