Space Girls

Panama | What on Earth is Synthetic Aperture Radar?

SAR for the Benefit of The Planet

High-Level Project Summary

SAR is a special type of radar that allows high-resolution images to be obtained over long distances, e.g. from space. Synthetic aperture radar (SAR) bounces a microwave radar signal off the Earth's surface to detect physical properties.Unlike optical technology, synthetic aperture radar (SAR) can "see" through darkness, clouds and rain, detecting changes in habitat, water and moisture levels, effects of natural or human disturbances, and changes in the Earth's surface following events such as earthquakes or sinkhole openings. Given the growing threats of environmental phenomena to national and global security, SAR can provide additional information to assess and respond to climate change.

Link to Project "Demo"

https://youtu.be/Km9kbCr_bHU

Link to Final Project

https://youtu.be/Km9kbCr_bHU

Detailed Project Description

¿WHAT IS RADAR SYSTEM?

Radar systems have long been used to detect different types of natural events and are very promising because they provide detailed information on certain factors. There are multiple types of radars depending on the area being studied. Radar systems show great promise in providing the detailed information needed to direct a clearing operation, especially in the presence of clouds. Clouds obscure satellite images (except radar) and also reduce contrast for aircraft flying below them.

Radar systems basically work by transmitting electromagnetic or radio waves. Most objects reflect radio waves, which can be detected by the radar system. The frequency of the radio waves used depends on the radar application. Radar systems are often designated by the wavelength or frequency band in which they operate. The choice of frequency depends on the requirements of the application. The minimum antenna size is proportional to the wavelength and inversely proportional to the frequency.

¿WHAT IS A SAR?

SAR stands for Synthetic Aperture Radar. SAR is a type of active data collection in which a sensor produces its own energy and then records the amount of that reflected energy after interacting with the Earth. While optical imaging is similar to interpreting a photograph, SAR data requires a different way of thinking in that the signal responds to surface features such as structure and moisture.

Figure 1: ERS-1 Satellite equipped with SAR.

Radar Aperture:

Aperture refers to the aperture used to collect the reflected energy used to form an image. In the case of radar, this is the antenna. The radar antenna first transmits electromagnetic energy towards the Earth and then receives the return energy after it is reflected off objects on the planet. The data collected by the radar antenna is then transmitted to another type of antenna on Earth, such as the ASF satellite tracking ground station antennas, so that it can be stored and processed.

Synthetic Aperture

In general, the larger the antenna, the more unique information scientists can obtain about an object, and the more information, the better the image resolution. However, antennas in space are not large. Therefore, scientists and engineers have come up with a clever solution: synthetic aperture. In this concept, a sequence of acquisitions from a shorter antenna is combined to simulate a much larger antenna, thus providing higher resolution data.

SAR takes advantage of the Doppler history of the radar echoes generated by the forward motion of the spacecraft to synthesise a large antenna. The Doppler effect is an effect of wave physics that occurs when a moving source emits waves. This allows high azimuth resolution in the resulting image despite a physically small antenna. As the radar moves, a pulse is transmitted at each position. The return echoes pass through the receiver and are recorded in an echo store.

SAR requires a complex integrated array of onboard navigational and control systems, with location accuracy provided by both Doppler and inertial navigation equipment.

Figure 2: Synthetic Aperture.

How does it work?

The instrument measures the distances between the sensor and the point on the Earth's surface where the signal is backscattered.

After the radar sends its microwave signal toward a target, the target reflects part of the signal back to the radar antenna. That reflection is called backscatter. Several properties of the target affect the amount of signal backscatter. This distance is a slant range, which can be projected onto the ground representing the ground range. The direction of flight is also known as the direction along the path or azimuthal direction, and the direction perpendicular to the flight path is the transverse or range direction. The angle between the direction in which the antenna is pointing and the nadir is the angle of view. The angle between the center of the radar beam and the normal to the local topography is the angle of incidence.

Polarization Mechanism

Polarization refers to the orientation of the plane in which the transmitted electromagnetic wave oscillates. While the orientation can occur at any angle, SAR sensors generally transmit linearly polarized. Horizontal polarization is denoted by the letter H and vertical polarization is denoted by V.

The advantage of radar sensors is that the polarization of the signal can be precisely controlled in both transmit and receive. Signals emitted in vertical (V) and received in horizontal polarization (H) would be indicated by a VH. Alternatively, a signal that was emitted horizontally (H) and received horizontally (H) would be indicated by HH, and so on. Examination of the signal intensity of these different polarizations conveys information about the structure of the image surface, based on the following types of scattering: rough surface, volume, and double bounce (see figure below):

Rough surface scattering, such as that caused by bare ground or water, is more sensitive to VV scattering.
Volume scattering, for example, caused by leaves and branches in a forest canopy, is more sensitive to cross-polarized data such as VH or HV.
The last type of scattering, double bounce, is caused by buildings, tree trunks or flooded vegetation and is more sensitive to an HH polarized signal.

Benefits:

Earth observation satellites have been powerful tools for scientists and policymakers to understand planetary changes. In particular, Earth observation data play a central role in environmental monitoring by allowing users to compare images of the Earth over time and examine large-scale phenomena such as melting sea ice, deforestation, droughts, and floods.

SAR images provide more detail about the Earth's surface because of the way SAR signals interact with particular surfaces (e.g., buildings, trees, mountains, lakes, etc.).

Since SAR instruments do not rely on the Sun's energy to collect surface data, SAR satellites can operate equally well during the day or night. In addition, SAR signals can penetrate through clouds to "see" the surface cover below, allowing satellites to have a complete view of the Earth's surface regardless of atmospheric or lighting conditions. SAR can also "see" through other types of cover, such as smoke, vegetation, snow or sand, depending on the satellite's designated operating band (which indicates the frequency and wavelength associated with the sensor).

Given the increased threats of environmental phenomena to national and global security, SAR can provide additional information to assess and respond to climate change, ecosystem loss, natural disasters, and more. Below are just a few examples of how SAR is being used for such purposes:

Agriculture. Differences in surface roughness are indicative of field ploughing, soil tillage, and crop harvesting.
Floods. Differences in surface reflection can help distinguish heavy flooding, light flooding, urban areas, and permanent bodies of water.
Land subsidence. Differences in measurements over time can reveal displacements of land, such as sinking ground caused by the extraction of underground natural resources.
Snow cover. Differences in surface reflection can help forecast snowmelt by distinguishing wet snow, dry snow, and snow-free areas.
Wildfires. Penetration through thick smoke can provide more accurate and timely information about the extent of a forest fire and can help quantify vegetation loss.
Wetlands. Penetration through wetland areas can reveal flooded vegetation where land is covered by shallow water.

Space Agency Data

To develop our challenge we used

NASA Earth Data :

https://earthdata.nasa.gov/learn/backgrounders/what-is-sar

https://asf.alaska.edu/information/sar-information/what-is-sar/

European Space Agency:

https://earth.esa.int/documents/10174/2700124/sar_land_apps_1_theory.pdf

Both space agencies provided valuable information to advance our challenge.

Hackathon Journey

This is our first time participating in the space app challenge.

Marisabell: It has been a great experience, being able to participate and work on a topic which is not very related to my field of study made it very interesting because I could learn many new things that I did not know existed and put my mind to work quickly in order to organize ideas and capture everything in the development of our challenge.

Genesis: The simple fact of participating for me is already a gain. I really liked the experience, it has been a lot of fun, and it is very helpful since it helps to maintain our creative mind, apart from that it challenges us to work with tools that we did`t know, always learning something new is valuable.