The attitude and spin rate information were observed by a Sun sensor and a three-axis magnetometer.

[3] Satellite performance was satisfactory except for a partial power failure in May 1966, which reduced data acquisition time to about half the nominal amount.

Responsibility for standby monitoring of the satellite was given to the ESSA telemetry station at Boulder, Colorado, on July 8, 1969.

During this standby operation, experiment data were collected only once on 1 October 1969, for 9 minutes from the electrostatic probe for use in studying a red arc event.

Each sensor was basically a Langmuir probe consisting of a collector electrode extending from the central axis of a cylindrical guard ring.

The two sensors were mounted on opposite sides of the spacecraft, and were perpendicular to the spin axis and in the orbit plane.

The probe current-voltage characteristics were investigated by means of the same modulation technique that was used in the spherical ion-mass spectrometer.

The amplitude and depth of modulation of the resulting carrier current were then measured as a function of probe potential.

The output ion current from the multiplier was measured by a logarithmic electrometer amplifier and converted to a voltage.

The instrumentation was a modified Langmuir probe in which unwanted ion and photo-current components were eliminated through the use of a grid with appropriate bias.

The grid was mounted flush with the satellite surface and it received a sweep voltage of from -5 to +4 V. The collector was biased at +25 V. From the measured current-voltage data the electron density could be obtained with an accuracy of about 20%.