GEOG 271 Lecture 7-11 Notes PDF

GEOG 271 Lecture 7-11 Notes Lecture 7: Passive Microwave Remote Sensing + Applications Microwave Radiometry ○ All natural surfaces emit microwave energy ○ PM sensors detect this (and atmosphere's) at different frequencies, and convert this to brightness temperature (TB...

GEOG 271 Lecture 7-11 Notes Lecture 7: Passive Microwave Remote Sensing + Applications Microwave Radiometry ○ All natural surfaces emit microwave energy ○ PM sensors detect this (and atmosphere's) at different frequencies, and convert this to brightness temperature (TB) ○ TB = A descriptive measurement of radiation of a hypothetical blackbody which emits an identical amount of radiation at the same wavelength. Why use Passive Microwave Remote Sensing ○ Here are some advantages that microwave remote sensing has over different parts of the EMS. They penetrate clouds, and therefore can be used as a measurement tool for any type of weather. Microwaves at low frequencies partially penetrate vegetation, and thus can be used for soil-moisture measurement on vegetated areas They partially penetrate soil surfaces, and can be used to detect soil dept since emitted signatures contain information regarding the depth of the penetrated soil They are partially absorbed by vegetation, so this can be used to collect properties based on vegetation due to emitted signatures having information regarding the properties of vegetation. They are independent of solar radiation, so they can be used during both the day and night. ○ Here are some key disadvantages of passive microwave remote sensing They have larger instantaneous field of view (5 to 50 km + of spatial resolution) in comparison to visible or active microwave sensors  Emissivity changes without knowledge of the in situ measurements (emissivity is not consistent for ground targets on the ground within an individual pixel) Polar orbiting satellites provide discontinuous temporal coverage of equatorial regions (can be an issue for weather observation, as weekly composites need to be created) Popular Satellites Spectral Sensor Microwave Imagery (SSM/I) Advanced Microwave Scanning Radiometer (AMSR, AMSR2) Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) Surface Interactions: Bare Soil ○ This shows the relationship between emissivity and volumetric water content Emissivity decreases as volumetric soil moisture increases  ○ ○ Brightness temperature curves follow the same pattern as the emissivity curve due to the relationship that soil moisture has on emissivity ○ Temperature of soil increase/decreased relative TB, but has no effect on curve due to soil moisture ○ ○  ○ Week 8: Active Microwave Signatures I Passive vs Active Systems Passive ○ Relies on the electromagnetic energy reflected or emitted from an external source (Landsat -> Sun) Active ○ Sensor emits its own source of EM radition This follows the process: Sensor transmits energy towards target Energy interacts with target, producing ‘backscatter Energy returns to sensor and is measured Radio Detection And Ranging A device that measures the distance to an object Consists of a transmitter, a receiver, antenna, and electronic systems Transmitter generates pulses of radiation (A), focused by the antenna onto a beam (B) ○ Wavelengths range from 1 cm to 1 m Measures the amount of returned energy (C) (intensity), and the delay or range  ○ Backscatter: portion of transmitted signal that returns from a target to the antenna Influence of Wavelength This image shows the effect of acquisitions made with a longer or shorter wavelength. Image is of a downpour of rain in Rondonia Brazilian rainforest (April 10, 1994) ○ X Band: 3.1 cm 10 GHz ○ C-Band: (5.65 cm 10 GHz) ○ L-Band: (22.5 cm 1.5 GHz) Scattering/absorption happens when the wavelengths of incident EMR approximates the size or scatters of targets The observed variation of backscatter (returned energy) is dependent on wavelength ○ Shorter wavelengths are influenced by presence of water droplets in this example (X-Band > C-Band > L-Band) Polarization of Signals Polarization = orientation of emitted energy from RADAR Systems Influences ○ The types of interaction which occur at different targets ○ The amount of backscatter (returned radiation) Filters are used for sensors to transmit as either horizontal or vertical.  These transmitted signals will interact with targets and return to the sensor, if filters allow for the received energy through this is recorded. Types of Polarizations If system transmits and receive the same polarization (HH, VV) it is referred to as co-polarized If system transmits and receives opposite polarizations it is termed cross-polarization There are some system such as RADARSAT2 which are able to be set to get transmit and receive all combinations which are (HH + HV + VH + VV), this is termed quad-polarized Geometry of a RADAR Scene Azimuth Direction describes the direction that the object used for sensing is going (aka flight line) Look/Range Direction: describes the pulses of energy being emitted at the right angle of the azimuth direction Depression Angle: This describes the angle between the horizontal plane of the sensor and the image being studied Look Angle: Is equal to the depression angle - 90 degrees, and describes the angle between the vertical plane of the sensor and target Primary Advantages of RADAR Has low frequencies and long wavelengths ○ This allows for RADAR to be able to penetrate clouds, which furthers its versatility as it can be used in all-weather conditions Operates at user-specified times (day or night) Allow for shallow look-angles, which results in differing perspectives of the same target, which cannot be completed by other sensors Senses outside visible/infrared, which can be useful when looking the the surface roughness, dielectric properties, and/or moisture content Allows for Synoptic Views of large areas (mapping cloud-shrouded countries) Secondary Advantages of RADAR Is able to penetrate through vegetation, snow and soil, so it can be used to measure snow depth, and soil depth. It has its own source of illumination (light), so images will be clear even when penetrating snow, vegetation and soil. Furthermore its angle of illumination can be controlled, which allows for a greater freedom when taking images. It allow for the resolution to be independent of the distance to the object being captured ○ The spatial resolution of Synthetic Aperture RADAR can actually be 1 x 1 meter Images taken use different types of polarized energy It may operate simultaneously at different wavelengths (frequencies) Difference between Range and Azimuth Resolution Range Resolution ○ Depression angle has a known range ○ Uses ranges between towers for calculations related to pulse length Towers 1 and 2 are not resolved, while towers 3 and 4 are resolved Uses distances between towers 1&2 and between towers 3 & 4 to calculate pulse length Azimuth Resolution ○ Depression angle is unknown ○ Uses distances between takes for calculations, and operates on a triangular plane instead of a straight line Tanks 1 and 2 are resolved, while tanks 3-4 are not resolved Uses distance between tanks 1 and 2 and between tanks 3 and 4 for calculations (tanks 1 & 2 are placed parallel to each other on the triangular place, the same thing goes for tanks 3 and 4) Active Microwave Signatures II Radar Backscatter Coefficient ◦ ○ Denoted as σ ○ Illustrates the effects of terrain on RADAR/SAR signals ○ Radar backscatter coefficient = RADAR cross-section that is reflected back to the receiver, per unit area of ground ○ Determines the amount of energy that is reflected back to the RADAR within a cell. ○ Can be affected by Certain terrain parameters including: surface roughness and moisture content RADAR System Parameters including: Wavelength, depression angle and/or polarization RADAR Target Interactions: Viewing and Target Geometry Local surface orientation has a large effect on backscatter ○ Is either darker with larger incidence angles or brighter with smaller incidence angles RADAR Target Interactions: Viewing and Target Geometry Look direction has an influence on RADAR imagery Since RADAR is an all-weather satellite ○ It does not operation on a sun-synchronous orbit  ○ Images that are produced from either ascending or descending passes will be different, so you need to understand the differences when looking at imagery. ○ RADAR Target Interactions: Viewing Geometry Geometric distions exist within the realm of RADAR remote sensing where topographical relief exists This includes ○ Foreshortening Occurs when slopes facing the RADAR sensor appear compressed due to the side-geometry of Synthetic Aperture RADAR. Happens because distances are represented as shorter than they actually are on inclined surfaces. Foreshortening allow for a compression effect to take place, rather than some information loss to take place which occurs during shadowing, and overlapping signals to occur during layovering ○ Layover Occurs when signals from the top of a tall feature or slope returns back to the sensor before the signal from its base, which cause images to appear flipped, or overlapped Is more extreme compared to foreshortening. Furthermore, signal overlap occurs with this, while  foreshortening involves compressing distances without any overlap, while shadowing involves results showcasing data gaps ○ Shadowing Occurs when RADAR beams does not successfully illuminate the terrain, thus causing shadows to appear within the image, and causes for those areas to not have any recorded signals. Happens when slopes are facing away from the RADAR sensor Results in dark areas being shown, while foreshortening or layovers cause distortion or overlapping RADAR Target Interactions: Roughness Understanding surface roughness is another key concept to understand when using RADAR Remote Sensing Smooth surfaces act as specular reflectors while rough surfaces act as diffuse reflectors. This means that smooth surfaces reflect energy  away from the sensor, while rough surfaces cause for energy to reflect towards the sensor The smoothness of a surface is influenced by wavelengths and viewing angle (incidence angle) RADAR Target Interactions: Relative Permittivty (ElectricalProperties) Permittivity describes the ability of molecules to become polarized in cases where an electric field (microwave) is applied to it. Complex permittivity: this describes the ability for a medium to reflect, absorb, and transmit microwave energy * * ' " ○ Calculated using this formula: ε (ε = ε * 𝑗ε ) ○ When moisture is present, permittivity increases, and when moisture decreases, permittivity decreases ○ When there is an increase in permittivity, there is also an decrease in penetration in the microwave spectrum, and when there is a decrease in permittivity, there is an increase in penetration in the microwave spectrum RADAR Target Interactions: Scattering Mechanisms There are three different mechanisms ○ Surface scattering Associated with flat surfaces like highways, as there is nowhere double bounce to take place, and has flat surfaces which make reflection simple. ○ Double Bounce Scattering Associated with urban building as there is presence of angular reflection and urban buildings have high permittivity ○ Volume Scattering Associated with snow, as snow has low permittivity Week 10: Active Microwave Platforms & Applications Microwave Wavelengths/Bands What impacts do different bands have on RADAR observations? ○ Examples of Spaceborne Synthetic Aperture RADAR Satellites ○ RADARSAT Constellation ○ TerraSAR-X ○ Sentinel-1 Applications ○ Fire Disturbance ○ Arctic Sea Ice Extent ○ European Alps Week 11: Other Remote Sensing Platforms & Topics Hyperspectral Remote Sensing Captures images from a wide range of the EMS. Technology divides light into hundreds of contiguous spatial bands, which typically range from visible to infrared wavelengths. Key characteristics ○ High Spatial Resolution: Hyperspectral sensors collect data in many narrow wavelengths usually between 5-10 nm ○ Contiguous Bands: Spatial bands are continuous and cover a large area of the EMS with no gaps ○ Data Cube: Hyperspectral data is illustrated as a 3D cube, made of 2 spatial dimensions and 1 spectral dimension Fundamental Principles ○ Spectral Signature: Different materials on Earth absorb, reflect, and emit light in unique ways, which create spectral fingerprints  ○ Detailed Analysis: High spatial resolution allows for precise identification and differentiation of materials, even if they appear small in conventional imagery Applications ○ Environmental Monitoring ○ Mineral Exploration ○ Agriculture and Forestry ○ Urban Planning Comparison to multispectral remote sensing ○ While multispectral remote sensing allows for the use of several discrete bands, hyperspectral remote sensing uses continuous bands for each pixel, allowing for more detailed analysis to be performed, and have the ability for more accurate material identification to be done. LiDAR Stands for Light Detection and Ranging, and is a type of active remote sensing, meaning that it gets energy from the sun, and uses laser lights to be able to measure distances and create detailed 3D representations of the Earth's surface and objects around it. Key Principles ○ Pulse Emission: Uses many rapid pulses of laser light towards a target ○ Time Measurement: The amount of time it takes for each pulse to return back to the sensor is recorded ○ Distance Calculation: Using the speed of light, distance to the specified target can be calculated using information regarding the travel time. ○ Point Cloud Generation: Millions of measurements are used to generate dense point clouds, which represents the 3D structure of the scanned area Applications ○ Topographic Mapping: LiDAR can help create high resolution digital elevation models ○ Forestry: Measuring tree heights, canopy structure, and biomass estimation  ○ Urban Planning: 3D Models of city can be created using LiDAR Remote Sensing GNSS-R Uses reflected signals from GNSS satellites to gather information regarding the surface of the Earth Key Principles ○ Signal Reflection: It exploits the GNSS signals which are reflected off the Earth's surface before reaching the receiver ○ Bistatic Radar: Operates a bistatic radar system, using assistance from GNSS satellites, which act as transmitters and specialized receivers on the ground, on aircrafts, or on satellites ○ Delay-Doppler Maps: Reflected Signals are processed to create delay-doppler maps that contain information regarding the reflecting surface Applications ○ Ocean Surface Monitoring: Measuring seas surface height, wind speed and direction ○ Soil Moisture Estimation: Assessing Moisture content in the top layer of soil ○ Ice and Snow Detection: Monitoring Ice Sheets and Snow Cover Altimetry Basics ○ RADAR altimeters are NADIR looking pulse radars ○ Measures distances between satellites and surfaces ○ Obtains waveform data which show power in response to time Applications ○ Sea Level Height Drones Surged over the last 20 years, and drones are becoming more and more compatible with more and more sensors, it cannot replace traditional remote sensing techniques Advantages ○ Offers flexibility in terms of monitoring ○ Ease of use  ○ High Quality/ Spatial Resolution ○ Some flexibility for sensors Applications ○ Precision Agriculture Machine Learning for Remote Sensing Branch of artificial intelligence and computer science which focuses on the use of data and algorithms to mimic the way humans learn, which gradually improves its accuracy Examples ○ Support Vector Machine ○ Random Forest ○ Linear Regression