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The Pointing, Acquisition and Tracking system of SILEX european program : a majortechnological step for intersatellites optical communication.

Michel BAILLY, Eric PEREZ

MATRA-.ESPACE, Automatic Control Direction37 avenue Louis Breguet

78140 Vélizy-Villacoublay, FRANCE

ABSTRACT

This paper is concerned with the overall architecture and the performances of the Pointing, Acquisition and Tracking (PAT)subsystem of the European SILEX program (Semiconductor laser Intersatellite Link Experiment). Starting from the descriptionof the mission constraints, it deals with the retained baseline built around a two-stages beam steering system, an open iooppoint-ahead control and a full digital implementation. Performance prediction is based upon signal processing, controlalgorithms and detailed modeffing of equipments and flexible structures. It shows how such an accurate performance in theorder of a microradian can be achieved with a correct allocation of the performance for each equipment of the system. Inparticular the rejection of the microvibrations coming from the platform when the Terminal is operating will be pointed out asa key-feature of the design. The PAT equipments and softwares - sensors, electromechanisms, control units - are presentedshowing which technological step is achieved (from current advanced CID phase programs) or still to be achieved.

1 -INTRODUCTION : SILEX MISSION

The Semiconductor Laser Intersatellite Link Experiment (SILEX) is a very promising mission of the European Space Agency(ESA) especially in view of the future generation of the Data Satellites network (13). The optical technology in the field ofcommunication systems allows very high data rates, potentially several hundred of Mbits/sec through wavelengthsmultiplexing, a great confidentiality of the transimission with a lighter and more compact payload than with other techniquessuch as RF transmission . The major concerns of this optical technology are first the choice and the qualification of the laserdiode devices and second the performances of the Pointing, Acquisition and Tracking system (PAT), due to the very narrowlight beam, less than 10 microradians, and to i06 Bit Error Rate (BER) required for the transmission. Studies to demonstratethe feasability of such control system have been started in an extensive way at MATRA-ESPACE since 1987 and recentlycame out into the availability of a System Test Bed (STh), composed of functional breadboards (5) and the successfull testingof PAT functions.

From now on mature, the project will enter its industrialization phase with the development and building of two ifight opticalTerminals, respectively mounted on SPOT4 (French Government low earth observation orbiter) and ARTEMIS (ESAexperimental geostationary platform) satellites.The mission will consist in an Inter Orbit Link (IOL), which is more demanding for the PAT design than an Inter SatelliteLink (ISL) between geostationary platforms, and therefore will lead to a full in orbit demonstration of the performances andgrowth potential of optical free-space communication systems.

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2 - PAT SYSTEM MiSSION AND ARCHITECTURE

The optical terminal

The optical Terminal is a fully integrated payload which guarantees to the host platform a complete operation autonomy, over24 hours period, with a limited programming message and a maximum decoupling (especially for the dynamic distrubancestorques). In addition, the design is basically common between Low Earth Orbit (LEO) and geostationary (GEO) configurationsand most of equipments are identical.Each terminal is made up of a mobile and a fixed parts (8). The mobile part is gimballed by a coarse pointing mechanism. Themobile part is designed around three subassemblies : an afocal telescope, an optical head by which the control and processingof the optical signal (emission and reception) is performed, and an electronic boxes compartment. Accurate and active thermalcontrol is achieved on the optical bench, while electronics equipments are only passively controlled. Not only collecting theincoming light, the telescope provides an optical magnification of the pointing error on the line of sight so that the controlequipments, sensors and electromechanisms, implemented on the optical bench may present relaxed performances with respectto the submicroradlian accuracy required on this line of sight to ensure the BER performance.

Figure 1 : optical terminal functional block-diagram

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LIII OpW-...hks PAT Sb.ysA EEl EAt!k.l IllllI1


PAT subsystem mission

It consists in acquiring and tracking the counter terminal incoming laser beam as well as in pointing terminal outcoming beamwith an accuracy which enables a performant data transmission between two satellites. This mission is decomposed in thefollowing objectives:

1 - Compensate for the initial beam pointing error due to spatial acquisition errors, mainly ephemeriserror and spacecraft location prediction errors. This is the acquisition phase.

2- Once the beam is acquired, track out local angular disturbances transmitted from the host platformand the dynamic elements of the payload with a submicroradian accuracy. This is the tracking phase.

3 - Point the terminal optical head towards the opposite satellite and compensate for relative platformmotions and finite transit time of light. This is the pointing phase.

144 / SPIE Vot 1417 Free-Space Laser Communication Technologies Ill (1991)

PAT subsystem architecture


3 - PAT EQUIPMENTDESCRIPTION AND PERFORMANCES

Coarse Pointing Assembly (CPA)

The CPA achieves three main functions:

- to gimbal and steer the mobile part of the optical payload towards the opposite satellite with large angular rotation (twoaxis capability greater than 2 H steradians), and under the control of the tenninal On-Board Processor (OBP).

- to gimbal autonomously the mobile part from any position to a predefined position called 'canonic position" on receipt ofa dedicated command referred to as emergency command.

- to maintain the mobile part in a fixed position when the CPA is not operated.

It consists in a Coarse Pointing Mechanism (CPM) which gimbals the mobile part and a coarse pointing drive electronics(CPDE) implemented on the host platform structure (fixed part of the terminal). This mechanism (2) is composed of twoSAGEM 57PPP59 stepper motors orthogonally mounted on a CFRP L-bracket structure with berylium motors mountingframes. A cable wrap mounted on each drive permits to route the interface harness through the hinge without torque disturbanceand whatever is the commanded rotation.

Each motor, without gear and mounted on large ADR ball bearings with a MOS2 dry lubrication, is directly driven in openloop through micro-stepping profiles stored in PROMs for harmonic defects compensation and in closed loop, using twoSUNSDTRAND accelerometers sensors per motor, for motor stiffness resonance damping. In addition a wired logiccompensates for the friction and cable wrap stiffness effects. Immobility is ensured by blocking devices composed of frictioncable wrapped on a disk and driven by redunded electromagnets which are active (releasing of mechanism) when the CPA isoperated.

All the drive electronics are located in the CPDE which interfaces with both unregulated primary power bus and terminal databus. CPDE is designed around a 80C85 microcomputer which gives usefull flexibility to perform drive control laws andpointing and mode management.

This equipment is developped and built by MATRA-ESPACE, with the cooperation of SHRACK Electronics (Austria) for theCPDE, and is directly derived from an already existing and qualified MATRA mechanism developed for the IOC (Inter OrbitCommunication) payload of EUIRECA satellite mission (European Retrievable Carrier).

The major characteristics and performances of the CPA have been issued during extensive B phase studies which have includedanalysis, breadboarding and tests activities (2).

iTEM PERFORMANCE REMARKSAngular coverage : azimuth

elevation180 deg90 deg

Kinematics : azimuthcapability elevation

2 deg/sec0,5 deg/sec

required by mission profile

Pointing accuracy 0,008 deg BOL0,012 deg EOL

Short term stability 15 microradian (3 ) over 30 m secDynamic disturbances 0,1 Nm (3 )

Motor resonance frequency between 1,4 Hz and 2,4 Hz tuning is possibleAbsolute pointing measurement better than 0,5 deg used only telemetry purpose

Mass 21kg CPM=l4kgCPDE = 7 kg

including harness and interface baseplatePower consumption 30 W

—

heatets require 13 W more whenoperated

reliability R>O,991 for 5 years mission

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Fine Pointing Assembly (FPA)

The EPA function is to provide deflection of the incoming and outcoming laser beams around two orthogonal axis and isoperated in order to provide:

- the sweeping of the beacon over a wide angular range. This is the scanning mode.

- afast deviation angle over a wide range to recenter the incoming beam on the tracking sensor as sensed by the acquisitionsensor. This is the acquisition mode.

- the control of the angular position of the incoming beam as sensed by the tracking sensor with high bandwidth andaccuracy. This is the tracking mode.

It is composed of an optomechanical device, called Fine Pointing Mechanism (RPM) which supports and positions one or twomirrors and includes mirrors position sensors. This RPM is direcity mounted on the optical bench of the SILEX terminal.Connected to this FPM is the associated drive electronics, called Fine Pointing Drive Electronics (FPDE) which implementsthe control and command stages of the mechanism as well as services such as power supply and TM/TC interface functions.This unit is mounted inside the electronic package of SILEX Terminal.

146 / SPIE Vol. 1417 Free-Space Laser Communication Technologies 111(1991)

Figure 2 : Coarse Pointing Assembly (IOC design)


This equipment is certainly one of the most demanding in performance and is then very critical for the SILEX mission. This ismainly due to the combination of potential conflicting acquisition and tracking performances, considering also the highrequirements on mass, volume and dissipated power. This is mainly pointed out by the large angular range to accuracy ratiowhich reflects both acquisition and tracking requirements.

Several design solutions are presently under competition for the final choice of the FPA supplier for CID phase. They arebased either on a single two active axis mirror or a two single axis mirror package. Therefore, detailed design will not bepresented in this paper.

However during SILEX B phase, extensive and comparative studies have been contracted together to TELDIX (Germany ),(10)and to CSEM (Switzerland) companies for developing EPA breadboards which have permitted to assess both feasibility ofperformances and EPA behaviour under space environments.

Figure 3 : Fine Pointing Mechanism (Breadboard from TELDIX Company)

The major specified functional performances and characteristics of the FPA ai detailed in the following table.

PARAMETER ACQUISITION MODE TRACKING MODEmaximum deflection range 160 mrd

______________________________

160 mrd after acquisition10 mrd steady state after

FPA offloadingstep response 15 msec -

frequency response - First order type up to 2,5 KHz, crossover frequency typically

30 hzaccuracy less than 5% (3 ) on final pointing

achievement-

noise - less than 5 microrad (1 )

Specified functional performances of the EPA

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optical quality:wave front error (WFE):

polarization degradation transmission

� ?/5Ø rms<4 iø->

for the whole FPA package

PPM mass 400 gFPDE mass 500 g for one channel only

PPM dimension 90 mm x 60 mm x 60 mmFPDE dimension 159 mm x 125 mm x 50 mm for one channel only

power consumption 1,5 w16,5 w

in tracking modein acquisition mode (20 ms max)

reliability > 0,995 for 5 years lifetime

Poind Ahead Assembly (PAA)

Main specified characteristics of the FPA

As for the EPA, the principle of the PAA is to deflect SILEX laser beam around two orthogonal axis but on the emission pathonly. The main difference is the required performances of this device since it shall accurately achieve the computed point aheadangle from an open ioop command. Its deflection range and control bandwidth are therefore limited but the pointing accuracy tobe achieved is very constraining. The PAA offset is calibrated every 24 hours by directing the emitted beam to the trackingsensor using a Rip Plop Mirror (PPM). It is composed of a Point Ahead Mechanism (PAM) and a Point Ahead DriveElectronics (PADE) with the same architecture as the FPA.

This equipment is procured from Marconi Space Systems Ltd( MSS, United Kingdom) with the cooperation of QueensgateInstruments Ltd (United Kingdom) for the design of the mechanism. PAA design is based on Bendix pivots suspended mirrors,actuated by high voltage driven piezo-transducers in a folded redunded configuration which are controlled in closed loop usingcapacitance micrometers sensing the mirror rotation. The rotation movement is produced by means of these linear actuatorsacting on lever arms with respect to the mirror position. In order to achieve the required thermal stability two separate capacitorelectrodes are mounted on the rear side of the mirrois and their outputs are independantly compared to a fixed reference capacitorbefore processing the error signal. In addition this configuration allows a straightforward implementation of the sensorsredundancy.A closed loop sensor control system maintains the error signal at zero. An important use of hybrids technology for control,TMTFC and position sensor conditionmg functions is the key feature of a very light and compact PADE.The mirror designuses a fused silica substrate on which a multilayer dielectric coaling is fabricated.A SILEX B phase contract was given to this company to build two PAA breadboards and to assess the feasibility of therequired perfonnances as well as the good behaviour under space environment. The final design is now based on the successfullissues of this contract (9).

Pigure 4 : Point Ahead Mechanism (breadboard from Queensgate Instruments Ltd)

148 / SPIE Vol. 141 7 Free-Space Laser Communication Technologies ill (1991)


A summary of the main functional perfonnances and characteristics is provided on the following table.

iTEM PERFORMANCES REMARKSmaximum deflection range 6,5 mrd

accuracy less than 5 microrad over a 24 hours periodnoise less than 0,5 microrad (1 )

bazxlwidth 50 Hzoptical quality :

wave front error (WFE)polarization degradation

transmission

� /5O rmsio-4� o 98,

for the whole PAA package

PAM mass 350 g use of beryliumPADE mass 900 g for two channels

PAM dimensions 90 mm x 60 mm x 60 mmPADE dimensions

-159 mm x 125 mm x 90 mm for two channels

power consumption 1 Wreliability > 0,999 for 5 years lifetive

Acquisition and tracking sensors (ASDU & TSDTJ)

These functions are based on the use of Charge Coupled Devices (CCD), either a THOMSON TH7863 with 288 x 384 pixelsfor the acquisition sensor or a THOMSON THX 31 160 with 14 x 14 pixels for the tracking sensor (6). The TH 7863 isoperated with a 3 MHz pixel readout frequency in order to sample the images at a 33 Hz frequency able to cope with the veryconstaining acquisition phase timing when a complete convergency of LEO and GEO tracking shall be perfonned in less than0,5 second. The ThX 3 1 160 produces 8 KHz sampled images, either within the complete 14 x 14 pixels FOV with 3 MHzreadout frequency or within a limited centied section of 2 x 4 pixels with a reduced 750 KHz zadout frequency. While the high3 MHz rate allows only classical thresholding techniques for an efficient signal processing, the latter is used for accuratedetermination of the incoming spot centre location based on a centrodic algorithm. Compensation of the optical backgroundsignal is also implemented.To limit the volume and the power dissipated on the optical bench, only detection units are implemented which include:detector pointing and stabilization system, clocks drivers, video preamplifiers stage, power supply regulator. The otherpart ofthe sensing functions, as video processing and docks generation functions are centralized in the FPSCE Equipment (seebelow). Shielding of the CCD is implemented on both units up to 15 mm aluminium equivalent to limit as much as possiblethe loss ofperformances induced by the radiations environment (7). The TSDU includes its own optics to focus the laser beam.The main concerns of these equipments are the combination of small physical budgets and high alignment and stabilityaccuracies which lead to critical areas is the mechanical design. This has been extensively studied during competitive B phasecontracts given to SIRA (UK), British Aerospace (UK) (11), Officine Galileo (I) (12), SODERN (F) and also MATRA-ESPACE (F) to build sensors breadboards and to assess feasibility of the specified perfonnances. Final choice of the contractorfor the CTh phase is not yet frozen.

Figuic4 : Tracking Sensor Detection Unit

(breadboard from SODERN company)

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Fine Pointing and Sensors Control Electrics (FPSCE)

This is the core of the fine pointing control stage within the mobile part of the optical terminal. This equipment provides thefollowing functions:

- the ASDU & TSDU operating and video signal processing- the fine control laws implementation- the FPA & PAA operating, command and measurement signals processing- theoperating modes management- the terminal data bus interface- the centralized power supply from an unregulated primary power bus to all PAT equipments of the mobile part.

The design developped by MATRA-ESPACE which is responsible of this key equipment of the SILEX mission, is based onan ADSP2100 digital signal processor, from Analog Devices, associated to a Remote Interface Unit (RIU) for main terminalbus interfaces. To achieve the required 8KHz sampling frequency for the fine control command implementation, the DSP is runat 32 MHz, thus requiring the use of fast access time RAM (8K x 8). The IDT 7164 product has been chosen for the baselinebut the fast evolution of these products technology may offer more performant devices in a very near future. The clocksgeneration and video processing chains are derived from an already existing design developped and qualified in the frame of theMATRA-SODERN SED12 star tracker. The command stage to the FPA, which requires in principle a 16 bits resolution, isorganized around a decomposition between coarse and fine 12 bits ADC stages.To achieve very constraining objectives on mass, volume and power budget due to the implementation within the mobile partof the terminal, the electronic design takes benefit from technologies implementation such as hybrids, ACTEL components(logical circuits with frequency lower than 10 MHz) and Surface Mounting Technology (SMT) for boards surface reduction.The software is coded in assembler language using a cross development system, the ADSP2100 Development System, runningon SUN work stations. Capacities of 6 K-24 bits words for program memory and 2K-16 bits words for data memory areimplemented. The operating system provides two periodical tasks (typ. 8 KHz and 50 Hz) to activate the cyclical algorithms.The applicative algorithms, especially for what concerns the control ifiters, are implemented using a state descriptionminimizing the cumulative numerical errors. A decomposition between pre-and post-processing functions offers a largereduction of the delay between measurement and command, which is the key feature of high control bandwidth. The criticalfunctional areas of the FPSCE, especially the interfaces with the sensor and the mechanism, the DSP implementation and thesoftware performances, have been studied during SILEX B phase contract with the development of a breadboard designed arounda TMS 320 C25 Digital Signal Processor from Texas Instruments.

PAT software

The PAT sofware implements the following functions : orbitography, pointing ahead (including host platform attitudeestimation), coarse pointing (from ephemeris and offloading commands), scanning, calibration, mapping, operating modesautomation, anomaly detection.Developped by MATRA-ESPACE, runs on a 1750 standard MAS281 processor from MEDL, coded in ADA language underthe Hermes Software Development Environment implemented on SUN work stations.

All services, such as operating system which is the MAThA product ASTRES 1750, and equipments interface functions areprovided by the terminal software for all applicative functions. Program and data memory for the PAT software represent 40 K-bytes words and 42 % of CPU load.The On board software has already been successfully developped during SILEX B phase and implemented on the System TestBed. The main differences with the flight software are in the choice of 68020 processor, C language coding and VRTX 32operating system.

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4 - PAT FUNCTIONAL ARCUTTECTURE

The reader would take benefit from references (3) (4) where the complete and detailed PAT control architecture is described. As asummary, two control stages are implemented:

- the fine control stage, which achieves the acquisition and tracking of the counter tenninal signal. It is organized around theacquisition and tracking sensor and the EPA. On one hand the issue is to fastly recentre the received spot from the ASDUto the TSDU and initialize the tracking control, on the other hand the system accurately compensates for the host spacecraftdisturbances transmitted through the terminal structure. In addition the emitted light beam is pointed ahead the receivedlight beam direction to compensate for the relative position variation of the two platforms and to the finite speed of light.This last function is achieved by the PAT software and the PAA.

- the coarse pointing stage, which allows to steer the terminal pointing direction from on-board computed ephemeris, to rallyany predefined position, to achieve open ioop pointing profiles, and to offload the FPA with the CPA. This function isachieved by the PAT software and the CPA.

Acquisition phase

The acquisition strategy relies on a beacon scanning sequence for the GEO terminal able to cover the uncertainty cone createdby the attitude errors of the two platforms. Once a terminal detects on its acquisition sensor the incoming signal from theopposite terminal, acquisition control algorithms are activated to realize a two-step recentnng to the tracking sensor.Once the spot is within the TSDU FOY, the rallying control algorithms are activated allowing a closed loop convergence tothe Quadrant Detector (QD) in less than 20 msec.

ACQUISITION SENSOR

TRACKING SENSOR

® First AS measurernen

Second AS measurement

Spot in TS field of view

integration beam on AS

ASintegralion ... I II]

AS readout --.----- ••_

FPA deflecon (15 ins)

TS rallying

Figure 5 : acquisition strategy principle at terminal level

inhibited measurements

SPIE Vol 1417 Free-Space Laser Communication Technologies Ill (1991) / 151


Trackin&phase

The first issue is to achieve a high control bandwidth able to compensate for the microvibrations transmitted from the hostplatform to the terminal line of sight. The key features of such a system rely on a 8 KHz sampling frequency of the TSDUmeasurements, a minimization of the delay between measurement and command (FPSCE), a resonance free EPA up to 2,5KHz. Pointing budget presented in the next section shows that all equipments, except the ASDU, contribute to the pointingerror and that a particular care has been taken since the beginning of this project to find the correct allocations and presentationof performances for each equipment. A major issue is also to assess structural transmission between the mechanical interfacewith the host platform and the line of sight. Simulation softwares, based on finite elements model and in house built dynamicpackage, so called DYCEMO, provide the transfer functions (1). The actual resonnances Q.factor to be considered are derivedfrom tests measurements (2).The steering of the line of sight is commanded with on-board computed ephemeris derived from a Keplerian model (J2 term isadded for the LEO), with a complete autonomy of 24 hours. In order to reduce EPA operating offset, EPA positionmeasurement is used to drive the CPA in order to compensate for long term components of FPA position. This is performedby the offloading control law implemented by PAT software to be combined with the ephemeris command. In order to avoidEPA fine command saturation, the EPA coarse command bias is slowly reduced to zero in open loop by the dedicated biasfilter function implemented by FPSCE software.

To achieve the submicroradian accuracy, the point ahead angle is computed on-board from the orbital data but also from anestimation of the host platform attitude error. This estimator is based on a constant gain Kalman filter where the state vector issized to estimate the bias and the two first orbital harmonics terms.

5 - PAT SYSTEM PERFORMANCES

Figure 6 : Functional PAT architecture in tracking mode

5. 1 - Acquisition success probability

Acquisition success probability is a major contributor to SILEX system availability rate. Considering the statistic of everyevent involved in PAT acquisition process leads to potential acquisition failure. The linked probability is mainly driven by thesuccess rate of two phases : Acquisition sensor detection process and recentring of beam within Tracking Sensor field of view.Since the latter phase involves only the combination of pointing errors and associated probability distribution, only the firstphase will be addressed in this paper.

152 / SPIE Vol. 1417 Free-Space Laser Communication Technologies /11(1991)


5.1.1 - Acquisition sensor detection process

Detection on AS is performed by comparing the most illuminated pixel to a fixed threshold. Detection failure occurs in twocases:

- no detection : threshold value is not exceeded by any pixel in the presence of useful signal- false alarm : threshold value is exceeded at least by one pixel in the absence of useful signal.

Consequences of no detection or false alarm events are different. In case of no detection, induced delay in the acquisitionsequence is such that full acquisition may not converge during available beacon illumination time. In case of false alarm, anuntimely acquisition sequence is initialized. During the time necessary to detect the false alarm (beam does not appear on TSafter FPA deflection) and to drive back the FPA to zero position, PAT system is not available so that an actual beam arrivalcan not be detected, thus leading to acquisition failure. Analysis of figure 5 shows that acquisition failure probability is givenby

AF5 FA +2ND

where AF is the PAT system acquisition failure probabilityFA is the false alarm probability for one AS imageND is the no-detection probability for one AS image

5.1.2 - Acquisitionsensor detection failure

Depending on beam spot location on CCD matrix, the percentage of total incoming power collected by the most illuminatedpixel may vary in significant proportions, as shown on figure 7. Assuming a uniform statistic for spot location, theprobability distribution of energy percentage is derived and shown on figure 8. In addition, due to opposite terminal pointingerror variations, the incoming optical power is not considered as constant. Statistic of pointing error being expressed as theprobability for the incoming power to be zero during any given millisecond, a complete and conservative statisticalrepresentation of most illuminated pixel received power is obtained. A given optical power being now given on a pixel, thefine modelling of the detection chain, including CCD behaviour under space environment, (7), electronics noise and signalquantization, as shown on figure 9, is used to derive both no-detection and false alarm probability for a given threshold.Global PAT detection failure is then plotted versus threshold tuning for minimum useful optical power, for any potentialbackground configuration and considering both beginning and end of life radiation effects, figure 10. Such a curve highlights athreshold tuning range ensuring required probability over any operational conditions.

C

0

>00I)

(I)

Probability

.0.2 0.4 0.6 0.8 1.0

Percentage of total power on the most illuminated pixel

Figure 7 : Percentage of incoming poweras a function of spot location on a pixel

Figure 8 : Probability of having agiven percentage of incoming power

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0.30.30.20.20.0.

2 4 6 8 10 12 14 1AS field of view iradian

0


o:-4 -

6 -

10 - -;iiiL----12 I! I

0

Detection threshold (LSBs)

Figure 10 : PAT detection probability

5.2 - Host Spacecraft disturbance rejection

Ability of PAT system to actively control the disturbances generated by host spacecraft is a major design concern. In fact,most of the PAT optimization, particularly TS sampling frequency, FPA frequency response or FPSCE software architectureare directly derived from required rejection capability. Therefore, host spacecraft disturbance proffle is a major design constraint.While in flight and ground analysis have permitted to define a reliable micro-vibration profile for a LEO platform, (3), onlyrecent in flight measurements were obtained by ESA on a European geostationnary satellite,(14). Typical disturbances arepresented in terms of 3 axis linear accelerations at SILEX interface and expressed as a PSD for steady state and as a time profilefor attitude control manoeuvres transients, as shown on figure 11. Additional harmonic disturbances generated by spacecraftrotating parts are also taken into account but are omitted for the sake of simplicity.

154 / SPIE Vol. 1417 Free-Space Laser Communication Technologies III (1991)

I dark signal1 detectionCCD noisel noise 1video measurement Ishot noise _________ noise

incomingpower (pW) EEto

Figure 9 : AS detection chain model

Probability (log) threshold tuning band

-2

co BOL, maximum optical background

EOL, minimum optical background

EOL, maximum optical background

® BOL, minimum optical background -8

10 20 30 40 50


transient

(thruster firing)

linear acceleration (mg)

time (s) frequency (hz)

Figure 1 1 : OLYMPUS microvibrations

Since host spacecraft disturbances are defined at SILEX attachment interface, transmission of the terminal up to optical benchhas to be considered. For this, a modal representation of the structural elements of the terminal is derived from a finite elementmodel while damping ratio of eigenmodes are derived from measurements performed under micro-acceleration environment.Linearization of the model then provides transfer function between any of the six displacements at SILEX attachment and lineof sight pointing error. Such transfer functions, when combined with disturbances spectrum provide the actual steady state lineof sight error to be compensated by PAT control ioop, figure 12. As far as transient disturbances are concerned, performancederivation is achieved by complete non-linear simulation, (1). Linearized PAT rejection curve is shown on figure 13.Corresponding PAT tracking error is presented in table 14. An interesting feature of SILEX tenninal behaviour is that structureacts as a ifiter for high frequency disturbances. It particular, effect of transient disturbances is found to be negligible becausemainly composed of high frequencies.

PSD of line of sight motion (jird/sqrt(Hz))

frequency (Hz)

1000

0

-20

-40

-60

-80

Rejection gain (dB)

- 10010 100

frequency (Hz)

1000 10000

Figure 12 : Line of sight error beforerejection due to GEOsteady state disturbances

table 14 : Tracking error due to GEO platform disturbances

SPIE Vol. 141 7 Free-Space Laser Communication Technologies Ill (1991) / 155

15

10

5

0

-5

-10

-15

steady state

linear acceleration PSD (mgLJ7

. - - . -—. ,I

2.5

1

0.1

0 . 013.0

! ! I

3.5 4.01 10 100 1000

10 100

Figure 13 : PAT rejection curve

steady statedisturbances (PSD)

0,01 jird (1 )

Harmonic disturbances 0,04 jtrd (1 )Transient disturbances 0,02 jtrd (1 )


PAT tracking budget

Table 15 presents a typical steady state tracking budget, pointing out the equipments contributions. A special attention,conforted by bnadboarcIs development as well as System Test Bed experience,was paid to define realistic alocalions. From thistable, the criticity is shown to be equally spread, without any major contributor on which the whole perfonnance would rely(except for TS static error on GEO which is due to earth background). Furthermore, the rejection capability is such as to reducehost spacecraft disturbances effect down to the level ofPAT equipment errors, therefore featuring an optimized design.

PAT TRACKING BUDGET

SOURCESLEO GEO

BIAS ERRORS (rnax

TSFPAPAACPACONTROL

0.110.000.170.000.06

0.300.000.170.000.06

- TOTAL(RSS) 0.21 0.35

HARMONIC ERRORS (0-peak)

TS -FPAPAACPACONTROL

0.000.150.000.000.10

0.000.150.000.000.07

TOTAL (RSS) 0.18 0.17

RANDOM ERRORS (1q

TSFPAPAACPACONTROL

0.100.160.030.080.03

0.120.160.030.080.01

TOTAL (RSS) 0.21 0.22

TOTAL STATIC ERRORTOTALRANDOM ERROR (1o)

0.330.21

0.420.22

7- CONCLUSION

table 15 Typical PAT tracking error budget

The extensive studies and technological development programs started in the frame of ESA SILEX project have now enableMATRA to establish a consistent PAT system baseline. Important effort was made to design, build and test equipmentbreadboards such as to increase concerned industries expertise in the new technologies required by optical pointing systems andto provide a feasible and consissent set of requirements. System analysis and simulations, continuously updated by testmeasurements both at equipment level and at system level, have now provided reliable performance predictions, thus ensuringthe PAT mission feasibility. The PAT system, a major challenge of the SILEX program, is now ready to enter developmentphase, with an expected flight configuration delivery in early 1993.

This work was sponsored by European Space Agency. The views expressed are those of the authors and do not necessarilyreflect the official position of ESA.

156 / SPIE Vol. 1417 Free-Space Laser Communication Technologies Ill (1991)


8 - REFERENCES

(1) "Simulation model and on ground perfoimance validation of the PAT system for SILEXprogram" F. COSSON, P. DOUBRERE, E. PEREZProc. SPIE, vol 1417 (Los Angeles) 1991

(2) 'tCoarse pointing assembly for SILEX program or how to achieve outstanding pointingaccuracy with simple hardware associated with consistent control laws'D. BUVAT, G. MULLER, P. PEYROTProc. SPIE, vol 1417 (Los Angeles) 1991

(3) "Pointing Acquisition and Tracking System for SILEX intersatellite optical link"M. BAILLY, J.M. PAJROT, E. PEREZSPIE Orlando 1989, Free-space laser communication technologies session

(4) "Système de pointage, d'acquisition et de poursuite pour liaison optique intersatellite"E. PEREZSecond SMAI conference (Société de Mathématiques Appliquées et Indusirielles),March 1989, Paris

(5) "System Test Bed for demonstration of the optical space communication feasibility"R. DUMAS, B. LAURENTProc. SPIE, vol 1218 (Los Angeles) 1990

(6) "THX 3 1 160 : a new area array CCD sensor for laser tracking applications"P. DAUTRICHE, G. BOUCHARLATProc. SPIE, vol 1 131 (Paris) 1989

(7) "Space radiations effects on CCDS"G.R. HOPKJNSONESA electronics component conference (ESTEC), Proc. ESA-SL-313, November 1990

(8) "How to meet intersatellite link mission requirements by an adequate optical terminal design ?"0. DUCHMANN, G. PLANCHEProc. SPIE, vol 1417 (Los Angeles) 1991

(9) "Precision Pointing Mechanism for Intersatellite Optical Communication", T. Hicks, B. 0. Sullivan,J. Russel, L. Scholl, Proc. SPIE, Orlando 1989.

(10) "Pointing, Acquisition and Tracking subsystem and components for optical space communication system"R.H. KERN, U. KUGELProc. SPIE, vol 1131 (Paris) 1989

(1 1) "Analysis of SILEX tracking sensor performance"R.P. MATHUR, C.I. BEARD, D.J. PURLLProc. SPIE, vol 1218 (Los Angeles) 1990

(12) "An acquisition sensor for optical communications in space" E. CORPACCIOLI, G. BORGRIProc. SPIE, vol 1131 (Paris) 1989

(13) "European SILEX project : concept, performance, status and planning"G. OPPENHAUSER, M.E. WITTINGProc. SPIE, vol 1218 (Los Angeles) 1990

(14) "In Orbit measurements of microaccelerations of ESA's communication satellite Olympus"M.E. WITTIG, L. VAN HOLTZ, D.E. TURNBRIDGE, H.C. VERMEULENProc. SPIE, vol 1218 (Los Angeles) 1990

SPIE Vol. 1417 Free-Space Laser Communication Technologies III (1991) / 157


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