many thanks Wooks61
I had seen those docs, but only looked casually as they all seem to assume some kind of receiver unit. I was hoping the mast units behaved in a similar fashion to the old fine offset unit, i.e they transmit a data stream every few seconds to a receiver unit. It's the format of the transmitted data stream that I'm after. Those docs seem to concentrate on data from the receiver units rather than the mast unit.
I may have missed something so I will read the docs in more detail.