Explorer 38

Explorer 38

Explorer 38 satellite
Names	RAE-A RAE-1 Radio Astronomy Explorer-1
Mission type	Radio astronomy
Operator	NASA
COSPAR ID	1968-055A
SATCATno.	03307
Mission duration	1 year (achieved) 56 years and 9 days (in orbit)
Spacecraft properties
Spacecraft	Explorer XXXVIII
Spacecraft type	Radio Astronomy Explorer
Bus	RAE
Manufacturer	Goddard Space Flight Center
Launch mass	602 kg (1,327 lb)
Power	25watts
Start of mission
Launch date	4 July 1968, 17:26:50GMT
Rocket	Thor-Delta J(Thor 476 / Delta 057)
Launch site	Vandenberg,SLC-2E
Contractor	Douglas Aircraft Company
Entered service	4 July 1968
End of mission
Disposal	Decommissioned
Last contact	July 4, 1969
Orbital parameters
Reference system	Geocentric orbit
Regime	Medium Earth orbit
Perigee altitude	5,851 km (3,636 mi)
Apogee altitude	5,861 km (3,642 mi)
Inclination	120.60°
Period	224.40 minutes
Instruments
Capacitance Probe Impedance Probe Planar Electron Trap Radio Bursts Receivers Step Frequency Radiometers
Explorer program

Explorer 38(also called asRadio Astronomy Explorer A,RAE-AandRAE-1) was the firstNASAsatellite to studyRadio astronomy.Explorer 38 was launched as part of theExplorer program,being the first of the 2 RAE-satellites. Explorer 38 was launched on 4 July 1968 fromVandenberg Air Force Base,California,with aDelta Jlaunch vehicle.

Spacecraft

Explorer 38 spacecraft measured the intensity of celestial radio sources, particularly theSun,as a function oftime,directionandfrequency(0.2 to 20-MHz). The spacecraft wasgravity-gradientstabilized. The spacecraft weight was 602 kg (1,327 lb), and average power consumption was 25watts.It carried two 230 m (750 ft) long V-antennas,one facing toward theEarthand one facing away from the Earth. A 37 m (121 ft) long dipole antenna was oriented tangentially with respect to theEarth's surface.

The spacecraft was also equipped with one 136-MHztelemetryturnstile. The onboard experiments consisted of four step-frequency Ryle-Vonberg radiometers operating from 0.45 to 9.18-MHz, two multichannel total powerradiometersoperating from 0.2 to 5.4-MHz, one step frequency V-antenna impedance probe operating from 0.24 to 7.86-MHz, and one dipoleantennacapacitance probe operating from 0.25 to 2.2-MHz. Explorer 38 was designed for a 12 months minimum operating lifetime.

The spacecrafttape recorderperformance 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 radiointerferencecoming from the Earth, both natural (aurorae,thunderstorms) and artificial.

Instruments

Explorer 38 has 4 antennas deployed in orbit:

• Two V-shaped antennas with each of the 4 branches being 229 m (751 ft) long and used by scientific experiments;
• A 37 m (121 ft)electric dipoleantennaused by scientific experiments;
• A cross-dipole turnstile antenna for the transmission oftelemetryon a frequency of 137-MHz.

The scientific experiments are:

• Four Ryle-Vonbergradiometersanalyzingfrequenciesbetween 0.45 and 9.18-MHz;
• Two multi-channel radiometers analyzing frequencies between 0.2 and 5.4-MHz;
• An impedance probe associated with 5 antennas analyzing frequencies between 0.24 and 7.86-MHz;
• A capacitance probe associated with the dipole antenna analyzing frequencies between 0.25 and 2.2-MHz.

Experiments

Capacitance Probe

Determine reactive and resistive components of antenna impedance as a function of localelectron density,electron temperature,magnetic field,and vehicle potential. The impedance measurements was made at 10 frequencies (0.25 to 8-MHz).

Impedance Probe

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).

Planar Electron Trap

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 reducephotoemissionof 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.

Radio Bursts Receivers

Thirty-two channel step frequencyradiometerswere connected to the lower 230 m (750 ft)-long antenna and to the 37 m (121 ft)-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 50dBabove 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.

Step Frequency Radiometers

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 37 m (121 ft) dipole, one to the lower 230 m (750 ft) 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.

Results

The following results are reported in 1971:

• Absolute spectrum and average cosmic noise up to the frequency 0.5-MHz.
• Collection ofradiodata transmitted during type IIIsolar radio burstsin the 0.2 to 5-MHz frequency band. These elements made it possible to obtain a first estimate of thesolar coronaelectrondensitygradient,thesolar windspeed and density inhomogeneities in the solar corona regions between 10 and 30solar radiis.A second radio broadcast ofhectometricfrequency was observed.
• An upper limit to the radio flux emitted byJupiter'sHigh frequency(HF) radio broadcasts was determined by the observations made during theMoon's occultations of the giant planet.
• Radio emissions from theEarthof natural andhumanorigin are both widespread and often very intense (40dBhigher than thecosmic background) on the frequencies observed (0.2 to 9.2-MHz).

See also

• Explorer 49
• Explorer program

External links

• Real Time Satellite Tracking and Predictions: Explorer 38 (RAE-A)N2YO
• Data cisplays of Radio Astronomy Explorer-1