GPS接收机仪器偏差的短期时变特征提取与建模

Short-term temporal variability of GPS receiver′s differential code biases(DCB):retrieving and modeling

卫星和接收机仪器偏差（Differential Code Biases,DCB）是利用GPS（Global Positioning System）研究电离层的两类主要误差源.由于所处的空间环境恒定,且可被全球跟踪站连续观测,GPS卫星的DCB具备长期稳定性和较高的估计精度.但针对各类型接收机而言,受测站环境、硬件设施等影响,其DCB可能会呈现明显的短期变化.精确地模型化接收机DCB的短期变化特征,将有助于提高GPS电离层产品的可靠性,以及基于这些产品反演空间和地球科学现象的准确性.采用零/短基线GPS数据,本文改进了提取和分析接收机DCB变化的现有方案.随后,本文推导了一种能直接估计接收机DCB的函数模型.当检验出接收机DCB的短期变化服从随机游走时,通过对比接收机DCB的直接估值与间接提取值之间的符合程度,可＂试探出＂过程噪声标准差的最优经验值.实验分析选用4台双频接收机（共形成1条零基线和2条短基线,间距最大为15m）多天的观测数据,主要结论包括：1）改进的接收机DCB提取方案能较好地克服低频伪距噪声和多路径效应的影响;2）针对零基线,其接收机DCB在各天内的变化量级小于1TECu,变化趋势则可采用过程噪声标准差为1.0～1.5mm的随机游走加以描述;3）对应于某短基线的接收机DCB在某天内的变化可达12TECu,当采用随机游走描述其趋势时,过程噪声标准差的经验值超过2mm.

The satellite and receiver differential code biases（DCB）combined,account for the main error budget of GPS-based ionosphere investigations.As the space environment onboard the GPS satellites is quite constant,the long-term stability of GPS satellite DCB has been observed.At the same time,continuous GPS data collection from receivers of global coverage makes it possible to estimate GPS satellite DCB with high accuracy.These two facts,however,do not hold true for a variety of receivers′DCB.As a result of various operating environments as well as distinct firmware versions,receiver DCB may experience short-term variations over time.Precisemodeling of receiver DCB′s variation can raise the reliability of ionosphere products determined from GPS data,as well as ensure the correctness of conclusions drawn based on these products when investigating atmosphere/space effects and geodetic phenomena.Given zero/short-baseline GPS data,the customary scheme used to retrieve receiver DCB is further modified as follows：1）Precise point positioning（PPP）has been implemented,respectively,using GPS data collected by each of the receiver that forms the baseline.The slant ionosphere delays biased by satellite and receiver DCB,and the undifferenced,float-valued ambiguities can be estimated,among other parameters.2）Those undifferenced ambiguities are then combined so as to form an independent set of double-differenced ambiguities that are fixable.3）After taking these fixed ambiguities into consideration,the slant ionosphere delays determined by means of PPP can be further refined.The between-receiver,single-differenced values of these delays are then used to retrieve a time series of receiver DCB,the time resolution of which is equal to that of GPS observations in use.In addition,the ionosphere-fixed model with estimable receiver DCB has been derived.By characterizing the dynamic model of receiver DCB as random walk,the consistency between both the estimated and the formerly retrieved receiver DCB forms a basis to identify an optimal empirical value of the STD of the process noise.Numerical tests make use of multiple days′dual-frequency GPS data collected by 4co-located receivers that form a total of one zero-baseline and two short-baselines,with a maximum separation of 15 m.The main conclusions include：1）The modified scheme outperforms the customary one,as being able to determine a time series of receiver DCB less affected by lowfrequency code noise and multipath effects;2）The intra-day variations of receiver DCB determined from the zero-baseline is less than 1TECu,without apparent day-to-day repeatability.A random walk with STDs of process noise between 1.0and 1.5mm is sufficient to characterize different days′variation behaviors;3）The size of receiver DCB variation corresponding to one of the shortbaselines can exceed 12TECu（roughly 2m）during one day.To model it with random walk,the empirical STD of the process noise should be set no less than 2mm.The proposed methods in this paper may serve as new ways to routinely monitor and calibrate receiver DCB.