PSN-L Email List Message

Subject: Re: Network time standard
From: ian ian@...........
Date: Sun, 10 Apr 2005 08:40:50 +0100


Hi,

an interesting discussion.  I note that a spec of  <0.1 seconds has been 
mentioned (below).  Could I ask, how is this derived?  Apologies if this 
is documented somewhere on the psn site.

I only ask because I know that using specs that are higher than needed 
can lead to costs that could have been avoided.  I guess one would start 
by determining what is a reasonable error in calculating epicentre 
distance that can be tolerated and working back from there to derive a 
time spec.

Another question is, which of the many factors influencing epicentre 
calculation is the limiting one?  I would imagine that the average speed 
from the epicentre to a psn station would vary from station to station 
since each station is (obviously) located on a different part of the 
Earth and (presumably) the wave will travel through different parts of 
the Earth at a slightly different speed for each direction. 

If all the psn stations were locked in time to less than 0.1 seconds, 
then the average speed of the wave would have to be no worse than this 
for the data to benefit.  For a teleseismic event which took, say, 15 
minutes to arrive, all the "rays" would have to travel at the same 
average speed to within about 0.01% of each other.  Is this possible?!

Ian Smith

ChrisAtUpw@....... wrote:

> In a message dated 10/04/2005, shammon1@............. writes:
>
>     My experience when working for TrueTime (Now a division of
>     Symmetricom) was that you could achieve 10 milliseconds or so if
>     the server was not too far away on the public internet.  If you
>     run a local time service on your own network, the timing is much
>     better than that.  It usually takes several messages to get there
>     due to the filtering.  Also, true NTP will perform much better if
>     you give it more than one server to work with.  The software makes
>     some attempt to evaluate the stability of each source and choose
>     the best.
>
> Hi Keith,
>  
>     I think that we maybe in danger of comparing grapes with bananas, 
> if we are not careful. There is a fixed cycle time for the interrupt 
> polling in a computer, both for the communications interface and for 
> the program switching. Your multi tasking computer still only runs one 
> program at a time - it just looks more capable since every cycle it 
> polls a list to determine which are active. On one of the old IBM type 
> computers that I checked, this was about every 10 milli sec, but it is 
> likely to be more frequent on the current offerings. There are also 
> different levels of interrupt priority.
>     Then there are network response delays and digital transmission 
> delays. Local phone networks are likely to be more accurate. I have 
> measured transmission delays of 3 sec from the NIST clock over the 
> internet to the 60 KHz radio time, but that was exceptional. The on 
> line clock seems to have stated error bands of 0.5 to over 1.5 sec.   
>  
>     I asked Steve what was the absolute accuracy that he had measured 
> over the network time servers, but he didn't seem to fully understand 
> my question. I quite believe that his own internal GPS network can 
> detect a 1 mS error, but this is not an indication of how badly the 
> network time servers and the communications programs are performing in 
> practice. 
>
>     If you don't have a GPS, as I said, the software comes with an
>     extensive
>     list of network time servers that can be accessed via the Internet
>     to obtain
>     accurate time. Overall, the experience with this software package
>     has been
>     very positive and after 30-days of testing I'm now recommending it
>     to other
>     members of the Public Seismic Network.
>
>     Does it give any timing accuracies in milli seconds for the 
> various time servers?
>  
>     We do need to get a reasonable approximation to Universal Time, 
> say better than 0.1 sec and this needs to be maintained at all times. 
> We also need to remember that our filters can give very significant 
> delays. A 6 pole 10 Hz Butterworth filter peaks at about 100 mS. A 6 
> pole 10 Hz Bessel gives about 40 mS. However, if you reduce the 
> cut-off to 1.5 Hz, the figures are about 500 and 260 mS respectively. 
> Part of this difference is the result of defining the cut-off as the 3 
> dB point, rather than matching up the ultimate slopes. The P waves may 
> roll in at ~10 Km / sec.
>  
>     We wouldn't be having this discussion if computers were fitted 
> with reasonably accurate clocks. The 4.194 MHz timing crystals can be 
> trimmed to a few seconds per fortnight, but high precision temperature 
> tracking modules can give 0.1 ppm. The lousy apology for a clock 
> fitted to my current computer has drifted 8 sec in the last 2 hours. 
> Even hourly updates would not give me anywhere near the precision 
> required. You used to be able to buy boards with clock modules on 
> them, but I haven't seen any about lately.
>  
>     Since you can get 60 KHz receiver modues and aerials, it would be 
> helpful if A/D boards were able to read and update their clocks 
> directly using WWVB signals. This should be maybe 1/3 the cost of a 
> GPS system and you would not be dependant on having a permanent phone 
> connection.
>  
>     Regards,
>  
>     Chris Chapman    




  
  


Hi,

an interesting discussion.  I note that a spec of  <0.1 seconds has been mentioned (below).  Could I ask, how is this derived?  Apologies if this is documented somewhere on the psn site.

I only ask because I know that using specs that are higher than needed can lead to costs that could have been avoided.  I guess one would start by determining what is a reasonable error in calculating epicentre distance that can be tolerated and working back from there to derive a time spec.

Another question is, which of the many factors influencing epicentre calculation is the limiting one?  I would imagine that the average speed from the epicentre to a psn station would vary from station to station since each station is (obviously) located on a different part of the Earth and (presumably) the wave will travel through different parts of the Earth at a slightly different speed for each direction. 

If all the psn stations were locked in time to less than 0.1 seconds, then the average speed of the wave would have to be no worse than this for the data to benefit.  For a teleseismic event which took, say, 15 minutes to arrive, all the "rays" would have to travel at the same average speed to within about 0.01% of each other.  Is this possible?!

Ian Smith

ChrisAtUpw@....... wrote:
In a message dated 10/04/2005, shammon1@............. writes:
My experience when working for TrueTime (Now a division of Symmetricom) was that you could achieve 10 milliseconds or so if the server was not too far away on the public internet.  If you run a local time service on your own network, the timing is much better than that.  It usually takes several messages to get there due to the filtering.  Also, true NTP will perform much better if you give it more than one server to work with.  The software makes some attempt to evaluate the stability of each source and choose the best.
Hi Keith,
 
    I think that we maybe in danger of comparing grapes with bananas, if we are not careful. There is a fixed cycle time for the interrupt polling in a computer, both for the communications interface and for the program switching. Your multi tasking computer still only runs one program at a time - it just looks more capable since every cycle it polls a list to determine which are active. On one of the old IBM type computers that I checked, this was about every 10 milli sec, but it is likely to be more frequent on the current offerings. There are also different levels of interrupt priority.
    Then there are network response delays and digital transmission delays. Local phone networks are likely to be more accurate. I have measured transmission delays of 3 sec from the NIST clock over the internet to the 60 KHz radio time, but that was exceptional. The on line clock seems to have stated error bands of 0.5 to over 1.5 sec.   
 
    I asked Steve what was the absolute accuracy that he had measured over the network time servers, but he didn't seem to fully understand my question. I quite believe that his own internal GPS network can detect a 1 mS error, but this is not an indication of how badly the network time servers and the communications programs are performing in practice. 
If you don't have a GPS, as I said, the software comes with an extensive
list of network time servers that can be accessed via the Internet to obtain
accurate time. Overall, the experience with this software package has been
very positive and after 30-days of testing I'm now recommending it to other
members of the Public Seismic Network.
    Does it give any timing accuracies in milli seconds for the various time servers?
 
    We do need to get a reasonable approximation to Universal Time, say better than 0.1 sec and this needs to be maintained at all times. We also need to remember that our filters can give very significant delays. A 6 pole 10 Hz Butterworth filter peaks at about 100 mS. A 6 pole 10 Hz Bessel gives about 40 mS. However, if you reduce the cut-off to 1.5 Hz, the figures are about 500 and 260 mS respectively. Part of this difference is the result of defining the cut-off as the 3 dB point, rather than matching up the ultimate slopes. The P waves may roll in at ~10 Km / sec.
 
    We wouldn't be having this discussion if computers were fitted with reasonably accurate clocks. The 4.194 MHz timing crystals can be trimmed to a few seconds per fortnight, but high precision temperature tracking modules can give 0.1 ppm. The lousy apology for a clock fitted to my current computer has drifted 8 sec in the last 2 hours. Even hourly updates would not give me anywhere near the precision required. You used to be able to buy boards with clock modules on them, but I haven't seen any about lately.
 
    Since you can get 60 KHz receiver modues and aerials, it would be helpful if A/D boards were able to read and update their clocks directly using WWVB signals. This should be maybe 1/3 the cost of a GPS system and you would not be dependant on having a permanent phone connection.
 
    Regards,
 
    Chris Chapman    

[ Top ] [ Back ] [ Home Page ]