Radar Flashcards
Spatial Resolution
- Ground size of the smallest recording it
- Smallest area on the ground that can be seen or resolved by a sensor
Spectral Resolution
The number and dimension of electromagnetic wavelength intervals to which a remote sensing instrument is sensitive
Temporal Resolution
- Revisit frequency for a particular area on the Earth’s surface
- Dependent on swath and orbit
Radiometric Resolution
- The sensitivity of a remote sensing instrument to differences in electromagnetic energy
- Signal vs. noise
RADAR: acronym and description
- RAdio Detection And Ranging
- Active Remote sensing
- Transmitter sends pulse of microwave energy of specific frequency and polarization
- Antenna receives energy scattered back to radar (echo)
- Receiver/system records the intensity (strength of echo) and the time delay (range)
Active remote sensing
Pulse, Echo, and Range
- Echo also called Backscatter
- Similar to acoustic but with microwaves and not sound
What are the 3 classes of radars used in EO? And what are other radar types?
1 - Imaging Radar (SAR)
2 - Scatterometer
3 - Altimeter
Other - Side-Looking Airborne Radar (SLAR) used in surveillance, and meteorological radar
How many frequencies are in one band of radar?
- Radar transmits and receives one frequency i.e. band
Time delay
- clock how long it takes a signal/pulse to return (echo)
Where did the technology, and therefore the terminology for radar originate?
- Military based
- Terms non-sensical to confuse spies during war times
What are 4 of the most important bands for RS w/ Radar?
- L (1 - 2 GHz)
- C (4 - 8 GHz)
- X (8 - 12 GHz)
- Ku (12 - 18 GHz)
Single-pol
- HH, VV, HV, or VH
Dual-pol
- HH and VV, HV and HH, or VV and VH
Quad-pol
- HH and VV and HV and VH
Polarimetric
- Quad-pol and phase info
Synthetic Aperture
- Forward motion of craft, combined w/ sensor’s ability to store the location of a target enables synthesis of a larger effective aperture (i.e. antenna length) and finer resolution
- Synthesizes a larger antenna w/o having to install a really big antenna
Imaging Radar: SAR
- Can be controlled by user needs
- Instrument can be set to run in a background mode
- B/c source energy is controllable, SAR’s have selectable beam modes
Beam mode
- Vary based on spatial resolution and swath coverage
- Beam and swath and spatial resolution is inter-related, therefore can be controlled
- Controllable energy = same amount of energy but transmitted over larger/smaller area
- Vary beam so swath is smaller and spatial res increases
- Increase swath for large coverage = decrease spatial res
What is the benefit of being able to run in background mode?
- Instrument can last longer before it runs out of power
Name some examples of Beam modes on Radarsat-1
- Fine, 8m,
- Standard, 30m
- Wide, 30m
- ScanSAR Narrow, 50m
- ScanSAR Wide, 100m
- Extended High, 18-27m
- Extended Low, 30m
What is the trade-off involved w/ varying swath?
- Trade-off w/ spatial res
- High spatial res with narrow swath
- Ex. 45km swath = 8m vs. 500km swath = 100m res
What are Constellations
- Multiple identical EO satellites that are separated in orbit
- Enhances re-visit freq for instruments that otherwise have small swath
- Increases temporal res
- More efficient monitoring based on temporal resolution for change detection
- Maximizes coverage
- Not exclusive to SAR, becoming more common
Constellation example
- Sentinel-1 SAR mission
- Launch 2014
- Sentinal-1A and 1B
- 2 Satellites on one orbit plane, spaced 180 degrees apart
- 12 day orbit w/ 6 day repeat freq
- Revisit frequency 3 days at equator, <1 day at high latitudes (Europe 2 days)
Radarsat Constellation Mission (RCM)
- SARs
- Launch 2018
- 3 sats, 1 orbit plane
- Spaced 120 degrees apart
- Approx. 4 scenes/day at polar, 1/day at equatorial
Scatterometer
- Side-looking, non-imaging radar used to measure backscatter quantitatively at low resolution and over large swaths
- Spatial res = low, several km, swath = several hundred km, comparable to radiometers
- Regular data acquisition, not controllable like SAR
- BS data can be mapped to grid and visualized like an image
- Ex. Wind speed retrieval from NASA’s QuikScat satellite scatterometer
Why use scatterometers if they have such poor fixed spatial res?
- Ability to form Earth surface images w/ increased temp res b/c of large swath like a radiometer but w/ active tech
- Good for large areas
Radar Altimeters
- Precise nadir measurements of distance from the satellite to the surface from time delay
- Infer topography
- Know height of sensor and time of signal to calc elev at surface
- Ex. Ice-sheet height change over time
3-polarization colour overlay
- HH = Red
- HV = Green
- VV = Blue
- Overlay and different polar combos highlights different features
Which platforms can provide distance baed on the time it takes a signal to return?
- SAR
- Scatterometers
What is the overarching goal of EO?
- Development of relationship btwn image measurements and geophysically useful information
- Ex. Forest biomass, SWE, Soil moisture, Sea-ice type
What needs to be considered when using Radar?
- What measurements radar provides
- What geophysical parameters these measurements can provide
- Whether they can be provided by radar alone, in combination w/ other radars or other sensor types such as optical, or other data sources
What are the simplest things SARs and scatterometers provide info about? How?
- Structure
- Roughness and Texture
- Wetness
- Distance (range) info
- Mainly by providing BS intensity and polarization information
Scattering Mechanisms
- Microwave interactions w/ Earth materials and BS are described by scattering mechanisms
- Surface (specular, water)
- Double Bounce (2 objects, very bright return)
- Volume (snow, veg)
What are scattering mechanisms and BS affected by?
- System Parameters (Incidence angle, resolution, frequency, polarization)
- Target Parameters (Target geometry, surface roughness, electrical properties i.e. moisture)
