Schreiter Lucas, Montenbruck Oliver, Zangerl Franz, Siemes Christian, Arnold Daniel, Jäggi Adrian
Astronomical Institute, University of Bern, Sidlerstrasse 5, 3001 Bern, Switzerland.
GFZ German Research Centre for Geosciences, 14473 Telegrafenberg, Germany.
GPS Solut. 2021;25(2):70. doi: 10.1007/s10291-021-01107-0. Epub 2021 Mar 9.
Gravity fields derived from GPS tracking of the three Swarm satellites have shown artifacts near the geomagnetic equator, where the carrier phase tracking on the L2 frequency is unable to follow rapid ionospheric path delay changes due to a limited tracking loop bandwidth of only 0.25 Hz in the early years of the mission. Based on the knowledge of the loop filter design, an analytical approach is developed to recover the original L2 signal from the observed carrier phase through inversion of the loop transfer function. Precise orbit determination and gravity field solutions are used to assess the quality of the correction. We show that the a posteriori RMS of the ionosphere-free GPS phase observations for a reduced-dynamic orbit determination can be reduced from 3 to 2 mm while keeping up to 7% more data in the outlier screening compared to uncorrected observations. We also show that artifacts in the kinematic orbit and gravity field solution near the geomagnetic equator can be substantially reduced. The analytical correction is able to mitigate the equatorial artifacts. However, the analytical correction is not as successful compared to the down-weighting of problematic GPS data used in earlier studies. In contrast to the weighting approaches, up to 9-10% more kinematic positions can be retained for the heavily disturbed month March 2015 and also stronger signals for gravity field estimation in the equatorial regions are obtained, as can be seen in the reduced error degree variances of the gravity field estimation. The presented approach may also be applied to other low earth orbit missions, provided that the GPS receivers offer a sufficiently high data rate compared to the tracking loop bandwidth, and provided that the basic loop-filter parameters are known.
通过对三颗蜂群卫星进行GPS跟踪得到的重力场显示，在磁赤道附近存在伪迹。在任务早期，由于跟踪环带宽仅为0.25Hz，L2频率上的载波相位跟踪无法跟上电离层路径延迟的快速变化。基于对环路滤波器设计的了解，开发了一种分析方法，通过对环路传递函数进行反演，从观测到的载波相位中恢复原始L2信号。利用精密轨道确定和重力场解来评估校正的质量。我们表明，对于简化动力学轨道确定，无电离层GPS相位观测的验后方均根误差可以从3mm降低到2mm，同时与未校正观测相比，在异常值筛选中可以多保留高达7%的数据。我们还表明，磁赤道附近运动轨道和重力场解中的伪迹可以大幅减少。这种分析校正能够减轻赤道伪迹。然而，与早期研究中对有问题的GPS数据进行降权相比，这种分析校正并不那么成功。与加权方法不同，对于受严重干扰的2015年3月，可以多保留高达9 - 10%的运动位置，并且在赤道地区获得更强的重力场估计信号，这可以从重力场估计的误差度方差减小中看出。如果GPS接收机提供的数据速率与跟踪环带宽相比足够高，并且基本环路滤波器参数已知，本文提出的方法也可应用于其他低地球轨道任务。
