TEST OF AN AUTOMATIC TRACKING SYSTEM FOR BALLOON-BORNE TELESCOPES

The objective of the flight was to test a new automatic tracking system for balloon-borne astronomical observation objects. It was tested coupled to an X-Ray telescope. The tracking system was developed at the Purple Mountain Observatory of the Chinese Academy Of Sciences.

Balloon-borne astronomical observations must consider that the telescope is always in motion. The balloon position is constantly changing and is interfered by many factors such as wind speed and wind direction. The celestial signal from space sources is weak and the target must be accurately tracked for a long time to obtain useful data. To overcome this constrains a new automatic tracking system was developed that could accurately track the measured celestial body for a time to keep the telescope aligned with the measured celestial body during any kind of movement.

The system was composed by a control computer system (based on a modular STD system designed according to the IEEE-961 standard, with an 80C196CPU motherboard and a digital I/O diffuser), an attitude energy stabilization system and a GPS receiver autonomously working in a closed-loop control system. A telemetry system sent all the pointing information to the ground station. The software used by the onboard computer was compiled using the PLM96 high-level language, using floating-point calculations to find the target and give the correction value to the telescope through an actuator. The gondola was stabilized by the attitude stabilization system, which provided a stable working platform for the telescope based on the geomagnetic sensor to point to the true north (0°) and clockwise from 0º to 360°. The azimuth was changed by rotating the entire gondola while the elevation angle was directly driven by the motor, so that the telescope can stably follow the target that the GPS receiver selected.

After reading the current real-time position of the telescope the computer calculated the tracking azimuth, the zenith distance and the elevation angle and then adjusted the telescope to the target. At the time, the novelty of this system was that it made full use of the GPS global positioning technology, obtaining the real-time geographic location from the positioning data once per second. It obtained the wind speed, wind direction and disturbance from the new position, and the automatic correction eliminated the accumulated errors, no longer needing the positioning obtained from a ground-based radar, which greatly simplified the system design.

Balloon launched on: 5/17/1998 at 00:52 local time
Launch site: Zhengding Airport, Hebei,China  
Balloon launched by: Institute of High Energy Physics (IHEP)
Balloon manufacturer/size/composition: Zero Pressure Balloon  
End of flight (L for landing time, W for last contact, otherwise termination time): 5/17/1998 at 4:00 local time
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): ~ 3 h

The balloon was launched by dynamic method from the Zendhing Shijiazhuang Airport at 00:52 local time on May 17 1998. The planned observations included the observation of 5 different X-Ray targets. However, at about 4:00 local time, due to the failure of the axonometric channel of the engineering system, in order to prevent loss of the payload , the cutting from the balloon was carried out immediately, and the observation was ended. In the map above can be seen the route followed by the balloon and at right the altitude curve.

During the flight, the system has tracked 3 targets, and recorded the data obtained. According to the analysis of the recorded data, the required tracking accuracy was achieved during most of the flight and even some small interferences were corrected within a few seconds. This was enough to prove that the design of the system was successful and fully met the observational requirements: the highest pointing accuracy of the system obtained during the flight was 0.1º. Enough accurate, this system however have great potential to improve some parameters such as increasing the elevation angle, improve the accuracy of the rotating parts and the stability of the attitude control system to be applied to missions with higher requirements. Also the balloon-borne GPS provides data such as route flight, height and time, which can be used to set functions such as automatic end of flight.

• The successful application based on a new automatic tracking system for balloon-borne astronomical observation objects Chinese Journal of Space Science , Volume 20, Issue 2: 172 - 176 (2000)