chapter 4: Meteorological Satellite Instumentation Flashcards
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.
basic elements of a radiometer
The optics, detectors, and electronics
Optics:
collect the radiation, separate or disperse the spectral components, and focus the radiation to a field stop.
Detectors:
located behind the field stop, respond to the photons with a voltage signal.
electronics
That voltage signal is amplified by the electronics and converted into digital counts.
Usually, a radiometer is further identified by
the portion of the spectrum it covers
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).
Earth emitted radiation is detected in
several spectral regions by radiometers where the spectral separation through one of the following approaches.
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.
There are two common types of radiometers:
imagers and sounders
Imagers:
A radiometer that has a scanning capability to provide a twodimensional array of pixels from which an image may be produced.
The imagers are utilized in satellite meteorology in two ways:
- 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.
- 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.
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
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.
Spatial resolution of passive sensors (we will look at the special case of
active microwave sensors later)
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)
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.
resolution cell
This area on the ground is called the resolution cell and determines a sensor’s maximum spatial resolution.
For a homogeneous feature to be detected
its size generally has to be equal to or larger than the resolution cell.
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.
Spectral resolution describes
the ability of a sensor to define fine wavelength intervals
The finer the spectral resolution, the
narrower the wavelength range for a particular channel or band
multi-spectral sensors
Many remote sensing systems record energy over several separate wavelength ranges at various spectral resolutions
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.