[time-nuts] LORAN-C demise

[David I. Emery]

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.

	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...

	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.

	One would clearly need to use a GPS based measurement to establish
the propagation based skews... 

> 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...

	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.

-- 
  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."