Occasionally the ISIS I and ISIS II spacecraft rendezvous in parts of the world where simultaneous telemetry coverage is feasible. Electromagnetic propagation between ISIS I and ISIS II has been studied using their topside sounders at medium and high frequencies. Above the gyrofrequency, the O and upper-frequency X modes have been identified often during separations of up to a few thousand kilometers. In a few instances, the lower-frequency X, or Z, mode has been observed with separations less than 1000 km. This mode is continuous below the gyrofrequency where the whistler mode is also seen. Intersatellite ionograms confirm that the O-, X-, and Z-mode pulses propagate largely as expected on the basis of magnetoionic theory applied to a smoothly varying medium, with little distortion except near cutoff. On the other hand, whistler-mode pulses are significantly elongated by the medium. Their interpretation is the subject of a companion paper, part 2.