[ntp:questions] How to keep Linux server in Chicago and Mumbai in sync to within 5 microseconds

Richard B. Gilbert rgilbert88 at comcast.net
Wed Jan 12 22:44:46 UTC 2011


On 1/12/2011 2:57 AM, unruh wrote:
> On 2011-01-12, Chris Albertson<albertson.chris at gmail.com>  wrote:
>>> Two GPS receivers ( Garmin GPS 18X for about $70 each and a bit of time
>>> soldering on an RS232 plug.
>>> )
>>> Certainly cheaper and much more accurate. And if you can see the cell
>>> tower to get the CDMA you should be able to see the GPS sattelites.
>>
>> Yes the GPS can be even cheaper than $70.  But the CDMA signal really
>> can go through walls that the GPS signal can't.  So I'd say the way to
>> deside is to bring a hnd held GPS to the proposed antenna ocation and
>> see what hapens.  Can it get a good view of the sky.   Most GPSes have
>> a display screen that will answer that question.
>>
>> Also the OP is asking for +- 5 microseconds.  That is moderately hard.
>
> No way you will get that out of CDMA AFAIK. The time from the tower is much
> longer than that.
> GPS will give that to you.
>
>>   It means each box can only have a worst case 2.5 uS error.  To get to
>> that level some care is required.)    The specs on the GPS 18X say the
>> pps is "+- 1 microsecond" which is not very good.  It uses to much of
>> his error budget for my taste,  For the same price you can have at
>> least 10X better.   and maybe 100X better with a $200 Thunderbolt GPS.
>>   Then in effect none of his error budget is used up outside of the
>> computer.
>
> The problem is that the entry to the computer (interrupt handling) takes
> a lot more time than 1usec. Ie, unless he has very special hardware he
> will be hard pressed to get his 2us, and there is no way with CDMA
> AFAIK.
>
>>
>> Then we have to know a little about the computer.  Hopefully it has a
>> real serial port and is running BSD or Linuix and has the pps kernel
>> API.
>
> Probably a special parallel port interrupt handler would be better.
>
>>
>> This is that kind of thing that will suck up the OP's time.  There are
>> a million little things that he was to learn and each takes 10 minutes
>> or an hour.    To me reading this stuff is "entertainment" and a good
>> way to avoid the TV and learn something.  But to another person it is
>> just a huge "time sink" (pun intended)
>>
>> Bottom line:  This is just the classic "build or buy" business decision.
>>
>> =====
>> Chris Albertson
>> Redondo Beach, California

There are a few companies that specialize in GPS timing.  Symmetricomm 
is one such.  They will sell you a GPS clock for big bucks.  The last 
time I looked, the bottom of the line cost about $1200 US. There are 
other companies though I can't think of their names at the moment.

At the other end of the scale, you can get a GPS timing receiver for 
around $100 US.  This is a "no frills" deal; you will need to know how 
to use a soldering iron.  You will also need a five volt DC power supply.




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