On Wed, May 24, 2017 10:45 am, Warren Young wrote:> On May 24, 2017, at 8:52 AM, Chris Adams <linux at cmadams.net> wrote: >> >> Once upon a time, Warren Young <warren at etr-usa.com> said: >>> a. It???s transmitting from a fixed location in a time zone you >>> probably aren???t in ??? US Mountain ??? being the least populous of >>> the lower 48???s four time zones. You therefore have to configure time >>> zone offset and DST rules, which means additional software if you want >>> it to track changes to these things. There were 10 batches of such >>> changes last year! >> >> This really has no bearing on time source; none of the commonly-used >> time sources (satellite, terrestrial radio, or network) carry time zone >> information > > In editing my prior reply, I removed the observation that GPS solves > problem ???a??? by telling you where you are in the world, as well as what > the UTC time is. > > It still has problems b and c, though. > >> (although WWVB does carry a bit to indicate if US DST rules >> are in effect). > > ???which is of no help when the DST rules are hard-coded into the clock, > as they are so frequently. I had to discard a few WWVB clocks when the > last DST rules went into effect. > >>> GPS time is a much better solution, but it???s power-hungry, as you >>> probably know from running GPS on your smartphone. This rules it out >>> for laptops. >> >> Not exactly; laptop batteries' capacity is an order of magnitude larger >> than phone batteries. > > Sure, but it???s still a market where people buy based on benchmarks. The > laptop that gets 20 minutes less battery life but has great time accuracy > even when not on the Internet will lose in the market. > >>> The GPS transmitters probably have a higher received signal strength >>> than WWVB >> >> No, GPS is lower signal strength than WWVB, at least for most of the >> continental US (although WWVB signal strength varies significantly based >> on the time of day, because it is a low frequency signal). > > I went looking, and you???re right. GPS satellites transmit at 0.5 kW and > WWVB at 70 kW, and WWVB has the additional advantage of less distance to > transmit. > > (21000 km from orbit to Earth surface vs ~3000 km from Fort Collins to > either Bangor, Maine or Miami.)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?). 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...). So, 70 kW vs 0.5 kW and the distance advantage still may not necessarily allow ground system beat hands down satellite based one (again, speaking only about S/N ratio for one vs another). It would be good to ask someone who can measure S/N for both (using the same $$ receiving radios) - which one is better. I would expect one disadvantage factor for GPS, as you have to receive and decode much wider signals for it as opposed to much slowed stuff for time purely ground based one... Valeri> > Regardless, that's another reason not to do this as a matter of general > policy. > _______________________________________________ > CentOS mailing list > CentOS at centos.org > https://lists.centos.org/mailman/listinfo/centos >++++++++++++++++++++++++++++++++++++++++ Valeri Galtsev Sr System Administrator Department of Astronomy and Astrophysics Kavli Institute for Cosmological Physics University of Chicago Phone: 773-702-4247 ++++++++++++++++++++++++++++++++++++++++
[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] 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!).
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!! Valeri> > 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!). > > _______________________________________________ > CentOS mailing list > CentOS at centos.org > https://lists.centos.org/mailman/listinfo/centos >++++++++++++++++++++++++++++++++++++++++ Valeri Galtsev Sr System Administrator Department of Astronomy and Astrophysics Kavli Institute for Cosmological Physics University of Chicago Phone: 773-702-4247 ++++++++++++++++++++++++++++++++++++++++