As an important infrastructure of space information service, the continuously operating reference system (CORS) has been widely used in high accuracy measurement, weather monitoring, crustal deformation, vehicle navigation and other fields in information-based society. Along with the regional operation of BeiDou Satellite Navigation System, it has become a research hotspot on the field of satellite navigation and positioning to establish the multi-constellation positioning augmented system and improve the positioning performance. For the purpose of promoting the development of multi-constellation fusion technology and realizing seamless connection between network RTK and precise point positioning (PPP), the following aspects were mainly carried out:
1. The equivalence theory of GNSS multi-frequency un-difference and differential model are summarized and developed.
Due to the influence of uncalibrated signal delay, the un-difference ambiguity lost its integer nature. The uncalibrated signal delay in multi-frequency and multi-GNSS observations are generally defined. The unified mathematical expression model is built for GNSS data processing. In order to solve the rank-deficient problem, the datums are introduced to realize the re-parameterization and decorrelation of unknown biases. Relying on the new difference operator, the equivalent transformation between un-difference and differential observation equation is developed.
2. A between-receiver single-difference Kalman filter is proposed and applied in the analysis on the short-term temporal variations of GNSS biases and multipath mitigation.
With the consideration of the time-invariant ambiguity, a between-receiver single-difference model is established. By re-parameterizing the biases and clock, the characteristics of time-delay biases are analyzed between different receivers, systems and frequencies. Meanwhile, A sophisticated multipath extraction and mitigation technique is proposed based on the repeatability of multipath. The results identify that the reduction of mean RMS values of coordinate components, from 13 consecutive days, maintain stable at about 30%~45%.
3. A new method for the extraction of un-differenced atmospheric delays and the recovery of long-distance baseline integer ambiguity is proposed based on uncombined precise point positioning technique.
Using the un-difference and un-combined precise point positioning technique, the un-difference atmospheric delay is estimated epoch by epoch. The least squares and the ionospheric regional polynomial model is used to separate the DCB and the ionospheric delay. The results show that, the accuracy of satellite hardware delay is mostly less than 0.2ns, and the positioning accuracy could reach 10 cm in the horizontal direction within 30~40 min for single-frequency static receiver; Furthermore, for dual-frequency static receiver, the accuracy of positioning in the horizontal direction could reach 10cm effectively and stably less than 10 min. The DD integer ambiguities are re-obtained by utilizing the float un-different ambiguities. And a robust model strength can be achieved compared with the conventional DD model. Using data from eight independent baselines with an average length of approximately 150 km, our method obtained more obviously centralised distributions and the successful fixed rate was 98.4% for the new method.
4. A full-rank un-differenced network system of observation equations for multi-constellation PPP-RTK is proposed.
By increasing the benchmarks including the delay deviation of receiver and satellite hardware, the clock delay of core site receiver, the phase deviation and initial ambiguity of core site receiver, a state space representation for satellite clock, phase biases and un-differenced ionospheric delay is built. The rank-deficient of whole network’s estimated parameters is resolved and the integer property of un-difference ambiguity is repaired. Futhermore, based on the regioanl enhanced information, an un-difference integer ambiguity resolution-enabled precise point positioning is proposed. Therefore, a novel high-precision positioning algorithm is developed by weakening the geometric location between the reference station network and the users. With the consideration of the reliability of the state space enhanced information, extre experiments concerning the positioniong accuracy and convergence time of the users are evaluated, and the variation characteristic of satellite uncalibrated signal delay are analyzed.
5. A multi-constellation compatible regional augmented system – EarthNet3.0 is programmed.
Utilizing the above algorithm and research, a multi-constellation compatible regional augmented system – EarthNet3.0 is programmed. The multiple core application layers and Advanced Message Queuing Protocol (AMQP) are applied in the system to realize the unified data transmission of PPP/RTK/RTD enhanced information. In the process of BDS triple frequency observables in Nanjing, the reliability and precision of enhanced information are tested. The results identify that the instantaneous ambiguity-fixed positioning results can be achieved and the dynamic accuracy could reach 1.76cm in plane and 2.22 cm in height for the users in the network. Meanwhile, it need 1~2 minutes to achieve the reliable ambiguity resolution because of the limitation of the space-state biases. The dynamic accuracy could reach 9.03cm in plane and 9.88 cm in height for the users outside the network.