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

[Bob Camp]

Hi

In this day and age, there is no absolute need to feed in something as broadband as a 1 pps edge. Yes, it is easy to just cable it “over there”. An alternative would be some sort of fairly simple phase modulated signal that is much more narrowband. Pick one you can generate with a couple chips. As you suggest, do the “correlation stuff” and come up with a very accurate time estimate of that signal. The advantage would be: You can take the “time signal” out with simple frequency domain filtering. 

More or less:

Time signal runs from 8 to 12 MHz (or maybe a lot more narrowband than that ….) 

Digital high pass shoves time signal “over there” and it gets processed. 

Loran is < 1 MHz

Digital low pass shoves loran “over here” than then it’s processed. 

No need to do the fiddly “subtract an exact image” stuff. 

Yes, there are a lot of fiddly details to consider. Picking 100 KHz x 100 for your time tone carrier might not be a great idea. You would need to do a bit of work to come up with the ideal signal. 

Bob


> On Sep 28, 2023, at 12:42 PM, Lux, Jim via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 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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