SDA Optical Communications Standard (DRAFT).docx

Optical Communications Standard

Developed by the

Space Development Agency

Office of the Under Secretary of Defense Research and Engineering (USD(R&E)) 3030 Defense Pentagon Washington, D.C. 20301

Email: OSD.SDA.Outreach@mail.mil

This document is a DRAFT document and is subject to change.

DISTRIBUTION A: Approved for Public Release: Distribution Unlimited

Revision History

Table of Contents

1. Introduction The purpose of this document is to provide the framework and protocol definitions for optical communications systems employed by the Space Development Agency (SDA) and its partners. The scope of optical communication links supported includes space-to-space (S2S, a.k.a. Optical Intersatellite Links or OISLs), space-to-air (S2A), space-to-maritime (S2M), and space-to-ground (S2G) optical communications.

The SDA OISL Standard [1] was first published in early 2020 [2] in preparation for the first iteration of the SDA spiral development process: Tranche 0. SDA’s Tranche 0 (T0) program added sufficient detail to the initial OISL Standard as to enable demonstration of multi-vendor interoperability. Since early 2020, significant advancements have been made across the Optical Communications Terminal (OCT) market.

These advancements have prompted the addition of several protocols to the OISL Standard. The change in this document’s scope prompted the document’s renaming as S2G, S2M, and S2A do not fit into the moniker “Optical Intersatellite Link.” This document now serves as a Space Optical Communications Standard (SOCS) and is intended to provide a framework for the engineering of systems that will provide optical communications where at least one endpoint is a space-based terminal.

This document is intended to provide standard framework to be used in the lifecycle interface specification for OCTs. This is intended to provide the framework and “definitions” that are referenced by the requirements levied on a specific system.

Backward Compatibility Backward compatibility is achieved through levying requirements on the OCT. This Standard does not require backward compatibility; rather, this document captures the definitions and protocols required to specify backward compatibility.

Normative Text The following conventions apply for the normative specifications in this Specification:

a. the words ‘shall’ and ‘must’ indicate a binding and verifiable specification

b. the word ‘should’ indicates an optional, but desirable, specification

c. the word ‘may’ indicates an optional specification

d. the words ‘is’, ‘are’, and ‘will’ indicate statements of fact Standard Definition Pointing, Acquisition, and Tracking (PAT) Lead and Follow PAT The PAT spatial acquisition shall employ the framework described in Section 2.3 of reference [3] and shall be referred to as a lead and follow strategy (referred to as master-slave in [3]). Either terminal in this method is capable of acting as either leader or follower.

Beacon Signal The beacon signal is defined as an amplitude modulation (AM) on the transmitted signal at a lower frequency than the data signal. The nominal AM Tracking Tone Modulation Index (MI) is defined as:

Sinusoidal Tone The Sinusoidal Tone is a sine wave modulation at a specified frequency.

Square Wave Tone The Square Wave Tone is a square wave modulation at a specified frequency.

Arbitrary Beacon Signal The Arbitrary Beacon Signal is defined as an array of values of length, n, at a specified frequency. These values may be either binary numbers, for which an MI must be specified, or they may be the range of values between 0 and 1.

PAT Acquisition Time The PAT Acquisition Time is the duration of time between entry into the PAT sequence and the successful communications channel standup.

Latency The Receive Latency is defined as the duration of time from the first photon of a message arriving at the OCT to the last bit of that message exiting the OCT.

The Transmit Latency is defined as the duration of time between transmission of the first photon of a message to transmission of the last photon of a message.

Re-Programming Physical Framing, Coding and Scrambling and Encapsulation shall be re-programmable on orbit. During reprogramming, the transmission of data may be interrupted and .

Protocols In addition to the T0 Protocol, SDA intends to leverage the availability of commercial off-the-shelf (COTS) components and intellectual property (IP) through inclusion of additional protocols. The included protocols are defined in the following documents:

1. SDA-T0 with Reed-Solomon (RS) FEC [4]

2. ITU-T Recommendation G.709 [5] with RS FEC for Optical Transport Network (OTN) data transmission

3. ITU-T Recommendation G.975 [6] and G.975.1 [7] with Reed Solomon (RS) Forward Error Correction (FEC) or Low-density Parity Check (LDPC) FEC

4. OpenZR+ [8] Multi-Source Agreement (MSA) Effective Data Rate and Protocols Protocols and the applicable data rates are shown in Table 21. The base protocols are defined in Table 23. The data rates are approximate and are defined in the associated reference documents.

Table 21

Physical Layer Spectral Grid Definition To permit operations of multiple communications channels through wavelength division multiplexing (WDM), the ITU-T G.694 [9] 100 Ghz channel grid as shown in Table 22, below, will serve as the basis for SDA Optical Communications channels. This grid follows the ITU-T G.694.1 recommendation and is limited to the 44 channels in the optical C-Band (1530 nm to 1565 nm).

Table 22

Spectral Grid Tolerances Spectral Grid Tolerances for ITU-T G.694.1 100 GHz This section specifies tolerances for signals operating under this standard.

Center Frequency Tolerance For the specified 100 GHz channel spacing, denoted n, transmitter center frequency shall be accurate to within a tolerance of ± 10 GHz as follows:

Laser Line Width The modulated laser linewidth shall be less than 10 GHz, measured at full width at half-maximum (FWHM) over a time scale of 100 milliseconds.

In-Band Spillover Emissions The laser shall transmit 95 percent of its energy within ± 20 GHz of its center frequency.

Timing Jitter Timing Jitter refers to the Root Mean Square (RMS) pulse-to-pulse timing jitter. Timing jitter requirements shall be specified for OCTs operating under this Standard.

Transmit and Receive Wavelength Selection The transmit and receive wavelengths shall be selectable through software.

Polarization Polarization requirements shall be defined for each protocol.

Note: The direct-detect and QPSK protocols are expected to have conflicting polarization requirements.

Protocols Table 23 defines the protocols, including the modulation, frame type, and forward error correction (FEC), along with a shorthand identifier (ID). These protocols are described in the associated reference documents.

Table 23

References

Appendix A: Current Channel Usage by OCTs on the ITU-T G.694.1 Spectral Grid This appendix was last updated on 4/12/2021.

The channels that are currently in use by one or more current-generation OCTs are shown in Table 41. OCTs are not required to implement all channels shown below.

Channels in Table 41 are for reference only and their inclusion does not indicate according to the reserving entity’s channel plan (e.g. SDA or another constellation owner). Each channel has a corresponding channel that is designated as its “pair.”

Table 41

* These channels are defined on the ITU-T G.694.1 50 GHz spectral grid rather than the 100 GHz spacing specified in this document. The allocated 100 GHz channels are listed in parenthesis.

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