Re: [Starlink] Time Synchronization in Satellite Networks

[Hesham ElBakoury <helbakoury@gmail.com>]

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Time from GPS is a one way transmission from the satellites down. The
relative motion must be accounted for. It is called the Sagnac effect.

On Mon, Apr 1, 2024, 3:22 PM Sebastian Moeller <moeller0@gmx.de> wrote:

> Hi Hesham,
>
>
> > On 2. Apr 2024, at 00:04, Hesham ElBakoury <helbakoury@gmail.com> wrote:
> >
> > Hi Christian,
> > The problems is that Satellites move, therefore,  the delay between the
> different directions is different which violates the condition to run NTP
> and PTP.
>
> But GPS Satellites themselves are not in geostationary oprbit, and still
> we can get precision time from them... so I would argue that must be a
> solved problem, no?
>
> Regards
>         Sebastian
>
> >
> > Hesham
> >
> > On Sat, Mar 2, 2024, 8:19 AM Christian von der Ropp <cvdr@vdr.net>
> wrote:
> > Hi Hesham,
> >
> > You do not acquire the time from a LEO satellite but directly from the
> GPS satellites which carry an atomic clock on board.
> > I'd not be aware of any LEO providing a GNSS signal but Xona plan such
> system (although not carrying proper atomic clocks but probably chip-sized
> atomic clocks that require frequent syncing with proper atomic clocks):
> > https://twitter.com/Megaconstellati/status/1708091536439673323
> >
> > There are efforts to build trapped-ion quantum clocks that are expected
> to become significantly smaller and cheaper than traditional atomic clocks
> while as accurate which would make it viable to put an atomic
> clock-equivalent on small LEO satellites. Once that happens you would have
> an independent alternative to the big GNSS birds in MEO but with stronger
> signals. I'm told that we are 5-10 years away from such trapped-ion quantum
> clocks.
> >
> > But for NTP clients, the described method (running a local NTP server in
> the satellite terminal synced to GPS) should be good enough.
> >
> > Christian
> >
> >
> > Am 2. März 2024 18:02:47 OEZ schrieb Hesham ElBakoury <
> helbakoury@gmail.com>:
> > Hi Christian,
> > How you synchronize the time of the satellites in the network? Are you
> saying each satellite has a master clock?
> >
> > Hesham
> >
> > On Sat, Mar 2, 2024, 7:38 AM Christian von der Ropp <cvdr@vdr.net>
> wrote:
> > Why not acquire the time directly from by the satellite terminal and run
> local NTP servers instead of syncing via the Internet? LEO satellite
> terminals always have onboard GNSS antennas for geolocation which is
> necessary to find the satellites, so integrating a local GNSS-disciplined
> Stratum-1 NTP server seems trivial to me.
> >
> >
> > Am 2. März 2024 17:25:59 OEZ schrieb Hesham ElBakoury via Starlink <
> starlink@lists.bufferbloat.net>:
> > Hi Sebastian,
> > Can we still use PTP and NTP for time synchronization in  Satellite
> networks or we need new protocols? If we need new protocols, do such
> protocols exist?
> >
> > Thanks
> > Hesham
> >
> > On Sat, Mar 2, 2024, 7:18 AM Sebastian Moeller <moeller0@gmx.de> wrote:
> > Hi Hesham
> >
> > > On 2. Mar 2024, at 16:03, Hesham ElBakoury via Starlink <
> starlink@lists.bufferbloat.net> wrote:
> > >
> > > Time synchronization, for satellite networks, faces several challenges:
> > > 1. Signal Propagation Delays: Unlike terrestrial networks where
> signals travel through cables at the speed of light,
> >
> > [SM] The speed of light in your typical glas fibers (and accidentally
> the information propagation speed in metallic conductors) comes in roughly
> at 2/3 of the speed of light in vacuum, while the speed of light in air at
> see level is a mere 90 KM/s slower than in vacuum.
> >
> > > satellite communication involves signals traveling vast distances
> through space. This creates significant delays.
> >
> > [SM] Sure distances might be larger, but propagation speed is around
> 100000Km/s faster... my main point is speed of light is a) dependent on the
> medium b) not the things that differentiates space from the earth's surface
> here, but mere geometry and larger distances on larger spheres...
> >
> > > 2. Clock Drift: Even highly precise atomic clocks, used in satellites,
> are susceptible to "drift" - gradually losing or gaining time. This drift,
> caused by factors like temperature variations, radiation exposure, and
> power fluctuations, can lead to inconsistencies in timekeeping across the
> network.
> > > 3. Signal Degradation: As signals travel through space, they can
> degrade due to factors like atmospheric interference, ionospheric
> disturbances, and solar activity. This degradation can introduce noise and
> errors, impacting the accuracy of time synchronization messages.
> > > 4. Limited Resources: Satellites have limited power and processing
> capabilities. Implementing complex synchronization protocols can be
> resource-intensive, requiring careful optimization to minimize their impact
> on other functionalities.
> > > 5. Evolving Technologies: As satellite technologies and applications
> continue to evolve, new challenges related to synchronization might emerge.
> For example, the integration of constellations with thousands of satellites
> poses unique synchronization challenges due to the sheer scale and
> complexity of the network.
> > > These challenges necessitate the development of robust and efficient
> time synchronization protocols for satellite networks and an integrated
> satellite and  terrestrial networks
> > > Are you aware of such time synchronization protocols?
> > > I would think that using Satellite simulators is the most viable way
> to develop and test these protocols given that using satellites is not that
> easy.
> > > Thanks
> > > Hesham
> > >
> > >
> > >
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