I am a student of a flying school which has 10 Cessna-152's. Since all the aircraft are not equipped with a transponder, there is no way to track them. I am searching for a way to track them like Flightradar24 does with the maps. This is a project assigned for a bunch of students, so we are looking for an inexpensive but still good quality system.
You could try to get some cheap Car GPS trackers on aliExpress. But they would need to be in range of cell towers. Flying to high will cause connection loss.
A more expensive one, which I found to be working great is this one.
But you can get android based trackers for about 10\$US.
It may be possible to use almost any Android mobile phone with GPS tracker app, somewhat like this (many others exist). Unfortunately the solution with mobile phones alone only works within the reach of cell towers, while otherwise it is both very simple and convenient.
If you have radio stations on these planes, it is possible to develop custom software stack so that mobile phone could communicate the coordinates via radio channel in some digital form, and the receiving side (another phone) should be able to decode. The audio jack on the phone has both audio output and microphone input, and data from there can make into software layer for digital processing. Converting between sound and digital data has long history and known approaches; early computers used ordinary cassete recorders to store data and software.
You need rather good software engineers in the team to realize this idea.