Spring Technical Meeting, Portsmouth, VA 05/03-07/2010
Spring Technical Meeting, Bordeaux, Fr
04/15-18/2013
First Flight:
Successful Use of a High Rate LDPC Code
With High Data Rate in a Restricted Band
H. Garon, V. Sank, W. Fong
NASA/GSFC
“Successful Use of a High Rate LDPC…”
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Statement of the Problem
LDCM Characteristic:
Sun-synchronous polar orbit (16 day
repeat cycle at nominally 705 km)
with a finite number of ground stations
Imaging mission with high data
rate of 384 Mbps in 375 MHz X band.
Requires low coding overhead.
CFDP with 1 G Byte file size but using
CFDP class where entire file is retransmitted if there is an error.
EESS X band
Abuts DSN X band
Restriction:
Imposed
Requirement:
Limited contact
time
Sensors generate
data at high data
rate
Limited time/capacity to
repeat transmissions and
large file size drives
BER < 10-12
Limited onboard
storage (4 Tbits)
Bandwidth limited
to 375 MHz
Power Spectral Flux
Density restriction at
Deep Space Network
antennas
Band pass filtering
aboard S/C to achieve
PSFD at DSN
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Selection of Codec
• Unlike DSN, expect excess signal margin operationally
• S/C science data rate, and limited storage capacity forces
high RF data rate.
• High Data Rate in restricted bandwidth channel, forces
high rate code (384 Mbps => 440 Msps)
• Large file and low retransmission, requires BER < 10-12
• High data rate restricts Complexity of encoder
– while decoder important, it is on the ground
• Absence of BER floor
LDPC 7/8 chosen even though there was no flight heritage.
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Will it work?
(Establishing link viability prior to flight)
• Questions …
– Flight experience for LDPC 7/8? Receiver/Decoder availability?
– Assuming LDPC 7/8 is used, what is the impact on truncating the
transmission spectrum to less than 375/440 in order to meet spectral
emission requirements?
– Similarly, impact of radiated spectrum asymmetrical about carrier?
– The spectrum truncation is accompanied by significant phase
distortion. Can we properly compensate for that distortion on the
ground?
– Can we achieve a BER < 10-12 ?
Absolute requirement to build both end-to-end
simulation and hardware emulation in order to
gain confidence in flight application.
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End-to-End Simulation Using Mathworks
Matlab/Simulink
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End-to-End Emulation Using Test Bench
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Key to hardware emulation
Custom bandpass filter designed using simulation to replicate distortion
introduced aboard spacecraft
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Emulation Spectrum Compared To Flight
Actual on orbit
LDCM spectrum
Original emulation
of LDCM spectrum
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Mission Events
• 2/11/13 Launch
• 2/14/13 X-band SSOH files have been successfully received at both Gilmore Creek
and Sioux Falls ground stations.
•
The Svalbard ground station corrected a configuration setting so they should be able to receive X-band files on future contacts. There was a
problem with getting S-Band SSOH back from Svalbard . The X-Band mission data from Svalbard uses a different link and is unaffected.
•
•
•
•
3/17/13
3/21/13
3/25/13
3/27/13
OLI & TIRS activation
Released its first X band images of Earth
X-band downlink with International Collaborators
LDCM hit a 400 scenes/day milestone
All with the rate 7/8 LDPC code.
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Conclusion:
• Demonstrated in flight
 A viable 440 Msps link with BER < 10-12 and implementation loss less
than 2 dB (including ground station loss) despite an asymmetrical
spectrum truncation to 375 MHz,
• Commercial off-the-shelf availability of
 Flight encoder chips qualified to Level 1 (GEO environment at 1 Gbps)
along with
 Commercial hardware using sliced data frame, now proven.
 Multiple vendors offering FPGA cores and high data rate receivers with
LDPC decoder (Zodiac, RT Logic, Avtec, …) (both sliced and not sliced)
 Successful cooperation within CCSDS to standardize the codec.
Now we can say that not only does it look good, but the rate 7/8 LDPC code
works well in practice.
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The Real Conclusion, an Error Free Image
http://www.nasa.gov/images/content/736042main_onion-skin-still-detail.jpg
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