Impacts of signal reflections, reflecting off cliff?


#1

Hi guys,

Finally got my dad to install a rig on the south coast of the Isle of Wight here in the UK…

He’s ordered the bits, I’ll help him set it up in a few weeks.

Question is, he’s got a good view out over the english channel, but a massive cliff behind him to the north.

Of course, dont expect to receive anything from the North (have run the usual - HeyWhatsThat model), but what (if any) impacts to be had from signals from the south, being reflected off the cliff to the north?

https://www.heywhatsthat.com/?view=LX9TKT1G


#2

Multipath might be an issue for mlat, but not really a big deal for regular ADS-B


#3

Thanks - be interesting to see when we get him online; it’s taken literally years to get him interested… :wink:


#4
  1. You know that the link to that map reveals your address when zoomed in? You could just take a screen shoot of that, if you care for privacy.
  2. Change the altitude of the planes to 38000 feet. You will probably “see” further above that cliff, not very far tough.
  3. If you want to provide a good MLAT, you can always put a metallic reflector between the antenna and the cliff, to block the reflections. It needs to be located at a short distance of the antenna (I would guess lambda/2 = 137.5 mm, but you can test it later).

#5

Thanks Sonic,

1… Yup, thats why it’s not his exact address… :wink:

3… Good idea, I’ll certainly have a play with that when the station’s up.

Just so I know, what “symptoms” would I see, that indicate reflection was indeed causing an issue, no MLAT lock for example?


#6

The reflection from the rockface should be quite a bit weaker than the original. It will probably drown in noise is my guess as the surface of the rock is irregular anyway.

Only a plane surface makes a good reflector as otherwise you will have lots of interference. Even if the rockface would be plane it would probably still make a very poor reflector.

A metal surface close to the antenna is tricky though. If you choose the distance wrong the radio waves will cancel each other out.
If you were to block the complete rockface the plate would have to encompass a rather large field of view.
That could mean depending on where the signal comes from the waves will either enhance or cancel out with the reflection.

So just start with a bog standard antenna and don’t worry about reflections i’m not quite sure if you would receive any reflected messages anyway.

One of the messages from the planes is at least 56 microseconds long anyway, in that time radio waves travel 15km anyway, so a reflected message would overlap with the original message anyway.

The worst case scenario with a highly reflective rockface therefore is that the message is not received at all.
If it is received it will be the directly traveling message as the reflected one is a lot weaker.

Hope this makes sense to you :slight_smile:


#7

If the plate is close enough, it will block a big angle. And yes, the distance will have to be tested, in the similar way a Yagi reflector would be positioned.