1 Pointing and Stabilization of Lightweight Balloon Borne Telescopes presented at the SwRI LCANS 09 Balloon Workshop on Bridging the Gap To Space Lightweight Science Payloads on High-Altitude Long-Duration Balloons and Airships 26 October 2009 Larry Germann Left Hand Design Corporation

2 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

3 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

4 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. Field of Regard (+- milliradians) System Precision (micro-radians) Mass-Stabilized Telescope Satellite, like HST Fine-Steering Mechanism (FSM) with a Coarse Steering Mechanism Coarse-Steering Mechanism Single Full-Aperture Flexure-Mounted Steering Mirror Single Full- or Reduced-Aperture Flexure-Mounted Steering Mirror Full-Aperture FSM Sensor Noise Limit FSM Sensor Noise Limit with 10x Optical Gain Friction Limit FSM Sensor Dynamic Range Limit Increasing Cost

5 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

6 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

7 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 or 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

8 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)

9 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 s ervo 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

10 CE50-35-CV-RC2 FSM Is 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 surface figure error 450 Hz -3dB closed-loop servo control bandwidth

11 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 wave surface figure error 250 Hz -3dB closed-loop servo control bandwidth Coating is highly reflective at 1.5um

12 FO AL Has Space-Flight Experience FO 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

13 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

14 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

15 Servo Functional Block Diagram

16 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

17 Servo Control Electronics Available in a VME-6U Single-Card Format SC02-BD Single-Card VME-6U Format Contains All Servo Functions - Pointing and Tracking Modes - Current-Referenced Driver - High-Temperature Driver Shutdown

18 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

19 Imaging Resolution Limit is Related 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

20 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

21 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

22 Fast Beam Steering is also Defined 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

23 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 / Bandwidth 2 Hysteresis-induced pointing error –Typically associated with ceramic actuators –Typically quantified in terms of % of travel range –Effect are similar to friction effects

24 Precision Pointing Systems Offer Many Benefits Extended Dynamic Range, –Up to 9 orders of magnitude –Up to 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/sec 2 acceleration –Up to 30 rad/sec rate

25 Many Precision Pointing Instruments are 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