[ldm-users] Dropped packets: what's the deal?

OK, so I've been getting some private messages that, with the new NOAAport signal, people are still dropping significant numbers of packets. What's going on?

So here's the deal: Broadcasting with a modulation coding of 2/3 16PSK is
a bear. Most engineers I know would use another word that is anything
but family-friendly to describe it. With the current legacy broadcast,
errors were tolerated more. Now, as NOAA and some of you found out, any
minor issues that can be ignored with the old broadcast...can't be ignored
with the new one. Let me explain...

If you had an issue with Terrestrial Interference (TI) that is minor or too small to notice with the legacy feed, it could be an issue now. I have found that the new broadcast is very intolerant to rogue signals or interference from anything from power lines, sodium lamps, to cell phone tower issues to goodness knows what else.

Since Patrick Francis has volunteered his information, and he's is experiencing some packet loss, let's look at his data:


Look at his signal strength. 78%! He's rocking it! Anything over 60% is good. And, look at the signal level. -36 dBm! That's excellent!


Look at his Carrier to Noise: 11.1.

Ruh roh!

Here's the ultimate standard the National Weather Service is looking for:
a Carrier to Noise, aka Signal to Noise of 17:1 or higher. And his
is 11.1:1 (the Novra just displays the number 11.1, what they mean is 11.1:1). So what?

Well, in a perfect world, in an idealistic realm for perfect equipment and nothing else is wrong with no interference, a C/N of 9:1 should give you
perfect data. But since we live with this thing called the sun causing
noise, dishes are imperfectly pointed, and mankind and space generates a
lot of terrestrial interference. So, the realistic C/N is considerably
higher than that for you to have a 100% packet capture. I would dare say
it's above 13:1. Most importantly: the NWS C/N standard is 17:1. Some NWS
offices have gotten theirs above 28:1...I assume those are out in the
middle of nowhere.

What I found is that when we "peaked" our dish, we peaked it for C/N,
and NOT signal strength. When that happened, we got a C/N of 19 for the
"legacy" NOAAport, and nearly 18 for the current one. As a result, our
signal strength dropped from 82% (peaking for signal strength) to 76%.
Our signal level, a much better indicator, went from -35 dBm to -37 dBm.
But...when we were at 82% signal strength, our CN was around 10 or 11, and we dropped packets like crazy, even under the legacy broadcast. Now? 100% packet captures with our C/N of nearly 18 on the new NOAAport broadcast.

So, here's how can you reduce TI, or the possibility of it:

Peak dish to C/N, not signal strength

Use RG-11 coax, not quad shielded RG-6, if coax length is longer than 50'
between the receiver and the LNB. This helped us considerably with a 75'
long run. It also improved our signal level by 3 dBm, a nearly 50%

Make sure your equipment has all the specs NWS requires: dish size of 3.8
meters is minimum (in the southern U.S. and near the borders, it's 4.5
meters or larger); LNB must have drift of +/-5 kHz, noise level must be
20K at most.

All of these specs assumes a sunny day. It will be lower with clouds/rain.

That's something for you to think about and chew on today as you switch
over to the new NOAAport broadcast. And, I like peanut butter.


Gilbert Sebenste                                                    ********
(My opinions only!)                                                  ******
Staff Meteorologist, Northern Illinois University                      ****
E-mail: sebenste@xxxxxxxxxxxxxxxxxxxxx                                  ***
web: http://weather.admin.niu.edu                                      **
Twitter: http://www.twitter.com/NIU_Weather                            **
Facebook: http://www.facebook.com/niu.weather                           *

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