Integrated navigation using multiple Global Navigation Satellite Systems (GNSS) is beneficial to increase the number of observable satellites, alleviate the effects of systematic errors and improve the accuracy of positioning, navigation and timing (PNT). When multiple constellations and multiple frequency measurements are employed, the functional and stochastic models as well as the estimation principle for PNT may be different. Therefore, the commonly used definition of “dilution of precision (DOP)” based on the least squares (LS) estimation and unified functional and stochastic models will be not applicable anymore. In this paper, three types of generalised DOPs are defined. The first type of generalised DOP is based on the error influence function (IF) of pseudo-ranges that reflects the geometry strength of the measurements, error magnitude and the estimation risk criteria. When the least squares estimation is used, the first type of generalised DOP is identical to the one commonly used. In order to define the first type of generalised DOP, an IF of signal–in-space (SIS) errors on the parameter estimates of PNT is derived. The second type of generalised DOP is defined based on the functional model with additional systematic parameters induced by the compatibility and interoperability problems among different GNSS systems. The third type of generalised DOP is defined based on Bayesian estimation in which the a priori information of the model parameters is taken into account. This is suitable for evaluating the precision of kinematic positioning or navigation. Different types of generalised DOPs are suitable for different PNT scenarios and an example for the calculation of these DOPs for multi-GNSS systems including GPS, GLONASS, Compass and Galileo is given. New observation equations of Compass and GLONASS that may contain additional parameters for interoperability are specifically investigated. It shows that if the interoperability of multi-GNSS is not fulfilled, the increased number of satellites will not significantly reduce the generalised DOP value. Furthermore, the outlying measurements will not change the original DOP, but will change the first type of generalised DOP which includes a robust error IF. A priori information of the model parameters will also reduce the DOP.

Interoperability of GNSS Open Services is a significant trend in the development of all the satellite navigation systems, and its performance will directly affect the users' security and reliability. Therefore, the monitoring and assessment of GNSS Open Services has become a focus of attention for all providers and users of GNSS. In this paper the elements and methods for the monitoring and assessment of GNSS Open Services are investigated first. Then the schemes designed for an international GNSS Monitoring and Assessment System (iGMAS) are proposed. The schemes are based on omni-directional antennas, multi-beam antennas and a high-gain paraboloid antenna to achieve Quadruple-overlap and Single-overlap coverage and sophisticated analysis respectively. Moreover, the scheme for the deployment of world-wide monitoring stations is also proposed. Finally, some related works that have been conducted to monitor and assess Beidou Open Services are introduced, which verifies the feasibility of the proposed system.

The evaluation of Beidou system time is one of the important tasks in constructing and developing the Beidou satellite navigation system. The evaluation methods based on two-way satellite time and frequency transfer method (TWSTFT) and GPS common-view method are proposed in this paper. Tests and performance evaluation of the Beidou time system (BDT) were conducted. Results show that BDT is stable and reliable. The BDT frequency deviation, Allan deviations and clock offsets, relative to UTC (NTSC), are 10e-14, 10e-15 and less than 30 ns respectively.

Compared with traditional GPS signals, modern GNSS signals are much more complex and have various new modulations. This introduces a difficulty in combining multiple signal components into a constant-envelope signal that maximises the power efficiency of High Power Amplifiers (HPA) on satellites. This paper first describes the fundamental Phase-Optimised Constant-Envelope Transmission (POCET) technique that searches the optimum combining solution for multiple binary navigation signals. Then the Compass B1/B3 signals are modelled by POCET. For the B1 band, a binary complex sub-carrier is adopted to implement the centre frequency difference between regional and global Compass navigation systems. Regional B1 Open Service (OS) signals and global TMBOC signals are combined with optimum loss of 1·0 dB. For the B3 band, Interplex modulation is proved to be the optimum method to combine QPSK (10) and BOC (15, 2·5) signals. Signal quality in the presence of finite word-length effects of Digital-Analog (DA) converters is analysed. Simulations for signal model validation are conducted. The result indicates that relative amplitude error less than 0·01 and angle error less than 0·1 degree can be achieved with 10 bit DA converters. The POCET method is demonstrated as an efficient solution for Compass signals.

As the technology of global navigation satellite system (GNSS) and augmentation systems are evolving rapidly, compatibility becomes a critical issue for system providers. By April 2011, China had successfully launched eight satellites of the Compass phase II (CP II) navigation system, which will provide positioning, navigation, timing and communication services to the Asia-Pacific region by the year 2012. Due to the limitations of available radio frequency bandwidths, it is important to assess the compatibility and to design signals based on the compatibility within these limited radio frequency bandwidths. This paper presents a modified compatibility assessment methodology, derived from the traditional methodologies that are based on the spectral separation coefficient (SSC) and the effective carrier-power-to-noise density ratio. The modified methodology takes into account additional factors including the Doppler offset, code tracking loop, and band-limiting, sampling and quantisation (BSQ) of the GNSS receiver. In the simulation section, the comprehensive compatibility assessment between CP II and other GNSSs, such as GPS, Galileo, Wide Area Augmentation System (WAAS) and European Geostationary Navigation Overlay Service (EGNOS) on L1 Band are carried out and presented with some new results. Simulation results reveal that CP II does not cause serious interference on GPS, Galileo, WAAS and EGNOS as the interference level is below the 0·25 dB threshold recommended by ITU.

Unlike a direct broadcasting satellite navigation system, the transmitting satellite navigation system developed in China uses transponders onboard communication satellites to retransmit navigation signals generated at a ground master station. The transmitting navigation satellite constellation consists of a number of inclined geosynchronous orbit (IGSO) satellites. Considering China's mainland coverage in the northern hemisphere occupies some 62 degrees in longitude, the inclination of the IGSO satellites cannot be too high, or its signals would not be received by the users in the middle and high latitude areas when the IGSO satellite travels over the southern hemisphere. Meanwhile, the latitude of the most southerly station in China mainland that can uplink navigation signals is about 18°N when the IGSO satellite travels to the southern hemisphere. Therefore, there is a need to consider the IGSO inclination to achieve balance between uplinking high-quality navigation signals and covering the high-latitude area. In this work, the navigation performance and availability of the IGSO satellite are examined when navigation signals are uplinked from the stations Lintong and Sanya.

In designing and developing navigation receiver embedded software, the correct scheduling of navigation tasks is critical for ensuring that a receiver works in the right way. If a receiver's timing scheduling becomes abnormal, it may lead to timing conflicts of the multi-tasking navigation receiver, which may seriously influence the system's performance of real-time features and functionalities. This paper models the timing scheduling of navigation receivers by extracting all important timing parameters including the execution time, period, deadline and priority etc., and focusing on the execution time. The maximum execution time of a navigation task relies on the design and implementation algorithms and the computing capability of the processor. The execution time parameter can be properly adjusted in receiver software design or even in the phase of the receiver requirement changes. On the other hand, the other parameters (i.e., the period, deadline and priority) mainly depend on the receiver's technical requirements, and usually these parameters are set to constant values. In this research, a novel off-line method based on the maximum allowed execution time dichotomy optimization is proposed, and the equilibrium rule is proposed to apply to the multi-task maximum allowed execution time dichotomy search and an analysis of time series of those parameters obtained from real observations of receiver tasks was also conducted. Results indicate that the relative equilibrium is more suitable to the multi-task scheduling of receiver navigation, compared to the absolute equilibrium. The proposed method provides a fast off-line scheduling approach for the top level task design of navigation receivers, and remarkably improves the development efficiency of receiver embedded software.

Signal-In-Space Error (SISE) is important for the system integrity of GNSS. Real-time monitoring SISE has been one of the effective solutions used for improving GNSS system-level integrity. As one of the typical solutions, the Galileo integrity concept uses Signal-In-Space Accuracy (SISA) and Signal-In-Space Monitoring Accuracy (SISMA) respectively to transmit the quality of SIS and the integrity alarming information to users. As an important component of the Galileo Integrity Concept, Worst User Location (WUL) is of great importance for the computation of SISA and SISMA. An improved WUL algorithm based on a geometrical model is given in this paper, which needs no iterative search and is simple when compared with the existing models. The WUL accuracy requirement is discussed based on an analysis of the magnitude of the orbit errors and SIS ranging resolution and error. The conclusion is that, considering the metre-level orbital error observed in the orbit determination process, as long as the orientation error of the WUL is less than three degrees, the projection error of SISE can be kept to centimetre levels. This will satisfy the requirements of most users. The algorithm is further optimised to omit the non-linear iterative parts based on the above assumption. The optimisation reduces computation load and enhances the real-time capability. GPS raw measurements are used to validate the correctness and reliability of the algorithms.

The status of the tropopause has impact on weather phenomena and climate change occurring in the atmosphere of the Earth. The investigation of structure and variation of the tropopause plays a significant role in an in-depth understanding of water vapour exchange, mass and chemical materials across the tropopause, and their impacts on climate change and ecological environment. With the advantages of high vertical resolution, global coverage, unbiased instrumentation, and long-term stability, GPS Radio Occultation (RO) data is ideal for the monitoring of tropopause structure. In this research, GPS RO data from the two missions, CHAMP and COSMIC, were used to assess and analyse the temporal and spatial variations in tropopause heights and temperature over China. The consistency of the precision of the GPS temperature profiles derived from the two missions were also statistically validated. The two types of tropopause, i.e. the Lapse Rate Tropopause (LRT) and the Cold Point Tropopause (CPT), were determined from the GPS RO temperature profiles, and the trend of the variations in tropopause heights and temperatures of the two types of tropopause were compared and analysed.

Centimetre-level RTK solutions are mainly influenced by satellite orbit errors, ionospheric and tropospheric delays, and measurement noise (including multipath effects). Estimation and mitigation of the main errors for the CM-level Compass RTK solutions over medium-long baselines are investigated. Tests conducted for this research lead to the following conclusions:

1. 1. For 100 km baselines, a 4 cm error in height component will be induced by a 10 m orbit error. For longer baselines, rapid precise ephemeris will be needed for CM-level accuracy RTK solutions.

2. 2. The residual ionospheric delay error can be eliminated using the optimal triple-frequency ionosphere-free linear combination with the coefficients of 2·6087, −0·5175 and −1·0912 respectively for observations on f1, f2 and f3 frequencies. This combination is optimal in terms of its noise level, e.g., the noise is only amplified three times. It can be used for high accuracy RTK positioning.

3. 3. The residual tropospheric delay can be resolved for the introduced relative zenith tropospheric delay (RZTD) parameters.

The GNSS-R technique brings out the need for signal receiver systems to process both direct and reflected GNSS signals. This paper describes the architecture of a new GNSS-R receiver system (GRrSv.2) that features enhanced capabilities for remote sensing based on reflected Global Positioning System (GPS) signals. Signal processing issues including DDM calculation, Carrier and Code Generation and DDM Synchronization are presented. Aircraft- and land-based verification experiments for ocean winds, ocean Significant Wave Heights (SWH) and soil moisture have been performed and some primary results are presented in this paper.

The estimation of GPS repeater jamming plays an important role in modern military applications. In this paper, based on the maximum likelihood estimation (MLE) method, a new approach for the estimation of GPS repeater jamming using Kalman filtering with coloured observation noise is proposed. The state equation and observation equation of the Kalman filter with the coloured observation noise are deduced and the generation process explained. The recursive formula for the Kalman filter used to iteratively filter for the MLE output is also derived. Experimental results show that the new design has a better performance than the MLE; hence it can be used for the precise estimation of repeater jamming signals under the condition that the carrier-to-noise density ratio is below 20 dB-Hz.

A number of methods have been developed to enhance the robustness of Global Positioning System (GPS) receivers when there are a limited number of visible satellites. Vector tracking is one of them. It utilizes information from all channels to aid the processing of individual channels to generate receiver positions and velocities. This paper analyzes relationships among code phase, carrier frequency, and receiver position and velocity, and presents a vector loop-tracking algorithm using an Extended Kalman filter implemented in a Matlab-based GPS software receiver. Simulated GPS signals are generated to test the proposed vector tracking method. The results show that when some of the satellites are blocked, the vector tracking loop provides better carrier frequency tracking results for the blocked signals and produces more accurate navigation solutions compared with traditional scalar tracking loops.

Autonomous orbit determination of a navigation constellation is the process by which the orbit parameters of navigation satellites are autonomously calibrated onboard the satellites without the need for external aids. It commonly uses a satellite onboard data processing unit and a filtering method to process the measurements of inter-satellite ranges. The onboard data processing unit is the main module of autonomous navigation systems. In this paper, the two main factors that affect the accuracy of autonomous orbit determination for a navigation constellation are discussed first, and then a distributed onboard algorithm for autonomous orbit determination of navigation satellites is proposed. This method is based on a long-term ephemeris prediction and is suitable for the satellite hardware capability. The main feature of this method is that both the distributed computing method and an onboard analytical state transition matrix are used to process inter-satellite range measurements. One of the main advantages of this approach is high-speed computing since the amount of calculations needed is significantly less than that of the centralised computing method and those distributed methods that need to use an onboard numerical integrator. Another advantage of this approach is that the use of the onboard analytical state transition matrix algorithm can save a great amount of resources for both ground-to-satellite data transmissions and data storage units in satellites’ hardware. This could result in substantial cost reduction for space missions. Finally, a simulation method used for testing the proposed algorithm is presented. Results of tests over a period of 90 days show that the user range error of autonomous orbit determination derived from the proposed method is less than three metres.

In this contribution, a novel un-differenced (UD) (PPP-RTK) concept, i.e. a synthesis of Precise Point Positioning and Network-based Real-Time Kinematic concept, is introduced. In the first step of our PPP-RTK approach, the UD GNSS observations from a regional reference network are processed based upon re-parameterised observation equations, corrections for satellite clocks, phase biases and (interpolated) atmospheric delays are calculated and provided to users. In the second step, these network-based corrections are used at the user site to restore the integer nature of his UD phase ambiguities, which makes rapid and high accuracy user positioning possible. The proposed PPP-RTK approach was tested using two GPS CORS networks with inter-station distances ranging from 60 to 100 km. The first test network is the northern China CORS network and the second is the Australian Perth CORS network. In the test of the first network, a dual-frequency PPP-RTK user receiver was used, while in the test of the second network, a low-cost, single-frequency PPP-RTK user receiver was used. The performance of fast ambiguity resolution and the high accuracy positioning of the PPP-RTK results are demonstrated.

The LAMBDA method for integer least-squares ambiguity resolution has been widely used in a great variety of Global Navigation Satellite System (GNSS) applications. The popularity of this method stems from its numerical efficiency and its guaranteed optimality in the sense of maximising the success probability of integer ambiguity estimation. In the past two decades, the LAMBDA method has been typically used in cases where the number of ambiguities is less than several tens. With the advent of denser network processing and the availability of multi-frequency, multi-GNSS systems, it is important to understand LAMBDA's performance in high dimensional spaces. In this contribution, we will address this issue using real GPS data based on the Bernese software. We have embedded the LAMBDA method into the Bernese software and compared their ambiguity resolution performances. Twelve day dual-frequency GPS data with a sampling interval of 30 s was used in the experiment, which was collected from a network of 19 stations in the Perth area of Western Australia with an average baseline length of 380 km. Different experimental scenarios were examined and tested with different observation spans, which represent the different ambiguity dimensions. The results showed that LAMBDA is still efficient even when the number of ambiguities is more than 100, and that the baseline repeatability obtained with the ambiguities resolved from the LAMBDA method agreed well with that of Bernese. Therefore, for future dense network processing, the easy-to-use LAMBDA method should be considered as an alternative to baseline-per-baseline methods as those used in e.g. the Bernese software.

New signals from the modernised satellite navigation systems (GPS and GLONASS) and the ones that are being developed (COMPASS and GALILEO) will present opportunities for more accurate and reliable positioning solutions. Successful exploitation of these new signals will also enable the development of new markets and applications for difficult environments where the current Global Navigation Satellite Systems (GNSS) cannot provide satisfying solutions. This research is aiming to exploit the improvement in monitoring, modelling and mitigating the atmospheric effects using the increased number of signals from the future satellite systems. Preliminary investigations were conducted on the numerical weather parameter based horizontal tropospheric delay modelling, as well as the ionospheric higher order and scintillation effects. Results from this research are expected to provide a potential supplement to high accuracy positioning techniques.