The best way to do this within the current code would be to hook up the led to a analogue input and set it up as a rssi input. Then you would get logging of rssi level rather than voltage but would defiantly be a good proof of concept.
this would be done by setting
RSSI_TYPE to 1 for AnalogPin
RSSI_ANA_PIN to your pin connected to the led (you will have to be careful about voltage, must be less than 3.3v)
I’m not sure if this is available, or if it is that the settings would be the same for the older firmware version that runs on the APM, would defiantly recommend getting some current hardware.
I don’t have any experience of the best way to post process the data. But here are a couple of ideas
For reading the data out the quickest method may be to overwrite the altitude a data with the logged value, you could then get a snail trail giving a idea of the sensor readings and plot that on google earth.
The next step up would be to plot a 2d contours of the data and manually overlay that on google earth. (this may actually be less work than the first idea).
The real professional way to do this would be with commercial software such as px4d. The advantage here is the level of automation and the accuracy with which the data would be aligned to the map.