Trans. JSASS Space Tech. Japan Vol. 7, No. ists26, pp. Td_19-Td_28, 2009 Precision Orbit Control of the Advanced Land Observing Satellite (ALOS) for SAR Interferometry By Takanori IWATA and Masanobu SHIMADA Japan Aerospace Exploration Agency (JAXA), Tsukuba, JAPAN (Received May 8th, 2008) Earth observation satellites are typically inserted into orbits which rendezvous with ideal target orbits and are pre- cisely controlled to form and maintain formations with virtual spacecraft on the ideal orbits. As Earth observing instru- ments achieve higher spatial resolution, their subsatellite cross-track deviation requirements become more stringent, and a new application by SAR interferometry requires a further improvement in orbit control accuracy. The Advanced Land Observing Satellite (ALOS), which was launched on January 24, 2006 and has been operated successfully on orbit, required precision orbit control for high-resolution Earth observation and SAR interferometry under various practical constraints. This paper presents the ALOS orbit control strategy with a particular emphasis on requirements and practical constraints, and demonstrates the resulting on-orbit performance in which equator crossing points have ¼ been regulated within ¦ km from reference ground paths, and altitude variations over the same geo-locations have ¼ been kept within ¦ km. Key Words : Orbit Control, Orbit Maintenance, Frozen Orbit, ALOS, SAR Interferometry 1. Introduction Earth observation satellites are typically inserted into or- bits which rendezvous with ideal target orbits and are pre- cisely controlled to form and maintain formations with vir- tual spacecraft on the ideal orbits. The satellite’s semima- jor axis, inclination, eccentricity, and in-plane phase are ini- tially and periodically adjusted to bring the satellite to a tar- get orbit on which the satellite flies in a corridor specified with respect to ground-fixed reference paths. Cross-track deviations from the reference paths are regulated and recur- rence is maintained. Local sun time is kept and sun syn- chronicity is maintained. Altitude variations over the same geo-locations are minimized and the orbit is frozen. Fig. 1. ALOS on-orbit configuration As Earth observing instruments achieve higher spatial res- olution, their subsatellite cross-track deviation requirements On January 24, 2006, the Advanced Land Observing become more stringent. As a new application by synthetic Satellite (ALOS) was launched by an H-IIA rocket from the aperture radar (SAR) interferometry emerges, a further im- Tanegashima Space Center into a sun-synchronous orbit at provement in orbit control accuracy is required. To keep an altitude of ¾ km (Fig. 1). Since then, ALOS has been coherence in interferometry, a satellite with SAR must fly operated successfully on orbit, delivering a variety of high- through a narrower corridor that has a cross section with resolution images in numerous quantities and contributing smaller cross-track deviation and altitude variation. A Eu- to disaster management support many times.3)4) These ac- ropean Earth observation platform, ENVISAT, launched in complishments include several cases of crustal deformation 5) ½ 2002 reported that its orbit was maintained within ¦ km monitoring by SAR interferometry. of its reference ground track for SAR interferometry. 1) A ALOS is a Japan Aerospace Exploration Agency German SAR satellite, TerraSAR-X, which was recently (JAXA)’s flagship for high-resolution Earth observation. ¾¼ launched requires a ground track repeatability within ¦ With the mass of 4000 kg and the power of 7 kW, ALOS m per repeat cycle for repeat-pass interferometry. 2) is the largest low Earth orbit satellite that Japan has ever Despite this trend, however, there are a few perturbing built, and is designed to contribute to cartography, regional forces and several practical constraints against accomplish- environment monitoring, disaster management support, and ing precision orbit control. A challenge for improving orbit resource survey. In order to accomplish this mission, ALOS control accuracy, therefore, requires numerous engineering has three mission instruments: Panchromatic Remote Sens- efforts to solve for a practical solution under these realistic ing Instrument for Stereo Mapping (PRISM), Advanced constraints. Visible and Near-Infrared Radiometer-2 (AVNIR-2), and Copyright© 2009 by the Japan Society for Aeronautical and Space Sciences and ISTS. All rights reserved. Td_19 Trans. JSASS Space Tech. Japan Vol. 7, No. ists26 (2009) Phased Array Type L-band Synthetic Aperture Radar (PAL- Corridor SAR). Pass2 Pass1 B This paper presents the ALOS orbit control strategy em- h Bp ployed to meet precision orbit control requirements for high- Bh resolution Earth observation and SAR interferometry under Incidence r1 various practical constraints. Their operational results and r2 Angle on-orbit performance are also reported. Earth Surface Elevation z 2. Orbit Control Requirements 2.1. Orbit control for Earth observations Geocenter Earth observation satellites are typically inserted into or- Fig. 2. Geometric concept of SAR interferometry bits which rendezvous with ideal target orbits with great ac- curacy and are precisely controlled to form and maintain for- Table 1. Parameters of ALOS nominal observation orbit mations with virtual spacecraft on the ideal orbits. In an Orbit Type Sun-Synchronous initial orbit insertion achieved by a launch vehicle and sub- Subrecurrent sequently by a satellite’s propulsion system, the satellite’s Frozen Orbit ¦ ½ Local Sun Time of ½¼¿¼ semimajor axis, inclination, eccentricity, and in-plane phase Descending Node Minimum Change are adjusted to bring the satellite into a target orbit in which Recurrent Day Å ½ Recurrent Revolution ½ the satellite flies in a corridor specified with respect to fixed Daily Revolution Æ ½ reference ground paths called Reference System for Plan- Daily Leap Number Ä Equatorial Orbit Altitude ½ km ½ ning (RSP). After the initial phase, the satellite’s semimajor Inclination deg ¾ ¦¼ axis and in-plane phase are periodically controlled by in- Equator Crossing Point Deviation ¦ km km(target) ¾ ¦¼ Repeat-Pass Altitude Variation ¦ km km(target) plane velocity increment maneuvers to regulate cross-track deviation from the geo-fixed reference paths. This in-plane control forces the satellite’s ground track to follow the RSP section above prescribed ground paths in order to maintain paths and thus maintains the equator crossing points and coherence in interferometry from repeat-pass observations. recurrence. The satellite’s inclination is occasionally con- To enable such flight, three orbit control strategies are trolled by out-of-plane maneuvers to keep local sun time typically practiced. First, subsatellite cross-track deviation of the descending node within a specified tolerance and to should be tightly regulated via a more precise altitude con- maintain sun synchronicity governed by the inclination and trol. Second, the satellite must be inserted into a frozen by the semimajor axis. orbit to minimize altitude deviation above the prescribed As Earth observing instruments achieve higher spatial res- ground paths, and its eccentricity vector must be regulated olution, their observing swaths typically become narrower with small changes. Third, the inclination change must be and their subsatellite cross-track deviation requirements be- regulated to avoid large subsatellite cross-track deviation in come more stringent to secure overlap of swaths and to ob- high latitude regions. serve the specified target areas. Thus, tighter subsatellite cross-track regulation is required via a more precise altitude 2.3. Orbit control requirements for ALOS control. In order to ensure swath overlaps for PRISM, AVNIR-2, 2.2. Orbit control for SAR interferometry and PALSAR and to acquire target areas requested by a mis- sion operations plan, ALOS’ orbit is required to regulate its The trend for precise orbit control becomes more evident cross-track deviations from geo-fixed reference RSP paths ¾ when a new application by SAR interferometry (InSAR) at the equator within ¦ km. The RSP paths are defined emerges. SAR interferometry is a technique that exploits by subsatellite traces of a Keplerian orbit based on the nom- a pair of SAR data sets acquired by two observations over inal observation orbit given in Table 1. Given ALOS’ orbital the same ground target and extracts the phase difference be- parameters for recurrence, ALOS flies 671 RSP paths in 46 tween the two data sets to yield the heights of the observed recurrent days, as shown in Fig. 3. terrain.6) It can measure the surface topography and defor- In order to ensure lighting conditions for PRISM and mation with an accuracy that depends on various factors AVNIR-2and thermal and communication conditions for the including baseline distance and orbit determination accu- satellite system, ALOS’ orbit is required to maintain its local 7) ¦ ½ racy. A geometric concept of SAR interferometry is shown sun time of the descending node within ½¼ ¿¼ min- in Fig. 2. SAR interferometry can be accomplished by for- utes and to keep sun synchronicity. In addition, it is required mation flight of two SAR satellites or by repeat-pass recur- to minimize changes in the local sun time of the descending rent observations of a single SAR satellite, which ALOS node, given a condition that inclination control campaigns PALSAR performs. For the repeat-pass interferometry, a cannot be carried out earlier than two and half years