This 16-tone PSK tactical data link, also called TADIL-A and "Alligator," is used by the US Navy and allied units to share radar tracking data. It's one of the more distinctive sounds on HF.
Here we're looking at the classic waveform, as opposed to the newer single-tone one. The mode's distinctive bonging sound is due to the hard keying of the 605-Hz doppler tone, which initiates at the start of the databurst and then continues through the burst. The lines up at the top near 2900 Hz are the sync tone, which also initiates at the start of the burst. The other lines in the middle show this mode's 14 data tones, the phase-shift keying of which makes the classic buzz that really does sound quite a bit like an alligator happily making little 'gators. The rest of the grunge is distortion, phase noise, and general crud of various origins in the receiving and plotting process.
Note the variable length of the data bursts, something of a characteristic of this mode. The long-short pattern varies, but always has a rhythmic sound.
The peak-holding spectrum analysis at the bottom is of several minutes of bursts. It clearly shows where things are in classic link-11.
Plots made with GRAM.EXE and SkySweeper.