>10% drop in flights during rain: normal or antenna problem?

FlightAware ADS-B Flight Tracking
1

Hi all,

How strongly are ADS-B signals attenuated by rain and other inclement weather?

I’ve noticed 10-20% drops in the number of flights and position reports observed when it’s rainy (and usually on the day following the rain) at one of my feeder sites and I’m trying to figure out if the reduction is due to ATC routing traffic away from the area, a leak in the antenna, or rain attenuation of the signals. My site sees enough air traffic that counting statistics should be reasonably good (~6,000 flights and ~370,000 position reports per day) and week-to-week air traffic is usually quite consistent.

My thoughts are:

  • ATC re-routing is possible, but I don’t know how much of an effect this would have: the local area gets only a little bit of traffic but the site is located at the top of a 3000ft foothill and has clear line of sight in many directions out to ~300nm. The typical coverage area includes areas with lots of air traffic that are far enough away to avoid local weather patterns. In one particular sector (NNE at 200-250nm) there’s a ~50% drop in position reports seen on rainy days compared to the same day a week earlier. Closer traffic traffic shows a smaller reduction (~15% or so at 50-100nm and 10% at <50nm)

  • A leak is possible, but I took reasonable precautions against the weather: the antenna is a stock FlightAware antenna mounted outdoors. The cable uses a compression-fit type-N connector. I applied liberal amounts of silicone grease to the connectors, threads, and o-ring, then wrapped the connection (including several inches up the base of the antenna and down the cable, past the compression fit) with self-amalgamating tape. The antenna tube didn’t have cracks, and the cap at the top was secure. If there was a leak, the water trapped in the antenna might take a day or two to evaporate, which would explain the traffic increasing back to normal over the day or two following the rain.

Anyone have any insight into what might be causing the reduction in observed traffic? I tried comparing my observations with those of other sites nearby, but other sites in the area don’t have the same long-distance coverage so those sites don’t show the same changes in traffic.

Thanks in advance!
-Pete

2

Pete,
I see about the same drop in stats during rainy periode (abt 10%). I contribute this to a row of trees at 10m from the antenna. I guess the leaves get wet and/or the tree sucks up the moisture (don’t really know how fast that works - must have slept through the biology class way back)
/paul

3

Actually, from my experience, after the rain, the reception is quite better then average. I think this has two reasons:

  1. The air is more ‘clean’. The raindrops tend to form around the dust particles, so after after the rain, all the dust and dirt is ‘washed out’.

  2. The wet surface will reflect the radio signals better than the dry surface. So you will get ‘more signals’ then usual. This means, even when they come from reflections, it can result in slightly better numbers.

So, if you have poor results after the rain, it is highly possible that there are some humidity issues present within your antenna/cable/connectors. Self-amalgamating tape, normally is perfect. Silicone actually, I would never use for this purpose, maybe graphite, but I don’t think that any kind of grease is necessary or good here. Never used any of it with HF-connectors.

During the rain, worse reception results are completely normal @1GHz. Just have a look at satellite TV @10GHz. Heavy rain, and you will see no more TV program with a standard size dish.

Regards,

Kabuse

4

Could be rain fade. Satellite TV suffers from this to a much larger extent due to the weaker signals.

Living in Florida we have extremely heavy rainstorms in the afternoons during the summer and I regularly see a drop on range bit definitely see rerouting of traffic around the storms which affects the qty of planes observed.

5

Thanks for the info about rain fade: I had no idea it’d be that significant at 1GHz – I normally associate it with heavy rain at satellite frequencies (though I don’t have a satellite receiver).

As for the grease, I didn’t flood the connector: I just put a light wipe of silicone grease on the center pin of the N connector, a bit on the o-ring, and a bit more on the threads to ensure a watertight joint and to allow me to easily separate things later if necessary. I’ve found silicone grease works well at this, and I’ve not had any problems with reliability of connections even after years of service in low-power connections like receive-only antennas and cables, automotive connections, etc. Your mileage may vary, of course.

After looking at weather maps for the rainy days in question and the day or two following, it appears that the issue may be a combination of (a) rain fade and (b) larger-scale weather affecting flights hundreds of miles away. After the weather moves through the local area, local flights return to normal but the weather is still affecting more distant flights. So, it seems that the mystery is, if not solved, at least better understood.

6

A suggestion (and one I’ve made before).

It’s hard to tell what helps and what hurts in optimizing station performance, because: the number of birds in the air changes all the time, from day to day. Painful fact.

I work around this by selecting a station in my immediate area (Nicely provided as Nearby ADS-B sites on my status page) to use as a reference station.

To be useful as a reference, such a station should: (1) be in a similar topology, suggesting a similar pickup pattern, (2) have similar and consistent numbers on a day-to day basis, (3) have been around for a while, with those visible daily numbers in the past looking consistent.

I track my daily numbers, and the reference station’s daily numbers. The spreadsheet I use shows me percentage comparisons as well as raw numbers.

If I’m not changing things, those percentages stay within +/- 2% to 3%, even over the weekends (when numbers in our area at least drop a good bit) and other traffic changes induced by weather, holidays, smoke from fires (a sad but real issue in Northern California).

Now when I collect numbers and enter them into the spreadsheet, I don’t freak out because my numbers dropped from the previous day – I can look at the percentage comparison with my reference and see that the percentages are about the same; the reference saw the same relative drop.

Or, adjusting SDR gain levels, and being able to get a reasonably good idea of what a gain change does (on a daily basis), being able to pretty much ignore those daily fluctuations in traffic.

The major risks of such an approach are: (1) my reference station goes away and I have to find another, or (2) my reference station starts tweaking up their works as well, no longer giving me that stable baseline.

If you’re already collecting data, collect a little more – comparing your station’s performance against a reference works. Doing that will help you separate the effects from changes you’re making from changes in traffic.

bob k6rtm

7

Bob,

I like the idea of a spreadsheet. I’m going to have the wife work on something like that. Maybe do weekly ave. and so on. She is the smart one in that field.

8

Good to know I’m not the only one. ~3000/day but the last 2 days of rain in NE Florida really hurt–down 500 on day 1 and around 800 on day 2 (arguably the more rainy and stormy of the 2 days) Picked up nicely today however, so I guess it’s a hazard Florida Living just has to deal with :slight_smile:

PS. Love the idea of the “reference Station” I’m lucky to have several in the general vicinity!