Heather Hunter • February 24, 2017

Radar in Earth and Planetary Science: An

Intro

If you’ve ever been on a ship or been to an airport, you’ve likely seen a radar. Or, if you’ve

watched the weather report on your local news, you’ve probably heard about “Doppler radar.”

But if you haven’t heard about or seen any of these things, what exactly am I talking about?

OAR / ERL / National Severe Storms Laboratory (NSSL)

Example of one of NOAA’s Doppler radar

Radar stands for “[RA]dio [D]etection [A]nd [R]anging.” It’s a sensor that generates microwave

radiation (let’s call it a “signal”) and uses that signal to detect and locate objects. A radar can do

this because the signal it sends out (or “transmits”) bounces off material in its path in different

ways. How the signal bounces off the material depends on the composition of the material, from

what direction the signal is traveling, the frequency of the radar’s transmitted signal, and the size

and shape of the object.

What kinds of things are radar used to observe? The first applications of radar devices were the

upper atmosphere and lightning, and, of course, military-related (i.e.: to locate air, ground, and

sea targets). These days, a typical object for a radar might still be an aircraft or a ship, but also

natural objects, like precipitation (as with your weatherman’s Doppler radar), ice, aurora,

spacecraft, and celestial objects. Your radar might be on the ground, attached to the bottom of an

aircraft, or part of a payload on a satellite in space.

Regardless of whether your radar is on the ground or in space, when a radar signal reaches its

intended object, a couple of things happen. As with the optical light you see with your eyes, a

radar signal will either reflect off the object back to the radar, scatter in many different

directions, or bend around the object in a process called “diffraction”. Depending on the objects

being observed, the wavelength of the signal, and how the signal gets back to the radar, the radar

will discern different properties of the object, including location, size, roughness, and speed.

ESA

Scattering mechanisms for radar

In fact, one of the most unique features of a radar is its ability to determine the distance to its

target. It does this by measuring the time it takes for its transmitted signal to return back to the

radar, and that’s done by digitally applying a “time marker” to the signal before its transmitted.

But what if your target is moving? To determine how fast a non-stationary target is moving,

radar systems take advantage of the “Doppler effect”. You may have heard about this in

reference to sound waves, with the familiar example of an ambulance siren. The sound waves

originating at the siren have some kind of frequency (or pitch) and some wavelength (or spacing

between crests in the sound wave).

As the ambulance approaches you, the siren’s pitch seems higher. The actual frequency and

wavelength of the siren’s sound waves didn’t change, but, because the ambulance is moving, the

apparent frequency and wavelength changed.

Schematic of the Doppler Effect

In other words, the ambulance moved location during 1 wavelength (or 1 period, or 1 cycle) of

the siren’s sound wave, effectively shortening the wavelength of the sound wave, as it appears to

you. Similarly, as the ambulance moves away, the effective wavelength grows, making the pitch

of the siren lower. So, for a radar, a moving object will appear to reflect the transmitted signal at

a different frequency than it was originally sent. By analyzing the change in frequency, the radar

can then determine the velocity of the object.

Radars operate in a part of the electromagnetic spectrum called the “microwave band”. This

range of frequencies spans from 3 megahertz (MHz), or 3,000,000 hertz (Hz) to approximately

40 GHz (or 40,000,000,000 Hz). In the range of 3 MHz to 30 MHz, you have what are called

“HF Radar”. These low frequency radar are famously used on coasts and boats to track and

measure ocean waves and currents. For example, the National Oceanic and Atmospheric

Administration (NOAA) has many HF radar in several locations along the East and West coasts

of the United States.

Radar frequency bands

The higher frequency radar, from about 300 MHz to 40 GHz, are often used on aircraft or

satellites that look down at Earth. What would be the benefit of these radar over regular, optical

satellite sensors? As we’ve learned, radars generate their own radiation. Optical sensors, like the

one on GOES-16, for example, rely entirely on radiation from the sun to illuminate a given

scene. What this means is that optical sensors can’t see at night. On the other hand, since radars

use their own “light” source, they can observe the Earth without help from the sun!

Another pitfall of optical sensors is that they can’t see through obstructions in the atmosphere,

like clouds, smoke, or dust. This is because the wavelength of the optical light is either smaller or

the same size as the particles in the atmosphere, depending on which particles are in the way.

The particles either absorb, transmit, or simply reflect the incoming light depending on their size

and molecular structure. On the other hand, light in the microwave band of the electromagnetic

spectrum has a comparatively longer wavelength, meaning the microwave energy won’t be as

sensitive to the small particles in the atmosphere. This allows microwave radiation to go all the

way to the Earth’s surface, except during torrential rainfall.

Given what we’ve learned about how a radar operates versus a typical optical sensor, what kind

of information do you think you see when you look at a radar image?

ESA

Spaceborne radar

Example of a spaceborne radar image of ice in Tibet, from the ESA spacecraft Sentinel-1A.

Optical sensors capture solar energy that’s been reflected off an object on the Earth. At an optical

sensor, this information comes at many different wavelengths between violet and red. An

“optical image” gives you information on an object’s composition, temperature, and other

physical properties, including roughness or texture, to a degree.

An “image” from a radar gives you an image of what is called the “radar backscatter”, or the

amount of energy the radar records after a transmitted signal is bounced off a given object or

scene. It indicates how rough surfaces are, which is especially useful when looking over the

ocean.

Now that we’ve learned the basics of how radars work, how have they actually been used on

satellites, either to study the Earth or other planets? One type of radar, ubiquitous in space

exploration, is the Synthetic Aperture Radar (SAR). From ocean waves on Earth to the surfaces

of Venus and Saturn’s moon, Titan, SAR has given us unique views of our solar system that just

aren’t possible with regular optical sensors.

And SAR is what we’ll discuss next time!

NASA / JPL-Caltech / ASI

Titan by radar

The surface of Titan from the Radar Mapper on the Cassini spacecraft.

Source: https://www.planetary.org/blogs/guest-blogs/2017/0222-radar-in-earth-and-planetary-

science.html

Heather Hunter • May 12, 2017

Radar in Earth and Planetary Science, Part 2

In part one of our introduction to radar in Earth and planetary science, we briefly discussed some

basics. We learned that radar stands for “radio detection and ranging,” and that it is a sensor that

generates its own electromagnetic energy, usually in the microwave portion of the

electromagnetic spectrum. We also learned that radar is generally impervious to weather, so if

your target is behind clouds or rain, you’ll still be able to “see” your target.

But what if we can improve our radar to take interpretable images? What if, instead of only

seeing brief echoes of the radar energy reflecting off a given scene, we could create high

resolution images of the reflected energy?

NASA / JPL-Caltech Washington D.C.

Image of Washington, D.C. taken by the Spaceborne Imaging Radar-C/X-band SAR (SIR-C/X-

SAR) aboard the space shuttle Endeavor on April 18, 1994.

Let’s begin our discussion by imagining an aircraft carrying a radar. Images taken by radars on

aircraft or satellites are acquired in long “strips” as the platform moves along its flight path, or

track. Unlike optical sensors, a radar is typically pointed off to the side of the flight track, so that

the radar’s energy is transmitted obliquely. The “footprint” of the radar’s energy, emanating

from the within the radar’s antenna “beam” has dimensions in terms of a “range” direction and

an “azimuth” direction. The term “range” refers to the dimension crossing the flight track.

Alternatively, you can think of it as the dimension coming straight out from the antenna’s beam,

but projected onto the ground. The term “azimuth” refers to the dimension along the flight track,

projected onto the ground. These terms will be key to our subsequent discussion.

Heather Hunter Example of geometry for a side-looking radar on a spacecraft

In remote sensing, the basic measure of the spatial quality of an image is its “resolution.” The

term “spatial resolution” refers to the ability of our radar (or any sensor) to differentiate between

objects. It is a measure of finest detail our radar can see and is typically measured in meters.

There’s a measure of resolution for both dimensions of the radar image: “azimuth resolution”

and “range resolution,” which refer to how well the radar can resolve objects in either direction.

Synthetic aperture radar (SAR) exists because conventional radar suffers from poor azimuth

resolution—in other words, conventional radar is not very good at distinguishing objects in the

direction along the aircraft’s flight direction. The azimuth resolution of the radar is dependent on

the physical dimensions of the radar’s antenna—in fact, they have an inversely proportional

relationship. This means that, as the radar’s antenna size grows, the resolution improves, though

it also depends on other variables, like the radar’s transmitting wavelength and the distance to the

target or scene in question.

How do we make the azimuth resolution better? Based on the relationship I’ve just described, we

can guess a few ways this can be done: 1) increase the along-track antenna length, 2) increase the

antenna’s transmitting wavelength, or 3) get the aircraft closer to the target.

It’s clear that the last option is not viable, especially for a SAR on a spacecraft. The second

option is also not viable, because as we increase the wavelength of the radar’s transmitted signal,

the radar requires more power to generate that signal, and we’d like to keep the power

requirements for our radar as low as we can.

That just leaves us with the first option—and it’s the key behind SAR. But adding an antenna

that might be 25 to 100 meters long to an aircraft or spacecraft is generally impractical, so what

do we do? We use computers to simulate a long antenna using a conventional antenna, which

might be just a few meters long, instead.

How is a single SAR antenna able to emulate a very large antenna? The basic concept is as

follows: the SAR transmits a signal, receives the echo of its transmitted signal from a target—

while the SAR is moving—and maps that received signal to a specific position. In order to

effectively map the positions of whatever the received signals have bounced off of (the targets),

SAR exploits the motion of its platform to determine the direction of the signal, also known as

the signal’s “phase”. Without the relative motion between the SAR and the target or scene, the

SAR would not be able to locate the target(s) in question.

Heather Hunter SAR operation

Simple cartoon of basic SAR operation, depicting measurement of multiple return signals from

the ground.

Once the SAR has obtained multiple (usually thousands) of echo signals and mapped them to

specific positions, it combines all signals corresponding to each position. With some

sophisticated signal processing, the SAR concentrates the signals at each position, resulting in a

synthesized image with a much improved spatial resolution than that of its regular radar

counterpart. Better yet, this process generates an image using the adequately-sized, conventional

antenna, meaning that we didn’t require a huge antenna that might not even fit on an aircraft or

satellite.

NASA / JPL-Caltech Venus

SAR image from the Magellan mission of a crater on the surface of Venus.

It turns out that SAR is extremely useful in both Earth and planetary sciences, for more reasons

than just its ability to see through clouds, fog, and bad weather. Since it makes careful

calculations of the relative locations of objects in a scene, it can be used to determine how things

have changed over time. For example, SAR imagery can help us monitor glacier movement,

landmass movement after Earthquakes, movement of man-made objects, flood extent, and even

the motion of ocean waves and ocean currents (though the latter is a complex discussion best

saved for another day).

SAR is also used to make high-fidelity digital elevation maps (topography maps), it helps us see

and track oil spills, and it can even make measurements of soil moisture. Because SAR, unlike

optical images, gives us information on the intensity and direction of the signals its receives—

and the behavior of the signals transmitted by SAR depend largely on the structure and dielectric

properties of the SAR’s intended target (that is, how well it conducts electricity or interacts with

an electric field, like a SAR signal)—we can glean a wealth of information unseen to the human

eye, which gives SAR an edge over conventional sensors which rely on the visual part of the

electromagnetic spectrum.

DLR Oil spill

SAR image of an oil spill in the Timor Sea, northeast of Australia.

Now that I’ve sung the praises of SAR, how are some of the world’s leading space agencies

making use of it?

Heather Hunter

A short list of some SAR used for Earth- and planetary-science

Owing to its ability to peer through clouds, the SAR on the Magellan probe had no problem

mapping the Venusian surface. It is the perfect, earliest example of SAR’s advantage over optical

systems when it comes to thick atmospheres. Similarly, the SAR aboard the Cassini probe gave

scientists first-ever views of Titan’s surface. Recall, Titan is, like Venus, covered in clouds and

haze, and SAR is the perfect fit for a cloudy atmosphere!

NASA / JPL-Caltech Change detection on Titan

Example of detecting changes in surface features on Titan using the Cassini’s SAR.

When it comes to the Earth, the European Space Agency (ESA) leads the way in the number of

operational SAR among their Earth-observing fleet. The most recent of these include the Sentinel

satellites, which are a collection of satellites that monitor the entire globe and provide data on

everything from the status of Arctic sea ice, to oil spills, to other environmental information that

allows the development of maps required during crisis situations and humanitarian aid

operations. Sentinel-1A was launched in 2014, Sentinel-1B was launched in 2016, and the rest of

the Sentinels (2–6) are either not fitted with SAR or have not yet been launched.

Now you know the very basics of SAR, and how scientists are using it study both the Earth and

other bodies in the solar system. If you’re interested in learning more technical details, or you

want to learn more about the applications of SAR, here are a handful of online resources to help

you get started:

1. For more details on how SAR is used in meteorological and oceanographic applications:

NOAA’s Synthetic Aperture Radar Marine User’s Manual

2. For more details on the physics of SAR, as well as access to SAR data sets: University of

Alaska Fairbanks Satellite Facility

3. UAF-ASF also provides a list of free and commercial SAR image viewing tools

4. For more details on the Sentinel missions, as well as access to technical guides, data

guides, and access to data: ESA Sentinel Online

5. Technical write-up for Cassini, from NASA

6. Access to Cassini SAR data, from NASA

7. Access to Magellan SAR data, from NASA

Document Source: https://www.planetary.org/blogs/guest-blogs/2017/radar-in-earth-and-

planetary.html