Introduction to Remote Sensing The Electromagnetic (EM) Spectrum

Electro-magnetic radiation is a wave

Radiation All objects emit electro-magnetic radiation in some form This radiation moves through space until it hits something The thing it hits may then absorb the radiation and get its energy Alternatively it may deflect, scatter or reflect the radiation It may even simply allow the radiation to pass through - transmission

Wavelength We can describe the radiation by: –Wavelength The actual length (metres) between wave peaks. Wavelengths for radiation vary greatly –radio waves (100 cm to 160 metres) –Light (10 -9 metres). –I will often talk in cm or mm for microwaves –μm for IR –nm or Ǻ for visible

Frequency –The number of wave crests that pass by a point per second (Hz). Often we will talk in terms of GHz (10 9 ) We also sometimes refer to a wavenumber; this is 1/λ, or the number of wavelengths in a unit length (normally in cm -1 )

λ &  These two quantities are inversely proportional because Where c is the speed of light (which varies with the medium, but not very much for the media we will consider)

Radiation The distance between wave crests is the wavelength. Shorter waves: x-rays, UV, visible light Longer waves: infrared, microwave, radar, TV, radio One Wavelength

Solar Spectrum max = 0.55  m © 1998 Wadsorth Publishing -- From Ahrens Essentials of Meteorology

Passive instruments These are instruments that simply receive radiation. –Eg radiometer, camera, retina –Most satellite instruments are passive

Active instruments These send out a signal and measure what is returned to them –Eg radars, lidars –We will pretty much ignore these –We will occasionally present examples of both active and passive images for discussion

Other classifications We can also classify instruments by their position: –Ground based Fixed Mobile –Airborne –Satellite borne Geostationary Other orbits

Ground based Most common is radar, but also profilers, ceilometers etc Advantages? Disadvantages?

Satellite borne Mostly passive visible and radiometers Advantages? Disadvantages?

More spectrum

Wavebands We often, in this field, talk of wavebands. These can be broad, e.g. the visible band (400nm - 700nm) or narrow, e.g. the radar S-band (around 10cm) These are related to the sensitivities of particular receivers

Today’s quiz What is the speed of light? What is your favourite colour? What is its wavelength? What is its frequency?

Units You will become familiar with many concepts, including things we measure. These will have units. Temperature: in Kelvin (K) Radiance in Wm -2 Albedo (unitless) And many more

Why do things have colour? Emission Scattering (reflection) and absorption

Interactions with matter When radiation hits matter 3 things can happen –Scattering (radiation bounces off – includes reflection which is a special kind of scattering) –Transmission (radiation passes through) –Absorption (radiation is… erm… absorbed)

Can define more things Absorptance a λ = radiation absorbed / incident Reflectance ρ λ = radiation scattered / incident Transmittance T λ = radiation transmitted / incident A λ + ρ λ + T λ = 1

What we need to do How do we get values of temperature and albedo (reflectance) using the instruments on the satellites?

Albedo Albedo varies with wavelength Many substances have high albedo (reflectance) in the visible (e.g. snow), but low albedo in the microwave (e.g. snow) Can also have different albedo for different colours and therefore appear coloured (e.g. leaves)

Specular and Lambertian reflection

Reflection/scattering The type of reflection is determined by the smoothness of the surface –This is really the ratio of the wavelength of the radiation to the size of uneven features Very few natural surfaces produce true specular reflection Give me some examples!!

Solid angles The solid angle is the proportion of the surface area of a sphere subtended by the 2 dimensional angle. (See picture drawn on board) It is measured in steradians – sr.

Measuring Reflectance (albedo) Measure the energy impacting the sensor in the visible waveband channel In Wm -2 sr -1 μm -1 Energy reflected per unit time per unit area: Normalised for width of waveband and solid angle view.