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Dr. GarverGEO 420
Sensors
Sensors
So far we have discussed the nature and
properties of electromagnetic radiation
Sensors - gather and process information
detect and measure photons.
Most air/space sensors are spectroradiometers
The term spectroradiometer is reserved for sensors
that collect the dispersed radiation in bands rather
than discrete wavelengths. Spectroradiometry is the measurement of absolute radiometric quantities in narrow bands of
wavelength
All sensors reside on a platform
Ground based sensors are used to compare with info collected by satellite sensors.
Sensor Technology
EMR is reflected or emitted from
target, through atmosphere,
monitored by sensor.
Sensors measure photons.
Critical component - the detector.
Photoelectric effect (Albert Einstein)
The release of electrons that occurs when electromagnetic
radiation comes in contact with a metal.
Plate
EMR
Photoelectric effect
electrons
Signal
Photoelectric effect (Albert Einstein)
Emission of electrons when a negatively
charged plate of light-sensitive material
is subjected to a beam of photons.
Electrons flow from plate, collected, and
counted as a signal.
Magnitude of electric current is
proportional to light intensity.
In 20th century physics two ideas stand out as being totally revolutionary: Relativity and quantum theory.
Although Einstein is best known for his theory of relativity, he also played a major role in developing quantum theory.
His contribution to quantum theory - explaining the photoelectric effect
Nobel Prize in 1921.
Theory of relativityA theory developed by Albert Einstein which says that the way that anything except light moves through time and space depends on the position and movement of someone who is watching
Quantum theoryA theory in physics based on the principle that matter and energy have the properties of both particles and waves, created to explain the radiation of energy from a blackbody, the photoelectric effect, and the Bohr theory, and now used to account for a wide range of physical phenomena, including the existence of discrete packets of energy and matter, the uncertainty principle, and the exclusion principle.
photoelectric effectThe emission of electrons from a material,
such as a metal, as a result of being struck
by photons. Some substances, such as
selenium, are particularly susceptible to
this effect. The photoelectric effect is used
in photoelectric and solar cells to create an
electric potential.
Radiometer is a general term for any instrument that quantitatively measures EMR.
Most sensors are spectroradiometers. radiation collected in narrow spectral
bands.
Prism or diffraction grating - breaks radiation into discrete wavelengths.
SENSOR SYSTEMSPlatforms - Ground based ,Airborne , Satellite
Sensor Types
Passive, active
Imaging, nonimagingPassive Sensors
Photographic
spectroradiometers Passive microwave systems
Visible, infrared, and thermal imaging systems
Active Sensors - Radar, Lidar
Two classes of sensors Passive - radiation received comes
from external source, Sun.
Active - energy generated from
within sensor system, beamed
outward, and fraction returned is
measured.
Passive Sensors
Sun provides source of energy
reflected (vis, near IR)
absorbed and re-emitted (thermal IR)
Passive sensors can only be used to detect
energy when the sun is illuminating the Earth.
thermal infrared - detected day or night.
Active sensorssensor emits radiation which is directed
toward target.
radiation reflected from target is detected
and measured by sensor.
Active sensorsAdvantage
measurements anytime, regardless of time
of day or season.
can be used for examining wavelengths not
sufficiently provided by the sun, such as
microwaves.
non-imaging - measures radiation and
reports result as electrical signal
imaging - electrons released are used to
excite or ionize a substance like silver
(Ag) in film or to drive an image producing
device like a TV or computer monitor.
Sensors can be
Orbits
path followed by a satellite
orbits are matched to the objective
of the sensor(s) they carry.
geostationary orbits
Satellites at very high altitudes
views same portion of Earth's
surface at all times
revolves at speed which matches rotation of
Earth
observes and collects information
continuously over specific areas
weather & communications satellites
near-polar orbits
north-south
covers most of Earth's surface over a
period of time.
sun-synchronous
covers each area at the same time of day
consistent illumination conditions
important factor for monitoring changes between
images or for mosaicking adjacent images together
Most satellite platforms today are in near-polar orbits
Swath
Area imaged on the
ground
Imaging swaths for
different sensors vary
from tens and hundreds of
km wide.
Earth is rotating (from west to east).
satellite swath covers new area with
each consecutive pass.
Allows complete coverage of Earth's
surface.
Cross Track - oscillating mirror sweeps
along a line that is long (km) narrow (m)
Whiskbroom
Scanning
Along Track mode - line of detectors
stacked side by side.
Pushbroom scanning
4 types of resolution
1. Spatial Resolution
The detail discernible in an image is
dependent on the spatial resolution
of the sensor.
Spatial Resolution
Pixel size of satellite images
High spatial resolution: 0.5 - 4 m
Medium spatial resolution: 4 - 30 m
Low spatial resolution: 30 - > 1000 m
Landsat spatial resolution = 30m
2. Temporal Resolution
Specifies the revisiting frequency of a satellite
sensor for a specific location.
High temporal resolution: < 24 hours - 3 days
Medium temporal resolution: 4 - 16 days
Low temporal resolution: > 16 days
Landsat = 16 days
3. Spectral Resolution
Number and position of bands in the
electromagnetic spectrum that the sensor
measures.
High spectral resolution: - 220 bands
Medium spectral resolution: 3 - 15 bands
Low spectral resolution: - 3 bands
Landsat = 7 bands
3. Spectral Resolution
4. Radiometric Resolution
The actual information content in an image.
The sensitivity of the sensor to the magnitude of
electromagnetic energy determines the
radiometric resolution
refers to the smallest change in intensity level
that can be detected by the sensing system.
4. Radiometric Resolution
In a digital image, the radiometric resolution is
limited by the number of discrete levels used to
digitize the continuous intensity value.
Digital Number (DN) - each pixel has a
discrete value made by converting the
analog signal to digital values of whole
numbers over a finite range.
Landsat system range is 28, 0 to 255
Landsat Thematic Mapper
7 channel sensor mounted on the Landsat platform
sun-synchronous, near-polar orbit
altitude 705 km.
16 day repeat coverage
30 m ground resolution across a swath of 185 km
except for thermal data -120 m ground resolution.
7 Bands - Landsat
Landsat Thematic Mapper Bands
BLUE (0.45-0.52 µm): water body penetration,
coastal water mapping, soil/vegetation
discrimination, forest type mapping, cultural feature
identification.
GREEN (0.52-0.60 µm): green reflectance peak of
veg. for discrimination and vigor assessment,
cultural feature identification.
Landsat Thematic Mapper
RED (0.63-0.69 µm): chlorophyll absorption region
aiding in plant species differentiation, cultural feature
identification.
NEAR INFRARED (0.76-0.90 µm): determining
vegetation types, vigor, and biomass content,
delineating water bodies, soil moisture
discrimination.
Landsat Thematic Mapper
MID-INFRARED (1.55-1.75 µm): vegetation
moisture content and soil moisture, differentiation of
snow from clouds.
MID-INFRARED (2.08-2.35 µm): discrimination of
mineral and rock types, vegetation moisture content.
THERMAL INFRARED (10.4-12.5 µm): vegetation
stress analysis, soil moisture discrimination, and
thermal mapping applications.
Recommended