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Pointing and Stabilization of Lightweight Balloon Borne Telescopes

presented at the

SwRI LCANS 09 Balloon Workshop onBridging the Gap To Space

Lightweight Science Payloads on High-Altitude Long-Duration Balloons and Airships

26 October 2009

Larry GermannLeft Hand Design Corporation

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The Purpose of a Precision Pointing System

• Perform line-of-sight stabilization

– Correct atmospheric turbulence

– Correct vehicle base motion

– Correct vibration of optical elements

– Correct force or torque disturbances

– Correct friction-induced pointing errors

• Perform scanning function to extend the Field of Regard beyond the telescope’s Field of View

• Perform chopping function

• Perform dither function

• Quickly slew and stare among a field of targets

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When a Precision Pointing System is Needed

• When the required pointing stability cannot be achieved by the platform attitude control system

• When the field-of-regard requirement is larger than the instrument’s achievable field-of-view

• When chopping is required to calibrate the optical sensor

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Precision Pointing Systems Cover Large Ranges of

Precision and Field-of-Regard

• Fields-of-Regard from 100 microradian to continuous rotation are considered.

• Precision is defined as positioning resolution, stability and following accuracy.

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System Precision (micro-radians)

Mass-Stabilized Telescope Satellite, like HST

Fine-Steering Mechanism (FSM) with a Coarse Steering Mechanism

Coarse-Steering Mechanism

Single Full-ApertureFlexure-Mounted Steering Mirror

Single Full- or Reduced-ApertureFlexure-Mounted Steering Mirror

Full-Aperture FSM Sensor Noise LimitF

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FSM Sensor Dynamic Range Limit

Increasing Cost

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Line-Of-Sight Stabilization, Stability Correction Ratio

Correction Ratio Amplitude (f) = Base Motion (f) / Residual LOS Jitter Requirement (f)

Pointing System Cost is Related to the Correction Ratio Spectrum

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Dominant Sources of Vehicle Base Motion

• LEO Spacecraft

– Thermal Shock from Transitions into & from Umbra

– Attitude Control System (ACS) exciting vehicle bending modes

– Solar Array Drives

• High-Altitude Lighter-Than-Air

– ACS exciting pendulum & suspension cable bending modes

– Payload Mechanisms

– Station-Keeping Propulsion, if applicable

• High-Altitude Heavier-Than-Air

– Air Turbulence exciting vehicle bending modes

– Propulsion

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Typical Precision Pointing System Components

• The components of a typical precision pointing system include:

– Beam-expander telescope

– Fine-steering mechanism or fast-steering mechanism: two-axis reduced-aperture, full-aperture steering mirror or isolation system

– Coarse-pointing mechanism: vehicle attitude control system, two-axis gimbaled telescope or full-aperture steering mirror

• Payload motion sensor suite: inertially or optically referenced

• In general, both fine-and course-pointing mechanisms are required when system dynamic range >10^5 @1kHz or >10^6 @10Hz is required, exceptions include a mass-stabilized satellite ACS for the single pointing stage

• Flexure-mounted fine-steering mechanism is required when system following accuracy requirement exceeds friction- or hysteresis-induced limits

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Fine- and Coarse- Pointing Mechanisms

• Coarse-Pointing Mechanism

– Performs large-angle motions

– Can be vehicle ACS or a bearing-mounted mechanism

– Keeps FPM near the center of its travel range

• Fine-Pointing Mechanism

– Performs high-frequency portions of pointing motions

– Performs high-acceleration motions

– Accurately follows commands

– Corrects or rejects base motion and force and torque disturbances

– Can be reaction-compensated (a.k.a. momentum compensated)

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2-Axis Fast-Steering Mechanism Technology is Mature

• Apertures for beam sizes from 15mm to 300mm are available, 116 x 87mm for a 75mm beam shown

• -3dB closed-loop servo control bandwidth up to 5,000 Hz

• Range of travel up to +-175mrad (+-10degrees) is available

• A variety of mirror substrate materials are proven– Aluminum– Beryllium (shown here)– Silicon Carbide– Silicon Carbide Foam– Zerodur– BK-7

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CE50-35-CV-RC2 FSMIs Simple, Robust and Mature

•The CE75-35-BK SN140

•BK-7 mirror

•76.2mm diameter aperture

•+-35mRad travel

•120 Rad/Sec2/rootW efficiency

•2,300 Rad/Sec2 acceleration

•wave PV @633nm surface figure error

•450 Hz -3dB closed-loop servo control bandwidth

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CE75-35-ZD Represents LHDC’s line of Cost-Effective FSM

•CE75-35-ZD SN147, Zerodur mirror

•76.2mm diameter aperture

•+-26mRad travel

•A custom abbreviated frame

•9,000 Rad/Sec2 acceleration

•120 Rad/Sec2/rootW efficiency

•0.165 wave PV @633nm surface figure error

•250 Hz -3dB closed-loop servo control bandwidth

•Coating is highly reflective at 1.5um

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FO50-175-ALHas Space-Flight Experience

•FO50-175-AL SN106

•Aluminum mirror

•80.7 x 60mm polished aperture

•+-175mrad travel

•380 Hz -3dB closed-loop servo control bandwidth

•7,000 Rad/Sec2 acceleration

•Proven in low-earth orbit

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FO50-35-SC-RT7 Achieves Record Servo Control Bandwidth

•FO50-35-SC-RT7 SN133

•Silicon carbide mirror

•80.7 x 60mm polished aperture

•+-5mrad travel with the reduced-travel option

•5,000 Hz -3dB closed-loop servo control bandwidth when base-referenced

•6,000 Hz -3dB closed-loop servo control bandwidth when optically referenced

•3,300 Rad/Sec2 acceleration

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The Fine-Steering Mechanism Can Be An Active Isolation System

Non-Contacting 6-DOF Active Isolation System

• Non-Contacting electromagnetic actuators

• Non-Contacting sensors

• Highly flexible umbilical transfers signals with <0.1 Hz suspension resonant frequency

– minimal transfer of base motion forces

• Accelerometer- and position-referenced stabilization servos

• IS2-10 Isolation System

– Occupies a 25mm thick disk

– ±2mm travel in 3 axes

• IS5-40 Isolation System used here as a base-motion simulator

– ±5mm travel in 3 axes

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Servo Functional Block Diagram

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Flight-Format Servo Control Electronics is Available

• SC03-BD

• 2 Channels Servo Control

– Position-Referenced Loops

– Current-Referenced Drivers

– Optical Tracking Reference

– Position Sensor Reference

• Light Weight

– 150 Grams

• Full Military Temperature

• Up to +-45V, 10A Driver Capability

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Servo Control Electronics Available in a VME-6U

Single-Card Format

SC02-BDSingle-Card VME-6U Format

Contains All Servo Functions- Pointing and Tracking Modes- Current-Referenced Driver- High-Temperature Driver Shutdown

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Components of Pointing Accuracy

• Fine- and course-steering mechanism pointing accuracy is defined in several ways:

– Positioning resolution and position reporting resolution

– Line-of-sight jitter and position reporting noise

– Short-term positioning drift and position reporting drift

– Long-term positioning drift and position reporting drift

– Positioning thermal sensitivity and position reporting thermal sensitivity

– Positioning linearity and position reporting linearity

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Imaging Resolution Limit isRelated to Altitude and Aperture

• Imaging resolution is constrained by the optical diffraction limit, which is a function of altitude and telescope aperture

• Image resolution is defined as a distance on the ground from 30km altitude

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Positioning and Reporting Linearity

• Positioning linearity is defined as the difference between commanded and achieved position over the operating ranges of travel and temperature

– Dominated by friction, disturbances and position sensor error

– Position sensor error is dominated by thermal sensitivity

– Typically not much better than 0.04% of travel

• Reporting linearity is the difference between reported and achieved position over the operating ranges of travel and temperature

– Dominated by position sensor error

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Fast Beam Steering is Defined as Servo Control Bandwidth

• Fast beam steering is defined as the ability to follow a small-amplitude sine wave at various frequencies

• Generally defined as the frequency at which the closed-loop servo response falls by 3dB

• Alternately defined as the 0dB open-loop frequency

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Fast Beam Steering is alsoDefined as Acceleration Capability

• Fast Beam Steering is sometimes defined as the highest frequency at which the mechanism can perform a full travel sine wave

• This is limited by the mechanism’s acceleration capability

• Acceleration is shown here in terms of peak and continuous capability

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Non-Linear Characteristics Limit Positioning Accuracy

• Friction-induced pointing error

– Typically associated with ball or sleeve bearings

– Peaks at turn-around condition (stick-slip)

– Friction-induced error amplitude can be readily estimated

• Peak Pointing Error ~ 2 * Friction Torque / Inertia / Bandwidth2

• Hysteresis-induced pointing error

– Typically associated with ceramic actuators

– Typically quantified in terms of % of travel range

– Effect are similar to friction effects

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Precision Pointing Systems Offer Many Benefits

• Extended Dynamic Range, – Up to 9 orders of magnitude– Up to +-180 degree Field of Regard– As low as nanoradian line-of-sight stability

• High servo control bandwidth, up to 5,000 Hz– Correct disturbances up to 1,000 Hz

• Stable Line-of-Sight– Correct for platform vibrations– Correct for aero turbulence

• Agile Beam-Steering for scanning, chopping, dither, etc.– Up to 15,000 rad/sec2 acceleration– Up to 30 rad/sec rate

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Many Precision Pointing Instrumentsare Suitable for

Near-Space Platforms

• LIDAR measurements of forest canopy

• LIDAR measurements of foliage, carbon stock under canopy

• LIDAR measurements of targets under foliage or camouflage

• LIDAR topology measurements under foliage

• 0.1m resolution over a 20km circle on ground from 100km altitude

• 0.03m resolution over a 6km circle on ground from 30km altitude