Precision Orbit Control of the Advanced Land Observing Satellite (ALOS) for SAR Interferometry

Trans. JSASS Space Tech. Japan Vol. 7, No. ists26, pp. Td_19-Td_28, 2009

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 precisely controlled to form and maintain formations with virtual spacecraft on the ideal orbits. As Earth observing instruments 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

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 orbits which rendezvous with ideal target orbits and are precisely controlled to form and maintain formations with virtual spacecraft on the ideal orbits. The satellite’s semimajor axis, inclination, eccentricity, and in-plane phase are ini- tially and periodically adjusted to bring the satellite to a target 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 recurrence is maintained. Local sun time is kept and sun synchronicity 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 resolution, 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

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 accuracy 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

reference ground paths called Reference System for Plan- Daily Leap Number Ä

Equatorial Orbit Altitude ½ km

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 mission 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

¦ 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 after SAR satellite must ﬂy through a corridor with small cross the launch to maximize observation opportunities.

Td_20 T. IWATA et al.: Precision Orbit Control of the Advanced Land Observing Satellite (ALOS)

bit control does not typically regulate short-period effects.

A favorable perturbation, which actually achieves sun syn- Â

chronicity, is caused by the ¾ zonal harmonics term repre- senting the Earth’s oblateness. This perturbation rotates the

orbit plane one revolution per year according to

Ê ¿ ª

Ò Â Ó×

¾ (1)

Ø ¾ Ò

where ª is the longitude of the ascending node, is the orbit

rate, is the Earth’s equatorial radius, is the semimajor

Fig. 3. ALOS’ RSP groundtrack axis, and is the inclination. The sun synchronicity and resulting local sun time of the 1km descending node is disturbed by the sun’s attraction via the Flight 1km Corridor decrease of the inclination: Pass1 RSP Orbit

Flight Corridor Pass2 ¾

¿ Ò

r r1

2 ¾

×Ò ½· Ó× ¯ ×Ò ¾ ´ª « µ

× (2)

Ø Ò

Earth Surface

Equator ¯ RSP Groundtrack Ò

where is the Earth’s revolution rate, is the obliquity of «

the ecliptic, and × is the right ascension of the sun. The

ª Ø

Fig. 4. Virtual corridor that ALOS ﬂies through change in causes a change in , resulting in a change

Ì Ø

in the local sun time of the ascending node, × . Mid- In order to enable interferometry by PALSAR, a require- term inclination controls are typically performed to reset the ment is originally imposed that altitude difference over the inclination to its initial value. same ground areas on the same RSP paths in different re- The subrecurrence is mainly disturbed by atmospheric

drag via the decrease of the semimajor axis: ¾ current cycles should be within ¦ km. This requirement

necessitated the use of a frozen orbit to avoid excessive alti-

tude changes over the same latitude induced by the rotation (3) Ï

of an eccentricity vector (i.e. migration of perigee). Post- Ø

launch assessment of PALSAR interferometry data, how- where is the drag coefﬁcient, is the satellite effective

ever, revealed that further regulations of orbital deviations area, Ï is the satellite weight, is the atmospheric density, were necessary to enhance coherence. Therefore, an addi- and Î is the satellite velocity. The decrease of yields a

tional requirement was imposed that ALOS should ﬂy in a shorter orbital period and thus a shift of equator crossing

¼ corridor of ¦ km sides over the same ground areas, as il- points. Routine velocity increment controls compensate for lustrated in Fig. 4. Note that this requirement is not speciﬁed this decrease and maintain subrecurrence of the orbit and

with respect to the reference RSP paths but rather regulates track of RSP paths.

¢ £

Ó× ×Ò

the peak-to-peak difference between a pair of identical RSP The eccentricity vector de-

paths. In addition, this requirement does not assume that ﬁned by eccentricity and argument of perigee is mainly

Â Â ¿ the center of the corridor follows Keplerian RSP paths. The perturbed by the ¾ and zonal harmonics terms of the center can be an osculating path determined by the mean of Earth’s gravity potential according to

deviated osculating trajectories. This requirement, however,

Ê ¿ Ò

is not strictly enforced for high-latitude portions of the or- ¾

Â ×Ò ½ ×Ò Ó×

Ø ¾

µ bit, because ALOS’ satellite design and mission operations ´½

do not assume frequent inclination control maneuvers. (4)

Ê ¿Ò

Â ½ ×Ò

¾ (5)

3. Astrodynamics and Constraints ´½

¾ ¾

Ê ×Ò Ó× ×Ò Â

½·

Despite the trend in control requirements, however,

¾Â ´½ µ ×Ò Earth observation satellites on sun-synchronous subrecur- ¾ (6) rent frozen orbits are subject to disturbances and practical By selecting a particular combination of initial eccentricity

constraints against accomplishing precision orbit control re- Â

values, there will be no variation to the order of ¿ in eccen- quired by high-resolution Earth observing instruments and SAR interferometry. tricity and argument of perigee with time, and the perigee rotation is stopped. The orbit will be frozen in the inertial 3.1. Perturbations space, and the orbit altitude history will repeat with every revolution. When the initial values are near the frozen-orbit Astrodynamics exerts both favorable and unfavorable per- values, which represent more realistic cases, the eccentric- turbations on polar-orbiting satellites. Only long-period ef- ity vector circulates about the frozen point. Therefore, the fects and secular effects are discussed here, because or- nearly frozen orbits maintain an almost constant altitude

Td_21 Trans. JSASS Space Tech. Japan Vol. 7, No. ists26 (2009)

profile over the oblate Earth from revolution to revolution, change is allowed only at a node in eclipse to minimize with very small altitude variations. In order to achieve the battery discharges. In addition, these limitations not vicinity of a particular combination for the initial eccen- only restrict the ability to freely manipulate the eccentricity, a series of initial orbit controls are performed. Be- tricity vector but also produce disturbing effects on the cause the frozen characteristics are affected by the solar ra- eccentricity vector. diation pressure and atmospheric drag perturbations, small orbit maintenance maneuvers have to be performed periodi- 6) Eccentricity vector control requires proper timing of or- cally to offset those perturbations.8) 9) bit control executions especially when constraint 5) is effective. Therefore, initial acquisition and reacquisition of the frozen point may require a rather long opera- 3.2. Practical constraints tion involving a series of orbit controls. Implementing a In addition to the astrodynamic perturbations, there are strict frozen orbit sometimes accompanies mission op- several practical constraints, mostly in a satellite side in im- erations’ sacrifices. plementing precision orbit control: 7) In order to maximize observation data acquisition and 1) Since orbit control thrusters are typically assembled on to avoid operational complications, orbit control may only one side of a satellite, the satellite can produce a often be executed on particular days of a week and thrust vector only in one direction with respect to the temporal intervals between orbit controls typically have satellite body. It must make thruster-based attitude ma- minimum days and finite resolutions. This means that neuvers causing uncounted adverse velocity increments orbit controls may not be performed as frequently as to implement deceleration and inclination changes, un- desired even in solar maximum period and then fre- less it carries large angular momentum exchange de- quency has to be compensated by velocity-increment vices to make wheel-based attitude maneuvers. magnitude resulting in a larger deadband in orbit control accuracy. 2) Since the light load mode and the safe mode typically use thruster-based attitude control in most Earth obser- 8) Low solar activity in an 11 year solar cycle makes con- vation satellites, the use of thrusters for attitude constraints 4) and 7) more significant. trol significantly disturbs orbits. A series of orbit controls to reinsert the satellite into its target RSP path and 3.3. ALOS’ practical constraints frozen point are necessary before resuming operational observations. ALOS is not an exception to the difficulties in achieving precise orbit control. Like other Earth observation satellites, 3) For a limited duration, the attitude control system of- it is also subject to the practical constraints suggested above. ten uses thruster-based attitude control to settle down

its attitude which is disturbed during the implementa- 3.3.1. Thruster conﬁguration Î tion of ¡ . The use of thrusters for attitude control

even for a short period of time, however, applies an un- The ALOS Reaction Control System (RCS) has four Î desired small ¡ to its orbit, and makes precise orbit pieces of 4N thrusters, eight pieces of primary 1N thrusters, regulation a difﬁcult task. To avoid this adverse effect, and eight pieces of redundant 1N thrusters (Fig. 5). Two

the satellite must carry large reaction wheels which can pairs of 4N thrusters are designed to produce orbit con- Î produce a large torque, or alternatively it must improve trol ¡ together, while at the same time each of the two attitude control performance in its orbit control mode. pairs takes charge of providing pitch and yaw attitude control torque respectively. In one set of eight 1N thrusters, four 4) Since there is a minimum thrust size and a minimum thrusters are assigned to control roll attitude. The remaining ﬁring duration because of thruster line-ups, thruster re- two pairs are primarily designed to produce pitch and yaw sponse characteristics, and the control software pro- attitude control torque respectively, but also play the role of cessing cycle, the satellite can produce only a coarse orbit control thrusters as a backup for the two pairs of 4N

finite impulse bit that prevents it from achieving fine thrusters. As seen in Fig. 5, all of the thrusters are assem- orbit control. 1N thruster is the smallest choice even bled on a panel and are canted to guarantee sufficient

for small to medium-sized satellites. At least one pair torque margins. Therefore, they produce translational force of thrusters must be ﬁred simultaneously. The time de- in the · axis even when the ﬁring objective is merely to

lay and the minimum ON/OFF time are a few tens of produce rotational torque for controlling attitude. ½ msec. The control cycle is typically ½¼ to Hz. 3.3.2. Attitude acquisition mode 5) The satellite can make a deceleration maneuver and an inclination-change maneuver only in limited orbital po- Due to the thruster conﬁguration, thruster ﬁring for apply- sitions because of power, lighting, and thermal condi- ing attitude control torque in the attitude acquisition mode of

tions. An inclination-change maneuver can be applied the Attitude and Orbit Control System (AOCS)10) inevitably

Î · only at the ascending or descending nodes. Sometimes, produces an unwanted net velocity increment ¡ in the a deceleration maneuver involving yaw-around attitude direction and perturbs the orbit to a larger one, even when a

Td_22 T. IWATA et al.: Precision Orbit Control of the Advanced Land Observing Satellite (ALOS)

sition mode for ½¼½ min. This resulted in an altitude in-

crease of ¡ m and the estimated velocity increase of

Î ¼¼¾

¡ m/s. This velocity increment caused the max-

¾ imum equator crossing point deviation of ¾ km 29 days later, but the eccentricity vector was not disturbed. No ad- justment orbit control was performed. Another attitude reacquisition event that took place was intentional. In order to reprogram the attitude control software to update its solar array paddle drive law, the attitude control mode was temporarily transferred to the reacquisi-

tion mode for ¾¼¾ min in April 20, 2007. Since a larger

¼ semimajor axis increase of ¡ m and a subsequent drift and violation of equator crossing points were expected, a series of in-plane deceleration and acceleration controls were executed to recover the groundtrack. Although the eccentricity vector was not moved during the reprogramming operation, the recovery deceleration and acceleration

controls disturbed the eccentricity vector. Since the alti-

¼ tude variation was still within a ¦ km tolerance, we did not employ two-part maneuvers for eccentricity control, but

rather decided to adjust the eccentricity vector by choosing Î ¡ locations in the subsequent half year.

Fig. 5. ALOS RCS conﬁguration Î 3.3.3. Post ¡ idling

7083.5 In order to settle down attitude variation caused by contin-

7083 Î uous thruster ﬁring during orbit control ¡ , attitude is con-

7082.5 trolled by thrusters for ½¼ sec. This thruster-based attitude Î control caused additional ¡ . Fig. 7 shows the equivalent

7082 Î time required to produce the adverse ¡ by 1N thrusters.

7081.5 No obvious dependency was observed on date and orbital position, and only a loose dependency was observed on the

7081

¡Î ¡Î

Semimajor Axis (km) planned . The effect of this additonal is signiﬁcant, Î 7080.5 Attitude Acquisition Mode especially when the planned ¡ is very small, which is typ- in Critical Phase ically the case in this solar minimum period. Although this 7080 06/01/24 06/01/25 06/01/26 06/01/27 06/01/28 06/01/29 effect is partially counted, complete compensation is difﬁ-

Date Î cult because of randomness of the additional ¡ , as seen in

Fig. 6. Semimajor axis history in critical phase Fig. 7. Î 3.3.4. ¡ resolution 3-axis attitude is established and the purpose of the attitude control torque is purely to compensate for the external dis- For orbit control, ALOS was originally designed with 4N turbance torque. The longer the attitude acquisition mode thrusters as primary thrusters. 1N thrusters were considered

as backup thrusters, because 4N thrusters have extended Î continues, the larger the total ¡ is added to the forward

velocity. component life in terms of pulse number and total ON-time. Î This limitation constrains ALOS’ ¡ resolution. However, During the ﬁrst 3.5 days on orbit in the critical phase, since the H-IIA rocket inserted ALOS precisely, we could ALOS had been in the 3-axis submode of the thruster-based omit major portions of initial orbit controls. Therefore, we initial attitude acquisition mode. Fig. 6 shows a history of its were able to exploit life allocation of those portions for con-

semimajor axis during the ﬁrst 5 days. Although the eighth Î tinuous ¡ and started to use 1N thrusters for producing

ﬂight of the H-IIA rocket precisely inserted ALOS into an

Î ¡Î

¡ so that we could obtain ﬁner resolution.

¼½

orbit with the achieved accuracy of ¼ ( ) and ( ), Î Another limitation that prevents from pursuing finer ¡ the altitude had been increased by about ¿ km by thruster resolution is minimum firing time. Due to the control soft- firing for attitude regulation until the start of the reaction- ware that processes commands, we cannot specify a contin- wheel-based normal attitude control mode.

uous thruster ﬁring shorter than ¾ sec. The precisely regulated orbit was also disturbed by the thruster-based attitude reacquisition mode. On December 3.3.5. Attitude maneuver 12, 2006, ALOS automatically transferred to the light load

mode involving the attitude reacquisition mode to protect In order to perform deceleration orbit control or inclina-

¦¼ itself against an operation error. It had been in the reacqui- tion orbit control, ALOS has to make ½¼deg or deg

Td_23

Trans. JSASS Space Tech. Japan Vol. 7, No. ists26 (2009)

¡Î

Ó× ´ · ´ Ø µµ

+Δ a Maneuver (1N)

¡ ¾

+Δ a Maneuver (4N) (10)

· ´ Ø µµ 60 ×Ò ´

½ 40 V Time (sec)

Δ Î

where ¼ is the original orbital velocity. Based 20 on this simple model, changes in the ec-

Equiv. 0

06/01/01 07/01/01 08/01/01 centricity vector are estimated to be ¡

¢ £

Date Ì

½¼¿ ¢ ½¼ ¼ ¢ ½¼ for the ½¼deg maneu-

¢ £

+Δ a Maneuver (1N) Ì

¼ ¢ ½¼ ¼¾ ¢ ½¼

Δ ¡

60 + a Maneuver (4N) ver and for the ¼ deg maneuver. 40 V Time (sec) Δ 20 3.3.6. Other constraints

Equiv. 0 0 50 100 150 200 250 300 350 Acceleration control is nominally allowed above polar re- ω + f (deg) gions to minimize accumulated perturbations of the eccentricity vector but it is not strictly limited to polar regions. On the other hand, deceleration control and inclination control are allowed only at the ascending node to alleviate the bat-

1 tery’s depth of discharge and to minimize changes caused by 10 thermal distortion. As for operational frequency, orbit controls are usually allowed only on Saturday (JST). Finally, V Time (sec)

Δ ALOS was launched in the solar minimum period, in which

¡Î

Equiv. the resolution is more critical in terms of accomplishing

0 10 orbit control accuracy.

1N 2sec Firing 4N 2sec Firing +Δ a Maneuver (1N) +Δ a Maneuver (4N) 4. Control Strategy

-3 -2 -1 0 10 10 10 10

Δ V (m/s) 4.1. Original design

Î ¡Î Fig. 7. Equivalent ¡ caused by post idling A control strategy for ALOS’ orbit acquisition and regulation consists of three parts: yaw-around attitude maneuver by 1N thrusters. Since the 1) RSP control (altitude control) by acceleration and ma- rotational torque and the resulting velocity increments are neuvers for regulating cross-track deviations with re- applied by thrusters at particular arguments of latitude along spect to geo-ﬁxed reference paths,

the orbit, the eccentricity vector is perturbed by the yaw- ¼ around attitude maneuver. In addition, the ¦ deg yaw- 2) eccentricity control consisting of acceleration and de- around maneuver brings a net along-track velocity incre- celeration maneuvers and phasing for achieving a

ment, while, in the ½¼deg yaw-around maneuver, positive frozen orbit and regulating altitude variations above and negative along-track velocity increments are canceled to geo-ﬁxed reference paths, and yield a zero net along-track velocity increment. A change in the eccentricity vector caused by the yaw- 3) inclination control by out-of-plane maneuvers for keep-

around maneuver is derived below. Along-track transla- ing the local sun time of the descending node.

´Øµ tional force induced by the yaw-around maneuver The RSP control or the altitude control assumes the initial

torque is acquisition of the RSP paths and subsequent routine alti-

´Øµ Ó× ´ ´Øµ µ

¼ (7)

tude controls to compensate for atmospheric drag. In the

´Øµ where ¼ is the thruster force, is the yaw angle, and is check-out phase, the initial acquisition of the RSP orbit was

the thruster cant angle. The along-track velocity increment

performed for the original RSP control criteria of ¦ km

¡Î

caused by this force is at the equator. The eccentricity control assumes the initial

½ acquisition of the eccentricity vector and subsequent semi-

¡Î ´Øµ Ø ½

(8) passive orbit control with minimum adjustments in routine

Ñ Ø

× altitude control to compensate for perturbations caused by

´Øµ ØØ

solar radiation pressure and atmospheric drag. In the check-

Ø Ê

(9) Ø

out phase, the initial acquisition of the eccentricity vector

´Øµ Ø Ø

× was performed for the original altitude variation criteria of

Ñ Ø ¦¾

where is the satellite mass, × is the attitude acceler- km. The inclination control assumes the initial acquisi- Ø

ation/deceleration start time, is the attitude acceleration of the inclination and the re-acquisition no earlier than

Ø ¡Î

tion/deceleration end time, and is the centroid time. two and half years in order to establish sun-synchronicity

A change in the eccentricity vector ¡ caused by the yaw- and minimize changes in local sun-time of the ascending around maneuver is given by node. The initial inclination acquisition was not performed

Td_24 T. IWATA et al.: Precision Orbit Control of the Advanced Land Observing Satellite (ALOS)

7090

5000 5000 7080

a (km) 7070

(km) 0 (km) 0 7060

ECR ECR 0 1 2 3 4 5 6 7 8 9 y z 4 t (sec) x 10 -5000 -5000 98.22 98.2 98.18 i (deg) -5000 0 5000 -5000 0 5000 98.16 x (km) x (km) ECR ECR 98.14 0 1 2 3 4 5 6 7 8 9 4 -73 t (sec) x 10

Fig. 8. Trajectory with respect to ECR frame (one day) -73.5 (deg)

Ω -74 7090 -74.5 7085 0 1 2 3 4 5 6 7 8 9 t (sec) 4 7080 x 10 7075 ¦r¦ (km) 7070 Fig. 10. Orbital elements (one day) 7065 7060 -3 0 1 2 3 4 5 6 7 8 9 x 10 4 3.5 t (sec) x 10

7.51 2.5

7.5 2

¦v¦ (km/sec) 7.49 1.5 y

7.48 e 1 0 1 2 3 4 5 6 7 8 9 4 t (sec) x 10 0.5

0 Fig. 9. Radius and velocity (one day) -0.5

-1 because of good insertion accuracy, and the ﬁrst inclination -1.5 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 e -3 control campaign is planned from June to August 2008. x x 10

4.2. Improvement efforts Fig. 11. Eccentricity vector trace (one day) Three updates to the strategy were implemented: ground-based GPS differential positioning using measure- 1) The primary orbit control thrusters for the altitude con- ments from an on-board dual-frequency carrier-phase mea- trol were changed from 4N thrusters to 1N thrusters, suring GPS receiver.3)4)11) The results presented in Sec- tion 5.3. were assessed with posterior orbit estimates from

2) the altitude variation criteria for the eccentricity vector range and range-rate data.

¼ control was revised to ¦ km, and

3) the RSP control criteria for the routine altitude control 5.2. Short-period behavior

¼ were revised to ¦ km. An ALOS’ flight trajectory with respect to the Geocen- The first update, which was applied to the routine altitude tric geofixed frame is shown for a one-day period (July 31,

control from August 18, 2006, intended to meet the original 2007) in Fig. 8. Time histories of magnitudes of ALOS’

Ö Ú RSP criteria and to improve SAR interferometry. The sec- position and velocity vectors ( and ) are shown in Fig. 9. ond and third updates intended to improve SAR interferom- Time histories of ALOS’ orbital elements for the same day etry performance. A series of orbit controls for the second are shown in Fig. 10. Fig. 11 gives a trace of ALOS’ eccen- update were carried out on August 5 and 7, 2006. The third tricity vector for the same day. These ﬁgures demonstrate update regarding the RSP control criteria became effective a typical short-period behavior of ALOS’ orbit and indicate from the routine altitude control on February 2, 2007. that variations of the orbital parameters during one revolution are much larger than a mean behavior of those orbital parameters over the longer term, which will be given in the 5. Flight Performance next sections. This short-period behavior also implies the need for a frozen orbit to avoid large altitude variation over 5.1. Orbit determination accuracy long period of time. The results presented in Sections 5.2. and 5.4. were as-

sessed with the posterior precision orbit estimates with

´¿ µ an accuracy of ½ m that were obtained from the

Td_25 Trans. JSASS Space Tech. Japan Vol. 7, No. ists26 (2009)

-3 -3 7100 x 10 x 10 01IP 02IP 06IP 07IP 5 2.5 7090 03IP 04IP05IP 7080 7070 4 a (km) 7060 2 03IP 7050 3 02IP 30 -3 40 50 60 70 80 90 x 10 4 t (day) 04IP

y 05IP y

e 2 e 1.5 3 06IP 07IP

e 2 01IP 1 1 1 0 30 40 50 60 70 80 90 0 t (day) 98.22 -1 0.5 -4 -3 -2 -1 0 1 2 -1 -0.5 0 0.5 1 98.2 -3 -3 e x 10 e x 10 98.18 x x i (deg) 98.16 98.14 30 40 50 60 70 80 90 Fig. 14. Initial orbit acquisition (eccentricity vector trace) t (day)

7070.2 4N e Control RSP Control Update

Fig. 12. Initial orbit acquisition (, , , 1 revolution average) 7070 7069.8

a (km) 7069.6 7069.4 400 LLM LLM ACFS Update 7069.2 100 -3 200 300 400 500 600 700 800 300 x 10 2 t (day) 01IP 02IP 03IP 04IP05IP06IP 07IP 200 (km) e Δλ 100 1.5 R 0 1 -100 100 200 300 400 500 600 700 800 30 40 50 60 70 80 90 98.22 t (day) t (day) 98.2 5 98.18

i (deg) 98.16 98.14 100 200 300 400 500 600 700 800 (km) 0 t (day) Δλ R

-5 Fig. 15. Orbital elements (revolution average) 30 40 50 60 70 80 90 t (day) 40

30 ments for 2 months after the critical phase. Fig. 13 gives a time history and a phase plane trace of the cross-track de- 20 04IP 05IP viation of equator crossing points. Fig. 14 shows traces of 10 the osculating (left figure) and mean (right figure) eccentric- 0 ity vectors for the same 2 months. These figures illustrate that the RSP equator crossing points were first acquired with

)/dt (km/day) -10 Δλ 01IP 03IP 01IP and 02 IP, and then the frozen point eccentricity vector

d(R -20 was acquired with 03IP to 07IP. -30

-40 5.4. Control performance -50 -100 -50 0 50 100 150 200 250 300 350 RΔλ (km) The two types of orbit control with the three revisions have been implemented for ALOS in the past 27 months. Fig. 13. Equator crossing points drift (revolution interval) The performances of these orbit controls are presented in Figs. 15 to 20. Fig. 15 shows the time histories of the orbital 5.3. Initial orbit acquisition elements, and Fig. 16 gives a time history of the cross-track deviation of equator crossing points. The behavior of the A series of test maneuvers and initial orbit acquisition ma- equator crossing points incorporated with the effects of the

neuvers are presented in Figs. 12 to 14. Four test maneuvers altitude controls is also presented in Fig. 16 in the phase

·¡ ¡ ¡ consisting of 1N·¡ ,4N ,1N , and 1N ma- plane. Fig. 17 shows a trace of the mean eccentricity vec- neuvers were tested in order to verify the health of AOCS tor. Finally, repeat-pass altitude and cross-track deviations and RCS and to validate the operational procedures. Seven are shown in Figs. 19 and 20, based on a deﬁnition given in in-plane controls for initial orbit acquisition (01IP to 07IP) Fig. 18. Fig. 19 presents a long-term trend for the altitude

were performed to acquire RSP paths and a frozen point. and cross-track variations over the same ground locations Ý The ﬁrst and second in-plane orbit controls (01IP and 02IP) with different arguments of latitude. Changes in and inserted ALOS into the RSP paths and then the third to sev- represent the altitude and cross-track variations. Short-term enth in-plane orbit controls (03IP to 07IP) moved the ALOS’ variations with a time constant less than one day, as seen in eccentricity vector near the frozen point. Fig. 19, are attributable to different RSP paths. Therefore, Fig. 12 shows the time histories of ALOS’ orbital ele- actual altitude and cross-track variations for the same RSP

Td_26 T. IWATA et al.: Precision Orbit Control of the Advanced Land Observing Satellite (ALOS)

3 e Control LLM h 2 RSP Control Update 4N ACFS Update Flight Corridor 1 Y Cross Section Δ (km) 0 rrp

Δλ Actual Flight

R -1 Trajectory -2 X -3 100 200 300 400 500 600 700 800 RSP Reference t (day) Orbit 0.5 Z rp rr

(km) 0 Δλ R Geocenter

-0.5 100 200 300 400 500 600 700 800 t (day) Fig. 18. Differential position coordinate 1.5 0.15 2007.6.16 to 2008.2.14 8 e Control 0.1 4 deg 1 6 0.05 40 deg 4

0.5 h (km) 0 90 deg )/dt (km/day)

)/dt (km/day) 2 Δλ Δλ d(R

d(R -0.05 0 0 100 200 300 400 500 600 700 800

-0.1 t (day) 2 -0.5 -4 -3 -2 -1 0 1 2 -0.6 -0.4 -0.2 0 0.2 0.4 0 RΔλ (km) RΔλ (km) 4 deg -2

Fig. 16. Equator crossing points drift (top: revolution average, bottom: one y (km) -4 40 deg -6 day average) 90 deg RSP Control Update -8 100 200 300 400 500 600 700 800 -3 x 10 t (day)

1.4 Fig. 19. Altitude and cross-track deviation trend

8 1.3 Path No.: 68 6 Eccentricity y 4 e 1.2 Control (2006/8) h (km) Post e Control 2 Post-ACFS-Update Deceleration Control 1.1 0 (2007/4) 0 50 100 150 200 250 300 350 ω+f (deg) 10 1 5 Post RSP Control Update -2 -1 0 1 2 0 e -4

x x 10 y (km) -5

Fig. 17. Eccentricity vector trace (two day average) -10 0 50 100 150 200 250 300 350 ω+f (deg) paths are slightly less than those in Fig. 19, as shown for Fig. 20. Altitude and cross-track deviation along orbit Path 68 (Hokkaido-Tokyo Path) in Fig. 20. Due to the RSP control, the equatorial cross-track devi-

ation of the ALOS’ subsatellite traces with respect to the ALOS’ altitude deviation above the RSP paths, which the

ground reference RSP paths has been controlled within ¦ eccentricity control ought to regulate, had been maintained

½¿¾

km except from June 28 to July 18, 2006. To reduce peak- within ¦ km until August 5, 2006. This altitude devia-

¾¼ ¢ ½¼ to-peak change of the cross-track deviation from the RSP tion corresponds to an eccentricity error of ¦ rel- paths and therefore to reduce the risk of exceeding its toler- ative to the frozen point. For a better SAR interferometry, a ance, the altitude control thrusters were changed from 4N to series of major orbit controls were performed from August 5

1N on August 18, 2006, and then the peak-to-peak change to 7 to improve the frozen level of the orbit and equivalently

was reduced from ¿ km to km. For an improvement in to reduce the eccentricity error. Since then, the altitude devi-

¼¾¾

SAR interferometry, the RSP control criteria were revised ation and the eccentricity error have improved to ¦ km

¾ ¦¼ ¦¿½ ¢ ½¼ from ¦ km to km on February 2, 2007, and since and respectively. The deceleration control after

then, the equatorial cross-track deviation has been regulated the ACFS reprogramming perturbed the eccentricity vector,

¼ ¦¼¿¾ within ¦ km, as shown in Figs. 16, 19, and 20. and the altitude deviation of about km and the eccen-

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Trans. JSASS Space Tech. Japan Vol. 7, No. ists26 (2009)

¢ ½¼ tricity error of about ¦ have been continued since 4) Iwata, T., Ishida, H., Osawa, Y., and Tomioka, T.: Advanced Land Observing Satellite (ALOS): Development and On-Orbit Status, J. of Space Technology and then. These can be observed in Figs. 17, 19, and 20. Science, 23 (2007), pp.1-13. However, as expected based on the design, cross-track 5) Shimada, M., Isoguchi, O., and Miyagi, Y.: Deformation Monitoring Using the deviation for high latitude regions showed a large change PALSAR and the Related Activity, Proc. of Global COE Workshop2, pp.23-27, (Figs. 19 and 20). The implementation of the inclination 2007. control to reset this change is planned to start on June 11, 6) Fujimura, T., Kimura, T., and Ito, N.: The Case Study of Interferometric SAR by 2008, i.e. 2.5 years after the launch. PALSAR, IEICE Technical Report SANE2006-19 (2006-4), pp.97-102, 2006.

7) Shimada, M., and Hirosawa, H.: Slope Corrections to Normalized RCS Using SAR Interferometry, IEEE Trans. on Geoscience and Remote Sensing, 38 (2000), 6. Recommendations for Future Satellites pp.1479-1484.

8) Vallado, D.: Fundamentals of Astrodynamics and Applications, Second Edition, Finally, the following recommendations are derived for Microcosm Press, 2001. the design of future SAR interferometry satellites with more stringent requirements to orbit control accuracy: 9) Chobotov, V., et al.: Orbital Mechanics, 3rd Ed., AIAA, 2002. 10) Iwata, T., et al.: Precision Attitude and Orbit Control System for the Advanced Land Observing Satellite (ALOS), AIAA Guidance, Navigation, & Control Conf., ¯ Thruster and wheel conﬁguration causing no translational force in producing attitude control torque. AIAA-2003-5783, Austin, U.S.A., 2003. 11) Nakamura, R., et al.: Precise Orbit Determination for ALOS and the Accuracy Veriﬁcation by SLR, J. of Space Technology and Science, 23 (2007), pp.14-19. ¯ Thruster selection and control software design enabling precise impulse bit.

¯ Flexible or autonomous orbit control operations capability in orbital position, timing, and resource management.

¯ Short-time inclination and deceleration control operations capability.

7. Conclusions

ALOS has already completed 27 months on orbit with successful operations. For high-resolution Earth observation and SAR interferometry, precision orbit control was required for ALOS under various practical constraints. Three types of control with three updates were implemented for ALOS to meet the requirements. The ﬂight results demon-

strated that the requirements were achieved: equator cross-

¼ ing points had been regulated within ¦ km from refer-

ence ground paths, and altitude variations over the same geo-

¼ locations had been kept within ¦ km.

Acknowledgments

The authors wish to express their gratitude and appre- ciation to past and current members of the JAXA Flight Dynamics Group, in particular, Mr. Masatoshi Saito, Ms. Chikako Hirose, Mr. Hirohiko Dosho, Mr. Takashi Uchimura, Ms. Maki Maeda, and Mr. Kazunori Someya, for their work in the ALOS’ orbit control planning and operations.

References

1) Bargellini, P., Garcia Matatoros, M. A., Vetimiglia, L., and Suen, D.: ENVISAT Attitude and Orbit Control In-Orbit Performance: An Operational View, Proc. of 6th Int. ESA Conf. on GN&C Systems, ESA SP-606, Loutraki, Greece, 2005.

2) Werninghaus, R., Buckreuss, S., and Pitz, W.: TerraSAR-X Mission Status, IEEE Int. Geoscience and Remote Sensing Symp. 2007, Barcelona, Spain, 2007.

3) Iwata, T.: Advanced Land Observing Satellite (ALOS): On-Orbit Status and Platform Calibration, IEEE Int. Geoscience and Remote Sensing Symp. 2007, Barcelona, Spain, 2007.

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