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Lecture 2 Remote Sensing: Radiation Theory and Solar Radiation

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Title: Lecture 2 EMS Author: menglin Last modified by: menglin Created Date: 10/27/2009 2:43:46 AM Document presentation format: On-screen Show Company – PowerPoint PPT presentation

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Title: Lecture 2 Remote Sensing: Radiation Theory and Solar Radiation


1
Lecture 2 Remote Sensing Radiation Theory and
Solar Radiation
  • Professor Menglin S. Jin
  • Department of Meteorology and Climate Science
  • San Jose State University

2
This is to review
  • How much energy is emitted by some medium?
  • What kind of energy (what frequency/wavelength)
    is emitted by some medium?
  • What happens to radiation (energy) as it travels
    from the target (e.g., ground, cloud...) to the
    satellites sensor?

3
Brief history
  • Since the 1960s, most remote sensing has been
    conducted from satellites
  • Prior to that remote sensing is associated mainly
    with aerial photography, using cameras mounted in
    aircraft that fly at various altitudes (with
    scale emcompassed)
  • Aircraft remote sensing continues through today
    but is usually directed towards specific tasks
    and missions.

4
"Remote" and "Proximal" Sensing
  • Remote sensing involves making measurements and
    collecting data for (and from) objects, classes,
    and materials that are not in contact with the
    sensor (sensing device) whereas the Proximal
    sensing includes making direct contact with these
    targets

5
if the objective is to measure a person's bodily
temperature
  • the proximate approach would be
  • the remote approach would be

to place a thermometer in or on the body
to hold a radiometer sensitive to thermal energy
at some distance from the body
  • Both need Calibrated

its response as a sensor must be transformable
into a good approximation of the actual
temperature by determining the response using a
target whose temperature range is specifically
known.
6

7
Passive and Active Remote Sensors
  • Remote sensing systems which measure energy that
    is naturally available are called Passive
    Sensors. (Sun, surface emission, etc)
  • Active sensors, on the other hand, transmit short
    bursts or 'pulses' of electromagnetic energy in
    the direction of interest and record the origin
    and strength of the backscatter received from
    objects within the system's field of view.
    Passive systems sense low level microwave
    radiation given off by all objects in the natural
    environment.

8
Example of passive and active remote sensing

In this figure, find out passive and active
remote sensing environment
9
diagram for remote sensing for surafce (1) solar
radiation
10
diagram for remote sensing for (2) longwave
emission
11
Electromagnetic Spectrum
  • Electromagnetic radiation can be described in
    terms of a stream of photons, which are massless
    particles each traveling in a wave-like pattern
    and moving at the speed of light. Each photon
    contains a certain amount (or bundle) of energy,
    and all electromagnetic radiation consists of
    these photons.

12
A photon has no mass, but its energy is E h?
E h? hc/? (since c ??)
  • h Plancks constant, which is a VERY small
    number(6.63 x 10-34 in our units).

13
Electromagnetic Spectrum
  • Remote sensing relies on measurements in the
  • electromagnetic spectrum (except sonar)
  • Remote sensing of the ground from space
  • Need to see through the atmosphere
  • The ground must have some feature of interest
    in that spectral region
  • Studying reflected light requires a spectral
    region where solar energy dominates
  • Radar approaches mean we need frequencies that we
    can generate
  • Also need to ensure that we are not affected
    by other radio sources
  • Atmosphere should be transparent at the
    selected frequency

14
(No Transcript)
15
  • The wavelengths we are most interested in for
    climatology and meteorology are between 0.01 and
    100 µm

16
The Wavelength of Visible Light The typical unit
of measurement for ? is the Ångstrom (Å) 10-10
m 0.1 nanometer Red 6500 Å Yellow 5800
Å Green 5300 Å Blue 4800 Å
17
Units (in case you need)
  • The hertz (symbol Hz) is the SI unit of frequency
    defined as the number of cycles per second of a
    periodic phenomenon
  • One hertz simply means "one cycle per second"
  • 1 micrometer 1 000 nanometers 1 µm 1 000 nm
  • 1 µm 10-6 meter

kHz (kilohertz, 103 Hz) MHz (megahertz,
106 Hz) GHz (gigahertz, 109 Hz) THz (terahertz,
1012 Hz)
18
Class activity
  • PM frequency is 860x106/s what is its wavelength
    in light? (light speed is 3x108m/s,
    nmnanometer10-9m)
  • Sound wave is v 343 m/s, what is its wavelength
    in sound wave?

19
Need to know
  • Solar constant
  • solar radiaiton at TOA
  • TOA radiation budget
  • Basic definition

20
Measuring energy (Important!)
  • Radiant energy Total energy emitted in all
    directions (J)
  • Radiant flux Total energy radiated in all
    directions per unit time (W J/s)
  • Irradiance (radiant flux density) Total energy
    radiated onto (or from) a unit area in a unit
    time (W m-2)
  • Radiance Irradiance within a given angle of
    observation (W m-2 sr-1)
  • Spectral radiance Radiance for range in ?

21
Radiance
Toward satellite
Normal to surface
Solid angle, measured in steradians (1 sphere
4? sr 12.57 sr)
Radiance is what satellite sensor can measure,
but in specific wavelength
22
Blackbody radiation
  • Examine relationships between temperature,
    wavelength and energy emitted
  • Blackbody A perfect emitter and absorber of
    radiation... does not exist

23
Stefan-Boltzmann Law
M BB ??T 4
Total irradiance emitted by a blackbody (sometimes
indicated as E)
Stefan-Boltzmann constant
The amount of radiation emitted by a blackbody is
proportional to the fourth power of its
temperature Sun is 16 times hotter than Earth
but gives off 160,000 times as much radiation
24
Plancks Function
  • Blackbody doesn't emit equal amounts of radiation
    at all wavelengths
  • Most of the energy is radiated within a
    relatively narrow band of wavelengths.
  • The exact amount of energy emitted at a
    particular wavelength lambda is given by the
    Planck function

25
Plancks function
First radiation constant
Wavelength of radiation
c1?-5
B ? (T)
exp (c2 / ?T ) -1
Absolute temperature
Second radiation constant
Irridance Blackbody radiative flux for a single
wavelength at temperature T (W m-2 ?m-1)
Total amount of radiation emitted by a blackbody
is a function of its temperature c1
1.19x10-16 W m-2 sr-1 c2 1.44x10-2 m K
26
Planck curve
27
Weins Displacement Law
?mT 2897.9 ?m K
Gives the wavelength of the maximum emission of
a blackbody, which is inversely proportional
to its temperature
Earth _at_ 300K Sun _at_ 6000K
10 µm 0.5 µm
28
Rayleigh-Jeans Approximation
B? (T) (c1 / c2) ??-4 T
When is this valid 1. For temperatures
encountered on Earth 2. For millimeter and
centimeter wavelengths At microwave wavelengths,
the amount of radiation emitted is directly
proportional to T... not T4
B? (T)
TB
(c1 / c2) ??-4
Brightness temperature (TB) is often used for
microwave and infrared satellite data,
where it is called equivalent blackbody
temperature. The brightness temperature is equal
to the actual temperature times the
emissivity.
29
Emissivity and Kirchhoffs Law
??????????????
Actual irradiance by a non-blackbody at
wavelength ?
Emittance Often referred to as emissivity
Emissivity is a function of the wavelength of
radiation and the viewing angle and) is the ratio
of energy radiated by the material to energy
radiated by a black body at the same temperature
????????absorbed/ ????incident?
Absorptivity (r? , reflectivity t? ,
transmissivity)
30

31
Solar spectrum composition
  • The spectrum of the Sun's solar radiation is
    close to that of a black body with a temperature
    of about 5,800 K.
  • The Sun does, however, emit X-rays, ultraviolet,
    visible light, infrared, and even Radio waves
  • UV 0.1-0.4 µm
  • visible 0.4-0.7 µm (namely so called light)
  • infrared 0.7 µm 1mm

32
Intensity and Wavelength of Emitted Radiation
Earth and Sun
33
window
Atmosphere Window
34
Solar constant (see Wallace ch. 4)
Question How to calculate solar
radiation? Assuming suns surface temperature is
5780K, Average distance between Sun-Earth is
1.5x108 km mean Sun radius is 7x105 km.
  • The solar constant is defined as the quantity of
    solar energy (W/m²) at normal incidence outside
    the atmosphere (extraterrestrial) at the mean
    sun-earth distance. Its mean value is 1367.7
    W/m². The spectral distribution is given in the
    figure.
  • Energy from the Sun (E)
  • Using the Stefan-Boltzmann law, calculate t
  • the average irradiance of the sun.

b. Reverse law
The inverse square law is used to calculate this
constant So E(sun) x (R(sun)/r)2
The solar constant includes all wavelengths of
solar electromagnetic radiation, not just the
visible light
35
How to calculate solar radiaiton at TOA?
The Earth receives a total amount of radiation
determined by its cross section (pRE²), but as
it rotates this energy is distributed across the
entire surface area (4pRE²).
Hence the average incoming solar radiation is
one-fourth the solar constant (approximately
342 W/m²)
36
Is solar radiation at TOA a real constant?
Does this value vary with latitude and season,
and local hour?
Answer At any given moment, the amount of solar
radiation received at a location on the Earth's
surface depends on the state of the atmosphere
and the location's latitude.
37

Sun Spot numbers
solar energy incident to earth
38
Solar Zenith Angle (important)
The angle between the local zenith and the line
of sight to the sun.
39

40
In general, the (solar) azimuth angle varies with
the latitude and time of year and the full
equations to calculate the sun's position
throughout the day. There is equation for
calculation this angle (not discussed in this
class)
http//www.pveducation.org/properties-of-sunlight/
azimuth-angle
41
  • the "solar constant" images above, from a variety
    of calibrated satellite instruments aboard SOHO.
  • SOHO was launched in December 1995 by an Atlas
    Centaur rocket and became operational in March
    1996. SOHO weighs about two tons and with its
    solar panels extended stands about 25 feet
    across.  SOHO will continue operating well past
    the next solar maximum in 2001. (Image credit
    Alex Lutkus)

42
HW2 solar radiation and Black Body
  • IDL tutorial for HW2
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