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Sweep-frequency backscatter records have proved to be of great value in identifying the sources of backscatter seen on a fixed frequency by demonstrating the development of the echo as frequency and range increase. The most commonly observed scatter is ground scatter propagated via the F2 layer, but it is also evident that the other layers propagate ground scatter and that scatter from the distant E region may at times be important. In one group of observations over an 1,150-km path on three undisturbed days, the values of F2-layer maximum usable frequency scaled from midpoint vertical-incidence ionospheric records and those determined by backscatter delay assuming ground scatter agreed almost within experimental error. In another three-day group characterized by a low-latitude ionospheric disturbance with low geomagnetic indexes but considerable sporadic activity, values of muf determined from scatter were much too high under the ground-scatter assumption, errors of about 30% being not uncommon.