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What would cause speed (Field 8) in GPRMC to remain constant for some amount of time?

GPRMC message reports speed over ground (SOG) in field 8. If going through a GNSS outage, the position status would report PROPAGATED.

When the solution is in PROPAGATED it is just propagating the last known position forward in a constant straight line, hence why the "speed over ground" and the "track made good" fields remain constant for those epochs. Depending on how many satellites are available the solution may attempt to use the available satellites to reconverge on a good solution, which is why it is possible for the speed over ground to change while in PROPAGATED.