[time-nuts] Re: As there a hobby application of precise time or frequency measurement except for being a time nut?

Fri Apr 25 15:13:59 UTC 2025

On 2025-04-25 07:34, Paul Boven via time-nuts wrote:
> As Jim (and others) have mentioned: Radio Astronomy.
> 
> I'm a volunteer at the historic 25m radio astronomy dish in Dwingeloo, 
> the Netherlands, and I would consider myself our local 'time nut', 
> amongst the many hats I wear in this group.
> 
> Some of the things we do which require accurate time or frequency 
> signals:
> 

Dwingeloo also participated recently in planetary radar, receiving 
returns from Venus, which also requires accurate station time.

https://www.camras.nl/en/blog/2025/first-venus-bounce-with-the-dwingeloo-telescope/

> Hydrogen line observing - the Doppler shift of the 21cm line at 
> 1420.40575 MHz can be used to model the spiral arms of our Milky Way. 
> We also observe the redshift to other galaxies, and other spectral 
> lines such as the OH lines. Accuracy requirements are quite modest, on 
> the order of a PPM.
> 
> Satellite tracking - Doppler information is crucial to tracking your 
> spacecraft. We were an official NASA (volunteer!) ground station during 
> the ARTEMIS I mission, and are planning to join the future missions as 
> well. The accuracy requirement here is much higher, ideally sub-Hz at 
> 2.5 GHz.
> 
> Pulsar timing - measuring the time of arrival of the pulsar signal 
> allows you to determine many physical parameters, such as the size of 
> the Earth's orbit, a very precise location of the pulsar in the sky, 
> and the slowdown of the pulsar as it loses its energy. Here the 
> requirement is very good stability over many years.
> 
> Interferometry: We've participated in VLBI observations, together with 
> other professional dishes in the European VLBI network, and got great 
> fringes. For us, this requires phase stability at up to 5 GHz, for 
> periods of up to 15 minutes (after that, the ionosphere starts to 
> destroy coherence anyway).

VLBI famously requires a very good local oscillator - you need phase 
stability to << a wavelength over the VLBI integration interval 
(typically, 1 second). At cm wavelengths that means something like 
10^-13 at one second is the goal, which
is tough to do with cell or beam atomic clocks, and which is why most 
VLBI networks use hydrogen masers as their frequency standards.

I have done X band VLBI with a Rb standard, and the data were OK, but 
with a noticeable increase in short term frequency noise.

However, not only do the atmosphere (water vapor!) and ionosphere cause 
a loss of coherence at periods of tens of minutes (at cm wavelengths - 
it's tens of seconds at 1 mm) but longer term clock instabilities can be 
estimated from the VLBI data itself.

That has led to interest in using things like active optical fibers 
(without frequency stabilization) as local oscillators.

Regards
Marshall Eubanks

> 
> We're putting up a new 3m dish, and are planning to do interferometry 
> and holography of our big dish this way.
> 
> We have several Rubidium clocks, but mostly we use a dark fiber White 
> Rabbit link of 35km length, which is connected to a 'nearby' Hydrogen 
> maser at the WSRT radio telescope. We ended up trenching and splicing 
> the last 250 m of fiber ourselves, in 2018.
> 
> Regards, Paul Boven - 73 de PE1NUT.
> 
> On 4/11/25 19:26, Jim Lux via time-nuts wrote:
>> 
>> 
>> 
>> 
>> 
>> A lot depends on what you consider "precise" of course...
>> 
>> Radio astronomy - more and more people (yeah, probably < 1/2 dozen, 
>> but..) are setting up interferometers and precise time knowledge is 
>> important.
>>   Radio amateurs on microwave frequencies with narrow band signals 
>> need good frequency control (1ppm at 10 GHz is wildly insufficient - 
>> you'd like to be within 10 Hz - 1E-9)
>> 
>> There seems to be a bunch of people doing direction finding with a 
>> distributed system (perhaps collaborative drone flight) - it's unclear 
>> what their requirements are.
>> 
>> There is a hobby radar community with bistatic radars - both at low 
>> frequencies (chirp sounders) and at microwave frequencies. The 
>> performance of the radar depends on both long term stability (for SAR) 
>> and closein (phase noise).
>> 
>> I know a guy with a LTE base station he built in his house - granted 
>> he's not putting up multiples, so he doesn't need precise timing 
>> (yet).
>> 
>> I suspect none of these are hundreds or thousands of people, but they 
>> all have some need to understand time and frequency to a "more than 
>> you get with a TinySA or oscilloscope" kind of performance.
>> 
>> 
>> 
>> On Fri, 11 Apr 2025 12:46:12 +0200, Erik Kaashoek via time-nuts 
>> <time-nuts at lists.febo.com> wrote:
>> 
>> Maybe the subject line is a bit clickbait but this is a serious 
>> question.
>> Hobby VNA are used to measure impedance for antenna, amplifiers,
>> filters, PCB's and cables. Many applications for a VNA outside the
>> professional world.
>> Hobby spectrum analyzers are user for hunting RFI, measuring harmonics
>> of active devices, assessing what part of the spectrum is still
>> available for wireless devices, etc... Again many applications.
>> But for time and frequency measurement the situation seems to be
>> different. What are hobby applications for accurate assessment of
>> stability, time or frequency?
>> For radio amateurs that operate in the GHz bands the accurate 
>> assessment
>> of the frequencies of their generators is such an application but what
>> other applications do exist and what are their requirements regarding
>> accuracy?
>> I'm purposely excluding the applications where the accuracy is the 
>> goal
>> instead of some usage for accuracy.
>> Any input is welcome.
>> Erik.
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