Remote Sensing “How we know what we know” A Brief Tour

Dr. Erik Richard Dr. Jerald Harder LASP

Remote Sensing – Space Science Teachers Summit 2008 Richard 1 Remote Sensing

• The measurement of physical variables (usually light or sound) from outside of a medium to infer properties (other physical variables) of the medium.

• Electro-magnetic radiation which is reflected or emitted from (or absorbed by) an object is the usual source of remote sensing data. However any media such as gravity or magnetic fields can be utilized in remote sensing.

Remote Sensing – Space Science Teachers Summit 2008 Richard 2 Measurement Fundamentals

• Key Instrument Components

– Sensing device, or sensor – Transducer • Translates a sensed quantity (i.e. photons, acoustic waves, etc.) into a measurable quantity (e.g. voltage, current, displacement etc.) – Readout device

Remote Sensing – Space Science Teachers Summit 2008 Richard 3 Everyday example: Digital camera

Remote Sensing – Space Science Teachers Summit 2008 Richard 4 Functional Classes of Sensors

Remote Sensing – Space Science Teachers Summit 2008 Richard 5 Element of optical sensors characteristics

Sensor

Spectral Characteristics Radiometric Characteristics Geometric Characteristics

Detection accuracy Field of view Spectral bandwidth (λ ) Signal to noise Instan. Field of view Resolution (∆ λ ) Dynamic range Spectral band registration Out of band rejection Quantization level Alignments Polarization sensitivity Flat fielding MTF’s Scattered light Linearity of sensitivity Optical distortion Noise equivalent power

Remote Sensing – Space Science Teachers Summit 2008 Richard 6 Resolving Power Na spectral lines

Na D-lines

D1=589.6 nm Instrument & Detector D2=589.0 nm

Remote Sensing – Space Science Teachers Summit 2008 Richard 7 Schematic Wave of Radiation

Electromagnetic (EM) energy at a particular wavelength l (in vacuum) has an associated frequency f and photon energy E. Thus, the EM spectrum may be expressed equally well in terms of any of these three quantities: χ c = φρεθυενχψ? ωαϖελενγτη ? λ = φ c = 299,792,458 µ / σεχ −34 ηχ η = 6.626069 ? 10 ϑ?σεχ E = η φ Ε = ? ? λ

Visible Spectrum

0.4 0.5 0.6 0.7 Wavelength (µm)

Remote Sensing – Space Science Teachers Summit 2008 Richard 8 The electromagnetic spectrum

• Remote sensing uses the radiant energy that is reflected and emitted from Earth at various “wavelengths” of the electromagnetic spectrum • Our eyes are only sensitive to the “visible light” portion of the EM spectrum • Why do we use nonvisible wavelengths?

Remote Sensing – Space Science Teachers Summit 2008 Richard 9 Passive or Active?

• Passive sensor – energy leading to radiation received comes from an external source • e.g., direct Sun, reflected Sun, thermal emission etc.

• Active sensor – Energy generated from within the sensor system, beamed outward, and the fraction returned is measured. • e.g. laser LIDAR, microwaves, RADAR, SONAR, etc.

Remote Sensing – Space Science Teachers Summit 2008 Richard 10 Operational Classes of Sensors

Remote Sensing – Space Science Teachers Summit 2008 Richard 11 Scanning or Non-scanning?

• Scanning mode – Motion across the scene over a time interval (think of your video recorder)

• Non-scanning – Holding the sensor fixed on the scene or target of interest as it is sensed in a brief moment (think of your digital camera)

Remote Sensing – Space Science Teachers Summit 2008 Richard 12 Scanning Types

Remote Sensing – Space Science Teachers Summit 2008 Richard 13 Multi or Hyper-spectral?

• Multidimensional data “cube” – Spatial information – Spectral information • Full spectrum – Hyperspectral • Partial spectrum – Multispectral

Remote Sensing – Space Science Teachers Summit 2008 Richard 14 EM derived information

Remote Sensing – Space Science Teachers Summit 2008 Richard 15 Spectral Reflectance

• Spectral reflectance is assumed to be different with respect to the type of land cover. This is the principle that in many cases allows the identification of land covers with remote sensing by observing the spectral reflectance (or spectral radiance) from a distance far removed from the surface.

Remote Sensing – Space Science Teachers Summit 2008 Richard 16 Spectral Reflectance

• Shown below are three curves of spectral reflectance for typical land covers; vegetation, soil and water. As seen in the figure, vegetation has a very high reflectance in the near infrared region, though there are three low minima due to absorption. Soil has rather higher values for almost all spectral regions. Water has almost no reflectance in the infrared region.

Remote Sensing – Space Science Teachers Summit 2008 Richard 17 Earth’s Albedo

•Albedo is defined as the reflectance using the incident light source from the Sun

Remote Sensing – Space Science Teachers Summit 2008 Richard 18 MODIS

• MODIS: MODerate-resolution Imaging Spectroradiometer

• NASA Terra & Aqua satellites – Launched 1999, 2002 – 705 km polar orbits, descending (10:30 am) & ascending (1:30 pm) • Sensor Characteristics – 36 spectral bands ranging from 0.41 to 14.385 µm – Cross-track scan mirror with 2330 km swath width – Spatial resolutions • 250 m (bands 1-2) • 500 m (bands 3-7) • 1000 m (bands 8-36) – 2% reflectance calibration accuracy

movie

Remote Sensing – Space Science Teachers Summit 2008 Richard 19 Black Body Radiation

• An object radiates unique spectral radiant flux depending on the temperature and emissivity of the object. This radiation is called thermal radiation because it mainly depends on temperature. Thermal radiation can be expressed in terms of black body theory.

• Black body radiation is defined as thermal radiation of a black body, and can be given by Planck's law as a function of temperature T and wavelength

Remote Sensing – Space Science Teachers Summit 2008 Richard 20 Blackbody Radiation Curves

Remote Sensing – Space Science Teachers Summit 2008 Richard 21 The Sun’s spectrum

UV Vis IR

Radiometric definitions Irradiance : Radiant power incident per unit area upon a surface (W/m2) Spectral Irradiance : Irradiance per unit wavelength interval (W/m2/nm) Remote Sensing – Space Science Teachers Summit 2008 Richard 22 The Sun’s spectrum with Planck distributions at different temperatures

UV Vis IR M. Planck

Remote Sensing – Space Science Teachers Summit 2008 Richard 23 Black body radiation

• Planck distributions

Hot objects emit A LOT more radiation than cool objects

QuickTimeﾪ and a 2 4 YUV420 codec decompressor I (W/m ) = σ x T are needed to see this picture.

The hotter the object, the shorter the peak wavelength λ T x max = constant

Remote Sensing – Space Science Teachers Summit 2008 Richard 24 Spectral Characteristics of Energy Sources and Sensing Systems

Remote Sensing – Space Science Teachers Summit 2008 Richard 25 Emissivity

• In remote sensing, a correction for emissivity should be made because normal observed objects are not black bodies. Emissivity can be defined by the following formula-

Ραδιαντ?ενεργψￊοφￊανￊοβϕεχτ Emissivity= Ραδιαντￊενεργψￊοφￊαￊβλαχκￊβοδψ ωιτηￊτηεￊσαµ εￊτεµ περατυρεￊασￊτηεￊοβϕεχτ

Remote Sensing – Space Science Teachers Summit 2008 Richard 26 Atmospheric Absorption in the Wavelength Range from 1 to 15 µm

Remote Sensing – Space Science Teachers Summit 2008 Richard 27 Atmospheric Observation Modes

Remote Sensing – Space Science Teachers Summit Richard ￊￊ Transmittance of the Atmosphere

• Transmission of solar radiation through the atmosphere is affected by – Absorption – Scattering

• The reduction of radiation intensity is called σ extinction (expressed as extinction coefficient, ext)

Remote Sensing – Space Science Teachers Summit 2008 Richard 29 Optical thickness

τ • The optical thickness of the atmosphere ( t) σ is the integrated value ext with altitude τ (l ) = s dz t ? ext 0

Total attenuation in a vertical path from the top of the atmosphere down to the surface

Ι −τ (λ) T = = ε τ Ιο

Remote Sensing – Space Science Teachers Summit 2008 Richard 30 Atmospheric absorption of solar radiation

~99% penetrates )

m to the troposphere k (

e d u t i t < 2% R l E A stratosphere

troposphere

Altitude “contour” for attenuation by a factor of 1/e

I(km) = 37% x Io

Remote Sensing – Space Science Teachers Summit 2008 Richard 31 Global Ozone Monitoring

• The Total Ozone Mapping Spectrometer (TOMS) samples backscatter UV at six wavelengths and provides a contiguous mapping of total column ozone.

Remote Sensing – Space Science Teachers Summit 2008 Richard 32 Composition of atmospheric transmission

Remote Sensing – Space Science Teachers Summit 2008 Richard 33 Atmospheric Scattering

• Factors influencing atmospheric transmittance

– Atmospheric molecules (size << λ)

• CO2, O3, N2, etc. – Aerosols (size >λ) • Water drops (fog & haze), smog, dust, etc.

Remote Sensing – Space Science Teachers Summit 2008 Richard 34 Scattering

• Rayleigh scattering – Scattering by atmospheric molecules with size << λ σ – Scattering coefficient s 1 σ ? s l 4

The strong wavelength dependence of the scattering (~λ-4) means that blue light is scattered much more than red light.

Scattering by aerosols with larger size than the wavelength is called Mie scattering (think of a movie projector with dust)

Remote Sensing – Space Science Teachers Summit 2008 Richard 35 Radiometry

• Radiant energy – Energy carried by EM radiation (J)

• Radiant flux – Radiant energy transmitted per unit time (W)

• Radiant intensity – Radiant flux from a point source per unit solid angle in a radial direction (W sr-1)

Remote Sensing – Space Science Teachers Summit 2008 Richard 36 Radiometry con’t

• Irradiance – Radiant flux incident upon a surface per unit area (Wm-2) • Radiant emittance – Radiant flux radiated from a surface per unit area (Wm-2) • Radiance – Radiant intensity per unit projected area in a radial direction (Wm-2sr-1)

Remote Sensing – Space Science Teachers Summit 2008 Richard 37 Understanding the Earth’s Energy Budget

Solar radiation is the Earth’s only incoming energy source. The balance between the Earth’s incoming and outgoing energy controls daily weather as well as longterm weather patterns (i.e. climate). Since we are dealing only with electromagnetic radiation as a heat transfer mechanism, we can start by applying the basic laws of radiation physics to begin to understand the Earth-Sun system and the Earth’s energy budget

Remote Sensing – Space Science Teachers Summit 2008 Richard 38 Radiation Balance

Remote Sensing – Space Science Teachers Summit 2008 Richard 39 Radiation Balance

Remote Sensing – Space Science Teachers Summit 2008 Richard 40 Radiation Balance

Remote Sensing – Space Science Teachers Summit 2008 Richard 41 Earth’s Energy Balance

Remote Sensing – Space Science Teachers Summit 2008 Richard 42 So, just how “bright” is the Sun?

If T = 5780 K @ Sun’s Then the Sun’ssurface emission from the photosphere is = σ ξ Τ 4 ISun ? ￊ

2 ISun ~ 63,000,000 W/m (6.3 kW / cm2)

What does this mean for Earth?

Remote Sensing – Space Science Teachers Summit Richard 2 Surfaceareaￊ=?4π Ρ1ΑΥ

2 Surfaceareaￊ=?4πρΣυν

2 How much 63 MW/m r = 696,000?κµ here Sun here? = 149,600,000 κµ R1AU ?

1360 Ω / µ 2 I@Earth ? ?

Historically know as “Earth’s Solar Constant”

Remote Sensing – Space Science Teachers Summit 2008 Richard 44 “It is ridiculous to try to measure variations in a constant”

- Dove & Maury (ca. 1890) famous oceanographers

Remote Sensing – Space Science Teachers Summit 2008 Richard 45 SORCE Solar Radiation and Climate Experiment

http://lasp.colorado.edu/sorce/

A Mission of Solar Irradiance for Climate Research Launched January 25, 2003

Daily measurements of • Total Solar Irradiance (TSI) • Solar Spectral Irradiance (SSI) 0.1 nm-27nm & 115 - 2400 nm

Remote Sensing – Space Science Teachers Summit Richard Total Irradiance Monitor (TIM)

Four TIM Instrument Radiometers

Detector Head Board

Vacuum Door Heat Sink

Shutter Radiometer Vacuum Shell Precision Aperture Light Baffles (Cone)

Remote Sensing – Space Science Teachers Summit 2008 Richard 47 1360 W/m2 QuickTimeﾪ and a YUV420 codec decompressor are needed to see this picture.

Remote Sensing – Space Science Teachers Summit 2008 Richard 48 30 year TSI record from space

The “constant” variable

Remote Sensing – Space Science Teachers Summit 2008 Richard 49 Solar Cycle 0.1% = 1.4 W/m2

∆ T of ~1.5 °C on Sun

Remote Sensing – Space Science Teachers Summit 2008 Richard 50 Clouds and the Earth’s Radiant Energy System (CERES) • NASA, TRMM, Terra & Aqua – launches 1997, 1999, 2002 – 350 km orbit (35° inclination), 705 km polar orbits, descending (10:30 a.m.) & ascending (1:30 p.m.) • Sensor Characteristics – 3 spectral bands » Shortwave (0.3-5.0 µm) » Window (8-12 µm) » Total (0.3->200 µm) – Spatial resolution: » 20 km – ±78° cross-track scan and 360° azimuth biaxial scan – 0.5% calibration accuracy – onboard blackbodies & solar diffuser CERES Swath Movie Remote Sensing – Space Science Teachers Summit 2008 Richard 51 CERES Results

• Longwave (thermal) radiation

• Longwave (thermal) & simultaneous Shortwave (reflected) radiation

Remote Sensing – Space Science Teachers Summit 2008 Richard 52 “If the Sun had no magnetic field… it would be as boring as most astronomers seem to believe it is”

- R. Leighton Astrophysicist, CalTech

Remote Sensing – Space Science Teachers Summit 2008 Richard 53 The Sun’s magnetism is ultimately responsible for all manifestations of solar activity Erupting prominences CME’s Sunspots

Coronal loops Flares

Remote Sensing – Space Science Teachers Summit 2008 Richard 54 The Sun’s spectrum

UV Vis IR

Remote Sensing – Space Science Teachers Summit 2008 Richard 55 Magnetic Fields and Sunspots

P. Zeeman G. E. Hale

λ G.E. Hale, June 1908 Remote Sensing – Space Science Teachers Summit 2008 Richard 56 The formation of sunspots Animation Hale provided the first proof that sunspots are the seats of strong magnetic fields

QuickTimeﾪ and a YUV420 codec decompressor are needed to see this picture.

TRACE image

Remote Sensing – Space Science Teachers Summit 2008 Richard 57 The Sun’s Magnetic Cycle

Hale’s polarity Law (1919)

Well-organized large scale magnetic field

Changes polarity approximately every 11 years (22 year magnetic cycle)

N S

S N t = 0 t = 3 yrs t = 9 yrs t = 11 yrs

Remote Sensing – Space Science Teachers Summit 2008 Richard 58 “Seeing” the Sun’s magnetic fields

QuickTimeﾪ and a YUV420 codec decompressor are needed to see this picture.

SOHO MDI Magnetograms Remote Sensing – Space Science Teachers Summit 2008 Richard 59