FlightAware Discussions

Clear view of the sky

I wrote elsewhere:

There are endless discussions and tips on antenna types and reception optimisation, which can sometimes overshadow the basic advice “make sure that you have a clear view of the sky”. If a picture can say more than a thousand words, here are two pictures to make that point:

The image on the left was made with tar1090 and shows all the tracks recorded by my feed in the preceding eight hours. The black dot on the image on the right shows the position of the antenna.

The antenna is the absolutely simplest possible and sits on the balcony railing of a 3rd-floor apartment in a six-floor building, essentially half-way down an urban canyon. Look at the direction of the street and compare it to the spread of the recorded tracks. That £5-antenna has been picking up signal from 100nm away where the view is unobstructed, but can’t even reach 20nm out across the street.

Until I saw that track spread, I was pondering getting a better antenna, a preamplifier, generally better equipment. Looking at the spread makes it clear that just moving the entire rig to the roof of the building will do a world more good than throwing any pile of money at it in its current location. Especially since this happens to be the tallest building at the very top of this particular hill.

Ergo: a clear view of the sky beats a good antenna hands down. Having both is ideal, but a clear view should always be the first priority.

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Fun to see what you can get out of a setup. Give https://www.heywhatsthat.com/ a try as well. Goto new panorama, type your correct coordinates in, wait a few seconds and look at the map after clicking the “up in the air” button - zoom out. There you will see accurate distance lines for your specific location at whatever altitudes you choose. I think it defaults to 10k/30k feet. You can also take that data and overlay it onto your dump1090-fa/tar1090 maps.

An example of a 24 hour timeline for my location, outer ring and tar1090 filtered to 40k feet.

To overlay the ring on your tar1090 map:

sudo wget -O /usr/local/share/tar1090/html/upintheair.json "http://www.heywhatsthat.com/api/upintheair.json?id={CODE}&refraction=0.25&alts=3048,12192"

Replace {CODE} with the heywhatsthat code provided after you create your panorama.

Sorry to rehash this if it was already known to you. Perhaps it would be of help to one person reading regardless. I know there are existing threads explaining this as well.

Well, the credit for me finding tar1090 and the subsequent insights it provided, is all yours :wink:

I did already, I happened to stumble on a link to it yesterday. Unfortunately I put in my elevation as 20m higher than it really is, so the resulting theoretical graph is slightly skewed and I can’t find a way to correct the elevation, other than by creating a completely new profile.

Wow, 200+nm, only clipped to the south at lower altitudes. Your FA page places you in north Scottsdale and OSM shows some elevations just south of you, which explains the clipping, which heywhatsthat did map quite accurately. I have a similar little obstacle to my east, which I share with almost all other Athens-based trackers. There’s a 4-million population this side of the mountain and only a few tens of thousand inhabitants on the other side of it, so the two airport-side-based trackers are a really rare and precious species in this context.

Absolutely. Sooner or later it will be.

I said it numerous times. ADS-B signals are not limited by weakness like a normal radio station would be. They are limited by obstructions and obstructed horizon. Planes have powerful enough transmitters that can be received with simple antennas.
LNA and filters help to compensate for long antenna cables and respectively local RF interference.
Raise the antenna above roofs and you’ll enjoy a bigger range in all directions.

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I first thought I’d put the whole rig on the roof. That turned out very impractical, so I put the antenna on the roof and kept the rest in the house. Doing so required 17 metres of RG-213 and one metre of RG-58 cable, plus three coaxial connectors, with all the attenuation that they and my lack of expertice in piecing them together brought.

The previous antenna was “the absolutely simplest possible”, but the new one is even simpler. It is not an antenna at all, but just the last 69 millimetres of the 17-metre connecting cable stripped of its outer insulation and shield braid (but not of the inner dielectric) at the top of a 4,8-metre mast. Like this:

This view from up there is as best as one can get from Athens, stuck as the city is in a basin with mountains on three of four sides. Below is a 360° panorama from the roof (made with hugin). North is just slightly left of the middle of the image. If you zoom in you can see Aigina across the sea to the south and even spot the Akropolis at about 10 o’clock:

And this is the result as tar1090 tracks for 24 hours:

The range has only increased slightly, but the spread has increased immensely towards NW and SE. The two blue irregular rings show the heywhatsthat.com theoretical range for 7.500 and 40.000 feet. No matter what, with that mountain just east of here, my range towards the east will never improve. In all other directions, a better antenna should bring major further improvement.

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At least make a quick spider: QUICK SPIDER - No Soldering, No Connector

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Heh, yes, I already had that and this and this and a few others bookmarked, but I have another problem to deal with first: my Raspberry, an original Model B, is already running at its limits as it is. dump1090 uses 65-70% CPU and the average load is never below 1.3, most of the time around 1.6. In other words, a better antenna will kill the pi. The question is, should I spend €60 on a new rpi 4B, or should I get something better than a Raspberry in the first place? I’m trying to understand how much I/O is being done on the SD-card, whether performance and card durability would improve by putting some stuff in a tmpfs in RAM and, if so, which stuff.

Also, I’m pondering whether it makes any sense to save the data in a database. In the past few days I have seen an incredible number of bizjets coming, going or passing, many more than I had expected, some with weird trajectories such as ETAR-LGEL. Many have concealed identity, e.g. a Turkish-registered GLEX taking off from LGAV the other day in the direction of Ankara in the middle of a diplomatic crisis and a war of words between Greece and Turkey. On Saturday we had a bushfire very close to where I live and I could follow five different Erickson S-64 on my own dump making the rounds, but I could also see them with my eyes from my balcony. Those beasts can carry 10.000 litres of water each and can fill up again from the sea in less than a minute, but one was flying around with a 1-ton bucket hanging from a wire, while another was in the air for hours without ever picking up any water. So what exactly were they doing with that fire, apart from burning taxpayers’ money? Could a database in the long run provide any interesting insights on the basis of just my own data? Questions like these could affect the hardware, which is precisely why I haven’t ordered a new rpi 4B yet.

So lets say you had

a) a clear view of the sky outside a window.
b) a clear view of the sky from a balcony (surrounded by deck walls)

Would the best option be a) as you have more clear sky to see from?

Neither a) nor b) qualify as “clear view of the sky”, given that the house on which the window or the balcony sits obstructs about 180° of the surroundings both horizontally and vertically. But yes, if you can see a bit more sky from the one than from the other, that’s probably the better of the two.

Then again, there are so many other factors at play that make generalisations tricky. The window of a wooden cabin at the top of a mountain does not compare to the window of a 1st-floor flat in a massive building in the middle of a city, nor does a balcony facing open sea compare to a balcony facing another building. Besides, one sector of the sky might be much more interesting than another, e.g. if your balcony looks towards an airport while your window is on the other side of the same house. If your question is not purely academical, I would suggest you test every possible antenna position at your location and see what gives you the best results.

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Summed up nicely :+1: :+1: :+1:

 

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Thanks to wiedehopf (a) I can now see my CPU usage at a glance without logging in and running top and (b) it went up even more.

~# top -bi |egrep '(dump1090|rrd)'
      398 dump1090  15  -5   29796   7332   2228 R  73.1   1.7   1691:54 dump1090-fa
      398 dump1090  15  -5   29796   7332   2228 R  64.7   1.7   1691:56 dump1090-fa
      398 dump1090  15  -5   29796   7332   2228 R  59.0   1.7   1691:58 dump1090-fa
    24075 root      39  19   46680   3680   3188 R   3.5   0.8   0:00.11 rrdtool
      398 dump1090  15  -5   29796   7332   2228 R  55.1   1.7   1692:00 dump1090-fa
    24081 root      39  19   51624   9388   8396 R   8.9   2.1   0:00.28 rrdtool
      398 dump1090  15  -5   29796   7332   2228 R  42.7   1.7   1692:01 dump1090-fa
    24104 root      39  19   51628   9176   8256 R   7.6   2.1   0:00.24 rrdtool
      398 dump1090  15  -5   29796   7332   2228 R  56.2   1.7   1692:03 dump1090-fa
    24113 root      39  19   53020  11980   9904 R  16.6   2.7   0:00.52 rrdtool
      398 dump1090  15  -5   29796   7332   2228 R  51.7   1.7   1692:05 dump1090-fa
    24121 root      39  19   53040  12108   9976 R  20.0   2.7   0:00.63 rrdtool
      398 dump1090  15  -5   29796   7332   2228 R  52.7   1.7   1692:06 dump1090-fa
    24180 root      39  19   51504   8272   7500 R   6.0   1.9   0:00.19 rrdtool
      398 dump1090  15  -5   29796   7332   2228 R  49.2   1.7   1692:08 dump1090-fa
      398 dump1090  15  -5   29796   7332   2228 R  51.6   1.7   1692:09 dump1090-fa
      398 dump1090  15  -5   29796   7332   2228 R  60.1   1.7   1692:11 dump1090-fa
      398 dump1090  15  -5   29796   7332   2228 R  48.7   1.7   1692:13 dump1090-fa
    24220 root      39  19   54412  13356  10004 R  38.1   3.0   0:01.18 rrdtool
      398 dump1090  15  -5   29796   7332   2228 R  49.7   1.7   1692:14 dump1090-fa
    24240 root      39  19   53392  12392  10068 R  21.6   2.8   0:00.70 rrdtool

Hmm. Should it really be running as root? Hadn’t noticed that before.

Thank you for taking the time to reply. I’ll do just that.

And another two. The lousy antenna at the top of this thread was replaced by a rather imperfect eight-legged spider, seen here. The rest of the rig is the same. Range went up by 100nm and is very close to the theoretically achievable (outer blue ring is 40.000 ft theoretical range according to heywhatsthat):

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