What is the SBAS message (RXM-SRFBX) time-of-validity?

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The UBX-RXM-SFRBX message (Broadcast Nav Data Subframe) does not containing any timing information. But when decoding and applying SBAS data, I need to know the time-of-validity of the message. The SBAS msg tov is defined as the start of the epoch when the first bit was transmitted from the GEO. So, normally, this is one second prior to when the SBAS data message was received. Is there a way to determine when the SBAS subframe was received? Section 9 of the Protocol & Spec (Broadcast Navigation Data) simply states "a separate message is generated every time the receiver decodes a complete subframe of data from a tracked signal." I need the time when that message was generated.

Thanks!
by tcashin asked Aug 9
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0 votes
The other SFRBX reporting contain an HOW/TLM
by clive1 answered Aug 9
How does that help? I need the tov for SBAS frames, not GPS/GLONASS/Galileo frames. And there is no way that I know of to tie the time of one SFRBX message to the time of another.
Well the navigation data isn't some spurious transmission, it's part of a sequenced synchronous stream.
Time is relatively linear, you have a common view of it, the local time, and time of transit have been established into the nano-second level. Even in a relatively crude implementation one should be able to establish a sub-microsecond perspective on when a particular sub frame arrived at the receiver, and when it left the satellite.
I know it can be done onboard the chip, but I only have access to the messages that are sent to a port, in whatever order that ublox decides to send them. Sounds like you are asking me to assume that a GPS subframe (which arrive every six seconds) would be sent before the SBAS subframe for the corresponding GPS z-count.  (BTW, SBAS frames are sent every second) But, I have no idea if these messages are buffered, synchronous, or asynchronous. Is there somewhere in the spec that states this?
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