[CentOS] System Time Source

Thu May 25 16:37:28 UTC 2017
Valeri Galtsev <galtsev at kicp.uchicago.edu>

On Thu, May 25, 2017 11:16 am, Lamar Owen wrote:
> [Going a bit off-topic here, and going to do a bit of a deep-dive on RF
> stuff, but maybe it will be useful to Chris]

Lamar, thanks a lot for very instructive write-up!!


> On 05/24/2017 12:20 PM, Valeri Galtsev wrote:
>> It is insightful, yet... There are a bunch of other factors that may
>> need
>> to be taken into account. Angular transmission pattern of satellite
>> (horn?
>> or is it yagi? antenna) vs ground based (monopole? or dipole? antenna -
>> which one is used there to transmit in HF?).
> WWVB uses a two-element phased array, where each element is a 400-ft
> top-loaded vertical monopole.  The ERP is listed as 70kW, so the antenna
> gain is already applied to the transmitted signal's specification and
> thus doesn't need to be considered. (Lots of technical data can be found
> in NIST's report on the 1998 upgrade:
> http://ws680.nist.gov/publication/get_pdf.cfm?pub_id=50031 ).
> Please see http://gpsinformation.net/main/gpspower.htm for the relevant
> data on GPS (25.6W output, 13dBi gain, EIRP 27dBW (about 500W), free
> space loss of 182dB, -130dBm receive signal strength (0.1 femtowatts, if
> I've done the calculation correctly)).
>> Ground effect (attenuation)
>> along the whole path or propagation for ground based HF vs ground effect
>> only at the receiption point, but much higher for much higher
>> frequencies
>> of GPS; pre-amplifier Signal to Noise ratio (S/N; which can technically
>> be
>> achieved to be much better at much higher GPS frequencies...).
> WWVB's signal is at 60kHz, which is LF, not HF. LF signals are not
> significantly attenuated by ground conductivity effects, so a simple
> inverse-square-law free-space path loss calculation is a close
> approximation; the loss to a point halfway around the world (~20,000 km)
> is about 94dB (82dB for 5,000km); the ERP is 70kW (78.45dBm); the
> minimum power available anywhere on the surface of the world is
> -15.55dBm, or 0.03mW and the minimum power available within 5,000km is
> about -3.55dBm, or about 0.44mW.  Half a milliwatt is quite a bit to
> work with, excepting the noise effects of 1/f ("pink") noise and local
> interference.  Higher-gain receive antennas are easy at 60kHz (iron-core
> loopstick or a multi-turn loop).  According to NIST's site, however,
> WWVB is currently running at half-power (35KW ERP; 75.45dBm) so cut the
> available power in half at the moment.
> However, WWVB's signal _is_ 60kHz, and so any building of metal
> construction, even sparse-spaced rebar in concrete, will effectively be
> a very high attenuation 'waveguide-beyond-cutoff' attenuator, and so a
> very effective shield, even with the very high power available to the
> receiver.
> GPS receiver module manufacturer u-Blox has an informative paper on GPS
> receiver antenna design that might answer some other questions:
> https://www.u-blox.com/sites/default/files/products/documents/GPS-Antenna_AppNote_%28GPS-X-08014%29.pdf?utm_source=en%2Fimages%2Fdownloads%2FProduct_Docs%2FGPS_Antennas_ApplicationNote%28GPS-X-08014%29.pdf
> I'm running an NTP setup here with our secondary being a CentOS box
> using an Agilent Z3816 GPS-disciplined OCXO with timecode and 1PPS
> outputs.  Our primary is a Datum/Symmetricom SSU2000 modular system with
> a cesium PRS, a rubidium stratum 2E secondary clock, and an OCXO stratum
> 3E tertiary clock.  The cesium PRS is down at the moment, but the
> rubudium is close enough for current work.
> The CentOS box runs very well for this purpose, and the interface wasn't
> too difficult.  I have not implemented the 1PPS discipline for the
> kernel clock as yet, however, since the SSU2000 is up.
> As far as cost is concerned, I would think CDMA, GSM, or LTE timecode
> receivers would be a bit less expensive to integrate than GPS receivers,
> but u-blox and others have really gotten the cost down for GPS modules.
> GPS is already supported by the NTP server shipped with CentOS, where I
> don't think any CDMA/GSM/LTE timecode receivers are (but I reserve the
> right to be wrong!).
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Valeri Galtsev
Sr System Administrator
Department of Astronomy and Astrophysics
Kavli Institute for Cosmological Physics
University of Chicago
Phone: 773-702-4247