Electronic emission if that works for you.How does it work aboard the drone? Does it use a pair of crossed ferrite loopsticks and a sense antenna (to remov the 180 deg ambiguity that crossed loopsticks present) tuned to the band of interest (80 Meters/3.5 to 4 MHz in this case)?
What's the resolution bandwidth (RBW) of the receiver - can it resolve two or more signals within 200 Hz of each other (doubtful)?
You know, it's one thing to look for the uplink of a satphone being used by an Al Qaeda member (the signal will be somewhat broad, isolated by itself and therefore unique as viewed spectra-wise and posses definite directional properties to a 'monopulse' antenna design), quite another to go weak-signal sniffing for something 200 Hz away from a strong broadcast carrier on the HF bands (like where a QPSK31 transmission might hide) ...