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Date: Sat, 28 Jan 2012 21:10:45 +0000
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From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: WOLF DATA MODE S/N FIG ?
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Hi Jim ..

If it "had" to reacquire the PN sequence every frame, there could be
no coherent addition.   Since the data transmissions are contiguous, I
assume it will be continuously locked to the running code, using a
sliding correlator.    Provided the code is being generated locally,
it can be always compared against that received and a continuous
correlation performed.  Teh result of this nudges the locally
generated copy to keep it in phase

Its what GPS does.  The underlying GPS data is modulated at 1kHz which
is the PN code repeat rate, and is the bandwidth required (at least in
the first two generation receiver types) that initial lockup works
with.

Once the system is locked, it can drop to the lower 50Hz effective
bandwidth corresponding to the transmitted data.   So, until the third
generation three dimensional deep search receivers arrived round about
2006 - 7, GPS lockup needed 13dB more signal than for tracking.

Wolf would appear to behave similarly.

Andy
www.g4jnt.com


On 28 January 2012 20:30, James Moritz <james.moritz@btopenworld.com> wrote=
:
> Dear Andy, LF Group,
>
>
>> So by continuously looking at repeated message, the effective
>> bandwidth will get lower by the average of the number of times it has
>> been repeated. =A0 Two repeats 3dB or half BW, 4 repeats 6dB , 8
>> repeeats 9dB =A0etc etc
>>
>> Does this seem about right ?
>
>
>
> Well, I cannot claim any expert knowledge, but I'm sure that must be the
> intention. Presumably the limit to which this can be done is determined b=
y
> the ability of the decoder to successfully detect the pseudo-random
> sequence. I guess it has to do this separately for each 96 second message=
 in
> order to get the carrier phase for that message.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>