Satellites Use 'This Weird Trick' To See More Than They Should - Synthetic Aperture Radar Explained.
Summary
TLDRScott Manley discusses the advancements in commercial synthetic aperture radar (SAR) satellite technology, highlighting its ability to produce detailed imagery regardless of light or weather conditions. SAR's capability to penetrate clouds and operate at night, along with its high-resolution imaging, is revolutionizing Earth observation. The video explores SAR's technical aspects, its applications in various fields, and the commercial interest in this technology for generating valuable data.
Takeaways
- 🛰️ Capella Space launched a small satellite on a Rocket Lab Electron vehicle for a mission named 'I Can't Believe It's Not Optical', showcasing impressive radar imagery.
- 🚀 The mission is part of a growing trend of commercial ventures in radar satellite technology, with multiple companies planning satellite constellations for Earth observation data services.
- 🌐 Optical imagery services, like those from Planet Labs, provide global coverage with daily updates, but are limited by clear daytime skies; radar satellites offer 24/7 and all-weather capabilities.
- 🔬 Synthetic Aperture Radar (SAR) technology, though not new, has been miniaturized and optimized for use on smaller spacecraft, matching the resolution of optical data.
- 📡 SAR works by emitting radio signals and timing their reflections to determine distances, creating images with high accuracy by combining data from various viewpoints.
- 🌌 SAR systems overcome the limitations of large antenna size for high resolution by using the motion of the antenna to effectively create a larger aperture.
- 🛠️ SAR processing involves complex algorithms, including Fourier transforms, which were historically done using optical analog computers and are now performed digitally.
- 🏞️ SAR images can reveal details about the surface, such as the dielectric constant and surface geometry, which are crucial for understanding the radar reflections.
- 🌳 SAR can penetrate certain surfaces, like dry snow or sand, and is sensitive to the polarization of radio waves, providing a wealth of information for analysis.
- 🏢 Commercial interest in SAR data is high due to its ability to provide high-resolution images in any lighting condition and its applications in various fields, including agriculture and resource monitoring.
- 🛡️ While SAR has military applications, it also offers significant benefits for civilian use, with public archives like the Alaska Satellite Facility providing access to a vast amount of data.
Q & A
What is the name of the small satellite built by Capella Space?
-The satellite's mission name is 'I Can't Believe It's Not Optical', but the transcript does not provide the specific name of the satellite.
What is Synthetic Aperture Radar (SAR) and how does it differ from optical imaging?
-Synthetic Aperture Radar is a technology that uses radar to generate images. Unlike optical imaging, which relies on visible light and clear skies, SAR can work at night and through clouds, providing high-resolution images regardless of lighting conditions.
How does SAR technology overcome the limitation of radar's longer wavelengths compared to optical photons?
-SAR overcomes this limitation by moving an antenna over long distances and combining all the viewpoints into a single radar image with an effective aperture much larger, using the motion of the antenna to create a larger aperture.
What is the role of the Doppler effect in SAR imaging?
-The Doppler effect shifts the frequency of the radar echoes based on the movement of the object relative to the antenna. This frequency shift can be used to determine the distance ahead or behind the vehicle, improving the localization and quality of the SAR image.
How does the polarization of radio waves affect SAR imaging?
-Polarization is a key aspect of SAR systems as it can affect how radio waves are reflected and modify the returned radio waves' polarization. Different surfaces reflect different polarizations more strongly, which can be used to analyze the surface properties.
What are some of the artifacts or distortions that can occur in SAR images?
-Some artifacts in SAR images include foreshortening, where mountains appear to lean towards the viewer, and layover, where the tops of steep slopes appear closer than the base. Additionally, large objects can cast shadows, creating black areas in the image.
How does the dielectric constant of a substance affect its appearance in SAR images?
-The dielectric constant, an electrical property of a substance, affects how well it reflects radio waves. Substances with a high dielectric constant reflect radio waves more strongly, while those with a low constant may allow radar to penetrate and reveal subsurface features.
What are some practical applications of SAR technology on Earth?
-SAR technology can be used to monitor changes in farmland to estimate crop yields, observe land deformation around volcanoes, identify archaeological sites through subsurface reflections, and analyze storage facilities to determine the amount of crude oil being stored.
How has the miniaturization and optimization of SAR technology impacted its use in space missions?
-The miniaturization and optimization of SAR technology have allowed it to be flown on smaller spacecraft, enabling missions like Capella Space's to produce high-resolution imagery from a small satellite platform.
What are some of the limitations of SAR technology?
-SAR technology has limitations such as its resolution being ultimately limited by the size of the radio wave photons, the assumption that the motion of the vehicle is known and nothing else is moving, and its inability to penetrate most buildings to provide useful interior imagery.
What resources are available for those interested in exploring SAR data from publicly funded science missions?
-The Alaska Satellite Facility archives a wealth of SAR data from publicly funded science missions, providing a resource for those interested in analyzing the world through SAR imagery.
Outlines
🛰️ Introduction to Commercial Radar Satellites
Scott Manley introduces Capella Space's small satellite, launched via Rocket Lab's Electron, as part of a mission humorously named 'I Can't Believe It's Not Optical.' The satellite utilizes Synthetic Aperture Radar (SAR) technology to produce high-resolution images, which is a significant advancement in the commercial satellite industry. SAR's ability to operate in any weather conditions and at night contrasts with optical imaging, which is limited by clear skies. The script also mentions the growth of the commercial radar satellite market, with companies like Planet Labs already providing optical imaging services. The potential of SAR to match optical resolution and its historical development since the 1950s are highlighted.
📡 Understanding Synthetic Aperture Radar
This paragraph delves into the technical workings of Synthetic Aperture Radar. It explains how SAR overcomes the limitations of large antenna size for high-resolution imaging by moving the antenna over a long distance, effectively creating a larger aperture. The explanation includes the fundamental physics of imaging with radio waves, the difference in wavelengths between radar and optical photons, and the concept of interferometry in achieving high-resolution images. The paragraph also discusses the historical use of SAR in missions like Magellan to Venus and the evolution of SAR processing from analog to digital methods.
🌐 SAR Imaging Process and Its Challenges
The script explains the process of SAR imaging, starting with the emission of radar pulses that reflect off surface features and are picked up by the antenna. It clarifies that the radar beam is angled, not pointed straight down, to capture different reflection times from various parts of the surface. The concept of the range migration curve is introduced, which accounts for the changing distance of the radar return as the antenna moves. The paragraph also touches on the use of the Doppler effect to improve image localization and mentions the artifacts unique to SAR processing, such as foreshortening and layover, which can distort the appearance of tall objects or steep slopes.
🌳 Surface Reflection Analysis in SAR
This section discusses the importance of radar reflection in analyzing surfaces, focusing on the dielectric constant's impact on radio wave reflection and penetration. It explains how the geometry of a surface affects reflection, using calm water as an example where the radar pulse is reflected away from the antenna, resulting in lakes appearing black on SAR images. The paragraph also explores the significance of the scale of surface bumps relative to radar wavelengths and the role of polarization in SAR systems, which can reveal different surface properties and is useful for creating false color images that enhance the analysis of the surface.
🚀 Applications and Limitations of SAR Technology
The final paragraph outlines the diverse applications of SAR technology, from revealing the terrain of Venus to identifying hydrocarbon lakes on Titan, discovering buried archaeological sites, monitoring land deformation around volcanoes, and assessing crop yields and crude oil storage levels. It also addresses the commercial interest in SAR data and clarifies common misconceptions about SAR's capabilities, such as its inability to penetrate most buildings or track moving objects accurately. The script concludes by highlighting the availability of publicly funded SAR data for curious observers and ends with a signature sign-off from Scott Manley.
Mindmap
Keywords
💡Capella Space
💡Synthetic Aperture Radar (SAR)
💡Optical Imaging
💡Radar Resolution
💡Planet Labs
💡Dielectric Constant
💡Polarization
💡Doppler Effect
💡Radar Interferometry
💡Layover and Shadowing
💡Commercial Satellite Constellations
Highlights
Capella Space launched a small satellite on a Rocket Lab Electron to demonstrate high-quality radar imagery.
The mission was humorously named 'I Can't Believe It's Not Optical' reflecting the clarity of the radar images.
Commercial radar satellites are being launched to provide Earth observation data to paying customers.
Planet Labs offers optical imaging services with daily global updates and higher resolution imagery on demand.
Radar satellites can operate at night and through clouds, offering advantages over optical imagery.
Synthetic Aperture Radar (SAR) technology has been miniaturized and optimized for use in small spacecraft.
SAR uses radio signals to create images with high resolution despite the larger wavelengths compared to light.
The principle of SAR involves moving an antenna to synthesize a larger effective aperture for high-resolution imaging.
SAR systems account for the curvature of the Earth and the motion of the antenna to accurately locate radar reflections.
Doppler effect is utilized in SAR to improve image localization by analyzing frequency shifts in radar echoes.
SAR processing involves complex algorithms, historically done with optical analog computers, now done digitally.
SAR images can have artifacts like foreshortening and layover, which can be corrected with advanced processing.
The dielectric constant and surface geometry affect how objects reflect radar, providing insights into the surface.
Polarization in SAR systems is a valuable tool for analyzing surface properties and can be visualized in false color images.
SAR technology has been used for diverse applications, from revealing Venus' terrain to identifying hydrocarbon lakes on Titan.
Commercial interest in SAR is growing due to its ability to provide valuable data for various industries.
SAR has limitations, such as reduced resolution compared to optical photons and challenges with moving objects in the scene.
Publicly funded science missions offer a wealth of SAR data available for free, such as from the Alaska Satellite Facility.
Transcripts
hello it's scott manley here
earlier this year a small satellite
built by capella space
launched on a rocket lab electron launch
vehicle
in a mission named i can't believe it's
not optical
which accurately represented the many
people's reactions to seeing the imagery
which this satellite has been able to
produce
using radar specifically a technology
called
synthetic aperture radar but it's
actually just
one of many new commercial ventures
commercial radar satellites are being
launched right now
with multiple companies planning on
constellations of satellites
that will be able to generate earth
observation data for customers
who want to pay for it now there are
already companies like planet labs which
sell
optically optical imaging from space as
a service
a few years ago i took a close look at
their dove cubesats
which can take three meter resolution
imagery from a satellite about this size
and they can do this for the entire
planet and update it on a
daily basis they also have the ability
to take higher resolution imagery from
other satellites on demand but optical
imagery it relies
on clear daytime skies the radar
satellites
can work at night and they can work
through clouds
and thanks to some clever mathematics
they can now match the resolution of
optical data from similar size
spacecraft
so synthetic aperture radar isn't a new
technology it first
was conceived in the 1950s but over the
years it's been miniaturized and
optimized
and it flew on many spacecraft like
magellan for example is one of my
favorites it went to venus and it used
its radar to penetrate the clouds
and finally reveal the terrain on this
planet
which never has clear skies so i want to
talk about
why it's called synthetic aperture radar
and to understand this you have to
understand how it works
so radar as you probably know is a
remote sensing system where you
send out a radio signal and you listen
for reflections of it coming back
and the timing between the signal being
sent and received tells you
the distance with great accuracy of the
object reflecting
so classic use of this is to have a
rotating radar beam
so that when it points at a target it
gives you both the range
and the direction based upon where the
beam was pointing
so the aperture part means that it's a
radar system designed for imaging
just like cameras and telescopes which
have an aperture and
focusing elements designed to collect
imaging images using
optical photons you can do a similar
thing with radar radio photons i mean
remember light and radio waves are both
photons but with different wavelengths
just as optical telescopes can have
mirrors focusing
on arrays of photo sensors you can make
images with radio reflectors by having
you know radial reflectors an array of
antenna right
but there's a fundamental bit of physics
that shows the amount of detail that you
can get from an image
is limited by the wavelength of the
photons divided by the size of the
aperture right
so the wavelengths used in radar are
usually in the range of 1 to 10
centimeters while
optical photons are you know hundreds of
nanometers so the wavelengths of radio
waves is like a hundred thousand times
longer than light and that means you
need larger radio telescopes to get the
same
angular resolution as an optical res
optical telescope right like a hundred
thousand times bigger
the largest single radial dishes can't
possibly match
optical telescopes but it is possible to
use
interferometry between radio telescopes
thousands of miles apart to get better
resolution than optical
and that's how the event horizon
telescope was able to construct their
famous
black hole image it isn't possible to do
that kind of
interferometry with optical images yet
so anyway the requirement for a large
antenna to get high resolution
is a problem if you want to put your
radar imaging system on a spacecraft or
a satellite where the size is limited
so synthetic aperture radar tries to get
high resolution with a small antenna by
moving that antenna
over long distances and combining all
the viewpoints into a single
radar image with an effective aperture
that's much
larger using the motion of that antenna
to create a larger
aperture so synthetic aperture radar
systems
fly over terrain either on an aircraft
or on satellites and they synthesize
radar responses to create detailed
images
okay so now you know what sar means
how does it actually work well we have a
vessel
either an aircraft or a spacecraft
carrying the radar
over a surface the radar is pointed
sideways and downwards so that it covers
a patch of the surface
as the radar pulses are emitted they
reflect upon surface features on this
flat surface
and the reflections are picked up by the
antenna
as it returns to the satellite now
because the radar beam
is covering a finite section of the
surface at an
angle objects on one side of the patch
may be closer
to objects on the other side and
therefore the reflections will come
at different times now to be clear it's
a common misconception that satellite
radar mapping systems point the radar
straight down underneath the satellite
but that doesn't work
because you need to have a different
time from different parts of your patch
and if you point it straight down they
all come back at the same time
so we know the distance to a feature on
the landscape
but the radar beam is a finite width i
mean remember to focus
radio waves you need a large antenna and
that applies to whether you're receiving
or to broadcasting
so you can draw an imaginary equidistant
line on
this flat surface where the object could
be
inside that radio beam we need to
approximate the
shape of the surface of course you know
for aircraft you assume it's flat
for satellites you have to at least
account for the curvature of the planet
so as the vessel is carrying the radar
across the landscape
the distance to each radar return will
change it'll start
further away and it'll decrease towards
a minima when that's closest
and then it'll start increasing before
it moves outside of the radial beam
so this curve is called the range
migration curve
and since we know the motion of the
antenna and the shape of the surface
we can figure out exactly where the
reflection should be
on that surface and of course this can
work for multiple radar reflections just
fine and this is an example i made i
have a circle of reflectors
and when the antenna moves by the
reflected pulse timings
change showing all these different nice
curves
and for each reflection we can then
build up the probability of the object
in the area we're covering you know and
draw this out as a sort of probability
map
and when we add all these together we
get bright spots corresponding to the
objects and this is a really simple
piece of code this isn't even a proper
sar implementation now there's a second
bit of information you can use to
improve on this
because the antenna is moving across the
surface
the radar echoes will be shifted by the
doppler effect
so as an object is ahead of this
aircraft
the reflection will be higher
frequencies and as it is behind
the frequency will be lower so by
looking at the frequency of the response
you can figure out the uh the distance
ahead or behind of the vehicle
and localize your your get make your
image a whole lot better
and so you can take these radar signals
and turn them
into these fantastic images now this
involves a lot of
processing power these days and you
might wonder how they did this in the
past
before we had computers that were able
to do this well
some smart people actually figured out
that this whole
set of processing actually boils down to
a bunch of fourier transforms
and they could do this in an optical
analog
computer they would record the radio
signals
onto photographic film and they would
use these in a facility where they would
shine lasers through the film
uh through set of diffraction gratings
and
lenses optical elements and out the
other end
all the interference patterns would boil
down to the image that you actually
wanted
now of course these days we do all that
in computer because it means you can
have a lot more control about the
process
and tweak things without having to
reshape all your lenses
now there are also going to be artifacts
which are peculiar to the
sar processing for example we start by
assuming that the surface is flat and
things which rise up vertically among
this
that means they will be projected onto
that flat surface so that means
the tops of tall objects can appear
closer
than they should be this causes
mountains to lean towards the viewer
this is an effect called foreshortening
and if the slope on the near side of the
mountain is steeper than the
angle of the illumination the top of the
mountain will actually appear closer
than the base
so this detail on the slope upwards will
actually be reversed
and this is a problem called layover you
also can get large
objects shadowing things behind them so
you'll have black areas where you know
there's nothing or you don't know what's
there
but there are ways to correct for all of
this especially if you have
data from multiple angles or if you say
use a spotlight mode where you
track the thing for much longer time
over multiple
angles okay so at this point i've just
been talking about things reflecting
radar as simple reflectors but
the way in which things reflect is
really important and it tells us
a lot about the surface so the radar
reflection depends on something called
the dielectric constant which is an
electrical property of a substance
the higher it is the better it will
reflect radio waves
things with a low a dielectric constant
will allow the radar to
penetrate down which can be useful if
you're interested in looking at things
below the surface
the geometry of the surface is also
critical if a surface is
perfectly flat it reflects like a mirror
and unless that surface is
oriented to reflect back at the radar
the reflection will be sent off in some
other direction and not seen
a good example of this is the surface of
calm water
water has a pretty high dielectric
constant it's really good at reflecting
the radio waves
and it forms flat surfaces that means
that the radar pulse
is going to be reflected away from the
antenna so lakes will appear black on
sar images this is of course a this
reflection is also a basic principle in
the design of stealth aircraft where
they reflect the
radar away from the receiver
but natural terrain usually has a rough
surface on it and the little bumps on
the surface
scatter the radio pulses in all
directions producing an isotropic
scattering
but the scale of those bumps in relation
to the wavelength of the radar
is important if they're much larger than
the radio waves then they will reflect
more like
curved mirrors if they're much smaller
then the radio waves won't scatter from
them
so sar images at different wavelengths
can show different brightnesses and
different properties depending upon the
type of surface they're hitting
finally the way radio waves are
reflected can depend
upon the polarization of the radio waves
and it can modify the polarization of
the returned radio waves as well
so the polarization is a really useful
thing in sar systems
the polarization is measured relative to
the surface it can either be horizontal
or it can be vertical and modern sar
systems
will generate different pulses and they
can receive
both types of reflections so like
rough surfaces generally reflect
vertically polarized radio waves more
strongly
urban environments with a lot of flat
surfaces mean you can get
horizontal waves reflected really well
and if you have say
thick vegetation like forests canopies
you can see
multiple reflections bouncing around and
randomizing the polarization
so you will see a change in the
polarization
this is a really useful tool for
analyzing the surface
it's quite common to see false color
images in sar that use
one channel for horizontal one for
vertical and one
for where the polarization was changed
and it's really good
analog for rgb color so
using all of this together now there is
a wealth of
information to be gleaned from radar
images for example we saw
under the clouds of venus to finally
reveal the terrain
cassini had a radar and it used it to
identify hydrocarbon lakes on the
surface of titan
on earth we've seen archaeological sites
that were buried under sand they've been
found
now thanks to subsurface reflections
we can watch the land around volcanoes
as it rises and falls by millimeters
as lava moves around under the ground
by observing farmland over the season
you can measure
the changes due to growing crops and
figure out crop yields
and hey you can look at tank farms
storing crude oil and
analyze them to figure out exactly how
much is being stored and now
with that you can see why there's a
number of commercial companies
interested in generating this kind of
data for customers
so this radar technology lets you get
high resolution images using a
small satellite it works when the target
is in darkness
it doesn't get stopped by clouds indeed
there are cases when you can use it to
see through other
optically opaque things like dry snow
sand or safe through the side of tents
that are designed to hide vehicles
it can see through thin walls but
generally you can't
see more inside most buildings it's very
that's something i want to be clear most
buildings are not
penetrated enough by radar to get a
useful in image
that's despite what some people on the
internet say
sar does have limitations its resolution
is
ultimately going to be limited by the
fact that it uses big
fat radio wave photons rather than
finely detailed
optical photons it assumes that the
motion of the
vehicle is known and nothing else is
moving so if there's moving things in
the scene
then they don't get captured correctly i
remember seeing
some like conspiracy theorists saying
that sar
was being used to track somebody as they
drove around
because of the strong reflection from a
ring they were wearing
and this is ridiculous
because it's an object smaller than the
radio waves being used and it would be
inside a vehicle which is a
radar opaque thing and it would be
moving
all of these things are what sar fails
at
but you know of course there are
military uses for sure
especially these days when you can take
a a
landscape image and then run machine
learning over it
to have it highlight and classify all of
the part
vehicles aircraft and ship for you but
you know on a civilian viewpoint if you
are a curious observer
there are already huge amounts of data
from publicly funded science missions
available
for free if you're interested i suggest
looking at the alaska satellite facility
which archives all of this stuff
has everything you need to begin looking
at the world
in a different light i'm scott manley
fly safe
[Music]
you
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