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1

define passive radiometers

  • The instruments flown on-board the satellites measure electromagnetic energy that is either reflected or emitted by our planet
  • An instrument that quantitatively measures the intensity of electromagnetic radiation in some bands (wavelength regions) within the spectrum.

2

basic elements of a radiometer

The optics, detectors, and electronics

3

Optics:

collect the radiation, separate or disperse the spectral components, and focus the radiation to a field stop.

4

Detectors:

located behind the field stop, respond to the photons with a voltage signal.

5

electronics

That voltage signal is amplified by the electronics and converted into digital counts.

6

Usually, a radiometer is further identified by

the portion of the spectrum it covers

7

Usually, a radiometer is further identified by the portion of the spectrum it
covers; for example: 

  • visible (0.4 – 0.7 um),
  • infrared (0.7 to 3.0 um – reflected IR and 3.0 to 100um – thermal IR), or
  • microwave (1 mm to 1 m).

8

Earth emitted radiation is detected in

several spectral regions by radiometers where the spectral separation through one of the following approaches.

9

Earth emitted radiation is detected in several spectral regions by radiometers
where the spectral separation through one of the following approaches.

  • Prisms separate the incoming radiation as refraction changes with wavelength (bending angle depends on index of refraction that is a function of wavelength; longer wavelengths are deflected less)
  • Band pass filters, using internal reflections within the filter, can separate the infrared spectrum into roughly 20 cm-1 segments.
  • Grating spectrometers and interferometers which are capable of spectral resolutions (λ/Δλ) of about 1/1000 also have been used for remote sensing of the earth.

10

There are two common types of radiometers: 

imagers and sounders

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Imagers: 

A radiometer that has a scanning capability to provide a twodimensional array of pixels from which an image may be produced. 

12

The imagers are utilized in satellite meteorology in two ways:

  1. To measure the amount of visible light from the sun reflected back to space by the earth's surface or by clouds, to produce visible imagery.
    • Visible images are the same thing we would see with our naked eye and require daylight.
  2. To measure the amount of infrared radiation emitted by the earth's surface or by clouds, to produce ir imagery
    • Infrared images depend on the amount of radiation an object emits. The obvious advantage to having infrared capability is that weather systems can be monitored both day and night.

13

Sounders: 

measure the infrared radiation, emitted by:

  • the earth's surface or
  • by clouds,

provide:

  • vertical profiles of temperature,
  • pressure,
  • water vapor and
  • critical trace gases in the earth's atmosphere

14

The detail visible in an image is dependent on

  • the spatial resolution of the sensor and
  • refers to the size of the smallest possible feature that can be detected.

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Spatial resolution of passive sensors (we will look at the special case of

 active microwave sensors later)

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Spatial resolution of passive sensors (we will look at the special case of active microwave sensors later) depends primarily on their

Instantaneous Field of View (IFOV)

17

The IFOV is 

the angular cone

  • (A) of visibility of the sensor and determines the area
  • (B) on the Earth's surface which is "seen" from a given altitude at one particular moment in time.
  • The size of the area viewed is determined by multiplying the IFOV by the distance (C) from the ground to the sensor.

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resolution cell 

This area on the ground is called the resolution cell and determines a sensor's maximum spatial resolution.

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For a homogeneous feature to be detected

its size generally has to be equal to or larger than the resolution cell.

20

For a homogeneous feature to be detected, its size generally has to be equal to or larger than the resolution cell.
 If the feature is smaller than this

it may not be detectable as the average brightness of all features in that resolution cell will be recorded.

21

Spectral resolution describes

the ability of a sensor to define fine wavelength intervals

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The finer the spectral resolution, the

narrower the wavelength range for a particular channel or band

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multi-spectral sensors

Many remote sensing systems record energy over several separate wavelength ranges at various spectral resolutions

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hyperspectral sensors

Advanced multi-spectral sensors that detect hundreds of very narrow spectral bands throughout the visible, near-infrared, and midinfrared portions of the electromagnetic spectrum.

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Advanced multi-spectral sensors called hyperspectral sensors, detect hundreds of very narrow spectral bands throughout the visible, near-infrared, and midinfrared portions of the electromagnetic spectrum.
 Their very high spectral resolution facilitates

fine discrimination between different targets based on their spectral response in each of the narrow bands.

26

The radiometric resolution of an imaging system describes

its ability to discriminate very slight differences in energy.

27

The finer the radiometric resolution of a sensor, the

more sensitive it is to detecting small differences in reflected or emitted energy.

28

Imagery data are represented by 

 positive digital numbers which vary from 0 to a selected power of 2

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Imagery data are represented by positive digital numbers which vary from 0 to a selected power of 2. This range corresponds to

the number of bits used for coding numbers in binary format.

30

Each bit records 

an exponent of power 2 (e.g. 1bit=21=2)