Satellites Use 'This Weird Trick' To See More Than They Should - Synthetic Aperture Radar Explained.

Summary

Scott 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

00:00

🛰️ 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.

05:01

📡 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.

10:03

🌐 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.

15:03

🌳 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.

Keywords

💡Capella Space

Capella Space is a company that builds small satellites. In the context of the video, it is mentioned as the builder of a satellite that launched on a Rocket Lab Electron launch vehicle. This company is part of the new commercial ventures in the field of satellite technology, particularly focusing on radar imaging from space.

💡Synthetic Aperture Radar (SAR)

Synthetic Aperture Radar is a technology that uses radar signals to generate high-resolution images of the Earth's surface. The video explains that SAR works by moving an antenna over long distances and combining the radar signals into a single image with an effective aperture much larger than the physical antenna. This technology is central to the video's theme of satellite imaging and is exemplified by Capella Space's satellite.

💡Optical Imaging

Optical imaging refers to the process of capturing images using visible light, typically from the sun. The video mentions that companies like Planet Labs sell optical imaging from space as a service. However, optical imaging is limited by weather conditions such as cloud cover and daylight, unlike SAR which can operate in any lighting condition.

💡Radar Resolution

Radar resolution is the level of detail that can be discerned in radar images. The video script explains that modern SAR technology has been optimized to match the resolution of optical data from similar-sized spacecraft, despite the longer wavelengths of radar signals compared to light.

💡Planet Labs

Planet Labs is a company that provides optical imaging from space as a service. The video script mentions Planet Labs' Dove CubeSats, which are capable of capturing imagery with three-meter resolution for the entire planet on a daily basis. This company represents the existing commercial ventures in space imaging prior to the advent of more advanced SAR technology.

💡Dielectric Constant

The dielectric constant is an electrical property of a substance that affects how well it reflects radio waves. In the context of SAR, the video explains that the dielectric constant influences the reflection of radar signals, which in turn can reveal information about the surface being imaged.

💡Polarization

Polarization refers to the orientation of the radio wave's electric field as it is transmitted and received. The video script discusses how the polarization of SAR signals can be horizontal or vertical, and how different surfaces reflect these differently, providing valuable information about the surface characteristics.

💡Doppler Effect

The Doppler Effect in the context of SAR is the change in frequency of the radar signal based on the relative motion between the satellite and the object being imaged. The video explains that by analyzing this frequency shift, the position of objects in the image can be more accurately determined.

💡Radar Interferometry

Radar interferometry is a technique that uses the phase difference between two or more radar signals to extract elevation information. The video script mentions that while it's possible to achieve high-resolution imaging using interferometry between radio telescopes, it's not yet possible with optical images.

💡Layover and Shadowing

Layover and shadowing are artifacts that can occur in SAR imaging. Layover is when the top of a tall object appears closer than it should be due to the angle of illumination, while shadowing occurs when a large object blocks the radar signal, causing areas behind it to appear black. The video script uses these terms to explain some of the limitations and challenges in interpreting SAR images.

💡Commercial Satellite Constellations

Commercial satellite constellations refer to the networks of multiple satellites launched by companies for various purposes, such as Earth observation. The video script discusses the growing interest of commercial companies in launching constellations of satellites capable of generating Earth observation data for paying customers.

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]

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