L4 Active Remote Sensing Flashcards
What is active remote sensing?
When the subject being measured is illuminated by an artificial energy source coming from the device that is also measuring the subject
What technology does active remote sensing use to measure changes?
Radar
What wavelength does radar measure?
Radiowaves
When was radar first used?
WW2
What is the simple equation that describes the relationship between distance and radio wave return time?
r (distance from source) = [speed of wave x return time] / 2
What is the device used to collect radar data?
Radio altimeter
What is a limitation of radio altimeter?
It does not provide image forming capabilities
What is side-looking airborne radar (SLAR)?
A plane will usually have an altimeter attached to the side at a specific angle. By combining the side-looking technology and the down-looking technology, you can start to form an image
What is the ‘swath’?
the dimensions of a beam that is doing the scanning form an altimeter
What is diffuse and specular reflection?
the form/direction of reflection back from a surface that will either be rough or smooth
With SLAR, how does the beam work/operate?
The altimeter will send out a beam at an angle which means that parts of the ground closer to the near side of the altimeter will return first. This means that data collection from one of the swath measurements will be returned over a period of time from the first to the last measurement.
How does an SLAR create a topographical image?
It combines the data collected from the ground that returns over time with the factored specular or diffusive reflection.
What are the 3 topographically generated errors that can occur when collecting SLAR data?
Layover, foreshortening, shadowing
What is the layover topographic error?
Points that are higher up than points which are lower but nearer the shorter side of the swath survey area will return to the sensor fast because they have less distance to travel down to the ground. This means the sensor may misinterpret their ground position
What is the foreshortening topographic error?
Two distances on the ground may be the same length but due to the angle of the altimeter sensor, it may distort these lengths so that they are received as different in length. This means a terrain feature with equal force on fore and back slopes may be imaged to have shorter and steeper sides than the other when in nature they are symmetrical
What is the shadowing error?
An uneven topography can create a shadow behind it that can prevent the sensor radar from hitting those parts of the ground which means that they are not accounted for
What is radar resolution?
Larger antennas can provide finer resolution measurements within the swath as they can focus on smaller parts of the ground
What equation describes the radar resolution?
Beam width = wavelength divided by antenna length
What is synthetic aperture radar (SAR) and how does it differ from normal radar?
SARs use the multiple reflective measurements taken as the plane moves over an area to construct a synthetically larger antenna for a satellite i.e. the measurements combine over an area to simulate a larger antenna
Why was SAR invented?
Because it was not feasible to increase the spatial resolution of normal radar simply by in creasing the size of the antenna on the airplane
Why does SAR result in a finer spatial resolution?
because the multiple measurements will have some overlap with each other that combine to provide greater synthetic focus on multiple points
How does the antenna know which beam it is collecting data from if it is an older/ delayed returning beam?
The doppler effect allows us to work out how far the returning wave has travelled from. This is dependant on knowledge of the wavelength that we initially sent out from the sensor.
What information goes in to the calculation for the doppler effect?
initial wavelength sent out, speed of wave sent out, doppler effect relationship, start of pulse time and location
What topographic information can be identified from using SAR?
Roughness
What information do we use to determine the roughness of a surface?
The phase shift and the return time of the radar waves
What is phase shift and how does it tell us about roughness?
Whether the wave that returns back to the sensor is at the same point as when it was sent out. If it returns at the same point then it is a smooth surface, but if it returns at a different point then this tells us that there is an element of roughness
What is InSAR?
It utilises the information in radar signals to acquire information about topography and surface deformation
How do non-imaging system measurements work?
They have a swath measurement tool that they pan over a survey line
What are non-imaging system measurements commonly applied for?
Ice penetration data or sonar depth scanning
What is sonar?
Uses soundwaves to collect data
What medium is commonly used to collect sonar data? and what does this mean for the measurements it takes?
Boats - this means measurements are a function of water depth and instrument beam angle
What is the process of interpolation?
taking known data points to estimate values inside of the ‘convex hull’
What is the process of extrapolation?
Estimation of values lying outside of the convex hull
What is the purpose of interpolation and extrapolation?
To enable simulation/modelling of what might occur
What is the inverse distance weighted method of interpolation?
Assumes that each input point has a local influence that diminishes with Euclidean distance (points closer to the processing cell are allocated a greater weight than those further away)
What are 3 pros of inverse distance weighted interpolation method?
estimation of extreme changes in terrain, dense evenly spaced points are well interpolated, can increase or decrease amount of sample points to influence cell values
What is a con of inverse distance weighted interpolation method?
cannot estimate above maximum or below minimum values so is not good for mountains
What is spline interpolation?
Estimates the values using a mathematical formula that minimises overall surface curvature, resulting in a smooth surface that passes exactly through the input points
What are 2 pros of spline interpolation method?
useful for estimating above max and below min points, smooth surface is created
What are 2 cons of spline interpolation method?
Poor representation of cliffs, not good when you have a lot of dense points that exhibit large changes in elevation
What is krigging interpolation method?
Works out the relative influence of the different known points but does this by calculating and combining the distance and degree of variation
What are 4 pros of krigging method?
directional influence accounted for, can exceed min and max point values
What are 2 cons of krigging?
does not pass through any points which causes interpolated values to be higher or lower than any real values, computationally expensive
What is natural neighbour?
Basic equation idnetical ot the one used in IDW, but uses areas of overlapping polygons to define the weights. Local coordinates define amount of influence any scatter point will have on output cells
What are 4 pros of natural neighbour?
Exact, continuous everywhere except at the data points, performs well when data is highly anisotrophic, handles large datasets well
