Polar Plot anomalies

[steve-b]

Got it, thanks - I should have worked that out for myself! :-)

[DemonLee]

Good news, and I'm sure it will be even better when the mast is up. I have had two "spikes" which I think were real because they are reflected in the Maximum Distance figure, but the rest aren't.

Pardon my ignorance, but how did you check the data to confirm the long-distance hits?

As Patrick stated, you hover your mouse over the stats spike and it will give you this figure.... on the latest image attached, I have hovered the mouse over 364nm and you will notice it clearly states at that distance, it received 268 hits.. obviously 1 could be an error, but not 268...

I think this is because now we are getting over the Carboneras Mountain Range and hitting more flights over the Med into North Africa into Algeria... at the rear, because the new mast is not up yet, we are not yet getting over the Sierria Nevada Mountains, but if our calculations are correct, the mast will resolve this and fingers crossed, if I can get the same sort of distance in that direction, it will mean a huge coverage area.... I actually think however the temporary site with the Roasting Tin, the roasting tin is acting as a reflector, allowing the mast to pick up weaker signals....!!!

08102015-16-08-UTC-3.jpg

[steve-b]

In my case the histogram confirms that (with a couple of exceptions) the long-distance spikes on the polar plot here are spurious - the histogram doesn't go nearly as far as the polar plot says it should. You are certainly putting my setup to shame, but it shows what a difference the antenna position can make.

[DemonLee]

In my case the histogram confirms that (with a couple of exceptions) the long-distance spikes on the polar plot here are spurious - the histogram doesn't go nearly as far as the polar plot says it should. You are certainly putting my setup to shame, but it shows what a difference the antenna position can make.

Steve don't be glum I am new to ADS-B although played with all types of Antennas and receivers in the past, this is why I chose to apply for the FR24 Kit out of the box as it made the learning curve a little shorter.....!!!

My location for many (Antas, Almeria, Spain) would not be an obvious choice. Although I have a number of Airports in the region, Almeria and Malaga to the South/South West and Murcia and Alicante to the North/North East, I also have to large Mountain Ranges to deal with, the Carboneras and the Sierra Nevada and a Military NO FLY Zone to contend with, I knew I was initially going have a few issues due to the 10m limit on the cable. So I set it up exactly as I had advised FR24 I would on delivery and ran it for 3 days. This highlighted 2 dead spots despite Antas being 120m above Sea Level and the Antenna fitted to the roof of a 3 story block of Apartments. It was my friend, a surveyor that did some calculations of the mountain ranges heights and distances that gave me a different location on the roof with a taller mast solution.

My first stage to overcome was the 10m limit on cable, so instead of amplification, I used 20m of standard Electrical Cable with Earth to get power to the new mast location, used 20m of Cat 5E Cable to get the signal from the mast to my PC... cost of cables, plugs and fittings around €53 instead of around €350 for amplification equipment. The new mast and brackets cost about €60 and hopefully, weather permitting and the availability of friends, we should get the mast up within the week.

Now years ago as a poor student, I had a Dipole Antenna for Short Wave Radio and no money for a proper mast, so I simply purchased 4 Broom Handles and some brackets, fitted them end to end and made a mast for around £8 GPB.

Where are you based, what are your issues with reception - height? cost? obstructions?

[steve-b]

Steve don't be glum

Where are you based, what are your issues with reception - height? cost? obstructions?

Oh, I'm not downhearted - just realistic!

We are in south west England, near the Bristol Channel. Our home is 6m above sea level with higher ground immediately to the north, so there are some restrictions we can't do much about. There is more high ground in some directions at a distance of 20-30nm, but I think that would only be an issue for very long ranges.

I use a DVB USB stick with a home-made discone-type antenna which is close to the ceiling on the first floor - so about 5m above ground level, and to some extent masked by solid walls. According to VirtualRadar I can get aircraft positions in some directions up to 120nm, in other directions only to about 50nm. Given the location of the antenna I don't think that's too bad, but I know I could do better.

We need to do some work in the roof space, after which I will be able to put a long USB cable up there and mount the DVB and antenna close to the apex of the roof. Unfortunately mounting the antenna outside on a mast would be a lot of work - more than I'm prepared to consider at the moment - but even with losses caused by the roof tiles I think putting the antenna high up in the roof space will give me a big improvement. No point in doing it at the moment, though, as it would have to come out again when the work is done.

[DemonLee]

Hi Steve, see the problems you have, hope you can work everything out - do you have an exterior TV Aerial on a Chimney mast? If for example you applied to FR24 for one of their kits, you could mount their antenna above the TV aerial on the same mast.

[steve-b]

Hi Steve, see the problems you have, hope you can work everything out - do you have an exterior TV Aerial on a Chimney mast? If for example you applied to FR24 for one of their kits, you could mount their antenna above the TV aerial on the same mast.

No external TV aerial here, and from the PC to the best position for an external mast would be quite a long and awkward cable run - that's one of the reasons I would rather avoid it if I can. An antenna in the roofspace would be much easier, and if it gives me a sensible amount of coverage then I'll settle for that.

I wouldn't qualify for one of FR's free kits - there are other receivers around here and two regional airports within about 15 miles, so it wouldn't make sense for FR to support me. I already run a PC 24/7 for other reasons, so a DVB stick and the FR software costs me next to nothing and works quite well even with the antenna limitations. When I can move the antenna I'll be able to contribute a bit more, but at the moment I don't see any need to buy one of the dedicated boxes.

[DemonLee]

Shame about not have an external T Aerial as it would have been an ideal mast.... don't assume you won't qualify, to be honest I was not sure I would with about half a dozen quite close to me, but the way FR24 look at it is that by having blanket coverage near airports, it gives more accurate data, especially for M-LAT and of course it builds in 'redundancy' should one or more of the others go down for any reason, but I can see your points about the 24hr PC and costs issue.

Well good luck with the works with the loft, hope it all goes to plan and you can get your aerial moved... do consider using the method I have used by feeding power into the loft space and running Cat5E cable as the Cat5E will run up to 235 ft without any loss whereas the distance on the USB power is much more restrictive.

[DemonLee]

I think I know what is causing the Polar Plot Anomalies some have been getting and it was yesterday and today that gave me the clue when using VirtualRadar.

Over the last few days I noticed a number of what appeared to be random spikes in FR24, but when I went to VirtualRadar they were showing as being 'tracked' in the list but did not appear anywhere on the map - they were flying at 30K plus feet and were Military planes from various countries.

3x Boeing KC135R Stratotankers (US Military)
1x McDonnell-Douglas KC10A Extender (US Military)
1x McDonnell-Douglas KC10 (Netherlands Military)
1x Cessna 560XLS Citation (Morrocan Military)
1x Panavia Tornado GR4 (UK Military)
1x Unknown make/model (UK Military)

With all these various Military Flights going on, they would all be using IFF (Identity Friend or Foe) which just happens to use 1030MHz to ping another aircraft that would respond on 1090MHz so although they do not appear on our maps, we cannot get speed, Call Sign or Reg in most cases, our Antennas are picking up the IFF signals on 1090MHz as they respond on 1090MHz to identify themselves as 'friendlies'...

As I list my aircraft by furthest distance, then height, all these aircraft roughly matched the spikes I got on the Polar Plot without it registering the distances that were all over 300km....!

[steve-b]

That may be it, though I'm still surprised I would receive signals from such big distances with the setup I have. I have a couple of 350nm points on the polar plot today.

Things have moved on, though. This afternoon I moved the antenna into the roofspace, just below the apex - about 3m of extra height, but more importantly much less brickwork for the signal to pass through! I have put the DVB receiver in the roofspace as well to keep the coax run short, and then run a 12m USB cable (the type that boosts the signal to allow for the long run) back to the PC.

It's only been running like that for about four hours, but my initial assessment is that I'm seeing about double the distance (200nm instead of 100nm) and reporting about three times as many positions. It will be interesting to see how things look after a full 24-hour period with the new setup.

I've left VirtualRadar running to see how the new coverage looks. I have a gap to my north because there is a 150m hill about 1km north of here - not much I can do about that! Another gap might be caused by a group of very big trees about 100m SE of the antenna and quite a bit higher - it will be interesting to see what happens when the leaves fall. There some other anomalies that I can't account for, but as I say it's early days yet - I'll see how things look tomorrow. It's a big improvement, anyway.

[DemonLee]

Hi Steve, nice to hear the move into the roof space was productive, always a good thing when you see the results change for the better. Since I installed Phase 1 of the new Antenna Mast the range has improved greatly to the South East over the Med and into North Africa, however as I have mountains on 3 sides of me anything from 50-250km away, I still have a few 'flat' spots (See Image) that I have marked with red circles and as the mast is of the type where you can add extensions, this weekend I will undertake Phase 2 and add another 3 metres, making 9m in total. This I have calculated will overcome some of the flat spots, though I expect to lose some of the traffic below 10,000ft...

I think with the anomalies, we are not taking enough into account of 'reflection' and 'deflection'... being in the middle of three airports, from 11am to around 6pm is the peak flow of aircraft in the region and it is possible that Aircraft over 35,000 feet some distance away and normally out of 'range', their signals are hitting aircraft at lower altitudes and our antennas pick these random signals up and unless it gets another one to confirm within X Seconds, the Polar Plot Records the distance, but FR24 software ignores it in the main as it cannot get a second confirmation signal.... we have had some very clear weather this week and I suppose I was lucky enough to pick up the Military Craft especially as one recorded 368nmi on FR24, not just on the Polar Plot, but as having 9 Locations recorded, even though I could not see it on Virtual Radar.

Radar Plot & Obsticles.jpgLongest Confirmed Track 3.jpg08102015-16-08-UTC-3.jpg

[steve-b]

Right, after the first 24 hours with the antenna in the new position I have a better idea of how things are going. According to the FR24 page I submitted 134,044 positions yesterday, compared to a daily average of around 25k-30k before the change. The maximum distance was 339nm with about 100 positions reported between 330-339nm (so that looks like a genuine distance, not an anomaly) but apart from that the realistic range is up to 200nm.

In the plots below (if I've uploaded them correctly!) you can see the increase in "Aircraft seen" after I made the change on Thursday.

FR_stats_20151023.jpg

I've run VirtualRadar for several hours and the range plot looksl ike this:-

VR_Plot.jpg

The gap to the north and north-east is because of a hill to the north and a group of tall trees to the NE - the leaves are falling now, so that might improve. The deep notch near Bristol is a chimney blocking the signal. I think the gap to the South West is where the signal has to pass through the apex tiles on the roof at an oblique angle. The other smaller notches are consistent with some other large trees (lots of trees round here!).

I know this is nothing like as good as I would get with an external antenna, but it's a big improvement on what I had before. For an indoor setup that cost <UKP20, I'm quite pleased!

[DemonLee]

That's a significant improvement, I only hit around 450-500 aircraft a day this time of year being out of season as flight numbers are greatly reduced, from June to September next year I should be getting significantly more if I have everything ready...

As a suggestion, have you considered removing a tile and having the antenna outside of the roof. This can be done by replacing the tile with a vented tile insert, run the cable out through the vent and install the antenna on top of it?

[steve-b]

. . . As a suggestion, have you considered removing a tile and having the antenna outside of the roof. This can be done by replacing the tile with a vented tile insert, run the cable out through the vent and install the antenna on top of it?

Now there's a thing - we had some ridge tiles re-bedded about three years ago. I wish I'd thought of it then.

Actually the benefits might not be all that great. It would remove the big gap to the South West, but the bulk of the air traffic here is either on a north-south line from Brittany through Cardiff and east Wales, or a SE-NW line from London towards southern Ireland. Relatively little goes across the SW peninsula, although some traffic between Europe and Dublin or Belfast goes that way. Hence filling that big south-western gap would increase my area of coverage by, say, 25% - but I'll guess that the increase in traffic monitored might be something like 10% or less.

Within the limitations caused by the roof I think I'm covering the busy E-W and N-S routes fairly well, except for the gap to the north caused by the hill - and I'd need a 150m mast to get past that. To be honest, clambering around a roof isn't really my thing - so I think I'll stick with what I've got for now.

[DemonLee]

That's a shame..... managed to get another 2m on the mast today before the rains set in again, the reading before were: (Degrees,Distance)

0,213nmi / 45,213nmi / 90,210nmi / 135, 254nmi, / 180,136nmi / 225,121nmi / 270,106nmi / 315, 126nmi

and after

0,213nmi / 45,216nmi / 90,216nmi / 135, 256nmi, / 180,145nmi / 225,122nmi / 270,115nmi / 315, 128nmi

So not a massive difference, but when we can get it to the full height with the other 3-5 metres, then I think we are finally getting over the mountains...!