Industry Insights

Radar Part 2 - Understanding Synthetic Aperture Radar

2020-09-04  |  6 min read 

In part 1, we discovered what radar is, and what frequency bands it commonly operates on. We’ll now turn our attention to one of the specific types of radar – synthetic aperture radar (SAR) – and its use in satellites. We’ll cover why it was developed, the basic principles of its operation, and what it’s used for.

The radar’s spatial resolution, or the amount of detail it can see, is a function of the antenna size. To see smaller or more distant things clearly, you need a bigger antenna. While this remains practical for terrestrial applications, making the antenna larger is not possible for satellites. China’s 305-meter-wide FAST telescope is roughly as large as the fictional Space Station V from 2001: a Space Odyssey, but launching that much material into space would be prohibitively expensive. The international space station is roughly a fifth the size of Space Station V and cost a whopping $150 billion to develop and assemble in orbit.

So why would we want to use radar imaging sensors in satellites, why not just stick with a conventional camera? Cameras are cheap, they see everything, and even the biggest camera in space is not 300 meters in diameter! Well, consider this: radar works in nearly all weather conditions, it can see in the dark, it can penetrate clouds, treetops, soil, sand and some types of rock. It gives us a vastly richer set of data than visual images alone ever could.

Now we know why we want to have a radar in space, looking down at earth. At a typical satellite’s altitude (around 300-600 km) to make 1 pixel of your image equivalent to 10 m, you’d need an antenna 4,250 m long! However, we’re also aware of the prohibitively high cost of launching a large antenna into orbit. So, we have to assume that satellites must have smaller antennae, and indeed they are. How, then, can we get a useful spatial resolution from such a distance with such restrictions on antenna size? The answer is SAR.

What is SAR?

SAR is a technique used when increasing the size of a radar antenna is not possible. Rather than increasing the antenna size, a larger antenna can be effectively simulated by moving a smaller antenna in an arc, such as the orbital path of a satellite. By moving along the arc, the sensor can image more of the target, and, due to parallax, it can capture images from multiple perspectives. This adds additional depth data to the image, creating not pixels, as visible light imaging sensors do, but volumetric pixels, or voxels. The data collected from the many samples as the satellite’s sensor traverses the earth builds up a rich 3D image of the target.

What is SAR used for?

Use Case

Example

Military surveillance

Monitoring weapons deployment

Oceanography

Wave height, water depth, ocean floor topography

Forestry

Mapping forest cover, wetlands, deforestation

Agriculture

Monitoring photosynthesis, crop yield forecasting

Urban surveying

Monitoring construction activity, infrastructure stability, mapping disasters, urban growth

Glaciology

Monitoring ice volume in glaciers and oceanic ice sheets

Geology

Monitoring earthquakes and landslips, cartography

Topography

Mapping vertical features of land

Archaeology

Ground and vegetation penetration imaging of former urban sites

Table 1: Examples of uses for SAR.

While the principles of radar may be fairly simple, developing a SAR application is far from trivial. Being able to model the radar transmit and receive signals accurately in a realistic simulation is critical. For ground-based applications, maintenance is always available, and replacing a part is possible. Satellite-based SAR applications are, however, mission critical.

Mission-critical designs have to work flawlessly, and there is no chance of replacing something after the launch. The W1905EP radar model library for Keysight’s PathWave system design software enables you to overcome these development hurdles and ensure that your designs will function as intended in the real world.

As we’ve learned, the variety of applications SAR supports is wide: military, commercial, and archaeological are but a few. It is a technology that can not only help predict future climate conditions, but also see into our past to uncover forgotten cities. It provides us with information about our world to help us understand the scale of crises big and small, and it helps us put food on the table. As technology advances, improvements in SAR image processing algorithms and antenna design improve, the applications of SAR will become more diverse, and I, for one, am excited to see what new things we will learn about the planet we call home.