direct stabilisation for laser communications

Direct stabilisation for laser communications

P.E.G. Cope, D.M. Priestley

MATRA MARCONI SPACE, Anchorage Road, Portsmouth,

Hampshire PO3 5PU, UK

Abstract

Laser communication between spacecraft allows high data rates for low powerdue to the immense antenna gains involved, but at the expense of greatdifficulties in Pointing, Acquisition & Tracking (PAT). Classically this isachieved using a steering mirror in a wide bandwidth tracking loop. This paperdescribes the experimental investigation of an alternative approach in which thewhole telescope is isolated from disturbances so that much lower trackingbandwidth becomes sufficient. The system was tested on a motion tablecovering the whole ESTeC microvibration environment specification andshowed a worst error of 500nrad rms, this at lOOHz. This is compatible with"burst error" criteria. The problem of making fast acquisition movements wasaddressed & solved. Many valuable lessons were learned.

Plate 1: Overall view of Equipment & Test Apparatus

Transactions on the Built Environment vol 19, © 1996 WIT Press, www.witpress.com, ISSN 1743-3509

608 Structures in Space

1. Background

Following earlier studies of optical communications techniques using gas anddiode laser sources and the launch of the pioneering Silex full scaledemonstration, ESTeC issued an ITT for the investigation of an alternativePointing, Acquisition & Tracking (PAT) technology, "Stabilised Platform forAcquisition & Tracking" (SP1AT), followed by a contract to MSS (now MatraMarconi Space, MMS) at Portsmouth under which the work reported in thispaper was carried out. Thanks are due for the valuable criticism and assistanceprovided by, initially, Dr.A.Hahne and subsequently Dr.A.Popescu andcolleagues, of ESTeC.

This study was to use what MMS had called "direct stabilisation" of theoptical antennae, that is to say the light beam was to be stabilised to therequired accuracy by direct stabilisation of each antenna telescope rather thanindirectly using a fast steering mirror to attenuate the effects of telescopemotion after they had been impressed upon the light beam. The final form ofthe apparatus used in the hardware study and accuracy demonstration is shownin Plate 1 below, which is a photograph of the equipment and test facility.

An earlier low-cost study carried out for Intelsat using very inexpensivebreadboard equipment had indicated a capability to stabilise a telescope inertiato an accuracy inside lOOnrad rms in the face of a roughly representativemicrovibration environment.

The accuracy requirement for a 250mm diameter antenna at 850nmwavelength with 0.5dB allowable pointing loss is around 2500nrad, dependingon illumination criteria. In order to obtain the .0001% so-called "burst error"probability favoured by the CCllT for satellite links it is necessary to makesome hard-to-justify assumptions about the normality of the pointing errordistribution and to operate at around 4.89 standard deviations. This leads to anrms pointing error requirement of around 510nrad and a need for anyexperimental test to be run for at least 10 million times the response time of thetracking loop in order to obtain a statistically significant number of independenterror samples to justify such a low probability. This normally amounts to aweek or more of continuous error assessment without any unforeseendisturbance to the test environment.

Some earlier pieces of Marconi experience which proved valuable in thiscontext concerned stabilised mounts for the use of TV cameras in helicoptersdeveloped by Marconi Research at Great Baddow and Stratospheric balloonastronomy gondolas developed by Marconi Space at Frimley. In both examplesa dual bearing drive method (or module) (DBDM) concept had provedvaluable. This comprised a wide angle conventional bearing and driveassembly combined with a small angle flexural pivot assembly to providemicrovibration isolation. Baddow refined this approach by feeding forwardspring deflection as a torque compensation in the final drive whilst Frimley hadused a feedback method to the main drive to maintain the required (nominallyzero) deflection in the flexural bearings.


Structures in Space 609

2. SPAT Fundamentals

The control task in laser communications is known as the PAT task since itcomprises Pointing, Acquisition & Tracking. Each of these sub-tasks has itsown peculiar difficulties. The sub-tasks may be defined as follows:

* Pointing: Directing the telescopes at each other open-loop prior to* Acquisition: Enabling each telescope to bring the other to mid-beam* Tracking: Maintaining alignment within the burst error criteria

Pointing considerations

Pointing prior to acquisition is a particularly difficult task for the followingreasons:

* Beacon power is dispersed at the receive end as (pointing error)** Acquisition detector background varies as (pointing error)** CCITT non-availability <.01% forces looking within 1.2° of the sun* Lack of definitive information on attitude errors or telescope scatterIt was recognised very early in the project that the, effectively 4th power,

dependence of acquisition link budget margin on pointing error puts a very highpremium on achieving good open loop accuracy. To that end MMS proposedfrom the outset to add inertia! sensors within the SPAT equipment whichwould enable additional filtering of the attitude measurements originating fromthe spacecraft's own AOC sub-system and which would contribute a reducedsensitivity to satellite variations in AOC performance (important with a generalapplication equipment).

Fig.l: Possible spectra for attitude error Filtered by ADS or FLIRT



Fig. 1 illustrates the way in which an additional inertial sensor within theSPAT can be used to minimise the effect of the error in the AOCS sensors. It isa log/log angular rate power spectral density diagram covering 14 decades ofpower spectral density vertically and 8 decades of frequency horizontally. Thelikely AOC sub-system attitude sensing error is represented by the LEO andGEO IRES curves whilst the curves labelled SD8301 and SPAT FLIRTs showthe noise equivalent angle (NEA) spectrum (and extrapolation) of the simplebut accurate angular displacement sensors (simple inertial sensors) made bySystron Donner and used on Landsat for post-hoc image deblurring and anMMS breadboard equivalent (FLIRT). An NEA spectrum for a moreexpensive Ferranti type 125 HI Gyro is included for comparison.

On an SD/frequency diagram of this kind it is possible to add contours ofconstant rms angle by taking account of both angle* noise density and low-passcut-off frequency. The curves show that, by using the available LEO opticalsensor and the FLIRT inertial sensor each in its better regime, a modestimprovement from O.Smrad to 0.2mrad may be obtained. This represents a linkbudget saving of nearly 16dB e.g. from a 5W beacon to a 125mW beacon. Nosimilar improvement is possible, however, in GEO since the slowness of theradiance variation component in this case eludes the smoothing capabilities ofeven the Ferranti inertial sensor. On the other hand there will typically be 10 to20 LEO acquisitions during the long period radiance variations which give riseto the error peaks seen from GEO. This implies that, by noting the acquisitionerror on each occasion and propagating it by trend analysis to the nextacquisition a similar reduction in effective error magnitude could be obtained.Such reductions are of the utmost importance in containing the acquisition linkbudget which can otherwise demand impossible beacon power levels.

Acquisition considerations

The narrow-angle sun-scatter problem imposed by the CCITT availabilityrequirements is normally encountered only in very special astronomicalinstruments like coronagraphs. An examination of the design of theseinstruments was indeed helpful in identifying the need for ultra-clean opticswhich led quickly to the conclusion that the limiting case for SP1AT wouldprobably arise when a paint flake, detached by atomic oxygen degradation orsome similar mechanism, was electrostatically attracted onto the primary optic,so contaminating the main beam with scattered sunlight. A single 5mmdiameter white paint flake was arbitrarily assumed in order to constructsomething approaching a realistic acquisition budget. The immense difficultiesin obtaining better interface data and/or system level inputs for these vitalsizing calculations were a characteristic of all the laser communications studiesat the time.

The resulting link budget demonstrated a feasible acquisition performanceby making use of spacecraft attitude measurement uncertainty cones reduced byinertial sensing (LEO) or by trend analysis (GEO) and also making use ofnarrow band (lOu wide) optical filtering to assist in rejecting scattered sunlight.Detector dark current was not a significant factor in the budget, so confirmingthe 4th power dependence on uncertainty angle.



Tracking considerations

The impact of the lack of interface data extended seriously into the trackingdesign also and several changes to the baseline microvibration environmentrequirement had to be made during the course of the work, for example to takeaccount of results available from the Olympus PAX experiment. Neverthelessthe final state of the microvibration requirement remained open-ended in termsof total power content since a roll-off in micro vibration angular amplitudeabove 40Hz of only -40dB/decade remained in the specification leading to awhite torque spectrum with no specified cut-off frequency.

Since realistic link delays and sight-line attenuation could not beinvestigated in the laboratory, the main emphasis of the SPAT experimentalwork was not on pointing or acquisition aspects but on the demonstration thatthe specified levels of microvibration were not incompatible with the bursterror criteria for successful optical communication.Fig.3 shows an outline diagram of the elevation dual-bearing drive module(DBDM) which combines a ball-bearing/geared stepper motor wide-angle drivewith a flexural pivot/slab D.C. motor narrow angle drive. The flexural pivotdeflection is measured by a pair of linear variable differential transformer(LVDT) pick-offs arranged to cancel translation and to double rotation effects.The wide angle drive gear backlash is removed by means of a "tensator" (reel-to-reel) pre-load spring.

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abscissa against frequency as ordinate. The specified microvibration in Ref. 1 isgiven in (angle)* power spectral density terms and the integral of the givenspectrum is 13^rad rms. Also shown on the diagram are the filteringattenuation due to the mechanical resonance of the telescope inertia supportedin the flexural pivots of the DBDM and the filtering attenuation due to theclosure of a tracking loop with a 5Hz crossover frequency (compatible with50Hz tracking detector field scan frequency). The attenuating effect of each of


these processes on the specified microvibration is shown as well as the residuewhen both effects are operating.

This residue meets the burst error requirements at all specified frequencies.The additional margin achievable by feed-forward flexure stiffnesscompensation is also illustrated. The flexure deflection is measured by theLVDTs and a proportional signal is fed-forward into the drive motor in thenegative stiffness sense so as to offset the flexure stiffness and lower thenatural frequency of the mechanical filtering element. This compensation iscomplicated by the hysteresis forces in the drive motor but a worthwhileadditional safety margin is obtainable.

The need for evidence that burst error probabilities will be as low as 1E-6has an unfortunate implication when the tracking loop bandwidth is as low as5Hz. The autocorrelation function of the tracking noise will be significantlyabove zero at regression times up to 50msec with the result that 50msecseparated samples, some 10 million in number, will be needed to get anystatistically significant idea of the burst error probability. This amounts to some6 days or more of testing without unrealistic external influences. In the MMStests, automatic monitoring of all possible environmental influences had to beused to exclude any such unrealisms.

Design trade studies

An important trade study at the outset of the design was the selection of theMRC feed-forward isolation method used on helicopter TV mounts versus theMMS feedback isolation method used in the Marconi stabilised balloonplatform and the Intelsat PAT breadboard. The MRC feed-forward methodwas selected, although feed-forward is usually less accurate, because in thiscase no large improvement factor was required and because, in the interests ofavoiding gimbal lock singularities in the middle of the coverage hemisphere,the azimuth/cross elevation DBDM had to be split across a gimbal soprecluding the tight torque feedback gain essential to the correct functioning ofthe feedback method.

The next decision concerned the form of the acquisition and trackingdetectors. Parallel studies had indicated the immense value of a staring arrayacquisition detector over a simple quadrant detector such as might be used if abeacon wavelength in excess of lOOOnm had been chosen. The quadrantdetector needs six or eight iterations of light exchange between the participantsbefore it can home in to the required tracking accuracy whereas a staring CCD,for example, can reach the accuracy in two steps, at the most. Staring arrays ofHgCdTe coupled to Silicon by Indium bumps, which can function at 1500nm,were considered to involve too many defence complications to be a satisfactoryalternative to CCDs.

With the availability of beacon laser diode arrays up to 0.5W in output at860nm, the final obstacle to the use of a Silicon CCD was removed. Since thatchoice was made, both the radiation hardness and the dark currentcharacteristics of CCDs (with MPP) have been improved, further consolidatingthe position.

For tracking, it was immediately attractive to see whether the centre fourpixels of the acquisition detector could serve as a tracking detector withoutfurther complication. These pixels are read out at 50Hz continuously and


acquire a good working signal when looking at a focused laser diodetransmitter package (LDTP) even at 40Mm range. A 50Hz data rate is adequateto close a 5Hz tracking loop which is all that SPAT needs. The singlecombined acquisition and tracking CCD detector was therefore chosen.

3. The Experimental Apparatus

An examination of Plate 2, below, a close up of the equipment under test,will assist in the description of the apparatus. A fixed optical table mounted tothe solid footings of the laboratory carries a fixed 250mm Newtonian telescopeof diffraction-limited quality (entrance aperture visible at the right of thephotograph). This serves as a collimator allowing the electro-optical devices inits focal plane to appear as if at infinity (or 60Mm, the other side ofgeostationary orbit). Another, similar 250mm telescope (large cylinderoccupying the centre of the apparatus) represents the laser terminal antennaunder investigation. The small gap between the two telescopes was closed,later, by a simple labyrinth cuff to inhibit convective disturbances of the air inthe path (which would not be a source of problems in orbit).

The Equipment under Test

The telescope under investigation is mounted in the DBDM gimbals and iscarried on two-axis flexural pivots backed up in each axis by a conventionalball bearing geared drive servo so arranged as to minimise the deflection of theflexural pivots. (The full DBDM in the elevation axis is visible in the centre ofthe plate. There is a more detailed drawing of it in Fig.3).

Plate 2: Equipment under Test, SPIAT Telescope & MountBy ballasting the rear of the primary mirror with lead masses (numbered

discs at centre left) representing the optical bench in a real optical link, it waspossible to give the telescope a full 90°+ elevation clearance in the gimbals so



that gimbal lock avoidance could be explored. The gimbal assembly itself wasmounted, not to the main optical table but to a subsidiary motion table (lowerforeground) again resting on the same solid footing but free to move about avertical axis of rotation under the constraint of a flexural support system with asingle-degree-of-freedom. This motion table could be driven about this axis offreedom by use of a D.C. motor and flywheel mounted to the underneath of thetable top, to simulate microvibration of the host spacecraft. Dead-zone in thismotor which might otherwise have suppressed microvibration was avoided bysuperimposing a steady rotation.

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For convenience, the acquisition/tracking detector CCD, which had arather stiff cable, was mounted at the focus of the fixed telescope (visible onlyon plate 1) so that only a pinhole LED source was needed at the focus of themovable telescope (top, right of centre). The electrical readout collimator(telescope tube visible looking into mirror, lower front) was arranged to bemovable easily from reading the motion of the telescope balancing masses(main mirror) to measuring the movement of the motion table (using lowermirror behind and below main mirror) so that direct comparisons of the inputand decoupled motions could be made. Other features visible in the plate arethe fluid loop inertial sensor and readout head (on top of the ballast discs) andthe thin flexure lead wires carrying signals across the elevation axis to the innergimbal (atop the elevation DBDM).

In the elevation DBDM drawing, Fig.3 three concentric hollow shafts areevident, the inner shaft being connected to the inner, narrow angle, telescopegimbal. The cantilever bracket from the azimuth drive carries the outer tubecontaining the ball-bearing journals for the middle tube. A geared, micro-stepdriven stepper motor provides the drive to the ball bearing axis which needsonly ±50° freedom in the elevation axis. The inner tube can rotate up to ±5°inthe middle tube by virtue of the crossed spring flexural pivots connecting thesetwo tubes. The slab D.C. motor drives the inner tube w.r.t. the middle tube.



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Fig.4: Block Diagram of Flexure Compensation Feed-Forward & wide angle Feedback

The feed-forward connection of the DBDM used by MRC is shown inFig.3(and in more detail in Fig.4). Torque is applied to the telescope by the slabD.C. motor in parallel with the flexure pivots. Since this needs to operate overonly a small angle, flying leads can be used in place of brushes so removing thelast traces of Coulomb friction in the drive. To avoid the build-up of largeflexure deflections, the non-telescope side of the flexure pivots is rotated inconventional bearings under the control of the LVDT pick-offs using a steppermotor and gear drive with a reasonably fast (O.ls) response. It is this gearingwhich is preloaded by the tensator spring. In order to destiffen the flexurepivots to reduce the effect of residual errors in this follow-up loop, thedeflection measured by the LVDTs is also processed and fed at wide bandwidthto the slab D.C. motor in such a sense as to produce the required level ofnegative stiffness. Since the whole loop is under the tight control of thetracking optics the compensation is not critical to stability and over-compensation by this feed-forward has no worse effect than under-compensation. The LVDT signal processing is able to take into account theD.C. motor hysteresis as well as the flexure stiffness.

The value of inertia! sensing in reducing pointing error has already beenmentioned. It also has some value in reducing tracking error by enablingadditional tracking detector output filtering. Reference to Fig. 1 shows that thenoise level of the tracking detector within the 5Hz bandwidth of the trackingloop is of the order of SOOnrad rms whereas, if the SD8301 or the FLIRT isused in an electro-optical sensing blend, accuracies of 80 or 55 nrad rms shouldbe obtainable, respectively, the tracking detector use bandwidth having beenreduced accordingly. The blending algorithm used allowed individualadjustment of the proportional, 1/s and 1/s* terms of the inertia! sensor responsebut was erroneously adjusted as far as the tracking detector blend wasconcerned. This did not come to light until after the final set of test results hadbeen recorded and led to an unnecessarily high level of noise contaminating thedecoupling isolation measurements.



The test equipment

The test equipment comprised the following items:* Target ccllimator* Fixed optical table* Motion table* Read-out collimator* Test Control Station (TCS)* TCS software* Standard metrology (auotocoll., X-Y microscope, interference mic.)* Standard electronics (oscilloscope, spectrum analyser, plotter &c.)* Environmental monitoring instruments for long-duration PDF testsThe target collimator was a twin 250mm Newtonian telescope to the

telescope under test. This telescope was carefully adjusted for centration (i.e.to ensure that it was being used with the axis of the paraboloid passing throughthe centre of the CCD) by observing the comatic distortion of the image ofPolaris and by progressive adjustment of two of the three centration screwsuntil all trace of coma had been removed. This telescope was then used for thecentration of the gimballed telescope in the laboratory. Again, the mirror cellscrews were adjusted until all trace of coma had again been removed from acentral LED/pinhole image.

The fixed optical table stood on the oversite concrete of the ground floor ofthe laboratory. Additional diagonal leg bracing was added to suppress a high-Q3Hz resonance otherwise occurring about a vertical axis. A troublesome aircompressor in a nearby building also had to be remounted on rubber feet beforethis table became sufficiently stable for its purpose.

The motion table is built from two circular table tops each 1 metre indiameter mounted horizontally and connected by three equally spaced verticalrods constraining vertical translation and rotation about two horizontal axes.Three chains connecting the feet of the rods on one disc to the centre of theother disc constrain the two horizontal translations and apply preload via thediaphragm compliance of the table top. This leaves only rotation about thevertical axis relatively unconstrained. The loadings due to the mass of theequipment on the table and the preload in the chains both have a destiffeningeffect on the vertical rods but a good margin of positive stiffness remains toprevent collapse and equipment damage. The table was fitted with both driveand readout transducers on its periphery but these were abandoned in favour ofa rotary motor and flywheel, for drive, and a mirror/collimator, for readout, inorder to avoid exciting or measuring spurious translational movements of thetable.

The read-out collimator was purpose-built from a commercial 1 metrefocal length refracting telescope by replacing the eyepiece with a high intensityLED adjacent to a TO-5 encapsulated quadrant Silicon detector. This provideda narrow-angle (Imrad ptp), wide-band (0 - 500Hz) read-out of very highaccuracy (lOnrad rms NEA).

The Test Control Station (TCS) was built around an Intel 286 PC withadditional interface cards allowing it to communicate simulated telecommandsand to receive telemetry from the similar PC running the "embedded" Pascalsoftware for the investigation.



The TCS also used Pascal software which could be used to set up testconditions, initiate tests, provide real-time displays of test results and logresults for post-processing and evaluation.

Standard metrology equipment used on the tests included Hilger & Wattsautocollimator for absolute angle measurements (supported by a Minidekkorfor quick-look measurements), a two-axis travelling microscope with vernierposition read-out and an interference microscope which was typically used withwhite-light illumination and a ball-bearing target so that the dark central groupof three fringes unequivocally identified the zero path difference, for along-sightline measurements.

Minor equipments used at times included magnets and coils for applyinglocal forces and step disturbances and a simplified fluid loop with a spring-supported permanent magnet "float", surrounded by a 5000-turn copper read-out coil used for motion table velocity measurements.

In addition to the usual storage scopes, transfer function analysers,spectrum analysers, recorders and plotters, the prolonged probability densityfunction (PDF) tests involved the construction of some special environmentalmonitoring equipment. Mains and room temperature variation measurementswere easily arranged but a vertical-filament tungsten bulb with enveloperemoved operating in a bridge circuit formed a very sensitive micro-anemometer whilst a large version of the FLIRT (known as the "Python"), witha big area ratio between the large main tube and the small read-out sectionformed a very sensitive seismometer. No direct measurement of the noise levelwas possible in the local seismic environment but indirect indications were thatread-out noise did not exceed 3nrad rms in a lOHz bandwidth.

4. Results Obtained

Base Motion Rejection (BMR) Tests

Preliminary tests had indicated the following equipment performances:* Tracking detector noise: 350nrad rms in 25Hz bandwidth (b/w)* FLIRT noise: 120nrad rms in lOHz b/w* Inner gimbal mechanism noise TOnrad rms in 5Hz b/w* State monitor noise (instrumentation): lOnrad rms in 500Hz b/wWhen the equipment was assembled and the tracking loop was closed the

overall noise measured nearly 500nrad rms which was disappointing since amuch cruder earlier breadboard had shown noise levels as low as 60nrad rms,albeit using effectively noise-free sensors. It was found later that the SP1ATtracking state estimator was actually mistuned for the noise variancesprevailing at the time but this was not discovered until too late in themeasurement series for it to be corrected. The base motion rejectionmeasurements could still be made to adequate accuracy by the use of a transferfunction analyser, at ESTeC's suggestion, to give coherent detection of thecoupling terms amongst the noise.

The composite results curve, Fig.5, requires some explanation. The ESTeCrequirement specification spectrum in use at the time, translated into "worst-case" spot rms values, is the smooth curve labelled ISL-SY-IF-2 (See Ref.l). Itshows a uniform base motion level below IHz and a roll-off at -20dB/decade



until 400Hz when, after the PAX results became available, it was indicated thatthe roll-off could be taken to increase to -40dB/decade. The torque levelrequired to move the motion table in accordance with this requirementincreases at 20dB/decade over the range 1 to 400Hz and remains level above.In fact, even using quite a powerful drive motor on the motion table, it was notpossible to reach the specified micro vibration level at 400Hz.

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At lower frequencies, however, it was easy to exceed the required level and, inorder to get residual motion readings significantly above noise, a 20dBexaggeration of the disturbance was used in this region. The measured levelswere then reduced by 20dB (or the appropriate factor) to produce the thickcurve in the diagram. The l-_ requirement level (based on 5-_ equating to theburst error level) is shown on the diagram as a thick dashed line and it isevident that the rejection is compliant at all the frequencies tested.

What is at first surprising is that there is not more margin around lOOHz.The reason for this was anticipated by Dr.Popescu when he pointed out quiteearly in the programme that linear vibrations of the mounting could passthrough the flexural pivots to the telescope and there generate angular errorsdue to the flexible behaviour of the telescope structure. This effect occurredparticularly acutely in the SP1AT demonstration because the "garden model"telescope used had a particularly flexible structure and because the motion tableexhibited many additional degrees of freedom in the lOOHz region, some ofwhich were translational.



Error Probability Density Function (PDF) Tests

The next task was to demonstrate, within the limitations of the programmeresources, that the burst error criteria could be met. To this end some specialon-line post-processing software was written which counted error peaksexceeding a number of specified levels. Ideally, then, a search would be madefor an error level showing, say, 10 error transgressions over a period involving10 million independent error samples. This level would then roughly representthe 1E-6 probability error level and could then be compared with the burst errordepointing allowance in the communications link error budget.

A test of this type was carried out. During the period of the test, only some24 hours, the mains voltage, room temperature, air currents and seismicenvironments were continuously recorded since it was impracticable and evenundesirable (because of movement and attention lapses) to have humanmonitors in place during this period.

A technique was found for plotting the logarithm of the cumulativedistribution of error and of l-(the cumulative distribution) which very usefullydisplayed the improbable events such that a Gaussian distribution tended to astraight line asymptote at the low probabilities. One of these plots, a result ofthe test, is reproduced in Fig.6.

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Fig.6: Probability Density Function Test Results: Burst Errors

Although resources did not permit to continue the test until valid resultswere obtained at the 1E-6 level it is clear that the results actually obtained werenot inconsistent with a 1E-6 level of about l.Tmicrorads which represents anadequate margin below the 2.5microrads depointing allowance for burst errors.



Time-Optimal Acquisition Slew Tests (TOAST)

Although the SP1AT test set-up was not suitable for comprehensive pointingand acquisition tests, some aspects could be explored.

One of the more obvious advantages of the indirect stabilisation approach(using a fast beam steering mirror) in comparison with the indirect approach isthat, when a rapid realignment of the beam is required during the acquisitionsequence, the fast steering mirror can respond very much more quickly thanwhen moving the whole telescope structure. In order to determine the extent ofthis limitation the flexural gimbal motors selected were as powerful aspracticable and some tests were mounted to see if they behaved as requiredunder bang-bang control.

The test actually carried out was a much simplified version of a realacquisition slew in that an uncomplicated double square pulse of full drive wasapplied to the gimbal drive motors. The + and - durations were then adjusteduntil a slew of the required amplitude was obtained and with zero finalvelocity, making use of the two degrees of freedom in the durationsadjustments. A series of such time-optimal slews was then carried out toestablish the degree of amplitude scatter to see if it was consistent withexchanging the narrow communications beams with the responding spacecraft.

The results showed that the amplitude scatter, from the very edge to thecentre of the acquisition field in two axes simultaneously, resulted in a finalerror scatter of no more than ±1 pixel (in 150 pixels); this after 160ms, total, ofslewing time. The final settling transient would then take less than 10msec.

An important aspect of this performance is that the large double impulse oftorque given to the telescope as so far described would also react upon themounting and, in orbit, upon the spacecraft. In fact the mounting table in theSP1AT lab had to be quite rigidly constrained during the TOAST exercise toenable the consistent results to be obtained. Such a level of disturbance wouldbe quite unacceptable on a spacecraft conducting optical observations at thetime. Fortunately a remedy is available by mounting the stators of the gimbalmotors in bearings themselves. This would remove the inertia! reaction on thespacecraft and substitute only the friction. The motor stators would not rotatefar (under 180°) since conservation of angular momentum would ensure that,when the telescope stopped the motor stator also stopped. Also, the velocityreached would not be sufficient seriously to detract from the torque availablefrom the motor, however, the motor could no longer use just flying leads butwould have to be commutated by brushed or brushless methods. Accumulationof stator velocity could be resisted by feeding stator angle into the geared driveloop in parallel with the LVDT feedback. This would lead to a gentle wind-upof the flexures which, if correctly sensed, would bring back the stator to therequired rest position in the normal course of feeding forward the wind-uptorque.



5. Conclusions

The main conclusions of this preliminary system and subsequent hardwarestudy are as follows:

* The worst-case base motion rejection performance is compliant* The noise PDF test appeared to be compliant in the 24hr test* The acquisition transient response was compliant in the allowed time* Valuable lessons learned:

- Motor hysteresis is significant and needs compensation- More care is needed in state estimator optimisation- A rigid telescope structure should be used in any further test- Acquisition budget involves (AMA)4 making AMA a big driver- Combined Acq & Track on a single CCD works fine in SP1AT- Despite being inferior to Ref 2, it meets the need!

In short, it can be concluded that the use of this approach in anexperimental laser terminal could proceed in the confidence that the laser linkwould not be affected by thruster firings, wheel speed resonances with lightlydamped structural modes microwave switch operations or other identifiedsources of microvibration. Furthermore no technology advance (like thedevelopment of special CCDs) is called for in its implementation.

Acknowledgements

The authors wish to thank igain Dr. A.Hahne and Dr. A.Popescu, of ESTeC,and their colleagues for their valuable criticism, patience and assistance duringthe course of the study as the various system and hardware problems wereencountered and resolved.

References1. ESTeC specification ISL-SY-EF-2 Issue 22. "Experimental assessment of laser tracking noise angle" SPEE Proceedings,Vol.1191, 1989


direct stabilisation for laser communications

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