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.
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
