[spam]I'm not a real special agent or anything, obviously.  It was just a weird thing to say some time ago.  I ran into "classified" technology many years ago and have been losing my mind severely, obviously.  I'm aware of hundreds of other people in that situation.

A cheap imu has too much drift to be a good source of truth on its own.  So you need like a gymbal or the phone camera.  There are free visual-inertial-odometry software systems that can get highly accurate location data from a camera and an imu.  They are used for drones and 3d mapping.

I'm not a radio engineer, but if I wanted to send an undetectable radio signal, I would measure noise already being scattered by my equipment, and emit rare signals that appeared to be that noise without amplifying it.  The other party would know the timing and incoming direction of my signals, and integrate them to amplify my signal over the real noise. You could also split your weak noise into different parts to be reconstructed together from how they interfered at the receiver.

Usually when I think of anonymous radio I just think of putting a transmitter somewhere distant from its owner.  Seems simplest.

To start a covert communication project, I would try to just demodulate any old signal you send to yourself.  Make sure you understand working with the raw i/q data so you can do tricks later to hide it.  I'm guessing it might work fine if you think of EM as being complex-valued sinusoids that fill space and get summed by antennas, but don't mistake those numbers for being the electric and magnetic field components, they're just the electric voltages at your antenna sampled in such a way that they can be frequency shifted with an fft.  Then set up a shared key, synchronise your transmitter and receiver clocks, and send secret messages by tuning at cryptographically determined times. Then make your data signal a little too weak to be received, but send redundant data, and recover the data at the receiver by summing the redundant timepoints together so that the signals add and the noise cancels.  Then work on making your signal be indistinguishable from existing signals unless you are the intended recipient, even if an adversary knows exactly what you are doing.

I'm not a cryptographer, but it seems if there were some cryptographic function that worked like xor or a stream cipher, but sustained its properties under different linear transformations of each datapoint, such that data could be recovered after decryption of data transformed into noise if the linear transformations were then inverted, it would be incredibly helpful.  Then you could decrypt redundant noise and combine it to amplify a signal.  Decrypting the signal after amplification leaves a path for others to identify the redundancy in the air.

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