Galileo local element augmentation system
Background & Objectives
The target for the applications includes the following potential user groups: - Safety-of-life users (SoL): requiring high performance requirements both in accuracy and safety; - Medium-accuracy and low-safety users (MA-LS), public/private transport, traffic management: here, the low cost constraint for terminals could help in the diffusion of local messages providing information useful for accuracy enhancement.
The project addresses new methods and algorithms to locally predict, monitor and possibly improve in near real time the performance of the positioning service offered by the Galileo satellites. The proposed application in the GALILEA project can be considered as a Galileo local element which augments the global Galileo service in a local area.
The following complementary research directions were investigated:
- Efficient computational methods for generating high-accuracy local data in a very short time making use of only local reference stations;
- Innovative data fusion techniques to merge local data with global/regional data, in order to predict and monitor SISE, tropospheric and ionospheric corrections;
- Efficient local communication architecture able to disseminate the SISE and the derived information with the lowest latency time using Internet or UMTS technology.
The error prediction and correction module (EPCM), whose architecture is reported in Figure 2, is the GALILEA software designed to generate the SISE prediction and the iono/tropo error corrections to be transmitted to the user. The software can be split into four computation modules:
- IGS module: its task is to compare the coordinates from the IGS ephemeris with the ones calculated from the navigation message in order to estimate the signal in space error due to orbit and clock errors, called OCE.
- Error prediction module: this computes the pseudorange residuals at each time and for each satellite-station combination, fits them with a modified RBTB model and computes the instantaneous error value at satellite level with a multiple station approach.
- Iono/tropo correction module: this computes, for each reference station and satellite, the iono-free pseudoranges and the coefficients to correct the first and second (only if three frequencies are available) order refraction effects, both in the two (GPS) and three (Galileo) frequency cases. It also calculates a grid of ionospheric and tropospheric corrections for the network of reference.
- Data fusion module: this acquires the IGS SISE estimation from the IGS module and the instantaneous error value from the EPM. It computes the SISE correction to be applied to the IGS SISE estimation using the instantaneous error trend. Finally, it computes a SISE prediction to be output to the user.
Integrity monitoring As part of GALILEA, three candidate concepts for Galileo local element integrity monitoring were discussed based on service volume simulations:
- Generation of local error corrections and relay of the Global Galileo integrity information to the users: the local processing facility generates ionospheric and tropospheric error corrections and broadcasts these corrections to the users for improving the accuracy. Since no improved SISA, SISE and SISMA values are determined, it is not possible to increase the integrity performance as compared to the global system.
- Generation of local error corrections and also locally improved Galileo SISA/SISE values while the SISMA values are simulated: in this case the integrity performance can be improved. The level of the integrity performance improvement is dependent on the number and geometry of regional ground sensor stations and the access to the global ground sensor station data in (near) real time keeping the necessary TTA (time-to-alert). In this case e.g. CAT-1 aircraft precision approach requirements seem to be achievable.
- Adaptation of the GBAS concept to Galileo: the highest levels of integrity can only be reached with a local reference station architecture as is the case for GBAS. Most likely the final architecture will become a multi-constellation GPS and Galileo local element.
The operational scenario reported in Figure 1 is composed of: Reference stations: each station has to produce observation and navigation files (in RINEX format) with high rate, and transmit them in real time to the processing facility. The necessity of more than one station for the GALILEA application has been stated for two principal reasons: the transmitted ionospheric and tropospheric corrections can be user-location dependent only if a grid in space is defined; a SISE prediction can be independent on signal errors relative to a specific station only if a multiple station approach is selected.
Processing facility: the facility has to collect data from different sources and elaborate them.
- Error prediction and correction module: this tool generates the SISE predictions, and ionospheric as well as tropospheric corrections that have to be transmitted to the user.
- Offline integrity monitoring module: the offline integrity monitoring computes the integrity risk and/or the user protection level in the local service area and compares them with the envisaged service levels. Weather stations: local meteorological information (pressure, temperature, humidity) can significantly improve the tropospheric modelling accuracy. Radio broadcast facility: direct radio transmission to SoL users is via a dedicated broadcast facility, while communication with MA-LS users is performed using the Internet.
Work performed & results
The following SISE prediction results were achieved simulating several operational scenarios: SISE prediction refresh rate: 60[s] for SoL and 20[s] for MA-LS SISE prediction duration: 300[s]for SoL and MA-LS SISA validity time prediction accuracy: 20% for SoL and 30% for MA-LS Local area radius: 50[KM] for SoL and 100[KM] for MA-LS.