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<p>There's a bit to unpack here, so see below.<br>
</p>
<div class="moz-cite-prefix">On 25/11/2024 6:59 am, Colin_Higbie
wrote:<span style="white-space: pre-wrap">
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<pre wrap="" class="moz-quote-pre">I know we see significant rain fade with geostationary satellites, which I have long assumed is at least in part because from our latitude (around 44 degrees north), the angle to a geostationary satellite is so small that it's going nearly horizontally through hundreds of miles of clouds. In contrast, Starlink satellites, nearly overhead, punch almost vertically straight through the clouds. This means it has far fewer water droplets and clouds to pass through to reach a satellite in the same weather. I assume this is at least partially responsible for why Starlink is VASTLY more reliable at holding a connection in bad weather than geostationary links.
First, is that correct?</pre>
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<p>Partly. Clouds don't reach more than about 10 km in normal
circumstances, with thunderstorms sometimes going up to about 20
km. At 44 deg north, you're just over 3000 km above the equatorial
plane (ballpark), and the geostationary orbit sits around 39,000
km away from the point where your location projects onto the
equatorial plane. Assume a GEO sat on the same longitude as you
for a start. Do an arctan on 3,000/39,000 and you get the angle
between a geostationary sat's line to you and the equatorial
plane, about 4.4 deg. Deduct this from 90 deg and you get the
angle between the line between you and the sat and the projection
from your location to the equatorial plane. So about 85.6 deg. The
angle between the tangential surface plane at your location and
the line of projection onto the equatorial plane is 90 deg minus
your latitude, so 46 deg. Subtract that from the 85.6 deg and you
get the highest elevation of the geostationary arc as viewed from
your location. Makes 39.6 deg. </p>
<p>Assume we're working with a 20 km path through a thunderstorm
cloud straight up. Now going at an elevation of 39.6 deg instead
of 90 deg through a 20 km cloud layer results in a path of under
32 km. Not hundreds of miles. Now that path stretches a bit
obviously if you are aiming your antenna at satellites that are
not on the same longitude as you, and that's where you could get
into the hundreds of miles potentially if they're at a longitude
that's far off yours.</p>
<p>Noting here that Starlink talks upwards of an elevation of 25
degrees - so there's at least some overlap between LEO and GEO
domain here in terms of potential path lengths through rain
clouds.</p>
<p>The reason why GEO connections struggle compared to LEO is the
path loss over the distance - a factor of about 1000 in terms of
power, order of magnitude, over Starlink. This is why GEO either
needs very powerful transmitters in space (sat TV) or big antennas
with high gain on the ground (everything else pretty much). High
gain antennas also tend to be pretty sensitive in terms of
directionality. If you can't track electronically as Dishys do,
then anything (wind, atmospheric refraction) that knocks the beam
off course means lower signal at the receiving end and more
trouble keeping connected.<span style="white-space: pre-wrap">
</span></p>
<p><span style="white-space: pre-wrap">Basically, rain fade gets worse the higher you go in frequency - so Ka is worse than Ku, no matter whether it's a GEO or a LEO link. That's why C band was more popular than Ku until it filled up. And now you have Ku being more popular than Ka but it's also filling up.
</span><span style="white-space: pre-wrap">
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<pre wrap="" class="moz-quote-pre">Second, does this have any bearing on your point about Ka not being good in the rain? I.e., maybe it's not as good in the rain as Ku, but because of the angle of communication and therefore reduced signal attenuation, it can still get through typical cloud cover and moderate rain, still "good enough"?</pre>
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<p>The standard technical response to rain fade (regardless of band)
is to change modulation when it gets bad. So you trade bit rate
against robustness by downgrading from, say 64QAM to 16QAM or
somesuch. <span style="white-space: pre-wrap">
</span></p>
<p><span style="white-space: pre-wrap">Or you can up transmit power.
</span></p>
<span style="white-space: pre-wrap">
</span>
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<pre wrap="" class="moz-quote-pre">While obviously none of us want to lose connectivity or signal in the rain, I'd rather drop to some fractional capacity (whatever fits on Ku) during occasional bad storms, if it meant that the rest of the time, I could have much more bandwidth with added Ka support. But I acknowledge that I don't know how all these factors interact for final results.</pre>
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<p>The main issue with "added Ka support" is that Dishy isn't
dual-band, and I'm not sure if or when we'll ever see a Ka-band
Dishy. Period.</p>
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<pre wrap="" class="moz-quote-pre">
</pre>
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<pre class="moz-signature" cols="72">--
****************************************************************
Dr. Ulrich Speidel
School of Computer Science
Room 303S.594 (City Campus)
The University of Auckland
<a class="moz-txt-link-abbreviated" href="mailto:u.speidel@auckland.ac.nz">u.speidel@auckland.ac.nz</a>
<a class="moz-txt-link-freetext" href="http://www.cs.auckland.ac.nz/~ulrich/">http://www.cs.auckland.ac.nz/~ulrich/</a>
****************************************************************
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