Explorer 27 (or BE-C or Beacon Explorer-C, Beacon-C or S-66C) was a small NASA satellite, launched in 1965, designed to conduct scientific research in the ionosphere.

The program will determine the total electron content of a vertical cross-section of the ionosphere located between the satellite and the Earth.

Accomplishing this objective will aid in establishing the behavior pattern of the ionosphere as a function of latitude, time of day, season, and solar cycle".

[4] S-66 mounted 360 25 mm (0.98 in) "cube corner" retroreflectors made of fused silica,[4] so that the satellite could be tracked via lasers beamed from mobile stations at Wallops Flight Facility (WFF).

[9] Explorer 22's as-yet unnamed backup was scheduled for launch in March/April 1965 to extend the geodetic experiments of its predecessor.

[10] Upon the backup's launch on 29 April 1965 at 14:17:00 GMT via Scout X-4 from Wallops Island, this third S-66 satellite was designated Explorer 27.

This experiment performed nominally from launch until 13 August 1968, when solar cell degradation resulting from radiation prevented operation of all systems on the satellite.

The plane of polarization of the three lower frequencies underwent an appreciable number of rotations due to electron concentration.

[13] Two coherent, unmodulated CW transmitters, operating on frequencies of 162 and 324 MHz, allowed the Tranet Doppler Network to obtain data for studies of dynamic geodesy.