[time-nuts] LORAN-C demise

[bg at lysator.liu.se]

Hi,

Well, most of this discussion is about GPS backup. The GLONASS system has
received global coverage (again) for navigation. Timing is less demanding
if we consider the static and known position case. In some years we will
have Galileo (Euro) and Compass (China). Perhaps also regional Japanese
and Indian (global?) systems. These are systemwise independant of GPS.
Looking at spectral redundancy - we have L2C available on some GPS SVs, L5
will come. The other coming systems are also multi carrier frequency. One
could argue that a jammer on the whole of the 1GHz to 2GHz will take all
services. But such a wide band jammer is very much less efficient than a
narrowband L1 carrier version.

How about using SBAS (WAAS, EGNOS, MSAS) for single satellite timing use?
That should be possible? The SBAS do provide a ranging signal if running
in the right mode, and that should be usable for a timing receiver. Having
a GEO signal, there should be no problem in using a high gain (parabol)
pointing directly to the GEO. This will steal many dB from the jammer
power budget.

Perhaps the way to redundancy is getting GNSS receivers using multiple
carrier frequencies and multiple constellations. Even though it is sad
Loran-C seems to go away soon.




--

    Björn

> 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."
>
>
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