1 Pointing and Stabilization of Lightweight Balloon Borne Telescopes SwRI Balloon Workshop on Low Cost Access to Near Space 27 April 2007 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 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

5 Dominant Sources of Vehicle Base Motion LEO Spacecraft –Thermal Shock from Transitions into & from Umbra –Attitude Control System –Solar Array Drives High-Altitude Lighter-Than-Air –Attitude Control System –Payload Mechanisms –Station-Keeping Propulsion, if applicable High-Altitude Heavier-Than-Air –Air Turbulence –Propulsion

6 Typical 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 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

7 Fine- and Coarse- Pointing Mechanisms Coarse-Pointing Mechanism –Performs large-angle motions –Can be vehicle ACS or 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)

8 2-Axis Fast-Steering Mechanism Technology is Mature Apertures for beam sizes from 15mm to 300mm are available Servo control bandwidths to 5000 Hz Range of travel up to +-175mrad (+-10degrees) are available A variety of mirror substrate materials are proven –Aluminum –Beryllium –Silicon Carbide –Zerodur –BK-7 –LEBG

9 The FSM Can Be An Active Isolation System Non-Contacting 6-DOF Active Isolation Systems are Available 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 Shown here is an IS2-10 Isolation System –±2mm travel in all axes Shown here is an IS5-40 Isolation System used as a base-motion-simulator –±5mm travel in all axes

10 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 3,000 Hz Stable Line-of-Sight –correct for platform vibrations –correct for aero turbulence Agile Beam-Steering –up to 15,000 rad/sec 2 acceleration –up to 30 rad/sec rate

11 Precision Pointing Systems Cover Large Ranges of Precision and Field-of-Regard Fields-of-Regard from 1 milliradian to continuous rotation are considered Precision is defined as positioning resolution, stability and following accuracy Field of Regard (+- milliradians) Precision (micro-radians) Mass-Stabilized Telescope Satellite 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

12 Near Space Relative to Lower-Altitude Aircraft The primary advantages of near-space deployment relative to operating on lower-altitude (up to 18km) aircraft platforms include: –Additional FOV and FOR for earth observation and surveillance missions –Potentially quieter platforms The primary difficulties are: –Additional LOS stability is required for the same resolution on the ground

13 Near Space Relative to LEO Platforms The primary advantages of near-space deployment relative to operating on LEO platforms include –increased resolution for earth observation and surveillance missions –relaxed environmental requirements –the ability to loiter over an area of interest –less hardware is required to cool actuators and servo control electronics The primary difficulties associated with near-space deployment relative to LEO platforms include –aircraft and UAV encounter atmospheric turbulence and the resulting line- of-sight and platform disturbances –reduced FOV and FOR

14 Components of Pointing Accuracy Fine- and course-steering mechanism pointing accuracy is defined in several ways: –positioning resolution –position reporting resolution –line-of-sight jitter –position reporting noise –short-term positioning drift –long-term positioning drift –positioning thermal sensitivity –position reporting thermal sensitivity –positioning linearity –position reporting linearity

15 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

16 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

17 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

18 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

19 Non-Linear Characteristics Limit Pointing Accuracy Friction-induced pointing error –Typically associated with ball or sleeve bearings –Peaks at turn-around condition (stick-slip) –Error amplitude can be readily estimated 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

20 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 5-10m resolution over a 60km circle on ground from 100km altitude 1-3m resolution over a 20km circle on ground from 30km altitude