There seem to be two ways to achieve roll steering, also known as
GPSS.
1) The GPS (which contains the planned route, or the IAP waypoints
etc) generates the turning parameters internally, and transmits them
directly to the autopilot; typically using ARINC. In this case, the
autopilot is in NAV mode to get GPSS.
2) There is an add-on box, which takes the data stream coming out of
the GPS (which contains the route, as in 1) above) and this box works
out when to turn. It then generates a faked heading bug signal (which
I believe is a quadrature sine/cosine signal - what does a normal HSI
output for the heading bug setting?). In this case, the autopilot is
in HDG mode to get GPSS.
Method 2) can be used with a GPS, or with an autopilot, if the two
won't talk to each other. But it still needs the add-on box to be fed
with the currently active waypoint details, and the one that follows
it, otherwise how could it calculate the turn?
Is the above description correct? Are there any other variations?
Next Q is: How does GPSS happen if you have an EHSI? Perhaps this is
method 3). In this case, the EHSI course pointer flips automatically
at each waypoint, to the new track. But something somewhere has to do
a turn calculation, otherwise the autopilot would not do a smooth turn
onto the new track. In this case, does the EHSI work like the add-on
box in case 2) and the course pointer flipping to the new track is not
related to the info fed to the autopilot? If this is true, the AP
would be in HDG mode with an EHSI but I believe this is wrong; NAV
mode is used. Maybe the EHSI takes the route data from the GPS, does
the turn calculation, and sends the turning data to the (compatible)
AP using ARINC, as in case 1) above.. ??
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