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.
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.
The radiometric resolution of an imaging system describes
its ability to discriminate very slight differences in energy.
The finer the radiometric resolution of a sensor, the
more sensitive it is to detecting small differences in reflected or emitted energy.
Imagery data are represented by
positive digital numbers which vary from 0 to a selected power of 2
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.
Each bit records
an exponent of power 2 (e.g. 1bit=21=2)
Each bit records an exponent of power 2 (e.g. 1bit=21 =2). The maximum number of brightness levels available depends on
the number of bits used in representing the energy recorded.
Thus, if a sensor used 8 bits to record the data, there would be
28 =256 digital values available, ranging from 0 to 255. However, if only 4 bits were used, then only 24=16 values ranging from 0 to 15 would be available. Thus, the radiometric resolution would be much less.
Thus, the difference in the level of detail discernible (visible) depends on
the radiometric resolution
Many electronic remote sensors acquire data using scanning systems, which
employ a sensor with a narrow field of view (i.e. IFOV) that sweeps over the terrain to build up and produce a two-dimensional image of the surface.
multispectral scanner (MSS)
A scanning system used to collect data over a variety of different wavelength ranges
There are two main modes of scanning:
across-track and along-track scanning.
Across-track scanners
scan the Earth in a series of lines. The lines are oriented perpendicular to the direction of motion of the sensor platform (i.e. across the swath).
Across-track scanners scan the Earth in a series of lines. The lines are oriented perpendicular to the direction of motion of the sensor platform (i.e. across the swath).
Each line is scanned from
one side of the sensor to the other, using a rotating mirror or an array of detectors.
Across-track scanners scan the Earth in a series of lines. The lines are oriented perpendicular to the direction of motion of the sensor platform (i.e. across the swath).
Each line is scanned from one side of the sensor to the other, using a rotating mirror or an array of detectors.
As the platform moves forward over the Earth
successive scans build up a two dimensional image of the Earth´s surface.
…………………………………………………. determine ……………………………… and thus the spatial resolution
The IFOV of the sensor and the altitude of the platform determine the ground resolution cell viewed
The angular field of view is
the sweep of the mirror, measured in degrees, used to record a scan line, and determines the width of the imaged swath
Along track scanners
- also called pushbroom scanners
- along track scanners use a linear array of detectors
Along-track scanners (also called pushbroom scanners) use a linear array of
detectors.
Each individual detector measures
the energy for a single ground resolution cell and thus the size and IFOV of the detectors determines the spatial resolution of the system.
…………………………………….. is required to measure each spectral band or channel.
A separate linear array
A separate linear array is required to measure each spectral band or channel.
For each scan line,the energy detected by each detector of each linear array is
sampled electronically and digitally recorded.
Along-track scanners with linear arrays have several advantages over acrosstrack mirror scanners.
- The array of detectors combined with the pushbroom motion allows each detector to “see” and measure the energy from each ground resolution cell for a longer period of time.
- This allows more energy to be detected and improves the resolution.
The second generation (started in 1994) GOES satellites have
separate imaging and sounding instruments.
The GOES Imager
The imager has five channels (Table) sensing visible and infrared reflected and emitted solar radiation. The infrared capability allows for day and night imaging.
…………………………………………………….enable imaging of an entire hemisphere, or small-scale imaging of selected areas
Sensor pointing and scan selection capability
Sensor pointing and scan selection capability enable imaging of an entire hemisphere, or small-scale imaging of selected areas.
• The latter allows meteorologists to
monitor specific weather trouble spots to assist in improved short-term forecasting.
The GOES Imager data are ……………………………. resolution
10-bit radiometric
the GEOS imager data are 10-bit radiometric resolution, and can be transmitted
directly to local user terminals on the Earth’s surface
The GOES Sounder
the …… channel sounder
19
The GEOS 19 cahnel sounder measures:
- emitted radiation in 18 thermal infrared bands and
- reflected radiation in one visible band
The 19 channel GEOS sounder measures:
• emitted radiation in 18 thermal infrared bands and
• reflected radiation in one visible band The GOES Sounder
These data have a spatial resolution of
8 km and 13-bit radiometric resolution
Sounder data are used for:
- surface and cloud-top temperatures,
- multilevel moisture profiling in the atmosphere, and
- ozone distribution analysis
The primary sensors on board the NOAA satellites, are the
- Advanced Very High Resolution Radiometer (AVHRR) and
- the High Resolution Infrared Radiation Sounder (HIRS)
The AVHRR
It has sensor with 5 bands
The AVHRR – It has sensor with 5 bands (Table), detects
radiation in the visible, near and mid infrared, and thermal infrared portions of the electromagnetic spectrum, over a swath width of 3000 km.
AVHRR data can be acquired and formatted in
four operational modes, differing in resolution and method of transmission
AVHRR data can be acquired and formatted in four operational modes, differing in resolution and method of transmission. Data can be
transmitted directly to the ground and viewed as data are collected, or recorded on board the satellite for later transmission and processing.
AVHRR data are widely used for
weather system forecasting and analysis. The sensor is also well-suited to observation and monitoring of land features.
AVHRR data is used extensively for
- monitoring regional,
- small-scale phenomena,
- including:
- mapping of sea surface temperature, and
- natural vegetation and
- crop conditions.
(HIRS) is short for
The High Resolution Infrared Radiation Sounder
The High Resolution Infrared Radiation Sounder (HIRS):
The HIRS is a scanning radiation detection sounder with 20 detectors in the infrared spectrum.
The High Resolution Infrared Radiation Sounder (HIRS) detects and measures
energy emitted by layers of the atmosphere to construct a vertical profile of temperatures from the earth surface to an altitude of about 40 km.
HIRS has ………………… channels ( ) and
19 infrared channels (3.8-15 µm) and one visible channel
HIRS has 19 infrared channels (3.8-15 µm) and one visible channel. The swath width is
2160 km, with a 10 km resolution at nadir
HIRS has 19 infrared channels (3.8-15 µm) and one visible channel. The swath width is 2160 km, with a 10 km resolution at nadir. HIRS uses
CO2 absorption bands for temperature sounding.
HIRS also measures water vapor, ozone, N2O, cloud and surface temperatures
Microwave Remote Sensing
Because of their long wavelengths (1cm to 1m), compared to the visible and IR, microwaves have special properties that are important for remote sensing.
Longer wavelength microwave radiation can
penetrate through cloud cover, haze, dust, and all but the heaviest rainfall, as the longer wavelengths are not susceptible to atmospheric scattering which affects shorter wavelengths.
Longer wavelength microwave radiation can penetrate through cloud cover, haze, dust, and all but the heaviest rainfall, as the longer wavelengths are not susceptible to atmospheric scattering which affects shorter wavelengths.
• This property allows
detection of microwave energy under almost all weather and environmental conditions so that data can be collected at any time without interruption.
Passive microwave sensing is similar in concept to
thermal remote sensing. All objects emit microwave energy of some magnitude, but the amounts are generally very small.
A passive microwave sensor detects
the naturally emitted microwave energy within its field of view. This emitted energy is related to the temperature and moisture properties of the emitting object or surface.
Passive microwave sensors are typically ……………… or …………………….
radiometers or scanners
Passive microwave sensors are typically radiometers or scanners and operate in
much the same manner as systems discussed previously except that an antenna is used to detect and record the microwave energy.
The microwave energy recorded by a passive sensor can be:
- emitted by the atmosphere
- reflected from the surface
- emitted from the surface or
- transmitted from the subsurface
for passive microwave sensors
because the wavelengths are so ….., the ……………………………………
long, the energy available is quite small compared to optical wavelengths
Because the wavelengths are so long, the energy available is quite small compared to optical wavelengths.
Thus,
the fields of view must be large to detect enough energy to record a signal. Most passive microwave sensors are therefore characterized by low spatial resolution.
Active microwave sensors provide
their own source of microwave radiation to illuminate the target
Active microwave sensors are generally divided into two distinct categories:
imaging and non-imaging
The most common form of imaging active microwave sensors is
RADAR
The RADAR transmits
a microwave (radio) signal towards the target and detects the backscattered portion of the signal
The strength of the backscattered signal is measured to
discriminate between different targets and the time delay between the transmitted and reflected signals determines the distance (or range) to the target.
Non-imaging microwave sensors include
altimeters and scatterometers
Radar altimeters transmit
short microwave pulses
Radar altimeters transmit short microwave pulses and measure
the round trip time delay to targets to determine their distance from the sensor.
Generally altimeters look
straight down at nadir below the platform and thus measure height or elevation.
Radar altimetry is used on
aircraft for altitude determination and on aircraft and satellites for topographic mapping and sea surface height estimation.
Scatterometers
generally non-imaging sensors
Scatterometers are also generally non-imaging sensors and are used to
make quantitative measurements of the amount of energy backscattered from targets
The amount of energy backscattered is dependent on
- the surface properties (roughness) and
- the angle at which the microwave energy strikes the target
Scatterometry measurements over ocean surfaces can be used to estimate
wind speeds based on the sea surface roughness
Ground-based scatterometers are used extensively to
accurately measure the backscatter from various targets in order to characterize different materials and surface types.
