David I. Emery wrote:
On Sun, Nov 29, 2009 at 12:25:06PM -0800, Mark Spencer wrote:
The main difference is that receiving lf signals is challenging in
many areas built up areas and the doppler shift of hf via sky wave
reduces the accuracy considerably, while there are already a large
number of exisiting high power transmitters that can be locked to an
external time base and have line of site paths to many locations in a
typical metropolitan area. If a sutiable receiver existed this might
be a feasible means of distributing accurate frequency info and then
with a suitable reciever you could generate a 1 pps signal. That being
said a dedicated uhf or shf transmiter that could send accurate 1 pps
signals (as well as providing a very accurate carrier frequency) might
be an easier solution. In any event if there was a market for such a
system I believe it would have emerged by now.
As I suggested earlier, I believe that relatively simple tweaks
to a broadcast ATSC transmitter/modulator/mux chain to lock both carrier
frequency, symbol clock, and say PCR clock to a local cesium standard
with time of day based (initially) on GPS would probably be quite
practical and perhaps even little more than using the 10 Mhz (or 27 MHz
derived from it) from the cesium as clock input for existing plant and
setting some firmware settings correctly.
There's a standard for ATSC SFN configuration.
OBVIOUSLY as others have pointed out someone has to pay for this
even though the actual costs might be very small compared to the other
operating and engineering costs associated with the broadcast
transmitter plant. It is hard to think of a more powerful signal for
time sync in a metro area...
Unless SFN operation is applied, it is fairly unreasnoble to expect that
it would happend.
As for receivers, existing ATSC tuner/demod chip sets and a FPGA could
no doubt supply all the usual timing signals (10 MHz, 1 PPS, time of day
in some standard format). One imagines sub microsecond PPS accuracy
(once propagation skew is measured) is quite possible.
The propagation delay from the transmitter tower and any delay-offsets
of the tower would add up.
One would clearly need to use a GPS based measurement to establish
the propagation based skews...
Certainly.
I imagine you could even design a gps timing receiver that could also
receive terristerial signals as a backup, but again it does not seem
there is a market for this (:
Network effects apply here - if there is no signal to lock to,
then there is no market for a receiver - if there is no receiver even rather
low cost changes to TV plant aren't gonna happen or be justified...
Only within SFN regions the receivers would be usefull.
And I suppose the bottom line is that we'd better hope that no
natural (or perish the thought deliberate man made) event takes out
enough of GPS to cause GPS based timing to fail. And system designers
had better start thinking about very local jamming of GPS timing
receivers at targeted sites that might cause a vital system (say public
safety radio) to degrade or fail.
There are several man-made things which may cause loss of signal,
including firmware errors, bad antenna connections, water in antenna
(actually happend to me) etc. etc. There are many reasons for the full
function to fluke out.
Cheers,
Magnus
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