[time-nuts] Simulating LORAN-A GRI Timing Generator

paul swed paulswedb at gmail.com
Sat Jan 8 09:33:33 EST 2011


Loran c was complicated by the fact that there were phase reversals in the
patterns that had to be emulated it doubled the size of the code. But to
answer your question John its all done in simple readable basic language.
Think of it as a endless loop pattern. The real trick is using a good
reference. I think Magnus said 10 KC. What you do is count the 10 KC
reference. Using a Parallax SXB chip that can run all the way up to 80 Mhz.
The code runs so fast that 99.9% of the time the micro is just waiting and
counting edges.
I like to create dumb simple stupid, reliable as heck programs like that.
The Loran C sim allows the Austrons to compare to their limit of 1 e -13.
There is a bit more cleverness in the C simulator external to the program.

I thought there were different frequencies at about 1.8 Mhz.

I remember the gone-ometer (I still think thats a disease sailors can get),
green crt black box about 2 ft tall.
If you really wanted to do a location you might need 2 or 3 chips at
different GRIs and phases. But at $3.56 not a really big deal. Lastly there
must be some overall sync relationship between the chains that has to be
maintained I might guess.

Oddly enough I emailed the group several times at the ship and never
received a response.
Figured they were all set.
Regards
Paul.

On Sat, Jan 8, 2011 at 1:27 AM, Magnus Danielson <magnus at rubidium.dyndns.org
> wrote:

> On 08/01/11 06:54, Magnus Danielson wrote:
>
>> John,
>>
>> On 08/01/11 05:04, J. Forster wrote:
>>
>>> Magnus,
>>>
>>> See my post of a few minutes ago.
>>>
>>> The GRI and Delay counters need to be 5 decimal digits. My current
>>> thinking is manual entry of the numbers via BCD coded thumbwheel
>>> switches.
>>>
>>
>> If you only need a static location that will be fine. Naturally you tune
>> it up to the ships current (final?) position. Good exercise for the
>> visitors. :)
>>
>>  LORAN-A has 24 possible GRIs; in three blocks of 33 PPS; 25 PPS; and 20
>>> PPS. It also has 4 RF frequencies.
>>>
>>
>> I did a fashinating read-up here:
>> http://www.jproc.ca/hyperbolic/loran_a.html
>>
>> GRI is a LORAN-C term if I get it correctly, where as PRR is used
>> together with frequency indication in LORAN-A.
>>
>
> Pondering some more on this, doing some calculations (spread-sheets) the
> actual scheme emerge... the range of 0 to 7 is not selected by accident.
>
> The base pulse repetition rate is 30 ms, 40 ms and 50 ms. The pulse
> repetition rate modifiers modifies the repetition rate by -100 us per step.
> It would only take two (or possibly three) wheels to select PRR.
> This explains the high rate starts with 33 3/9... it also makes the
> receiver design easier to understand.
>
> So a 10 kHz source would suffice for PRR settings. A phantastron divider
> from a 100 kHz standard should do it in those days technology, if not a 10
> kHz standard was used directly.
>
> Only the delayed pulse would need the full set of thumb-wheels.
>
>
> Cheers,
> Magnus
>
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