Synthetic aperture radar makes the Earth's surface viewable throughout the week

Synthetic aperture radar capabilities are just one example of how much progress is being made with all kinds of sophisticated sensors and data processing systems needed to interpret its output.

The first time humans observed a battlefield from a celestial point was in June 1794, nearly a decade after the Montgolfier brothers invented the hot-air balloon. The French Air Corps, a diverse crew of chemists and carpenters, launched a hydrogen balloon over the battlefield at Florus, in what is today Belgium, to report the movements of their Austrian enemies via the semaphores to their colleagues below.

The British magazine "The Economist" said in a report that the success of the hot air balloon in itself was not pioneering, and its importance was short-term. When war broke out again in the Low Countries in 1914, the airship had already begun to give way to the aircraft, and was no longer in use by the end of the war. By the time NATO moved its military headquarters to Mons, 40 kilometers west of Floros, in the 1960s, satellites had broken into the field.

Sensor Challenges

But even as the flight method changed, the sensors remained the same, with two obvious problems: darkness at night and clouds; Half the world is in a state of darkness at any time of the day, while in most areas the sky is sometimes or almost always overcast.

More than half of Europe is covered by clouds at any time of the year. And in parts of South America, cloud-free images only appear every decade or so, according to Adam Maher of Ursa Space, a startup based in Ithaca, New York that uses satellite imagery for business intelligence.

Planet, which aims to capture one-meter-resolution images of the entire Earth's surface every day, says that about 70% of the Earth's surface is overcast at any given time.

But in the past few years, an alternative to visible wavelengths has made huge strides. Orbital radars illuminate the surface using wavelengths hundreds of thousands of times longer than visible light. These wavelengths easily penetrate clouds, fog and smog.

Among the advantages of long wavelengths; ability to penetrate. The accuracy of the sensor depends on the wavelength and on the size of the aperture, the mirror or lens in the case of a camera or telescope and the antenna in the radar. And if you extend the wavelength, you increase the size of the aperture you need to reach a certain resolution.

Synthetic aperture radar

Synthetic-aperture radar offers a way around this problem. The satellites move very quickly, usually in low orbit, about 25,000 kilometers per hour. By taking all radar shots of a specific target as it passes over it and processing them into a single image, synthetic aperture radar gives an accurate result.

This technology has been available since the 1960s, and spy satellites have been in use since the 1980s, but it was limited, expensive, and highly classified. And it wasn't until the late 2000s, when both India and Israel had their own military satellites, that the Pentagon's right-hand man, the US National Reconnaissance Office, declassified the existence of such satellites.

Civilian space agencies in America, Canada and Europe have used this technology for various environmental missions, but not as a routine way to track human activities. Advances in electronics, spacecraft engineering and orbital computing mean that synthetic aperture radar systems with accuracy of a few meters or less can now be placed on small satellites at an affordable cost.

In 2018, two start-ups - Capella Space of California and ICEYE of Finland - launched commercial satellites carrying synthetic aperture radar. Realizing that trying to stop their use would be futile because other countries had similar capabilities anyway, the Pentagon backed them in order to bolster his influence.

The kings of the industry

Jack O'Connor, who has been retired from the National Geospatial-Intelligence Agency since 2013, says synthetic aperture radar coverage was insufficient to provide a normal analysis.

The magazine noted that the ability to see the ground even when the weather is bad - whether it is day or night - allows tracking of moving objects such as a military buildup on the border of Ukraine. The images may not be clear enough to determine the type of armor, but they are good enough to count the number.

Changes at a specific location can also be analyzed with remarkable accuracy thanks to radar systems that collect data from the phase of the waves they use in ways that exceed the capabilities of photovoltaic systems that use ambient light. Correlative change sensing can even show momentary differences.

When the United States discovered that it was losing soldiers in Iraq and Afghanistan to roadside bombs, scientists at Sandia National Laboratory - one of the facilities responsible for American nuclear weapons - developed the Copperhead system, a drone equipped with a synthetic aperture radar to detect changes in The soil, where the "rebels" (according to the magazine's description) were planting explosive devices or detonating wires. Similar technologies allow satellites to detect slight subsidence of the Earth's surface when building tunnels for nuclear testing.

Spotting differential elements as they occur is useful when fighting a war, where it is necessary to quickly assess whether the bombs hit the right targets and what damage they did.

India used its burgeoning fleet of synthetic aperture radar (SAR) satellites for this purpose after an airstrike on Pakistan in February 2019.

The US Air Force did the same in tests conducted in December 2020, explicitly stating that the motive behind their use It was the weather conditions in Europe and the Pacific Ocean.

Population at all times and places

The magazine emphasized that the ability to see a site on a regular basis - regardless of weather conditions - allows the armed forces to create archives that will be useful in the future.

One example of this is another system developed by the Pentagon to counter improvised explosive devices called the Gorgon Star System, which allowed drones to capture almost continuously video clips of entire cities over weeks and months. The idea was to find out when the IEDs were planted and track members of the responsible group by re-viewing the videos recorded after an attack occurred.

The magazine quoted Payam Benazadeh, founder of Capella, that the six satellites that his company has now in orbit can check any spot on the planet every 6 hours. As the company's constellation of satellites grows, it plans to reduce the re-visit time to a maximum of 15 minutes, which is less time than it takes for a country to launch a ballistic missile.

He also pointed out that the satellites cannot operate 24/7 because the antennas are powered by solar panels, and their batteries are not large enough to run continuously at night.

The magazine stated that the time it takes to transfer information from space to those who need it is in turn diminishing with the increase in the number of data companies such as "Amazon Web Services" and the number of ground stations. In 5 years, it will be possible to order and receive a satellite image in minutes, says Jo Morrison of Umbra, a California-based company.

Umbra hopes to fuel the growth of an industry that makes it possible to sell analysis based on synthetic aperture radar data to civilian and government customers by making that data cheaper. It plans to sell the images, covering 16 square kilometers, with a resolution of one meter, for $500, under a license that allows buyers to do whatever they like with the product.

The huge amounts of satellite imagery at relatively low prices provide an ideal input to the machine learning algorithms that have underpinned recent advances in artificial intelligence.

The magazine noted that synthetic aperture radar capabilities are just one example of how much progress is being made with all kinds of sophisticated sensors and data processing systems needed to interpret its output.