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Session E8: PNT Open Systems Architecture 1

Novel Radial Clock Card Architecture for SOSA Systems
Luke Littleton-Strand, Tim Kelly, Microchip; Eddie Byrne, Jeremy Warriner, Agalti; Andy Baster, AFRL/RYWN; John Raquet, IS4S
Location: Room 1-3
Date/Time: Wednesday, Jun. 5, 8:35 a.m.

Under the Joint Multi-Int Precision Reference (JMPR) program, the AFRL sensors directorate is developing an OpenPNT Testbed to investigate the use of mixed synchronization models in airborne systems. Current systems almost exclusively employ centralized architectures for Positioning, Navigation, and Timing (PNT) dissemination within and between systems. This centralized architecture imposes PNT requirements on all the systems in its hierarchy which can be costly to change after the design phase. Incorporating decentralized PNT architectures into a mixed synchronization model eliminates the burden of defining system hierarchy at design time in favor of a run-time definition or asynchronous operation of peer devices. Thereby increasing flexibility and decreasing timelines to meet new sensing objectives.
The key to employing a mixed synchronization model within the OpenPNT Testbed is a new and novel radial clock card design. The OpenPNT Testbed is comprised of multiple VPX-based chassis aligned to the Sensor Open Systems Architecture (SOSA) version 1.0 specification. As such, the radial clock card is the centralized source of timing within the chassis. The radial clock card also provides synchronization between the chassis, but the novelty of this design is that it does so in a decentralized manner where each chassis in the system operates as a peer to the others. This enables a system to establish hierarchical relationships for synchronization at run-time or allow the chassis to operate asynchronously with one another. All while still allowing a system to use and benefit from PNT devices in other chassis, even though it may be operating asynchronously to the local chassis.
The decentralized architecture between chassis is enabled by using Asynchronous Two-Way Time Transfer (ATWTT) between the radial clock cards. The ATWTT methodology enables asynchronously operating devices to perform TWTT without first coordinating their exchanges. This allows each device to operate independently in terms of the rate and timing of its TWTT transmit events while simply measuring and reporting the times at which it received timing events from the remote device. Each device then independently pairs its transmit and receive events according to which ones occurred “most” simultaneously according to the simultaneity equation. After determining the optimal pairing of events, time and range offsets between the devices can be calculated using the traditional TWTT process.
Decentralizing the OpenPNT Testbed enables federating the PNT devices and resources across the entire network of chassis. Typical SOSA systems rely on a PNT system being constituted from PNT devices (GNSS receiver, IMU, atomic clock, OCXO, etc.) in a single chassis and then distributed from there, but our decentralized architecture allows the PNT devices to be in any chassis and still contribute to any of the sensing system’s PNT solutions. With this ad-hoc distribution of PNT devices comes significant improvement in flexibility and resiliency, but also the challenge of organizing the PNT devices at run-time. To solve this challenge, the HORDE protocol was created to enable the discovery of PNT devices, subscription to PNT measurements, and routing of ASPN messages through the system.
This presentation provides a more detailed description of our approach to mixed synchronization architectures in airborne systems and how we envision decentralization extending beyond individual platforms. Current test results for the synchronization of multiple
SOSA chassis connected via fiber optic ATWTT links will be shown, along with capability demonstrations that highlight the utility of mixed synchronization architectures. These capability demonstrations include a fully federated timing system, a timescale filter using distributed oscillators, and the dynamic allocation of PNT resources during system operations.
Public Release Note:
APRS-RYW-2024-01-00013
Case Reviewer: Mary Allen
The material was assigned a clearance of CLEARED on 29 Jan 2024.



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