A Rigid Message Scheduler for SBAS

Satellite Based Augmentation Systems (SBASs) broadcast a series of 250-bit messages that together contain a full set of differential correction and integrity information. Each message requires one second to send and contains only a small subset of the full set of required information. Different types of data have different update periods, ranging from six seconds for the satellite integrity information, to five minutes for the ionospheric corrections. A key component of an SBAS is deciding which message needs to be sent at any given second. Each message has an associated time-out period that is approximately two times the update period. It is possible to get into a situation where several messages will time out within a very short interval and it may not be possible to update all of them within the desired timeframe. Further, if there is a change in integrity status that requires an alert to be broadcast, these alert messages will preempt and delay the broadcast of otherwise intended messages. The Wide Area Augmentation System (W AAS) currently uses a dynamic message scheduler that actively determines which messages should go out over the next several seconds. As the corrections or confidence levels change, some messages may change in priority. This scheduler has been carefully tuned for the current number of GPS satellites and ionospheric grid points, but the algorithm is not adaptable should either value change appreciably. Further, with the advent of L5 messages that have their content organized very differently (and that have no ionospheric corrections) a new message scheduling algorithm is needed.