Tuesday, May 14, 2019

Line Of Sight: Aircraft Edition

Definition: Line of Sight

Aircraft and radio waves are two great examples to talk about regarding line of sight.

Simply put, the higher the aircraft flies, the further a ground station can receive transponder signals at 1090 MHz or 978 MHz originating from an aircraft.  Obstructions on the ground such as mountains and tall buildings can obstruct the ability to receive all ADS-B transmissions at the ground level too.

The technology covered in this article is known as automatic dependent surveillance broadcast or ADS-B for short. A fancy GIF animation at the end of this article will help visualize reality from theory even further!

Compare:  Fact and Fiction   

The website "Hey, What's That!" is useful for identifying mountain peaks and line of sight for different forms of communication. The below map shows the theoretical range of ADS-B communications up to 10,000 and 30,000 feet elevation from the chosen center point at a known height.

Poughkeepsie aircraft hobby remote control plane drone

Ground based obstructions have been accounted for on the map as noted by some local summits with the red push pins that make for great amateur radio SOTA destinations. 

A calculation of 47 foot above ground level for a total height of 256 feet was used for the same latitude and longitude coordinates six digits past the decimal point on the "Hey, What's That!" website in order to match the location of the HVDN ADS-B antenna location with high accuracy.

boeing aircraft KPOU IBM helicopter
The "Blue" center dot clearly is not in the center. of the reception range. Its location was moved to calculate the distance to all aircraft from where I was located while I was authoring this article. This is one of many features available to those that gain access to the HVDN VRS. Details about HVDN membership can be found at the end of the article.

Comparison of the theoretical and real world values are very, very close when looking at both data sources.

Had the ADS-B antenna been located outside and above the roof line just a few feet higher, its possible that the two results would have been even closer.

The dark red "splat" is the historical reception report over time for aircraft up to 30,000 feet. The pink splat is for aircraft under 10,000 feet.

However, there is one thing to take in mind here, aircraft are not flying in every possible location which will account for some gaps in coverage when looking back at the theoretical map.

Is it worth the extra work in adding an outside antenna to gain the additional coverage benefits? This certainly is shaping up as a classic case of diminishing return, so read on to show how this was validated.

Detail:  Spot checking some results

The top photos are captured from the "Hey, what's that!" website for theoretical "potential" results. The lower photo is zoomed in from the HVDN Virtual Radar Server to complete the comparison against fact to cover real life reception results.

10,000 Foot Elevation Connecticut Comparison:   The projected range spike towards and over Springfield was accurate from the projection and actual reception results.

Add caption

30,000 Foot Elevation Finger Lakes Comparison: Coverage was just short of the prediction over Cayuga Lake, but awfully close.


10,000 Foot Elevation Long Island East End Comparison: Coverage fell short of the North Fork at 10,000 feet but air traffic is captured as noted by pictured aircraft which were at 14,000 and 18,000 feet pictured to right of coverage overlay.

hamptons 2m repeater hVDN

30,000 Foot Altitude Over Delaware Bay Comparison: Coverage was pretty accurate based on traffic spikes compared to theoretical results.

Further Results:  Comparing Other ADS-B Feeder Sites

According to Flightaware's website, there are a number of close proximity feeder sites to where the HVDN primary site is located in the Hudson Valley of New York.

Only the two highlighted receivers on the map (Orange) have similar coverage of HVDN which must mean that the height above sea level for all three stations must be similar and may even be using near identical reception equipment to that of HVDN (Red)

Here is further evaluation of the public details each of the three stations provide.

Receiver Site #1:  Montgomery, NY - Flightaware 87114

Receiver #2: Poughkeepsie, NY - Flightaware 77960

Receiver #3: Middletown, NY - Flightaware 83104

For Fun:  What if.....

It would seem for the Hudson Valley, few interested seriously in ADS-B reception can really improve reception without a huge height increase or locating the antenna outside.

Steve K2GOG of HVDN thought it would be fun to model the benefits of increased antenna height, so check out our amazing use of animation skills below to see how much higher you need to go to literally receive every aircraft possible via line of site up to 10,000 (Orange line) and 30,000 (Blue line) feet.  

hvdn aircraft, ads-b range distance

Perhaps it may be worth while putting the antenna up another 30 feet located outside. Interested in setting up your own ADS-B aircraft receiver?  Helpful links follow:
Another approach may just be easier to recruit ADS-B feed providers to just aggregate into the HVDN Virtual Radar Server to increase the range of reception for aircraft under 10,000 feet. Contact the author if there is interest in contributing to this.

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