WATCH (Wide Angle Telescope for Cosmic Hard X-Rays)

WATCH acronym for Wide Angle Telescope for Cosmic Hard X-rays was an instrument designed for accurate localization of cosmic gamma-ray burst sources employing a novel approach based on the rotation modulation collimator principle. The detector developed at the Danish Space Research Institute was intended for satellite applications, but was flown on balloon flights during the development phase.

WATCH balloon payload is shown schematically on the figure at left (click for more details of the detector). Unlike conventional rotation modulation collimators, which required an additional monitor counter to determine burst time structures, this instrument eliminated the need for such a counter by utilizing an innovative detector arrangement. It incorporated two interleaved detector types, arranged in a mosaic of alternating NaI(Tl) and CsI(Na) scintillator bars, viewed by a single phototube. This setup allowed electronic separation of signals based on the differing scintillation pulse decay times of the two detector materials.

The instrument achieved modulation by replacing the second grid of a traditional modulation collimator with the interleaved detector system. The sum of the counting rates in the two detectors provided the burst time structure, while the ratio of one detector's signal to the total signal yielded a modulation pattern independent of variations in source flux. This design doubled the system's throughput relative to classical modulation collimators by removing the second grid, thereby enhancing sensitivity and efficiency.

The crystal mosaic used in the detector measured 65 mm in diameter, with individual scintillator strips 5 mm wide and 2 mm thick. A shadow grid composed of 0.5 mm tungsten backed by 1.0 mm tin spanned a diameter of 240 mm and was positioned 50 mm above the mosaic. This configuration resulted in a field of view of 120° full width at half maximum, allowing the instrument to observe approximately one-quarter of the sky at a time. The energy range suitable for modulation detection extended from 40 to 130 keV, as higher-energy photons could penetrate the shadow grid while lower-energy photons suffered excessive atmospheric scattering.

The balloon payload included the instrument along with a spin motor that rotated it at approximately 15 revolutions per minute. A Sun detector monitored the spin rate and measured the Sun's elevation angle, providing data that allowed position determination to an accuracy of about 20 km. The payload also featured a command receiver for flight termination, a ballast release system, and pressure and temperature sensors. A microprocessor served as the central data collection unit, managing sensor inputs and generating the telemetry format. The microprocessor also controlled ballast release based on pressure sensor readings and monitored the X-ray count rate to detect strong transients. If a transient signal was identified, the count rate record was stored in memory until a subsequent transient event was detected, ensuring the capture of gamma-ray bursts even when the balloon was out of immediate contact with a ground station.

The instrument operated with a power consumption of approximately 5 watts, supplied by solar panels. The total weight of the balloon package, including 5 kg of ballast, was 18.5 kg.

Balloon launched on: 8/13/1979 at 16:25
Launch site: Ny-Alesund, Svalbard, Norway  
Balloon launched by: Danish Meteorological Institute
Balloon manufacturer/size/composition: Zero Pressure Balloon  
End of flight (L for landing time, W for last contact, otherwise termination time): 8/29/1979
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 17 d

As the experiment aimed to achieve long-duration balloon flights in the Arctic to detect rare gamma-ray bursts based on prior research conducted by the Danish Space Research Institute, in collaboration with other Scandinavian groups a flight campaign was planned with a launch site at Ny �lesund on Svalbard, located at 79°N latitude. The campaign incorporated three downrange telemetry stations positioned across Greenland and Canada, strategically placed to maximize the time during which the balloon remained within range for data transmission.

The launch took place on August 13 at 16:25 GMT. Shortly after ascent, unexpected oscillations in the pressure sensor readings triggered premature ballast release, likely depleting the entire ballast supply within the first 24 hours. This malfunction prevented the intended long-duration level flight, as evident from the recorded pressure and Sun elevation data for the first 180 hours of the mission. Despite this setback, the data confirmed that minimal ballast was required to sustain altitude over extended periods. The balloon's trajectory indicated that it became caught in a high-altitude wind shift west of Thule, resulting in an unexpected southward turn. After vanishing from tracking for approximately a week, it reappeared near Thule on August 28 and 29, roughly 400 hours after launch. Despite efforts, no signals were received from Mould Bay, even with extensive search attempts.

During the flight, the recorded X-ray count rate was surprisingly low despite the negligible geomagnetic cut-off in the Arctic region. For the first 100 hours, the count rate remained at approximately 25 counts per second within the energy range of 40 to 130 keV. However, a subsequent solar particle event caused a significant increase in background radiation, raising the count rate to approximately 300 counts per second at the peak of the event. No cosmic gamma-ray bursts were detected throughout the mission, though the inconsistent altitude profile likely would have impeded accurate source localization even if bursts had been observed. Following the results of this mission, preparations began for a new campaign in the summer of 1980, incorporating two instrument payloads to refine and improve data collection capabilities.