Is ADS-B truly line of sight only?

[CYYZGUY]

The reason I am asking is this.  Right now I have my antenna, sitting on a tripod in my back yard surrounded by houses.  I am still picking up some aircraft 60 to 70 Nm away from me, some being directly behind my house...so its physically impossible for the line of sight signal to hit my antenna.   Is it possible for ADS-B signals to bounce off of objects?


regards,

Jason

[Runway 31]

Hi Jason,

Yes I would suggest that any radio signal at specific wavelengths can bounce off structures and even clouds in some cases.  Google radio wave propagation and see what you come up with.

There are people on here with good radio experience who will I am sure be able to give a better answer on how it all works.

Alan

[CYYZGUY]

Thats what I figured Alan but was not too sure.

I can't wait until Saturday (2:30PM) eastern time when the tripod installer shows up to mount my antenna on the roof.  I have been waiting too long!

Jason

[RodBearden]

When I stayed at a Heathrow hotel a few years ago my RadarBox view was line of sight except for a pronounced lobe behind me that must have been caused by a reflection off a large hangar door opposite me.

Good luck with the new mount.

[CoastGuardJon]

Hi Jason,
As the Frequency gets higher , the Wavelength gets shorter ADSB works 1090MHz/27.5cms - microwave - which doesn't pass through much thicker than cloud, so yes 109MHz is pure line of sight, BUT you can get ducting where a signal can get "funnelled" through between the different atmospheric layers.  As a matter of interest, if you watch the sunset (especially over a sea horizon) as the sun appears to touch the sea, the sun has actually set and is below the horizon out of sight, the image of the sun is ducted through the atmosphere due to light refraction, the same can happen with RF.   In the mid 70s, I was caught up in the early days of CB, using illegal rigs on AM and SSB, conversations with USA and other more exotic lands were frequent due there being a "lift" on, when the signal would get bounced off one layer or another in the atmosphere.    So, in summary, yes 1090MHz is line of sight, but can be reflected in certain circumstances - HTH Jon

[dudbaker]

Hello

A few points to add.

1.  Tropospheric ducting or lift conditions are not common so 1090mhz is not going to do much bouncing off the atmosphere.
2.  Hotels tend to have reinforced concrete walls.  Steel bars in the concrete which tends to block radio signals
3.  1090mhz is direct wave or line of sight.  27 MHz is in the HF band which is mainly ground wave and sky wave.  Travels along the ground or bounces off the ionosphere.  This is what gives long range rather than a lift.

[CoastGuardJon]

Yes, good points Dudley which I should've made clearer.   I believe the newer heat retaining/reflecting glass such as Pilkington K also block and reflect RF - any clarification on that one anyone?   Under certain weather conditions I used to pick up US taxis and other services in the 30 - 40MHz and also get foreign transmissions in the VHF bands 100 - 110MHZ region, but very few "strangers" above that, so wouldn't expect to find anything around 1090.   Just hope I haven't muddied the issues Jason!

[dudbaker]

Hello

Sorry to press the point but it is important.

What happens at 30 and 40 Mhz which allows transatlantic comms is about ionosphere and sunspots and is known as skip.

What happens at higher frequencies is about weather; particularly atmospheric pressure and is known as lift.

There is another one call sporadic E when 100 MHz will bounce off the ionosphere E layer but this is rare and may only last a few minutes.

[CYYZGUY]

All points taken into consideration for sure...

But as we speak, I have an airbus about 10 Nm to the front of my house...where my antenna is in the back of the house in the back yard, completely blocked from line of sight, and I am still picking him up moving closer and closer..

So for me, there still is no explanation if according to what dudbaker is true....

cheers,

Jason

[neroon79]

Light is (or to be more precise: it can be viewed as) an electromagnetic wave just like the 1090MHz signals. All electromagnetic waves be subject to the following effects:

- absorpstion (in this case the wave energy is converted to lowest energy level: heat radiation)

- reflection/refraction (wave "bounces" off the surface, usually with partial absorption). Especially metallized Windows, sheet metal or metal-foils are very good surfaces for reflection. Behind this survaces the reception is close to zero, which is logical as the bulk of the as signal was reflected. Reflection takes place on stone-walls too, but here the part of absorption is much higher. Short: the ratio between absorption and reflection depends on the material of the surface and the angel in wich the wave "hits" the surface. Refraction and reflection does always happen at the media-interface of different (in case of light optical) densitiy. In gases or liquids density variation is also caused by temperatur, ionisation or composition of the media.

- diffraction (can/is generated by electromagnetic fields or gravitation. Simply spoken: This is why we have a core shadow and a half shadow behind earth). The diffraction will not be a huge influence on the 1090MHz signal, but it may allow you to receive an aiframe a few miles longer although its already below the horizon.

I've to add some details about steel in concrete blocking (absorbing) radio signals: It's not only the steel, but the conctrete itself. I live in an 100 year old house made of full-density bricks (almost no air holes). In the "inner-core" I have almost NULL radio reception of any kind (Radio, TV, mobile phone or in our case: ADS-B). Next Mobile-Phone antenna would be in line of sight if there weren't some (steel-free) walls in between. Nevertheless very weak mobile phone reception. If the steel would be the only part of blocking the radio signals I shall have good reception. I remeber that there is a relation between the mesh density that a metal-mesh must have to block a signal of a particular wave length (1090MHz ~ 27cm). At lower Frequencies it's as far as I remember 1/10 of wave length. If it's still valid at 1090MHz the steel-mesh must have a density of about 3 cm/mesh-hole. As far as I know the mesh density of steel mesh in concrete is much lower.

Ingo

[Kenny]

RF frequency can bounce.  It is up to frequency and type of surface.  Good example is our backyard satellite dish.  The old big dish was 4GHz and the smaller one (Ku Band) is 12 GHz.  The LNB is at the focal point of the dish for maximum signal strength from signal bouncing from the dish.

[dudbaker]

Hi

Sorry, I got away from the original question.  Radio waves will travel through materials.  Radoms  are a case in point.  Radio waves do bounce of buildings - this is what used to cause guosting on analogue TVs.  Line of sight is a straight line; you do not have to be able to see it.