ABSTRACT
Long before rockets were developed with the capability of reaching the ionosphere at the altitude of about 100 km, it was firmly believed that the smooth and regular variations of the geomagnetic field on the so-called quiet days, were caused by electric currents flowing in the ionosphere. Consequently, there was enthusiasm to launch rockets into the ionosphere as soon as possible to confirm the existence of these currents and to study their characteristics. The data measured by instruments aboard the rocket are telemetered to the ground equipment for further processing. The simplest form of telemetry is to use the data to modulate the frequency or the phase of a high frequency transmitter aboard. Analog signals are usually first converted from analog to frequency or analog to digital before modulating the transmitter carrier and this adds to the possible errors. Rockets provide the most direct of all the methods of studying ionospheric currents, because their measurements are made in situ.
