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show PALSAR antenna PFM and PALSAR characteristics,
respectively.
Figure 4. PALSAR antenna PFM under integration.
E
3. MISSION SUPPORTING SUBSYSTEMS
For supporting these observation sensors and achieving mission
objectives, the ALOS bus subsystems have some unique
characteristics. First one is highly stable attitude control system.
In order to minimize geometric distortion of the image, satellite
attitude (angular velocity) is stabilized within 2x10-4 degree per
5sec (equivalent to 2.5m distortion within 35km2 scene).
Disturbances from major vibration source, such as, data relay
communication antenna pointing mechanics, AVNIR-2 pointing
mirror drive mechanics, and solar array drive mechanics are
carefully controlled using feed forward technique and onboard
parameter tuning. Second is autonomous position and attitude
determination system. High precision satellite position and
attitude determination system enables users to locate exact
position of each image pixel without any ground reference
points. This capability is crucially important for cartography for
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B1. Istanbul 2004
unmapped area. This is achieved with accurate star trackers,
dual-frequency carrier phase tracking GPS receivers, and jitter
sensors on board. Combining the data from dual-frequency
carrier phase tracking GPS receiver onboard ALOS and that of
ground GPS reference stations, ALOS position is determined
within 0.2m accuracy.
Third is mission data handling system. Optical sensors' data are
compressed real-time up to 1/9 using Discrete Cosine
Transform for lossy compression and Differential Pulse Code
Modulation for lossless compression in order to fit its
communication link bandwidth. Inter-satellite communication
capability allows real-time data acquisition via Japanese data
relay satellite (DRTS) in 240 Mbps (Hihara, 2002). This
capability is of great importance for disaster monitoring mission.
The ALOS has a conventional 120 Mbps X-band direct down
link for back up and for additional services to foreign receiving
stations. Bit error rate is drastically reduced to the order of 107
with Reed-Solomon error correction code. The ALOS's solid-
state data storage has 768Gbits memory and allows 50 minutes
recording at 240M bps.
4. ALOS DATA DISTRIBUTION
The observation data produced by the ALOS on a daily basis
will be about one tera byte (one trillion bytes per day) which is
beyond the capabilities of any single agency to manage,
however there is worldwide interest in the use of the ALOS data,
JAXA proposed the concept of the "ALOS Data Nodes (ADN)'.
The benefits of the ADN concept are:
— to increase capacity for ALOS data processing and
archiving;
— to accelerate practical and scientific use of ALOS
data;
— to increase international cooperation including joint
validation and joint science study activities;
— toenhance service for potential users.
The current concept envisages four nodes worldwide, shown in
Table 5, in order to achieve the necessary global coverage.
Each Node, associated with a geographical zone shown in
Figure 5, has responsibility for the physical residents therein as
potential ALOS users. The Antarctica is a common region to all
nodes' agencies.
Table 4. PALSAR characteristics.
Operation mode FBS FBD Direct downlink ScanSAR Polarimetric
Chirp bandwidth 28MHz 14MHz 14MHz 14MHz, 28MHz 14MHz
Polarization HH, VV HH+HV, HH, VV HH, VV HH+VV+HV+VH
VV+VH
Incidence angle 8-60deg. 8-60 deg. 8 - 60 deg. 18 - 43 deg. 8 - 30 deg.
Range resolution 7-44 m 14-88 m 14-38 m 100 m (multi Look) 24-89 m
Observation swath 40-70 km 40-70 km 40 - 70 km 250-350 km 20-65 km
Bit length 5 bits 5 bits 34.5 bits 5 bits 3/5 bits
Data rate 240Mbps 240 Mbps 120 Mbps
scene : | dB / orbit : 1.5 dB
Radiometric accuracy
Center frequency L band (1270MHz)
120Mbps, 240 Mbps 1240 Mbps
