INTERFEROMETER

The objective of the flight was to perform measurements of the submillimetre wavelength emission spectrum of the stratosphere above 30 km, using a Michelson interferometer and Fourier spectroscopic techniques.

The basic instrument was a single-output Michelson interferometer, based on the NPL Grubb-Parsons versior available commercially, but reduced from about 20 to 8 kg in weight by dispensing with unnecessary bulk metal, and by a careful design of the optical components. The moving mirror, was mounted on a nonrotating shaft connected to a 25 mm diameter micrometer, with a 25 mm traverse. This micrometer was driven by a synchronous motor. The chopping was accomplished using phase modulation and to do this, the second mirror of the interferometer was mounted on a loudspeaker vibrator coil.

The detector was a commercial Golay cell, which used a Ga-As lamp in the internal optical lever mechanism. Radiation was collected from the sky using a Cassegrain telescope of 350 mm aperture, set at an angle of 4º above the horizontal. The sky radiation received was limited to submillimetre wavelengths by using a polyethylene scatter filter and 2 pieces of 100 µm black polyethylene before the detector and 100 µm black polyethylene at the input aperture of the interferometer. The beam splitter was 100 µm thick Melinex.

The system was designed to be almost fully automatic, so that a minimum of transmitted commands would be required during the flight. The electronics supply and control system, was contained in a sealed aluminium can embedded in polystyrene foam, for thermal insulation and crash-padding. The data obtained by the instrument was relayed to the ground using an EMI UOM1O solid-state type transmitter operating at 444 MHz. It was embedded in the polystyrene on a carefully designed heat sink, at the bottom of the package, immediately above the aerial and ground plane. Power for the complete system was obtained from two types of Ag-Zn alkaline cells which could supply 10 and 15 A-hr, respectively.

The command receiver unit was a FM tone-modulator operating at 71 MHz that allowed to send commands to the instrument and also carried a channel for the cut-down command. This was powered by its own battery supply.

The whole experiment was contained in a cylindrical gondola constructed from 25 mm diameter tubular aluminium. It had a diameter of 1.20 meters, a height of 1 meter and was filled with expanded polystyrene. The interferometer was suspended from cross-struts, using antivibration mounts to reduce shocks. The interferometer was thermally insulated using 10 mm thick polystyrene sheets, but no attempt was made to evacuate or to seal the instrument. Dry nitrogen was leaked through the interferometer from a small supply dewar embedded in the polystyrene, to remove contaminant water vapour. The total weight of the gondola was 50 kg.

Balloon launched on: 10/2/1971 at 8:05 utc
Launch site: Cardington, Bedfordshire, England  
Balloon manufacturer/size/composition: Zero Pressure Balloon  
End of flight (L for landing time, W for last contact, otherwise termination time): 10/2/1971 at 11:50 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 3 h 45 m
Landing site: Near Rushden in Northants, England
Payload weight: 50 kg

The launch took place at 8:05 utc on 2O October 1971 at Cardington, England. The apparatus was switched on manually some minutes before launch, and interferogram data were received continuously from then on. The launch caused a jolt to the apparatus, and a consequential slight misalignment of the interferometer. However, this was not serious. The data received at the ground were recorded in analogue form on a chart recorder, and also in digital form on paper tape. The ascent rate was approximately 4 meters per second, and the maximum altitude attained at 10:00 utc was 105,000 ft while the balloon moved slowly to the NE. At this point the upper winds began to change and eventually moved to the south. At 11.05 UTC, a fault developed in the command receiver which resulted in a loss of further data. The flight continued for one more hour while efforts were made to reactivate it, without success. Finally the flight was terminated at 11.50 UTC as the balloon was approaching restricted air space. The payload descended under parachute for about 1.25 hr and was recovered by 13.30 UTC from a field near Rushden in Northants, England, roughly 20 miles northwest of Cardington.

• Measurements of the submillimetre stratospheric emission spectrum from a balloon platform Infrared Physics, Volume 13, Issue 2, p. 149