An analysis of intersystem biases for multi-GNSS positioning
An analysis of intersystem biases for multi-GNSS positioning
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DOI：
10.1007/s10291-014-0388-2
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发表时间：
2015-04-01
期刊：
影响因子：
4.9
通讯作者：
Caporali, Alessandro
中科院分区：
文献类型：
--
作者：
Dalla Torre, Andrea;Caporali, Alessandro
商业接收机跟踪多个全球导航卫星系统（GNSS）的能力带来了参考框架和时间尺度相互对准的问题。系统偏差可能应提前计算并告知用户，这样对于码点定位而言，唯一的未知量就是接收机坐标以及相对于唯一时间尺度的接收机时钟偏移。我们分析了2013年4天内来自五个GNSS星座（GPS、格洛纳斯、伽利略、准天顶卫星系统和北斗）、九个欧洲测站以及四种不同GNSS接收机的数据，目的是：（a）在有精确轨道和时钟数据时（GPS、格洛纳斯、伽利略、准天顶卫星系统），评估广播星历相对于精确轨道和时钟的质量；（b）估计每个GNSS所保持的时间尺度相对于GPS的系统偏移；（c）研究可能与接收机相关的系统偏差。我们发现用广播星历计算的卫星坐标与精确轨道之间在米级上相符。对于GPS（国际GNSS服务组织产品），广播时钟与精确时钟之间的差异可达±几纳秒（1纳秒 = 10⁻⁹秒）。对于格洛纳斯，广播时钟与俄罗斯（IAC）SP3值的差异也小于±10纳秒，但当广播时钟与国际GNSS服务组织的综合IGL产品比较时，该差异会增大到约200纳秒。对于准天顶卫星系统，广播时钟与日本宇宙航空研究开发机构SP3产品的差异也限制在±10纳秒以内。对于处于测试模式下广播数据的伽利略，根据F - NAV或I - NAV卫星时钟模型，我们得到不同的值：利用慕尼黑工业大学（TUM）的SP3产品，我们发现广播时钟与SP3之间存在几十纳秒的差异。系统时间偏差综合了与卫星相关的差分码偏差、与接收机相关的系统间偏差以及给定GNSS时间尺度相对于参考（我们假定为GPS）时间尺度的偏移。对于格洛纳斯，系统时间偏差通常约为360 - 380纳秒，与广播星历或精确（IAC）星历相当一致。对于伽利略，偏差更具变化性，且取决于使用广播数据还是精确数据。对于准天顶卫星系统，我们评估出准天顶卫星系统相对于GPS的系统时间偏差在时间上几乎恒定。其值根据SP3数据源（TUM或日本宇宙航空研究开发机构）的不同会有大约20纳秒的差异。对于北斗，我们发现时间系统偏差约为10 - 100纳秒，具体取决于日期和接收机类型。在几个不同测站中，相对于GPS与接收机相关的时间系统偏差相当明显，且在不同接收机类型之间具有合理的一致性。最后我们强调，有必要对这种与GNSS相关和与接收机相关的GNSS间偏差进行监测活动，并且要设计适当的方法将这些偏差提供给最终用户。
The capability of commercial receivers to track multiple GNSSs poses the problem of mutual alignments of reference frames and time scales. System biases should possibly be computed in advance and made known to the user, so that the only unknowns for code point positioning are the receiver coordinates and the receiver clock offset relative to a unique time scale. We have examined data from the five GNSS constellations GPS, GLONASS, Galileo, QZSS and BeiDou for 4 days in 2013, nine European stations and four different GNSS receivers, with the goals to (a) evaluate the quality of the broadcast ephemeris relative to the precise orbits and clocks, when available (GPS, GLONASS, Galileo, QZSS), (b) estimate systematic offsets of the time scales kept by each GNSS relative to GPS and (c) investigate possible receiver-dependent system biases. We report an agreement at the meter level between coordinates of satellites computed with broadcast ephemeris and precise orbits. Differences between broadcast and precise clocks are up to +/- A few nanoseconds (1 ns = 10(-9) s) for GPS (IGS products). For GLONASS, the difference of the broadcast clock to the Russian (IAC) SP3 values is also smaller than +/- 10 ns, but rises to about 200 ns when the broadcast clock is compared to the combined IGL product of IGS. For QZSS, the difference broadcast to SP3 products of the JAXA are also confined to +/- 10 ns. For Galileo, for which data are broadcast in a test mode, we report different values depending on the F-NAV or I-NAV satellite clock model: Using the SP3 products of Technical University of Munich (TUM), we report discrepancies between broadcast and SP3 of some tens of nanoseconds. System Time Biases lump together satellite-dependent Differential Code Biases, receiver-dependent Inter System Biases and offsets of a given GNSS time scale to a reference (we assume GPS) time scale. The System Time Bias is typically of the order of 360-380 ns for GLONASS, quite consistently with broadcast or precise (IAC) ephemeris. For Galileo, the biases are more variable and depend on the use of broadcast or precise data. For QZSS, we evaluate a QZSS to GPS System Time Bias very nearly constant in time. Its value differs of about 20 ns depending on the SP3 data source (TUM or JAXA). For BeiDou, we find Time System Biases of the order of 10-100 ns depending on day and receiver type. Receiver-dependent Time System Biases relative to GPS are quite evident in several different stations and are reasonably consistent across receiver type. We conclude by emphasizing the need for a monitoring activity of such inter-GNSS biases, both GNSS dependent and receiver dependent, and that appropriate means are devised for making them available to the final user.