Satellites and instruments How RS works

This section More reflection Sensors / instruments and how they work

Today Some random points about reflection Homework return and guidance how we form images Comments on discussions

Energy transitions Electrons in atoms are constrained to certain energy levels When a photon is absorbed it must move an electron from one level to another (quantisation) But quantum physics is a wonderful thing and Heisenberg said that everything is uncertain so energy bands have width

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!!

Errors The measurement of albedo has errors due to the scattering and absorption of radiation in the atmosphere This is pretty constant and can be corrected for (unless a volcano has erupted)

Review 1: EM What we’ve done to date –Various parts of the EM spectrum (esp. those used in RS) –What objects produce what types of EM (esp. things on and around the Earth) –What happens to the light as it encounters matter (esp. the atmosphere) –How wavelength is related to temperature

Forming Images How do sensors take data How do we transform that data into an image

Scanning How sensors on satellites actually observe the Earth Concentrating on geostationary satellites

Another Quiz Who first put forward the idea of geostationary satellites and what is he most famous for?

Geostationary scanning Spin scanning – satellite spins (like a gyroscope this stabilizes the satellite) and detector spins with it and therefore scans

Spin scan

Old Spinning satellite

Spinning Spin scanning – satellite spins (like a gyroscope this stabilizes the satellite) and detector spins with it and therefore scans Not efficient scan (half of the time pointing into space)

Rocking Rock Mirror scanning –Rotating mirrors provide the scanning –Satellite must be stabilized –More efficient scan

Polar orbiter scanning

Coverage by polar orbiters

Coverage

True Colour

Photography Very pretty Not that useful in analysis Need numerical measures of known energy distributions to quantitatively deduce things

Imagers What we will be looking at (mostly) are imagers These were the first instruments put into orbit and remain the basic tool of atmospheric (and other) remote sensing These instruments record a 2D image This is usually presented as a image constructed of pixels Could get the actual values and analyse

GOES Imager 4 or 5km resolution –Depends on wavelength –Shorter wavelengths have better resolution Or worse resolution –Depends on position on Earth’s surface relative to that beneath satellite position (nadir)

NOAA Polar Orbiter Imager Advanced Very High Resolution Radiometer (AVHRR) Nominal 1km resolution –Worse at extremities of scan

AVHRR Channels

Review 2: Instruments Where satellites are –Why particular orbits are useful Imagers –Scanning and image formation The wavebands used by imagers (why? Esp. in relation to issues in review 1) What objects are recognised at which wavelengths

Instantaneous field of view and ground resolution GRE = IFOV x H where IFOV is measured in radians H IFOV GRE

Total field of view Image width = 2 x tan(TFOV/2) x H where TFOV is measured in degrees H

Image scan lines

Image pixel array

Imaging array concept H

Recording of values Energy measurements are converted into 8 or 10 bit for storage and transmission. This has the disadvantage of only recording certain discrete values and introduces ‘quantisation’ errors.

Converting data We know all about the sensor from pre-flight calibration So instead of actually finding the energy and then converting we do a straight conversion from the byte value to albedo This will be something like

Real albedo Have to take account of illumination as albedo is the proportion reflected If incoming solar changes, then reflected energy changes We should know incoming solar, based on time of year, day and latitude

Radiometric resolution Bit

Exercise Contrast to Polar orbiters Put up slide of polar orbits and coverage!! What are the advantages and disadvantages of geostationary satellites compared to polar orbiters