ALOS ("Daichi") was a Japanese Earth-observation satellite, developed by JAXA (Japan Aerospace Exploration Agency, Tokyo; formerly NASDA), manufactured by NEC, Toshiba, and Mitsubishi Electric Corp. The objectives call for an optical and an active microwave sensor payload who's high-resolution data may be used for such applications as cartographic mapping, environmental and hazard monitoring (within 48 hours).

Having developed a power generation anomaly and unable to recover communication with the satellite, JAXA decided to complete operations with the spacecraft and ended the mission. ALOS was retired on 12 May 2011.

The intent was to provide the user community with data of sufficient resolution to be able to generate 1:25,000 scale maps. This in turn required observational data of 2.5 m horizontal resolution for the determination of land conditions, and a 3-5 m vertical accuracy for contour mapping. Multispectral data with 10 m horizontal resolution was needed for the classification of land cover (vegetation, forests, etc.). Short-term hazard monitoring (within 24 hours on average) was  accommodated by the use of pointing mechanisms. Particular application features of the ALOS mission were:

• The mapping of land areas (without the need for ground control points) for cartographic applications
• The monitoring of disasters on a global scale (as a complement to the capabilities of other spacecraft)
• Resource surveying

Schematic illustration of the ALOS spacecraft (image credit: JAXA)

The S/C structure consists of CFRP (Carbon Fiber Reinforced Plastic) trusses and aluminum fittings. The approximate S/C dimensions in the stowed configuration are: 6.4 m x 3.4 m x 4.3 m (x, y, z); the in-orbit configuration dimensions are: 8.9 m x 27.4 m x 6.2 m (x, y, z, where x is in the velocity direction and z is toward nadir).

The ALOS S/C requires in particular precise attitude and position determination to minimize image quality degradation. Attitude was sensed by the following devices: STT (Star Tracker) triplet configuration, IRU (Inertial Reference Unit), ESA (Earth Sensor Assembly), and carrier phase tracking of a GPS receiver (dual frequency). In addition, a laser corner-cube reflector was being used for SLR tracking services to calibrate the GPS receiver. The actuators used were: reaction wheels, magnetic torque rods and 16 hydrazine thrusters of RCS (Reaction Control Subsystem), each with a thrust of 1 N. A short-term attitude stability of ±0.00002º/0.37 ms (3 sigma) is provided; the long-term stability is ±0.0002º/5 s (3 sigma). The pointing accuracy knowledge is ±0.0002º, and the spacecraft position accuracy knowledge is ±1.0 m (a posteriori). The dual-frequency carrier-phase tracking GPS receiver of Toshiba Corp. was used for orbit determination.

The S/C mass is about 4000 kg (at lift-off, 180 kg of hydrazine), the largest satellite ever for Japan. Its solar array (size 22 m x 3 m) generates power of 7 kW (EOL). ALOS has five sets of NiCd type battery (BAT). On orbit, a paddle drive rotates the solar array into the sun for maximum efficiency. A combination of active/passive thermal control subsystem is used. The spacecraft design life was 3 years with a goal of 5 years.

Launch: A launch of ALOS took place on January 24, 2006 by a Japanese H-IIA rocket from the Tanegashima Space Center, Japan.

Orbit: Sun-synchronous near-recursive circular orbit, altitude = 692 km, inclination = 98.16º, repeat cycle = 46 days (with a sub-cycle of 2 days for event monitoring), local time at descending node 10:30 AM (±15 min), period = 98.7 min, orbits/day = 14 27/46.

STT (Star Tracker). The ALOS STT was a key element in achieving required attitude determination accuracy. It was a precision fixed-head attitude sensor with a CCD detector design. STT had 3 optical heads (STO) and used 2 optical heads simultaneously. It detected positions and the brightness of stars in its two fields of view (FOV), calculated corresponding addresses and intensity levels of the stars in its star tracker coordinate frame, and provided the results to AOCE (Attitude & Orbit Control Electronics) an onboard computer.

STT characteristics/specifications

Major specifications of the AOCS

The precision GPSR (GPS Receiver) was a key component to the position determination requirements of ALOS. JAXA developed the dual-frequency spaceborne GPS receiver that provided carrier phase measurement. GPSR measures pseudoranges and carrier phases of both L1 and L2 signals. The pseudorange of the L1 signal is used for the onboard, stand-alone (i.e., non-differential) position solution. AOCE compiled the navigation solution of GPSR as the "Payload Correction Data," and provided accurate position information via TT&C to other subsystems such as the mission sensors. In addition, GPSR generated the precise reference time pulse, and served as the reference clock of the intra-satellite time management.

RF communication and data distribution:

The primary data transmission link was via DRTS (Data Relay and Test Satellite of Japan) in Ka-band for mission data at 240 Mbit/s, and S-band for TT&C data. In addition there was an X-band downlink for maximum data rates of 120 Mbit/s. This was considered a backup only for AVNIR-2 data. ALOS was equipped with a solid-state recorder with a capacity of 768 Gbit using 64 Mbit DRAM technology. The storage capacity was sufficient for a 50 minute recording of a 240 Mbit/s data stream. The data rate capabilities of the recorder were: 360 Mbit/s for recording and 240 Mbit/s for readout of playback data.

The observation data produced by ALOS amounted to about 1 TByte/day. In view of this large amount of data, JAXA proposed the concept of ADN (ALOS Data Node) to the international EO community. The benefits of this concept were:

• 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
• To enhance service for potential users
• The current concept envisages four nodes worldwide:

(An excerpt from Dr Herbert J. Kramer's book: Observation of the Earth and its Environments)