Abstract: An improved modem system for HF data transmission over a varying communication channel transmits data in packets formatted into successive frames, each having two identical training sequences following a sequence of data. Each training sequence has a time length longer than the expected time length of the multipath dispersion in the communication channel, in order to isolate one training sequence in each frame from multipath interference from the preceding data sequence. The "clean" training sequence allows the receiving modem system to derive very accurate channel estimates for each frame by autocorrelation to the known symbol sequence stored at the receiver. By interpolating across the channel estimates for the current and previous frames, the modem system can track the communication channel by interpolated estimates for any part or any point in time of the data sequence.