The comparative performance of several digital data modems over a high-frequency (HF) radio link is described in this document. During April and June 1964, field testing of evaluation models of new phase-shift keyed (PSK) modems was conducted over a leased 6900-nautical mile HF radio circuit from Pretoria, South Africa, to Riverhead, New York. The superior bandwidth economy of PSK over frequency-shift keyed (FSK) modulation was demonstrated. This work was accomplished as part of the ESD program to improve missile test range HF digital data transmission.