Air-to-ground lasercom system demonstration design
overview and results summary
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Citation
Walther, Frederick G. et al. “Air-to-ground lasercom system
demonstration design overview and results summary.” FreeSpace Laser Communications X. Ed. Arun K. Majumdar &
Christopher C. Davis. San Diego, California, USA: SPIE, 2010.
78140Y-9. © 2010 SPIE.
As Published
http://dx.doi.org/10.1117/12.864262
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Society of Photo-optical Instrumentation Engineers
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Final published version
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Thu May 26 18:27:48 EDT 2016
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http://hdl.handle.net/1721.1/61648
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Air-to-Ground Lasercom System Demonstration
Design Overview and Results Summary
Frederick G. Walther, Steven Michael, Ronald R. Parenti, and John A. Taylor
Massachusetts Institute of Technology Lincoln Laboratory, Lexington, MA 02420-9108
ABSTRACT
We present an overview of an air-to-ground laser communications demonstration performed at MIT Lincoln
Laboratory. Error-free communication at 2.5 Gb/s was demonstrated along a 25-km slant path between a 1-in
transmit aperture on an aircraft at 12 kft altitude and ground terminal with 4 separate 1-cm receivers. Power
fluctuations from turbulence-induced scintillation are mitigated in the spatial domain by use of the multiple
ground receivers and in the time domain by the use of forward error correction and interleaving. The optical
terminals are monitored by multiple high-rate sensors which allow us to quantify total system performance.
Keywords: free-space optical communications, atmospheric turbulence, optical scintillation, pointing and tracking, forward error correction, interleaving, diversity, field demonstration
1. INTRODUCTION
An important future application of lasercom is support of tactical intelligence, surveillance, and reconnaissance
(ISR) missions, delivering near real-time sensor data to the operational theater. Representative objective requirements include the ability to transport data at rates up to tens of Gb/s over a range of roughly 50 km. The
air terminal should have low size, weight, and power (SWaP), and low integration impact on the aircraft. Use
of standard terrestrial network protocols can simplify the implementation, particularly at the client interface. A
key advantage of lasercom is its ability to provide high bandwidth without adding to RF congestion in theater.
An example of such an application, currently under development at Lincoln Laboratory, is the Multiple
Aperture Sparse imager Video System (MASIVS), an imaging system with 880 megapixels generating in excess
of 20 Gb/s of raw data that can be near-losslessly compressed to about 2 Gb/s. The work described in this paper
is directed towards providing an air-to-ground optical link that will allow the MASIVS system to relay data to
users on the ground in real time.
Optical links based on single-mode fibers in the 1550-nm band are commonly used in commercial telecom and
provide a readily available base of COTS components, including efficient pre-amplified receivers and components
that support a wide range of modulation formats. In free space communications the fiber connectivity between
transmit and receive nodes present in terrestrial telecom is replaced with a power delivery subsystem the sole
purpose of which is to deliver power from the transmit fiber to the receive fiber as efficiently as possible.
Communications performance depends on both the mean power in fiber and the fluctuations of power in fiber.
Design approaches must maximize average power and minimize its variance in the presence of atmospheric
turbulence and platform jitter.
Figure 1 diagrams the effects of the channel on communications performance. Power in fiber is a product of
both power delivered to the receiver aperture and the coupling efficiency from aperture to fiber. Transmitter
platform jitter and turbulence in the far field of the receiver both lead to fluctuations in power delivered to
the aperture, the well-known scintillation problem. In addition, turbulence near to the receiver causes phase
distortion in the aperture that reduces the efficiency of power coupling into fiber. Traditionally, phase distortion
can be corrected with adaptive optics; the lowest order distortion, tilt, can be corrected with fast tracking
systems. Moreover, as long as the receiver aperture is small compared to the phase distortion scale, r0 , tilt
tracking is sufficient to ensure high coupling efficiency, avoiding need for higher-order phase corrections.
The design used in this work exploits 1-cm ground receiver apertures which use tilt-only tracking and which
have 8-m diffraction limited imaging resolution at a 50-km slant range to the aircraft. This provides the flexibility
to use multiple apertures at the aircraft, if desired, to avoid structural blockage or to allow seamless handover
between multiple aircraft apertures with limited field of regard. The small apertures ensures that high-order phase
aberrations are negligible; scintillation is the dominant atmospheric turbulence effect, and must be mitigated.
Free-Space Laser Communications X, edited by Arun K. Majumdar, Christopher C. Davis,
Proc. of SPIE Vol. 7814, 78140Y · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.864262
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A similar optical module is used in the ground terminal, as depicted in Figure 8. The beam after the FSM is
directly in object space; no further transmit optics are used on the ground. One stabilization technique, reported
earlier,3 is to dither the FSM in a conical scan, allowing the incoming fiber power to be detected synchronously
providing a nutation tracking discriminate for boresight alignment of fiber to camera by adjusting the beam
splitter during operation. As on the aircraft optical module, a flip-in retro reflector provides the means of
initially boresighting the fiber to the tracking camera.
Two such modules are assembled on each side of a plate; two such plates are combined to form the 4 aperture
ground module. The unit is integrated with a commercial pan-tilt unit to form a wide steerable opto-mechanical
system. A broad uplink beacon and a wide field of view acquisition camera are integrated on the pan-tilt unit.
The completed assembly is mounted in a rotatable optical dome mounted on the roof of a lab building directly
above the ground terminal test facility.
4. PERFORMANCE OF THE POWER DELIVERY SYSTEM
−4
−4
−2
−2
−2
−2
−2
0
2
4
−4
0
2
−2
0
λ/D
2
(a) Aircraft
4
4
−4
0
2
−2
0
λ/D
2
4
(b) Gnd Terminal 1
4
−4
λ/D
−4
λ/D
−4
λ/D
−4
λ/D
λ/D
For initial signal acquisition, the pointing burden is placed on the aircraft since it has all the information required
to point to the ground terminal. The ground terminal requires knowledge of the aircraft position, which can
be obtained through a RF downlink if available. For this work, the aircraft was acquired optically by flying
through predetermined waypoints. After initial acquisition, any reacquisition can be achieved by propagating
the aircraft motion. In an actual ISR mission with the aircraft in a pre-known orbit, a wide acquisition camera
on the ground would suffice to acquire the downlink beacon.
0
2
−2
0
λ/D
2
(c) Gnd Terminal 2
4
4
−4
−2
0
λ/D
2
(d) Gnd Terminal 3
4
4000
3000
0
2000
2
1000
4
−4
−2
0
λ/D
2
4
0
(e) Gnd Terminal 4
Figure 9: Focal plane images of received bean at aircraft and ground. Center of white circle indicates track
point; circle diameter is 2 λ/D.
Sample frames from tracking camera on the aircraft and the four tracking camera on the ground are shown in
Figure 9. The tilt-only tracking system is sufficient to deliver well-contained distributions in the focal plane. The
beam spot is smaller for the ground terminal, with nearly diffraction limited performance. The aircraft power
distribution is reasonably well contained, but is spread a bit in elevation. A known handover issue between the
fast steering mirror and the gimbal contributes to this distortion on the aircraft The potential impact of airframe
boundary layer turbulence is still under investigation, although it appears to be modest. Not shown in Figure 9
is the track jitter for the air and ground terminals, which we measure to be less than 0.3 λ/D.
Figure 10(a) overlays probability densities of power-in-aperture and power-in-fiber. With good tracking and
low higher-order phase distortion, the densities should closely match. A stiction-induced tracking disturbance in
the wide area beam director caused occasional mirror jumps. Contours of the residual centroid log probability are
shown in Figure 10b. The low probability jumps along the direction of the two gimbal axes are easily identified.
The result is deep fades in the fiber power distribution that do not appear in the aperture distribution, resulting
in the rising tail seen in Figure 10a. We can remove these jumps by ignoring points where the plane orientation
(as measured by the IMU) is changing quickly. The result is well matched distributions of fiber and aperture
power, indicating good tracking performance and the absence of higher-order phase disturbances.
The distributions in Figure 11 illustrate the single and combined 4-aperture distributions for two cases: 50km range afternoon and 25-km range evening conditions. Overlaid on the charts are wave propagation model
simulations for the respective cases, scaled with a multiplier for best fit relative to the well known Hufnagel-Valley
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0
10
−4
Elevation = −4.9 deg
Azimuth = −26.2 deg
−2
−2
10
−1
λ/D
Probability Density
−3
−4
0
1
10
2
Intensity on Focal Plane
3
σ = 0.29 λ / D
az
σel = 0.25 λ / D
4
−4
−3
Power in Fiber
Power in Fiber (Gimbal Jumps Removed)
−6
10
−15
−10
−5
0
5
Normalized Power (dB)
10
(a) Power Distributions
−2
−1
0
λ/D
1
2
3
4
(b) Contours of Residual Centroid
Log Probability
Figure 10: Coupling Efficiency
0
10
−1
10
0
10
σ21 = 2.44
Single Channel
4−Channel Sum
Simulation
σ24
Single Channel
4−Channel Sum
Simulation
−1
= 0.65
10
Probability
Probability
ratio = 3.7
−2
10
−3
10
−4
−5
σ2 = 0.10
4
−3
10
ratio = 3.6
−4
10
10
−70
σ21 = 0.36
−2
10
10
H−V 5/7 Multiplier = 1.1
−60 −50 −40 −30 −20 −10
Received Power in Fiber (dBm)
(a) Afternoon, 50-km range
−5
0
10
−70
H−V 5/7 Multiplier = 0.69
−60 −50 −40 −30 −20 −10
Received Power in Fiber (dBm)
0
(b) Evening, 25-km range
Figure 11: Measured Scintillation with Simulation Overlay
5/7 (HV 5/7) turbulence model.4 Afternoon conditions fit well to 1.1 x HV 5/7, while evening conditions fall to
0.7 x HV 5/7. Over the range of conditions measured, from 20 to 60 km and for various times of day, the HV
5/7 channel model fits the data with multipliers from 0.4 to 2. In all cases, spatial diversity reduces the variance
in received power.
5. COMMUNICATIONS SYSTEM PERFORMANCE
The data flow used for communications performance evaluation is shown in Figure 12. Stored 6 minute data
files of MASIVS imagery were delivered in ethernet format and encapsulated in Synchronous Optical Networking
(SONET) frames to match the input format of our existing high speed electronics (HSE) hardware. The the
HSE system performed OTU1 data encapsulation, adding RS(255,239) FEC and interleaving over a 1.25-s span,
with inter-symbol spacing of 5 ms. On-off keying was chosen as the optical modulation format. Detection and
clock recovery were followed by deinterleaving, FEC decoding, delivery of SONET data and extraction of the
original ethernet data stream for display processing and storage.
One metric of communications performance is the ability to deliver error free data files of significant length.
Our basic unit was a 6 minute image file of 100 Gbytes. With 1 Watt of optical transmit power, the link delivered
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5
55
6
50
4
45
2
0
19:40
19:45
19:50 19:55
10/22/2009
20:00
40
20:05
Dropped Frames
8
60
Dropped Frames
Distance
Distance (km)
Dropped Frames
10
8
x 10
Dropped Frames
Distance
6
44
43
42
4
41
2
0
13:50
(a) Moderate Turbulence
Distance (km)
5
x 10
40
13:55
14:00
10/19/2009
39
14:05
(b) Strong Turbulence
Figure 13: Frame Errors
The short 6-month development time for this experiment left little room for system optimization. Several
improvements are planned as we move towards a deployable system. The HSE system will undergo a significant
SWaP reduction, even as the data rate is increased to 10 Gb/s. A more efficient optical waveform and larger
diameter ground apertures will allow the higher data rate without a corresponding increase in transmit power.
The control system for the ground station will be reduced in size and the entire terminal will be ruggedized and
packaged to be transportable. The optical bench on the aircraft will be reduced in size and the control computer
reduced to a single 4-in cube stack. The beam director will be moved outside the aircraft below the fuselage to
allow for hemispherical coverage. The use of passive isolation will be considered and compensation bandwidths
increased as the aircraft terminal moves from the soft ride of a damped optical table within the Twin Otter
to the harsh angular and linear vibration environment of a deployable aircraft. We are also considering the
use of alternative commercial beam directors that provide inertial stabilization and have a heritage of aircraft
integration.
REFERENCES
[1] Moores, J. D., Walther, F. G., Greco, J. A., Michael, S., William E. Wilcox, J., Volpicelli, A. M., Magliocco,
R. J., and Henion, S. R., “Architecture overview and data summary of a 5.4 km free-space laser communication
experiment,” Free-Space Laser Communications IX 7464(1), 746404, SPIE (2009).
[2] Greco, J. A., “Design of the high-speed framing, fec, and interleaving hardware used in a 5.4km free-space
optical communication experiment,” Free-Space Laser Communications IX 7464(1), 746409, SPIE (2009).
[3] Murphy, R. J., Volpicelli, A. M., William Wilcox, J., Crucioli, D. A., and Williams, T. H., “A conical scan free
space optical tracking system for fading channels,” Free-Space Laser Communications IX 7464(1), 74640P,
SPIE (2009).
[4] Smith, F. G., ed., [The Infrared & Electro-Optical Systems Handbook, Volume 2: Atmospheric Propagation
of Radiation ], SPIE Optical Engineering Press (1993).
This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA872105-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the authors and are not
necessarily endorsed by the United States Government
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