Earth Observation Research and Application
Earth Observation Research and Application Center (EORC),
Japan Aerospace Exploration Agency (JAXA), Japan
ALOS is follow-on of the Japanese Earth Resources Satellite-1 (JERS-1) and the Advanced Earth Observing Satellite (ADEOS) to upgrade satellite-based land observing technology. In case of big disasters, ALOS will attempt to capture images of the disaster area by any of its instruments within a few days
The mission objectives of Advanced Land Observing Satellite (ALOS), that is planned to be launched in 2005, include cartography, regional observation, and disaster monitoring. Geographic information such as elevation, topography, land use, and land cover maps in particular are necessary basic information in many fields of practical application and research areas. However, less than 30% of the worldwide terrestrial area has been covered by 1:25,000-scale topo maps.
ALOS has three mission instruments, i.e., two optical imagers and an L-band Synthetic Aperture. The optical imagers are the Panchromatic Remote-sensing Instrument for Stereo Mapping (PRISM) that consists of 3 telescopes of forward, nadir and backward view with 2.5 m spatial resolution and the Advanced Visible and Near Infrared Radiometer type 2 (AVNIR-2) that has 10 m spatial resolution, multi-spectral imaging and cross-track viewing capabilities. The radar is the Phased Array type L-band Synthetic Aperture Radar (PALSAR) that has up to 10 m spatial resolution, variable off-nadir angle beaming and full polarimetric capabilities.
ALOS is follow-on of the Japanese Earth Resources Satellite-1 (JERS-1) and the Advanced Earth Observing Satellite (ADEOS) to upgrade satellite-based land observing technology. The satellite mass is approximately four tons and the design life is three to five years. ALOS will be in a sun-synchronous orbit with an inclination angle of 98.16 degrees, 691.65 km of altitude at the equator and a repeat cycle of 46 days. In case of big disasters, ALOS will attempt to capture images of the disaster area by any of its instruments within a few days. Considering the very high speed data produced from each instrument, ALOS is designed to use a 240 Mbps inter-satellite link by JAXA’s Data Relay Test Satellite (DRTS), for its major data downlink. ALOS also has an X-band direct downlink capability to the local ground stations. However, this downlink data rate is only 120 Mbps due to the limitation of current available frequency resource for this satellite.
Characteristics of mission instruments
PRISM will be used to generate a digital surface model (DSM) with high spatial resolution, which is one of the mission objectives of ALOS. PRISM consists of three independent optical systems for a forward, nadir and backward view which mutual viewing alignment is fixed to achieve the stable elevation extraction accuracy of 5 m for an accurate DSM. PRISM basically acquires three images of the same point on the ground in the same orbit with a 2.5 m spatial resolution.
Table 1: PRISM characteristics
|Number of bands||1 (Panchromatic)|
|Wavelength||0.52 – 0.77 micrometers|
|Number of optics||3 (Nadir, Forward and Backward)|
|Base to height ratio||1.0 (between Forward and Backward)|
|Spatial resolution||2.5 m (Nadir)|
|Swath width||70 km (Nadir or Nadir + Backward), 35 km (Triplet mode)|
|Signal to noise ratio||> 70|
|Number of detectors||28,000 (70km swath), 14,000 (35km)|
|Pointing angle||-1.5 to +1.5 deg. (Triplet mode)|
|Bit length||8 bits/pixel|
|Data rate||960 Mbps (Triplet mode)|
|Data compression||Lossy, JPEG extension (onboard)|
|Data downlink rate||240 Mbps (1/4.5 compression)
120 Mbps (1/9 compression)