Active Imaging Techniques

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.

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.

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).

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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