Experimental Emulation of Air-to-Space Laser Communication Links

1. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (1)
PRESENTED AT THE
LEOS 2008 Annual Meeting
Newport Beach, CA
13 November 2008, Paper ThAA 2
J.M. Roth, R.J. Murphy, W.E. Wilcox, and R.A. Conrad
<jroth@ll.mit.edu>
Advanced Lasercom Systems & Operations Group
MIT Lincoln Laboratory, Lexington, MA
This work was sponsored by the Department of the Air Force under Air Force Contract FA8721-05-C-0002. Opinions, interpretations,
conclusions, and recommendations are those of the author and are not necessarily endorsed by the United States Government.
Experimental Emulation of Air-to-Space Laser
Communication Links

2. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (2)
Bandwidth and Antenna Gain Advantages for
Lasercom Deployment
• 15 THz of predominantly unregulated spectrum
• Lasercom can supports 2.5- to 40-Gb/s user rates
• High antenna gain provides high power-on-target and increased security
• Leverages telecom industry standards and components
Lasercom provides significant bandwidth increase at relatively low cost

3. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (3)
Links of Interest for Free-Space Laser
Communication (Lasercom)
Link Type Approx. Range Application
Interplanetary (e.g., Earth ↔ Mars) ~200,000,000 km Data relay for deep-space missions
GEO ↔ GEO 50,000 – 84,000 km Backbone ring network
GEO ↔ LEO, Aircraft 41,000 km Data extraction from tactical users
Aircraft ↔ Aircraft 100 – 400 km Mobile networking among tactical users
Lasercom applicable to wide variety of links

4. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (4)
Tracking Testbed emulates
these effects to provide
understanding of channel
impact on air-to-space
lasercom
Air-to-Space Lasercom Challenges

5. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (5)
Outline
• Background
• Lasercom Tracking Testbed
– Emulation capability
– Tracking terminals and software control
– Tracking model
• Experimental testing
• Compact aircraft terminal
• Acknowledgements and summary

6. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (6)
Coarse-Pointing Evaluation
Inertial measurement
unit 3-axis gyro
mounted on Yoke
Agile Eye II commercial beam director
IMU
Movie
Provides coarse-pointing performance evaluation
Motion Emulator Testbed Target

7. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (7)
Platform & Channel Emulation
CCT: command, control, and telemetry
Optical signal connecting two tracking terminals

8. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (8)
Platform & Channel Emulation: Jitter
Platform Jitter Mirrors
Host-specific,
programmable
platform jitter applied
Aircraft Spacecraft

9. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (9)
Platform & Channel Emulation: Far-Field
Propagation
Pinhole spatial filter generates point source

10. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (10)
Platform & Channel Emulation: Far-Field
Propagation

11. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (11)
Platform & Channel Emulation: Far-Field
Propagation
Over-filled fiber launch
assembly (Telescope)
Implements far-field propagation by sampling small fraction of beam

12. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (12)
Platform & Channel Emulation: Delay
Optical-to-Electrical
conversion
Programmable delay emulates latency
Delay and re-transmit Electrical-to-Optical
conversion

13. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (13)
Photonics
Terminal
Control
Implementation
• Analytical and numerical modeling produces fading environment
Modeling of Atmospheric Scintillation
Far-field images at GEO, Dt = 35 mm
• Fading impacts both comm and PAT through time-varying power fluctuations
Simulation
Technique
Elevation: 45o 10o20o
"Light" "Moderate"
"Heavy"
("Light") ("Moderate") ("Heavy")
Rx aperture size: ~3 x 10-4 km
(Cn
2 profile: 4xclr1; Altitude: 29 kft)

14. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (14)
Platform & Channel Emulation: Scintillation
Implements scintillation time series
Scintillation
time series Real-time control of
variable optical attenuators

15. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (15)
• Window interface geometry strongly affects strength of boundary layer distortions
Emulation of Boundary Layer Distortions
Beam
Director
Optical module
Window Interface
Beam
Director
Optical module
Turret Interface
90o Azim.
45o Elev.
No BL
Elevation
Azimuth
• Significant distortions cause beam break-up
• Computational Fluid Dynamics (CFD) simulations produce time-dependent flow fields

16. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (16)
Platform & Channel Emulation: Boundary Layer
Effects
Boundary Layer Implemented With Deformable Mirror
Commanded mirror shape
12x12 deformable mirror
with 2-KHz frame rate
Phase distortion
time series

17. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (17)
Complete Emulation Of Air-to-Space Links
Platform & Channel Emulation

18. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (18)
Outline
• Background
• Lasercom Tracking Testbed
– Emulation capability
– Tracking terminals and software control
– Tracking model
• Experimental testing
• Compact aircraft terminal
• Acknowledgements and summary

19. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (19)
• Full-duplex tracking system
• Beacon and comm beam functionality
• FPA (acquisition and coarse track) and QC (fine track) sensors; fiber receiver
• Terminals well characterized and calibrated
Tracking Testbed Terminal Architecture
Abbreviations
FFS: far-field simulator
EON: electro-optic nutator
FSM: fast-steering mirror
PAM: point-ahead mirror
FPA: focal plane array
QC: quad cell
Channel
Emulation

20. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (20)
Control and Telemetry Software Capabilities
Telemetry Archive &
Displays
Automated Data
Product Generation
Aircraft terminal
realtime control
Spacecraft terminal
realtime control
CFD data on
Boundary Layer
Emulator
Round trip delay (O – E – O),
fading, & platform emulation
Command & Control
SPACECRAFT AIRCRAFT

21. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (21)
Outline
• Background
• Lasercom Tracking Testbed
– Emulation capability
– Tracking terminals and software control
– Tracking model
• Experimental testing
• Compact aircraft terminal
• Acknowledgements and summary

22. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (22)
Tracking Model Objectives
System
Model
Disturbances
Tracking
Metrics
• Model provides temporally-, frequency-, spatially-resolved simulation of
Tracking Testbed components
Objectives:
1. Benchmark experimental data produced by Testbed
2. Rapid evaluation of proposed Testbed system upgrades
3. Investigate scenarios not realizable with current Testbed configuration

23. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (23)
• Model includes tracking sensors, actuators, and impairments
• Model includes air-to-space impairments
Tracking Model Capability
• Model validation using FSM, Quad Cell, and FPA experimental data
• Plan to include state machine behavior to model acquisition sequences
Model provides high-fidelity simulation of Testbed hardware

24. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (24)
Outline
• Background
• Lasercom Tracking Testbed
– Emulation capability
– Tracking terminals and software control
– Tracking model
• Experimental testing
• Compact aircraft terminal
• Acknowledgements and summary

25. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (25)
Air-to-Space Link Acquisition Sequence
• Both terminals are now tracking narrow
comm beam from other user. Aircraft
Spacecraft
Acquisition beam
• Aircraft scans beacon toward
estimated satellite position; Orderwire
indicates where and when satellite should
look for aircraft beacon.
PAT Stages
• Satellite detects aircraft beacon and returns
narrow beam to that location (after adding
calculated point-ahead offset).
Downlink narrow beam
• Aircraft detects satellite downlink,
stops scan and points stable beacon to
satellite. On stable return from satellite, A/C
returns comm beam.
Uplink narrow beam
PAT: pointing, acquisition, and tracking
Sequence emulated by Tracking Testbed

26. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (26)
Tests Performed with Platform Jitter and Fading
#
Platform
Jitter
Light
Fading
Moderate
Fading
Heavy
Fading
Boundary
Layer
Success
Rate
1 X 11/11
2 X X 11/11
3 X X 11/11
4 X X 11/11
5 X X X
6 X X X
7 X X X
• High success rate for acquisition trials with
fading and jitter
• Modification of state machine parameter to
accommodate heavy fading [Altitude = 29 kft]
Azimuth = 90o
Elevation = 20o
Azimuth = 90o
Elevation = 10o
4xclr1
"Moderate"
"Heavy"
"Light"

27. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (27)
No Fading Acquisition Timeline (1/4)
Aircraft begins spiral scan

28. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (28)
Acquisition Timeline (2/4)
Spacecraft enabled and detects beacon

29. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (29)
Acquisition Timeline (3/4)
• Aircraft starts tracking out jitter
• Increase in power at Spacecraft

30. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (30)
No Fading Acquisition Timeline (4/4)
Both terminals acquire each other and attain
fine track modes

31. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (31)
Power Fluctuations for Light to Heavy Fading
• Fading effects observable on QC and FPA

32. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (32)
Power-on-Aperture Tracking Performance across
Fading Environments
Increased fluctuation in received power-on-aperture as expected
Fading
Condition
Power on Aperture†
Std. Dev. [dBW/m2]
Spacecraft Aircraft
No Fading -77.7 -60.5
Light Fading -66.2 -48.3
Moderate Fading -64.3 -47.8
Heavy Fading -61.7 -44.2
†Measured by
quadrant-cell
detector
• Baseline mean for Spacecraft: -61 dBW/m2
• Baseline mean for Aircraft: -43 dBW/m2

33. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (33)
Residual Jitter Tracking Performance for Fading Tests
Fading
Condition
Residual Jitter
[% Beam-width, θF]
Spacecraft
(θF = 6.5 μrad)
Aircraft
(θF = 52 μrad)
No Fading 3.8% 8.9%
Light Fading 4.7% 9.5%
Moderate Fading 6.6% 13.7%
Heavy Fading 7.8% 16.5%
Increased residual jitter loss with stronger
fading conditions

34. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (34)
Acquisition Times Summary
Multiple trials showed weaker-than-expected correlation between
fade strength and acquisition time

35. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (35)
Boundary Layer Distortion Tests: Success
Rates
#
Platform
Jitter
Light
Fading
Moderate
Fading
Heavy
Fading
Boundary Layer Success
Rate
1 X
2 X X
3 X X
4 X X Forw. Side Back Total
5 X X 1/1 3/3 2/2 6/6
6 X X 1/1 2/4 2/2 5/7
7 X X 1/1 0/2 0/2 1/5
Boundary Layer
Emulator enabled
• Studied single window interface geometry
• Forward view angles (±45o azim.) not problematic for all fading conditions
• Moderate fading for side or back views: coarse track only
• Heavy fading links for side and back views: links could not acquire

36. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (36)
• Boundary-layer mitigation using algorithms
without use of adaptive optics
• Experiments performed over hemispherical field
for different tracking algorithms
Tracking Algorithm Performance in Presence of
Boundary-Layer Turbulence†
†Ross Conrad, "Impact of the Boundary Layer on Pointing and Tracking...", Master's Dissertation, M.I.T., June 2008
• Observe advantages of centroid
over peak tracking
• Plan to implement additional
algorithms and different window
geometries

37. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (37)
Outline
• Background
• Lasercom Tracking Testbed
– Emulation capability
– Tracking terminals and software control
– Tracking model
• Experimental testing
• Compact aircraft terminal
• Acknowledgements and summary

38. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (38)
Compact Aircraft Terminal Optical Module†
12 in
18 in
Rx
Tx
QC
• 2x reduction in size; 4x reduction in area
• Continuously-adjustable transmitter beam divergence
– approximately 10-msec seamless transition for 5x change in beam divergence
• Platform-jitter rejection using quad-cell tracking
• Evaluated new compact fast-steering mirror
• Developing electro-optical nutator fiber-tracking capability
FSM
†Steven Rose and Bradley Scoville, "Design, Build, and Testing of an Aircraft Lasercom Terminal Compact Optical Module...",
Major Qualifying Project, W.P.I., October 2008

39. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (39)
Outline
• Background
• Lasercom Tracking Testbed
– Emulation capability
– Tracking terminals and software control
– Tracking model
• Experimental testing
• Compact aircraft terminal
• Acknowledgements and summary

40. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (40)
• Lasercom Tracking Testbed has been designed and built to assess unique
PAT challenges in the air-to-space environment
• Disturbance models feed into experimental hardware Testbed emulator
– Platform jitter
– Atmospheric channel
– Boundary layer distortions
– Propagation delay
• Tracking model capabilitity extends hardware capabilities
• Demonstrates end-to-end acquisition sequences and performance over
wide trade space
Summary of Capabilities and Results

41. MIT Lincoln Laboratory
LEOS 2008
Roth, ThAA2 (41)
Acknowledgements
• Bruce Beighley
• Ross Conrad
• Dave Crucioli
• Steven Michael
• Tom Miller
• Frank Mola
• Bob Murphy
• George Nowak
• Steven Rose
• Bradley Scoville
• Marilynn Semprucci
• Scott Stadler
• Todd Ulmer
• Fred Walther
• Bill Wilcox
• Tim Williams
• Alicia Volpicelli