Weather Monitoring by Satellite

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NADIR:

Direction toward the center of the Earth. Opposite of zenith. e.g. A satellite measurement taken from a point on the earth's surface directly below the spacecraft.

History

The world's very first satellite launched was Vanguard 2, on February 17, 1959. It was designed to measure cloud cover and resistance, but due to its unstable axis rotation, the observed data could not be used.^[2]

The world's first successful weather satellite was TIROS-1, launched by NASAon April 1, 1960. Which operated 78 days and was much more successful than Vanguard 2. ^[3]

Geostationary & Polar-orbiting Satellites

Geostationary Operational Environmental Satellites (GOES)

Geostationary provides a continuous monitoring. GOES circle the Earth in a geosynchronous orbit at a speed which matches the Earth’s rotation. This makes it possible for the satellites to hover continuously over one position. The geosynchronous plane is about 35 800 km (22 300 miles) above the Earth; allowing a full-disc view of the Earth.

As GOES stays above a fixed spot on the surface, it makes it possible to provide a constant vigil for weather conditions; such as flood, storms, movements of sea and ice, etc. It’s a vital help for meteorologists to estimate weather conditions. GOES primary instruments is the Imager, which is a radiometer designed to sense radiant and solar reflecting energy, and the Sounder, which is also a radiometer to sense specific data parameters for atmospheric temperature and moisture profiles, surface and cloud top temperature, and ozone distribution.^[16]

Polar-orbiting Operational Environmental Satellites (POES)

Polar-orbiting satellites make nearly polar orbits 14 times per day, approximately 537 km (520 miles) above the surface of the Earth. Due to Earth’s rotation it is possible for POES to see a different view with each orbit; additionally each satellite yields two complete views of weather around the world per day.

The typical POES instruments include: Advanced Very High Resolution Radiometer (AVHRR) and Advanced TIROS Operational Vertical Sounder (ATOVS) suite. The EUMETSAT-provided Microwave Humidity Sounder (MHS) complete the ATOVS suite. For example the AVHRR/ATOVS provide visible, infrared and microwave data that is used for many different applications, such as: determination of surface properties, humidity profiles, and cloud and precipitation monitoring. POES data provides a vast range of different monitoring applications; climate research and prediction, weather analysis and forecasting, sea surface temperature, ocean dynamics research, atmospheric soundings of temperature and humidity, volcanic eruptions monitoring, forest fire detection, vegetation analysis, search and rescue etc.^[17]

Basic Mechanisms for Observation

Observation in weather monitoring satellites is carried out by the channels of electromagnetic spectrum, especially by visible and infrared specturms. Channels typically vary from 0.6 μm to 13.4 μm.

Visible Spectrum

Visible images record, as the name suggest, visible light from the sun that is reflected by land, oceans, snow, ice and clouds. Main differences between visible and infrared are: obviously the fact that one is visible one is not, but main difference is that visible images records reflected light rather than emitted radiation; they describe how reflective an object is, but nothing about its temperature.

Infrared Spectrum

Infrared radiation is invisible radiant energy. It is an emission of electromagnetic waves from objects with temperature above absolute zero. It is presented as a conversion of thermal energy into electromagnetic energy. Thermal energy is the collective mean kinetic energy, of the random movements of molecules and atoms in subject. Firstly it is imporant to reflect on the distribution of energy possessed by a molecule at any given time, defined as the sum of the contributing energy terms:

E_total = E_electric + E_vibration + E_rotation

The electronic component is tied to the energy transitions of electrons as they are distributed throughout the molecules either in specific bonds or delocalized over structures. The vibration component is explained by the absorption of energy by a molecule as the component atoms vibrate about the mean center of their bonds. Rotational energy is observed as the tumbling motion of a molecule. Rotational energy is the result of the absorption of energy in the microwave region. Everyone of these energies can be emitted if electronic, vibrational or rotational excitation is presented in the mass microstructure.

In meteorology; thermal radiation is an imporant subject in thermodynamics as it is partly responsible for heat transfer between objects, as warmer bodies (Earth's surface) radiate more heat than colder ones (clouds).

α + ρ + τ = 1

Where; α is the spectral absorpion factor, ρ is the spectral reflection factor and τ is the the spectral transimmion factor. These factors also depend on wavelength (λ). Spectral absorption factor is the same value an emissivity (ε). Surface is called a black body if the formula α = ε = 1, in all frequencies, can be applied.

Observation instruments / sensors in weather satellites

Visible Infrared Imaging Radiometer Suite (VIIRS)

VIIRS is a scanning radiometer, which collects visible and infrared light. It also does radiometric measurements of the land, atmosphere, cryosphere and oceans. Data that VIIRS collects is used to analyze cloud movements, water (sea and ice) amounts and temperatures and other visible phenomena’s. Data is also used to better understand the climate change.

It is a wide-swath (3,040 km) instrument with spatial resolutions of 370 m and 740 m at nadir. Its 22 bands span the spectrum between 0.412 micrometers and 11.5 micrometers.

Mass: Approximately 275 kilograms
Average Power: 200 Watts
Development Institutions: Raytheon Company
Purpose: To collect measurements of clouds, aerosols, ocean color, surface temperature, fires, and albedo.

VIIRS extends and improves upon a series of measurements initiated by the Advanced Very High Resolution Radiometer (AVHRR) and the Moderate Resolution Imaging Spectroradiometer (MODIS) ^{[4] [5]}.

VIIRS and Ocean Science

Sea surface temperature and ocean pigment concentration measurements were started at 1978 with Nimbus-7 weather satellite and few years after 1981 continued with NOAA-7 weather satellite. These satellites gave benchmark information to researches behind Sea-viewing Wide Field-of-view Sensors (SeaWiFS) and the Moderate Resolution Imaging Spectroradiometer (MODIS), which both provided good quality data by increasing the spectral range and adjustment exactness.

VIIRS is alike to MODIS a multi-disciplinary sensor delivering data from oceans, surface, aerosol and clouds. VIIRS allows similar data to SeaWiFS from sea surface temperature, which is one of the most essential climate variables. VIIRS provides a global coverage every two days, which is similar to SeaWiFS and MODIS.

The VIIRS design includes equivalent turning telescope assembly as SeaWiFS, which shields the optical gears from on-orbit pollution. This design gives in better on-orbit steadiness. VIIRS alike to MODIS has a solar diffuser installed with a permanency observer for tracking on-orbit functioning in observable wavelengths, and a MODIS-like black body adjustment object for the infrared bands.

Typical necessity for ocean ecology and carbon research is a two-day coverage as microscopic marine plants are usual to vary, especially in coastal areas. VIIRS with a 750-meter resolution scan gives double coverage compared to MODIS and SeaWiFS, which is important improvement for coastal and estuarine research. VIIRS also provides additional shortwave infrared bands that give data that can be used for turbid water aerosol corrections.

VIIRS helps with the measurements of pigment concentrations; water clarity, suspended particulates and other properties coastal zone controlling, fisheries organization, and naval setups. Likewise, precise estimations of sea temperatures are needed for many purposes like hurricane prediction and weather forecasting ^[4][5].

VIIRS and Land Science

VIIRS mainly helps in energy and water balance, vegetation dynamics, land cover and the cryosphere. Energy and water balance analyses involve measuring surface albedo, photo synthetically active radiation (PAR), land surface temperature, evapotranspiration and the associated radioactive forcing and surface atmosphere exchanges. This information is used to parameterize local to global scale climate and hydrological simulations ^{[4] [5]}.

VIIRS and Cloud Science

Already from 1980, weather satellites have included both imagers and sounders. These sensors record data by using different wavelengths to measure information of clouds all around Earth. The information is used to predict cloud formation, make weather forecast and gather information of clouds.

VIIRS provides data from clouds, aerosol and surface properties at a spatial resolution of 750 meters for most spectral measurements. VIIRS records data at a set of separate wavelengths from the ultraviolet (0.45 micrometers) to the infrared (12 micrometers). Cross-track Infrared Sounder

(CrIS) is a hyper spectral (> 1000 spectral wavelengths) sensor, which delivers complementary data from clouds, exclusively in difficult areas such as the poles, over bright surfaces such as snow and ice and in zones that have large temperature inversions ^{[4] [5]}.

Instrument

The VIIRS is a 5-channel cross-track scanning radiometer that measures radiance in five bandwidths from the visible to the infrared spectral regions: 0.63, 1.6, 3.75, 10.80, and 12.0 micrometers at 2km resolution. Although the VIIRS mechanism is meant mainly to collect data from clouds and precipitation, it is also capable of recognizing active fires. The collected data is summarized every month and it is used to monitor natural and man-made fires in the Tropical and Sub-tropical areas (+/- 40 degrees from the equator) ^[6].

Moderate-Resolution Imaging Spectroradiometer (MODIS)

MODIS is a main device inside the Terra (EOS AM) and Aqua (EOS PM) satellites. Terra MODIS and Aqua MODIS are screening the whole Earth's surface every one to two days, getting data in 36 spectral bands, or groups of wavelengths. This information will increase our understanding of global dynamics and processes happening on the land, oceans and in the lower atmosphere. MODIS has a vital role in policy making by giving accurate information of environmental changes, which helps policy makers to make right decisions ^[7].

Orbit: 705 km
Scan Rate: 20.3 rpm, cross track
Swath Dimensions: 2330 km (cross track) by 10 km (along track at nadir)
Telescope: 17.78 cm diam. off-axis, afocal (collimated)
Size: 1.0 x 1.6 x 1.0 m
Weight: 228.7 kg
Power: 162.5 W (single orbit average)
Data Rate: 10.6 Mbps (peak daytime); 6.1 Mbps (orbital average)
Quantization: 12 bits
Spatial Resolution: 250 m (bands 1-2), 500 m (bands 3-7), 1000 m (bands 8-36)
Design Life: 6 years

Design

MODIS offers high radiometric sensitivity (12 bit) in 36 spectral bands ranging in wavelength from 0.4 micrometers to 14.4 micrometers. The responses are custom tailored to the individual needs of the user community and provide exceptionally low out-of-band response. Two of the bands are imaged at a small resolution of 250 meters at nadir; five bands are imaged at 500 meters, and the lasting 29 bands at 1 km. A ±55-degree scanning design at the EOS orbit of 705 km achieves a 2,330-km swath and offers worldwide exposure every one to two days.

The optical system is built of a two-mirror off-axis afocal telescope, which leads energy to four refractive objective assemblies; one for each of the VIS, NIR, SWIR/MWIR and LWIR spectral regions to cover a total spectral range of 0.4 to 14.4 micrometers.

The first MODIS device was installed on the Terra (EOS AM-1) satellite. Terra was launched on the end of 1999. The second MODIS was installed on Flight Model 1 (FM1), which is installed on the Aqua (EOS PM-1) satellite; it was launched on May 2002 ^[7].

Advanced Very High Resolution Radiometer (AVHRR)

AVHRR is a sensor used usually in polar orbiting satellites. It measures the reflectance of the Earth usually in five or six different spectral bands. Where the first and second are focused into red and near infrared regions. The third one is located approximately on 3.5 micrometers and the last two are sampling thermal radiation emitted by Earth on around 11 micrometers and 12 micrometers. The more detailed information of the spectral bands can be seen from the “AVHRR and Science” –section [9].

The number of active channels has been changing during the history of AVHRR. The first AVHRR used only four channels; it was launched on 1978 with the TIROS-n weather satellite. The latest version of AVHRR was launched in May 1998, carried by NOAA-15 with 6 active channels [8].

The latest launched AVHRR, (AVHRR/3) weights around 33 kilograms and the size is approximately 30cm X 37cm X 80cm and it consumes 28.5 watts power [8].

AVHRR and Science

The key purpose of AVHRR-instruments is to monitor clouds and thermal emission of the Earth, which basically means cooling of the surface. Instrument surveys land surfaces, land-water boundaries, snow and ice surfaces, and temperatures of water.

Results and data from AVHRR-instruments have been used to study Climate Change and environmental degradation, because the data have been recorded and stored for over 20 years and it’s easily comparable. Still, the limitations dealing with the old technology used is causing some challenges with the long period data processing [9].

Example of the spectral bands used in AVHRR/3 instrument and purposes of different channels can be seen from the following table [8].

Channel Number	Wavelength (µm)	Typical purpose of use
1	0.58 – 0.68	Daytime cloud and surface mapping
2	0.725 – 1.00	Land-water boundaries
3	1.58 – 1.64	Snow and ice detection
4	3.55 – 3.93	Night clouds and temperature of sea surface
5	10.30 – 11.30	Night clouds and temperature of sea surface
6	11.50 – 12.50	Temperature of sea surface

Night clouds and sea surface temperature is measured using different wavelengths to achieve more precise results. Assessment of data from two different channels is used to detect features or measure various environmental parameters. The three channels operating within the infrared band are used to detect the heat radiation from the temperature of land, water, sea surfaces, and the clouds above them [8].

Spinning Enhanced Visible and Infrared Imager (SEVIRI)

Data from SEVIRI is used to track several different measurements in the atmosphere and the environment. SEVIRI is capable of measuring the temperature on the surface and atmosphere, atmospheric water vapour content, cloud formations, storms, hurricanes, heavy rains and fog. SEVIRI is the main instrument used in the MSG satellite which is a cooperation between EUMETSAT and ESA. [12]

Instrument

SEVIRI instrument has 12 spectral channels from the visible to the infra-red spectrum, capable of measuring different things. It produces an image of the Earth roughly every 15 minutes.

Spectral Range:

•0.4 – 1.6mm (4 visible/NIR channels)

•3.9 – 13.4mm (8 IR channels)

Resolution from 36 000 km altitude:

•1 km for the high resolution visible channel

•3 km for the infra-red and the 3 other visible channels

Focal plane is cooled to 85/95 k

245 000 images over 7 years nominal lifetime

Dimensions:

•2.43 m height

•1 m diameter without Sun Shield

Instrument mass: 260 kg

Power consumption: 150 W

Working principle

The scan mirror is used to move the instrument Line Of Sight towards the South-North direction. The telescope sends the collected radiation to the focal plane where it is divided into twelve different channels of the electromagnetic spectrum and transferred to 42 sensors. The Channel separation is performed at the telescope focal plane, by folding mirrors. Periodically the calibration unit is placed in front of the instruments field of view to calibrate the IR sensing correctly. The Imaging data is directly transferred to the main detection unit. Earth imaging is obtained by a bi-dimensional Earth scan combining the satellite spin and the scan mirror rotation. [13]

Atmospheric infrared sounder (AIRS)

AIRS measures temperature at an accuracy of 1°C in layers 1 km thick and humidity at an accuracy of 20% in layers 2 km thick in the troposphere in order to allow meteorologists to improve and extend weather predictions from the current five-day forecasts to over a week into the future and to observe changes in Earth's climate.

The Atmospheric Infrared Sounder (AIRS), is a cross-track scanning instrument. Its scan mirror rotates around an axis along the line of flight and directs infrared energy from the Earth into the instrument. AIRS can be found inside the space craft Aqua, as it moves a mirror sweeps the ground creating a scan that extends roughly 800 km. Within the AIRS instrument the infrared energy is separated into wavelengths. This information is sent from AIRS to the Aqua spacecraft, which relays it to the ground.

The term "sounder" in the instrument's name refers to the fact that temperature and water vapor are measured as functions of height. AIRS also measures clouds, abundances of trace components in the atmosphere including ozone, carbon monoxide, carbon dioxide, methane, and sulfur dioxide, and detects suspended dust particles. AIRS measures the infrared brightness coming up from Earth's surface and from the atmosphere. Each infrared wavelength is sensitive to temperature and water vapor over a range of heights in the atmosphere, from the surface up into the stratosphere. By having multiple infrared detectors, each sensing a particular wavelength, a temperature profile, or sounding of the atmosphere, can be made. [14]

Instrument

Spatial resolution:

IR:13.5 km

Vis/NIR: 2.3 km horizontal at nadir

1km vertical

Mass:177 kg

Power: 220 W

Thermal control:

IR detectors: active cooler at 60 K

Passive radiator at 150 K

Electronics at ambient

Thermal operating range:

20-25 degrees C

Field of View:

± 49.5 degrees cross-track

Sensor Description:

Sensor

Temperature(K)

Temperature (K)

IR Spectrometer

Multi-aperture, non-Littrow echelle array grating spectrometer configuration
Two-stage passive radiative cooler with retractable earth shield.

IR spectral Coverage:
3.74 - 4.61 µm /2169 - 2674 cm
6.20 - 8.22 µm /1265 - 1629 cm
8.80 - 15.40 µm /649 - 1136 cm
AIRS IR Spectral Coverage Chart

Spectral Resolution:l/Dl = 1200 nominal:
(900-1400 required)

Sensitivity: < 0.20 K from 3.7 to 13.4 µm
< 0.35 K from 13.4 to 15.4 µm

Channel Knowledge:

Frequency Knowledge: 0.01 Dl

Wavelength Stability: 0.05 Dl/ 24 hour.
On-board spectral calibrator required.
In-flight alignment adjust capability.

Radiometric Calibration:
< ±3% absolute error.
On-board two-point calibration every scan.

155K

Stability:

< 0.7mK/scan

Maximum: 350K

VIS/NIR Photometer

Four channels, from 0.4 - 1.0 µm:
Wavelengths at 50% peak response:
Channel 1 0.41 µm - 0.44 µm
Channel 2 0.58 µm - 0.68 µm
Channel 3 0.71 µm - 0.92 µm
Channel 4 0.49 µm - 0.94 µm
AIRS Vis/NearIR Spectral Coverage Chart

Contiguous ground coverage at nadir with IR.
No redundancy.
Data Rate (12-bit ADC): 119 kbps before formatting.
Three clocks for sample, start, amd reset.

300K

Working principle

low-noise detectors and read-out electronics for the AIRS wavelength range of 3.7 to 15.4 microns. The second is the low-vibration and long lifetime pulse tube cryocooler used to maintain the IR detectors near 58 K. These are described in more detail in later pages.

The heart of the instrument is a cooled (155 K) array grating spectrometer operating over the entire AIRS IR spectral range at a spectral resolution (lamdba / delta lambda) of 1200. A grating disperses infrared energy across arrays of high-sensitivity HgCdTe detectors. The concept requires no moving parts for spectral encoding and provides 2378 spectral samples, all measured simultaneously in time and space. Simultaneity of measurement is an essential requirement for accurate temperature retrievals under partly cloudy conditions.

AIRS looks toward the ground through a cross-track rotary scan mirror which provides +/- 49.5 degrees (from nadir) ground coverage along with views to cold space and to on-board spectral and radiometric calibration sources every scan cycle. The scan cycle repeats every 8/3 seconds. Ninety ground footprints are observed each scan. One spectrum with all 2378 spectral samples is obtained for each footprint. A ground footprint every 22.4 ms. The AIRS IR spatial resolution is 13.5 km at nadir from the 705.3 km orbit. There is also a set of visible and near infrared detectors (Vis/NIR) divided into four intermediate and broadband spectral channels. The Vis/NIR spatial resolution is approximately 2.3 km. The Vis/NIR channels provide a diagnostic imaging capability for observing low-level clouds.

The Vis/NIR light goes through a four-color imaging photometer at ambient temperatures to the Vis/NIR detectors. The stability of the Vis/NIR photometer is monitored using on-board lamps.

At the same time, the IR light passes through a multi-aperture spectrometer whose elements are passively cooled to about 155 K by a two-stage radiator assembly. Using space views and a view of a hot on-board blackbody every scan line, the science software (running on the ground) calibrates the IR radiances to absolute accuracy 3% or better. The wavelengths of each channel are calibrated using observations from scene data of well-understood atmospheric spectral lines, with help from an on-board spectral source, a sheet of thin Parylene film.

The IR detectors are divided into 12 modules containing 17 linear arrays distributed in a two-dimensional pattern on the cold focal plane. The detectors are of two types\photovoltaic (PV) and photoconductive (PC). It was originally hoped that the entire focal plane could be PV, a more advanced and compact technology. But PV detectors sensitive to the longest AIRS wavelengths were not available by the time the instrument design had to be frozen. So two of the twelve detector modules (for 13.7 to 15.4 micron wavelengths) are PC. [15]

Infrared Atmospheric Sounding Interferometer (IAS)

Purpose is to provide atmospheric emission spectra with deriving the temperature and humidity profiles with high vertical resolution and accuracy. It uses the Michelson interferometer technology with spectral coverage between 3.6 and 15.5 micrometers. IASI can also measure the cloud top temperature, pressure and the fractional cloud cover. [11]

As an example, polar orbiting weather satellite Nadir uses IAS at intervals of 25km along and cross tracks samples maxing a maximum diameter of 12km. [10]

References:

[1] http://www.noaa.gov/satellites.html

[2] http://www.nrl.navy.mil/accomplishments/rockets/vanguard-project/

[3] http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1960-016A

[4] http://en.wikipedia.org/wiki/Visible_Infrared_Imaging_Radiometer_Suite

[5] http://npp.gsfc.nasa.gov/viirs.html

[6] http://wdc.dlr.de/sensors/virs/

[7] http://en.wikipedia.org/wiki/Moderate-Resolution_Imaging_Spectroradiometer

[8] http://noaasis.noaa.gov/NOAASIS/ml/avhrr.html

[9] http://en.wikipedia.org/wiki/Advanced_Very_High_Resolution_Radiometer

[10]http://www.class.ngdc.noaa.gov/saa/products/search?datatype_family=IASI

[11]http://www.eumetsat.int/website/home/Satellites/CurrentSatellites/Metop/MetopDesign/IASI/index.html