Skip to content Skip to navigation

OpenStax_CNX

You are here: Home » Content » Background

Navigation

Lenses

What is a lens?

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

This content is ...

Affiliated with (What does "Affiliated with" mean?)

This content is either by members of the organizations listed or about topics related to the organizations listed. Click each link to see a list of all content affiliated with the organization.
  • Rice University ELEC 301 Projects display tagshide tags

    This module is included inLens: Rice University ELEC 301 Project Lens
    By: Rice University ELEC 301As a part of collection: "ELEC 301 Projects Fall 2007"

    Click the "Rice University ELEC 301 Projects" link to see all content affiliated with them.

    Click the tag icon tag icon to display tags associated with this content.

Also in these lenses

  • Lens for Engineering

    This module is included inLens: Lens for Engineering
    By: Sidney BurrusAs a part of collection: "ELEC 301 Projects Fall 2007"

    Click the "Lens for Engineering" link to see all content selected in this lens.

Recently Viewed

This feature requires Javascript to be enabled.

Tags

(What is a tag?)

These tags come from the endorsement, affiliation, and other lenses that include this content.
 

Background

Module by: Tianlai Lu. E-mail the author

Summary: This module talks about background of the synthetic aperture radar (SAR): the two types of SAR, resolution of advantages of SAR, as well as a general overview of imaging radars.

Synthetic Aperture Radar: Background

What is Synthetic Aperture Radar?

Synthetic Aperture Radar (SAR) is a microwave imaging system that is used to obtain high resolution pictures of large areas of terrain. The radar can be either airborne or spaceborne. As the platform moves, closely spaced pulses are transmitted and the reflected signals are received and processed using Fourier methods. The processed data resembles data taken with a system that has a very large antenna, thus allowing extremely high resolution.

Synthetic Aperture Radar was first developed in the early 1950’s. The earliest type of SAR is called strip-mapping mode SAR. It is primarily used for imaging large areas of terrain, such as the surface of a nearby planet. This mode emits the radar pulses at a constant “look” angle to the surface while traveling along a flight path or orbit. This process creates a strip of mapped ground, and can be repeated along a polar orbit to map the entire surface of a planet.

Spotlight mode SAR is a newer form of SAR and was developed in the early 1980’s. It is more widely used today than strip-mapping mode and it is what our project deals with. In spotlight mode, the radar is steered continually as the carrier of the radar flies over a patch of ground. In another word, the “look” angle is constantly adjusted so that a single patch of ground is always illuminated. This method allows for higher resolution in the azimuth “travel” direction of the platform but is not able to image as large of an area as strip-mapping mode.

How Does Imaging Radar Work?

As mentioned earlier, we use the Synthetic Aperture Radar processing technique because of its advantages when it comes to imaging large areas at high resolutions. However, why do we even use radar to image things in the first place?

Radar is used in imaging because of the minimal constraints that is has on time-of-day and atmospheric conditions. The area of imaging does not have to be illuminated by sunlight in order to obtain a picture. This allows for continuous mapping regardless of the position of the sun, which saves time and therefore, money.

Radar also has the ability to penetrate cloud cover because one can choose a wavelength that is not absorbed by water. This fact is what allowed scientists at NASA to provide stunning images of the surface of Venus, which is completely shrouded in cloud-cover.

Imaging radar works by emitting a signal and then recording the strength of the reflected signal (scattering coefficient) for that area. The pulses are emitted at an angle to the surface such that if they strike a smooth, flat surface, very little of the signal will be reflected back towards the antenna which corresponds to a darker spot on our scattering coefficient image.

When the radar pulse strikes uneven surfaces such as urban areas or vegetated areas, the signal gets reflected numerous times and there is an increased likelihood that the radar antenna will eventually receive a large portion of the signal back, corresponding to a whiter spot on your image. Scientists use this fact to determine the extent of flooding in urban areas or to discern how much an oil spill in the ocean has grown.

Synthetic Aperture Radar and Microwave Imaging System

The resolution of an image taken from an imaging system is usually determined by the size of the Aperture (lens for optical systems and antenna for radar). Conventional radar systems use passive methods deployed with optical or short-wave infrared sector that rely on sunlight reflection. On the other hand, synthetic Aperture Radar uses a microwave imaging system. Two important advantages resulting from using microwave pulses are that cloud cover can be penetrated and the imaging process can be performed at night.

However, antenna size limits one from applying microwave imaging systems. A very large size of antenna is required to obtain satisfactory resolution. Therefore, the size of the antenna makes it impractical for the radar carrier.

Synthetic Aperture Radar solves the problem by “simulating” a large Aperture. The radar sends and receives signals from a relatively small antenna while the platform traveling along a flight path. One can then use the digital signal processing techniques to combine the data into a coherent image. The result is the same as if one has used a very large antenna.

Content actions

Download module as:

PDF | EPUB (?)

What is an EPUB file?

EPUB is an electronic book format that can be read on a variety of mobile devices.

Downloading to a reading device

For detailed instructions on how to download this content's EPUB to your specific device, click the "(?)" link.

| More downloads ...

Add module to:

My Favorites (?)

'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

| A lens I own (?)

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

| External bookmarks