一种能实现单频PPP-RTK的GNSS局域参考网数据处理算法

全球范围内大量布设的GNSS(Global Navigation Satellite System)参考网为精密定位、导航和授时等应用提供了丰富的数据资源.基于局域参考网,先后发展了若干侧重实现双频精密定位的技术,如NRTK(Network Real Time Kinematic),PPP(Precise Point Positioning)和PPP-RTK等.其中,PPP-RTK融合了NRTK和PPP的技术优势,是目前相关研究的热点.本文改进了利用局域参考网提取各类改正信息的算法,以便于实现单频PPP-RTK,具体步骤包括:1)逐参考站实施非组合PPP,并固定已知站星距和卫星钟差,预估电离层延迟、浮点模糊度等参数;2)联合所有参考站的PPP模糊度预估值,通过重新参数化,形成一组双差整周模糊度和接收机、卫星相位偏差;3)固定双差整周模糊度,精化求解卫星相位偏差和各参考站PPP电离层延迟.基于网解中用到的卫星轨道和钟差,以及网解所提供的卫星相位偏差和(内插的)电离层延迟,参考网内的单频流动站即可实施PPP-RTK.基于澳大利亚某连续运行参考站网和流动站的实测数据,考察了:1)参考网数据处理中,双差模糊度的固定成功率(98.89%)和卫星相位偏差估值的时间稳定性(各连续弧段优于0.2周);2)流动站处电离层延迟的内插精度(优于10cm);3)单天内任一历元起算,固定静态(动态)单频PPP整周模糊度所需时长(均不超过10min);4)模糊度固定前后,单频动态PPP的定位精度(模糊度固定后,平面和天顶RMS分别优于5cm和10cm;模糊度固定前,相应RMS仅为28~53cm).

A method for processing GNSS data from regional reference networks to enable single-frequency PPP-RTK

Global Navigation Satellite System (GNSS) data from reference station networks deployed globally can facilitate positioning, navigation and timing applications. To enable precise positioning for dual-frequency users, several representative methods relying on GNSS reference networks have been developed, such as Network Real Time Kinematic (NRTK), Precise Point Positioning (PPP) and PPP-RTK. The state-of-the-art PPP-RTK integrates the advantages of customary NRTK and PPP, and has become an important topic in current research. In this contribution, a network processing method is proposed to achieve single-frequency PPP-RTK. The elementary procedures are as follows: 1) A Kalman-filter-based customary PPP is implemented station by station, with known geometric ranges and satellite clocks fixed. The estimable unknowns consist of, among others, the ionospheric delays and the float-valued carrier-phase ambiguities. 2) After measurement-update, the filtered PPP ambiguities of all stations are incorporated and reformulated into three sets of new parameters, namely, double-difference (DD) ambiguities, receiver and satellite carrier-phase biases. 3).The reformulated DD ambiguities are resolved into integers, and then the satellite carrier-phase biases as well as those filtered ionospheric delays are further updated. On the user side, by applying the satellite phase biases and (interpolated) ionospheric delays, the integer ambiguity resolution enabled single-frequency PPP-RTK is fulfilled. Numerical tests using daily GPS data collected by an Australian Continuous Operating Reference System (CORS) network and a single-frequency (u-blox) rover receiver show that success rate of CORS network ambiguity resolution is as high as 98.89%. In addition, the stability of estimated satellite carrier-phase biases is better than 0.2 cycles over every continuous satellite arc. By confronting the ionospheric delays interpolated from the CORS with that determined from a dual-frequency receiver co-located with the rover receiver, interpolation error of 10 cm has been verified. Re-initialization of Kalman-filter-based single-frequency static/kinematic PPP-RTK is attempted at every epoch, and the resulting time-to-first-fix values, as a measure of the time required for integer ambiguity resolution, are never more than 10 min. With the aid of resolved integer ambiguities, the RMS of single-frequency kinematic PPP-RTK positioning errors becomes as good as 5 cm for horizontal component and 10 cm for vertical component. Before ambiguity resolution, these RMS values vary from 28 to 53 cm.Although it is developed with the goal of enabling single-frequency PPP-RTK, the network processing method proposed does not lose its ability to attain dual-frequency PPP-RTK capability. More importantly, this method also reserves simplicity as well as flexibility in multi-frequency, multi-GNSS applications.