This Month´s Cover...

Loran has provided navigation service since 1958. Though not originally designed with data broadcast capabilities, Loran´s versatility has enahled data to he broadcast with great benefits. Research in the last two decades has resulted in a tremendous increase in the data capability of Loran, thereby increasing its utility. Currently, a modernized Loran is being evaluated for its capability to back up GPS; data modulation is an integral part of this Loran design. Loran Data Modulation: Extension and Examples by S.C. Lo, B.B. Peterson, P.K. Enge, and P. Swaszek [ibid., 4.3, 2 (April 2007), 628-644] discussed three basic techniques for placing data on the Loran signal, of which one is Pulse Position Modulation (PPM). PPM shifts the arrival time of the Loran pulse to transmit data. Small time shifts, when implemented correctly, generally have little adverse effect on navigation receivers. It is also relatively reasonable to implement in nearly all Loran transmitters. This includes both the solid state and tube type transmitters (SSX and TTX, respectively). Since time shifts must be small, this generally limits the data capacity. The cover illustration is the time domain of 32 state PPM.