Circling the earth, high above our heads, satellites are messengers and observers in the sky. They relay telephones calls, watch the weather, guide ships and aircrafts and carry out tasks that are impossible on the ground.
The satellites typically scan the earth using different wave lengths (channels). Current INSAT geostationary meteorological satellites have 3 channel imager with the following channels:
A satellite's orbit is the curved path it follows around earth. The pull of gravity is stronger closer to the earth, so a satellite in a low orbit must travel faster than one in a geostationary orbit.
Earth observation satellites are satellites specifically designed to observe Earth from orbit, similar to reconnaissance satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc. Geostationary satellites hover over the same spot, providing continuous monitoring to a portion of the Earth’s surface. Polar orbiting satellites provide global coverage, but only twice per day at any given spot.
INSAT –3D meteorological geostationary satellite will be launched in the latter half of 2009 by ISRO.
The date and time stamps are located at the top or bottom of every image, along with other information. For example, G-10 IMG 01 3 Jun 00 TIME=00:30UTC RES=4km NWS/WR=SSD, is a typical date-time stamp which appears on satellite images. The date and time, colored blue in the above example, is in Universal Coordinated Time (UTC). UTC is also known as Greenwich Mean Time (GMT) or Zulu Time (Z). Along the West Coast, we are about 8 hours behind the UTC during Pacific Standard Time (PST).
During Daylight Saving Time (PDT), from mid April through mid October, we are about7 hours behind the UTC. In the example above, it’s June 3rd 2000, 30 minutes past midnight in Greenwich England. In order to get local time, you subtract7 hours to get June 2nd 2000, 5:30PM PDT. The next calendar day has already started in Greenwich England!
Due to the rotation of the Earth, it is possible to combine the advantages of low-altitude orbits with global coverage, using near-polar orbiting satellites, which have an orbital plane crossing the poles.
These satellites are launched into orbits at high inclinations to the Earth's rotation (at low angles with longitude lines), such that they pass across high latitudes near the poles.
Weather Satellite is a type of satellite that is primarily used to monitor the weather and climate of the Earth. Satellites can be either polar orbiting, seeing the same swath of the Earth every 12 hours, or geostationary, hovering over the same spot on Earth by orbiting over the equator while moving at the speed of the Earths rotation. These meteorological satellites, however, see more than clouds and cloud systems. City lights, fires, effects of pollution, auroras, sand and dust storms, snow cover, ice mapping, boundaries of ocean currents, energy flows, etc., are other types of environmental information collected using weather satellites.
Geostationary satellites are positioned at an height above the earth about 36000 Km. At this height they rotate around the earth at the same speed as the earth rotates around its axis, so in effect remaining stationary above a point on the earth (normally directly overhead the equator).
As they remain stationary they are ideal for use as communications satellites and also for remote imaging as they can repeatedly scan the same points on the earth beneath them.
Polar Orbiting satellites by comparison have a much lower orbit, moving around the earth fairly rapidly, and scanning different areas of the earth at relatively infrequent periods.
Tether satellite is a satellite connected to another by a thin cable called a tether. The space tether idea had its origin in the late 1800s. The idea became more popular in the 1960s, and subsequently NASA examined the feasibility of the idea and gave direction to the study of tethered systems, especially tethered satellites.
Clouds and atmospheric gases such as water vapor radiate infrared energy to space. The amount of energy radiated by clouds and gasses relates directly with its temperature. This relationship, know as the Stephan-Boltzmann Law, allows us to “see” clouds at night. The atmosphere generally cools with height. A cloud that radiates low energy is higher in the troposphere than a cloud that radiates higher energy.
On a standard linear enhancement curve, bright white represent cold ice-crystal clouds, while clouds colored with light shades of gray are warm water filled clouds in the mid and lower troposphere. There are a number of enhancement curves that color pictures of clouds based on temperature. These curves accentuate temperature ranges that allow us to discern high clouds from mid and low clouds. At the bottom of every infrared image there is a key telling you the temperature range for that color. For example, on the infrared pictures provided on this web site, red represents cloud tops with a temperature of -40C which in a standard atmosphere is about 40,000 ft.
To study the weather condition of a particular region, we can select that area which is known as sector. We are interested in the weather of India, so we select that area.
The following products are being derived from INSAT satellites
Outgoing Longwave radiation (OLR)
Sea surface temperature (SST)
Quantitative precipitation estimate (QPE)
Cloud Motion vectors (CMV)
Water Vapour Wind (WVW)
Cloud Top Temperature (CTT)
Visible Channel Image
Infrared Channel Image
Colour Composite Channel Image
Water vapour Channel Image
Meteorological satellites provide important information to be used in more accurate weather forecasts and global climate monitoring.
A geostationary satellite is positioned above the Equator and orbits the Earth at the same rotation speed as the Earth itself, making it appear stationary from the point of view of an observer on the Earth’s surface. It flies very high above the surface of the Earth (altitude almost 36000 kilometers), and thus is able to capture the whole Earth disc at once. A polar orbiting satellite circles the Earth at a near-polar inclination, meaning that it always passes almost exactly above the poles.
The satellite passes the equator and each latitude at the same local solar time each day, meaning the satellite passes overhead at essentially the same solar time throughout all seasons of the year. The low Earth orbit (800 - 850 kilometers) is much closer to Earth than a geostationary orbit, and thus can see a smaller part of the Earth below than a geostationary satellite, but in finer detail.
A communications satellite (sometimes abbreviated to COMSAT) is an artificial satellite stationed in space for the purpose of telecommunications. Modern communications satellites use a variety of orbits including geostationary orbits, Molniya orbits, other elliptical orbits and low (polar and non-polar) Earth orbits.
In order to check the validity of the satellite measurements, it has to be compared against ground measurements (balloon soundings, lidar measurements, UV measurements etc.). This process is called validation.
Kalpana-1 located at Longitude 74° E and Insat-3A located at 93.5° E both geostationary satellites are being used to monitor the weather of Indian region.
For meteorological observation, INSAT-3A carries a three channel Very High Resolution Radiometer (VHRR) with 2 km resolution in the visible band and 8 km resolution in thermal infrared and water vapour bands. In addition, INSAT-3A carries a Charge Coupled Device (CCD) camera which operates in the visible, near infra Red and short wave infrared bands providing a spatial resolution of 1 km. A Data Relay Transponder (DRT) operating in UHF band is incorporated for real-time hydro meteorological data collection from unattended platforms located on land and river basins. The data is then relayed in extended C-band to a central location. Kalpana -1 Satellite has a 3- Channel VHRR and DRT similar to INSAT -3A Satellite.
GMT stands for Greenwich Mean Time, now called UTC (Universal Coordinated Time), and is the local time at Greenwich-England, which is at 0° longitude. Weather observations, including satellite images, are recorded in GMT as a way of solving the problem of trying to use weather data from different time zones.
Water vapor in the mid and upper troposphere absorbs infrared energy at discrete wavelengths. Using this information, water vapor can act like a tracer of atmospheric circulation in the mid and upper troposphere. Dark regions on water vapor pictures are generally areas where the air is sinking and drying. Light colored regions are areas of rising motion and moisture. Water vapor images are useful in locating jet streams and short-wave troughs and ridges in the mid and upper troposphere, but are lousy when trying to see clouds near the ground such as stratus.
Anti-satellite weapons (ASAT) are space weapons designed to incapacitate or destroy satellites for strategic military purposes. Currently, only the United States, the former USSR (now Russia) and the People’s Republic of China are known to have developed these weapons. On September 13, 1985, the United States destroyed US satellite P78-1 using an ASM-135 ASAT anti-satellite missile and malfunctioning US spy satellite USA-193 using a RIM-161 Standard Missile 3 on February 21, 200@On January 11, 2007, China destroyed an old Chinese orbiting weather satellite.
It will have two payloads- a 6-channel Imager and a 19 –channel Sounder almost similar to GOES satellites of USA. It will also have a Data Relay Transponder (DRT) similar to Kalpana-1 and INSAT-3A.
Unlike previous geostationary meteorological satellites launched by India, this satellite will permit programmable scanning of a special sector, with defined N-S and E-W coordinates. Scanning will also be faster and the data will be at a higher resolution as compared to the currently operational satellites. So the geophysical products derived during the scanning will be more accurate and of higher resolution.
Basically, water vapor images and loops/movies show how moist or dry the middle and upper atmosphere is. They also show the air circulation in the middle and upper atmosphere.
It is a pictorial representation measuring the electromagnetic energy recorded by a sensor, not by photography. A photograph is normally taken within a certain spectral range (visible light). Satellites take images outside this limited range.
As we know oceans are the major storage of heat in the earth climate systems. Sea Surface Temperature (SST) is one of the key controllers of climate variability and acts as a vast thermal reservoir. SST regulates the transfer of long wave radiation to the atmosphere as well as the latent and sensible heat fluxes into the lower atmosphere.
Miniaturized satellites or small satellites are artificial satellites of unusually low weights and small sizes, usually under 500 kg (1100 lb). While all such satellites can be referred to as small satellites, different classifications are used to categorize them based on mass.
It is one of the key meteorological parameter. A detailed knowledge of its distribution in space and time is essential for understanding weather & climate. Information about rainfall is of great value in variety of discipline beside being control to human survival scientifically The latent heat released during the process of condensation water into cloud and rain drops is one of the significant energy source responsible for atmospheric heat engine.
A space station is an artificial structure designed for humans to live and work in outer space for a period of time.
To date, only low earth orbital (LEO) stations have been implemented, otherwise known as orbital stations. A space station is distinguished from other manned spacecraft by its lack of major propulsion or landing facilities—instead, other vehicles are used as transport to and from the station. Current and recent-history space stations are designed for medium-term living in orbit, for periods of weeks, months, or even years. The only space station currently in use is the International Space Station. Previous stations include the Almaz and Salyut series, Skylab and Mir.
A biosatellite is a satellite designed to carry life in space.
NASA launched three satellites specifically named Biosatellite (1, 2 & 3) between 1966 and 1969.
A space observatory is any instrument in outer space which is used for observation of distant planets, galaxies, and other outer space objects. This category is distinct from other observatories located in space that are pointed toward the earth for the purpose of reconnaissance and other types of information gathering.
Types of Satellite are as under:
Majority of meteorological sensors operate in long wave range of radiation so that:
Keeping the above into mind the outgoing flux of long wave radiation at the top of atmosphere is an important parameter in the earth atmosphere radiation budget. This parameter can be derived by physical/statistical algorithm from the narrow band.
The pictures sent back to us from Weather Satellites are simply photographs taken from outer space. We get visible images during the daylight when the earth reflects sunlight back to outer space. Visible pictures show the amount of light reflected back to outer space. Thick water rich clouds, such as stratus (common along the North Coast during Summer months), show up as bright white on visible pictures. Snow pack over the Sierra Nevada during the Winter is another good reflector of sunlight. During the night, visible pictures are completely black as there is very little or no visible light to reflect!
A spy satellite officially referred to as a reconnaissance satellite is an Earth observation satellite or communications satellite deployed for military or intelligence applications.
These are essentially space telescopes that are pointed toward the Earth instead of toward the stars. The first generation type took photographs, then ejected canisters of photographic film, which would descend to earth.
Global Navigation Satellite Systems (GNSS) is the standard generic term for satellite navigation systems (“sat nav”) that provide autonomous geo-spatial positioning with global coverage. GNSS allows small electronic receivers to determine their location (longitude, latitude, and altitude) to within a few metres using time signals transmitted along a line-of-sight by radio from satellites. Receivers calculate the precise time as well as position, which can be used as a reference for scientific experiments.
In general, there are five steps in the derivation of Cloud Motion Vectors :
INSAT-based CMVs are particularly useful in the analysis of upper winds during the monsoon season to study the formation of eddies, cross –equatorial flow, approach of the two separate branches of the monsoon.
Three levels of the INSAT CMV are:
Low Level CMV – 1000 hpa – 700 hpa.
Medium Level CMV – 700 hpa – 300 hpa
High Level CMV – 300 hpa and above.
IR stands for infrared. On an image, IR is usually followed by a wavelength in micrometers (e.g. 10.7). In the IR spectrum, clouds at different heights show up very well as differences in radiances (quantity of light energy detected) from ground level (radiances vary with cloud height). Radiances can then be converted into temperatures with some calculation. So what we see on an IR image is the distribution of temperatures as detected by the satellite's sensor, and the temperature in the legend corresponds to the temperature of whatever the satellite sensor "sees" (clouds at different heights, sea surface, and earth surface).
VIS stands for visible. A VIS satellite image (taken in the visible spectrum) is a picture of the earth from space, just as you would see it if you were looking out the window from a spacecraft in orbit. During nighttime the picture is dark.
There are times when there are glitches in our system, but a more likely explanation is the problem in reception or dissemination of the image. The problems in reception include, an artifact effect in the images, patch temperature variation, or satellite maintenance problems. However, these are relatively rare.
Satellite in a circular orbit about 36000 km above the equator move in time with earth. Satellites in this orbit are called geostationary because they are stationary with respect to the earth and appear to be fixed in sky.
Satellite Meteorology refers to the study of earth's atmosphere and oceans using data obtained from remote sensing devices flown onboard satellites orbiting the earth. Satellite make measurements indirectly by sensing electromagnetic radiations coming from the surfaces below.
As they are positioned at such a high altitude the spatial resolution (i.e. amount of detail shown) of their images (typically 2.5 Km per pixel) tends to be not as good as some polar orbiting satellites (typically 1 Km to 50m per pixel), which are much closer to the earth.
However the advantage of their great height is that they can view the whole earth disk below them, rather than a small subsection, and they can scan the same area very frequently (typically every 30 minutes). This makes them ideal for meteorological applications.
One big problem with Geostationary satellites is that since they are always positioned above the equator they can't see the north or south poles and are of limited use for latitudes greater than 60-70 degrees north or south. The farther from the equator the lower the spatial resolution of each pixel and the greater the possibility of being hidden by the earth's curvature.