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

[Lux, Jim]

On 3/25/22 10:45 AM, jeanmichel.friedt at femto-st.fr wrote:
> 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


There's a good chance that the codes are non maximal length. More likely 
they are composite, Gold or Kasami codes, for instance. They give good 
cross correlation properties (as well as good autocorrelation) - which 
is why GPS uses them.

One can prove the orthogonality in an analytical sense, but these days, 
for a short 10,000 bit sequence, brute forcing it is probably as good as 
anything. Back in the 80s, a mathematical proof was faster.