Operations and Data Handling

W

ith the launching of the satellites, the work was only half done. Tracking the new birds, operating them, collecting their data, and processing and analyzing the data were of equal importance. The first several of those tasks are addressed in this chapter, with the data analysis effort being deferred to the next chapter.

As in the case of Chapter 5, the details in this chapter may be beyond the interest of some readers. They are included for those who may have a historical interest, or who may find them useful in their professional work. The more casual reader may wish to skip to the next chapter.

Explorer I operation

It was late evening, 31 January 1958, on the U. S. East Coast when Explorer I was launched. By then, it was early morning of the next day in Greenwich, England. It has been common for those most comfortable with local time to mark the launch date as 31 January, and for those preferring universal time to ascribe it to 1 February. Throughout this book, the 31 January convention is used.

A network of ground stations had been established for the Vanguard satellites that could provide tracking and data telemetry contact at least once each orbit. Starting with that basic array, the network evolved over time to make the coverage increasingly robust.

As an interesting side note, the Soviets did not establish a comparable worldwide network for their early Sputniks, but depended on a combination of stations within the USSR, plus coverage by radio amateurs and other volunteers who supplied some

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tracking information. As mentioned previously, an onboard data recorder was included on Sputnik 3, but it failed before launch and was not repaired. An examination of Soviet papers reporting early results from Sputnik 3 reveals that their scientific data coverage was almost entirely limited to the region 20 degrees to 145 degrees east longitude and 42 degrees to 63 degrees north latitude, that is, over the Soviet Union.1

The ground paths of the first several Explorer I orbits are plotted in Figure 11.1, along with the locations of the ground stations. Although based on sparse initial tracking data, the plot from which this figure was derived was accurate enough for early satellite operation, data acquisition, and processing efforts.2 Orbit number 0 (zero) began with the satellite’s orbital injection off the Florida coast and lasted until it passed its first ascending node, that is, its first northbound crossing of the geographical equator. Subsequent orbits were numbered sequentially as the satellite passed each ascending node.

The first public report of data from the Explorer I cosmic ray instrument was issued about four weeks after the launch.3 Although that report provided an excel­lent summary of the situation as it was known then, it was based on very early and incomplete telemetry data. Much of its content dealing with the character of the cosmic ray data was modified later as data analysis progressed, especially af­ter the first data were recovered from the onboard tape recorder in Explorer III on 26 March.

The final assessment of Explorer I performance was pieced together from a large number of sources that were written over a substantial period. The most authoritative was the full tabulation of Explorer I data that was finally published about three years after the launch.4 In summary: [7]

FIGURE 11.1 The ground tracks for the first orbits of Explorer I. This version was released less than 12 hours after the launch and was used for initial operations. Throughout the Explorer I lifetime, the orbit was refined repeatedly, as more and more extensive and accurate tracking data were obtained. Locations of the Explorer I and III tracking, data receiving, and commanding stations have been added to the original plot to indicate their relationship to the orbital tracks. (By Wilbur S. Johnston and the author, after a map produced by the Vanguard Computing Center, Naval Research Laboratory.)

OPENING SPACE RESEARCH

Unexplainably, the high-power transmitter signal reappeared 11 days later, and sparse but partly readable data were obtained during four days from 24 to 27 February. It disappeared for the final time on 27 February.

There was a partial failure of the high-power transmitter’s antenna. That antenna consisted of four flexible stainless steel cables, with the base of each cable anchored at the high-power antenna insulator; a donut-shaped ring was located between the instrument payload and the fourth-stage rocket motor. The four antenna cables (which we usually referred to as whips) were connected to the transmitter through a phasing harness and served as the active elements of that circularly polarized antenna. The flexible cables can be clearly seen in the drawing of Figure 8.4 and the photograph of Figure 8.5.

It had been expected that those flexible whip elements would be held in ax­ially semirigid positions by the centrifugal force resulting from the spinning of the satellite about its long axis. That did not occur. A Jet Propulsion Lab­oratory (JPL) report dated 21 February stated that the final spin rate during rocket ascent had been as designed, at 750 revolutions per minute (rpm). That was ascertained by direct measurements via the launch vehicle telemetry system and was verified by observation of the amplitude modulation of the high-power signal caused by the normal irregularities in its radiation pattern. An abrupt drop in satellite spin rate to 570 rpm occurred about one second after comple­tion of the final fourth-stage burn. That change in spin rate was accompanied by a concurrent increase in the amplitude of the spin modulation of the radi­ated high-power signal. Those facts pointed to the loss of one of the antenna elements.5

There was a departure of the satellite’s motion in free space from that ex­pected. It showed up as a slow periodic variation in the received signal strengths from the high-power transmitter superimposed on the faster variation resulting from the satellite spin. The period of that slower variation was 6.9 seconds, corresponding to a frequency of 8.4 cycles per minute. The slower modula­tion was first seen following the postburning transient mentioned above and grew in amplitude during the first few orbits. As the slower modulation grew in amplitude, the faster modulation decreased, and it disappeared entirely after several orbits. The new slow modulation continued for the rest of the satellite’s lifetime.

That phenomenon resulted from an unanticipated resonance coupling between the spin rate of the satellite and the free-pendulum oscillation rate of the whip antennas. Those two rates turned out to be nearly the same, with the result that the whip antennas were driven to swing violently back and forth. That bending of the whips resulted in a higher-than-expected dissipation of the rotational energy of the satellite. The original spin was around the body’s lowest moment

CHAPTER 11 • OPERATIONS AND DATA HANDLING 291

of inertia—its long axis. The dissipation of kinetic energy, combined with the law of conservation of angular momentum, caused the satellite to precess in a cone of increasing opening angle. Ultimately, the satellite rotated about its transverse axis, the axis of greatest moment of inertia.6

In simple terms, the satellite was tumbling end over end, rather than spinning around its long axis, as planned.

The unexpected modulation of the transmitted signals did make it somewhat harder for the receiving stations to acquire the signals and receive the data and resulted in more data dropouts than would have occurred otherwise. That situation, fortunately, caused only a minor reduction in the usefulness of the scientific and engineering data.