fields could be measured, however, because of the spacecraft's low apogee. The instrument was similar to that flown on Pioneer 1 and consisted of a single search coil mounted so that it measured the magnetic field perpendicular to the spacecraft spin axis. The instrument had a range of 0.6 nT to 1200 nT. No inflight calibration was provided. Some degradation of the telemetry signal occurred due to ionospheric effects. Insufficient ground observations on the electron content of the ionosphere prevented correcting the data for these effects. The experiment had both digital and analog outputs. The magnetometer amplitude and phase were sampled continuously for analog transmission and intermittently (every 2 minutes, 15 seconds, or 1.9 seconds, depending on satellite bit rate) for digital transmission. The magnetometer worked until loss of the telemetry signal in early October 1959. TV Optical Scanner The TV optical scanner flown was an improved version of the TV system first employed on Pioneer 2. The experiment consisted of an optical unit containing a concave spherical mirror and phototransistor, a video amplifier, timing and logic circuits, and telemetry. The experiment was designed to test the feasibility of using such instrumentation to obtain low-resolution daylight cloudcover photographs. The scanner also served as a forerunner to the TV camera systems carried on later, more advanced satellites. The scanner's optical axis was directed 45° away from the spacecraft spin axis, which was parallel to the orbital plane. The vehicle's spin furnished the line scanning, and the spacecraft's forward motion along its trajectory provided the frame scanning. During a scan (one spacecraft revolution), a single scan spot (element) on Earth was viewed and transmitted back to Earth. During the next spacecraft revolution, an adjacent spot was scanned. This procedure was repeated until a line of 64 such spots was formed. Then the process was repeated to form an adjacent line of elements, and so on, until a frame, or picture, was obtained. The system could produce useful photographs only when the spacecraft's velocity and orbital position were such that successive lines overlapped. (At apogee, for example, the TV lines were separated by a distance about equal to their length, and hence no meaningful picture could be obtained). Data obtained from this experiment are limited and of extremely poor quality. Proper spacecraft orientation was never achieved, resulting in a considerable amount of blank space between successive scan lines. The scanner's logic circuits also