| Explorer 38 | |
| Names List: | RAE-A RAE-1 Radio Astronomy Explorer-1 |
| Mission Type: | Radio astronomy |
| Operator: | NASA |
| Cospar Id: | 1968-055A |
| Satcat: | 03307 |
| Mission Duration: | 1 year (achieved) (in orbit) |
| Spacecraft: | Explorer XXXVIII |
| Spacecraft Type: | Radio Astronomy Explorer |
| Spacecraft Bus: | RAE |
| Manufacturer: | Goddard Space Flight Center |
| Power: | 25 watts |
| Launch Date: | 4 July 1968, 17:26:50 GMT[1] |
| Launch Rocket: | Thor-Delta J (Thor 476 / Delta 057) |
| Launch Site: | Vandenberg, SLC-2E |
| Launch Contractor: | Douglas Aircraft Company |
| Entered Service: | 4 July 1968 |
| Disposal Type: | Decommissioned |
| Last Contact: | 4 July 1969 |
| Orbit Reference: | Geocentric orbit[2] |
| Orbit Regime: | Medium Earth orbit |
| Orbit Inclination: | 120.60° |
| Orbit Period: | 224.40 minutes |
| Apsis: | gee |
| Instruments: | Capacitance Probe Impedance Probe Planar Electron Trap Radio Bursts Receivers Step Frequency Radiometers |
| Programme: | Explorer program |
| Previous Mission: | Explorer 37 |
| Next Mission: | Explorer 39 |
Explorer 38 (also called as Radio Astronomy Explorer A, RAE-A and RAE-1) was the first NASA satellite to study Radio astronomy. Explorer 38 was launched as part of the Explorer program, being the first of the 2 RAE satellites. Explorer 38 was launched on 4 July 1968 from Vandenberg Air Force Base, California, with a Delta J launch vehicle.
Explorer 38 spacecraft measured the intensity of celestial radio sources, particularly the Sun, as a function of time, direction and frequency (0.2 to 20-MHz). The spacecraft was gravity-gradient stabilized. The spacecraft weight was and average power consumption was 25 watts. It carried two long V-antennas, one facing toward the Earth and one facing away from the Earth. A long dipole antenna was oriented tangentially with respect to the Earth's surface.[3]
The spacecraft was also equipped with one 136-MHz telemetry turnstile. The onboard experiments consisted of four step-frequency Ryle-Vonberg radiometers operating from 0.45 to 9.18-MHz, two multichannel total power radiometers operating from 0.2 to 5.4-MHz, one step frequency V-antenna impedance probe operating from 0.24 to 7.86-MHz, and one dipole antenna capacitance probe operating from 0.25 to 2.2-MHz. Explorer 38 was designed for a 12 months minimum operating lifetime.[3]
The spacecraft tape recorder performance began to deteriorate after 2 months in orbit. In spite of several cases of instrument malfunction, good data were obtained on all three antenna systems. The small satellite observed for months the "radio sky" in frequencies between 0.2 and 9.2-MHz, but it was subjected to the continuous radio interference coming from the Earth, both natural (aurorae, thunderstorms) and artificial.[3]
Explorer 38 has 4 antennas deployed in orbit:[4]
The scientific experiments are:
Determine reactive and resistive components of antenna impedance as a function of local electron density, electron temperature, magnetic field, and vehicle potential. The impedance measurements was made at 10 frequencies (0.25 to 8-MHz).[5]
Determine reactive and resistive components of antenna impedance as a function of local electron density, electron temperature, magnetic field, and vehicle potential. The impedance measurements was made at ten frequencies (0.25 to 8-MHz).[6]
There were two planar electron traps mounted on opposite sides of the spacecraft. The trap consisted of a collector, positively biased in order to repel incoming ions and to reduce photoemission of electrons from the collector. A sawtooth voltage was applied to a grid, and the resulting current to the collector was telemetered. Electron density was obtained by analysis of the grid voltage-collector current profile. The electron density representing the ambient value was that obtained from the probe facing the direction of satellite motion. The spacecraft attitude for this purpose was determined either from the electron density or from the solar and magnetic sensors on the spacecraft. The data were tape recorded and telemetered once each orbit. These sensors operated nominally since launch and were providing electron density mapping data at spacecraft altitude.[7]
Thirty-two channel step frequency radiometers were connected to the lower -long antenna and to the -long dipole via high-impedance preamplifiers. The burst radiometer on the dipole was stepped rapidly through 32 discrete frequencies between 0.2 and 5.4-MHz to generate dynamic spectra. The radiometers measured the amplitude, rate of change of frequency, and decay time of solar burst and other rapidly varying noise in the 0.2 to 5.4-MHz band. Operating in two sensitivity modes, these receivers could measure signals up to 50 dB above the cosmic background level. The 32 channels were cycled every 7.7-seconds. The chief advantages of the burst radiometers were high time resolution and relatively few components for high reliability. The radiometer was a simple total-power receiver consisting of an input balun, a power divider, and several parallel tuned-radio-frequency strips. After about 18 months of operation, one of the preamplifiers on the lower V burst radiometer failed, reducing the sensitivity and changing the antenna pattern for that radiometer.[8]
This experiment used four Ryle-Vonberg radiometers connected to the three spacecraft antennas to provide high accuracy and long-term stability necessary for the sky mapping over many months of operation. One was connected to the dipole, one to the lower V-antenna, and two to the upper V-antenna. The Ryle-Vonberg radiometers used on the V-antennas were connected via balun transformers that provided an approximate match to the antenna impedance. Each radiometer was successively tuned to nine different frequencies in the band 0.48 to 9.18-MHz. Precise, automatic, and continuous calibration was inherent in this type of design. The intensities of celestial radio sources were measured by this experiment. The "fine" output channel of the Ryle-Vonberg radiometers failed after 3 to 9 months of operation. The Ryle-Vonberg "coarse" output channels provided good data without interruption, however.[9]
The following results are reported in 1971: