[time-nuts] Re: Is there an application for time-stamping with absolute time?

[Tom Van Baak]

 > I added time-stamping with absolute time to my timer/counter.

There are a number of GPSDO that include such a UTC time tag feature. 
The earliest example I know is the hp 59551A, with 3 time tagging 
inputs. I'm told power companies used these products to locate faults 
along transmission lines, a clever use of GPS. More info here [1].

See also the Pendulum CNT-91 timestamping counter for SCPI examples of 
time capture.

The latest GPSDO from SRS (FS740) includes UTC time tagging. Check the 
user manual for their SCPI implementation.

Yes, some mechanical clocks are timed against UTC. But most DIY and 
commercial timers in that space already employ GPS for this.

I've used UTC timestamping for mains frequency logging. That too is a 
niche application but it makes a good test case because of the unusually 
high data rate. It's also a good test case because multiple users on the 
same grid can directly compare their raw timestamp files.

There are lots of time formats to choose from. Start with one of the ISO 
8601 formats. Another popular one is based on unix time_t: unsigned 
non-leap seconds since 1970-1-1 and 9 decimal places of nanoseconds. For 
example, right now is 1720810149 and if you add nanoseconds the 
timestamp looks like 1720810149.123456789. One problem with that format 
is that it looks like an innocent floating point number but even if you 
use 64-bit double you lose a couple hundred nanoseconds due to loss of 
precision. So some systems output two integers instead, one for seconds, 
one for nanoseconds.

Assuming your code is open source, maybe make it easy for the user to 
create their own application specific format.

One last note, many vintage telecom related GPS receivers had support 
for timestamps. Mostly output (time of day), sometimes input (time 
tagging). The feature to note is that they often included a TFOM (time 
figure of merit) in their data format. This is kind of important for a 
GNSS based device. A smart GPSDO can know pretty well how accurate (at 
least to order of magnitude) the timestamps are or aren't. In other 
words, it's not so much about the format of the timestamp but the 
confidence of the timestamp. Power up, loss of lock (holdover), low 
visibility, mobile use can all affect TFOM.

/tvb

[1] http://leapsecond.com/museum/hp59551a/