L4 Active Remote Sensing

Question 1

What is active remote sensing?

Answer 1

When the subject being measured is illuminated by an artificial energy source coming from the device that is also measuring the subject

Question 2

What technology does active remote sensing use to measure changes?

Answer 2

Radar

Question 3

What wavelength does radar measure?

Answer 3

Radiowaves

Question 4

When was radar first used?

Answer 4

WW2

Question 5

What is the simple equation that describes the relationship between distance and radio wave return time?

Answer 5

r (distance from source) = [speed of wave x return time] / 2

Question 6

What is the device used to collect radar data?

Answer 6

Radio altimeter

Question 7

What is a limitation of radio altimeter?

Answer 7

It does not provide image forming capabilities

Question 8

What is side-looking airborne radar (SLAR)?

Answer 8

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

Question 9

What is the ‘swath’?

Answer 9

the dimensions of a beam that is doing the scanning form an altimeter

Question 10

What is diffuse and specular reflection?

Answer 10

the form/direction of reflection back from a surface that will either be rough or smooth

Question 11

With SLAR, how does the beam work/operate?

Answer 11

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.

Question 12

How does an SLAR create a topographical image?

Answer 12

It combines the data collected from the ground that returns over time with the factored specular or diffusive reflection.

Question 13

What are the 3 topographically generated errors that can occur when collecting SLAR data?

Answer 13

Layover, foreshortening, shadowing

Question 14

What is the layover topographic error?

Answer 14

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

Question 15

What is the foreshortening topographic error?

Answer 15

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

Question 16

What is the shadowing error?

Answer 16

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

Question 17

What is radar resolution?

Answer 17

Larger antennas can provide finer resolution measurements within the swath as they can focus on smaller parts of the ground

Question 18

What equation describes the radar resolution?

Answer 18

Beam width = wavelength divided by antenna length

Question 19

What is synthetic aperture radar (SAR) and how does it differ from normal radar?

Answer 19

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

Question 20

Why was SAR invented?

Answer 20

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

Question 21

Why does SAR result in a finer spatial resolution?

Answer 21

because the multiple measurements will have some overlap with each other that combine to provide greater synthetic focus on multiple points

Question 22

How does the antenna know which beam it is collecting data from if it is an older/ delayed returning beam?

Answer 22

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.

Question 23

What information goes in to the calculation for the doppler effect?

Answer 23

initial wavelength sent out, speed of wave sent out, doppler effect relationship, start of pulse time and location

Question 24

What topographic information can be identified from using SAR?

Answer 24

Roughness

Question 25

What information do we use to determine the roughness of a surface?

Answer 25

The phase shift and the return time of the radar waves

Question 26

What is phase shift and how does it tell us about roughness?

Answer 26

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

Question 27

What is InSAR?

Answer 27

It utilises the information in radar signals to acquire information about topography and surface deformation

Question 28

How do non-imaging system measurements work?

Answer 28

They have a swath measurement tool that they pan over a survey line

Question 29

What are non-imaging system measurements commonly applied for?

Answer 29

Ice penetration data or sonar depth scanning

Question 30

What is sonar?

Answer 30

Uses soundwaves to collect data

Question 31

What medium is commonly used to collect sonar data? and what does this mean for the measurements it takes?

Answer 31

Boats - this means measurements are a function of water depth and instrument beam angle

Question 32

What is the process of interpolation?

Answer 32

taking known data points to estimate values inside of the ‘convex hull’

Question 33

What is the process of extrapolation?

Answer 33

Estimation of values lying outside of the convex hull

Question 34

What is the purpose of interpolation and extrapolation?

Answer 34

To enable simulation/modelling of what might occur

Question 35

What is the inverse distance weighted method of interpolation?

Answer 35

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)

Question 36

What are 3 pros of inverse distance weighted interpolation method?

Answer 36

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

Question 37

What is a con of inverse distance weighted interpolation method?

Answer 37

cannot estimate above maximum or below minimum values so is not good for mountains

Question 38

What is spline interpolation?

Answer 38

Estimates the values using a mathematical formula that minimises overall surface curvature, resulting in a smooth surface that passes exactly through the input points

Question 39

What are 2 pros of spline interpolation method?

Answer 39

useful for estimating above max and below min points, smooth surface is created

Question 40

What are 2 cons of spline interpolation method?

Answer 40

Poor representation of cliffs, not good when you have a lot of dense points that exhibit large changes in elevation

Question 41

What is krigging interpolation method?

Answer 41

Works out the relative influence of the different known points but does this by calculating and combining the distance and degree of variation

Question 42

What are 4 pros of krigging method?

Answer 42

directional influence accounted for, can exceed min and max point values

Question 43

What are 2 cons of krigging?

Answer 43

does not pass through any points which causes interpolated values to be higher or lower than any real values, computationally expensive

Question 44

What is natural neighbour?

Answer 44

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

Question 45

What are 4 pros of natural neighbour?

Answer 45

Exact, continuous everywhere except at the data points, performs well when data is highly anisotrophic, handles large datasets well