Where do microwaves come from

Active remote sensing

= produce microwaves artifically

Anything that has a physical temperature

will emit EM radiation

A conceptual idea is a “blackbody”

= absorbs and remits all incident energy

Reflectivity

ratio of power reflected from surface to the incident power in a given direction.

Complete reflections

gives value of 1

Transmissivity

ratio of power transmitted through a medium to incident power

transparent

=1

Opaque

=0

absorptivity

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

the planck function

five different thermodynamic temperatures

temperature increases on total amount of radiated energy

it will increase and peak moves to higher energy

higher temperatures

will emit higher frequency radiation

where do microwaves come from

naturally + artificially EM waves are generated by transformation of energy from other forms

Artifically

• causes the movement of electrical charge

• initate em fields

for producing artificial microwaves

must have some way of controlling transformation of energy

lasers and masers

amplfying light through a proccess called stimulated emission.

lasers

amplify visible light while masers act on microwaves.

microwaves generation

generated using electron tubes to generate a variable/magnetic field which is guided to an antenna

magnetron

best known device for microwaves

radar purposes

important requirement when generating microwaves conherence need to generate a stream of coherent pulses

atmosphere

if we measure earths surface need to understand the influence

clear theoretical justification

to observe the earth since it links measurable properties and physical attributes

surface or atmosphere

important to understand how radiation is altered as it travels through homegenous medium rt theory.

microwave and visible atmospheric windows

1 THz atmospheric wall

Microwaves frequencies less than 10GHz

possibly ignore earth atmosphere all together

greenhouse affect

effects microwaves in the real world

remote sensing of earths atmosphere

always dealing with media with a high eneough temperature to emit significant microwave radiation

temperatures within the lower/middle atmosphere

weel above -73 degrees Celsius

at low microwave frequencies

usually assume non-atmosphere to be non-scattering non-refractive and in equlibrium

simplifies everything

no need to account for water vapor or aerosols

for imaging

using remote sensing to describe the layers on the ground

propagation of radar microwaves

little consideration so far given ro what medium the waves are propagating through and what properties effect EM waves

materials that have an effecr on the em radiation with correlation to visible light

• transparent

• opaque

• highly reflective

transparent materials

may refract or only be transparent of visible wavelengths

three terms to chacterise EM properties of materials are:

• electriv permittivity

• magnetic permebility

• electrical conductivity

EM waves cannot propagate in conducting material

this is because the elctric field induces currents in the material that disipate the wave energy

perfecct conductor the elctric field

this will be zero everywhere inside the material when the EM wave strikes such a material. it is totally reflected. no internal current generated no energy lost

no perfect conductors in remote sensing

always some penetration, energy loss and partial reflection

magnetic permeability

for nonmagnetic materials such as the earths atmosphere and most objects on the earth value of magnetic permability is 1. its the measure of a materials ability to become magnetised in repose to an applied magnetic field.

Electric permittivity

• most solid materials encountered in remote sensing are non-conducting. such as materials known as dielectric

• usual to hear dielectric properties from scientists when talking about a target or an object.

• usually refer to the electric permittivity of the material