Editing SimTX API/Tracking and pointing (section)

== Rotators ==
The rotator of a steerable antenna turns at finite speed. The simulation uses its actual heading, not the commanded one.
* '''Rates''' come from the antenna's pointing, each axis on its own. 0 selects the defaults: 3 °/s for an <code>AZ</code> mount; 6 °/s in azimuth and 4 °/s in elevation for an <code>AZEL</code> mount.
* '''Stops:''' the azimuth stays within 0 to 360°, with the stop at north. A turn never crosses north, so a turn from 350° to 10° goes the long way round. Elevation is 0 to 90°.
* '''Fixed headings:''' an [[SimTX API/Packets/AntennaPointingUpdate|AntennaPointingUpdate]] with <code>POINTING_MODE_FIXED</code> turns the rotator from wherever it is at that moment: parked, mid-turn or mid-track. [[SimTX API/Packets/SessionStatus|SessionStatus]] <code>UPDATED</code> reports the commanded heading in <code>pointing</code> at once; a client that draws the rotator animates the turn from the rates. Repeating the current heading does not restart the turn.
* '''Tracking:''' a session that starts with <code>point_antenna</code>, or switches to <code>POINTING_MODE_TRACK</code> or to a new target, starts slewing at that moment from where the rotator is, and then follows the target within its rate limits. The azimuth and elevation of a <code>TRACK</code> request are ignored.
* '''Slew start:''' while a rotator follows a target, <code>tracking.since_epoch_seconds</code> and <code>since_nanos</code> in SessionStatus <code>BOUND</code> and <code>UPDATED</code> give the simulation time at which it began slewing towards the target. With the rates they give the rotator's position for display. Clients leave these fields unset in their own requests; servers ignore them there.
* '''Paths:''' the actual heading applies to every propagation path: HF skywave and groundwave, VHF and UHF terrestrial paths, Earth-space links and the sky-noise beam. While the rotator turns, the server re-plans the link about once a second until the rotator arrives.
* '''Lag:''' a tracking rotator lags a fast satellite. On a 500 km overhead pass a 6 °/s rotator falls up to 16° behind just after closest approach, and a 3 °/s rotator up to 32°.

The pointing error reduces the antenna gain towards the target as the antenna pattern says.