Microwave Remote Sensing Overview

Microwave Remote Sensing

• Microwave remote sensing employs electromagnetic radiation with wavelengths between 1 cm and 1 m (microwaves).
• Microwaves penetrate clouds, fog, and ash/powder effectively, unlike visible and infrared radiation. This facilitates observations in challenging atmospheric conditions (e.g., volcanic eruptions, collapsed buildings).
• Microwave remote sensing systems are categorized into passive and active systems.

Passive Microwave Systems

• Passive systems detect naturally emitted microwave energy from surfaces.
• The emitted energy correlates with the temperature and moisture content of the emitting object/surface.
• Microwave energy, emitted, reflected, or transmitted from the surface or atmosphere can be recorded.

Active Microwave Systems

• Active microwave systems generate their own microwave radiation signal, directing it toward the target and measuring the backscattered portion.
• This principle determines range and allows differentiation among targets.
• This category includes radars, mainly divided into imaging and non-imaging systems.

RADAR (Radio Detection And Ranging)

• RADAR is the most prevalent imaging system.
• It sends a microwave signal, measures the backscattered portion, distinguishes among targets, and calculates distance via signal delay.

Non-Imaging Microwave Systems

• Non-imaging systems encompass altimeters and scatterometers.
• Altimeters measure the time delay of microwave pulses to determine the distance of targets from the sensor.
• Scatterometers quantify backscattered energy from targets based on surface properties and the incident angle of the microwave (roughness is a key factor.)

Development of Active Microwave Remote Sensing

• The first demonstrations of microwave transmission and reflection from objects date back to 1886 (Heinrich Hertz).
• Early radar systems were primarily for detecting ships.
• Ground-based pulsed radar systems were developed in the 1920s and 1930s for distant object detection.
• World War II saw advancements in imaging radars to detect and locate aircraft and ships.
• Side-looking airborne radar (SLAR) systems emerged after WWII to map extensive terrain from airborne platforms.
• Synthetic aperture radar (SAR) developed in the 1950s enhanced image resolution.
• These technologies were later declassified and applied to civilian applications.

SAR (Synthetic Aperture Radar)

• Two kinds of SAR: Real Aperture Radar and Synthetic Aperture Radar.
• Real Aperture Radar (brute-force radar) employs a fixed antenna length.
• Synthetic Aperture Radar leverages a smaller antenna, but computationally constructs a larger virtual antenna to achieve high resolution from a distance.
• 11m wide SAR antenna in a satellite system can generate a long synthetic antenna, simulating a much longer one.

Active Microwave System Components

• Active microwave systems consist of a pulse generator, transmitter, duplexer, antenna, and receiver.
• Recording devices (like high-density digital tape/hard disks) and CRT monitors assist in data recording/verification.

Wavelength, Frequency, and Pulse Length

• Microwave radiation emitted by radar systems has certain wavelengths and durations.
• Wavelengths are often measured in centimetres.
• Early naming conventions (due to World War II secrecy) employed alphabetic designations (e.g., K-bands). Shorterest wavelength (e.g., K-band ) can be absorbed by water vapor. Shorter wavelengths are often used in areas needing greater detail.
• Some radar systems use multiple frequencies.

Radar Band Designations

• A range of frequency bands (Ka to P) are used in microwave remote sensing; Each exhibits unique characteristics and optimal performance for specific applications.

Antenna, Azimuth, and Flight Direction

• Antennas are typically mounted beneath the aircraft for radar systems.
• Azimuth refers to the aircraft's direction of travel, used to locate the radar beam's path across the land.
• The plane (or satellite) flying is designed in such a way that the pulses illuminate strips of terrain at right or parallel angles to travel path. Pulses of energy illuminate a strip, often at right angles to the aircraft's direction of travel.

Range, Depression, and Incidence Angles

• Range is the distance from the sensor to the target.
• Depression Angle : the angle between a point at the surface of Earth and a line from the antenna to that point, projected onto the horizontal plane.
• Incident angle, is the angle between a line from the antenna to a point on the ground and a line perpendicular to that ground point.

Polarization

• Polarization describes the orientation of electromagnetic waves' vibrations within the radiation.
• Radars usually transmit, and receive linearly-polarized signals (e.g., horizontal or vertical polarization).
• Analysis of backscattered waves based on polarization combinations constitutes radar polarimetry.

Speckle

• Speckle in radar images is graininess or random variations due to the coherent nature of the radar signals (interference).
• Speckle can be mitigated using techniques like multi-look processing.

Relief Displacement (Foreshortening and Layover)

• Relief displacement, refers to the horizontal shifts in an object's position in the image due to elevation.
• Foreshortening and layover are two phenomena that are important to consider when analyzing radar images.

Range and Azimuth Resolution

• Range resolution is affected by the pulse duration of the microwave.
• Azimuth resolution depends on the width of the radar beam and the antenna length.
• Shorter wavelengths lead to improved resolution.