Using a drone to hunt radio beacons opens up some radiolocation techniques which aren't necessarily available to a human tracker on foot. With the drone's ability to quickly and easily fly grid search patterns over any terrain, one possible radiolocation technique I'm keen to try out is generating an RF heatmap.

RF heatmap showing WiFI signal strength in a building.

RF heatmaps aren't a new thing, but they mostly seem to be used as a way of showing the signal strength of WiFi in a building, or cell network coverage. They're a way of visualising the strength of radio signals over a geographical area, with a colour scale to represent the strength of the signal at that point on the map.

My plan is to fly a grid pattern over a given search area, and use an omnidirectional antenna mounted to the drone to take periodic measurements of the signal strength of the beacon. By pairing the measurements with the GPS coordinates of the drone, this should produce a map with a big red area showing the location of the transmitter. By using the stepped attenuator discussed previously to reduce the signal strength when close to the beacon, I'm hoping this technique should be able to provide a very accurate location for the becon.

This should act as a complimentary radiolocation technique to the radio direction finding (RDF) techniques discussed in a previous log. The RDF techniques are best used when initially looking for the beacon - they should be able to determine a reasonably small area (few hundred square meters) in which the transmitter is located. When the drone has reached this area, the RDF may become overwhelmed by the signal strength and not be of much further use. In this case, the drone can switch to the omnidirectional antenna and begin flying the grid pattern to produce a heatmap.

To see if this concept works as expected, I'm first going to knock together a system on my latop to test it out on foot. I plan to use a NavSpark Mini GPS reciever (and microcontroller), since there's already one plugged into my computer for another project, and an RTL SDR dongle. A small C program will periodically call rtl_power to take signal strength readings from the SDR, and retrieve the current location from the NavSpark, storing to a CSV file. I've had a look at the available heatmap generating libraries and none of them seem to do what I want, so I may well also have to write a quick script to generate a KML file containing the heatmap for use in Google Earth/Maps.

Development of this test setup should take place in the next day or two, so stay tuned!