[3] Explorer 18 was a solar cell and chemical-battery powered spacecraft instrumented for interplanetary and distant magnetospheric studies of energetic particles, cosmic rays, magnetic fields, and plasmas.

[4] A charged-particle, solid-state telescope was used to measure the range and energy loss of galactic and solar cosmic rays.

The output from two 128-channel, pulse-height analyzers was obtained for one incident particle every 41 seconds and read out along with the detector accumulations.

The second detector system consisted of two Geiger–Müller tube (GM) telescopes oriented parallel to and perpendicular to the spacecraft spin axis.

The parallel, perpendicular, and omnidirectional count rates were obtained for one 40-seconds accumulation interval during successive normal 81.9-seconds telemetry sequences.

Measurements consisted of 22 instantaneous current samples, each separated by 0.16 seconds (spanning more than one satellite rotation).

Interference was encountered from refracted particles (with the most pronounced effect at about 70° incidence to cup normal), from secondary electrons, and from Ultraviolet radiation.

[7] Each of two uniaxial fluxgate magnetometers, having dynamic ranges of ± 40 nT, sampled the magnetic field 30 times within each of six 4.8-seconds intervals every 5.46 minutes.

[8] The instrumentation for this experiment, designed to measure fluxes of geomagnetically trapped particles, consisted of a 7.6 cm (3.0 in) diameter, Neher-type ionization chamber and two Anton 223 Geiger–Müller tubes.

The second Geiger–Müller tube had no direct access to the space environment and responded omnidirectionally to background electrons and protons with E>6 and E>52 MeV, respectively.

It was mounted normal to the spacecraft spin axis and had an effective look angle of 5 sr. Coarse and fine resolution modes were programmed for both ions and electrons.

These modes consisted of 15 steps each for retarding voltages of 0 to 28 V and 0 to 100 V. The entire ion and electron sequence was repeated once every 10.92 minutes, and each 15-step spectral analysis required 5.4 seconds.

[13][14] The DDP allowed the different onboard digital experiments to store results into accumulators which were then read out on a repeating cycle and encoded into pulse-frequency modulation (PFM) signals to be sent to ground stations.

[12] In addition to the digital data sent in PFM format, a little over half of the normal transmission cycle (9 of 16 "frames") was used for sending analog signals.

[15] The processor used Series 51 chips from Texas Instruments, specifically the SN510 (a clocked SR latch) and the SN514 (dual 3-input NAND/NOR gates), which both came in 8-pin flatpack packages and used resistor-capacitor-transistor logic (RCTL).