The Geiger Tube Telescope is a scientific instrument that measures the intensities, energy spectra, and angular distribution of energetic electrons and protons in interplanetary space and near Jupiter and Saturn.
On Pioneer 10, the instrument used an array of seven miniature Geiger-Müller tubes, collectively known as a Geiger Tube Telescope (GTT). Each tube was a small gas-filled cylinder. When a charged particle passed through the gas, an electrical pulse was generated by the applied voltage. Individual pulses from five of the tubes and coincident pulses from three combinations of the seven tubes were transmitted. Protons of energy greater than 5 MeV and electrons with energies greater 40 keV were detected.
On Pioneer 11, one Geiger-Müller tube was replaced by a thin silicon wafer to detect protons in the specific energy range 0.61 to 3.41 MeV. Other minor changes were made to improve the characteristics of the detector system.
The trains of pulses were passed through quasi-logarithmic data processors and then to the radio telemetry system of the spacecraft. Angular distributions were measured as the spacecraft rotated. This telemetry data was transmitted to the earth by an 8 watt S band transmitter within the Pioneer probe at one of eight data rates (from 16 to 2048 bits per second).
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—Robert Frost (1874–1963)