Introduction
Types of Satellites
Interpreting Satellite Imageries
Introduction
Satellites are one of the most useful and important observational tools for continual monitoring of weather and environmental conditions. Regional smoke haze surveillance is carried out by using weather and environmental satellites to detect and monitor fire hotspots and smoke haze in the region. The hotspots and smoke haze are identified by employing selected electromagnetic spectral bands of sensors onboard satellites and highlighting them
Types of Satellites
There are two types of satellites: polar-orbiting and geostationary.
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Polar-Orbiting Satellites
Polar-orbiting satellites circle the Earth in an almost north-south orbit, passing close to both poles. The orbits are sun synchronous, allowing the satellites to cross the equator at the same local time each day. They orbit at a height of between 830 and 880 km and take about 100 minutes to complete an orbit. These satellites pass through the same region twice daily and are able to provide data with high spatial resolution due to their relatively low altitude. They are also equipped with more sensors onboard and therefore able to map the weather and atmospheric conditions in greater detail than geostationary satellites. |
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Geostationary Satellites
Geostationary satellites circle the Earth above the equator at altitudes of about 36,000 km in a geosynchronous orbit i.e. they orbit the equatorial plane of the Earth at a speed matching the Earth's rotation. This allows them to remain stationary with respect to the rotating Earth and thus can capture images of the entire hemisphere below continuously. |
The ASMC receives, processes and delivers satellite information in real time to different end-users for operational monitoring of fire hotspots and smoke haze in the region. Its antenna stations receive data from the following satellites: NOAA, EOS Terra and Aqua (polar-orbiting satellites), MTSAT-1R and FY-2C (geostationary satellites). |
Source: NOAA |
NOAA-series The NOAA-series satellites operated by the US National Oceanic and Atmospheric Administration (NOAA) are polar-orbiting satellites located at a height of about 850 km above the Earth. With its swath width of 2,600 km, about one half the width of the Indonesian archipelago stretching from Sumatra to Kalimantan can be covered in one image with a spatial resolution of 1.1 km at the sub-satellite point. One of the primary sensors of the satellites is the Advanced Very High Resolution Radiometer (AVHRR) which measures the reflectance of the Earth in 5 spectral bands with a spatial resolution of 1.1km at the satellite sub-point. The first two are centred around the visible (0.6 µm) and near-infrared (0.9 µm) regions, the third one is located around the mid-infrared region (3.5 µm), and the last two sample the thermal radiation emitted by the Earth in the long infrared region around 11 and 12 µm. The latest satellite in the series, NOAA-18, was launched in May 2005. Presently, NOAA-15, NOAA-16, NOAA-17 and NOAA-18 are in operation with NOAA-14 in standby. |
Source: NASA |
EOS Terra Launched in December 1999, Terra is a multi-national NASA scientific research satellites in sun-synchronous orbit around the Earth. It is the flagship of NASA’s Earth Observing System (EOS). The name “Terra” comes from the Latin word for earth. One of Terra’s five onboard remote sensors is the Moderate-resolution Imaging Spectroradiometer (MODIS) which capture data in 36 spectral bands ranging in wavelength from 0.4 µm to 14.4 µm and at varying spatial resolutions (2 bands at 250 m, 5 bands at 500 m and 29 bands at 1 km). With its 2300km wide viewing swath and altitude of 700 km, MODIS sees every point on the globe every 1-2 days. MODIS is ideal for monitoring large-scale changes in the biosphere and the environment and its spectral bands are particularly sensitive to fires. |
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EOS Aqua Launched in May 2002, Aqua is the second major component of NASA’s Earth Observing System (EOS following on Terra (launched in 1999) and followed by Aura (launched in 2004). Like Terra, Aqua is a multi-national NASA scientific research satellite in sun-synchronous orbit around the Earth. The name “Aqua” comes from the Latin word for water. Aqua carries six remote sensors which are designed for studies of water on the Earth's surface and in the atmosphere. One of the sensors is MODIS, which is also aboard Terra. |
Source: JMA |
MTSAT-1R The Multi-functional Transport Satellite (MTSAT) are a series of geostationary satellites with meteorological and aviation functions. Launched in February 2005, the MTSAT-1R is owned and operated by the Japan Meteorological Agency and the Japan Civil Aviation Bureau. The satellite provides coverage for the hemisphere centred on 1400E and relays images to the ground station every half or one hour. It carries an imager which provides imagery in five wavelength bands – one visible and four infrared, including a water vapour channel. The visible image and infrared image have a spatial resolution of 1km and 4km respectively at the sub-satellite point. One of the channels operates at the mid-infrared spectral band of 3.5-4.0 µm which makes it possible for detecting fire hotspots. |
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FY-2C Fengyun-2C (FY-2C) is a geostationary satellite launched by China in 2004. It is positioned above the equator at longitude 1050E, thus providing an overhead coverage of Southeast Asia. |
Interpreting Satellite Imageries
To help identify and highlight different features such as hotspots, smoke plumes/haze and clouds, the data from a combination of spectral channels are used to form a multi-spectral image. A hotspot is a mark on an infrared image that contains the spectral signature of high temperature bodies on the ground such as land and forest fires, straw burning after a harvest, and industrial targets such as waste gas flares. The dimension of each hotspot corresponds to the spatial resolution of the spectral channel used to detect the hotspot e.g. 1.1 km by 1.1 km at the sub-satellite point for the NOAA-series satellites.
By assigning the primary colours (red, green, blue) to selected spectral channels, hotspots are depicted in red, smoke plumes and haze in shades of yellow, and clouds in either white or blue-white. The colour assignment is not unique and various colour-to-channel combinations can be used which will result in the different features to be highlighted in a different colour. If the hotspot appears at the origin of a smoke plume, it is almost certain to be associated with an active fire. The smoke plumes will show a conical shape typical of fire emissions, with the vortex over the hot spots. Hot spots with no corresponding smoke plumes may be associated with very small scale fires or fires at the early stages of ignition.
Overlying clouds will invariably obscure areas with smoke haze and hotspots. In the presence of low clouds or fog, the smoke detection technique may not succeed and differences in the shape and texture between smoke and cloud/fog will need to be considered. Strong reflection of sunlight can also give rise to reddish shades over sea areas and these are not related to hotspots. Besides cloud presence, other situations which cause some hotspots and smoke haze to escape detection by satellites include fires below forest canopy, and the relative size and temperature of fires being insufficient to mark the fire hotspot.
The satellite image above is an example of a multi-spectral image that depicts the various features such as clouds, hotspots and smoke plumes/haze. The validity time of the image is indicated in UTC (Coordinated Universal Time) format, which is 8 hours behind Singapore.
