Tropo event October 2023

[g7ruh]

During October 2023 I captured a long-lived tropo event.

It lasted from 7-10 October. The high pressure was steady and moved little during this time. This shows what it was like on 9th October:

My normal reception distance is 30-50 NM, sometimes enhanced conditions give 100-120 NM. This time it was much more.

This is from 7th Oct 0900 GMT, about 170 NM to south-west:

and 3 hours later:

Distance chart from 7th at 1926 GMT. You can see that distance started to increase about 20 hours before:

The following day (8th) at 0835 GMT:

It was much the same at 1854 GMT the same day, picture not included.

The following morning it was much the same at 0704 GMT

The rest of the day conditions dropped back a bit and were more like they were on 7th October, then on 10th October at 0658 GMT
things got better:

and better, at 0849 GMT: note the message count for OOCL Felixstowe:

This is the end of the event, messages per hour graph time was 10th October 1511 GMT and messages started to drop off about 15 hours before:

So AIS-catcher can certainly be used to detect tropo events. 560 NM is certainly not bad. I was not watching the screens all the time, so there may have been a greater distance received. That is where some kind of automation could help.

My normal AIS-catcher web screens are two local ones, around Southampton Docks and Isle of Wight. Two others showing English Channel about 120 NM east and west, the final one is the same as the picture above with the OOCL Felixstowe detail panel.

A quick glance at the last two gives a fair idea of reception conditions. I often see as far as Calais and sometimes as far as The Netherlands, but not as good as last October!

Roger G7RUH

[jvde-github]

Super interesting. If we have a bit more critical mass with feeders, a tropo map based on AIS reception could be interesting.

[jvde-github]

Oh wow. I would be interested in having a look at some of this raw historical data at some point. I am interested in detecting spoofing and jamming algorithmically to see if we can infer the vessel position from other data.

[g7ruh]

"Sometimes a ship mooring 1km from my station appears off the coast of Scotland or Bosnia and Herzegovina"

@MaxX850 Interesting. I have seen several vessels report a strange position (e.g. Scotland or The Netherlands) and I recall seeing them in Southampton (my local port). When I look, that vessel moored in Southampton but I have reports from further away. I started to think this was "single bit errors" and may well be, but you add a different dimension to this if the vessel's GPS is being "modified" by jamming. I guess jamming is also subject to tropo enhancement.

Thank you for the reply and I am sorry you suffer from that sort of interference, most frustrating.

Regards
Roger G7RUH

[MaxX850]

I have been interested in digital broadcasts for many years. I have been receiving AIS for over 10 years. During this time, I used various hardware and software. It happened to me that I picked up the position of a ship that was really far away - off the coast of Norway or Denmark, but these positions could be confirmed on websites such as fleetmoon or MT based on photos uploaded from that place or port calls. However, since last year I have started observing such short changes in the position of individual units, reaching several hundred nautical miles. Sometimes even several thousand NM. I also have a commercial AIS receiver - I need to turn it on and check if it will also record such jumps. Some programs, such as shipplotter, have a filter that can limit the recording of unit positions above a user-defined distance from the station. I assume that large websites such as MT and others can also filter such data uploaded by users. I'm not sure if what I'm seeing is GPS signal jamming, but my region is listed in warnings for airline pilots as being at risk for this type of interference. Also on flightradar24 I can see strange things.

[MaxX850]

Here I can see a medium gps interference at my home location.

[Phil353556]

You're right, I also follow adsb data and the website (your screenshot) is very interesting: https://gpsjam.org/ I discovered it a few months ago.
Kind regards
Philippe

[g7ruh]

@MaxX850

Thanks, your Marco Polo picture certainly looks like a position being reported as good data from a GPS but a vessel cannot move that fast or over land! I saw that sort of thing on a smaller scale when GPS was subject to Selective Availability "errors" years ago. I was researching / testing amateur radio APRS accuracy at the time.

Here is an example which triggered my curiosity. I put it down to the "single bit error" type of issue at the receiving web site, clearly wrong but interesting to me. Screen grabs taken at the same time. One web site wrong the other OK and agreed with my AIS.

So not like yours, but a strange event.

Regards
Roger

[jvde-github]

No rush can be any time

[MaxX850]

Until now, it seemed to me that most of these types of incidents were caused by one ship - Taurus, a small port tug. However, now I caught something much bigger - Mogens Maersk, a 400-meter long container ship, jumping around the port basin and sometimes even reaching the coast of Great Britain. It's hard for me to dump a NEMA message with an incorrect location because these jumps are very short and I usually only see them when I turn on the ship's route display.

[jvde-github]

Thanks! Maybe I can find something in the data stream tomorrow. Thanks for flagging

[jvde-github]

I see the same jumps also with other aggregators, so that makes it less likely that it is on the receiver end but something with the location broadcasted by this sender. Makes it difficult to pinpoint.

[jvde-github]

Another explanation I am pondering over whether there is a bug that mixes up the ordering of the path points. The points outside the port is an anchorage area where the ship might have been at anchor.

[g7ruh]

Update April 13th 2024

Jasper has changed the number of vessels pop-up to separate Class A and B (thanks Jasper).

This does two things at this location:

1. it makes it easier to see when there is a major sailing event taking place: the number of Class B (pleasure craft) under way is near or exceeds the number moored.

2. it makes it easier to see enhanced reception conditions as the number of Class A vessels under way is significantly higher than average.

The above, from today, gives an example of both 1) and 2).

Maybe the most simple way to detect enhanced reception conditions is to count the number of fixed structures. These by their nature do not move (!!). When the number goes up from the "normal level" which for me is 4 or 5 then it can trigger an alarm. If the average number of fixed structures over a month or more ( I think 3 months) is recorded, then an alarm can be triggered when the current number reaches (for example) 10 - 15 in my case, then I know the wider map is worth looking at.

If the total number of vessels circle changes from the white background to another colour (say red) when the condition is met then viewing the web page shows the change in an obvious way.

The examples from last October above have an even higher total of fixed structures.

If the average number is stored and the trigger point is provided as a parameter then the number can be adjusted as needed, but the average will not change too much over time as enhanced propagation does not normally last for more than a few days.

I think it is better than trying to monitor the number of Class A vessels as the number is always changing.

It would be helpful if the alarm colour is configurable, e.g. to help those who may be colour-blind.

Roger