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.