[9] Y Kneizys zyxwvutsrqpon et zyxwvutsrqpon a1 zyxwvutsrqpon , “Atmospheric transmittanceiradiance Computer code Lowtran 6,” AFGL-TR-83 0187, Hanscom, MA, 1983 [lo] T Hosomura zyxwvutsrqpon et al , “Performance of MESSR data for landcover classification,” in Proc 3rd Symp on MOS-I Verification Program, NSDA, vol 1, pp 198-213, 1989 zyxwvutsrqp Results From Marine Observation Satellite-1 (MOS-1) Operation KOHEI ARAI Abstract-The Marine Observation Satellite (MOS-1) carrying three types of earth observation sensors: two multispectral electronic self- scanning radiometers, a visible and thermal infrared radiometer, and a microwave scanning radiometer, as well as a data collection system transponder, was launched at 01:23 universal time (UT), February 19, 1987. The sensors, subsystems, solar panels, and batteries continue to perform well, and sufficient fuel is on board that the mission may be extended for an additional two years. I INTRODUCTION The National Space Development Agency of Japan (NASDA) launched the Marine Observation Satellite-1 (MOS-1) on February 19, 1987 and placed it into the planned World Reference System (WRS) orbit The satellite carries the following mission instru- ments two multispectrum electronic self-scanning radiometers (MESSRs), a visible and thermal infrared radiometer (VTIR), a microwave scanning radiometer (MSR), and a data collection sys- tem (DCS) transponder The functions and performance of the mis- sion and bus instruments were well validated during the first three months on orbit The overall satellite specifications, functions, and performance are described in detail in [ 11, while the results of the assessment of the radiometric performance and geometric fidelity of the mission instruments are described in [4]-[8]. MOS-1 has been operating routinely for more than two years, its design life 11. DETAILED OPERATION HISTORY A. Orbit and Attitude Control All functions of the attitude and orbit control subsystem were confirmed during the initial three-month mission checkout period. The attitude control accuracy specified and achieved are compared in Table I. MOS-1 orbit control, satellite attitude control, and in- clination control functions were confirmed 15 times in two years. The attitude wheel unloading function was confirmed during the mission checkout period. In particular, when the satellite went into the safe hold mode in July 1987, the aforementioned functions were confirmed. Fig. 1 shows ground track deviation from the nominal WRS or- bit. With four satellite attitude manoeuvres, it has been possible to maintain the MOS-1 orbit within f10 km for the nominal orbit except in two cases. Attitude control was lost and the satellite went into safe hold (sun acquisition mode) on July 48, 1987 when the earth presence signal fell below threshold over Antarctica. To avoid this, MOS-1 has been operating without the earth presence check from June to September when the abnormal attitude error signal may occur. The threshold level of the earth sensor on board MOS- 1b has been changed to avoid this problem. On another occasion, Manuscript received October 7, 1989, revised January 3, 1990 The author was with the Earth Observation System, National Space De- velopment Agency of Japan, 1401 Ohashi, Hatoyama, Saitama, Japan. He is now with the Science and Engineering Faculty, Saga University, 1 Honjo, Saga-city, Saga, Japan 840 IEEE Log Number 9036041 zyxwvutsr Dec.’87 Nov.‘88 allowable range .................... ..................................... -Apr. ‘88 Ju1.‘87 -41h M J S N J M M J S N J H ‘88 zyxwv ’89 *ai Fig. 1. MOS-1 orbit control history. TABLE I THE SPECIFIED AND ACHIEVED ATTITUDE ERROR AND ATTITUDE CHANGE RATE Specified Achieved Specified Achieved Attitude Attitude Attitude Attitude Error (”) Error (”) Rate (O/s) Rate (‘/s) Roll 06 0 25 0 02 0 014 Pitch 0.6 0 33 0 02 0 004 Yaw 10 0 69 0.05 0 006 760 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL 28, NO 4, JULY 1990 0196-2892/90/0700-0760$01 .00 @ 1990 IEEE when the satellite orbit inclination correction was performed in Apnl 1988, the orbit exceeded the zyxw f 10-km specification. The local mean time (LMT) of MOS-1 orbit has been maintained between 10:lO and 10:30. Fig. 4 shows LMT for the past (actual) and the future (predicted). B. Telemetry, Tracking, and Control (TTC) Subsystem X-band real-time telemetry of MESSR and VTIR data has per- formed to specification, and ranging signals have been well quali- fied. VHF real-time telemetry, playback telemetry, and beacon sig- nals were determined to be within specification. A single-event upset (SEU) occurs when a charged particle from space resets randomly a bit in the stored command register. This occurs approximately once a day. To avoid this, the stored com- mand memory has been used only on paths that do not cross the South Atlantic anomaly area The stored command memory on board MOS-lb has been modified to prevent SEU’s. C. Solar Array Paddle Although the specification for power generation is greater than 640 W at the beginning of life and greater than 540 W at the end of life, more than 700 W of power has been generated routinely during the two-year design life of the satellite. zyx D. Power Supply The stability of the bus voltage was specified to be in the range of 48.4-49.6 V. Results showed 48.9 V for eclipse, and 49.3 V in daylight. The battery specification calls for about 10 000 charge- discharge cycles to a depth of discharge (DOD) of 45 %. Results show an average DOD of approximately 13% for standby mode operation (no sensors active), and 18% for VTIR mode in eclipse (in the worst case). A slight degradation of output voltage at the end of discharge is apparent in Fig. 3. Based on power supply per- formance to date, the MOS-1 mission can be extended for another two years.