VLBI测控信号群时延及相时延解算方法研究

Resolution of VLBI Group Delay and Phase Delay from Telemetry Signal

由于探测器天线没有对准地球,遥测信号时有时无或卫星飞离地球更远,导致地面接收到的测控信号带宽变窄,使VLBI群时延测量误差增大,野值点增多。针对这些情况比较了剔除野值点加权幅度的直线拟合、剔除野值点加权幅度平方的直线拟合、剔除野值点加权幅度3次方的直线拟合和剔除野值点加权幅度4次方的直线拟合在求解测控信号群时延时的优劣,得出剔除野值点加权幅度平方的直线拟合3基线群时延闭合效果最好和可决系数平均值较大的结论,并作为测控信号群时延拟合方法。为了进一步减小VLBI时延测量误差,还研究了将含有整周模糊度的相时延在整个观测期间内进行连接,并整体移至群时延中间位置的处理方法,从而得到了含微小偏移量的相时延数据。含微小偏移量的相时延随机误差与群时延相比大幅降低,系统误差相对来说更易在定轨处理中解算。这为嫦娥二号奔向远方等信号变弱情况下的时延测量精度的提高提供了一种新方法。

The misalignment of spacecraft antenna with the Earth, the discontinuity of the telemetry signal and the distance between spacecraft and the Earth farther and farther etc,which narrow the signal bandwidth and reduce the signal strength, contribute to the increase in VLBI group delay error and outliers. In order to acquire better group delay measurements in these situations, we studied four fitting methods, namely weighted linear fitting with amplitude after outlier-elimination, weighted linear fitting with squared amplitude after outlier-elimination, weighted linear fitting with cubic amplitude after outlier-elimination and weighted linear fitting with quartic amplitude after outlier-elimination. By evaluating the pros and cons of these four methods, we found that by using the second method can minimize the closure error of delays from three baselines and can relatively maximize the average coefficient of determination of group delay fitting. In order to further reduce the error of VLBI delay measurement, we introduce a method to connect the phase delay with cycle ambiguity in the entire observation period, and then eliminate its offset by minimizing its difference from the group delay. In this way, we are able to get phase delay measurements with relatively small offsets. Since the random error of phase delay is usually much lower than that of the group delay, the systematic error of phase delay can be reduced more easily, and be better applied in orbit determination process.