"collecting signals from a total of 5 aircraft,"
Are you bloody kidding me?
My FlightAware account run from a basic RTL-SDR on a Raspberry Pi gets that amount of different aircraft on the screen AT ALL TIMES, let alone for the research of a paper. 1000's a day - light aircraft, airline traffic, commercial, private, etc. coming in and out of range, doing everything from circling learners to high-altitude straight routes that just plough through my range within a few seconds.
You're going to need to do a mite more testing than 5 aircraft to make that work, and if all you need are RTL-SDR traces, ask anyone on FlightAware who seems to be picking up far more aircraft than you are every minute.