The daily observing and tracking of the smallest changes over time opens a new view into the processes and activities that are happening on our planet. Synthetic-aperture radar (SAR) technology has the unique capability of visualizing even changes that are smaller than the size of a pixel. ICEYE’s SAR satellite constellation enables persistent monitoring and very accurate change detection analysis by taking daily images with precisely the same imaging geometry from space. This was addressed in our last technical blog. The goal of this blog is to dig deeper into a type of change detection called ‘Coherent Change Detection’ (CCD) and to explain how ICEYE’s one-day repeating ground tracks offer a novel capability in the field of earth observation. First, we need to do a bit of preparation work to make sure we are all at the same level of understanding. Let’s get started. As we know, radar pulses are made from electromagnetic radiation. Each ICEYE radar pulse is centered at 9.6 GHz. The satellite antenna transmits a pulse for a short period of time and measures the amount of time it takes to reflect from an object and return back to the radar sensor. Because we know the speed that the pulse travels – the ‘speed of light’ – and the time the journey took, we can calculate the range to the object that it was reflected from. Objects further away take longer for the pulse to travel to it, reflect from it and arrive back at the receiver. Here, the rising and falling electromagnetic wave of the returning pulse is measured. Figure 1 shows the example of the rise and fall of a wave that doesn't complete a whole number of wavelength cycles and finishes one third of the way through the last wavelength. We call this fraction of a wavelength, the phase, and we store the information as an additional number in the pixel of the image. Figure 1. Phase of a transmitted electromagnetic wave Because the phase repeats every wavelength, it's easy to think of it as an arrow rotating around in a circle with the length of the arrow representing the brightness or amplitude of the pixel and the angle between the arrow and the horizontal x-axis as the sample’s phase. We call this arrow a ‘phasor’, as it represents not just the amplitude of the measurement but also the phase (Figure 2). Figure 2. Each sample has an amplitude value A and a measure of the fraction of a wavelength φ (phase). By convention, the term for a pixel or sample that contains both amplitude and phase information is called a ‘complex’ sample. Images that are made up of many complex samples are called ‘complex images’. So now we have an intuitive understanding of how a SAR sensor measures the amplitude and phase of a single pixel. The total brightness of a pixel can be dominated by one or a small number of scatters, or it can be made up from many million individual scatterers, like blades of grass, rocks or pieces of dirt, all adding together. Each pixel therefore has its own amplitude and phase, which are characterized by the complex scattering of the many miniscule objects within each pixel. We usually look at the amplitude of each pixel, as this forms an image of the radar reflectivity of the ground. This is what we usually think of as a SAR image. But it's equally valid to look at only the phase information in each pixel. Figure 3. The two aspects of a complex SAR image. The more common view shows the amplitude of each pixel (left), but the phase of each pixel – ranging between 0 degrees (black) and 360 degrees (white) – can also be displayed (right). If we were to take two complex images of the same location, we can carefully compare the phase inside each pixel and see how much the phase has changed. We call this comparison a measure of correlation and we can produce a change detection image from the comparison called ‘Coherent Change Detection’. In these images, by convention, white means that the phase has not changed from one image to the next. Black (or shades of gray) means that the phase has changed to some degree, which, if the imaging geometry is the same, suggests that something inside that pixel is different today than it was yesterday. Figure 4 shows two coherence images over the Mosul Dam in Iraq created from three SAR images acquired on consecutive days. In this example, a person or vehicle moved equipment between the outer compound and the storage area along the wall between the first and second day (left image). Between the second and third day this same area was unused (right image). Figure 4. Change detection techniques applied to three images acquired over the Mosul Dam in Iraq. The total amplitude and phase of a pixel is completely dependent on the direction from which the satellite is looking at the pixel. If the direction is different, the range to each of the smaller objects within the pixel would be slightly different. That would make it impossible to detect small changes happening on the Earth’s surface. This is where the power of the Ground Track Repeat (GTR) comes in. Because each image is taken from precisely the same direction, all of the individual scattering elements within the pixel have exactly the same phase from one day to the next – as long as nothing has changed. These individual objects could be as small as blades of grass or rocks and pebbles. But things do change. The world is constantly changing, whether it’s as a result of weather, human activity or plate tectonics. The key to understanding and better responding to these changes is the time over which we’re able to track them. Through experience we have discovered that images taken four or five days apart often have many natural weather-based changes. But observing the ground every day, or eventually every few hours or minutes, reduces the chance of weather-driven change causing the phase of a pixel to vary. So if changes caused by weather and environmental effects are reduced, what does this leave? Human activity and dynamic events. And these are exactly the sort of things that we care about. Coherent Change Detection is incredibly sensitive but it's not without its drawbacks. One issue is that changes are dark or black, but so are shadows and areas of ‘SAR layover’. This is a real problem in traditional CCD analysis, as the analyst has to decide which changes are real and which are just unseen areas in the radar image. This is where the ‘power of the stack’ comes into play. No SAR satellite provider has ever been able to provide daily repeating CCD imagery before. This means that CCD exploitation has always been applied to a single CCD image, leaving analysts with an exploitation headache and reducing the overall utility of satellite CCD products. When we look at the same place, at precisely the same time every day and from the same direction (and with the same radar imaging mode and satellite configuration), we are able to remove a lot of the CCD uncertainty. Areas of shadow or layover are geometry-related, which means that the dark areas in the image always occur in the same location on every image – they are persistent in all images. But changes made by people on the ground are transient and occur intermittently throughout the stack (Figure 5). Figure 5. Excavation activity related to stemming at the Mosul Dam on 1st July 2021 is transient and shows up as a temporary change within the data stack. This allows a kind of ‘change detection of Coherent Change Detection’ technique to be applied and provides a really useful ‘persistent monitoring’ capability that allows us to reliably identify even the smallest changes. There is something really neat about this type of change detection. It's very different from the type of change detection used with optical systems or the type discussed in the first blog post. In those examples, changes were detected because something appeared different in one of the two original images. You could look at the image yourself and see the change. But CCD is different. The level of changes we detect here are those that change the phase information within a pixel. Think about that for a second. The phase is a fraction of a wavelength. And our wavelength is 3 cm. So if you have a satellite in a one-day repeating ground track that can isolate natural weather-based change, you can detect changes in the order of a fraction of 3 cm. From 800 km away in space. In any weather condition, day or night. Now if that's not seeing the invisible, I don’t know what is. Figure 6. ‘Change detection of CCD’ technique applied to images over the Mosul Dam in Iraq. In this post, we discussed the technology and value of daily coherent change detection for persistent monitoring enabled by ICEYE’s GTR data stacks. This capability can now be explored with a dataset consisting of three coherent Ground Track Repeat images acquired on consecutive days. Download the world’s first Daily Coherent Ground Track Repeat dataset: Download the The daily detection of millimeter-level changes on Earth from space – some even invisible to the eye – might sound impossible. But ICEYE's Daily Coherent Ground Track Repeat capability does just that.
Behind the Technology
Changes on Earth that Only Radar Can Reveal
Daily Persistent Coherent Change Detection - a World’s First
Monitoring Millimetric Changes from Space
Explore ICEYE Daily Coherent GTR Images
FAQs
Who can see the invisible can do the impossible? ›
"Only He Who Can See The Invisible Can Do The Impossible" Frank Gaines.
Do the impossible see the invisible quote? ›quote : "Only those who can see the invisible can do the impossible" Albert Einstein | Powerful quotes, Albert einstein, Einstein.
What does only those who see the invisible can do the impossible mean? ›When you are on a mission to achieve something that appears impossible or are looking to turn an impossible task into a reality. Then finding ways or methods is one of the toughest and most important aspects of your journey. You need to find your own ways and methods to succeed in your journey.
What does it mean to see the invisible? ›Seeing the invisible means living and walking with absolute trust in God and faith in His promises for all our needs. This is an attribute common to all the believers of old who experienced divine interventions in their times of need or became instruments of divine change.
What is the power of the invisible? ›Grand Master Lu is often heard saying, “The invisible is more powerful than the visible”. In other words, what you see with your two eyes isn't the whole truth. When we strain to see what is happening and focus only on the visible, we miss what isn't seen.
Who is the visible presence of the invisible God? ›Christ is the visible image of the invisible God. He existed before anything was created and is supreme over all creation, for through him God created everything in the heavenly realms and on earth.
What is the most important quote in Invisible Man? ›“I am a man of substance, of flesh and bone, fiber and liquids—and I might even be said to possess a mind. I am invisible, understand, simply because people refuse to see me.
What is a famous quote about impossible? ›- Nothing is impossible; there are ways that lead to everything, and if we had sufficient will we should always have sufficient means. ...
- Never tell a young person that something can not be done. ...
- Only he who can see the invisible can do the impossible.
Years ago, the television show Mission Impossible always began with a scene in which the team leader, Mr. Phelps, would receive a tape describing his next mission. The tape invariably began, "Your mission, should you choose to accept it..."...
Where in the Bible does it say nothing is impossible to those who believe? ›Luke 1:37: “For nothing will be impossible with God.”
As the angel told Mary that God intended her to bear His Son, the angel proclaimed, “Nothing will be impossible with God.” Lord, You are never hindered by anything.
What does it mean that God is invisible? ›
God is invisible. And not just that; you cannot bump into him or touch him, for he is intangible, neither does he make any sound. And he doesn't smell, either. In fact, he is absolutely undetectable by any of our senses, and cannot be found even by our most sophisticated scientific instruments. …
What is the deeper meaning of the Invisible Man? ›The invisibility of Ellison's protagonist is about the invisibility of identity—above all, what it means to be a black man—and its various masks, confronting both personal experience and the force of social illusions.
What does visible and invisible mean in the Bible? ›The visible church is the expression of Christianity that people can see: the gathering and practices of the individuals in various church buildings on Sundays. The invisible church is the true church, which only God can see: born-again believers, past, present, and future.
How do you deal with being invisible? ›- Step 1: Stop hiding. If you felt invisible growing up, later in life you may have unknowingly agreed to hide. ...
- Step 2: Heal your inner child. The next thing you can do is start a reflection practice with yourself. ...
- Step 3: Connect with your physical self. ...
- Step 4: Re-train your brain.
: an invisible person or thing : someone or something that cannot be seen or perceived.
What is the invisible man's weakness? ›Weakness. Power Limitation: Although he was invisible, he was unable to become visible again.
Why the Invisible Woman is so powerful? ›The Invisible Woman received her powers after cosmic radiation had triggered mutagenic changes in her body. Originally only able to turn herself invisible, Sue later discovered she could render other things invisible as well and project an invisible force field.
Why is invisibility a good superpower? ›Invisibility is special because it provides a level of not only anonymity, but secrecy. You can slink around without people knowing. You can sneak up on others or spy with the ultimate in inconspicuousness. A very cool, if not icky, superpower.
What God sees in secret? ›But when you pray, go into your room, close the door and pray to your Father, who is unseen. Then your Father, who sees what is done in secret, will reward you. And when you pray, do not keep on babbling like pagans, for they think they will be heard because of their many words.
Who is the God that sees everything? ›Later in the story, Hagar, out of gratitude, gives God a name: El Roi, the God Who Sees (Genesis 16:13).
What does image of the invisible God mean? ›
Colossians 1:15 tells us that Jesus is the image of God. We are able to recognize what is unseen — the invisible God — by what was seen: Jesus. Hebrews 1:3 tells us that. He is the radiance of the glory of God and the exact imprint of his nature, and he upholds the universe by the word of his power.
What is Ralph Waldo Emerson's most famous quote? ›“To be yourself in a world that is constantly trying to make you something else is the greatest accomplishment.”
What is the first line of The Invisible Man? ›“I am an invisible man.”
This is the opening line of Ralph Ellison's hit novel, Invisible Man.
Probably the most important motif in Invisible Man is that of blindness, which recurs throughout the novel and generally represents how people willfully avoid seeing and confronting the truth.
What is the most famous line of all time? ›| Rank | Quotation | Year |
|---|---|---|
| 1 | "Frankly, my dear, I don't give a damn." | 1939 |
| 2 | "I'm gonna make him an offer he can't refuse." | 1972 |
| 3 | "You don't understand! I coulda had class. I coulda been a contender. I could've been somebody, instead of a bum, which is what I am." | 1954 |
| 4 | "Toto, I've a feeling we're not in Kansas anymore." | 1939 |
- "I have a dream." ...
- "The greatest glory in living lies not in never falling, but in rising every time we fall." - ...
- "The way to get started is to quit talking and begin doing." - ...
- "So we beat on, boats against the current, borne back ceaselessly into the past." -
“Insanity is doing the same thing over and over and expecting different results.” That witticism—I'll call it “Einstein Insanity”—is usually attributed to Albert Einstein.
What is the quote about believing the impossible? ›Quotes The only way to achieve the impossible is to believe it is possible.
Who said the famous quote nothing is impossible? ›– Audrey Hepburn”
What are some inspirational quotes from Mission: Impossible? ›“I am the storm.” “What's done is done when we say it's done!” “If I let you know where I'm going, I won't be on holiday.” Lindsey Farris: “How many rounds do you got?”
How visible is the invisible man? ›
After becoming invisible, he becomes truly visible only after he is dead. Throughout the narrative, once invisible, Griffin makes himself visible through the use of clothes, bandages and other accessories. He takes these off whenever he wishes to use his invisibility. However, he becomes fully visible only at the end.
What is the quote about invisibility in Invisible Man? ›I am invisible, understand, simply because people refuse to see me. Like the bodiless heads you see sometimes in circus sideshows, it is as though I have been surrounded by mirrors of hard, distorting glass.
Who is the invisible guest in the invisible guest? ›
Although viewers can't see the invisible man, the film plays off the key importance that they know he's there. There are several early scenes that signal the presence of an unknown stalker, presumably Adrian, such as the foggy breath scene that served as a highlight moment in the trailer.
Can we see the shadow of an invisible person? ›No. A shadow is cast because the direct rays of light from a source are blocked by an intervening object.
What is the main message of Invisible Man? ›The message of "Invisible Man" is that a person's self-identification should depend on himself and not on society's views of him. He is real even if he feels that no one sees him.
Why did The Invisible Man go crazy? ›His insanity is purely a side-effect of the invisibility drug and his motivation for the experiment was a misguided desire to do good for science and mankind, born primarily out of his love for his fiancée.
What is the first line of the Invisible Man? ›“I am an invisible man.”
This is the opening line of Ralph Ellison's hit novel, Invisible Man.
The ultimate irony is that the Invisible Man, obsessed with the blindness of others, is blinded. He refuses to see the truth even when others point it out to him.
Why does the narrator call himself invisible? ›Because the narrator is black, whites refuse to see him as an actual, three-dimensional person; hence, he portrays himself as invisible and describes them as blind.
What was the twist in The Invisible Guest? ›
Tomás confirms that the second person in the car was Adrián when he sees his cigarette lighter. He sees the same lighter in the car on the day of the accident. Tomás tosses the lighter into a decorative pool and notices the expression on Adrián's face. He confirms that Adrián must have dumped Daniel's car in some lake.
Is The Invisible Man a ghost? ›Instead, he's a ghost. The invisible man is nowhere and everywhere, a malevolent, unstoppable, permeating force. Turning AG from a man into a nightmare is, as a metaphor for domestic violence, the most realistic aspect of the movie.
Who kills The Invisible Man? ›Cecilia hides an invisibility suit in her bathroom and invites Adrian over for dinner and kills him by slitting his throat with a knife.
Can an invisible person see himself? ›Invisibility says nothing about only being invisible to other people, so unless you can see invisible creatures somehow, no, you can't see yourself.
How does The Invisible Man end? ›Invisible Man ends with an epilogue in which the narrator decides that his “hibernation” has lasted long enough, and that he will finally leave his underground cellar to rejoin society. Prior to reaching this conclusion, the narrator chronicles Harlem's spiral into a chaotic riot.