Unusual, extraordinary, and momentary signal propagation

My PiAware receiver is located in the WV panhandle, about 33 miles from Dulles International Airport (IAD). Normally, I lose tracks descending into IAD as they pass through about 1,500 feet.

For a few brief seconds yesterday evening, however, I was receiving signals from not 1, but 7 or 8 aircraft either on the ground or just taking off.

This is extraordinary because the terrain here, as well as the distance and other surface obstructions can’t possibly allow that to happen.

Most my RF experience is in the HF or VHF arena; I don’t have enough insight as to what other environmental conditions might permit such unusual propagation of a GHz signal.

Any thoughts?

Sure it wasn’t TIS-B?

Please have a look at these threads:

(1) Sporadic ground reception

(2) Impact of atmospheric/weather conditions on range

(3) Radio Wave Propagation - VHF, UHF & SHF

Not sure what TIS-B is, but I gather it’s transmitted from the ground-based transmitters at airports. If this is the case, then my surprise over receiving signals from Dulles stands.

I’m familiar with tropospheric ducting and scattering, but those typically occur at extremely low elevation angles; and ducting under strong temperature inversions. I wasn’t paying too much attention to the weather conditions at the time (as a recovering AF meteorologist, I still dabble in the craft from time to time), but I don’t recall anything extraordinary about the weather that day. Plus, conditions that could lead to tropospheric ducting would be a little bit more long-lived than the few seconds I was seeing updates.

With the Blue Ridge mountains just 5 miles to the east of me (>1100’ above my elevation along the path), that’s a lot of refraction to get a ground-based signal from IAD to me.

I suppose it’s possible that some short-lived phenomena, coupled with knife-edge diffraction over the Blue Ridge could do that… Maybe even meteor scatter??? :wink:

Regardless, it was (for me) a fascinating and surprising event – it’s fun to see the unexpected and try to puzzle out WHY.

TIS-B is retransmission of traffic info for the benefit of aircraft that can’t see the traffic directly - either because it is using a different transponder type (UAT vs 1090ES) - though that is more correctly ADS-R - or because the transponder is Mode S or A/C only and so you need to be a SSR to (easily) get a position out of it.

The retransmission doesn’t necessarily happen directly from the receiving site, there’s a network of ground stations that exchange data, so it’s possible that you have line of sight to a nearby ground station and there is a TIS-B-requesting aircraft near Dulles that causes traffic in that area to be broadcast via TIS-B.

In the FA offices in Houston it’s fairly common to get sporadic TIS-B for aircraft on the ground at KIAH which can’t be heard directly.

Ah, that makes more sense – It looks like there might be a TIS-B tower near Winchester, VA – certainly within line of sight from here under typical conditions.

Shoot, I thought I had witnessed something remarkable… :confused: But I learned something new, none-the-less.

Thanks for the education! :smiley:

Take a look on the pilotsofamerica.com/forum/sh … hp?t=67935 it used to have a kml file with ADS-R antennas locations.