The Interoperability Working Group (IWG) was formed by the different SBAS service providers to ensure that SBAS systems are compatible and to plan future enhancements. Nowadays its main goal is to contribute to the elaboration of a new standard able to augment up to 4 GNSS constellations simultaneously and offering new services for dual frequency (L1/L5) users. For that purpose, two families of SBAS L1/L5 ICD models are currently under consideration in the IWG:
a) UDRE ICD family, aimed at defining a SBAS L1/L5 ICD model as similar to the current SBAS L1 ICD as feasible. Two different alternatives are currently under analysis in this family: the first one based on the use of a dynamic mask (with a maximum number of 51 satellites to be considered and keeping SBAS L1 ICD update intervals) and the second one, which proposes a relaxation of the fast correction update intervals and the elimination of the dynamic mask;
b) DFREI ICD family, whose main objective is, based upon a similar PRN mask as L1, to simplify the number of messages and structure as much as possible. Their common proposal is to eliminate the fast correction, to send a single correction message per satellite every 120 seconds (orbit, clock and their rates, covariance matrix, DFREI) and a MT6-equivalent message with DFRECI and updates of DFREIs every 6 seconds. The main difference between the two DFREI ICD alternatives is about the MT6-equivalent message structure.
This paper provides a summary of the qualitative trade-off performed between the two aforementioned ICD families in the frame of EC PROSBAS (Prototyping and Support to Standardization of SBAS L1/L5 Multi-Constellation Receiver) related activities. This trade-off includes a qualitative analysis of the vertical Protection Level performances achieved by each alternative. The aforementioned results have already been presented to IWG and IWG feedback has been taken into account in the analysis.
For this trade-off analysis the following strategy has been adopted:
a) Definition of Study cases: a complete definition of the SBAS L1/l5 ICD families (and the alternatives for each family), including an initial list of their weak and strong points, was generated;
b) Definition of trade-off criteria: the criteria used to compare the L1/L5 SBAS ICDs were defined. The trade-off criteria included the analysis of the following concepts:
1. Reuse of existing concepts/models
2. Simplicity of the model for the user equipment manufacturer
3. Simplicity of the model for the service provider
4. Semi-quantitative VPL performance
5. Capability to recover L1 legacy performance
6. Level of Consolidation of ICD models
7. Robustness
8. Dependency on core GNSS required specifications
9. Expandability
10. Definition of scenarios: the scenarios for which the qualitative trade-off has been performed were characterised, including the parameters used for the semi-quantitative VPL performance analysis (e.g. constellations´ almanacs, service area & expected DOP values, satellite related parameters and other correction bounding parameters). For the estimation of the vertical PLs, a simple model for the degradation of the integrity and clock parameters has been used.
11. Qualitative trade-off of the ICD families: a qualitative trade-off of the two SBAS L1/L5 families was developed covering the previously defined trade-off criteria and taking into account the scenarios definition already provided.
12. Critical analysis of the results: a critical analysis of the results was finally performed.
The main results for each trade-off criterion are presented in this paper, including a summary of the VPL performances achieved for optimistic, nominal and pessimistic cases, as well as for 1-4 constellations and with each ICD family.
To sum up, the following preliminary conclusions can be provided from all the analyses developed in this study:
1) One of the key aspects for the selection of a SBAS L1/L5 model is the effect of the integrity and clock parameters degradation. In this preliminary analysis a simple model for the degradation of the integrity and clock parameters has been taken into account;
2) Qualitative trade-off: there is no definitive advantage of one ICD with respect to the other, and strength or weaknesses are balanced;
3) Concluding scenario preliminary performances: A scenario including the effect of the integrity and clock parameters degradation and other key effects has been prepared and is to be used to analyse the joint impact on VPL performances of the main effects analysed in this study. A description of this concluding scenario as well as the main conclusions derived from it will be provided in the paper.
Further analysis are planned to consolidate and extend the preliminary conclusions presented in this paper. These activities are part of the EC project PROSBAS.