VNIPIvzryvgeofizika

Astronomical calibration is defined as determination of the constant attitude of digital cameras relative to the inertial measurement unit (IMU) using ground-based star observations. The first part of [1] focuses on the calibration of relative attitude of all cameras, which provides combination of their separate measurements into the observations of one virtual camera. The second part examines the calibration of virtual camera attitude relative to the IMU. The calibration is performed on a simple test bench, which is unable to set the accurate attitude of the calibrated unit relative to the stars. Therefore, the calibrated parameters are estimated by the attitude increments over a time period between the astronomical measurements. Formulas for calculating the attitude increments from the inertial and astronomical measurements and their covariance matrices have been obtained. It has been shown that not all calibration trajectories provide unambiguous estimation of the calibrated parameters based on the attitude increments. The necessary condition for selecting the required trajectories has been formulated. The formula for finding the error covariance matrix of IMU attitude by the astronomical measurements has been received. This formula considers the star configuration observed with the virtual camera, its calibration errors described in the first part of the paper, and calibration errors in its attitude relative to the IMU obtained in the second part. The RMS error of the attitude calibration is experimentally found to be about 6 arcsec.

Astronomical calibration refers to determination of the constant mutual attitude of digital cameras and the inertial measurement unit using ground-based star observations. The first part of this paper considers calibrating the attitude of all cameras relative to the selected one. The calibrated vector contains the useful parameters (three attitude angles of each camera relative to the selected one) and interfering parameters (three attitude angles of the selected camera in the Earth-fixed frame at the time of shooting each frame). The system of nonlinear equations for determining the calibrated vector is based on the differences in the coordinates of images of the identified stars, which are calculated in the image planes of different cameras for each frame and then combined into a common residual vector. Atmospheric refraction and velocity aberration of light are considered when projecting the identified stars from the star catalog onto the image plane. Before solving the system of equations, intrinsic parameters of each camera are determined. Calibrating the relative attitude of real cameras provides a virtual camera with an extended field of view, which significantly reduces the error in star-based attitude determination. A virtual camera error model is provided that takes into account the errors in astronomical calibration. The results from experimental verification show that the error in astronomical calibration of the cameras’ relative attitude does not exceed 2 arcsec for each useful parameter.

The structure and phase composition of welded joints of austenitic steel 08Kh17N10T and titanium alloy PT-3V formed by diffusion welding with an intermediate layer of ultrafine nickel powder have been studied. The microstructure of joints welded in various regimes was studied using metallographic and energy-dispersive x-ray microanalysis techniques. Good prospects of the application of ultrafine dispersed media for diffusion welding of steels and titanium alloys are shown.