Geostationary Satellite

A geostationary satellite is launched in such a way that it follows an orbit parallel to the equator and travels in the same direction as the earth's rotation with the same period of 24 hours. Thus, it appears stationary with respect to the earth surface. A satellite following a geostationary orbit always views the same area on the earth. Thus, it is possible to monitor continuously any location within the field of view of its sensor. A large area of the earth can also be covered by the satellite.

Satellites in the geostationary orbits are located at a high altitude of 36,000 km from the earth's surface. At this distance, only low resolution images are acquired. The geostationary orbits are commonly used by meteorological satellites. Currently, the available geostationary satellites are:

• GMS (Geostationary Meteorological Satellites) (Japan), over the Asia-Pacific region (140oE)
  • The currently operating GMS satellite is the GMS 5, launched on 17 Mar 1995. It carries the Visible and IR Spin Scan Radiometer (VISSR), with a visible band at 0.5-0.75 µm (1.25 km resolution) and an IR band at 10.5-12.5 µm (5 km resolution). There is an extra infrared channel to measure atmospheric water-vapour distribution. The instrument acquires the full earth image at a repeat frequency of 30 minutes.
• FY-2 (Fengyun-2) (China), over the Asia-Pacific region (105oE)
  • The first Chinese Geostationary meteorology satellite FY-2B was launched on 10 Jun 1997. The satellite is equipped with a visible and infrared spin scan radiometer (VISSR), a cloud coverage information system and a data collection and transmission system. The VISSR is a 3-channel instrument: the visible channel operating at 0.55-1.05 µm, the infrared (IR) channel at 10.5-12.5 µm and the water vapour (WV) channel at 6.2-7.6 µm. The resolution for the visible channel is 1.25 km while that of the IR and WV channels is 5 km. The VISSR obtains a full view image every 30 minutes. It provides daytime reflective radiation data of clouds and the earth surface in the visible channel, infrared radiation data day and night from clouds and the earth surface from the IR channel, and the water vapour content of the middle and upper atmosphere in the water vapour channel.
  • A second geostationary meteorological satellite FY-2C was launced on 25 Jun 2000. It is an improved version of the first FY-2B satellite.
• INSAT (Indian National Satellite System) (India), A series of geostationary satellites for meteorological observation and telecommunication over India and the Indian Ocean.
• GOES (Geostationary Operational Environmental Satellites) (USA), over the American continents (75oW and 135oW).
• METEOSAT (Europeean Space Agency), over Europe and Africa (0oE).

NOAA POES (Polar Orbiting Operational Environmental Satellite), USA

NOAA-POES is a series of polar orbiting environmental satelites operated by the US National Oceanographic and Atmospheric Administration (NOAA). It begins with the TIROS-N satellite in 1978. The NOAA-16 satellite(launched 21 Sep 2000) was declared the operational afternoon satellite on 20 March 2001, replacing NOAA-14 (launched 30 Dec 1994). Transmission of images from the AVHRR instrument on the NOAA-12 satellite (another afternoon satellite, launched 14 May 1991) is still available. It serves as a backup instrument for the operational AVHRR on NOAA-16. A new NOAA-17 satellite was launched on 24 June 2002. It is expected to be operational in late 2002.

The AVHRR instruments on NOAA-14 and 15 are functioning, but degraded. The NOAA-15 satellite was launched on 13 May 1998. After launch, the satellite experienced problems in deploying one of its antennas. Its imaging instrument failed in early July 2000. Beginning 20 March 2001, NOAA began to resynchronize the AVHRR on NOAA-15 once daily at 0730 UTC. During the time of the resynchronization, there is a very brief disruption on all data in the HRPT transmission. When the AVHRR is in synchronization, usable images may be obtained. When the AVHRR synchronization is out of limits, images are unusable.

The NOAA satellites provide at least four image acquisitions per day for any location on the earth. One of the main instruments carried on the satellites is the AVHRR (Advanced Very High Resolution Radiometer) for observation of clouds and land and sea surface at 1 km resolution in the visible and infrared wavelength regions. The earlier AVHRR instument on NOAA 9, 10, 11, 12 had 5 bands in the visible (band 1), NIR (band 2), MIR (band 3) and Thermal IR (bands 4, 5) regions. The newer AVHRR instrument starting from NOAA-14 has an extra band in the SWIR. This band shares the same transmission channel with the MWIR band which is designated Band 3A, while the SWIR band is Band 3B. Only one of the 3A or 3B bands is activated at any instant.

AVHRR data are acquired in three formats: High Resolution Picture Transmission (HRPT), Local Area Coverage (LAC), and Global Area Coverage (GAC). HRPT data are full resolution image data transmitted to a local ground station as they are being collected. LAC are also full resolution data, but recorded with an on-board tape recorder for subsequent transmission during a station overpass. GAC data provide daily subsampled global coverage recorded on the tape recorders and then transmitted to a ground station. Many weather stations around the world operate ground stations that routinely receive real-time HRPT data. LAC data are available from NOAA's Satellite Active Archive (SAA) through the world-wide web (http://www.saa.noaa.gov). Since on-board tape facility is limited, only a limited number of scenes are archived. GAC are subsampled on-board to about 4-km pixel separation at nadir for global fire monitoring. GAC data are also available from NOAA SAA.

  DMSP (Defence Meteorological Satellite Program), USA

The US Air Force (USAF) Defence Meteorological Satellite Program (DMSP) operates a series of satellites which carry very sensitive light sensors known as the Operational Linescan System (OLS) that can detect light emission from the earth surface at night. The digital data from DMSP-OLS were archived at NOAA's National Geophysical Data Center (NGDC) since 1992. The USAF initially imposed a 72 hours embargo on the DMSP-OLS before they could be released by NOAA. Since 21 Dec 1999, the 72 hours hold has been relaxed to 3 hours.

The DMSP-OLS has a ground swath of about 3000 km. It has two broad spectral bands, one covering the visible-near infrared region (0.5 - 0.9 µm) and the other is in the thermal infrared region around 10 µm. The OLS data are acquired in two spatial resolution modes: 'fine' and 'smoothed'. The full resolution fine data have a nominal spatial resolution of 0.56 km. On board averaging of five by five blocks of fine data produces "smoothed" data with a nominal spatial resolution of 2.7 km. Most of the data received by NOAA-NGDC is in the smoothed spatial resolution mode.

The DMSP-OLS has a unique capability to observe faint sources of visible- near infrared emissions present at night on the Earth's surface, including cities, towns, villages, gas flares, heavily lit fishing boats and fires. By analysing a time series of DMSP-OLS images, it is possible to define a reference set of "stable" lights, which are present in the same location on a consistent basis. Fires are identified as lights detected on the land surface outside the reference set of stable lights.

ORBVIEW 2, USA

The Orbview 2 satellite (also called SeaStar) was launched in 1 August 1997. It carries NASA's Sea-viewing Wide Field-of-view Sensor (SeaWiFS). The purpose of the SeaWiFS Project is to provide quantitative data on global ocean bio-optical properties to the Earth science community. The sensor is designed to observe subtle changes in ocean color, and to derive from these measurements various biological indicators (such as the chlorophyll concentration distribution in the ocean) and other useful scientific products.

The satellite is operated by Orbital Science Corporation (OSC). NASA has contracted with OSC to provide, for five years, the raw satellite data for research purposes. OSC owns the data rights for operational and commercial purposes.

The SeaWiFS Project team has developed algorithms for deriving various ocean parameters such as the sea surface chlorophyll distribution.

OCTS - Ocean Color Temperature Sensor (on ADEOS Satellite, Japan)

OCTS is an optical imaging radiometer on board the ADEOS satellite. It mainly serves as an observation sensor of the ocean conditions, including chlorophyll and dissolved substances in the water, temperature profile and cloud formation processes. The ocean has great effects on our lives as well as the natural environment. Ocean surface temperature substantially interrelates with the global temperature changes since approximately 70% of the earth is coverd with the oceans. It is also important to get information on ocean primary production.

OCTS has 12 bands covering visible and thermal infrared region. In the visible and near-infrared bands, the ocean conditions are observed by taking advantage of spectral reflectance of the dissolved substances in the water and phytoplankton. On the other hand, the sea surface temperature is accurately measured in 4 thermal infrared bands. As the swath width of OCTS is about 1,400km with scanning mirror (west-east) and OCTS also scans south and north, it can observe the entire earth surface for 3 days. The spatial resolution is about 700m.

Source : http://www.crisp.nus.edu.sg