The PRA instrument consists of an antenna and a radio receiver. The antenna is made up of two thin metal poles, each 10 meters long, extended from the spacecraft after launch at an angle of 90 degrees to each other. These are electrically connected to two receivers of extremely high sensitivity and broad frequency response: from 1.2 kilohertz to 40.5 megahertz. The PRA can operate in a number of modes, depending on the measurements desired. At its lowest level of activity, it monitors intensity in all 198 bands and transmits the data at 266 bits per second. In its highest mode, where searches are made for variations with very short time scales, the data rate goes up to 108 000 bits per second, essentially the same as that required by imaging. In fact, the high-rate PRA data actually use an imaging frame as a display form, and occasionally throughout the mission an unfamiliar looking “image” was transmitted that was actually a block of PRA data—a portrait of electrical events in the Jovian atmosphere and magnetosphere that only Warwick and his colleagues could interpret. [Illustration: Norman F. Ness, magnetometer Principal Investigator] [Illustration: Voyager’s 13-meter-long magnetometer boom is shown fully extended. In space, under zero gravity conditions, the triangular epoxy glass mast spirals from its housing and provides a rigid support for two magnetometer instruments—one at the end of the boom and another at about the midpoint. [260-181]] Magnetometer The first of the direct sensing instruments to be discussed is the magnetometer, designed to measure the magnetic fields surrounding the spacecraft. Such measurements can be interpreted to yield the intrinsic fields of Jupiter and its satellites and to characterize, in conjunction with data from particle and plasma instruments, the processes taking place in the magnetosphere of the planet. The Principal Investigator for this instrument is Norman F. Ness of the NASA Goddard Space Flight Center. Ness is an intense, competitive scientist with a great deal of previous experience in spacecraft magnetometers, primarily on board Earth satellites. Ness is also the only Voyager Principal Investigator who has previous experience at Jupiter; he was Principal Investigator on one of the two magnetometer instruments flown on Pioneer 11. For the Voyager investigation, Ness is joined by four colleagues from Goddard and one from Germany.