esa/dlr blue book proposal high-flux scenario esa presentation to ccsds optical communications...

ESA/DLR Blue Book ProposalHigh-Flux Scenario

ESA Presentation to CCSDS Optical Communications Working Group

11 November 2014

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• Most material was provided by European industry, specifically TESAT Spacecom at the Spring 2014 CCSDS meeting.

• Existing terminal implementation LCT is a commercially available product, with heritage extending back to 1997 (LEO-LEO) and a 2nd generation for LEO-GEO links developed since 2006.

• LCT was flown on NFIRE and the German TerraSAR-X satellites, and has been selected for EDRS, with first terminals already launched on Sentinel 1a and AlphaSat. Operational validation is successfully underway.

Evolution of Existing Implementation

• Current user bandwidth is 1.8 Gbps (2.8125 Gbps raw)

• Doubling the clock rate (already in space since 2007 on NFIRE and TerraSAR-X) results in 3.6 Gbps user data rate.

• Switching from BPSK to QPSK provides a user data rate of 7.2 Gbps. Benefit: Detector bandwidth remains within validated performance regime.

• Further scaling by WDM around 1064 nm. Yb amplifier bandwidth is ≥60 nm.

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1. Proposal Draft Blue Book Optical Communications Physical Layer

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Blue Book Optical Communications Physical Layer – Proposed Contents

1.1 Signal Acquisition1. Terminal A• The center frequency shall be 281594.0 GHz ± 0.14%• The transmit beam shall have a Left-hand Circular Polarization (LCP:

where the E-vector rotates counter-clockwise as viewed from the transmitter in the direction of propagation)

• During spatial acquisition, the transmit signal shall be amplitude modulated with a frequency of 1.0045 MHz +/- 200 ppm and a modulation index higher than 95%.

2. Terminal B• The center frequency shall be 281566.0 ± 0.14%• The transmit beam shall have a Right-hand Circular Polarization

(RCP: where the E-vector rotates clockwise as viewed from the transmitter in the direction of propagation)

• During spatial acquisition, the transmit signal shall be amplitude modulated with a frequency of 1.7578 MHz +/- 200 ppm and a modulation index higher than 95%.

1.2 Communication Signal (1/6)1. Terminal A

• The center frequency shall be 281594.0 GHz• The transmit beam shall have a Left-hand Circular Polarization

(LCP: where the E-vector rotates counter-clockwise as viewed from the transmitter in the direction of propagation)

2. Terminal B

• The center frequency shall be 281566.0 GHz• The transmit beam shall have a Right-hand Circular

Polarization (RCP: where the E-vector rotates clockwise as viewed from the transmitter in the direction of propagation)

1.2 Communication Signal (2/6)1. Both Terminals

• The TX laser shall be tunable within a range of +-10 GHz• The maximum laser tuning rate shall be 120 MHz/s in

communication mode• The un-modulated TX laser shall have a line width of less than

100 kHz over a time scale of 100 msec• The TX laser Relative Intensity Noise (RIN) shall be less than -

30 dB in the frequency range between 0.01 Hz and 1 MHz.

1.2 Communication Signal (3/6)1. Both Terminals• The spectral density of the RIN shall be better than given in

the Figure below

10 Hz 10 kHz 10 MHz 1.4 GHz

5 dB

-134 dB/Hz

-20 dB/decade

-20 dB/decade

Frequency

RIN/Δf

quantum noise

-74 dB/Hz

1.2 Communication Signal (4/6)1. Both Terminals• The frequency noise of the free running TX laser shall be less

than the limit in Figure below:

• The change rate of the laser phase noise shall be below 6400 rad/s (Δφ/ Δt).

1.2 Communication Signal (5/6)1. Both Terminals• Modulation scheme: Binary Phase-Shift Keying (BPSK) with Non-Return to

Zero (NRZ).• Nominal symbol rate: 2812.5 Megasymbols/sec (Msps).• The modulating base-band spectrum of the transmit signal shall fulfill the

spectral mask defined in the Figure below. This spectrum shall be generated by the Line Product Code (LPC) as defined in Ref. CCSDS 000.0-W-0 (Optical Communication Modulation and Coding Proposed Draft Recommended Standard White Book)

1.2 Communication Signal (6/6)1. Both Terminals• The base-band eye-pattern shall fulfill the requirements listed in the Table

and as defined by the Figure below.

Symbol Definition

UI (Unit Interval) 1 / 2812.5 Mbps (≈355.55 ps)

x1 0.1*UI

x2 0.15*UI

x3 0.85*UI

X4 0.9*UI

y1 0.10*mean level of logical 1

y2 0.90*mean level of logical 1

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Contents - Green Book Material Optical Communications Physical Layer

2.1 Spatial Acquisition

PAT• Mutual spatial tracking and, in the case of homodyne communications, locking and tracking

of the opposing terminals carrier frequency must be achieved.• Depending on the relative size of the open-loop pointing Uncertainty Cone (UC) and the

aperture dependent Tx beam divergence, acquisition sequences either assisted by a higher-divergence Beacon or “Beaconless” acquisition schemes may be appropriate.

• Detection and centering of a beacon requires utilization of a 2-D sensor (e.g. CCD or Quadrant Photodiode).

• High divergence beacon requires high optical transmit power.• Size of UC determines necessary acquisition strategy and speed. Contributing factors are:

1. Timing accuracy2. Position knowledge3. Attitude knowledge4. (Micro)-vibration5. Planning cycle and Telecommand opportunities

2.1 Spatial AcquisitionBeaconless PAT• A beaconless PAT scheme is implemented in the European Data Relay

System (EDRS) and has been verified in-orbit with the TerraSar-X NFire optical LEO intersatellite links.

• It uses the comms beam (i.e. same wavelength and beam divergence as the comms signal) and scans the UC to achieve spatial acquisition.

• The scanning methodology mandates a time tagged sequence of steps.• The transmitted beam is modulated to facilitate terminal identification and

to improve the PAT link budget.• Establishment of a link with the EDRS optical terminals requires

knowledge of only the normative characteristics of the optical signal (wavelength, modulation, etc.).

• The knowledge of the time tagged acquisition sequence is beneficial and recommended to be agreed upon in a mission-specific ICD.

2.2 Acquisition SequenceBeaconless PAT

Beacon-less acquisition sequence in a master-slave approach. M: master terminal, S: slave terminal

2.2 Acquisition SequenceBeaconless Spatial Acquisition

Simplified flow chart of the beaconless spatial acquisition algorithm

2.2 Acquisition Sequence

Frequency Acquisition• After successful spatial acquisition, the next step in the

acquisition sequence is to achieve homodyne tracking by performing frequency acquisition. The laser frequencies of the respective local oscillators must be adjusted such that they are phase-locked to the received signal.

• On completion, each terminal is coherently phase locked to the counter terminal. Homodyne tracking is thus achieved and the terminals are ready to initiate communication.

2.3 Acquisition Time

• Length of the acquisition time is determined mainly the size of the total UC of the link.

• Based on the size of the UC, a trade-off between reliability and speed of link acquisition is required.

• In a highly dynamic LEO-LEO intersatellite link configuration acquisition times of 8 s (and in specific cases 2 s) have been demonstrated in-orbit.

• For uncertainty cones of 1 mrad, acquisition times of 55 s are guaranteed for the EDRS intersatellite and ground link (including a safety margin).

• Shorter acquisition times are expected after in-orbit verification.

2.4 Optical Signal Characteristics

• Optical signal characteristics are valid for coarse and fine spatial acquisition. For tracking, communications signal characteristics apply (direct detection on acquisition sensors, but coherent detection on tracking detector).

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