Lesson 4, Topic 1
In Progress

Active Imaging Techniques

Learning objectives of this topic

  • Fundamental acquisition principles of microwave systems
  • Variations of active bands and their use
  • Important nomenclature
  • Types of scattering and implications on the returned signal

Benefits of active Remote Sensing Systems

By transmitting electromagnetic waves in the microwave domain of the EM spectrum, radar satellites exploit unique characteristics not present in Optical Remote Sensing Systems.

Active Systems

  • Provide illumination by sending out microwaves
  • Are largely weather (cloud) independent
  • Acquire images during day and night
  • Actively control wavelength, frequency and polarisation of the transmitted signal

Passive Systems

  • Rely on the illumination of the Earth by the sun (or artificial light sources)
  • Detect the reflection from the Earth’s surface
  • Are very sensitive to cloud cover
  • Need no internal energy source
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What is an active Remote Sensing System?

Radar satellites are active systems. Find out about their characteristics, and learn how they differ from passive systems.

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Benefits of active Remote Sensing Systems

By transmitting electromagnetic waves in the microwave domain of the EM spectrum, radar satellites exploit unique characteristics not present in Optical Remote Sensing Systems.

Active Systems

  • Provide illumination by sending out microwaves
  • Are largely weather (cloud) independent
  • Acquire images during day and night
  • Actively control wavelength, frequency and polarisation of the transmitted signal

Passive Systems

  • Rely on the illumination of the Earth by the sun (or artificial light sources)
  • Detect the reflection from the Earth’s surface
  • Are very sensitive to cloud cover
  • Need no internal energy source
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The SAR principle

Radar satellites images are based on the concept of actively sending out microwave pulses towards the Earth. The spatial resolution of these images depends on the size of the radar antenna. Synthesizing a larger Radar Antenna is a core prerequisite of the successful deployment of Radar technology in space. Learn how the principle of Synthetic Aperture Radar works in the next topic.

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What is an active Remote Sensing System?

Radar satellites are active systems. Find out about their characteristics, and learn how they differ from passive systems.

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What does the term ‘SAR’ mean?

Radar images are very often referred to as SAR images. The acronym SAR stands for Synthetic Aperture Radar and describes an engineering method that is used to achieve higher resolution radar images. Learn how this aperture synthesis works in radar systems in this chapter.

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Radar bands

Microwave frequencies are labelled in bands. Perhaps you stumbled on this seemingly confusing nomenclature of X-Band, C-Band or L-Band when dealing with radar images. Learn what these labels mean and how to remember the bands from the interactive graphic below.

Radar Imaging geometry

Radar systems have a very unique way of making measurements. Microwave pulses are sent out towards the Earth’s surface and the echoes are recorded back at the sensor.

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Why does the radar need to look to a side?

A radar system primarily makes a measurement of time. The antenna sends out pulsed microwaves and detects the time it takes for the echoes from the target scene to return back to the antenna. If a radar sensor transmits this pulse straight down towards the nadir direction on a flat surface, the distance of all the targets on this surface to the radar antenna would be almost identical. Therefore, the echoes would mostly return simultaneously, and no differentiation of the signals could be made.

By adopting a side-looking geometry and transmitting the pulses obliquely, the radar system is able to resolve distinct targets on the ground by detecting a different time delay for each of the targets, since the time delay is then approximately correlated with distance along the ground (in the range direction).


The nomenclature

Before processing and interpreting radar data, it is important to understand the vocabulary that is used to describe the imaging process. This knowledge will be crucial to understanding what you see in a radar image and how the microwaves interact with the surface.

Nadir

Nadir describes the direction below a particular location. In the context of remote sensing, it refers to the point directly below the satellite/aircraft. To be more precise, it can be defined as the local vertical direction pointing toward the force of gravity at a particular location. The opposite direction to the nadir is the zenith.

Geometric description of the nadir.

Swath

The term ‘swath’ has its roots in farming. It describes the width of a scythe. This analogy has been transferred into remote sensing. In this context, the swath width describes the area (width) on the ground that is covered by the sensor instrument of a satellite or aircraft.

Azimuth

In the context of radar remote sensing, azimuth describes the flight direction or direction of travel of the satellite/aircraft. It can also be referred to as the line of flight.
In an image, azimuth is also known as along-track direction, since it is the relative along-track position of an object within the antenna’s field of view following the radar’s line of flight.

Range

The range direction is the distance between the radar and each illuminated target. It is the dimension of an image perpendicular to the line of flight (azimuth). In radar remote sensing, we differentiate slant range and ground range. Slant range is the distance from the radar toward each target and measured perpendicular to the line of flight. Ground range is the same distance, projected using a geometrical transformation onto a reference surface such as a map.

Radar range geometry

Incidence Angle

The incidence angle is the angle defined by the incident radar beam and the vertical (normal) to the intercepting surface. In general, reflectivity from distributed scatterers decreases with increasing incidence angle. The incidence angle changes across the radar image swath; it increases from near range to far range.
A change in incidence angle often affects the radar backscattering behaviour of a surface. In the case of satellite radar imagery, the change in the incidence angle for flat terrain across the imaging swath tends to be rather small, usually on the order of several degrees. In the case of an inclined surface, the local incidence angle is defined as the angle between the incident radar beam and a line that is normal to that surface (a vector perpendicular to the surface at a given point).

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Scattering mechanisms

Microwaves interact with objects on the ground. In fact, that is what we are trying to measure with our radar satellites, in order to distinguish the various materials and objects on the ground. To be able to do that and to fully understand the backscatter signal, we have to understand the various types of scattering that can occur on the Earth’s surface.

The concept of scattering mechanisms is at the heart of understanding the signals that are returned to the sensor, after the microwave pulses hit the Earth’s surface. Let’s go through them again to internalise them further. You will need to remember them for the various practical application scenarios in the upcoming lessons.

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The scattering mechanisms in more detail

Specular reflection

If the radar pulse hits a smooth, flat surface (on the scale of the wavelength), most of the energy is scattered away in a specular direction. These areas will appear very dark in the radar image. Typical examples for specular reflection are smooth water surfaces or tarmac (e.g. roads, parking lots).

Specular reflection animation (click to start)

Surface scattering

A single bounce, or surface scattering, appears when the microwave hits a somewhat rough, homogeneous surface. Parts of the energy are scattered back to the sensor. Which surface is rough and which isn’t is determined by the wavelength, the incidence angle and the spatial resolution of the system. These variables are also referred to as sensor parameters. We will learn more about these parameters in the following topics.

Surface scattering animation (click to start)

An example of the relationships between surface backscatter and surface roughness is shown in the following figure. The simplest form of surface scattering is the specular reflection introduced previously. As we can see, increasing surface roughness increases the diffuse scattering. Furthermore, the longer the wavelength, the smoother the surface appears for a sensor.

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Double bounce

Double bounce, or dihedral scattering, occurs when the radar pulse hits two relatively smooth surfaces that are perpendicular to each other. The returned signal is particularly strong, due to the multiple transmission of the energy back into the direction of the sensor.

Typical examples where double bounce occurs are buildings and other artificial structures.

Double bound animation (click to start)

Volume scattering

Volume scattering occurs if the radar pulse penetrates into a 3-dimensional body. The energy is scattered multiple times in multiple directions, before parts of it are returned to the sensor.

Classic examples of volume scattering include dry snow surfaces, tree canopies or vegetated fields.

Volume scattering animation (click to start)

Scatter models

We have developed three scenarios for radar backscatter. These explorable explanations are designed to help you understand the scatter mechanisms. Play around with the sliders and menus to see how radar pulses behave as they hit certain surfaces. Go ahead and try it yourself!

 Field scenario

The term dielectric constant describes the electric permittivity of a given material as a ratio, relative to the permittivity of a vacuum. It is directly related to the water content of an object or surface, as well as the material.
The term dielectric constant can be misleading, since it is not constant over various materials, only for a specific material or object. Keep that in mind when you think about this parameter.

 City scenario

Snow scenario


Historic excourse: 30 years after the start of ERS-1, ESA’s first SAR sensor in space.

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Other sources of active imaging – LiDAR

While this will not be covered in detail in this course, there is also another source of active remote sensing science that becomes more and more relevant, are Light Detection and Ranging (LiDAR) devices. These instruments send laser pulses (optical and infra-red wavelengths) to the Earth’s surface and measure the return delay in order to estimate the position of the target on the ground. For more information on the techniques behind every LiDAR acquisition, you can access the PDF below and/or take a look at the informative video from NEON Science.

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How LiDAR works. Source: NEON Science

Sources & further reading

Elachi, C. & van Zyl, J. (2015²). Introduction to the Physics and Techniques of Remote Sensing. Hoboken, USA: John Wiley & Sons, Inc.

Jensen, J.R. (2007²). Remote Sensing of the Environment. An Earth Resource Perspective. Upper Saddle River, USA: Pearson Prentice Hall.

Rees, W.G. (2010²). Physical Principles of Remote Sensing. Cambridge, USA: Cambridge University Press.

Richards, J.A. (2009). Remote Sensing with Imaging Radar. Berlin, Germany: Springer.

Schowengerdt, R.A. (2007³). Remote Sensing. Models and Methods for Image Processing. San Diego, USA: Academic Press.

Woodhouse I.H. (2006). Introduction to Microwave Remote Sensing. Boca Raton, USA: Taylor & Francis Group.