PSN-L Email List Message

Subject: Re: Computer Timing Problems / Solutions
From: Larry Cochrane lcochrane@..............
Date: Sat, 16 Apr 2005 16:30:32 -0700


The time stamping of the data is done on the A/D board. If you have a time reference 
like GPS the time accuracy is one or two milliseconds. The board uses a 1 millisecond 
interrupt so the timing can't be any better then one ms. With WWV and WWVB the 
accuracy will be in the order of +-10 to +-20 ms depending on the signal quality.

If you have a time reference connected to the A/D board WinSDR can use the A/D 
board's time to keep the PC's time accurate to +- 250 ms. Since the time stamping is 
done on the A/D board there is no need to keep the PC's time accurate to less then a 
few seconds. WinSDR does use the system's time for things like file names so the PC's 
time should be near the A/D board's time.

If there is no timing reference available for the A/D board you can use the PC's 
time. In this mode the A/D board requests the current time from the PC every minute 
or so and uses this time for data time stamping. If the PC's time is keep accurate 
with a program like NTP the overall accuracy can be +- 30 or +-50 ms or about the 
same as WWV/WWVB.

With regard to the different time references. By far GPS is the best, if you can find 
a location for the antenna that will allow the receiver to see two or more satellite 
most of the time. The cost of a OEM GPS receiver with a 1 PPS signal is now below 
$100.00 USD. I am now selling the Garmin GPS 18 
( for this price. This is a very 
nice receiver that works very will with my A/D board. Given that the cost to use GPS 
keeps going down I not sure I see the need to support WWV/WWVB any more. The other 
problem is these signals are only available here in the States. Since half of my 
sales are overseas a lot of people will not be able to use this feature. And with 
most of the PSN station having a full time Internet connection now a days they always 
have the  option to use NTP etc as a time reference.


Larry Cochrane
Redwood City, PSN

ChrisAtUpw@....... wrote:
  >     Larry, I would like to suggest that you give  consideration to providing
> the 16-bit Serial Output A/D Board for WinSDR with an  option to receive and 
> fully decode WWVB signals, or the ability to read the  output of a MCM 
> microcontroller module. I have read your note, dated 1998, on  WWVB signal reception. 
>     I note that the serial board can currently use WWV  minute tone decode 
> signals, but you state that 'You will not get 24 hour  reception on any one 
> channel, as long as you can get 4 to 6 hours per day will  be fine. At my location 
> I get best reception on 5.0Mhz at night and during the  day 10.0Mhz or 
> 15.0Mhz.' 
>     Does the timing for SDR originate on the board, or  is it dependant on 
> the computer clock, please? My computers can NOT keep time  over a timing break 
> of maybe 12 hours, to better than about 2 sec. This is  certainly NOT good 
> enough for seismic work! Unlike WWV, WWVB has the  potential to provide accurate 
> timing signals over the full 24 hrs for most  places in the USA. The extreme 
> range daytime WWVB signal is certainly a lot  weaker than the nightime signal, 
> but with the possible exception of Maine,  it should be satisfactory
>     The ability to fully decode the WWVB signal could  cope with the 
> situation of a power outage. It wasn't until I bought a 60 KHz  radio corrected 
> digital clock that I realised just how bad my computer timing  systems were! Another 
> reason for doing this is to provide a timing  system totally independant of 
> the www. There are already  predictions of future communications problems on 
> the web. 
>     The total cost could well be about that of  just a GPS ANTENNA !


Public Seismic Network Mailing List (PSN-L)

[ Top ] [ Back ] [ Home Page ]