[time-nuts] Re: looking for MITREX modem manual/documentation

[jeanmichel.friedt at femto-st.fr]

Thank you all for the very fruitful exchanges and providing the references
I was looking for (by private email).

FYI I indeed tried the brute force search as documented at
https://github.com/oscimp/gr-satre/tree/main/reverse_code
but wanted to try to find some formal documentation demonstrating the orthogonality
of the codes beyond "trust me, we selected the best codes for maximum orthogonality".

Also if anyone is interested, I am looking into receiving TWSTFT exchange using TV
parabola (of course receiving only !). That is for fun since, for work the modem is
being assembled indeed, but that is off topic :)

Thank you very much, Jean-Michel

--
JM Friedt, FEMTO-ST Time & Frequency, 26 rue de l'Epitaphe, 25000 Besancon, France

March 25, 2022 5:40 PM, "Lux, Jim" <jim at luxfamily.com> wrote:

> On 3/25/22 7:45 AM, Azelio Boriani wrote:
> 
>> From the paper "PRESENT STATE OF LONG DISTANCE TIME TRANSFER VIA
>> SATELLITES WITH APPLICATION OF THE MITREX - MODEM", Hartl specifies
>> that "The PN-code is a truncated maximum length sequence of period
>> 10.000, instead of the 16.383 chips". In the Xilinx application note
>> XAPP052, we can find that the taps for an MLS of 14 bits should be
>> 14,5,3,1. In the paper "GEOSTATIONARY SATELLITE POSITION DETERMINATION
>> FOR COMMON-VIEW TWO-WAY TIME TRANSFER MEASUREMENTS" there is an
>> introduction that suggests the MITREX 2500 modem has a variable
>> pseudorandom noise sequence. Maybe the modem was built in different
>> versions wrt the PN sequence generation.
> 
> There's a fair number of of maximal sequences of length 2^14-1 . And
> it's possible that they chose a non-maximal sequence that had "better"
> properties. Maximal sequences will have a run of N ones and (N-1) zeros,
> for instance, which might not have enough transitions per unit time to
> let the receiver get a good lock.  Or they picked a "good" 10kbit
> sequence in the middle of a maximal sequence.
> 
> It could also be a composite code (the XOR of multiple PN sequences) -
> none of the papers seems to say anything about tap configurations.
> 
> I'd suggest seeing if you could find an email address for Professor
> Hartl - He'd be 94 now.  I think he retired from Univ Stuttgart in 1990.
> 
> At 2 Mchip/second, you could pretty easily build a MITREX type modem today.
> 
>> On Fri, Mar 25, 2022 at 2:37 PM <jeanmichel.friedt at femto-st.fr> wrote:
>>> Dear community,
>>> I am trying to find historical documents describing the pseudo random sequences used
>>> in the SATRE Two-Way Satellite Time and Frequency Transfer (TWSTFT) modem.
>>> K. Imamura & F. Takahashi (Two Way Time Transfer Via a Geostationnary Satellite,
>>> J. of the Comm. Research Lab., 39(1), March 1992) describe the code structure
>>> (14-bit long pseudo random sequence truncated to 10000 bit length) but the
>>> generator polynomial coefficients are not given. I have not been able to find this
>>> information in Timetech's SATRE manual nor in the publicly available literature.
>>> This paper cites "P. Hartl, A modem for microwave time and ranging experiments
>>> via telecommunication satellites, MITREX2500 Manual, Jan 1989" which I am unable
>>> to locate.
>>> Would anyone have such a document, or at least could tell me whether the generator
>>> polynomials are described there? Alternatively, does anyone have a description of these
>>> 14-bit polynomial generators?
>>> 
>>> Thanks, Jean-Michel
>>> 
>>> --
>>> JM Friedt, FEMTO-ST Time & Frequency, 26 rue de l'Epitaphe, 25000 Besancon, France
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