Principles of Imaging Radar: Components, Radar Equation, Imaging Radar Bands, and Microwave Polarizations (Especially for GATE-Geospatial 2022)

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Components and Working of Imaging Radar

Imaging radar systems include several components: a transmitter, a receiver, an antenna, and a recorder. The transmitter is used to generate the microwave signal and transmit the energy to the antenna from where it is emitted towards the Earth՚s surface. The receiver accepts the backscattered signal as received by the antenna, filters and amplifies it as required for recording. The recorder then stores the received signal.

Principle of Radar Equation

Imaging radar acquires an image in which each pixel contains a digital number according to the strength of the backscattered energy that is received from the ground. The energy received from each transmitted radar pulse can be expressed in terms of the physical parameters and illumination geometry using the so-called radar equation

Where

  • Pr is the received energy
  • G is the antenna gain
  • is the wavelength
  • Pt is the transmitted energy
  • σ is the radar cross section, it is a function of the object characteristics and the size of the illuminated area
  • R is the range from the sensor to the object

Main Factors Influencing Backscattered Received Energy

From this equation you can see that there are three main factors that influence the strength of the backscattered received energy:

  • Radar system properties, i.e.. , wavelength, antenna and transmitted power,
  • Radar imaging geometry, that defines the size of the illuminated area which is a function of i.e.. , beam-width, incidence angle and range,
  • Object characteristics in relation to the radar signal, i.e.. , surface roughness and composition, and terrain topography and orientation.

What Exactly Does a Radar System Measure?

  • To interpret radar images correctly, it is important to understand what a radar sensor detects. Imagine the transmitter creates microwave signals, i.e.. , pulses of microwaves at regular intervals, the Pulse Repetition Frequency (PRF) , that is bundled by the antenna into a beam. This beam travels through the atmosphere, illuminates a portion of the Earth՚s surface, is backscattered and passes through the atmosphere again to reach the antenna where the signal intensity is received. From the time interval the signal needs to pass twice the distance between object and antenna, and knowing the speed of light, the distance (range) between sensor and object can be derived.
  • To create an image, the return signal of a single pulse is sampled and these samples are stored in an image line. With the movement of the sensor, emitting pulses, a two-dimensional image is created (each pulse defines one line) . The radar sensor, therefore, measures distances and detects backscattered signal intensities.

Commonly Used Imaging Radar Bands (Microwave Spectrum and Band Identification by Letters)

  • Similarly, to optical remote sensing, radar sensors operate with different bands. For better identification, a standard has been established that defines various wavelength ranges using letters to distinguish among the various bands. In the description of different radar missions, you will recognize the different wavelengths used if you see the letters.
  • The European ERS mission and the Canadian Radar sat, for example, use C-band radar. Just like multispectral bands, different radar bands provide information about different object characteristics.
Microwave Spectrum and Band Identification by Letters

Microwave Polarizations

  • The polarization of an electromagnetic wave is important in the field of radar remote sensing. Depending on the orientation of the transmitted and received radar wave, polarization will result in different images.
  • It is possible to work with horizontally, vertically or cross-polarized radar waves. Using different polarizations and wavelengths, you can collect information that is useful for particular applications, for example, to classify agricultural fields. In radar system descriptions you will come across the following abbreviations:
    • HH: horizontal transmission and horizontal reception,
    • VV: vertical transmission and vertical reception,
    • HV: horizontal transmission and vertical reception, and
    • VH: vertical transmission and horizontal reception.

A Vertically Polarized Electromagnetic Wave; The Electric Field՚s Variation Occurs in the Vertical Plane in this Example

A Vertically Polarized Electromagnetic Wave

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