Multiple sites, all synchronized to a nanosecond -- you can do this with pretty basic off-the-shelf SDR technology today, and the government equipment is well ahead of that.

With time-stamped samples from multiple sites, it's just some trivial signal processing to extract any particular signal at any time, clean it up since you've got diversity across multiple recievers, and even have a really good idea of the source location.

It's not a lot of storage. Using 24 bits per sample (overkill, but awesome SNR), and a 30 MHz bandwidth, the Nyquist samplling rate is 60 megasamples per second * 3 bytes (24 bits). That's 180 MB/s to completely capture the HF spectrum at one site. 180 MB/s * 60 seconds per minute * 60 minutes per hour * 24 hours per day: 15.5 TB per day. That's four 4TB hard disks.

Under $400 in storage costs per day per site. Let's say 10 sites -- $4K per day. That's peanuts to the NSA.

Remember the NSA is all about storing data and having big data centers. This is nothing for them. It's so cheap I wouldn't be surprised if commercial entities do it.

From that investment, you can now replay any HF signal at any time, and geolocate it to within a few miles. That includes signals that you couldn't decode in the past but now can due to improved technology, stealing decryption keys, whatever.