Geostationary satellites, such as Multi-functional Transport Satellites (MTSAT), orbit the Earth over the equator at a height of approximately 36,000 km. They complete one orbit every 24 hours so that their period is synchronised with that of the Earth's rotation about its own axis. This ensures that geostationary satellites remain over the same location above the equator allowing frequent imaging of a given region of Earth's surface and atmosphere. A number of geostationary satellites are positioned around the globe for continuous global coverage.
Imaging capabilities of geostationary satellites vary based on model and age. Baseline capabilities include imaging in the visible and infrared part of the spectrum, commonly at a resolution of 1 km and 4 km, respectively. Due to the nature of its orbit, a geostationary satellite is capable of scanning almost an entire hemisphere, though geometrical considerations constrain useful images to 70° of latitude or longitude in each direction from the sub-satellite point (the point on the equator directly below the satellite). Geostationary satellites can be disadvantaged by their high orbital paths leading to lower spatial resolution, however advances in imaging technology are mitigating this issue.
Polar orbiting satellites
Polar orbiting satellites pass over the poles at a height of approximately 850km. Each satellite follows a nearly fixed orbit while the Earth rotates beneath. Additionally, polar orbiting satellites tend to be placed in a sun-synchronous orbit, which means that the satellite flies over any given location on Earth at a set local time.
The areas scanned by each pass (swath) are nearly adjacent at the equator on consecutive passes. Further pole-wards the passes progressively overlap. Imaging sensors carried on-board these satellites generally have a swath width of about 2600km and by completing 14 orbits a day one satellite can provide an almost complete coverage of the globe twice a day. The radiometer points continuously at the Earth and images are built up by a mirror on the satellite scanning from side to side at right angles to the orbit path.
One of the main benefits of polar orbiting satellites is that due to their low Earth orbit, the cost of their payload is more affordable than that of geostationary satellites, hence polar orbiting satellites tend to have a wider variety of equipment on board. While this may change over time with technological advances, their other benefit is that they deliver frequent coverage of polar regions, areas not visible from geostationary satellites.
With access to Japanese, American, Chinese and European meteorological satellites, Australia is well served by regular imagery to support operations and research of national weather services like the Bureau of Meteorology. China, Europe, Japan, and the USA are to be congratulated on their contributions to global free exchange of meteorological data. In particular, MTSAT represents a significant contribution by Japan to the space-based observations part of the World Meteorological Organization's Global Observing System and data is made freely available for common good.
In order to access this meteorological data the Bureau of Meteorology has satellite data reception sites at Melbourne, Darwin and Perth, as well as Casey and Davis stations in Antarctica. These stations provide national coverage of imagery and processed products such as solar radiation, sea surface temperatures, vegetation indicies, grassland curing, atmospheric motion vectors, volcanic ash, and atmospheric profiles of temperature and relative humidity. These products support the Bureau's analysis and forecasting service, and also provide data for more detailed studies of climate and climate related disciplines. While all five stations are operated by the Bureau, the Perth station is owned by the Western Australian Satellite Technology and Applications Consortium (WASTAC), which is a consortium involving:
- Bureau of Meteorology
- Landgate - Satellite Remote Sensing Services
- Curtin University of Technology
- CSIRO - Office of Space Science and Applications
- Murdoch University
- Geoscience Australia
Each reception site consists of one or more satellite antennas that receive data either in L-band or X-band region of the electromagnetic spectrum. The signals are processed by computers on site and the ingested satellite data is distributed to locations around Australia, as well as to partners worldwide.
Multi-functional Transport Satellites (MTSAT)
Multi-functional Transport Satellites (MTSAT) is a series of geostationary weather satellites operated by the Japan Meteorological Agency (JMA). The MTSAT series carry an aeronautical mission to assist air navigation, plus a meteorological mission to provide imagery over the Asia-Pacific region. The satellites also have the capability to relay weather data from remote Automatic Weather Stations.
MTSAT observations form the core of the Bureau's satellite data intake.
NOAA Polar Orbiting Environmental Satellite (POES) Series
National Oceanic and Atmospheric Administration (NOAA) has a long-running series of polar orbiting environmental satellites. One of the instruments on POES platforms is the Advanced Very High Resolution Radiometer (AVHRR) which measures the reflectance of the Earth in five spectral bands. High Resolution Picture Transmission (HRPT) data is transmitted continuously and can be picked up at reception sites within range of the satellite. Reduced resolution Global Area Coverage (GAC) data is stored for later transition to NOAA ground stations.
At each Bureau receiving station NOAA HRPT data is archived in real time using the Australian Standard Data Archive (ASDA) format (developed by Bureau and CSIRO) and is also split into its components for processing.
Aqua and Terra
Aqua and Terra are polar orbiting Earth observation satellites flown by NASA as part of a multi-agency programme called the Earth Observing System (EOS). Terra, flagship of the EOS, provides coverage during the local morning, while Aqua images are available early afternoon local time.
Both satellites carry the Moderate-resolution Imaging Spectroradiometer (MODIS) which can capture 36 spectral bands at varying spatial resolutions (250m, 500m and 1000m). Aqua carries an Atmospheric Infrared Sounder (AIRS) that measures the infrared signal emitted by the Earth's atmosphere and surface in 2378 channels. This signal can be used to estimate temperature and water vapour content throughout the atmosphere.
Feng Yun Polar Orbiting Satellites
China has launched a number of polar orbiting satellites. These include the original FY-1 series, as well as the more recent FY-3 series. On board the FY-1 series satellites is an instrument called a Multi-channel Visible and Infrared Scan Radiometer (MVISR), which has 10 channels. Four of these channels are in the visible region of the electromagnetic spectrum, three in the near infrared, one in the short infrared and two in the long infrared. The spatial resolution of the instrument is 1.2 km. The FY-3 series is an improved generation of polar orbiting satellites with an imaging resolution of 250 m.
Feng Yun Geostationary Satellites
China also has a strong geostationary satellite presence. The current arrangement is to have two active geostationary satellites, FY-EAST (located at 105° E) and FY-WEST (located at 86.5° E). The roles of FY-EAST and FY-WEST are currently filled by FY-2E and FY-2D, respectively, members of the original FY-2 series of geostationary satellites. These satellites monitor temperature and clouds above China and neighbouring areas and are able to provide meteorological information for the Asia-Pacific region.
The next generation of Chinese geostationary weather satellites is known as FY-4.
Meteosat and GOES Data
Through co-operation with international agencies, the Bureau receives data from satellites which cannot be ingested directly due to their location. These include the European Meteosat and the US Geostationary Operational Environmental Satellites (GEOS). The Bureau accesses real-time, low resolution Meteosat imagery from the European Organisation for Exploitation of Satellites (Eumetsat) for internal use only.
GOES-East and GOES-West imagery is accessed by mutual agreement with the Space Science and Engineering Centre (SSEC). The data is received electronically every three hours. This data is not archived by the Bureau.