20 JSTS Vol. 2:3, NO. I

ATTITUDE AND POINTING DYNAMICS OF THE ADVANCED LAND OBSERVING SATELLITE(ALOS):

FLIGHT RESULTS AND CHARACTERIZATION

Takanori IWATA Japan Aerospace Exploration Agency (JAXA) 2-1-1 Sengen, Tsukuba, Ibaraki, 305-8505, JAPAN Phone: +81-29-868-4079, FAX: +81-29-868-5970

E-mail: iwata.takanori@jaxa.jp

Abstract The Advanced Land Observing Satellite (ALOS) was launched on Januaiy 24, 2006 and has been operated successfully since then. This satellite has the attitude dynamics characterized by three large flexible structures, four large moving components, and stringent attitude/pointing stability requirements. Presented in this paper are flight data analyses of ALOS's attitude, rate, and linear acceleration, as well as a brief overview of the design approaches to attitude control and stabilization. The analyses not only yield attitude control and stabilization performances but also exhibit unique dynamic behaviors due to flexible structural modes, thermally induced paddle motion, paddle drive, inter-satellite communication antenna drive, and A VNIR-2 mirror drive. Efforts to improve attitude stability by updating feedback and fecdforward control laws arc also described.

1 Introduction On January 24, 2006, the Advanced Land Observing

Satellite (ALOS) was launched by an 11-llA rocket from the Tanegashima Space Center into a sun-synchronous orbit with the altitude of 692 km. Since then, it has been operated successfully on orbit, delivering a variety of high-resolution images and accomplishing a number of "firsts."

ALOS is a Japan Aerospace Exploration Agency (JAXA)'s flagship for high-resolution earth observation [!]. It is a large Earth observation satellite with the mass of 4000 kg and the power of 7 kW (Figure I). ALOS is designed to contribute to: cartography, regional environment monitoring, disaster management support, and resource survey. In order to accomplish this mission, ALOS has three mission instruments: panchromatic stereoscopic radiometers PRISM, a multi-spectral radiometer A VNIR-2 and a synthetic aperture radar PALSAR.

ALOS is the only Earth observation satellite that is capable of attaining two conflicting goals: global data collection with high resolution (2.5 m). In order to provide precise geometric accuracy for globally observed high-resolution images without ground control points (GCP), ALOS is required to achieve stringent pointing determination accuracy ( 4.0e-4 deg on-board and 2.0e-4 deg ground-based, 30 and I OHz), position determination accuracy (I m ground-based, 3 cr and I Hz), pointing stability ( 4.0c-4 deg/5sec, 3 cr), and pointing control accuracy ( ± 0.1 deg, 3 cr). For

achieving these requirements, a variety of platform and ground systems technologies were developed [2].

This paper presents flight data analyses of ALOS's attitude, rate, and linear acceleration, as well as a brief overview of the design approaches to attitude control and stabilization. The analyses not only yield attitude control and stabilization performances but also exhibit unique dynamic behaviors due to flexible structural modes, thermally induced paddle motion, paddle drive, inter-satellite communication (DRC) antenna drive, and AVNIR-2 mirror drive. Efforts to improve attitude stability by updating feedback and feedforward control laws arc also described.

Slnr Tracker Antenna

\ GPS Antenna ;.. )'·· __ ,-:-::·· mass. 4000kg '•• . . power 7 kW

< . \ ' '\ . ,.;,, "' /' t Phased Array type

L-band Synlhelic . Advanced V1s1b!e Aperture Rnd<ir Panchromatic and Near Infrared (PALSAR) Hemole-sensmg Radiometer type 2

Instrument for (AVNIR-2) Stereo Mapprng (PRISM)

Fig. ALOS On-Orbit Configuration

2 Attitude and Orbit Control System

2.1 Attitude and Orbit Control System Overview The ALOS Attitude and Orbit Control System

(AOCS) [3] is a key subsystem in achieving the pointing requirements. The requirements for AOCS are

the attitude control accuracy of ±0.095deg, the attitude stability of 3.9e-4 deg/5s, the on-board attitude determination accuracy of ±3.0e-4 deg, and data outputs for the ground-based attitude and position determination accuracy of ±l .4e-4 deg and 1 m.

In order to achieve the specified accuracies, AOCS introduced a precision Star Tracker (STT), a precision GPS Receiver (GPSR), high-performance on-board computer (AOCE) based on new 64-bits space-borne MPUs, extended-Kalman-filter-based precision attitude determination and control, phase stabilization of flexible modes, precision cooperative control, and precision orbit model. The AOCS hardware and software are integrated to constitute the closed loop shown in Figure 2.

2.2 Overview of AOCS Operational Status ALOS and its AOCS have completed its first 15

months on orbit with successful operation. All AOCS capabilities including initial acquisition mode, normal control mode, orbit control mode, and paddle sun-track control were verified on orbit. In addition, the cooperative controls for DRC, A VNIR-2, and MTQ, the DRC parameter identification, and yaw steering were verified. Calibration of the STT-based precision attitude determination system has been performed for continuous accuracy improvements.

Fig. 2 AOCS Closed-Loop Block Diagram

3 Dynamics Measurements and Estimates

3.1 Star Tracker ALOS Star Tracker (STT) [4] is a key component

for accomplishing the attitude determination accuracy. It provides the star position accuracy of 9.0 arcsec @ 6 mag (random error) and 0.74 arcsec (bias error), both in 3 cr. Typically, two sets of three optical heads are used simultaneously. Upon the Earth observing data collection, AOCS maintains STT to the track mode optimized for accuracy, and STT gives the positions and brightness of up to 5 stars for each optical head in every second.

21 JSTS Vol. 23, NO. I

3.2 Inertial Reference Unit ALOS has a standard Inertial Reference Unit (IRU)

which consists of three tuned dry gyros (TDGs) [3][8]. The internal temperature of TDG is measured and used for temperature dependence calibration. Both STT and IRU are placed on the optical bench whose temperature is tightly regulated.

3.3 Attitude and Rate Estimates Attitude angles and attitude rates are estimated on-

board at 10 Hz by a fo1ward extended Kalman filter in AOCS [3][5]. For accuracy improvements, they are also estimated on the ground at 10 Hz by repeated smoothers in the Precision Pointing and Geolocation Determination System (PPDS) [5][8]. Flight data analyses in this paper used these estimates.

3.4 Accelerometers ALOS has a set of accelerometers to evaluate the

flexible structural modes on orbit [2]. Nine low-noise accelerometers with ±8 mG range are used at 64 Hz sampling. Three accelerometers measure 3-axis acceleration of the body. Two are allocated for paddle out-of-plane and in-plane motions. Two are given to PALSAR, and two are used for DRC.

3.5 Angular Displacement Sensor ALOS is capable of measuring high-bandwidth

pointing jitter that cannot be measmed with the attitude sensors [2][5][6]. A 3-axis device named the Angular Displacement Sensor (ADS) can provide us precise angular information (±0.010 arcsec, rms) from 2 Hz to 500 Hz at the expense of measurement range. ADS is installed on the optical bench and its measmements are sampled at 675 Hz.

4 Attitude Dynamics

4.1 Characterization of Modes and Disturbances ALOS has three large flexible structures: a solar

array paddle (PDL) with the length of 23 m, a PALSAR antenna with the length of 9 m, and a Data Relay Communication (DRC) antenna with the length of 2 m. The lowest vibration modes of the flexible structures are 0.05 Hz for PDL, 0.5 Hz for PALSAR, and 3.5 Hz for DRC, in the free-free condition. Figures 3 and 4 illustrate major elements of ALOS's attitude dynamics and their relationship. Major sources of internal and external disturbances include the DRC drive, the A VNIR-2 mirror drive, the reaction wheels, the PDL drive, the PDL thermally induced motion, and the environmental disturbances. These disturbances not only affect the spacecraft rigid-body attitude, but also excite various dynamic modes of the DRC H/D gear and support boom, PALSAR, PDL, the spacecraft main structure, and the propellant sloshing.

DRC Antenna

PALSAR (Flexible, 2·axes spring coupling with body)

Solar Array Paddle ( F xible)

, .k:> Paddle Rigid Rotation

AVNIR 2 Mirror

Fig. 3 Attitude Dynamics Elements

Disturbance

>le Dynamics

Attitude

Fig. 4 Relationship of Dynamic Modes and Disturbances

POL OP 3rd, Torsion 1st, POL IP 1st [PALSAR 1st

... .. '' J. •l·! I I!

I I I 111 _I _i_f...--\1 LI I II I I I I . I I 1111 I I I 1'1,1·1 I 11 I I I I

I : : : : : L - : :- : : . I I . \Ii.ii 'Wrfl#llllll'I' I . ...1 I l 11 Lu ___ l __ LL _ llJ,J l I 1 I l I

lO-i Pitch. Attitude (deg.ls)

Fig. 5 Attitude Rate FFT (DRC not in Drive)

Roi: Attitude Rate (deg/s)

: : , _ l L!.,! .... ' .. :: .. L__ L l l I [ J;J I 1 , J. I 1

10·• freqency (Hi)

Fig. 6 Attitude Rate FFT (DRC in Drive)

22 JSTS Vol. 23, NO. I

4.2 Post-Deployment Dynamic Modes The post-deployment stmctural integrity of the large

flexible structural modes was assessed using the flight data obtained with the accelerometers and attitude sensors. Since the major low-order modes were within acceptable tolerances, we transferred the attitude control from the acquisition mode using thruster control with low gain, low bandwidth, and high robustness to the normal mode using wheel control with high gain, high bandwidth, and high precision in 82 hours after the launch. Figures 5 and 6 show the FFT responses of attitude rate estimates for one orbit period in the normal control mode with DRC not in drive and with DRC in drive respectively. Without the DRC drive, the PSD magnitudes over 2 Hz were 100 times smaller than these responses. These FFT responses confirm that the major low-order modes agree with the pre-flight analysis.

5 Normal Control Performance

5.1 Attitude Profiles and Attitude Control Accuracy

The ALOS's attitude control accuracy is mainly accomplished by a precise onboard orbit model and an attitude control based on the precision attitude determination system.

Typical attitude and rate profiles over one orbit period are shown in Figures 7 and 8. And, the distributions of corresponding attitude control errors are shown in Figure 9. These plots demonstrate good attitude regulation for the requirement of ±0.095deg. Large transient responses observed in both the attitude angles and the attitude rates at about 4400 sec and about 8300 sec are due to the thermally induced dynamics of the solar atrny paddle. These transient responses follow the same profiles in eve1y orbital revolution. Larger amplitudes in attitude rates from 5100 sec to 8300 sec are induced by the tracking drive of DRC. Table 1 showed that the attitude control accuracy assessed for multiple orbital revolutions achieved ±0.039 deg.

Table 1 Attitude Control Accuracy Item Axis Req. (Jo) 011-0rbit Acc. (3 CJ)

Attitude Control Roll 0.0±0.095 0.027 Accuracy deg Pitch 0.015

Yaw 0.039

OJ 005

-005

·QI

OJ

OJ

005

0 ;; -005 :{_ -0 I

4000 I

5000

I I.

5000

Rof: Altitude Estimate ;

I I__ i

6000 7000 8000 9000 Pitch_ Attitude Estimate

i I

----1 - ... - . - - ,._,,....._.

! -- .I 6000 7000 8000 9000

Yaw Attitude Estimate

; I i- ; i- : I

-: -;1.; --- - - --- J\1 - -' I I 1 I l 1- 1 1

4000 5000 6000 7000 8000 9000 Time(sec)

Roll Attitude Estimate

;;·, " • ' . l - - . i

o,'\/"v-:. I I . . . - ' ·"' . _; ;; -0005· ., v - I ' I \....- I

< -001· I: __ __L • • 1 4000 5000 6000 7000 8000 9000

Pitch- Attitude Estimate

l (\' - •. 11"'/-----.:,. 0005• ' v v : . . . . \J11 -0.Ql t I I V

4000 5000 6000 7000 8000 9000 Yaw Attitude Estimate

i\ < -001 . vv ; - I ·

4(100 5000 6000 7000 8000 9000 Time (sec)

Fig. 7 Attitude Angle Profile (2006/9/3 Rev I)

Roll Attitude Rate Estimate ... 001

1 0005·

o---J.. i-0005· I Ji -00! 4000

1 ••• ,, •• : I I I j - [ l

6000 7000 8000 9000 Pitch- Attitude Rate Estimate

i

........... .... ··--.... ·0065 1 I

-o OJ 4000 5000 7000 Attitude Rate Estimate

... 001 • 2. 0005

" & -0005

-001 4000

I I

r-..---.................... _._ I I I

I I I I I I

5000 6000 7000 8000 9000 Time(sec)

Fig. 8 Attitude Rate Profile (2006/9/3 Rev I)

Roll

2·

15·

05 .i ', 0 -o I -008 -006 --004 -002 0 002 004 006 008 01

25 • 10' Pitch

2· 15

' 05 I I I

i, I I I -008 ··006 --004 -002 0 0.02 004 0.06 008

• 10' Yow 25

I

15 j ,, 05·

0 I I I --01 -008 -006 --004 -002 0 002 004 006 008 OJ

At.titude Control Accuracy (deg)

Fig. 9 Attitude Control Error Distribution (2006/9/3 Rev!)

23 JSTS Vol. 23, NO. I

5.2 Attitude Stability The ALOS's attitude stability is achieved through a

variety of approaches such as phase stabilization of flexible structures, a precision cooperative control and parameter identification, an active damping of the antenna pointing control, a star-tracker based attitude control, an eigenvalue management, a wheel bias operation, and a randomized PDL array trim.

Typical attitude stability profiles for one orbit period are shown in Figures I 0 (DRC not in drive) and 11 (DRC in drive). Attitude stability distributions corresponding to each plot are also shown in Figures 12 (DRC not in drive) and 13 (DRC in drive). Table 2 summarizes the attitude stability assessed stochastically for multiple orbit revolutions. ALOS has different attitude stability requirements for 420 sec after the initiation of the thermally induced vibration of the solar array paddle and for the rest of the orbit period. The result indicates that ALOS meets the attitude stability requirements during the paddle's thermally induced vibration but does not meet the requirements in the rest of the orbit period. We identified 3 sources that violate the attitude and pointing stability requirements and 1 additional source that violates the pointing stability requirements only:

• Paddle diive (attitude & pointing stability)

• DRC tracking drive (attitude & pointing stability)

• DRC slew drive (attitude & pointing stability)

• A VNIR-2 mirror drive (pointing stability)

The violation to the requirements is mainly due to a 0.5 Hz vibration persistently induced by the paddle drive. In particular, the roll axis is excited significantly . In addition, the DRC tracking drive degrades the attitude stability beyond its budgets.

Table 2 Attitude Stability Performance Condition, deg/5s Axis Rcq. (3 Ii) On-Orbit Acc. (3 a)

Paddle DRC not Roll l.9c-4 7.5e-4 in drive in drive Pitch 5.5e-4

Yaw 6.0e-4 DRCin Roll 3.9c-4 9.4c-4 drive Pitch 6.8c-4

Yaw 6.7e-4 Post DRC Roll

slew Pitch I 4c-4_!Ecak) Yaw I

Thermally Roll 0.04 0.013 induced Pitch 0.01 0.0022 motion Yaw 0,07 0.021

Fig. 11 Attitude Stability Profile (2006/9/3 Rev 1)

• 10'

0 J

i_2:L, 0o , o.s

''°

6 3• z ' ti

05

0 05

-, -

·--:r ' I

" 2 25 Prteh

15 2 25 Yow

1.5 2.5 Attotode St•bl>ty (del/Ss)

j J5 ' x 10'

' J.5 .10)

j JS ' .101

Fig. 12 Attitude Stability Distribution (2006/9/3 Rev3)

• 10'

o, .10· 0.5

O OS

"

Holl T

2 Prteh

LS 2

I

"

Yow

2.5

2.5

Att(ude StabHy (deg/Ss)

J.5 x 10 '.

J.5

.. JS '

• 10'

Fig. 13 Attitude Stability Distribution (2006/9/3 Rev 1)

24 JSTS Vol. 23, NO. I

6 Characteristic Motions

6.1 Thermally Induced Motion of Solar Array Paddle The attitude motion thermally induced by solar

array paddle at an eclipse exit results in the attitude and attitude stability profiles shown in Figures 14 and 15. The stability performance during this motion is evaluated in Table 2. The magnitude, shape, and time constant of the response, which repeat well from a revolution to another, agree with the pre-flight analysis [7] (Figure 16).

oil ; -i--oosf 1 --1

-I '

-011 J I 162 1.63

0051 +

Q: I -0051

-01 I

01: ' 0.051

162

ol -oosl 1

-01 · I 162

I 163

163

Roll Attotude Estirna;te

I ' 1.64 1.65 Prtch Attitude Estimate

- .L

I 164 165 Yow Attitude Estimate

I I 64 1-65

Tome {see)

I

' I 1.66

I 1.66

I 166

' , __ 167

I 1.67

I 167

1.. 10'.

10'

)( 10'

Fig. 14 Attitude Profile due to Paddle's Thermally Induced Dynamics (2006/9/3 Rev3)

Roll Altitude Stat<l;ty

p J\' : 0-/',

,r1""' ' \_, I ' '---··- .

164 165 166 167 Potch Attitude Stabi!.ty •

J\r" .._J 1.63 I 64 165 1 66 ! 67

l 'J'o'

l,,y "\.,_,_,, 163 164 1.65 166 167

lime {sec}

Fig. 15 Fig. 15 Attitude Stability due to Paddle's Thermally Induced Dynamics (2006/9/3 Rev3)

yawan9f€.(de9)

o 1 oo 200 300 4Qo 500 600 tm€'(S€'C)

Fig. 16 Attitude Profile and Stability from Pre-Flight Analysis (Yaw)

25

6.2 Attitude Vibration due to Paddle Drive The persistent attitude vibration observed in

attitude, attitude rate, and attitude stability is typically shown in Figure 17. These plots are for the segment of argument of latitude that usually yields worst-case attitude stability. Although the vibration of 0.5 Hz has small amplitude, it is a dominant contributor to the degradation of attitude stability. In addition, the vibration of about 0.05 Hz, which seems to be the !st out-of-plane mode of the paddle, is observed in the yaw axis at this orbital position.

The main cause of the persistent attitude vibration is the step motor drive and its randomization of the solar atTay drive. Figure 18 shows the attitude stability profile obtained from an experimental operation in which the paddle drive is temporarily suspended, and Figure 19 is the corresponding accelerometer measurements (top: paddle in-plane, middle: paddle out-of-plane, bottom: PALSAR). This 20 mm operation without the paddle drive resulted in the significant reduction of attitude vibration, where attitude stability values are down to 25,.....,47% (Table 3). A significant difference between the on-orbit performance and a preflight analysis is due to an improper application of a step motor torque profile in an attitude stability analysis simulation. Although a combined dynamics of step motor, harmonic drive, gear, and paddle torsional mode was properly modeled in deriving an interface torque profile, this torque profile was simplified improperly in applying the torque to an attitude satellite dynamics model. Since the simplified open-loop torque profile had unrealistic equal positive and negative torque steps at higher rate, the slower 0.5 1-lz mode was not sufficiently excited in the simulation.

Figure 20 shows the attitude stability profile obtained from another experimental operation without the paddle drive randomization. The results were summarized in Table 3, in which the attitude stability values were reduced to 41 ,....., 75% while the randomization was suspended. The reason that the randomization degraded the attitude stability instead of improving it against its design purpose is a step change of short-term drive rate with a residual periodicity of 3 ,....., 4 sec in the randomization logic. Further, the mechanism of the significant excitation of roll axis is currently investigated under a working hypothesis.

JSTS Vol. 2:J, NO. 1

x 10·1 Attitude Estimate -- _•--15· I - -1 I I

I_ __!_______ I ___ L_ _ _ _ 19M 1957

• 10·11 Attitude Estimate _ _ A to'

'i -4 Si : -- : - - - ,

... · I I I I Ji -SS- I I I

-6, l L __ - "'- -·'-= -1-955 1.956 1957 1 958 I 959

• 10 Yew Attitude Estimate x io'

-=_· 2 : - :_ - /'·-:-'i-"f-,, - - , -Y- --',.v/-

-3- - , --J _____ j_ - J -- __ -] -- - = -· - -- - -

1.m 1sss 1ss1 1.sss 1sss lirne(sec) • 10'

• 10· 1 Roll Attitude Hate Esti-nate .. , ' ----------- 1 • --------- - ' ,- -- -

I I I • I

., - .\/\? i'1 1\ 1 :1!1' 11 I: ii , 01t\; • 0- Vl'\i q/:, _.__ 1'1\1111 1:M,; 1-\1 <:;_ ,; v 'JI' l'V 1 v\l1' · :1iY 1 •1 11V\,'\:1: '\i:,r·1r_

I -f _/ - -- -- f1 lJ_ -- - - - I . I I( I v J

+-P1tch Attitude Rate Estimate

--00595 ,---006 I I I I -

-006!5 I J - l J __ _ 1954 1M5 1956 19H

" 1 10-l, Yaw 0Allitudo _ ' ' __

} 05· : - . : . -- - - : : .

j o/-iN111•,i\/1fv'\i'1,.__, -'L •' , _, " ( T T r\I 1f 11 1 v

·05· I I 1

- I -lts:1 1 955 I Js6 1-957 - - -- 1:-H 1 Js9 (sec)

Hoff Attitude Stabir.ty

' ' I

. _.I 1 ' '---/,r ;_ ..... f' 1 I I I

I _L_ ______ l 1.955 1956 1957 !958

Pito;h Attitude Stabilty I --,--

' ii 05· I

I 1959

.10·

o : ,. ; \ ' ' - . l.: .. L L f \A \ ( \ I . I j I :

0 1 J54 1:9ss 1 1 1 Tin1e(sec) x10'

Fig. 17 Attitude Angle, Attitude Rate, and Attitude Stability (2006/9/3 Rev3)

Fig. 18 Attitude Stability at Paddle Drive Suspension

ACC1 slart@: 2006 4 12 3 12_J4 - -r - I

I _ _L_ ___ _L - ___..L J

400 600 soo 1000 1200 1400 1800 ACC2 lime heel

-, 1 1

I 1- I

_____L_ -- _l j 400 600 800 1000 1200 1400 1800

ACC3 lime (He) r ------.----r- I 5 - - - t-

of--!o---r+-' ""'"'-ol-!-'----!1 .. ... -5

i I_ I

I _ _L__ I _j___ __ _____j__ I 200 400 600 800 1000 1200 1400 !600 !BOO

time(sec)

Fig. 19 Acceleration at Paddle Drive Suspension

Randomization Randomization Suspension •i• Resumption

j 11x 10' I I 'I 1

f 05 . -1_- . : : _MWM __ : : : ·_

':l 5 ., 5.1 5.2 l.3 5.4 5.5 1,ux 10 \' I ---- -----.-_ Attrtf:dc -_ . I ' 1 Q_'

--;: I J I I : I j , I 1

pJ,10' ..... 05 I I I I I

(tj I I I ! I µ '

01- - - -5 5.1 .3 5.4 5.5

Time (sec) x 104

Fig. 20 Attitude Stability at Randomization Suspension

Table 3 Attitude Stability Contributions Condition, deg/5s Axis Rcq. (3 a) On-Orbit Acc. (3")

Paddle not in DRC Roll 3.5c-4 drive not in Pitch 2.2e-4

drive Yaw l .5e-4 Paddle in Roll 5.le-4* drive w/o Pitch 2.2e-4*

randomized Yaw 2.5e-4* Contribution of paddle Roll 6.7e-4

drive Pitch 5. le-4 Yaw 5.8c-4

Contribution of DRC Roll 3.6c-4 5.7e-4 drive Pitch 4.0e-4

Yaw 3.0e-4 *: dunng satellite noon -5 to -t-5 nun

6.3 Attitude Vibration due to DRC Tracking Drive When DRC is driven in tracking the DRTS

satellite, an additional vibration with the frequency of 2.5 "'4 Hz is present over the 0.5 Hz vibration and degrades the attitude stability further. This is shown in Figure 21 for the argument of latitude that usually yields worst-case attitude stability. The magnitude of this additional vibration depends on the ALOS1s argument of latitude and longitude of its sub-satellite point, as the resonance dependent on the azimuth and elevation configuration of DRC is the cause. The stability degradation due to the DRC tracking drive was

26 JSTS Vol. 23, NO. 1

derived from Table 2 and is listed in Table 3. Figure 22 is the corresponding accelerometer measurements (top: PALSAR2, middle & bottom: DRCl & 2).

Although these persistent attitude vibrations due to the paddle drive and DRC tracking drive degrade the pixel gcolocation determination accuracy of PRISM images, the effects are alleviated by the geometric correction using attitude estimates. Of the attitude stability with respect to 0"'5 sec time frame, the slower attitude stability element with respect to the time frame of about 0.5 to 5 sec can be c01Tected while the residual higher-frequency attitude stability element with respect to the time frame of 0 to about 0.5 sec stays in PRISM images. The residual attitude stability with shorter time frame is summarized in Table 4.

Roi Attitude Estimate

I I

] oJ,_ \ f:f\j\ A},:/\ 0/ /\ .. ,,,r•..-JVV'.•"-< _,. vV"'v l' -,w v Y, v v· , :: -IOr t <( I I l

7690 7700 7710 7720 7730 x 10 • Prtch Attitude Estimate

7690

-00595

7700 7720 l<me (sec)

1740 7750

7740 7750

'

-006151 · : : 1 I ·1590 7700 11 tO 7720 7730 7740 7750

x 10 1 Yaw. Attitude Rate Estimate I '

I

0

-05• · 1 .

;£ 7700 7710 1120 7130 77140 1750

'E O.Si

Time(sec)

Ho' Attitude Stability

''\ i I \ I • .fl I ',I I 1 I l\,

1\ I l I

1110 7720 7730 Pilch Attitude StaMty

I 1140

' I I I !· " fl , I _.., /I . ' •'I"\ • 1' I \ " f\ I

I I l I / . / , B 1100 1110 1120 nJO

• !O i Yaw Attitude Stabihy 'Oli I ' '

I I

7740

I I I "I I I ; 05! I flt-! IV \I v' I f".A. I a i ,,(' l\1\ r I \ I ' I ''\' I \ I " r· / I V I 1\ j t/ \

o· 1 I l l I ' Ji. 7690 7700 7710 1720 7730 7140

Time (see) mo

Fig. 21 Attitude Angle, Attitude Rate, and Attitude Stability (2006/9/3 Rev I)

ACC4. start 2006 2_ 26 4. 26. 30

!:>' I I . . .. . . . . . I

I l•m ... - ·:·" .§ 0

i t -5

0 soo 1000 1500 2000 2500 3000 3500 ACC5

soo 1000 1500 2000 2500 3000 3500

<J trno<m) 111. , HI -..§. 5 .. i I

1-: :--0 500 1000 1500 2000 2500 3000 3500

time(sec)

Fig. 22 Accelerations at DRC Tracking Drive

Table 4 Uncorrectable Attitude Stability Co11ditio11 Axis Req_ (3 a) U11corrcctablc On-Orbit

Ace, (3 a) Paddle DRC not Roll I .9c-4 deg/5s 2.4c-4 d<:ll_i0.5s in drive in drive Pitch 2.1 c-4 d<i!9.5s

Yaw I .6c-4 d<:ll_i0.5s DRCin Roll 3.9c-4 dcg/5s 5.2c-4 d<:ll_i0.5s drive Pitch 3.0c-4 d<:ll_i0.5s

Yaw 2.1 c-4 d<:ll_i0.5s

--. ' .

I I I 00 5120 5140 5160

' ' --- "'-! •. -I

I ' I 5040 5060 5080 5100 5120 5140 5160

Time (sec)

Roll Attitude Stability I I

I

'\' /'' 11' \ "• ! I

Fig. 23 Attitude Angle and Stability at DRC Slew Drive

27 JSTS Vol. 2:3, NO. I

6.4 Transient Attitude Motion due to DRC Slew Drive Although angular momentum exchange between

DRC and the ALOS body upon DRC slew motion is properly compensated by the cooperative control of AOCS, the slow attitude motion that violates the attitude stability requirements is observed right after the DRC slew motion. Figure 23 shows a typical flight example of this motion corresponding to about 5000 sec in Figure 11. This attitude motion is caused by the high-speed motion that DRC makes right after its slew motion in order to eliminate a pointing error between a slew target (DRTS orientation at slew start) and a current DRTS orientation. The AOCS cooperative control is not designed to compensate this motion, because a proper feed-forward signal is not available from DRC in this sub-mode.

The effect of this attitude motion on the geometric accuracy of PRISM images is minimum, because the geometric effect with relatively large time constant can be corrected by attitude estimates and the DRC slews are planned only about twelve times per day for the period in which PRISM is not operated.

6.5 Pointing Vibration due to A VNIR-2 Mirror Drive Occasionally, temporary cross-track image

fluctuations with the amplihtde of 2 to 3 pixels and the period of about 30 pixels are observed in PRISM images (Figure 24). This implies that the PRISM pointing axis vibrates with the angular displacement of 4e-4 to 6e-4 deg, p-p about the roll axis at about 80 to 90 Hz. The phenomena occurred only when the A VNIR-2 mirror was driven. Note that the A VNIR-2 mirror drive lasts at most for 10 sec and is executed at most several times per orbit revolution. Although not only the attitude estimates sampled at I 0 Hz but also the ADS measurements sampled at 675 Hz did not show the vibration having the equivalent magnitude, the ADS measurements indicate a vibration with the amplitude of only 0.2e-4 deg and the frequency of 80 Hz during the A VNIR-2 mirror drive (Figure 25). The FFT responses of the ADS measurements before the A VNIR-2 mirror drive and the measurements during the A VNIR-2 mirror drive are compared in Figures 26 and 27. The comparison indicates an additional mode of about 80 Hz due to the A VNIR-2 mirror drive.

A current estimate for this anomaly is a resonance of local elements (mirrors or CCD) in PRISM by the high frequency drive of the A VNIR-2 mirror step-motor. Using a finite element model, further investigation on the trans1111ss1on and excitation mechanism of the 80 Hz mode is in progress.

Fig. 24 PRISM Image at A VNIR-2 Mirror Drive

10·• Roll. ADS Measurement

7257 12575 7258 12585 1259 12595' 1260 12605 7261 12615

7256 5 7257 1257 5 1258 1258 5 7259 7259 5 1260 1260 5 7261 7261 5

.. ,H···:·_ -- i ;·- i -_ - - -.-_ -- - I I • = --- -1- I . - '

· I I I I I 1 Ji -1 - - I I I - - I - !

-2 __ • __ ] __ J J •---. _]_ J : ----72565 7257 72575 7258 72585 1259 72595 7260 1260.5 7261 1261-5

Ti-ne(sec:)

Fig. 25 ADS Measurements before and during A VNIR-2 Mirror Drive

Roll: ADS Measurement (dog) ·-1r_r11:1 I I 1 -·1·-i- 'I ''11'1 I I

111111 '-.I I I

>Ji 1 l-1 ,. rl\f1 ' ' II lllj\\fi I •• I I I I I II

l l I I I _J O. l I _ I l _ _ I : l I. I L. _ 1 O Pitch: ADS MeJ!Lremcnt (deg) 1 O

--r ii i·1 ! i · .. T:-11 i i 1:1 i ·r ·I lh(\I I Ill I I

I 1111 I ;·----1-' - h .. l II I I !

, ,,,[, : : ; : • 10° Yaw: ADS (deg)

i iiii'l i i · r· i ii 1 i i I I I 1111 I I I 1111 I

, _Lt J t il100

Fig. 26 FFT Response before A VNIR-2 Mirror Drive

Roll: ADS Measurement (dog) <;:- ,--:_;"I'- 1-1:!1'1 -I I' II 'I'

10 io 1 ':""' I

- I I! 111 IJj)1 i II Ill I I

n. 0 •. 1i.11.1 1 1 1 11 ,1. 1

lO Pitch· ADS MeJ&rement (dog) IO

0 1/) a_

<;:-

' -2 10 0 1/) a_

r i ii 1 i i ! I -i

" !J'--,(\.1 I I I 1 1 11 'Vr'\.JAi.\ 111,j, !JM1 1 I I I I I I 11Jif11 · I I I I I I I l 11

I J l 11 1 1 1 l 11 ! 1 100 Yaw ADS Mcalerement (deg) lO

ii "ii I i i ii ·1

' \/ V'

I I If I 1""'11\AJ \!J, I I I II I I I I I rr

.11100 , I J 11 :i'Qi l I I I , : 11:02 _ ,

freqency (Hz)

Fig. 27 FFT Response during A VNIR-2 Mirror Drive

28 JSTS NO. 1

7 Control Law Updates

7.1 Improving Attitude Stability In order to improve attitude stability against the 0.5

1-lz vibration, the AOCS feedback control law in the normal control mode was tuned up and upgraded on May 14, 2006. Figure 28 shows the Bode plots with the original control law and with the revised control law for the roll axis with 90 deg paddle angle. This control law update for 3 axes provides additional phase stabilizations for the 0.5 Hz mode and resulted in about 20 % improvement in attitude stability. The results presented in the previous sections were obtained with the updated control laws.

., ·IWt !!, ,.,. l ' -.li)'Jr·

POL PALSAR 1&1

'• 17 3<18 PAJ..SAA Ull:J

---- .. __ /\u , 212 I , t , lt J dB 111 'I'

l• .. ;

'" . ,t 11 9 3\JB PAL SAR 200

.i

'.1<11

Fig. 28 Roll Bode Plots (Top: Original, Bottom: Update)

7.2 Parameter Identification for DRC Cooperative Control In order to compensate the angular momentum

exchange between the ALOS body and DRC due to the occasional DRC slew motion, AOCS executes fccdforward control, called cooperative control, in accord with the DRC antenna motion. The fccdforward control uses a linear relationship derived from a 3-rigid-body antenna model and consisting of antenna angle and rate terms and mass property terms. The resulting fccdforward torque is applied to the wheel servo loop.

Three parameter identification operations using a special DRC slew profile were canicd out on orbit to

reduce the cooperative control error. The improvements through the identifications were observed and shown in Figure 29. A typical attitude stability performance upon a routine DRC slew after the parameter updates is shown in Figure 23.

Be-005 6e-005 4e-005 2e.005

-2e-005 -4a.005 -6e.005 -8e.005 -00001

-0 00012 -0 00014

Roll[degfsec) - -'fo1·=:.::..-

2nd 3rd-

0 20 40 60 80 100 120 140 160

00004 0.0003 -0 0002 -0.0001 -

-0.0001. -0 0002 --0 0003 -0 0004 .

limefsecl Pitch(deg/sec)

-00005-- - ---•--•--0 20 40 60 80 100 120 140 160

8c-005

6c-005 -

4e-005 ·

2e-005 ·

o--2e-005 ·

-4c-005 ·

-6c-005

-Be-005 -·

t1mefsecl

Yaw(deg/sec]

0 20 40 60 80 100 120 140 160 8 timefsecl

Fig. 29 Performance Improvements through DRC Parameter Identification Slews

8 Conclusions ALOS and its AOCS have completed its first 15

months on orbit with successful operation. All capabilities and perfonnances of the ALOS AOCS were verified during this period. ALOS attitude dynamics on orbit were characterized and analyzed with flight data. Although most on-orbit attitude motions agreed with pre-flight analyses, the unexpected attitude and pointing motions that exceeded the stability requirements were idcnti lied. The causes of these motions were summarized and will be reported in detail in future publications. The feedback control law update alleviated the attitude vibrations and the on-orbit DRC parameter identifications improved the cooperative compensation.

29 JSTS NO. I

References [I] Takanori Iwata: Sensing and Control in lligh-Rcsolution

Earth Observation Satellites: Present and Prospects for the 21st Century, Measurement and Control, Vol. 40, No. I, pp. 113--121 (200 I) (Japanese).

[2] Takanori Iwata, ct. al.: Precision Pointing Management for the Advanced Land Observing Satellite (ALOS), 23rd ISTS, ISTS2002-d-56, Matsue (2002).

[3] Takanori Iwata, ct. al.: Precision Attitude and Orbit Control System for the Advanced Land Observing Satellite (ALOS), AIAA GN&C Conf., AIAA-2003-5783, Austin, U.S.A. (2003)

[4] Takanori Iwata, ct. al.: Precision Star Tracker for the Advanced Land Observing Satellite (ALOS), 27th Annual AAS G&C Conf., AAS04-027, Breckenridge, U.S.A. (2004)

[5] Takanori Iwata: Precision Attitude and Position Determination for the Advanced Land Observing Satellite (ALOS), SPIE 4th International Asia-Paci lie Environmental Remote Sensing Symp., Honolulu, U.S.A. (2004)

[6] Takanori Iwata, ct. al.: High-Bandwidth Pointing Determination for the Advanced Land Observing Satellite (ALOS), 24th ISTS, ISTS-d-10, Miyazaki (2004).

[7] Takanori Iwata, ct. al.: Thermally Induced Dynamics of Solar Array Paddle: Experimental Study, 48th Joint Conf. Space Sci. Tech., Fukui (2004) (Japanese).

[8] Takanori Iwata, ct. al.: Ground-Based Precision Attitude Determination for the Advanced Land Observing Satellite (ALOS), 25th ISTS, ISTS-d-32, Kanazawa (2006).

[9] Takanori Iwata, ct. al.: Precision Attitude Determination and Control for the Advanced Land Observing Satellite (ALOS): Flight Results, 50th Joint Conf. Space Sci. Tech., Kitakyushu (2006) (Japanese).

[10] Takanori Iwata, et. al.: Attitude Dynamics of the Advanced Land Observing Satellite (ALOS): Flight Results, Proceedings of the 16 th Workshop on Astrodynamics and Flight Mechanics, !SAS, Sagamihara (2006).