FREE SPACE LASER
COMMUNICATION CUBESAT
PAYLOAD DEVELOPMENT
MIDN 1/C Colston Polly
Primary Advisor: Assoc. Prof. Tae Lim
Off-site Advisors: Drs. Rita Mahon and Chris Moore at
the U.S. Naval Research Laboratory
BACKGROUND
•
Significant benefits in optical communication
• Security
• Data rates
• Error minimization
•
Previous success
• LADEE
• OPALS
CHALLENGES WITH OPTICAL
COMMUNICATION
•
Beam divergence
•
Availability of ground stations
•
Earth’s atmosphere
•
Weather
WHAT WAS ACCOMPLISHED
IN SPRING/SUMMER 2015
•
MIDN 1/C Dods, Blanchard, and A. Kelly
•
Proof-of-concept demo for optically communicating 6U cubesat
•
•
•
Laboratory tabletop test (~ 6 ft)
Desired data rate
•
Data rates prediction
•
Experimental determination and comparison
Link Margin
•
Established working link margin
•
Solidworks Model
•
Lay foundation for future years’ work
•
Collaboration with NRL advisors at the Chesapeake Bay Detachment
(CBD)
•
Summer internship of MIDN 1/C Polly at NRL-CBD
PHYSICAL SETUP
TRANSMIT
•
Computer
• Connected through Ethernet to MiniMc
•
MiniMc (Media Converter)
• Signal modulator
• Single mode fibers
• Laser diode
•
Collimator
• Transmits signal across free space
RECEIVE
•
Copies transmit, but in reverse.
•
Collimator acts as light bucket
•
MiniMc demodulates signal
•
Multimode fibers
INEFFICIENCY LOSSES
Produced
Loss_Tx
Tx
Loss_Space
mW
0.55
0.01
0.54
0.03
dBmW
-2.60
0.08
-2.68
0.25
Rx
Loss_Rx
Computed
0.51
0.33
0.18
-2.92
4.52
-7.45
LINK MARGIN
Inputs
Calculated Values
Tx Power [mW]
0.550
Tx Power [dBmW]
‐2.60
Jitter [rad, rms]
Ideal Beam Divergence [rad]
Actual Beam Divergence [rad]
Tx Optical Loss [dBmW]
Wavelength [m]
Distance [m]
Rx Aperture Diameter [m]
0.000
Tx Gain [dBmW]
74.09
0.0003
Space Loss [dBmW]
0.0006
0.080
1.55E‐06
2.00E+00
Rx Area [m^2]
Rx Gain [dBmW]
Pointing Loss [dBmW]
Ideal Divergence [dBr]
3.50E‐03
Actual Divergence [dBr]
Atmospheric Attenuation [dBmW]
^Prediction extrapolated from MODTRAN
Rx Optical Loss [dBmW]
4.50
Power Penalty [dBmW]
Sensitivity [dBmW]
0.00
‐42.00
144.19
9.62E‐06
77.02
0.00
‐35.54
Difference potentially due to short distance (quantum saturation)
‐32.53
‐0.46
Computed Power
Com_Power [dBmW]
Measured Power [dBmW]
Difference [dBmW]
‐6.74
‐7.45
0.71
Difference [%]
0.151
Set
Future Spec
Needs test
Unsure/assumption
FUTURE WORK
•
This academic year and next (AY2015-2016)
•
Longer range tests
• 50 meter - Kilometer scale (Rickover Hallway to Across the Severn)
•
Signal acquisition
• Auto alignment system: position sensing detector and fast steering
mirror
•
Tracking system
• Keeping the free-space comm link between moving objects: two axis
gimbals
•
Future academic years (AY2016-2017)
•
Optical comm payload (3U) design for 6U cubesat
• Component design, integration, testing
• Pointing, acquisition, and tracking system design
• Developing concept of mission operations
WHO ARE WE LOOKING FOR?
•
Two (2) midshipmen who are interested in:
• Optical design for laser communication
• Link budget analysis
• Alignment system design and testing
• Tracking system design and testing
• Mission concept of operations and systems engineering for
cubesat payload development
• Back-Up Slides
LINK BUDGET TEST GOALS
1. Component Inefficiencies
• Determined
2. Actual beamwidth
• Needs further test distance to be determined.
3. Detector Sensitivity
• Needs further test distance/greater attenuation (likely around
10 dBmW more).
4. Adequate predictive capability
• Predicted -6.74 dBmW at media converter
• Got -7.45 dBmW at media converter
SNR-TO-DATA RATE RELATIONSHIP
•
Our system uses On-Off Keying (OOK)
•
To find data rate
1. Large ping between two computers
2. Use WireShark, a network-analyzing program, to
determine time delay
3. Attenuate signal over several trials
SNR-TO-DATA RATE RELATIONSHIP
Computed
Computed
Power [mW] Power [dBmW]
Data Rate
[MBps]
Ethernet
n/a
n/a
45.02
Trial 1
0.180
-7.45
1.45
Trial 2
0.110
-9.59
1.44
Trial 3
0.053
-12.76
1.45
Trial 4
0.011
-19.59
1.45
Trial 5
0.001
-30.00
1.45
SNR-TO-DATA RATE RELATIONSHIP
Data Rate [MBps]
“Waterfall” ~ approx. -40 dBmW
Max Data Rate (1.45 MBps)
Sensitivity Limit (-42 dBmW)
SNR [dBmW]
SOLIDWORKS
SPONSORS/HELP
•
NRL optical communications team
•
Chris Moore and Rita Mahon
•
Working on optical communication aboard naval platforms