24 hours, allowing inflight calibration. The instrument package included a circuit for demodulating the outputs from the sensors in the spin plane. The noise threshold was about 0.2 nT. The instrument had three ranges covering ± 20, 60, and 200 nT full scale for each vector component. The digitization accuracy for each range was 1% of the entire range covered. The magnetic field vector was measured instantaneously, and the instrument range was changed after each measurement. A period of 2.05-seconds elapsed between adjacent measurements and a period of 6.14-seconds elapsed between measurements using the same range. The instrument performance was normal. Explorer 35 provided important reference data for magnetic field measurements taken on the Moon during the Apollo program. Bistatic Radar Observations of the Lunar Surface The purpose of this experiment was to study the electromagnetic reflective properties of the lunar surface. The 136.10-Hz () telemetry transmissions from the spacecraft were scattered from the lunar surface and then recorded by use of the Stanford dish antenna. The reflected signal intensity depended upon the lunar reflectivity, the spacecraft altitude above the lunar surface, and the mean curvature of the Moon. The returned signal bandwidth was proportional to RMS lunar surface slopes. Occultation phenomena permitted a determination of the scattering properties of the lunar limb. The dielectric constant of the lunar subsurface in the scattering region below a depth of about was then determined from a profile of reflectivity values versus the angle of incidence on the Moon. The mean lunar slope over each area from which signals were reflected has also been inferred. The observations were located within about 10° of the lunar equator. Experiment operation was normal as of March 1971. Electron and Proton Detectors Three EON type 6213 Geiger–Müller tubes (GM1, GM2, and GM3) and a silicon solid-state detector (SSD) provided measurements of solar X-rays (GM1 only, between 2 and 12 A) and charged particles in the vicinity of the Moon. GM1 and GM3 measured electrons of energies greater than 48 to 50 keV and protons of energy greater than 740 to 820 keV, while GM2 was shielded by a cap with approximately 1 gram per cm2 (limiting its response to protons of energies greater than about 55 MeV). The SSD output was discriminated at four thresholds: (1) PN1, which detected protons between 0.32 and 6.3 MeV, (2) PN2, which detected protons between 0.48 and 3.0 MeV, (3) PN4,