Whistler‐mode signals: Group delay by crosscorrelation

Group travel times of 18.6 kHz whistler‐mode signal from NLK, Seattle, to Wellington, New Zealand, are now being measured using the normal FSK transmissions. This is done using a mini‐computer programmed to perform real‐time cross correlations between two receivers: one receiver gets its signal from a whip aerial on which the ground wave (subionospheric mode) dominates, while the other gets its signal from a loop oriented for minimum ground wave. Group travel time can thus be measured continuously while there are whistler‐mode signals present. Delays of 0.2–0.8 seconds have been found.