Ayush BansalAyush is a SAR Data Engineer at PierSight. He holds a B.Tech and M.Tech degrees in Electrical Engineering, Instrumentation, and Signal Processing from IIT-Kharagpur. He has been building solutions in the Aerospace-Defence and Radar Imaging domains for several years.
Shreya BoseShreya is a seasoned tech writer with deep experience in writing for the B2B ecosystem. She’s now set her sights on conquering space tech.
Table of Contents
- What is a Synthetic Aperture Radar (SAR)?
- How does SAR work?
- Understanding Aperture
- Understanding Synthetic
- SAR Image Interpretation
- Contrast in SAR Images
- Advantages of SAR technology for surveillance and imaging
- PierSight’s Constellation: A Step-Function Change in Maritime Surveillance
- Honey, I Shrunk the SAR Satellites
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It’s 2024, and we use satellites for everything from broadcasting, communication, navigation, astronomical observation, monitoring, disaster management, and internet access.
As of 2024, there are over 10,000 active satellites in orbit, two-thirds of which belong to Starlink. Most EO satellites use radio waves, infrared rays, and visible light for imaging. They are used mainly in the operational domains of weather, defense, and disaster prevention/management.
It’s high time that we add maritime monitoring to this list.
After all, it covers 70% of our planet's surface. More importantly:
- upward of 80% of global trade is carried by sea
- over a billion people consume fish as a source of protein
- ocean-based industries are projected to employ 40 million people by 2030
Satellites currently in operation are capable of maritime monitoring but cannot provide persistent monitoring. This is because many EO satellites rely upon reflected sunlight for imaging. Clouds, darkness, smog, and pollution obscure sunlight. Considering that the average cloud cover over land is 55% and 70% over the oceans, it’s quite clear that we’d need some sort of cloud-penetrating tech for persistent monitoring.
Say hello to Synthetic Aperture Radars (SAR), an active sensor that can ‘see’ through cloud cover and image at night. No prizes for guessing, the satellites we’re building here at PierSight are all equipped with SAR (and AIS).
This article will introduce Synthetic Aperture Radar and how it works, discuss its place in surveillance tech stacks, and outline its advantages over other satellite-mounted sensors.
What is a Synthetic Aperture Radar (SAR)?
What do you think of when you hear “satellite image”? Most people think of an optical image—a photograph taken by an incredibly powerful camera. However, not every satellite image is an optical image.
Synthetic Aperture Radar (SAR) uses radio waves, specifically microwaves, instead of visible light (which our eyes use to see things). SAR antennas transmit radio waves and capture them again once they have been reflected from the targeted imaging area. These reflected beams are referred to as backscatter.
SAR images and optical images are quite different. While optical imagery captures what we see (like a photo), SAR captures surface properties like texture and moisture, which can reveal hidden details. Think of SAR as a "texture map" that shows you the surface and tells you something about the “feel” - whether it’s smooth water, a dense forest, or rugged mountains.
SAR’s use of radio waves instead of visible light enables 24/7 functionality in all weather conditions, making it ideal for surveillance and environmental monitoring.
How does SAR work?
Imagine calling out in a canyon and hearing your voice echo back to you - SAR works similarly by sending a “ping” and listening for the “echo” that returns, which provides detailed information about the surface it hit.
The antenna transmits microwaves with wavelengths ranging from a few centimeters to a few meters that reflect off the Earth's surface and captured by receiving antennas on the same platform.
The captured microwave data is examined and rendered via digital signal processing, which generates a focused image from the large amount of acquired data.
Think of it this way: if you were trying to take a wide group photo with a small camera, you might take photos from different positions and stitch them together to get the whole picture.
SAR Platforms
Apart from satellites, Synthetic Aperture Radars can be mounted on aerial vehicles (drones and aircraft) as well as on ground-based systems. For the purpose of simplicity, this article will focus on satellite-based SAR.
Let’s break down the terms for a closer understanding.
Understanding Aperture
If you’re into photography, you’ve probably heard the word “aperture” thrown around liberally. The aperture of a lens, is the opening through which light passes into the camera. Replace ‘light’ with ‘radio waves’ and ‘camera’ with a ‘receiver’ and, voila, you have the aperture of a radar.
In SAR, aperture refers to the distance the satellite covers while emitting microwaves from the antenna. It is the distance between the satellite's orbital movement from one point to another, within it, it can emit and capture radar data. In the diagram above, notice the path of the satellite from point 1 to 2. That movement creates the aperture.
The antenna sends out thousands of pulses per second while orbiting. The larger the aperture (distance covered by the satellite), the better it can detect distinct, closely placed objects on the Earth’s surface (terrain or marine). However, the larger the antenna, the better the image resolution.
Understanding Synthetic
To quote EarthData by NASA, “From a satellite in space operating at a wavelength of about 5 cm (C-band radar), in order to get a spatial resolution of 10 m, you would need a radar antenna about 4,250 m long. (That's over 47 football fields!)”
Obviously, no one can mount such a massive antenna on a satellite. But since human innovation is infinite, scientists found a way. They designed a “synthetic” aperture.
As the satellite moves in space from one point to another, the system repeatedly transmits and receives microwaves (several thousands of times a second), which creates a larger virtual (or synthetic) aperture
By simply covering a larger physical distance with the entire satellite, the system is able to direct microwaves at a larger surface area and absorb reflected data—all without having to carry an impossibly long antenna into space.
An excellent way to understand this concept is to go through this video.
SAR Image Interpretation
Once the reflected waves are captured, they must be processed to create a coherent image.
SAR imagery doesn't just capture appearance but also phase and amplitude/intensity of backscattered signals. This information depends on wavelength, polarization, viewing geometry, and transmitted power. It also depends on the interaction between the microwaves and the surface, influenced by shape, terrain, humidity, roughness, etc.
There’s not much need to dive any deeper, unless you are a scientist or engineer who wants to get into the brass tacks of how SAR images are generated and why they differ from optical photography. If you are, here’s an article that will answer your curiosity: Learn Synthetic Aperture Radar (SAR) by Example.
Contrast in SAR Images
In SAR images, smooth surfaces like water or roads tend to scatter radio waves away from the sensor, creating dark areas in the image. In contrast, rough surfaces like mountainous terrain or metallic objects reflect more waves back, resulting in brighter spots. This contrast allows SAR to differentiate between different types of surfaces quickly and effectively.
In the below labelled images, it’s easy to spot a ship at sea and the subsequent detection of an oil spill.
Advantages of SAR technology for surveillance and imaging
There are multiple advantages that come with deploying SAR imaging over it’s current counterparts:
- All-Weather Imaging: Microwaves can penetrate clouds, dust, and other forms of atmospheric obstacles. This allows for round-the-clock surveillance, irrespective of weather conditions.
- Imaging at Night: Because it transmits its own radio waves and receives the backscatter, the absence of such light does not hinder the operation of SAR sensors.
- Coherency Information: Microwaves are sensitive to roughness, moisture, and other features of the target object. Backscatter contains coherency information - the phase and frequency of the received signal. This is used acquire richer information of the reflecting surfaces.
~ In this context, roughness indicates the variations in height that occur when imaging natural terrain or a cityscape. This is recorded by microwaves, as roughness determines how strong the reflected signal will be.
~ Smooth surfaces (water, roads, relatively plain ground) actually scatter incoming microwaves away from the sensors, resulting in low-strength reflected signals. In SAR images, such patches will appear black.
~ Rough surfaces actually reflect more microwaves back to the sensors, creating brighter pixels in images.
- SAR sensors can be used to create high-res 2D and 3D images, useful for reconstructing objects such as a landscape.
- SAR works particularly well for mapping land cover and land use in areas under some kind of perpetual cloud of vegetation cover, such as rainforests and places that receive heavy rainfall round the year.
- Enables detections of surface displacements, which is useful for industries like mining, oil and gas extraction, construction, excavation, etc.
- SAR assists disaster management with its ability to notice and track land disturbances. It can also be used to detect troop, vehicle, or artillery movements, which is ideal for defense and security operations.
PierSight’s Constellation: A Step-Function Change in Maritime Surveillance
Current Earth Observation solutions lack real-time situational awareness for our oceans. To address this gap, PierSight is building a constellation of miniaturized satellites, combining SAR and AIS technology for persistent maritime monitoring. This will be the world’s first constellation dedicated to monitoring the oceans.
In the context of maritime surveillance, SAR satellites provide a powerful tool for authorities and organizations. They can monitor vessel traffic, identify 'dark ships' that are not broadcasting AIS signals, and detect illegal activities such as unauthorized fishing, or environmental disasters such as oil spills.
Honey, I Shrunk the SAR Satellites
A typical SAR satellite is about the size of a school bus. But we can’t really launch a constellation of school bus sized satellites, can we? And so, our satellites will be backpack-sized (10x smaller) and 6x more cost effective.
In order to achieve persistent monitoring, we’ll need a constellation of satellites and it only makes sense to bring down the cost. Of course, there are trade-offs but by dedicatedly focusing on maritime, we’ve optimized to ensure a best-in-class for maritime monitoring.
To help you understand the size difference, take the upcoming NASA ISRO Synthetic Aperture Radar (NISAR) satellite. It carries a large 12-meter-long stationary antenna reflector mounted on a 9-meter boom. When transmitting microwaves, the reflector focuses and bounces radar signals by illuminating a 242 km swath.
Without a doubt, NISAR is significantly more “powerful” than our satellites. However, its goals and objectives are different from ours.
When designing space missions, less is more. By focusing on the key objective - persistent monitoring - our satellites (as a constellation) will be superior to any other satellite(s) for the specific target use case - persistent maritime monitoring.
Just as you can track your Uber ride with high accuracy in real time, our goal is to do this for ALL vessels at sea. We’re building “eyes and ears” for the ocean to track all human and industrial footprint at sea.
Written by
Ayush Bansal
Ayush is a SAR Data Engineer at PierSight. He holds a B.Tech and M.Tech degrees in Electrical Engineering, Instrumentation, and Signal Processing from IIT-Kharagpur. He has been building solutions in the Aerospace-Defence and Radar Imaging domains for several years.
Written by
Shreya Bose
Shreya is a seasoned tech writer with deep experience in writing for the B2B ecosystem. She’s now set her sights on conquering space tech.
