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@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 ++++++++++++++++++++++++++++++++++++++++ _______________________________________________ CentOS mailing list CentOS@centos.org https://lists.centos.org/mailman/listinfo/centos
