Re: [Starlink] Time Synchronization in Satellite Networks

[Alexandre Petrescu <alexandre.petrescu@gmail.com>]

some of the question is to what level of precision one wants the time to 
be maintained synchronized between entities, and for what application? 
Nano-second precision?  Less?  More is acceptable?  For what kind of 
application?  (I will not give examples).

I think links with hundred ms latency range and NTP can easily maintain 
nano-second synch'ed precision, from experience with ground links.


Le 02/03/2024 à 18:01, Alexandre Petrescu via Starlink a écrit :
>
> Le 02/03/2024 à 16:38, Christian von der Ropp via Starlink a écrit :
>> Why not acquire the time directly from by the satellite terminal and 
>> run local NTP servers instead of syncing via the Internet?
>
> Certainly it is possible to run ntpd servers and clients on satellites 
> and maintain synchronized times.  I would be surprised if some of them 
> dont already do that.
>
> The performance characteristics of some links between some satellites 
> are not very different than links here on ground where NTP is run 
> routinely.
>
> NTP was designed and tested at a time when ground links had inferior 
> perf. characteristics than many satcom links of recent years.
>
> Alex
>
>
>> 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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