Post 6725 Ole Ole: More thoughts on ping accuracy
- Inmarsat-C uses TDM (time division multiplexing).
- Symbol duration within the time slots is 8.64s/10368 symbols = 833 microseconds per symbol
Transmissions in adjacent time slots have to arrive at the receiver (satellite) within the accuracy of one symbol. => Propagation delay has to be known with min. accuracy of 833 microseconds which is corresponds to 250km.
That is imho the upper error margin for the ping accuracy. For reasons given earlier I'm still optimistic accuracy is far better.
The arcs with greater radius have more favorable geometry thus accuracy is better further out. Think cutting a tomato (earth) into slices. Put the tomato to the far left of the cutting board and imagine the satellite to the far right. Each cut through the tomato marks points that are equidistant to the satellite. The skin of the slices is the set of points on the surface that lie within the error margin. The outer slices (those closer to the satellite) have more skin, i.e bigger error.
Stand to be corrected.
No correction here, but compliments on the great tomato teaching analogy. CDMA would be better for the satellite to use than time division, since propagation delay does not affect it as much, since the bits know how to put themselves back in order at their destination, since they are encoded in a Walsh Code. Of course, it's not easy to upgrade a satellite as well as the many customer transceivers, unless they have an up-loadable modulation format. So it's probably just academic. Maybe the next set of satellites and customer equipment will have CDMA modulation. Of course, the very nature of time division multiplexing may make it easier to use the shift in the spaces between the bits, as the customer moves, for tracking purposes.