[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!).
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