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
