GPS network-based approach to mitigate residual tropospheric delay in low latitude areas

A strong spatio-temporal variation of the wet component in the troposphere leaves us in a peculiar predicament. The residual tropospheric delay will remain in the measurements and therefore affect the estimation of related parameters. In the areas of hot and wet climate conditions, especially in the equatorial or low latitude regions, the strong tropospheric effect on GPS measurements is unquestionable. This study proposes geometric modeling through the network-based approach to mitigate the residual tropospheric delay in such regions. A part of Southeast Asia is selected as a test area for the study, which covers Malaysia and Singapore. Tests are conducted in post-processing but in the “simulating RTK� mode, and evaluated by the number of ambiguity fixes and the accuracy of the coordinate results. Network-based RTK positioning in low latitude areas has shown that the proposed technique can enhance ambiguity resolution by pivoting the ionosphere-free measurements through the mitigated residual tropospheric delay

Stochastic modelling for network-based GPS positioning

Over the past few years the concept of network-based positioning has been developed in support of longer baseline processing compared with 'traditional' single reference station positioning. In fact network-based positioning enables the generation of so-called ‘virtual measurements’, which can significantly improve positioning results. Even though the virtual measurements are generated from the stochastic network estimates, the error propagation into the user position solution has not been investigated in any detail. The aim is to understand how the unique stochastic properties of the network corrections propagate into the uncertainties of the estimated parameters. Test results indicate that by using the virtual measurements and considering the propagation of the network stochastic properties can provide reliable results, both in terms of ambiguity resolution and baseline component estimation

Spatial and seasonal ionospheric error growth in DGPS measurement: a case study in Malaysia

This paper tackles the Equatorial ionosphere and its effects on Differential Global Positioning System (DGPS) error growth over Malaysia by using a network of GPS Continuously Operating Reference Stations (CORS). Seasonal variation of ionospheric delay has been examined and findings show that the effect of spatial variation of ionospheric errors in DGPS is very significant during the equinoctial seasons. Furthermore, a DGPS regression model was developed and tested during the solar maximum year in 2013 by using internet-based DGPS. The results show that the model is capable of estimating DGPS positional errors for distances of user to reference station less than 680 km

Application of Running Average Function to Non-Dispersive Errors of Network-Based Real-Time Kinematic Positioning

The GPS errors can be separated into a frequency-dependent or dispersive component (e.g. the ionospheric delay) and a non-dispersive component (e.g. the tropospheric delay and orbit biases). Dispersive and non-dispersive errors have different dynamic effects on the GPS network corrections. The former exhibits rapid changes with high variations due to the effect of free electrons in the ionosphere, whilst the latter change slowly and smoothly over time due to the characteristic behaviour of the tropospheric delay and the nature of orbit biases. It is found that the non-dispersive correction can be used to obtain better ionosphere-free measurements, and therefore helpful in resolving the long-range integer ambiguity of the GPS carrier-phase measurements. A running average is proposed in this paper to provide a stable network correction for the non-dispersive term. Once the integer ambiguities have been resolved, both dispersive and non-dispersive corrections can be applied to the fixed carrier-phase measurements for positioning step so as to improve the accuracy of the estimated coordinates. Instantaneous positioning i.e. single-epoch positioning, has been tested for two regional networks: SydNET, Sydney, and SIMRSN, Singapore. The test results have shown that the proposed strategy performs well in generating the network corrections, fixing ambiguities and computing a user’s position

Low latitude troposphere: a preliminary study using GPS CORS data in South East Asia

Hot and wet conditions in the equatorial or low latitude region degrade satellite positioning accuracy noticeably. The degradation is related to the strong tropospheric effect, especially the wet component which is approximately proportional to the content of water vapor in the troposphere and thus makes satellite positioning more challenging in this region. Despite the efforts to achieve better understanding of the signal delay in the low latitude troposphere, much more still remains to be improved. Knowing that the water vapor content is heavy in this region, it is of special interest for meteorologists to look into the tropospheric effect. Such knowledge is vital for understanding the global climate, whereas a short term variation of water vapor is very useful input to local weather forecasting. South-East Asia is selected in this study to investigate the effect of regional tropospheric delay, and broadly to understand the behavior of a low latitude troposphere. The study area covers Malaysia and Singapore where GPS CORS networks have already been established. Results from GPS data processing show that a wide variation of the tropospheric delay can be observed. As expected, the largest variation occurs during the North-East monsoon (November to early March) and the South-West monsoon (early May to August). Coordinate repeatabilities of the sites in the network are calculated to show the impact of the tropospheric delay on the precision of GPS positioning activities. In addition, the variations of the tropospheric delay estimated from a local and a regional GPS network are compared to the results from the global network

Analysis of residual atmospheric delay in the low latitude regions using network-based GPS positioning

The atmosphere in low latitude regions is of particular interest to GPS researchers because the propagation of GPS signals becomes significantly delayed compared with other regions of the world. Hence this limits GPS positioning accuracy in equatorial regions. Although the atmospheric delay can be modelled, a residual component will still remain. Reducing, or mitigating the effect of residual atmospheric delay is of great interest, and remains a challenge, especially in equatorial regions. Analysis of relative positioning accuracy of GPS baselines has confirmed that the residual atmospheric delay is distance-dependent, even in low latitude areas. Residual ionospheric delay is the largest component in terms of both absolute magnitude and variability. However it can be largely eliminated by forming the ionosphere-free combination of measurements made on two frequencies. The residual tropospheric delay is smaller in magnitude but rather problematic due to strong spatio-temporal variations of its wet component. Introducing additional troposphere “scale factors� in the least squares estimation of relative position can reduce the effect of the residual. In a local GPS network, the distance-dependent errors can be spatially modelled by network-based positioning. The network-based technique generates a network “correction� for user positioning. The strategy is to partition this network correction into dispersive and non-dispersive components. The latter can be smoothed in order to enhance the ionosphere-free combination, and can be of benefit to ambiguity resolution. After this step, both the dispersive and non-dispersive correction components can be used in the final positioning step. Additional investigations are conducted for stochastic modelling of network-based positioning. Based on the least squares residuals, the variance-covariance estimation technique can be adapted to static network-based positioning. Moreover, a two-step procedure can be employed to deal with the temporal correlation in the measurements. Test results on GPS networks in low latitude and mid-latitude areas have demonstrated that the proposed network-based positioning strategy works reasonably well in resolving the ambiguities, assisting the ambiguity validation process and in computing the user’s position. Furthermore, test results of stochastic modelling in various GPS networks suggests that there are improvements in validating the ambiguity resolution results and handling the temporal correlation, although the positioning result do not differ compared to using the simple stochastic model typically used in standard baseline processing

Implementing network-RTK: the SydNET CORS infrastructure

As is well known, the limitation of single-base real-time kinematic (RTK) GPS carrier phase-based techniques is the constrained distance between base receiver and the rover receiver due to distance-dependent measurement biases. For high productivity GPS surveying techniques, requiring very fast on-the-fly ambiguity resolution, the baseline length is generally restricted to less than 10km. However, techniques have been developed to overcome this distance dependence using a network of GPS reference stations. Because the measurement biases can be modelled and corrected for using multi-reference receiver data, the positioning accuracy will be almost independent of the inter-receiver distance. This class of techniques is now variously referred to as Network-RTK, Multi-Reference Station Positioning, Wide Area Positioning, and the Virtual Reference Station Technique. The authors will describe the basis of Network-RTK techniques, and discuss the challenges in implementing the infrastructure necessary to support Network-RTK users in Sydney. This paper will also describe the components of a continuously operating reference station (CORS) network currently being established in the Sydney basin area, suitable for supporting Network-RTK techniques

Mitigation of Distance-Dependent Errors for GPS Network Positioning

The effect of the atmosphere has been identified as the major problem of long-baseline carrier phase positioning. The effect of the ionosphere,however, can be neutralised with dual-frequency observations. Thus, the effect of the troposphere is the challenge of precise positioning and needs to be mitigated in some way. The network-based approach provides a non-dispersive correction that can be useful in reducing this effect. Applying the correction to the ionosphere-free combination improves the ambiguity resolution and hence guarantees higher accuracy. In addition, dispersive and non-dispersive corrections can be separately applied to the computation of the receiver position

Satellite Division of the U.S. ION, a Fellow of the International Association of Geodesy (IAG), a member of

interest is in network-based GPS positioning, network stochastic modeling and troposphere study