[time-nuts] ISS NTP operation problems.

[Lux, Jim]

On 1/8/21 2:15 PM, Warren Kumari wrote:
> This is a propagation path that I suspect NTP is just not designed to
>>>> deal with.
>>> Well, I wonder if NTP over that path is even the best solution. Taking
>>> time off a GPS/GNSS receiver onboard the ISS would be a significant
>>> improvement. Just having the PPS would help immensly.
>> That is probably harder than it seems.  There's a lot of isolation among
>> systems on ISS - partly for safety, partly from history, partly from
>> institutional inertia. My payload on ISS (SCaN Testbed) had a
>> MIL-STD-1553 connection and a unidirectional Ethernet connection (out of
>> payload only). There's multiple GNSS receivers on ISS, but not all are
>> visible to an arbitrary payload - their output might get packaged up as
>> telemetry and store/forward sent to the ground via episodic
>> transmissions on the Ku-band system.  One of the experiments on my
>> payload was to actually try to measure the time and position offsets
>> between our radio(which had S-band Tx/Rx and GPS receiver) and the
>> various time sources on the Station.
> I must admit that I'm very surprised that GPS receivers worked and
> were able to compute a fix.
> The majority of GPS receiver chipsets have altitude and speed limits
> built in, both because of assumptions/discarding pathological results,
> but also because of ITAR and similar regulations. Were these special /
> licensed receivers which didn't have the "Erk, I think I'm on an ICBM"
> logic?



Yes  - these are all flight qualified GNSS receivers specifically 
designed for space use.  JPL has been building receivers for this kind 
of application for decades.


As a practical matter, one can buy standard GPS receivers (like the ever 
popular Novatel OEM 6 and 7 series) that are enabled for space.  Whether 
they survive single event effects or total dose accumulation is another 
issue, but a lot of people fly them and they work pretty well.  I didn't 
have any problems with them on a cube-sat I flew.