Abstract
A theoretical study on the bottlenecks of GPS phase ambiguity resolution in a CORS RTK Network Crucial to the performance of GPS Network RTK positioning is that a user receives and applies correction information from a CORS Network. These corrections are necessary for the user to account for the atmospheric (ionospheric and tropospheric) delays and possibly orbit errors between his approximate location and the locations of the CORS Network stations. In order to provide the most precise corrections to users, the CORS Network processing should be based on integer resolution of the carrier phase ambiguities between the network's CORS stations. One of the main challenges is to reduce the convergence time, thus being able to quickly resolve the integer carrier phase ambiguities between the network's reference stations. Ideally, the network ambiguity resolution should be conducted within one single observation epoch, thus truly in real time. Unfortunately, single-epoch CORS Network RTK ambiguity resolution is currently not feasible and in the present contribution we study the bottlenecks preventing this. For current dual-frequency GPS the primary cause of these CORS Network integer ambiguity initialization times is the lack of a sufficiently large number of visible satellites. Although an increase in satellite number shortens the ambiguity convergence times, instantaneous CORS Network RTK ambiguity resolution is not feasible even with 14 satellites. It is further shown that increasing the number of stations within the CORS Network itself does not help ambiguity resolution much, since every new station introduces new ambiguities. The problem with CORS Network RTK ambiguity resolution is the presence of the atmospheric (mainly ionospheric) delays themselves and the fact that there are no external corrections that are sufficiently precise. We also show that external satellite clock corrections hardly contribute to CORS Network RTK ambiguity resolution, despite their quality, since the network satellite clock parameters and the ambiguities are almost completely uncorrelated. One positive is that the foreseen modernized GPS will have a very beneficial effect on CORS ambiguity resolution, because of an additional frequency with improved code precision.
Highlights
In order to provide the most precise corrections to users, the Continuously Operating Reference Stations (CORS) Network processing should be based on integer resolution of the carrier phase ambiguities between the network's CORS stations
We show that external satellite clock corrections hardly contribute to CORS Network RTK ambiguity resolution, despite their quality, since the network satellite clock parameters and the ambiguities are almost completely uncorrelated
The basis of high-precision GNSS Network RTK positioning is the presence of a network of Continuously Operating Reference Stations (CORS)
Summary
The basis of high-precision (cm-level) GNSS Network RTK positioning is the presence of a network of Continuously Operating Reference Stations (CORS). To obtain the most precise atmospheric error prediction for the users, the processing of the data of the CORS stations should be based on fixing of the integer carrier phase ambiguities Once these ambiguities are resolved, the high precision of the phase data will be reflected in the network atmospheric estimates. In this paper we will study the limiting factors for instantaneous CORS Network RTK ambiguity resolution, based on dual-frequency GPS data For this we will use the concept of Ambiguity Dilution of Precision (ADOP), a scalar diagnostic measure for the precision of the float ambiguities. An efficient practical approach for realtime CORS network processing is to make use of a Kalman filter implementation, based on un-differenced dual-frequency GPS phase and code observations and with the double-differenced (DD) ambiguities, (zenith) tropospheric delays, ionospheric delays, receiver and satellite clocks in the state vector. A further elaboration on the relation between success rate and ambiguity precision is given by Teunissen (2000)
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