Once upon a time, a receiver would actually need to hop. But I wonder if one can build a decent receiver for a scheme like this using SDR (e.g. a direct RF sampling receiver and some DSP. (The Xilinx RFSoC series looks like it could do this without coming close to breaking a sweat.). In the absence of jamming, I wouldn’t be surprised if a system like HAVEQUICK could be decoded without the key.

(In the presence of jamming, the dynamic range of the receiver could matter.)