ClockSync F´ project

Summary:

The TimeSync component provides accurate time synchronization for satellite operations by managing the Linux system clock through F Prime's PosixTime component. This ensures all satellite subsystems (navigation, communications, mission management, and experiment execution) maintain synchronized, accurate time.

Project Requirements:

Background: Maintaining accurate time is critical for satellite operations. Clocks drift over time and require periodic recalibration through two methods:

1. Coarse Synchronization - Ground station radio messages provide absolute timestamps
2. GPS PPS Synchronization - GPS Pulse Per Second signals provide precise second-boundary alignment

Features:

1. Uses F Prime’s PosixTime component to keep track of the system time. Other components can successfully ping this component to get the system wide time.

Implementation:

• PosixTime Intregration: Deployed Svc.PosixTime component as the single source of truth for system time
• Topology Configuration: All components automatically query time via time connections instance posixTime pattern
• System Clock Management: TimeSync updates the Linux system clock via clock_settime(), which PosixTime reads and distributes
• All components are connected to PosixTime in topology.fpp

Workflow:

1. Ground/GPS sends timestamp/PPS signal to TimeSync
2. TimeSync updates Linux system clock via clock_settime()
3. PosixTime reads Linux system clock via clock_gettime()
4. All components query PosixTime for current time

                    Linux System Clock
                     (CLOCK_REALTIME)
                            ↑ ↓
                     write  │ │  read
                            │ │
        ┌──clock_settime()──┘ └──clock_gettime()──┐
        │                                         │
   TimeSync                                   PosixTime
        ↑                                         ↓
        │                                         │
   Ground/GPS                              All Components

2. Coarse synchronization: getting a mock radio message with a timestamp, and using this to calibrate the system time.

Implementation:

• Command SET_TIME(seconds, microseconds): accepts a timestamp from the ground station and sets Linux system clock
• Calibrates system time via clock_settime()
• Located in TimeSync.cpp

3. Implement PPS time sync: using the PPS value to more accurately reset the system time.

Implementation:

• Software PPS performPPSSync(): rounds time to the nearest second.
• Hardware PPS: Linux PPS subsystem integration with kernel level timestamps
• Triggers:
  • ppsSignal: async input port (recieves GPS PPS pulses)
  • TRIGGER_PPS_SYNC: command for manual testing
  • ENABLE_PPS: command to enable/disable PPS sync

Component Setup:

ClockSync/Components/TimeSync/
├── TimeSync.fpp          # Component interface definition
├── TimeSync.hpp          # Component header
├── TimeSync.cpp          # Implementation
└── CMakeLists.txt        # Build configuration
ClockSyncDeployment
├── Main.cpp          
└── Top/topology.fpp

Commands:

• SET_TIME(seconds, microseconds): coarse time synchronization
• ENABLE_PPS(enable): enable/disable PPS synchronzation
• TRIGGER_PPS_SYNC: manual PPS sync trigger (testing)

Ports:

• ppsSignal(async input): recieves GPS PPS pulses

Events:

• TimeCoarseSync: logged when coarse sync executes
• TimePPSSync: logged when PPS sync executes
• PPSDrift: logged when significant drift is detected
• PPSStateChanged: logged when PPS enabled/disabled
• TimeSetFailed: error event for failed time updates
• TimeGetFailed: error event for failed time reads
• PPSInitFailed: error event for PPS initialization failure
• PPSTimeout: warning when PPS signal not recieved

Telemetry:

• LastSyncTime(U32): timestamp of last synchronization
• PPSEnabled(bool): current PPS enable state
• DriftMicroseconds(I32): measured time drift
• SyncCount(U32): total synchonizations performed

GPS Hardware Integration To-Do's:

• Connect GPS module to Raspberry Pi
• Test Linux PPS subsystem integration
• Note that PPS related code in topology.fpp is currently commented out to allow for build testing

Dependencies:

Hardware:

• Raspberry Pi
• GPS module with PPS output

Software:

• F' framework
• Linux operating system
• PPS kernal support
• pps-tools package (for testing)

Build Requirements:

• CMake 3.16+
• C++ compiler with C++14
• Python 3.6+ with fprime-tools

Resources:

• PosixTime Implementation: nasa/fprime/Svc/PosixTime
• F Prime Tutorials: https://fprime.jpl.nasa.gov/
• Linux PPS Documentation: https://www.kernel.org/doc/html/latest/driver-api/pps.html