where do microwaves come from

natrually

• if we were to observe microwaves in the natrual world we should know what causes themact

active remote sensing

produces microwaves artifically

EM radiation occurs

when there is anything with a physical temperature

blackbody

absorbs and re-emits all incident energy

bodies

rarely black bodies but usally grey.

reflectivity

ratio of power reflected from a surface to the incident power in a given direction. complete reflection gives a value of 1.

transmissivity

ratio of power transmitted through a medium to incident power.

transparent = 1

opaque = 0

absorptivity

ratio of power absorbed by a medium to incident power. “lossless” when equal to 0.

properties

may be directional quantities and all frequency dependent.

planck function`

for five different thermodynamic temperatures

temperature increases

the total amount of energy radiated (spectral radiance) increases and the peak moves to higher f (higher energy)

higher temperatures

emits higher frequency radiation. all temps emit microwave

infrared, samurais, glass blowers, furnace.

artficially

• natrually + artifically produced EM waves are generated by transformation of energy from other forms

• cause the movement of electrical charge.

• initiate EM waves

• for artifically producing microwaves, we must have some way of controllinh transformation of energy.

• ½ wave dipole antennas lasers

masers =

selective emissions

microwave and light amplification

stimulated emissision of radiation

microwave generation

using electron tubes to generate a variable electric/magnetic field which is then guided to an antenna.

magnetron

best known device microwave ovens

magnetic field utilisation

forces electrons to rotate resultinng in the generation of microwaves

radar purposes

one of the most important requirements when generating microwaves is coherence needs to generate a stream of coherent pulses

microwaves in the real world

• to make meaningful measirements using microwaves it is important to know what governs interactions with natrual world

• one material of paticular intrest is the atmosphere- we want to measure its properties but if we want to measure the earths surface we need to understand its infulence

• clear theoretical justification for using microwaves to observe the earth since it links measurable properties and physical attributes

surface of atmosphere

important to understand how radiation is altered as it travels through homogenous medium. RT theory

microwave and visible atmospheric

windows. 1 THz atmospheric wall

microwaves freqencies less than

10GHz possibly ignore atmosphere together. greenhouse effect

remote sensing of earths atmosphere

usually dealing with media with a high enough temperatures to emit significant microwave radiation

temperatures

within the lower/middle atmosphere are well above -73˚c (200k)

low microwave frequencies

usually assume atmosphere to be non-scattering, non-refractive and in equilibrium (same temps)

simplicity

no need to account for water vapour or aerosols (small realitive to wavelength)

imaging

use RT to describe the interaction with layers on the ground e.g. layers of vegetation or snow

local scattering as well as emission

for active microwaves sensors ignore emission (very low realitive to sigal)

propagation of Radar (micro) waves

very little consideration so far given to what medium the waves are propagating through anf what properties infulence EM waves

consider the ffects that the difference materials can have in the EM radiation we are mosr familar with

visible light

some materials are:

• transparent

• opaque

• highly reflective

transparent materials

may refract, or be transparent only to range of visible wavelengths

all concequence of EM

properties of materials in realtion to waves

three terms used to chacterise the EM properties of a material are:

• electric permittivity

• magnetic permability

• electrical conductivity