Extended Radiobeacon Almanac

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This is an optional message not considered in the baseline of EGNOS. Messages of type 27 are used to increase the UDRE values that are broadcast through messages of type 2 to 5, 6 or 24 over several selected areas. This degradation is incompatible with the one defined in message type 28. The message contains the value of δUDRE factor (a multiplier factor) to be applied to integrity monitoring algorithms depending on the user location: inside any defined region or outside of all regions. Each message contains up to 5 regions. If more regions are defined in the SBAS, more than one message type 27 will be broadcast. Different messages of type 27 are linked between them via IODS parameter (Issue of Data Service Message). Each time a parameter in any message type 27 of the group is changed, the IODS is incremented, being the effective range is from 0 to 7. Two messages with the same IODS have the same value for δUDRE factor outside.

Priority code is used to allow the overlapping of the regions. A user situated in the intersection of two or more regions will use the δUDRE factor for the region with the higher priority code. In case of equality in priority code, the user will use the smallest δUDRE factor as this results in better performance. Messages type 27 format is defined in the section A.4.4.13 of MOPS.