Coming 2 ADS-B Again

 

Published back on February 17th 2021, my article on "Analyzing Frozen Air Traffic in the Hudson Valley" got some attention thanks to the most excellent RTL-SDR.com blog a few days later. 

Today I was inspired to write this follow up after returning from a COVID safe family trip down to Arden Point this morning after hearing (and seeing) a United States Coast Guard HC-130J overfly our position this morning around 10:30 AM local Hudson Valley Time (HVT).  This volkssport activity helped clear our heads after watching Coming 2 America the night prior. 

Sadly the above photo is a stock photo and not of the actual USCG aircraft that overflew us while enjoying the nature just to the south from United States Military Academy at West Point, as shown here on APRS.fi 

Comparing Visualized Data

The author of graphs1090 named weidehopf of whom I used the graphing software to generate the charts in my original article sent me a private message to adjust my gain downward and find a balance between messages received and distance of furthest aircraft since my signals over 3dbm were WAY too high.

When I moved in 2019 roughly 20 miles south of my prior location, I mistakenly left my gain set to AGC which automatically adjusted my system gain since my antenna was not in an optimal location for ADS-B compared to what is installed now.

Changing my setting from the default AGC to a figure of 24, here is the before and after difference in received messages. No adjustments to the hardware or antenna have been made.

It is a pretty remarkable difference in number of total ADS-B messages received, almost 2.5x times more in fact.  

But, usually there is a trade off when it comes to maximizing the distance received and is something I have intentionally tried to manipulate so as to avoid some of the heavier airtraffic to my south via KJFK and KLGA worrying that my receiver may get flooded with too many messages. 

For the same calendar period as above, here is the results by reducing my gain.

So for the first few days after adjusting my gain much further, I raised it up ever so slightly based on the guidance of weidehopf via the information found in that Git repository along with further reading on the FlightAware message boards. 

I found my perfect balance of messages, aircraft and range now, so thanks for that tip. 

You can see here the signal levels and where I made my initial adjustments and then increasing them slightly after a few days where I found the perfect setting which has been untouched for the last two weeks in the below chart.

The most important benefit to optimizing the gain setting aside from drastically increasing my total messages received has also been a boost in Raspberry Pi performance. 

The total CPU usage has decreased a good amount which also lowered the core temperature during certain times of day.

Since I am only using passive cooling via a heat spreader and metal enclosure, my maximum core temperature now seems to track against major peak times of ADS-B traffic rather than stay at a more steady level. 

This may be interesting to reevaluate once summer temperatures arrive given the receiver is located in my attic for maximum height without going outside due to diminishing returns for effort needed as mentioned in past via Line Of Sight: Aircraft Edition.

If you missed it, the moral is, more gain is not a good thing. Use a visualization tool like that offered by Graphs1090 to fine tune your ADS-B system for maximum benefit and enjoyment. Hope you enjoyed this explanation. Here are some past ADS-B related HVDN articles...

• Analyzing Frozen Air Traffic in the Hudson Valley

• Tropical Storm Barry: Weather & Aircraft

• Line Of Sight: Aircraft Edition

• Are you posh or not? Tracking aircraft

• Rare Aircraft Sightings: What have you "heard" lately?

• Access the HVDN Locator here and consider a small donation

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.

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.

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.

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.  

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:

• HVDN Notebook February 22nd 2019:  Are you posh or not? Tracking aircraft

• HVDN Notebook April 6th 2018: Rare Aircraft Sightings: What have you "heard" lately?

• HVDN ADS-B & Related Section:  What is ADS-B?

• HVDN ADS-B & Related Section:  Even more about ADS-B!

• HVDN ADS-B & Related Section:  Donate for access

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.