Anomalies of the Geomagnetic Retardation of the Spin of Satellite Vanguard I (1958 Beta)

Abstract

Continuous observations of the spin rate decay of satellite Vanguard I (Beta 1958) have indicated anomalies that do not fit into the picture developed from the theory of exponential magnetic damping by eddy currents. The first anomaly was detected in December 1958; the decay changed to a very low rate; a rate similar to the previous decay was observed early in 1959. Other anomalies were found later in 1959 and in 1960. Some of the anomalous low decay rates might be related to the movement of the Earth around the sun. During the 2| years of observation the spin changed from 1 revolution in J sec to 1 revolution in 20 sec. The slope of the decay curve varied between 0.19 and 7.6 arbitrary log units during fall 1960; initially the slope varied between 4.1 and 5.0 units. The slope during first anomaly was 2.2 units. Spin FadingT HE METHOD of obtaining the spin rate of an artificial satellite by the counting of maxima or minima in a satellite's CW signal is well known. These "spin fades" are produced by the lobes or zeros in the pattern of the satellite's transmitting antenna. The effects of various types of satellite antennas (such as dipoles, turnstile or slot antennas) have to be considered as well as the influence of the receiver antenna, the polarization of that antenna, and the geometry of and the ionization along the propagation path. For these reasons, the spin fading observations are easily disturbed by other fading phenomena such as multiple path reception or Faraday fading. For example, Sputnik I showed a spin fading period of about 4.6 sec. The period of Faraday fading observed was 4.4 sec at 20 mc and 15.0 sec at 40 mc. Thus, it is sometimes difficult to differentiate both phenomena. However, with frequencies in the order of 100 mc the periods of Faraday fading are in the order of minutes and will not divert spin fading observation. Further, multiple path reception is much less probable with such higher frequencies than in the 20-and 40-mc range. Therefore, the observations of Vanguard I on 108 mc are rather free from errors due to other types of fading.When a satellite is placed into orbit, the effects of gravity are greatly reduced in the sense that the satellite can be considered a free gyroscope and under no constraint. Therefore, small forces become effective. Torques acting at the spin axis produce a decay of the spin rate. Other small torques not aligned along the spin axis cause precession and lead to tumbling. Such tumbling or precession period causes a complicated rotation of the antenna pattern with respect to the observer on Earth. The resulting ambiguity in the interpretation of the data is demonstrated by Fig. 1, which shows the spin and precession fades during the early life of Vanguard I. The ambiguity was solved in this case by independent telemetry signals measuring the initial spin before third-state separation during the launching operation. 2 The author is indebted to R. I. Easton and M. J. Votav for this communication.Many authors have described the relationship between the radiation...