[time-nuts] Re: Eloran long test from now to August or September.

[Lux, Jim]

On 9/27/23 8:15 AM, Lester Veenstra wrote:
> Agree very old and multi used idea.
> For the clever ones in DSP, penetrate a time coherent spread spectrum that can be inband but under the noise floor of the collection bandwidth.  Correlate on receive in the same sample environment as you are doing science.
>
> Lester B Veenst


It doesn't even have to be below the noise floor, since once you've 
correlated it, you can coherently subtract it.  In fact, with a strong 
signal, the uncertainty in your correlation is smaller, so when you do 
the subtraction, you're subtracting a "more precise" copy. One catch is 
that you need to recover the amplitude accurately as well as the timing, 
which means you might need some additional processing, since most 
acquisition and tracking codes, like for GPS, are concentrated on good 
timing performance and only getting a coarse estimate of the power.

If one wants to hunting the literature, I'd look for stuff on adaptive 
cancellation of interferers and jammers. Typically looking at using 2 or 
more GPS antennas/receivers to null out an interference source.



>
> -----Original Message-----
> From: Jim Lux via time-nuts [mailto:time-nuts at lists.febo.com]
> Sent: Tuesday, September 26, 2023 3:33 PM
> To: jeanmichel.friedt at femto-st.fr; time-nuts at lists.febo.com
> Cc: Jim Lux
> Subject: [time-nuts] Re: Eloran long test from now to August or September.
>
>
> 	
>
>
> Exactly.. That does work, although it requires opening up your RTL-SDR (and on V3, in HF receiver mode, the RF input is directly coupled to the Q input, without going through the front end downconverter.). It didn’t work well.
> And ultimately, I was trying to figure out something that would work at Ku band, so I could “calibrate out” the delay through the LNB, the coax, and the RTL-SDR.
> The basic technique has been around since the 1960s.  Bracewell used it at Stanford, and a similar scheme was used for the Jansky VLA in New Mexico.
>
>
> On Tue, 26 Sep 2023 19:24:21 +0000, jeanmichel.friedt at femto-st.fr wrote:
>
>> I did it with 4 RTL-SDRs, and I’ll scrounge up the measurements. My first attempt was just to
>> couple the 1pps from a GPS module into the inputs - that’s not easily detectable (I was hoping that
>> the fast edges would be sufficiently broadband to be detectable in the sampled data stream -
>> nope..)
> See attached: I capacitively inject the 1-PPS on the RTL2832 input (either I or Q), not
> the R820(T2) radiofrequency frontend. This allows for timestamping on the rising edge
> of GPS 1-PPS the datastream fetched by the ADCs of the RTL-SDR.
>
> Best, JM
>
>
>
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