Session Payload Subsystems

Taller de Diseño de Picosatélites (CUBESATS) Taller de Diseño de Picosatélites (CUBESATS) y Estaciones de Tierray Estaciones de Tierray Estaciones de Tierray Estaciones de Tierra

Session 3Session 3Payload & SubsystemsPayload & Subsystemsy yy y

J M l d l CJ M l d l C M d R iM d R iJuan Manuel del CuraJuan Manuel del CuraDirector de Director de ProyectoProyecto, SENER, SENER

DptoDpto. . VehículosVehículos AerospacialesAerospaciales, ,

Mercedes RuizMercedes RuizIngenieraIngeniera de de SistemasSistemas, SENER, SENER

[email protected]@sener.es

ETSIA. UPMETSIA. [email protected]@sener.es

1Taller de Diseño de Picosatélites (CUBESATS) y Estaciones de Tierra. J.M. del Cura

ContentContent

• Picosat=Picodesign?g• System Engineering process• Main elements of a mission/spacecraftp• System drivers • Picosat payloadsp y• Picosat subsystems

– Attitude and Orbit Control– Data Handling – Communications– ThermalThermal– Structure– Propulsion


– Power

WarningWarning

• Terminologygy– Acronisms– English terms

• Too many slides– Adaptable presentation

Out of focus– Out of focus

• Ackowledgements


Picosat=Picodesign?Picosat=Picodesign?



Y Chaser

Z IBDMTarget

Z IBDMChaser

Y IBDMChaser

XIBDM

Z TargetX Chaser

Z Chaser

X IBDMTargetY IBDM

Target

IBDMChaser

X Target

g

Y Target



• Main differencesMain differences– Learning process– Standard equipment– Some decissions predetermined– Organisation– Schedule



XIXI IVIVProf. Nakasuka

Program Director

Prof. Nakasuka

Program Director XIXI--IVIV

Y.Tsuda

Project Manager

Y.Tsuda

Project Manager

CCElectronicsElectronics CommunicationCommunication PowerPower StructureStructure EnvironmentEnvironment Ground Seg.Ground Seg.

AAU CubesatAAU CubesatY.ArikawaY.TsudaN.MiyamuraS.Ishikawa

Y.ArikawaY.TsudaN.MiyamuraS.Ishikawa

T.ItoY.KatoT.EishimaS.Ukawa

T.ItoY.KatoT.EishimaS.Ukawa

N.SakoT.EishimaY.ArikawaS.Ukawa

N.SakoT.EishimaY.ArikawaS.Ukawa

N.MiyamuraT.ItoS.Ogasawara

N.MiyamuraT.ItoS.Ogasawara

P.SeoN.SakoK.KanairoK.Muramatsu

P.SeoN.SakoK.KanairoK.Muramatsu

S.OgasawaraY.TsudaT.MurakamiY.Oda

S.OgasawaraY.TsudaT.MurakamiY.OdaS.Ishikawa

T.MurakamiE.HwanK.Kanairo

S.IshikawaT.MurakamiE.HwanK.Kanairo

S.UkawaS.IhikawaY.KuwataT.Yamamoto S.Ganryu

S.UkawaS.IhikawaY.KuwataT.Yamamoto S.Ganryu

S.UkawaR.FunaseS.Hori

S.UkawaR.FunaseS.Hori

K.MuramatsuK.Muramatsu Y.OdaI.IkedaY.OdaI.Ikeda


System Engineering ProcessSystem Engineering Process

0Mission

Analysis,Needs

AFeasibility

BPreliminaryDefinition

CDetailed

Definition

DProduction/

Ground QualificationTesting

EUtilization

FDisposal

ESA

Identifiedg

MDR PRR PDR CDR AR

Pre- A B C D E

A Phase AAdvanced

Studies

PreliminaryAnalysis

Definition Design Development Operations

MCR MDR PDR CDR ORR DR

NA

SA

C C O

Pre-Milestone

0Concept

Exploration

IDemonstration

IIEngineering andManufacturing

Production andDeployment

Operations andSupport

III

DoD

0 Needs AnalysisConcept Dev.

andValidation

Development

0 SRR PDR CDR1 2

3

D




From ”Understanding Space” by Jerry Jon Sellers


Specifications

SimulationDesign Testing

Analysis

Delivery



C

SystemsEngineering

Control Theory

Digital ElectronicsEngineering

Sensors and ActuatorsTecnology

Power Electronics

SoftwareEngineering

Tecnology

StructureTechnology

DynamicSimulation

DevelopmentMethodology

Thermal

MechanismTechnology

PropulsionTechnology

StandardsAnd Norms

TechnologyCommunicationTechnology

Mission


Analyst


CONCEPT EXPLORATION DETAILED DEVELOPMENT

RequirementsGeneration(Users and

Very

Broad Performance

ObjectivesRequirements

PRODU(

Operators)needs

j

Studies Prototyping Design & Test

UCTION

AcquisitionManagement(Developers)

Alternative

Concepts

Concept

SelectionStable Design

AND

D

Resource Requirements and Constraints

DEVELO

Planning,Programming,And Budgeting

(Sponsors)

Affordability

GoalsConstraints Firm Unit Costs

PMENT


Mission ElementsMission Elements

SubjectCommand, Control and

Communications

Subject

MISSIONCONCEPT

Orbits and constellationsMission OperationsMission Operations

G d l t


Ground elementSpace elementLaunch element

Spacecraft ElementsSpacecraft Elements



ACSSensor(s)

PayloadSensor

StructurePower

PWR5V

OBCDebug I/FRS422 RS232

( )

Analog orDigital I/F

Power

Com1MainDC-DC1

ROM TX TNC TXRX TNC

TX

OBC

OBC

Serial Synchronous RS422Clock and Data

RS422, RS232JTAG

Com2DC-DC3 OBC

OBC

Analog SW

TLM

TLMDC-DC2

COM

Para

llel B

us

ACS

Analog I/F

OBCRX TNC

CW Gen

RX

CWCharge Circuit

uSWFlight Pin

CMD

TLMACK

PCDU

ACSActuator

BatteryImportant Analog Sensors

Digital Sensors Antenna Latch

Solar Cell

Flight Pin PWR5V

.......

Regulated voltage outputs

SolarPanel(s)

Sensors

Analog Sensors

Antenna Latch

Battery


System DriversSystem Drivers

•Size•On orbit Weight

•Definition of the Preliminary Mission Concept•Definition of the Subject Characteristics•On-orbit Weight

•Power•Data rate•Communications

j•Determination of Orbit and Constellation Characteristics•Determination of the Payload Size and •Communications

•Pointing•Number of S/C•Altitude

Performance•Selection of the Mission Operations Approach•Design of the S/CS f f SAltitude

•Coverage•Scheduling•Operations

•Selection of the Launcher and Transfer System•Determination of Logistics, Deployment, replenishment and disposalC t E ti tiOperations •Cost Estimation


Payload Design and sizingPayload Design and sizing

•The payload is the combination of hardware and software that interacts with the subject to accomplish the mission objectives.

Spacecraft Mission Payload Example

Communications• Full‐duplex broadband•Message broadcast• Personal comm

• Transceiver• Transmitter• Transceiver

•Milstar, Intelsat•DirecTV, GPS• Iridium

Remote Sensing• Imaging • Imagers and cameras • Landsat, Space Telescope• Intensity measurement• Topographic mapping

• Radiometers• Altimeters

• SBIRS early warning,• Chandra, TOPEX/Poseidon

Navigation• Ranging • Nav signal

• Transceiver• Clock and transmitter

• TDRS• GPS, GLONASSg ,

Weapons• Kinetic Energy•Directed Energy

•Warhead•High‐Energy weapon

• Brillant peebles concept• Space‐based Laser concept

In Situ Science• Crewed• Robotic

• Physical and life sciences• Sample collection/return

• Space Shuttle, Mir•Mars Sojourner, LDEF

Other•Microgravity Manufacturing• Space power

• Physical plant and raw materials• Solar collector, converter and transmitter

• Space Shuttle• SPS


• Resource utilisation•Tourism• Space burial

• Lunar soil collector and processor•Orbital hotel• Remains container

• Lunar Base• Various• Pegasus

Payload Design and sizingPayload Design and sizing

•Selection of payload objectives•Payload performance objectivesPayload performance objectives

•Conduct subject trades•Subject definition and performance thresholds

•Develop the payload operations conceptE d t d t f ll i i h d ti d•End-to-end concept for all mission phases and operating modes

•Determine required payload capability to meet mission objectives•Required payload capability

•Identification of candidate payloadsp y•Initial list of potential payloads

•Estimation of the candidate payloads capabilities and characteristics•Assessment of each candidate payloads

•Evaluation of the candidate payloads and selection of the baseline•Evaluation of the candidate payloads and selection of the baseline•Preliminary payload definition

•Assessment of the life-cycle cost and operability of the payload and mission•Revised payload performance requirements constrained by cost or architecture li it tilimitations

•Identification of the payload-derived requirements•Derived requirements for related subsystems

•Documentation and iteration


•Baseline payload design

Payload Examples Payload Examples -- AAUAAU

•Missions sucess criterias • That the involved students have achievedThat the involved students have achieved

some useful knowledge of space technology. • That communication is establised with the

satellite and housekeeping information is retrieved.

• Take and download any picture. • Test ACS performance. • Take pictures of certain locations on earthTake pictures of certain locations on earth. • Take pictures of celestrial objects and

experiment with the various subsystems.


Payload Examples Payload Examples –– CANXCANX--11

Technology demonstration mission• Training next generation of space engineersTraining next generation of space engineers• Color and monochrome CMOS imager to be

used as star, moon and horizon sensor• Testing performance of a custom-built OBC• GPS receiver• Active magnetic control system• Data collection of GaAs solar cells and

Honeywell magnetometerHoneywell magnetometer



Main objectives:•Establish bus component design for pico satellitesEstablish bus component design for pico satellites•Reduce the total development cost by using commercialoff-the-shelf (COTS) components

•Educational•Separation mechanismCUTE-I missions:1) Communication mission 2) Sensing mission2) Sensing mission 3) Deployment mission


Payload Examples Payload Examples –– DTUSatDTUSat

Main objectives:•Bird-tracking missionBird-tracking mission•On orbit demonstration of a CCD camera (PICOCAM)•On orbit demonstration of a MEMS sun sensor


Payload Examples Payload Examples –– PharmaSatPharmaSat

Main objectives:•Provide life support such as sugars the yeast can consume and environmentalProvide life support, such as sugars the yeast can consume, and environmental control, such as temperature, for yeast growth in 48 independent micro-wells;

•Administer three groups of growing yeast with an antifungal agent at three distinct dosage levels, and one control yeast group with no antifungal dosage;

• Track the yeast population density and health in each microwell before, during and after administering the antifungal by using an optical density sensor and Alamar Blue, an agent that turns the yeast varying shades of blue and pink as they consume the sugars;consume the sugars;

• Transmit the yeast population and health data, and PharmaSat’s system status data to Earth for analysis;

• Measure and determine the effect microgravity has on yeast resistance to an antifungal agent.


AOCS SubsystemAOCS Subsystem

Objectives:Objectives:i l h h i l f i f b h bi d i d•It implements the three typical functions for both orbit and attitude:–Navigation–Guidance–Control

•To maintain the orbit parameters•To perform all orbit operations in all mission phases including•To perform all orbit operations in all mission phases, including

–Parking orbit operations–Orbit Transfer–Orbit Maintenance or Station‐Keeping

•To determine spacecraft attitude•To define the spacecraft attitude referenceTo define the spacecraft attitude reference•To control the spacecraft attitude fulfilling pointing requirements•To perform the spacecraft angular momentum management


•To perform all required manouvres


orientación de paneles solaresorientación de paneles solares

control de actitudcontrol de actitud

Main Components (I/VI)Main Components (I/VI)• SENSORS• ACTUATORS• CONTROL ALGORITHMS

controles en carga de pagocontroles en carga de pago

control de potenciacontrol de potencia

• ESTIMATORS• FILTERS• FAILURE MANAGEMENT control de control de t lt l

apunte de antenasapunte de antenas

• MODES MANAGEMENT

ActController System

órbitaórbitacontrol térmicocontrol térmico

Act.Controller System

Sens.Estimator



Main Components (II/VI)Main Components (II/VI)

disturbancesExample: LEO AOCS

• forces• torques• • Atmosphere

outputsreference

y data

• ... Atmosphere• Solar Radiation• Luni-solar• ...inputs

Act.Controller System

outputsy data

SE ti t

• Position• Attitude• ...

satellite

Sens.Estimator

parameters• Mass• Inertia• Geometry•


...


Main Components (III/VI)Main Components (III/VI)

Example: LEO AOCS

• Magnetic torquers• Propulsion

referencey data

p• Reaction Wheels• ...

Act.Controller Systemy data

SE ti t

accesories

Sens.Estimator

• Stellar Sensors• Sun Sensors•


• gyroscopes• ...


Main Components (IV/VI)Main Components (IV/VI)

Example: LEO AOCS

• Control Laws• Reconfiguration Logic•• Orbit Reference• Pointing Reference

referenceand data

...g• ...

commands

• Wheels angular rate• Activation times• Intensities• ...

Act.Controller Systemand data

SE ti t

Control Unit

Sens.Estimator



Main Components (V/VI)Main Components (V/VI)

Example: LEO AOCS

• Orbit Reference• Pointing Reference•

disturbances• forces• torques• • Atmosphere

referencey data

• ...

outputs

... Atmosphere• Solar Radiation• Luni-solar• ...inputs

Act.Controller Systemy data p

SE ti t

Control Unit accesories • Position• Attitude• ...

satellite

Sens.Estimator



Main Components (VI/VI)Main Components (VI/VI)• ATTITUDEPOSITION • ATTITUDE• Absolute Sensors

– Sun Sensor– Star Trackers

POSITION• Absolute Sensors

– GPSA l – Earth Sensors

– Magnetometer– Gyroscopes

GPS

– Accelerometers– Ground Tracking– Celestial bodies

R l i S – GPS

• Relative Sensors– Laser– Cameras

• Relative Sensors– Laser– Cameras

P d liCa e as

• Actuators– Propulsion– Reaction Wheels

– Pseudolites– Differential GPS

• Actuators– Control Moment Gyros– Momentum Bias– Magnetic Torquers– Solar Sailing

– Propulsion– Solar Sailing– Tethers


Solar Sailing


AOCS Design ProcessAOCS Design ProcessO bi Orbit

Solar/magnetic

MissionRequirements

MissionProfile

Orbit Insertion

S/C geometry

OrbitModels

MissionProfile

Definition of control modesDefinition of requirements Quantification of

Disturbance Environment

Selection of AOCS controlby control mode

P/L, Thermaland Power needs

Orbit, PointingDirection

Disturbance Selection and Sizing of

Pointingaccuracy

Orbit

Disturbance Environment

Selection and Sizing of AOCS H/W

S/CDefinition of AOCS

AlgorithmsALLgeometry Orbit

Conditions

Missioni

Lifetime

Pointingdirection

Slew

Algorithms

Iteration and


Requirements Slewrates

Iteration anddocumentation

ALL


Main RequirementsMain Requirements

b• Orbit Requirements– To maintain a certain altitude– To maintain a certain inclination– To maintain a certain inclination– To maintain a certain ground track repitibility– To perform orbit transfers– To minimise propellant consumption– To minimise time for some operationsA i d R i• Attitude Requirements– To maintain a certain pointing with respect to an object– To fulfil pointing requirements (Accuracy, range)To fulfil pointing requirements (Accuracy, range)– To fulfill stability requirements (Jitter, Drift)– To perform attitude manoeuvres (Settling time)



Main Control ModesMain Control Modes• Orbit InsertionOrbit Insertion• Acquisition• NormalNormal• Slew• Contingency

SeparationSeparationSAFESAFE

MODEMODE

• Contingency• Special TRANSFERTRANSFER

MODEMODEUNDOCKINGUNDOCKING

MATEDMATEDMODEMODE

RENDEZRENDEZVOUSVOUS DOCKINGDOCKING

UNDOCKINGUNDOCKINGMODEMODE

VOUSVOUSMODEMODE MODEMODE

COLLISIONCOLLISIONAVOIDANCEAVOIDANCE

O


MODEMODE


Main AOCS TradesMain AOCS TradesT f bili i

Mission• Type of stabilisation:

– Spin– 3‐axis

P i– Passive

• On‐orbit vs Ground Determination• Sensor selection

Thermal

• Actuator selection• Computer Architecture

Communications

PowerPower

Str ct resPropulsion


Structures


Selection of Attitude Control Type (I/III)Selection of Attitude Control Type (I/III)TypeType Pointing OptionsPointing Options Attitude ManeuverabilityAttitude Maneuverability Typical AccuracyTypical Accuracy Lifetime LimitsLifetime LimitsTypeType Pointing OptionsPointing Options Attitude ManeuverabilityAttitude Maneuverability Typical AccuracyTypical Accuracy Lifetime LimitsLifetime Limits

GG Earth LV Very limited ± 5 deg (2 axes) None

GG+MW bias Earth LV Very limited ± 5 deg (3 axes) Wheel bearings

MGT N/S Very limited ± 5 deg (2 axes) Noney 5 g ( )

Spin Inertial Expensive in terms of fuel ± 0.1 deg to ± 1 deg (2 axes)

Fuel

Dual‐Spin Inertial or LV limited by despun platform

Expensive in terms of fuel for Momentum bias

± 0.1 deg to ± 1 deg (2 axes). + Despun

FuelDS bearingsy p p ) p DS bearings

MW Bias LV pointing Expensive in terms of fuel for MW bias

± 0.1 deg to ± 1 deg Fuel, Wheel bearings

Zero Momentum + thrusters

Any No constraints. High rates possible

± 0.1 deg to ± 5 deg Fuelthrusters possible

Zero Momentum + RW

Any No constraints. ± 0.001 deg to ± 1 deg Fuel, Wheel bearings

Zero Momentum + CMG

Any No constraints. High rates possible

± 0.001 deg to ± 1 deg Fuel, Wheel bearingsCMG possible bearings



Selection of Attitude Control Type (II/III)Selection of Attitude Control Type (II/III)



Selection of Attitude Control Type (III/III)Selection of Attitude Control Type (III/III)• Mainly dependant on:Mainly dependant on:• Orbit insertion:

– Large impulsePlane changes– Plane changes

– Maintenance

• Payload pointing:Earth pointing– Earth pointing• Gravity gradient for low accuracies• 3‐axis with Earth LV reference

– Inertial pointing– Inertial pointing• Spin• 3‐axis

• Slew rates:• Slew rates:– None/low– Nominal– High


High


Quantification of disturbance torquesQuantification of disturbance torquesDisturbance Type Parameters Formula

Gravity Gradient Constant (Earth) or cyclic (Inertial)

S/C inertias, orbit altitude

Solar Radiation Cyclic (Earth) or constant (Inertial)

S/C geometry and cog locations, S/C surface properties

M ti Fi ld C li O bit ltit d d i li ti BDT)(cos)1( cgciqA

cFT pss

Ssp

)2sin(23

3 yzg II

RT

Magnetic Field Cyclic Orbit altitude and inclination, Residual S/C magnetic dipole

Aerodynamic Constant (Earth) or cyclic (Inertial)

Orbit altitude and S/C geometry and cog locations,

Uncertainty in cog Unbalanced and unwanted S/C geometry 1 to 3cm

BDTm

)(21 2 cgCAVCT pada

Uncertainty in cog Unbalanced and unwanted torques

S/C geometry 1 to 3cm

Thruster Misalignment “ “ 0.1 to 0.5 deg

Mismatch of thrusters output

“ “ ±5%output

Rotating machinery Stability and accuracy Depending on design can be compensated

Liquid sloshing Torques and variation of cog S/C and tanks geometry Depending on design, can be compensatedp

Dynamics of Flexible Bodies

Resonance and limited bandwidth

S/C geometry Depending on the S/C structure

Thermal Shocks on Flexible Appendages

Attitude disturbance when in eclipse transients

S/C structure Worst with long booms



Design parameters for selecting the sensorsDesign parameters for selecting the sensors• Decissions to be taken regarding the sensors:

Type– Type– Number– Layout– Sensing combinations

S l d di h f ll i f• Sensors are selected according to the following features:– Pointing Accuracy– Field of View– Redundancies– Location and Orientation– Power– Mass– Data Rate

Sensor Typical Performance Range Mass Range (kg) Power (W)

IMU 0.003deg/hr to 1deg/hr, 1 to 5x10‐6 g/g2

(from 20 to 60g)1 to 15 10 to 200

( g)

Sun sensors 0.005deg to 3 deg 0.1 to 2 0 to 3

Star sensors 1arcsec to 1arcmin 2 to 5 5 to 20

Earth sensors 0.1deg to 1deg (LEO) 1 to 4 5 to 10


g g ( ) 4 5

Magnetometer 0.5 deg to 3deg 0.3 to 1.2 <1


Locating the sensors Locating the sensors -- ExamplesExamples+Y

S l A

Star Trackers

+X Z XX +Z

LEO SSO12H –Earth Pointing

Solar Arrays

Earth

Sun+X -Z -X-X +Z

-Y

+Y

S l A

XY

Solar ArraysSun

+X -Z -X-X +ZZ

GTO Equatorial – Sun

Earth

St T kSt T k


-Y

Pointing at Equinox Star TrackersStar Trackers


Design parameters for selecting the actuatorsDesign parameters for selecting the actuators• Decissions to be taken regarding the actuators:

T– Type– Number– Layout– Actuation combinationsActuation combinations

• Actuators are selected according to the following features:– Disturbance compensation– Redundancies– Location and Orientation– Power– Mass

Actuator Typical Performance Range Mass Range (kg) Power (W)

Thrusters‐Hot gas 0.5 to 9000N Variable N/A‐ Cold gas <5N Variable N/A

Reaction and Momentum wheels

0.4 to 400 Nms0.01 to 1 Nm

2 to 20 10 to 110

CMG 25 to 500 Nm >10 90 to 150


CMG 25 to 500 Nm >10 90 to 150

Magnetic Torquers 1 to 4000 Am2 0.4 to 50 0.6 to 16


Preliminar Sizing of the actuatorsPreliminar Sizing of the actuatorsParameter Simplified equations

Torque from RW for Disturbance rejection

Parameter Simplified equationsThrust force level for external Disturbances

)arg()( factorinMTT DRW L

TF Drejection

Slew torque for RWThrust force level for slew rates (zero‐mometum)

Thrust force level

2)2(tITRW

LIF

hMomentum storage in RW

Momentum storage

Thrust force level for slewing a Momentum‐bias vehicle

Thruster pulse life Derivation of the total number of h l

24PeriodOrbitalTh D

P

LdhF

Momentum storage in MW

Torque from

thruster pulses

Thrust force level for momentum dumping

aD hPT 4

TD D LthF

Magnetic Torquers

Momentum storage in Spinner

dumping

PropellantB

D Lt

gIFtMsp

p


p

AOCS Examples AOCS Examples -- AAUAAU


AOCS Examples AOCS Examples –– CANXCANX--11

Main characteristics:• Imager pointingImager pointing• Rotating for capturing Earth and stars images• Based on 3 magnetometers• Magnetic torquers as actuators


AOCS Examples AOCS Examples –– CUTECUTE--II

Main characteristics:• No attitude controlNo attitude control• Sensing package:

•4-axis gyros•4-axis accelerometers•Sun sensor (CMOS)

• Ground attitude determination


Session Payload Subsystems

Documents

Transcript of Session Payload Subsystems

11 9 session 40

YLS – Session 1

Best Practice-Session

10 29 session 35

Live session 24_04_13_1

Management SUITE Systems Message · PDF fileUnternehmensprozesse und Systeme vor Störungen und reduziert ... Source Systemen wie Security Information Event Managers (SIEM), ... Subsystems,

10 21 session 30

OSGi Subsystems - EclipseCon Europe 2019 · OSGi Subsystems @glynnormington Application Subsystems Useful for top layer of the system Avoid duplicating contents • Reduces sharing

Ch.9a Session

Session 01 - Unsyiah

Session bchh13

B2B Networking Session

ERFA Rudern OS - Federal Council · 2020-06-12 · Session 3 PM Session 3 PM Session 3 PM Session 3 PM Session 3 PM Session 3 PM Session 3 PM OFF ROW WEIGHTS ERG ... 4 mins at 32

Session #kommunikativeprokura KrisenPRCamp 2014

Main Session Lotus Software

Konzept Jam Session

Fit für Workshops Session 3 - SKOPOS NOVA · FIT FÜR WORKSHOPS 2 Session 1: Mal ganz von vorne: Wofür brauchen wir Workshops? Session 2: Session 3: Schnell, schneller! Was sind

11 11 session 42

10 22 session 31

J2EEKurs - Enterprise JavaBeans---Einführung · Allgemeines Session Beans Inhalt Allgemeines Motivation Rollen Aufbau einer EJB Arten von Beans Session Beans Stateless Session Bean