Behind the Curtain

People have asked me how does NGØE’s VHF Propagation Map come up with the propagation information that it displays on its map. The following explanation comes right from the website’s source code.

This map shows actual radio propagation from stations operated near 144 MHz. It uses data gathered by Automatic Packet Reporting System-Internet Service (APRS-IS) from packet stations in the amateur radio service.

The map shows activity that has happened in the past hour. Paths are combined to create color-coded footprint indicating the distance VHF signals are likely to be traveling. Packet stations typically run low power into small vertical antennas. Better equipped stations should exceed the the distances these stations report.  The map is updated automatically, typically several times per minute.

The map is created using positions (latitude and longitude) reported by nodes in the packet radio system and the hops from node to node that the data travels. By correlating the hops with the position of each end of the hop, the distance can be inferred.

The following is a typical packet that illustrates the process:

STJOHN>APN391,K0SUN-10,AI4GI-3,N5ALC-3*,qAS,KE5NOB:3916.93N/09454.35WhPHG7330...

In this packet, station STJOHN reports its position as 39 degrees 16.93' north and 94 degrees 54.35' west.

The packet took the path STJOHN, K0SUN-10, AI4GI-3, N5ALC-3. If we previously received and recorded the locations of each of these stations, then we know the distance it traveled between each of the station pairs: (STJOHN, K0SUN-10), (K0SUN-10, AI4GI-3), (AI4GI-3, N5ALC-3).

Errors can be present in the data used to create the map. Some stations operate on HF frequencies, which result in much longer distances than VHF typically supports. Occasionally a packet radio station on a high altitude balloon or satellite appear. Some stations incorrectly report their position, often by hundreds of miles, causing local communication to be misrepresented.