Decomposition of the radar signal
In this topic you are going to learn about the concept of polarisation and the related technique ‘Radar polarimetry’. This technique is very useful to understand the scattering mechanisms of different targets on the Earth’s surface.
The majority of radar systems are capable of sending out and receiving electromagnetic waves in a controlled manner. Due to this feature, it is possible to gain very specific information about the targets on the ground, illuminated by the microwave pulses.
Here you will gain a basic understanding of the concept behind polarimetry that will help us later in the practical tutorials.
What exactly is polarimetry?
Polarimetry describes a technique which includes the measurement, the processing and the interpretation of the polarisation state of an electromagnetic wave.
The polarisation state of a backscattered wave from a natural surface can be linked to the geometrical characteristics like shape, roughness and orientation and the intrinsic properties of the scatterer like humidity /moisture, salinity or medium density.
If you get hold of one end of a rubber rope, tie the other end to a post, stretch it and then send a series of pulses down the rope the vibration travels down the rope. Although each successive pulse may be sent in a different plane each pulse only vibrates in one direction.
This is exactly the same as a source of light. Each quantum emitted has vibrations in one plane but because you receive many millions of quanta per second from a light source it appears that the wave is vibrating in all directions.
A wave in which the plane of vibration is constantly changing is called an unpolarised wave.
However if the vibrations of the electrical field of a transverse wave are in one plane only then the wave is said to be plane polarised.
Remember, electric and magnetic fields are perpendicular to each other
When light is plane-polarised the vibrations are made to occur in one plane only. Light is a transverse electromagnetic wave with the vibrations of an electric and a magnetic field occurring at right angles to each other and in any plane at right angles to the direction of travel of the light.
Polarisation is easily observed with the rubber rope experiment described above but it can also be shown with electromagnetic waves such as microwaves, TV, radio and light.
It is important to realise that transverse waves can be polarised while longitudinal waves cannot.
What does that mean for radar systems?
The electromagnetic waves, emitted by the radar antenna have a specified polarisation, in most cases either horizontal (H) or vertical (V).
The radar antenna, which receives the backscattered echoes, also receives horizontally or vertically polarised waves. Depending on the target on the Earth’s surface, which scattered the microwave signal back, the echo may still be polarised in the same direction, or it may be completely re-polarised in the opposite plane, or it could be a mixture of these cases. Either way, this allows us to gather more information about the properties of the targets on the ground and their interaction with the microwave radiation.
The radar system sends vertically polarised waves and receives vertically polarised waves. In this example, where the polarisation was not changed during the travel of the signal to the ground and back, the incoming wave passes through and is detected at the sensor.
Polarimetric radar systems
Polarimetric radar systems usually differentiate between CO-Pol and CROSS-Pol setups. Learn what the difference is and how it actually looks in a radar image.
The term CO-Polarisation defines the setup when the radar antenna receives polarised waves in exactly the same polarisation as it sent them out. The following combinations are possible:
- Horizontally sent – Horizontally received (HH)
- Vertically sent – Vertically received (VV)
The term CROSS-Polarisation defines the setup when the radar antenna receives polarised waves in the opposite polarisation as it sent them out. The following combinations are possible:
- Horizontally sent – Vertically received (HV)
- Vertically sent – Horizontally received (VH)
The resource repository holds a comprehensive presentation for a deeper understanding of radar polarimetry.
We recommend this resource for users who would like to delve deeper into the topic and want to understand the mathematical foundation of radar polarimetry.
Bourgeau-Chavez, L.L, Riordan, K., Powell, R.B., Miller, N., Nowels, M. , (2009): Improving Wetland Characterization with Multi-Sensor, Multi-Temporal SAR and Optical/Infrared Data Fusion, Advances in Geoscience and Remote Sensing, Gary Jedlovec (Ed.)