Radio Telescope Center Point Calibration Method Using Reference Objects

  Objectives  Deep space antennas are widely used in deep space and radio astronomical observations, and their precise position coordinates are required for research needs. In many cases, this precise position is given by very long baseline interferometry (VLBI) based on an initial position with millimeter‐scale accuracy. Global position‍ing system(GPS) alone is merely not sufficient to obtain the initial position of newly‐built antennas. Therefore, a method using other objects as reference is needed to calibrate observatory positions, especially antenna arrays, and this paper presents such a method for deep space and radio astronomical observations.

  Methods  This method employs an azimuth‐elevation telescope with an accurate position as a reference object to measure the center position of a target telescope. Other reference objects with satisfying position accuracy can also be used in this method. GPS receivers are adopted to establish a refer‍ence network with giv‍en scale and orientation by differential measurement. The next step is measuring the local positions of two telescopes (or objective and reference) in the reference network using total station instruments deployed in several different locations. Calculations are induced during the transfer of positions in the reference system to unify data from those measurements. The position of the target to the reference object in a local coordinate system is obtained. The final step is the calculation of the coordinates of the target telescope through the accurate position of the reference telescope. A series of processes are used to convert coordinate positions from the local reference system to the international terrestrial reference frame 2014 (ITRF2014) refer‍ence system.

  Results  Experimental measurement was carried out to obtain the coordinates of the 40 m diameter radio telescope in national observatory of China (NAOC) Miyun observatory with the 50 m diameter radio telescope also there as a reference object. Total station measurement was conducted at three locations around the reference telescope and four locations around the target telescope. Two reflecting prisms calibrated by GPS devices were used as scale and orientation reference. Total station targets were placed on azimuth and elevation structures of both telescopes. The target position data were obtained when the telescope rotated and tilted at multiple angles. More than 400 times of measurement were taken during a two-day field experiment. Round-fitting and sphere-fitting achieved by the trust-region method were used to process measured points, and the coordinates of the 40 m diameter radio telescope in NAOC Miyun observatory were given with a calibration error of 2.312 mm (root mean square error, RMSE) in the ITRF2014 system.

  Conclusions  This method provides center point location in mm or submm-scale precision in complex terrain situations with simple tools and low time cost. The measurement of total station instruments towards reference objects or target objects could be the direct or centering measurement. This method can be applied to obtain positions of single or multiple antennas for observation and further calibration. Observatories can apply this method in complex terrain locations such as mountains, rocky fields, and even a meteor crater on an extra-earth planet. Subsequent observations show that this calibration has met the need.