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Date: Mon, 10 Apr 2006 13:19:02 +0100
From: Stewart Bryant <stewart@g3ysx.org.uk>
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Subject: Re: LF: Rugby LORAN measurement?
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Status: O


Peter

Peter Martinez wrote:

> Stuart:
>
> The design of an MSF receiver, intended to recover the carrier 
> frequency to any great accuracy, needs to be able to reject the 
> existing 'unwanted' signals either side of the carrier. These are only 
> about 15dB down and at +/-1Hz. I am referring to the 1 sec timing 
> pulses (gaps). 

Building a narrow band filter is only one way of doing this.

Some receivers - for example the Decca MSF design - use a coherent 
technique.
They know when the is due pulse and put the phase comparitor into 
holdover for the
duration of the pulse.

Indeed if you are trying to get time and frequency out of the same 
receiver you have
to open up the bandwidth enough to resolve the pulses, and then the coherent
mechanism is probably more attractive than the ultra-narrow filter method.

> The disturbance from this 1Hz source has to be attenuated by a very 
> large amount if it isn't to affect the output of the frequency 
> standard. If we suppose that MSF receivers in service can achieve 
> this, they shouldn't suffer any problem from unwanted products which 
> are somewhat further away, at 6 and 8Hz, and are already a long way 
> down. I cannot detect them here which probably means they are more 
> than 70dB down.

The issue that I have with this is trying to figure out is how to 
convert the
voltage and amplitude of the interference into a time error. 70db is 
1e-7 power and
only 3e-3 voltage. This seems big compared to 5e-11.

This clearly needs some more thought.

> I know it would be nice to be rid of the LORAN interference, but this 
> line of reasoning may not be a means to that end, and in any case it 
> wouldn't help amateurs close to the other LORAN transmitters. 

I was certainly told by NPL at ITSF 2005 that they did not know whether 
there
was an effect or not, and that there was some concern that users near the
LORAN transmitter might not be getting the expected accuracy from the MSF
service. Hence my line of reasoning.

> I understand the Rugby site must close in a year or so and they are 
> looking for an alternative site for the 60kHz MSF transmitter. The 
> question becomes whether the LORAN transmitter will also move to that 
> new site or whether it will close down. The long term future of LORAN 
> has always seemed shakey but it doesn't look as if it will be 
> killed-off until there is a European second-source for satellite-based 
> position-finding, and that seems some years away yet.
>
I am not sure what is going on in Navigation circles, but in timing 
circles LORAN
is being sold in the US (by the LORAN providers) as an alternative 
source of
time reference/frequency reference.

Contra to the US position, European service providers do not want to rely
off-air frequency service for reference - they are very vocal on this. The
telecomms industry is therefore persuing: synchronous ethernet, IEEE-1588,
timing over packet and possibly NTPv5 to address this and the signs are
that we can get to the required 10ppb using packet network technology.

One can but hope that the absence of interest in LORAN by the
telecommunications industry in Europe will limit the economic viability
of LORAN over here and assist its early demise.

73

Stewart G3YSX

> 73


> Peter G3PLX
>
>> The accuracy of MSF is 2 parts in 10E12, so systematic interference 
>> even a long long way down
>> may well  have an effect. It will depend on the design of the 
>> receiver and whether it has a front
>> end filter that has high rejection at +6Hz, -8Hz, but prior to the 
>> LORAN transmitter.
>
>
>
>
>