[ntp:questions] Tracking the drift of a GPS clock relative to a HW clock

Richard B. Gilbert rgilbert88 at comcast.net
Tue Jan 20 15:25:30 UTC 2009


ryad.bek at gmail.com wrote:
> On Jan 20, 2:00 am, Unruh <unruh-s... at physics.ubc.ca> wrote:
> 
>>> I want to compare the outputs of an application when this app is fed
>>> with GPS input & when it is fed with HW input
>>> HW input = series of ("HW timestamp" + event)
>>> GPS input = series of ("GPS timestamps" + event)
>> So feed it with time stamp inputs and see what it does. Why does it care if
>> they came from HW or GPS or were some random numbers you concocted?
> 
> It's a distributed application (it merges tcpdump trace)
> and I want to compare my merging algorithm when it uses perfect inputs
> (In this case I run a GPS on all nodes) &
> when it uses non perfect timestamps (no GPS embedded)
> 
>>> I want the same list of event. Only the timestamp can differ in the
>>> inputs.
>>> So I have to run the exeperience only once (i don't want to run an
>>> experience with GPS and an experience whithout GPS)
>>> The only way to do that is to invent a way to convert GPS timestamps
>>> to HW timestamps
>>> (i.e. the timestamps that would have been generated if the GPS was
>>> disabled)
>> ???? Make them up.
> 
> Make them up (the english word "make up" is quite confusing for me):
> you mean correct the drift as Terje Mathiesen previously explained me?
> (cf "Well, you could take the readings of the frequency drift rate
> correction and integrated them up").
> 
"Make them up" is colloquial English meaning something like "invent 
them" or "falsify them".

I can't guarantee that that was what he MEANT but it certainly seems so!




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