[ntp:questions] Re: Philosophical question about strata
Wolfgang S. Rupprecht
wolfgang+gnus20051013T230207 at dailyplanet.dontspam.wsrcc.com
Fri Oct 14 06:30:20 UTC 2005
"Lee Sailer" <lee.sailer at direcway.com> writes:
> 1. If everything is working, stratum 1 is better than stratum 2.
> 2. If the stratum 1 hardware is suffering a slow degradation
> breakdown, then the stratum 2 will be better.
Well, this is a sad (but true) commentary on the state of consumer gps
firmware. The gps (x,y,z,t) calculations, if done correctly, will
tell you when things are going badly. One only needs to see 4
satellites to work out the 4 unknowns. Usually one sees 8-12. There
is an awful lot of redundant information in observing those extra
satellites that one can use to determine error bars for the calculated
x,y,z and time. There really isn't a good reason why a gps can't tell
you "Hey, these numbers are out to lunch, please disregard."
If the failing gps's in question put out EPE, HDOP or similar "quality
of observation" numbers, it might be good to double-check that the ntp
drivers really do ignore once the readings get bad.
Question to the hard-core ntp jocks: Would anything break within the
internal of ntp if the stratum of a reference clock were to be
forcefully changed from the traditional stratum-1 for a correctly
running reference to, say, stratum-5 for a gps with really crummy EPE
that might be giving good time, but then again might not? (I assume
this is within the spirit of "stratum" with stratum being an
estimation of goodness, but please speak up if this isn't so.) I'm
guessing that changing the stratum of an ntp reference clock should
just work, because remote servers/peers can change their stratum and
ntp automatically adapts to the new landscape.
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