Does Dark Rain Clouds Affect FM LEO Satellite Communication?

[Johnson Francis]

General information is that hydrometeor or rain particles being quite small, start affecting radio waves only in the microwave range from 3 GHz upwards, typically in the 10 GHz range where plenty of Rain Scatter QSOs are reported. It is also known that higher size of tropical rain particles can lower the minimum frequency for Rain Scatter propagation.

Most would agree that rain clouds will not affect LEO satellite communication in VHF/UHF range. But some of my VU friends have noted that they have poor reception for LEO satellite passes with same elevation when there are plenty of dark rain clouds.

Does anyone out there have similar observations?

73 de Jon, VU2JO

[David Eckhardt]

I can't address LEO satellites, but do have something I'd like to contribute to the statements that rain affects propagation only in the µwave region.

I live in Northern Colorado roughly 8 miles WSW of Loveland, Colorado (not the ski area) and 100 miles as the RF propagates from Colorado Springs.  There is a repeater site on Pikes Peak just above CS.  They have both a 2-meter and 70-cm repeater on the 14,000-foot peak.  There are a "few" hunks of granite and hog backs between me and the repeaters.  I do not have line-of-site from my antennas.  Generally, I can not key them on a clear, cloudless day with 50-watts.  However, with cloud cover, fog, snow, a bit of rain (not heavy), I have no problem working them with 10-watts.  Sure, even the fog or cloud cover allows for forward scatter at both 146 and 44x MHz.  The "conventional knowledge" that fog, light rain, light snow, or cloud cover affects only µwave frequencies is bogus.  I know a lot of our taxpayer monies have gone into the conclusions about moisture and scattering RF energy, but the conclusions of those (biased) studies are in error.

Dave - WØLEV   

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Dave - WØLEV

[Johnson Francis]

Thank you very much Dave for that valuable information. Maybe we will have more such observations in the future.

73 

de Jon, VU2JO

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[Dana Whitlow]

Dave,

Is it possible that the effect you're seeing at 2m and 70cm is not

due the the dark rain clouds in the air, but rather to the effect of

your "granite hunks" being wet down with a high dielectric constant

fluid (water)?

Dana

Kerrville, TX

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[Joseph Johnston]

Can I throw in the fact that I have noticed rain clouds apparently affecting Internet and phone signals. I believe that is the microwave band, but I thought it was interesting

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[Dave New]

We notice in SE Michigan that during the summer when the leaves are on
the trees, that the range of our 430 MHz (70cm) repeaters are reduced.
In the winter, when all the water-bearing leaves are gone, there is a
noticeable improvement in 70cm repeater coverage.

Also, the reason 700-800 MHz is so valued for first responders is
because it penetrates structures so well. First responders,
especially fireperns, need radio coverage inside large structures. If
not, the building owner is required to place repeaters inside the
building to assist, or else they may be denied fire insurance.

73,
-- Dave, N8SBE

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[David Eckhardt]

I can pretty much assure anyone who read my original post on this thread that this is not due to wet - high dielectric constant - rock faces.  Even non-precipitating fog will enable access to both the 2-meter and 70-cm Pikes Peak repeaters.  Absolutely nothing when it's clear.

Another somewhat related experience goes back to when we lived in the forest before the 2012 forest fire.  Frequency is L-Band.  If both the transmitter and receiver were located within the forest, the entire forest acted as the source of the energy on our L-band allocation in FM (not that the mode impacts the results).  The scattering was so effective that over a reasonable path, the RF came from everywhere, much unlike line-of-site.  

Dave - WØLEV   

Dave - WØLEV

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Dave - WØLEV

[John Magliacane]

Hi Jon (and the group).

Maybe it's not the clouds and the rain that are visible to the eye, but the underlying tropospheric conditions producing the clouds and rain that are ALSO affecting the propagation of RF through that same volume of atmosphere.


73 de John. KD2BD

[Jonathan]

In my experience with C band reception, clouds and precipitation had little effect on SNRs, especially when the signal was quite strong. Many times I've had very heavy precipitation and thick clouds, C band reception was only marginally affected, if at all, especially if reflector gain was quite good. It was obviously a different story when snow accumulated on the reflector, as that actually deformed the shape of the parabola electrically. 

I have been told that the edges of tree leaves can scatter C band signals quite well, however, so even an antenna sighted through an opening in trees could see reduced SNR. I know I did. 

With Ku, precipitation absorbs some of the signal, creating the infamous "rain fade", which I regularly experienced with Ku. Many commercial and critical Ku antennas have rain blowers that blow air over feed opening when it rains or snows.

Jonathan

KC3EEY 

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[Marcus D. Leech]

On 11/09/2024 11:12, Jonathan wrote:

In my experience with C band reception, clouds and precipitation had little effect on SNRs, especially when the signal was quite strong. Many times I've had very heavy precipitation and thick clouds, C band reception was only marginally affected, if at all, especially if reflector gain was quite good. It was obviously a different story when snow accumulated on the reflector, as that actually deformed the shape of the parabola electrically. 

I have been told that the edges of tree leaves can scatter C band signals quite well, however, so even an antenna sighted through an opening in trees could see reduced SNR. I know I did. 

With Ku, precipitation absorbs some of the signal, creating the infamous "rain fade", which I regularly experienced with Ku. Many commercial and critical Ku antennas have rain blowers that blow air over feed opening when it rains or snows.

Jonathan

KC3EEY

I've seen rain effects even at L-band.   NOAA estimates transmissivity during moderate rain at 96% in L-band.   That translates
  to an additional few K of path noise. Which for some types of radio-science is devastating!  The main problem being that it's
  *variable* on time-scales that are important to, for example, radio astronomy.  Which is one of the many reasons that
  professional radio observatories are often located in places where precipitation events are low.

Now, of course, we operate within a radome, so, it's important to distinguish between "rain along the line of site", and rain
  forming sheets on the radome.  Two different mechanisms, with "forming sheets on the radome" being the dominant effect.

On Wed, Sep 11, 2024 at 10:08 AM 'John Magliacane' via HamSCI <ham...@googlegroups.com> wrote:

Hi Jon (and the group).

Maybe it's not the clouds and the rain that are visible to the eye, but the underlying tropospheric conditions producing the clouds and rain that are ALSO affecting the propagation of RF through that same volume of atmosphere.


73 de John. KD2BD

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[Dana Whitlow]

At the Arecibo observatory where I worked until

the end of 2016, rain and clouds had very little

effect through at least S-band.  The director once

told me that this appears to place the notion of

"water resonance" (re microwave ovens) in the

myth category.

However, the system noise level rose significantly

at X-band, and if IIRC, the effect persisted even

after the rain stopped, until the dish surface had

dried off pretty well.  The upper end of our receiver

coverage ended around 9 or 9.5 GHz.

The system noise temperature, (Tsys), was routinely

checked for all the feeds and receivers every day,

and even more if problems were suspected.  Depending

on frequency, Tsys ran around 30(ish) K for most of the

microwave range we covered.  This included contributions

from the technical equipment (receivers, WG losses,

feeds, etc), as well as contributions from feed footprint

spillover onto warm terrain, blackbody radiation from

absorption from stuff in the warm atmosphere, and

(occasionally & amusingly) inadvertently a celestial source

in the beam.  This 30K figure was always a painful number

to contemplate, given that our LNAs had input referred

noise temps down around 3K.

Arecibo had no domes pe se, although most feeds &

receivers had a dielectric window in the WG path to hold

in the vacuum in the cryogenic dewars, which were in

most cases cooled to about 15K.  But these windows

were always very well protected from precipitation (rain

only).  We had our share of rain, but obviously no snow,

and I don't remember ever seeing any hail.

Dana

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[Johnson Francis]

Glad to see an active discussion on the topic, which has reached beyond LEO satellites. May be someone with in-depth knowledge in the field, possibly from an academic institution, can combine all the observations and give it as a consensus opinion of HamSCI. That will be a baseline information for potential future formal studies.

73 de Jon, VU2JO

[Marcus D. Leech]

On 11/09/2024 11:49, Dana Whitlow wrote:

At the Arecibo observatory where I worked until

the end of 2016, rain and clouds had very little

effect through at least S-band.  The director once

told me that this appears to place the notion of

"water resonance" (re microwave ovens) in the

myth category.

Absolutely.  Microwave ovens take advantage of dielectric loss, and indeed, early microwave
  ovens were in X-band.

Your experience differs from mine.  I've seen situations where the dish was fairly low, and while it wasn't raining
  *over the site*, it was pointing through clouds that were raining.  We could see fluctuations in the range of 500mK
  until the weather we were looking through cleared up.

With a larger dish (like Arecibo), your gain in terms of K/Jy will be large.  Like 10-15K/Jy of flux.   So, tropospheric effects
  that might be up to 1K in magnitude, aren't going to create large problems for you, because even for sources of a few Jy,
  that source will be creating significantly more noise temperature increase at your feed than the rain.

But as you go to smaller dishes, the effective increase in Tsys (and variability) from tropospheric effects becomes more
  pronounced--it becomes a larger fraction of what your feed is seeing.

That's my 5-minute analysis....

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[David Eckhardt]

Lot's of conjecture.  Little real data addressing low VHF into UHF other than what I emailed out to the group.  It goes counter to the conclusions of Uncle who spent millions of tax payer dollars "researching" the subject.  Yes, tropospheric conditions may well contribute to the forward scatter.  But no real data to prove one hypothesis over another.  Science requires data to prove and/or disprove hypotheses. 

Dave - WØLEV 

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Dave - WØLEV

[Dana Whitlow]

Thanks, Marcus.

'Tis a pity, however; the Arecibo telescope is not so

large any more.  There is still a 12m telescope which

is apparently in working order, and that figures in with

people wondering what to do with the site.  One thing

appears certain, however, which is that the big dish

will never be rebuilt or replaced.

Dana

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[Gerald Creager]

We've seen these phenomena before. The forest scintillation is the easiest one to explain. A LOT of the pine needles are approximating odd multiples ofa quarter-wave at L-band AND are filled with conductive liquid, water and pine sap. The net effect is a bunch of tuned passive radiators serving to scatter the incident signals. 

Ducting is a little more complicated. Temperature, pressure and humidity will change the index of refraction of the atmosphere and are frequency dependent. They're also frequently height-dependent. Those that provide a pathway at/near the surface can provide near magical paths, evidenced by a conversation I had once on 2M from central Texas to Florida via a 2-way tropospheric duct, just a little bit farther than Longmont (or so) to Cheyenne Mountain. The Florida repeater completely overwhelmed the local repeater, and if I recall correctly (this was while I was an undergrad) I wasn't even keying the local repeater a couple of miles away. Although ducting at microwave is possible, it's much harder to achieve. Note that the ducting phenomenon does NOT depend on wet surface dielectric effects at all. 

High-angle takeoff signalling, such as talking to the LEOs will generally bypass the ducts. But atmospheric scintillation and high electron-count secondary to solar events can affect things, AND, I've seen significant thunderstorms with a sufficient water content (or especially with significant hail) affect direct-path UHF. It'll still affect VHF but the effect will be more muted at VHF. One effort I worked on a long time ago (Hi, Phil!) was the use of GPS to determine columnar water vapor in the atmosphere by passive observation of multiple GPS signals. My efforts were based on earlier efforts by Ware and Rocken. Some meteorological deterministic models today still assimilate those rather sparse water vapor observations, even despite their limitations: The basis of these measurements is in looking at the signal delay introduced by water vapor, among other confounders, then teasing out just the water vapor element and quantifying it. Most modern multi-frequency surveying hardware/software looking at carrier-phase measurement can drop these "nuisance" variables out for further analysis.

That said, water vapor can delay RF signals and that's an element of the refractive change process seen in ducting, but simple delay is really frequency-dependent. Then again, if you get sufficient water in the way (as noted, a BIG thunderstorm with that greenish-black appearance that normally portends changes in the space-time continuum or bad tornadoes), you really can attenuate the fool out of your signal. And we really haven't even talked about the effect of solar events on total electron count or scintillation (although I mentioned it earlier) on scattering, multipath, and apparent signal attenuation. 

73

Gerry N5JXS

Maj Gerry Creager
Deputy Senior Program Manager. Health Services
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[John Ackermann N8UR]

Ducting due to temperature inversions is indeed magical. I grew up
along Lake Michigan in northeastern Wisconsin. Basically you could go
180 miles straight down the western shore of the lake to Milwaukee, and
another 80 miles to Chicago. Green Bay was the nearest TV station town,
at about 70 miles bearing a few degrees west Milwaukee/Chicago, but
within the beamwidth of even a big TV antenna. Three stories come out
of that setup:

1. I had a summer job where I drove to work along the lake every
morning at around 7:30 local time, and very often I could work Milwaukee
2M repeaters from my car, dead full quieting.

2. I had a contact on 220 FM simplex with a ham who was using a
handheld while standing in front of a north-facing window in his
upper-floor apartment in (I think) the Hancock Building. I had a 7
element yagi up about 40 feet at my end.

3. Chicago and Green Bay each had TV channels 2, 5, and 11. About half
the time during the summer evening hours, there was co-channel
interference on channel 2, often making Green Bay unwatchable from our
house. Channel 5 was interfered with maybe 10% of the time. Only once
or twice did I see channel 11 interference -- and one of those was the
evening I worked the guy with the handheld.

In the '60s the two channel 2 stations tried an experiment using
lab-grade OCXOs compared against WWVB via strip recorder to maintain a
fixed offset frequency -- IIRC, it was 10 kHz -- that was intended to
minimize the beat pattern. It didn't work very well and the experiment
was abandoned after a couple of years.

The cause of all the fun was an inversion caused by the difference in
land and water temperatures along the west shore of the lake. That
created a duct a few miles wide that was in place especially in the
morning, but often in the evening, during the summer months.

73,
John
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[Gerry Creager]

Oh, it’s not really magic but it sure seems like it sometimes. Somewhere, I’ve got a python package that takes the output from one of the weather models I’ve worked on and produces graphics depicting the ducting patterns. I need to resurrect that, and animate it in 3d on something like Google Earth. Or Science on a Sphere.

73

N5JXS

Gerry Creager N5JXS

It's kind of fun to do the impossible. -- Walt Disney

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