Tropospheric propagation. Analysis.

Maritime stations equipped with AIS, aircraft with ADS-B, and radio amateurs with APRS and LORA, are constant beacons that we use to analyze VHF/UHF tropo conditions. It is common practice for radio amateurs to have a small AIS or APRS receiving station to get an idea of ​​current propagation conditions.
Although the WSJT-X together with PSK-Reporter are an unbeatable tool in this aspect, since we can know where our signal is going to, there are not always stations listening, just as we are not performing TX 24/7.
Gone are the years of listening to CW’s microwatic beacons…
For fixed station operations, it is enough to monitor the conditions through the various web pages on AIS/APRS or PSKreporter, but how can we prepare our portable expedition to try to make a difficult QSO, or to participate in a contest most efficient way?
Analysis by layers is a solution that I present below.

Tropo ducting types

There are 2 types of tropospheric ducts used by radio amateurs to make long-distance communications or DX in VHF/UHF/SHF, I summarize them briefly:

Marine duct: It is the duct formed by two different layers of air.
Electromagnetic waves get trapped in it, overcoming the curvature of the earth. On surface there is a moist and cold layer, while as we ascend in altitude, we find a thermal inversion and a warm and dry layer. This phenomenon occurs frequently in the East Atlantic, very frequently in summer, when the Azores anticyclone is stable and strong.
The following figure shows a schematic of this duct.

Elevated Duct: It is one formed by 3 layers, in ascending order (Dry and cool / Wet and cold / Warm and dry).
Electromagnetic waves are trapped in this second layer, also overcoming the curvature of the earth.
In this scenario, it is much more efficient for both stations to be at the altitudes into the duct, and not as in the following diagram:

The layer theory demonstrated with AIS.

Over the past year, while monitoring my AIS receiving station, I noticed that the duct I was looking at was elevated.
My station is at 420 m. above sea level, and during that day I could only receive coastal stations in Portugal and Galicia whose antennas were in mountains above 300m.

Elevated duct. Coastal stations of La Guardia (305m.) / Detrelo da Malhada (1027m.) / Serra do Funchal (427m.)

The altitude above sea level could be calculated thanks to the geographical coordinates of the beacons received along with google earth.
The 3 stations in the image are as follows:

La Guardia
Detrelo da Malhada
Serra do Funchal

A day later, the duct was merely marine:

After this experience, my concerns were diverted to making a tool that would allow tropospheric ducts to be distinguished with the tools present.

Data collection and processing

The aforementioned systems allow us to know the conditions in different layers:

  • AIS : Ships, Marine layer ( 162 Mhz- VHF)
  • AIS: Coastal Stations , medium 200-1500m ( 162 Mhz- VHF)
  • ADS-B: Planes , high +4000m ( 1.090 Mhz – UHF)
  • APRS: All layers (144.800 Mhz R1/144.390 Mhz NA/145.575 Mhz SA)
  • LORA : All layers ( 433.975 Mhz – UHF QRP)

For data collection, the first problem is the interpretation of each protocol. Here I make special mention of Ian Renton, who designed a Java program that collects the data and formats it in Json, differentiating APRS stations / SHIPS / COASTAL STATIONS / AIRPLANES.
With part of the task already done, the next thing was to process the data. Using Python I was able to make several programs to process this information and generate the maps that I host on my website, and which are shown below:

La imagen tiene un atributo ALT vacío; su nombre de archivo es map.png
La imagen tiene un atributo ALT vacío; su nombre de archivo es plot.png
La imagen tiene un atributo ALT vacío; su nombre de archivo es polar.png
La imagen tiene un atributo ALT vacío; su nombre de archivo es maplineas.png

Live map on https://ea8cxn.github.io

Another important part is being able to save these daily receipts for future analysis. Python has given me that possibility and I save LOGS every midnight, as well as reset counters and maps to zero.

APRS, world without standard

With the APRS we find the problem that there is no global agreed frequency, not even continental.

La imagen tiene un atributo ALT vacío; su nombre de archivo es APRSVHFworldmap-1024x508.png
G6UIM freq Map

Due to the fact that transatlantic contacts have been made in the two previous seasons, covering distances greater than 5,000 km on VHF, it is of vital importance to monitor this traffic in order to study these ducts.
The implementation of multi-frequency listening is done economically today thanks to the low cost of SDRs. At this stage I am adding SDRs to monitor at least the 3 frequencies that are important from the Canary Islands.

Current status

Right now, in the banner on the right of my website www.ea8cxn.es, you will find the maps shown above, updated every 10 minutes, and a real-time map of the stations received. This is done autonomously by several Raspberry Pis, and a Raspberry Pi4 that is in charge of processing and making the maps to host them on the web.
I still have a long way to go to improve the interpretation of the data, as well as automatically generate warnings when the ducts are marine or elevated, but this task is merely cosmetic, hoping to have them operational soon.

César Regalado

Leave a Reply

Your email address will not be published. Required fields are marked *