Active RS

What range is the microwave portion of EMS

1mm to 1m

What does radar stand for and what does it transmit?

(radio detection and ranging); transmits microwaves

Describe Transmitter (Imaging Radar System

Generates high power microwave/radio frequencies and pulses and transmits pulses at given frequency

Describe Antenna array (Imaging Radar System)

• Transmits narrow beam of microwave energy

• Can transmit and receive echoed signal

Describe Receiver (Imaging Radar System)

Accepts reflected signal from antenna

Filters and amplifies as needed

Describe Recorder/signal processing (Imaging Radar System)

records or displays the signal of an image

analyzes the time it took for the signal to return and the change in frequency

Active remote sensing characteristics (7)

• All/near weather bc radar penetrates through clouds

• Synoptic view/good coverage

• Day/night capability

• Clear terrain and drainage views

• Good positional accuracy

• Aircraft or satellite

• Side-looking capability

Heinrich Hertz

studied propagation and properties in microwave/radio EMS portions

AH Taylor and LC Young

Significant discoveries of radio usage in marine navigation, inclement weather and military contexts

Radars conceived in 1920s with what discovery?

When radio waves found deflected from ships, aircrafts, and other objects

What 3 countries were leaders in radar systems?

US, UK, Germany

What is ground clutter and why was it a problem in WW2

Ground clutter aka backscatter was a problem for navigational radars

What happened with RS tech between WW2 and Cold War

Radar systems (Imaging radars) designed specifically to observe Earth’s surface

Describe Active RS

• Time delay between the time a signal is transmitted toward the

terrain and the time its echo is received

• Accurately measure the distance from the antenna to features on

the ground

• Sensor transmit at signal of known wavelength, it is possible to

compare the received signal with transmitted signal

Near vs far range of imaging radar

• Far range have more shallow depression angles than near range

  • Shadow depression angle leads to larger radar shadow bc can't reach other side of sloped feature

Side-looking airborne radar

Microwaves used for SLAR imagery characterized by WL l0ng enough to escape cloud interference

What is Radar shadow

occurs when signal can't reach other side of sloped feature

Radar Layover

• Geometric error when radar beam reaches top of tall feature before it reaches base

• Occurs when object is so tall that the returns from the tall object are placed on the image closer to the sensor (near range) while obscuring the other point  as if the top is overlayed on foot of mountain

• Remember Narendra Modi Stadium example when features overlaying parking lot

Radar foreshortening

Occurs when the radar beam reaches the base of a tall feature tilted towards the radar (e.g. a mountain or A in the figure) before it reaches the top

Think of isosceles triangle example, 2 sides the same length, but radar reaching tip first makes the sides appear unequal

Longest WL band of Radar Freq designation

P band (107-77cm)

Shortest WL band of radar freq designation

Ka band (1.18-0.75 cm)

Spatial resolution (Improves or declines) as wavelength becomes (longer/shorter) with respect to antenna length? With what system?

Improves/Shorter; Real aperture systems

Can radar penetrate vegetative cover or soil; which wavelengths best for soil?

yes; longer WL best for soil moisture conditions

Without moisture, skin/penetration depth increases with what wavelength

increasing

Penetration of wavelengths is greatest at what slopes and range edges?

Steeper angles and near-range edge

What is polarization

The orientation of the field of electromagnetic energy emitted and received by the antenna

Polarization: HH or VV “like-polarized”

horizontal transmit, horizontal return

Polarization: HV or VH “cross-polarized”

horizontal transmit, vertical return

Polarization: VV

Vertical transmit, Vertical receive

Polarization: VH

Vertical transmit, Horizontal receive

Multi-polarization

Comparison of the two images to identify features and areas that tend to

depolarize the signal

What is a depolarized surface?

Horizontally polarized signal back to the antenna as vertically polarized energy

Sources of depolarization

Moisture, rough surfaces, volume scattering (inhomogeneous surface e.g. subsurface inhomogeneity)

Effects of polarization on imagery (HV vs HH)

Brighter in the HV image and darker in the HH image

Look Direction to maximize radar shadow?

• Look directions perpendicular to topographic alignment (orientation) will tend to maximize radar shadow

Look Angle at steeper depression angles, spatial resolution, describe slant range

• At steeper depression angles, a radar signal illuminates a smaller area than does the same signal at shallow depression angle

• Spatial resolution varies with respect to depression angle

• Slant range: the way how we view sides

Why could a radar shadow be ideal

In areas of small relief, radar shadows may be desirable as a means of enhancing micro topography

Describe Real Aperture Systems and their processes

• AKA brute force systems

• Oldest, simplest, and least expensive of imaging radar systems

• Follow the general mode: transmitter generates the signal; antenna directs this signal and receives reflection; reflected signal is amplifies and filtered; moving film records the radar image line by line

Along-track (range) resolution

• helps us to know how well Radar separates objects in the direction of motion

Across-track (range) resolution

helps us to know how well Radar separates objects at different distances

Describe Synthetic Aperture Radar (SAR)

• SAR operates on the principle that objects within a scene are illuminated by the radar over an interval of time

• Receives the signal scattered from the landscape during this interval and saves the complete history of reflections (backscatter) from each object

• Reconstruction of the signals to form an image

Three classes of surfaces

Diffuse: rough, relative to wavelength

Specular: smooth, relative to wavelength

Corner reflectors: scattering from complex, geometric surfaces. Bigger sizes than the actual object

Satellite Imaging Radars

SeasatSAR: Ocean waves, Sea ice, Coastlines, Wind/Waves/Topography

Shuttle Imaging Radar (SIR)-A/B/C

RADARSAT SAR (all weather)

ERS-1 SAR

Interferometric SAR

• Employs pairs of high resolution SAR images to generate high quality terrain elevation maps using phase interferometry methods

• Two or more SAR images of the same region acquired from different positions

• By using the phase of the returned waveform, this technique can measure centimeter-scale (or possibly millimeter-scale) changes in land surface height

Shuttle Radar Topographic Mission

Short-term mission with extremely high levels of accuracy