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!