UNIT 2 – MODULE 7: Microwave & LIDAR Sensing

MICROWAVES & RADIO WAVES In this section, it is important to understand that radio waves and microwaves are used interchangeably. This is because microwaves are part of the radio wave spectrum. Microwaves are smaller radio waves.

MICROWAVE SENSING Microwaves are capable of seeing through the atmosphere under nearly all weather conditions. Can penetrate through snow, clouds, smoke, haze, etc. Also highlights different features from the visible portion of the EM spectrum. What appears “rough” in the visible could be “smooth” in the microwave. Recall: remote sensing systems can be active or passive.

RADAR Prior to becoming a word, radar was an acronym: RAdio Detection And Ranging. Radar is an active microwave sensor, which means it supplies its own energy. Radar transmits radio waves or microwaves that reflect from any object in its path. Credit: Credit: Credit: NOAA *

RADAR APPLICATIONS Radar is used on ships & airplanes to measure distance, altitude, direction and speed of moving & fixed objects. Radar is also used in weather forecasting, early warning detection and missile guidance. Radar is even used in the tracking of birds & insects, and self-driving vehicles. Credit: Volvo

SOIL RESPONSE The presence of water in soil can be detected by radar, but the water must be at the top of the soil (a few centimeters). Soil moisture usually limits the ability of radar waves to penetrate the surface. Signal penetration of several meters has been observed for extremely dry soil. Credit: Aalto University

VEGETATION RESPONSE Shorter wavelengths are best for sensing crop canopies (e.g. corn, wheat, etc.) or the top layer of a forest canopy. Longer wavelengths are best for sensing tree trunks, limb & soil. If plant canopy is dense, there will be stronger backscatter. Credit: hwww.crisp.nus.edu.sg/

PASSIVE MICROWAVE SENSORS Sensors that detect microwave radiation naturally (i.e. low energy levels) from the Earth. Clouds are poor emitters of microwave radiation, but not sea ice. Advantage: view surface day or night, regardless of clouds. Disadvantage: low energy levels require a larger coverage area for radiation collection. Credit: Matt Smith – Univ. of Alabama

MICROWAVE RADIOMETER A passive sensor. Measures microwave energy that’s being emitted at extremely low levels (sub-millimeter-to- centimeter wavelengths). Used primarily for spacecraft measuring terrestrial & atmospheric radiation, and water vapor. Credit: NASA

TYPES OF ACTIVE MICROWAVE SENSORS Non-Imaging Radar – Scatterometer – Altimeter – Doppler Radar Imaging Radar – Side-Looking Airborne Radar (SLAR) – Synthetic Aperture Radar (SAR) – Advanced Synthetic Aperture Radar (ASAR) Credit: NASA

NON-IMAGING RADAR

SCATTEROMETER A microwave radar sensor that measures the amount of energy being reflected from Earth’s surface. Can see through cloud cover. Has nearly the same imaging details as passive microwave imagery. Credit: NASA/JPL-Caltech Credit: NOAA

ALTIMETER A sensor that sends a radar pulse towards Earth and measures the time it takes to return to it. Used to measure the altitude of Earth’s surface features. Can determine the height of sea ice surfaces, which can be used to calculate total sea ice thickness. Credit: NOAA Credit: ESA

DOPPLER RADAR A specialized radar that uses the Doppler effect to acquire velocity data from objects at a given distance. Bounces microwaves off a target and analyzes how the target’s motion has impacted the returning microwaves. Credit: NOAA

IMAGING RADAR

SIDE-LOOKING AIRBORNE RADAR (SLAR) Images acquired by an antenna aimed from the side of an aircraft. Sends one pulse at a time and measures what it gets back. Since it’s an active sensor, SLAR can be used during the night or day. Credit: Credit: Clemson

SYNTHETIC APERTURE RADAR (SAR) An advanced version of SLAR. Instead of sending one pulse at a time, SAR sends multiple pulses at one time. Results in higher resolution. Can produce 2D or 3D images in day or nighttime settings. Provides complimentary information to optical systems. Credit: Sandia National Laboratory

ADVANCED SYNTHETIC APERTURE RADAR (ASAR) An upgraded version of SAR. Can see through all weather, day or night. Observational capabilities are enhanced (e.g. observing different light polarities), as are the range of measurements. Many maritime applications, including data on high ocean swells. Credit: NASA

SHUTTLE IMAGING RADAR

SIR-A (1981) The first of three space shuttle imaging radar (SIR) missions that imaged Earth using radar pulses. Furthered our understanding of geologic features with respect to how radar images them. Credit: NASA

SIR-B (1984) The second space shuttle imaging radar mission. Was the first spaceborne SAR with a tiltable antenna. This meant that radar images from multiple angles could be captured. Due to technical problems, SIR-B was active for only several hours. Credit: NASA

SIR-C (1994) The third space shuttle imaging radar mission. Found the remnants of ancient riverbeds in the Sahara desert that were buried beneath sand. Credit: NASA *

LIDAR

Stands for light-detection and ranging. Data can be collected from the ground, air or space. Air data is the most common. Green or near-infrared is used, as it is highly reflective off vegetation. Scans the Earth from side- to-side, so that it covers more ground. * Credit: Credit:

LIDAR APPLICATIONS There are a variety of lidar applications: – Agriculture – Archaeology – Forestry – Geology – Police – Robotics – Seismology – Self-Driving Vehicles * Credit: Credit:

GEOSCIENCE LASER ALTIMETER SYSTEM (GLAS) The first LIDAR instrument used for global coverage of the Earth. GLAS measures: – Ice Sheet Topography – Atmospheric Properties – Cloud Properties Credit: NASA