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

[Lux, Jim]

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 I am trying to find historical documents describing the pseudo random sequences used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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