天问一号星下点太阳高度角在轨实时计算方法

On-orbit real-time calculation method of solar elevation angle of sub-satellite point of Tianwen-1

由于太阳高度角和地面景物反射率等条件的变化,天问一号火星环绕器光学成像载荷在轨工作期间入瞳辐亮度变化范围很大,为了达到最佳的成像效果,需要光学成像载荷具备在轨自适应调整增益的能力。太阳高度角是设置时间延时积分电荷耦合器件积分级数的重要参数之一,积分级数是调整增益需要调整的主要参数。针对在轨工作期间太阳高度角实时变化、星历表文件较大等问题,本文提出一种基于傅里叶拟合的火星环绕器星下点太阳高度角在轨实时计算方法。首先,基于最小二乘原理采用8阶傅里叶逼近对火星惯性坐标系下太阳矢量的x,y,z坐标进行拟合,获得以时间作为变量的拟合方程。其次,根据制导、导航与控制系统发送的轨道参数获得火星惯性坐标系下环绕器的实时坐标。最后,基于夹角余弦公式即可在轨实时计算星下点太阳高度角。实验结果表明,在协调世界时2021-01-01 00:00:00至2024-01-01 00:00:00期间,采用本文方法获得的星下点太阳高度角实时计算结果最大绝对误差小于0.3°。满足天问一号高分辨率相机时间延时积分电荷耦合器件积分级数设置对太阳高度角计算结果的精度要求。基于该方法,天问一号高分辨率相机获取的火星影像细节丰富,亮度、对比度合理。

The entry-pupil radiance of the optical imaging instruments of Tianwen-1 varies considerably during its orbit operation,owing to the changes in the solar elevation angle and ground scene reflectance.To achieve the best imaging effect,the optical imaging instruments should utilize on-orbit adaptive adjustment gain. The solar elevation angle is an important parameter that is used to set the integral series of the time delay and integration(TDI)charge-coupled devices(CCDs). Furthermore,the integral series is the main parameter used to adjust the gain. This study presents a method for calculating the solar elevation angle in real time at the sub-satellite point of the Mars orbiter based on Fourier fitting,to mitigate the challenges of the real-time variation in the solar elevation angle and the large ephemeris file created during the orbiting period. First,an 8-order Fourier approximation based on the principle of least squares is utilized to fit the x,y,and z coordinates of the sun vector in the Martian inertial coordinate system,and a fitting equation is obtained as a function of time. Second,the real-time coordinates of the orbiter in the Mars inertial coordinate system are obtained based on the orbit parameters sent by the guidance and navigation control system. Finally,the solar elevation angle of the sub-satellite point can be calculated in real time on the orbit based on the cosine formula of the included angle. The experimental results show that the maximum absolute error of the real-time calculation results of the solar elevation angle obtained by using this method is less than 0. 3° during the period from 2021-01-01 00:00:00 UTC to 2024-01-01 00:00:00 UTC. The accuracy requirements of the calculation results of the solar elevation angle of the TDI CCD integral series of the Tianwen-1 high-resolution camera are satisfied. Based on this method,the Mars image obtained by the high-resolution imaging camera of Tianwen-1 shows rich details with reasonable brightness and contrast.