Weather monitoring by satellite
Weather satellite is primarily used to monitor weather and climate conditions on Earth. It can either be orbiting earth, or geostationary over a same spot on the equator.
Besides its primary use, It may also be used to monitor City lighting, fire, air and water pollution, and even energy waste. Depending on the type of sensor the satellite acquires.[1]
U.S. NOAA-N Prime weather satellite
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]
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].
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